Updated: 2022/Sep/29
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GCC(1) GNU GCC(1)
NAME
gcc - GNU project C and C++ compiler
SYNOPSIS
gcc [-c|-S|-E] [-std=standard]
[-g] [-pg] [-Olevel]
[-Wwarn...] [-Wpedantic]
[-Idir...] [-Ldir...]
[-Dmacro[=defn]...] [-Umacro]
[-foption...] [-mmachine-option...]
[-o outfile] [@file] infile...
Only the most useful options are listed here; see below for the
remainder. g++ accepts mostly the same options as gcc.
DESCRIPTION
When you invoke GCC, it normally does preprocessing, compilation,
assembly and linking. The "overall options" allow you to stop this
process at an intermediate stage. For example, the -c option says not
to run the linker. Then the output consists of object files output by
the assembler.
Other options are passed on to one or more stages of processing. Some
options control the preprocessor and others the compiler itself. Yet
other options control the assembler and linker; most of these are not
documented here, since you rarely need to use any of them.
Most of the command-line options that you can use with GCC are useful
for C programs; when an option is only useful with another language
(usually C++), the explanation says so explicitly. If the description
for a particular option does not mention a source language, you can use
that option with all supported languages.
The usual way to run GCC is to run the executable called gcc, or
machine-gcc when cross-compiling, or machine-gcc-version to run a
specific version of GCC. When you compile C++ programs, you should
invoke GCC as g++ instead.
The gcc program accepts options and file names as operands. Many
options have multi-letter names; therefore multiple single-letter
options may not be grouped: -dv is very different from -d -v.
You can mix options and other arguments. For the most part, the order
you use doesn't matter. Order does matter when you use several options
of the same kind; for example, if you specify -L more than once, the
directories are searched in the order specified. Also, the placement
of the -l option is significant.
Many options have long names starting with -f or with -W---for example,
-fmove-loop-invariants, -Wformat and so on. Most of these have both
positive and negative forms; the negative form of -ffoo is -fno-foo.
This manual documents only one of these two forms, whichever one is not
the default.
Some options take one or more arguments typically separated either by a
space or by the equals sign (=) from the option name. Unless
documented otherwise, an argument can be either numeric or a string.
Numeric arguments must typically be small unsigned decimal or
hexadecimal integers. Hexadecimal arguments must begin with the 0x
prefix. Arguments to options that specify a size threshold of some
sort may be arbitrarily large decimal or hexadecimal integers followed
by a byte size suffix designating a multiple of bytes such as "kB" and
"KiB" for kilobyte and kibibyte, respectively, "MB" and "MiB" for
megabyte and mebibyte, "GB" and "GiB" for gigabyte and gigibyte, and so
on. Such arguments are designated by byte-size in the following text.
Refer to the NIST, IEC, and other relevant national and international
standards for the full listing and explanation of the binary and
decimal byte size prefixes.
OPTIONS
Option Summary
Here is a summary of all the options, grouped by type. Explanations
are in the following sections.
Overall Options
-c -S -E -o file -x language -v -### --help[=class[,...]]
--target-help --version -pass-exit-codes -pipe -specs=file
-wrapper @file -ffile-prefix-map=old=new -fplugin=file
-fplugin-arg-name=arg -fdump-ada-spec[-slim]
-fada-spec-parent=unit -fdump-go-spec=file
C Language Options
-ansi -std=standard -fgnu89-inline
-fpermitted-flt-eval-methods=standard -aux-info filename
-fallow-parameterless-variadic-functions -fno-asm -fno-builtin
-fno-builtin-function -fgimple -fhosted -ffreestanding -fopenacc
-fopenacc-dim=geom -fopenmp -fopenmp-simd -fms-extensions
-fplan9-extensions -fsso-struct=endianness
-fallow-single-precision -fcond-mismatch -flax-vector-conversions
-fsigned-bitfields -fsigned-char -funsigned-bitfields
-funsigned-char
C++ Language Options
-fabi-version=n -fno-access-control -faligned-new=n
-fargs-in-order=n -fchar8_t -fcheck-new -fconstexpr-depth=n
-fconstexpr-cache-depth=n -fconstexpr-loop-limit=n
-fconstexpr-ops-limit=n -fno-elide-constructors
-fno-enforce-eh-specs -fno-gnu-keywords -fno-implicit-templates
-fno-implicit-inline-templates -fno-implement-inlines
-fms-extensions -fnew-inheriting-ctors -fnew-ttp-matching
-fno-nonansi-builtins -fnothrow-opt -fno-operator-names
-fno-optional-diags -fpermissive -fno-pretty-templates -fno-rtti
-fsized-deallocation -ftemplate-backtrace-limit=n
-ftemplate-depth=n -fno-threadsafe-statics -fuse-cxa-atexit
-fno-weak -nostdinc++ -fvisibility-inlines-hidden
-fvisibility-ms-compat -fext-numeric-literals -Wabi-tag
-Wcatch-value -Wcatch-value=n -Wno-class-conversion
-Wclass-memaccess -Wcomma-subscript -Wconditionally-supported
-Wno-conversion-null -Wctor-dtor-privacy -Wno-delete-incomplete
-Wdelete-non-virtual-dtor -Wdeprecated-copy
-Wdeprecated-copy-dtor -Weffc++ -Wextra-semi
-Wno-inaccessible-base -Wno-inherited-variadic-ctor
-Wno-init-list-lifetime -Wno-invalid-offsetof -Wno-literal-suffix
-Wmismatched-tags -Wmultiple-inheritance -Wnamespaces -Wnarrowing
-Wnoexcept -Wnoexcept-type -Wnon-virtual-dtor -Wpessimizing-move
-Wno-placement-new -Wplacement-new=n -Wredundant-move
-Wredundant-tags -Wreorder -Wregister -Wstrict-null-sentinel
-Wno-subobject-linkage -Wtemplates -Wno-non-template-friend
-Wold-style-cast -Woverloaded-virtual -Wno-pmf-conversions
-Wsign-promo -Wsized-deallocation -Wsuggest-final-methods
-Wsuggest-final-types -Wsuggest-override -Wno-terminate
-Wuseless-cast -Wvirtual-inheritance -Wno-virtual-move-assign
-Wvolatile -Wzero-as-null-pointer-constant
Objective-C and Objective-C++ Language Options
-fconstant-string-class=class-name -fgnu-runtime -fnext-runtime
-fno-nil-receivers -fobjc-abi-version=n -fobjc-call-cxx-cdtors
-fobjc-direct-dispatch -fobjc-exceptions -fobjc-gc -fobjc-nilcheck
-fobjc-std=objc1 -fno-local-ivars
-fivar-visibility=[public|protected|private|package]
-freplace-objc-classes -fzero-link -gen-decls -Wassign-intercept
-Wno-property-assign-default -Wno-protocol -Wselector
-Wstrict-selector-match -Wundeclared-selector
Diagnostic Message Formatting Options
-fmessage-length=n -fdiagnostics-show-location=[once|every-line]
-fdiagnostics-color=[auto|never|always]
-fdiagnostics-urls=[auto|never|always]
-fdiagnostics-format=[text|json] -fno-diagnostics-show-option
-fno-diagnostics-show-caret -fno-diagnostics-show-labels
-fno-diagnostics-show-line-numbers -fno-diagnostics-show-cwe
-fdiagnostics-minimum-margin-width=width
-fdiagnostics-parseable-fixits -fdiagnostics-generate-patch
-fdiagnostics-show-template-tree -fno-elide-type
-fdiagnostics-path-format=[none|separate-events|inline-events]
-fdiagnostics-show-path-depths -fno-show-column
Warning Options
-fsyntax-only -fmax-errors=n -Wpedantic -pedantic-errors -w
-Wextra -Wall -Wabi=n -Waddress -Wno-address-of-packed-member
-Waggregate-return -Walloc-size-larger-than=byte-size -Walloc-zero
-Walloca -Walloca-larger-than=byte-size
-Wno-aggressive-loop-optimizations -Warith-conversion
-Warray-bounds -Warray-bounds=n -Wno-attributes
-Wattribute-alias=n -Wno-attribute-alias -Wno-attribute-warning
-Wbool-compare -Wbool-operation -Wno-builtin-declaration-mismatch
-Wno-builtin-macro-redefined -Wc90-c99-compat -Wc99-c11-compat
-Wc11-c2x-compat -Wc++-compat -Wc++11-compat -Wc++14-compat
-Wc++17-compat -Wc++20-compat -Wcast-align -Wcast-align=strict
-Wcast-function-type -Wcast-qual -Wchar-subscripts -Wclobbered
-Wcomment -Wconversion -Wno-coverage-mismatch -Wno-cpp
-Wdangling-else -Wdate-time -Wno-deprecated
-Wno-deprecated-declarations -Wno-designated-init
-Wdisabled-optimization -Wno-discarded-array-qualifiers
-Wno-discarded-qualifiers -Wno-div-by-zero -Wdouble-promotion
-Wduplicated-branches -Wduplicated-cond -Wempty-body
-Wno-endif-labels -Wenum-compare -Wenum-conversion -Werror
-Werror=* -Wexpansion-to-defined -Wfatal-errors
-Wfloat-conversion -Wfloat-equal -Wformat -Wformat=2
-Wno-format-contains-nul -Wno-format-extra-args
-Wformat-nonliteral -Wformat-overflow=n -Wformat-security
-Wformat-signedness -Wformat-truncation=n -Wformat-y2k
-Wframe-address -Wframe-larger-than=byte-size
-Wno-free-nonheap-object -Wno-hsa -Wno-if-not-aligned
-Wno-ignored-attributes -Wignored-qualifiers
-Wno-incompatible-pointer-types -Wimplicit -Wimplicit-fallthrough
-Wimplicit-fallthrough=n -Wno-implicit-function-declaration
-Wno-implicit-int -Winit-self -Winline -Wno-int-conversion
-Wint-in-bool-context -Wno-int-to-pointer-cast
-Wno-invalid-memory-model -Winvalid-pch -Wjump-misses-init
-Wlarger-than=byte-size -Wlogical-not-parentheses -Wlogical-op
-Wlong-long -Wno-lto-type-mismatch -Wmain -Wmaybe-uninitialized
-Wmemset-elt-size -Wmemset-transposed-args
-Wmisleading-indentation -Wmissing-attributes -Wmissing-braces
-Wmissing-field-initializers -Wmissing-format-attribute
-Wmissing-include-dirs -Wmissing-noreturn -Wno-missing-profile
-Wno-multichar -Wmultistatement-macros -Wnonnull
-Wnonnull-compare -Wnormalized=[none|id|nfc|nfkc]
-Wnull-dereference -Wno-odr -Wopenmp-simd -Wno-overflow
-Woverlength-strings -Wno-override-init-side-effects -Wpacked
-Wno-packed-bitfield-compat -Wpacked-not-aligned -Wpadded
-Wparentheses -Wno-pedantic-ms-format -Wpointer-arith
-Wno-pointer-compare -Wno-pointer-to-int-cast -Wno-pragmas
-Wno-prio-ctor-dtor -Wredundant-decls -Wrestrict
-Wno-return-local-addr -Wreturn-type -Wno-scalar-storage-order
-Wsequence-point -Wshadow -Wshadow=global -Wshadow=local
-Wshadow=compatible-local -Wno-shadow-ivar
-Wno-shift-count-negative -Wno-shift-count-overflow
-Wshift-negative-value -Wno-shift-overflow -Wshift-overflow=n
-Wsign-compare -Wsign-conversion -Wno-sizeof-array-argument
-Wsizeof-pointer-div -Wsizeof-pointer-memaccess -Wstack-protector
-Wstack-usage=byte-size -Wstrict-aliasing -Wstrict-aliasing=n
-Wstrict-overflow -Wstrict-overflow=n -Wstring-compare
-Wstringop-overflow=n -Wno-stringop-truncation
-Wsuggest-attribute=[pure|const|noreturn|format|malloc] -Wswitch
-Wno-switch-bool -Wswitch-default -Wswitch-enum
-Wno-switch-outside-range -Wno-switch-unreachable -Wsync-nand
-Wsystem-headers -Wtautological-compare -Wtrampolines
-Wtrigraphs -Wtype-limits -Wundef -Wuninitialized
-Wunknown-pragmas -Wunsuffixed-float-constants -Wunused
-Wunused-but-set-parameter -Wunused-but-set-variable
-Wunused-const-variable -Wunused-const-variable=n
-Wunused-function -Wunused-label -Wunused-local-typedefs
-Wunused-macros -Wunused-parameter -Wno-unused-result
-Wunused-value -Wunused-variable -Wno-varargs -Wvariadic-macros
-Wvector-operation-performance -Wvla -Wvla-larger-than=byte-size
-Wno-vla-larger-than -Wvolatile-register-var -Wwrite-strings
-Wzero-length-bounds
Static Analyzer Options
-fanalyzer -fanalyzer-call-summaries -fanalyzer-checker=name
-fanalyzer-fine-grained -fanalyzer-state-merge
-fanalyzer-state-purge -fanalyzer-transitivity
-fanalyzer-verbose-edges -fanalyzer-verbose-state-changes
-fanalyzer-verbosity=level -fdump-analyzer -fdump-analyzer-stderr
-fdump-analyzer-callgraph -fdump-analyzer-exploded-graph
-fdump-analyzer-exploded-nodes -fdump-analyzer-exploded-nodes-2
-fdump-analyzer-exploded-nodes-3 -fdump-analyzer-state-purge
-fdump-analyzer-supergraph -Wno-analyzer-double-fclose
-Wno-analyzer-double-free
-Wno-analyzer-exposure-through-output-file -Wno-analyzer-file-leak
-Wno-analyzer-free-of-non-heap -Wno-analyzer-malloc-leak
-Wno-analyzer-null-argument -Wno-analyzer-null-dereference
-Wno-analyzer-possible-null-argument
-Wno-analyzer-possible-null-dereference
-Wno-analyzer-stale-setjmp-buffer -Wno-analyzer-tainted-array-index
-Wanalyzer-too-complex
-Wno-analyzer-unsafe-call-within-signal-handler
-Wno-analyzer-use-after-free
-Wno-analyzer-use-of-pointer-in-stale-stack-frame
-Wno-analyzer-use-of-uninitialized-value
C and Objective-C-only Warning Options
-Wbad-function-cast -Wmissing-declarations
-Wmissing-parameter-type -Wmissing-prototypes -Wnested-externs
-Wold-style-declaration -Wold-style-definition -Wstrict-prototypes
-Wtraditional -Wtraditional-conversion
-Wdeclaration-after-statement -Wpointer-sign
Debugging Options
-g -glevel -gdwarf -gdwarf-version -ggdb -grecord-gcc-switches
-gno-record-gcc-switches -gstabs -gstabs+ -gstrict-dwarf
-gno-strict-dwarf -gas-loc-support -gno-as-loc-support
-gas-locview-support -gno-as-locview-support -gcolumn-info
-gno-column-info -gstatement-frontiers -gno-statement-frontiers
-gvariable-location-views -gno-variable-location-views
-ginternal-reset-location-views -gno-internal-reset-location-views
-ginline-points -gno-inline-points -gvms -gxcoff -gxcoff+
-gz[=type] -gsplit-dwarf -gdescribe-dies -gno-describe-dies
-fdebug-prefix-map=old=new -fdebug-types-section
-fno-eliminate-unused-debug-types -femit-struct-debug-baseonly
-femit-struct-debug-reduced -femit-struct-debug-detailed[=spec-
list] -fno-eliminate-unused-debug-symbols
-femit-class-debug-always -fno-merge-debug-strings
-fno-dwarf2-cfi-asm -fvar-tracking -fvar-tracking-assignments
Optimization Options
-faggressive-loop-optimizations -falign-functions[=n[:m:[n2[:m2]]]]
-falign-jumps[=n[:m:[n2[:m2]]]] -falign-labels[=n[:m:[n2[:m2]]]]
-falign-loops[=n[:m:[n2[:m2]]]] -fno-allocation-dce
-fallow-store-data-races -fassociative-math -fauto-profile
-fauto-profile[=path] -fauto-inc-dec -fbranch-probabilities
-fcaller-saves -fcombine-stack-adjustments -fconserve-stack
-fcompare-elim -fcprop-registers -fcrossjumping
-fcse-follow-jumps -fcse-skip-blocks -fcx-fortran-rules
-fcx-limited-range -fdata-sections -fdce -fdelayed-branch
-fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fdevirtualize-at-ltrans -fdse
-fearly-inlining -fipa-sra -fexpensive-optimizations
-ffat-lto-objects -ffast-math -ffinite-math-only -ffloat-store
-fexcess-precision=style -ffinite-loops -fforward-propagate
-ffp-contract=style -ffunction-sections -fgcse
-fgcse-after-reload -fgcse-las -fgcse-lm -fgraphite-identity
-fgcse-sm -fhoist-adjacent-loads -fif-conversion -fif-conversion2
-findirect-inlining -finline-functions
-finline-functions-called-once -finline-limit=n
-finline-small-functions -fipa-cp -fipa-cp-clone -fipa-bit-cp
-fipa-vrp -fipa-pta -fipa-profile -fipa-pure-const
-fipa-reference -fipa-reference-addressable -fipa-stack-alignment
-fipa-icf -fira-algorithm=algorithm -flive-patching=level
-fira-region=region -fira-hoist-pressure -fira-loop-pressure
-fno-ira-share-save-slots -fno-ira-share-spill-slots
-fisolate-erroneous-paths-dereference
-fisolate-erroneous-paths-attribute -fivopts
-fkeep-inline-functions -fkeep-static-functions
-fkeep-static-consts -flimit-function-alignment
-flive-range-shrinkage -floop-block -floop-interchange
-floop-strip-mine -floop-unroll-and-jam -floop-nest-optimize
-floop-parallelize-all -flra-remat -flto -flto-compression-level
-flto-partition=alg -fmerge-all-constants -fmerge-constants
-fmodulo-sched -fmodulo-sched-allow-regmoves
-fmove-loop-invariants -fno-branch-count-reg -fno-defer-pop
-fno-fp-int-builtin-inexact -fno-function-cse
-fno-guess-branch-probability -fno-inline -fno-math-errno
-fno-peephole -fno-peephole2 -fno-printf-return-value
-fno-sched-interblock -fno-sched-spec -fno-signed-zeros
-fno-toplevel-reorder -fno-trapping-math
-fno-zero-initialized-in-bss -fomit-frame-pointer
-foptimize-sibling-calls -fpartial-inlining -fpeel-loops
-fpredictive-commoning -fprefetch-loop-arrays -fprofile-correction
-fprofile-use -fprofile-use=path -fprofile-partial-training
-fprofile-values -fprofile-reorder-functions -freciprocal-math
-free -frename-registers -freorder-blocks
-freorder-blocks-algorithm=algorithm -freorder-blocks-and-partition
-freorder-functions -frerun-cse-after-loop
-freschedule-modulo-scheduled-loops -frounding-math
-fsave-optimization-record -fsched2-use-superblocks
-fsched-pressure -fsched-spec-load -fsched-spec-load-dangerous
-fsched-stalled-insns-dep[=n] -fsched-stalled-insns[=n]
-fsched-group-heuristic -fsched-critical-path-heuristic
-fsched-spec-insn-heuristic -fsched-rank-heuristic
-fsched-last-insn-heuristic -fsched-dep-count-heuristic
-fschedule-fusion -fschedule-insns -fschedule-insns2
-fsection-anchors -fselective-scheduling -fselective-scheduling2
-fsel-sched-pipelining -fsel-sched-pipelining-outer-loops
-fsemantic-interposition -fshrink-wrap -fshrink-wrap-separate
-fsignaling-nans -fsingle-precision-constant
-fsplit-ivs-in-unroller -fsplit-loops -fsplit-paths
-fsplit-wide-types -fsplit-wide-types-early -fssa-backprop
-fssa-phiopt -fstdarg-opt -fstore-merging -fstrict-aliasing
-fthread-jumps -ftracer -ftree-bit-ccp -ftree-builtin-call-dce
-ftree-ccp -ftree-ch -ftree-coalesce-vars -ftree-copy-prop
-ftree-dce -ftree-dominator-opts -ftree-dse -ftree-forwprop
-ftree-fre -fcode-hoisting -ftree-loop-if-convert -ftree-loop-im
-ftree-phiprop -ftree-loop-distribution
-ftree-loop-distribute-patterns -ftree-loop-ivcanon
-ftree-loop-linear -ftree-loop-optimize -ftree-loop-vectorize
-ftree-parallelize-loops=n -ftree-pre -ftree-partial-pre
-ftree-pta -ftree-reassoc -ftree-scev-cprop -ftree-sink
-ftree-slsr -ftree-sra -ftree-switch-conversion -ftree-tail-merge
-ftree-ter -ftree-vectorize -ftree-vrp -funconstrained-commons
-funit-at-a-time -funroll-all-loops -funroll-loops
-funsafe-math-optimizations -funswitch-loops -fipa-ra
-fvariable-expansion-in-unroller -fvect-cost-model -fvpt -fweb
-fwhole-program -fwpa -fuse-linker-plugin --param name=value -O
-O0 -O1 -O2 -O3 -Os -Ofast -Og
Program Instrumentation Options
-p -pg -fprofile-arcs --coverage -ftest-coverage
-fprofile-abs-path -fprofile-dir=path -fprofile-generate
-fprofile-generate=path -fprofile-note=path
-fprofile-prefix-path=path -fprofile-update=method
-fprofile-filter-files=regex -fprofile-exclude-files=regex
-fprofile-reproducible=[multithreaded|parallel-runs|serial]
-fsanitize=style -fsanitize-recover -fsanitize-recover=style
-fasan-shadow-offset=number -fsanitize-sections=s1,s2,...
-fsanitize-undefined-trap-on-error -fbounds-check
-fcf-protection=[full|branch|return|none|check] -fstack-protector
-fstack-protector-all -fstack-protector-strong
-fstack-protector-explicit -fstack-check
-fstack-limit-register=reg -fstack-limit-symbol=sym
-fno-stack-limit -fsplit-stack -fvtable-verify=[std|preinit|none]
-fvtv-counts -fvtv-debug -finstrument-functions
-finstrument-functions-exclude-function-list=sym,sym,...
-finstrument-functions-exclude-file-list=file,file,...
Preprocessor Options
-Aquestion=answer -A-question[=answer] -C -CC -Dmacro[=defn] -dD
-dI -dM -dN -dU -fdebug-cpp -fdirectives-only
-fdollars-in-identifiers -fexec-charset=charset
-fextended-identifiers -finput-charset=charset
-fmacro-prefix-map=old=new -fmax-include-depth=depth
-fno-canonical-system-headers -fpch-deps -fpch-preprocess
-fpreprocessed -ftabstop=width -ftrack-macro-expansion
-fwide-exec-charset=charset -fworking-directory -H -imacros file
-include file -M -MD -MF -MG -MM -MMD -MP -MQ -MT
-no-integrated-cpp -P -pthread -remap -iprefix file
-iwithprefix dir -traditional -traditional-cpp -trigraphs -Umacro
-undef -Wp,option -Xpreprocessor option
Assembler Options
-Wa,option -Xassembler option
Linker Options
object-file-name -fuse-ld=linker -llibrary -nostartfiles
-nodefaultlibs -nolibc -nostdlib -e entry --entry=entry -pie
-pthread -r -rdynamic -s -static -static-pie -static-libgcc
-static-libstdc++ -static-libasan -static-libtsan -static-liblsan
-static-libubsan -shared -shared-libgcc -symbolic -T script
-Wl,option -Xlinker option -u symbol -z keyword
Directory Options
-Bprefix -Idir -I- -idirafter dir -imacros file -imultilib dir
-iplugindir=dir -iquote dir -isysroot dir -isystem dir
-iremapsrc:dst -cxx-isystem=dir -iwithprefix dir
-iwithprefixbefore dir -Ldir -no-canonical-prefixes
--no-sysroot-suffix -nostdinc -nostdinc++ --sysroot=dir
Code Generation Options
-fcall-saved-reg -fcall-used-reg -ffixed-reg -fexceptions
-fnon-call-exceptions -fdelete-dead-exceptions -funwind-tables
-fasynchronous-unwind-tables -fno-gnu-unique
-finhibit-size-directive -fcommon -fno-ident -fpcc-struct-return
-fpic -fPIC -fpie -fPIE -fno-plt -fno-jump-tables
-frecord-gcc-switches -freg-struct-return -fshort-enums
-fshort-wchar -fverbose-asm -fpack-struct[=n] -fleading-underscore
-ftls-model=model -fstack-reuse=reuse_level -ftrampolines -ftrapv
-fwrapv -fvisibility=[default|internal|hidden|protected]
-fstrict-volatile-bitfields -fsync-libcalls
Developer Options
-dletters -dumpspecs -dumpmachine -dumpversion -dumpfullversion
-fcallgraph-info[=su,da] -fchecking -fchecking=n -fdbg-cnt-list
-fdbg-cnt=counter-value-list -fdisable-ipa-pass_name
-fdisable-rtl-pass_name -fdisable-rtl-pass-name=range-list
-fdisable-tree-pass_name -fdisable-tree-pass-name=range-list
-fdump-debug -fdump-earlydebug -fdump-noaddr -fdump-unnumbered
-fdump-unnumbered-links -fdump-final-insns[=file] -fdump-ipa-all
-fdump-ipa-cgraph -fdump-ipa-inline -fdump-lang-all
-fdump-lang-switch -fdump-lang-switch-options
-fdump-lang-switch-options=filename -fdump-passes -fdump-rtl-pass
-fdump-rtl-pass=filename -fdump-statistics -fdump-tree-all
-fdump-tree-switch -fdump-tree-switch-options
-fdump-tree-switch-options=filename -fcompare-debug[=opts]
-fcompare-debug-second -fenable-kind-pass -fenable-kind-pass=range-
list -fira-verbose=n -flto-report -flto-report-wpa
-fmem-report-wpa -fmem-report -fpre-ipa-mem-report
-fpost-ipa-mem-report -fopt-info -fopt-info-options[=file]
-fprofile-report -frandom-seed=string -fsched-verbose=n
-fsel-sched-verbose -fsel-sched-dump-cfg
-fsel-sched-pipelining-verbose -fstats -fstack-usage
-ftime-report -ftime-report-details
-fvar-tracking-assignments-toggle -gtoggle
-print-file-name=library -print-libgcc-file-name
-print-multi-directory -print-multi-lib -print-multi-os-directory
-print-prog-name=program -print-search-dirs -Q -print-sysroot
-print-sysroot-headers-suffix -save-temps -save-temps=cwd
-save-temps=obj -time[=file]
Machine-Dependent Options
AArch64 Options -mabi=name -mbig-endian -mlittle-endian
-mgeneral-regs-only -mcmodel=tiny -mcmodel=small -mcmodel=large
-mstrict-align -mno-strict-align -momit-leaf-frame-pointer
-mtls-dialect=desc -mtls-dialect=traditional -mtls-size=size
-mfix-cortex-a53-835769 -mfix-cortex-a53-843419
-mlow-precision-recip-sqrt -mlow-precision-sqrt
-mlow-precision-div -mpc-relative-literal-loads
-msign-return-address=scope -mbranch-protection=none|standard|pac-
ret[+leaf +b-key]|bti -mharden-sls=opts -march=name -mcpu=name
-mtune=name -moverride=string -mverbose-cost-dump
-mstack-protector-guard=guard -mstack-protector-guard-reg=sysreg
-mstack-protector-guard-offset=offset -mtrack-speculation
-moutline-atomics
Adapteva Epiphany Options -mhalf-reg-file -mprefer-short-insn-regs
-mbranch-cost=num -mcmove -mnops=num -msoft-cmpsf -msplit-lohi
-mpost-inc -mpost-modify -mstack-offset=num -mround-nearest
-mlong-calls -mshort-calls -msmall16 -mfp-mode=mode
-mvect-double -max-vect-align=num -msplit-vecmove-early
-m1reg-reg
AMD GCN Options -march=gpu -mtune=gpu -mstack-size=bytes
ARC Options -mbarrel-shifter -mjli-always -mcpu=cpu -mA6
-mARC600 -mA7 -mARC700 -mdpfp -mdpfp-compact -mdpfp-fast
-mno-dpfp-lrsr -mea -mno-mpy -mmul32x16 -mmul64 -matomic -mnorm
-mspfp -mspfp-compact -mspfp-fast -msimd -msoft-float -mswap
-mcrc -mdsp-packa -mdvbf -mlock -mmac-d16 -mmac-24 -mrtsc
-mswape -mtelephony -mxy -misize -mannotate-align -marclinux
-marclinux_prof -mlong-calls -mmedium-calls -msdata
-mirq-ctrl-saved -mrgf-banked-regs -mlpc-width=width -G num
-mvolatile-cache -mtp-regno=regno -malign-call -mauto-modify-reg
-mbbit-peephole -mno-brcc -mcase-vector-pcrel -mcompact-casesi
-mno-cond-exec -mearly-cbranchsi -mexpand-adddi -mindexed-loads
-mlra -mlra-priority-none -mlra-priority-compact mlra-priority-
noncompact -mmillicode -mmixed-code -mq-class -mRcq -mRcw
-msize-level=level -mtune=cpu -mmultcost=num -mcode-density-frame
-munalign-prob-threshold=probability -mmpy-option=multo -mdiv-rem
-mcode-density -mll64 -mfpu=fpu -mrf16 -mbranch-index
ARM Options -mapcs-frame -mno-apcs-frame -mabi=name
-mapcs-stack-check -mno-apcs-stack-check -mapcs-reentrant
-mno-apcs-reentrant -mgeneral-regs-only -msched-prolog
-mno-sched-prolog -mlittle-endian -mbig-endian -mbe8 -mbe32
-mfloat-abi=name -mfp16-format=name -mthumb-interwork
-mno-thumb-interwork -mcpu=name -march=name -mfpu=name
-mtune=name -mprint-tune-info -mstructure-size-boundary=n
-mabort-on-noreturn -mlong-calls -mno-long-calls -msingle-pic-base
-mno-single-pic-base -mpic-register=reg -mnop-fun-dllimport
-mpoke-function-name -mthumb -marm -mflip-thumb -mtpcs-frame
-mtpcs-leaf-frame -mcaller-super-interworking
-mcallee-super-interworking -mtp=name -mtls-dialect=dialect
-mword-relocations -mfix-cortex-m3-ldrd -munaligned-access
-mneon-for-64bits -mslow-flash-data -masm-syntax-unified
-mrestrict-it -mverbose-cost-dump -mpure-code -mcmse
-mfix-cmse-cve-2021-35465 -mfdpic
AVR Options -mmcu=mcu -mabsdata -maccumulate-args
-mbranch-cost=cost -mcall-prologues -mgas-isr-prologues -mint8
-mdouble=bits -mlong-double=bits -mn_flash=size -mno-interrupts
-mmain-is-OS_task -mrelax -mrmw -mstrict-X -mtiny-stack
-mfract-convert-truncate -mshort-calls -nodevicelib
-nodevicespecs -Waddr-space-convert -Wmisspelled-isr
Blackfin Options -mcpu=cpu[-sirevision] -msim
-momit-leaf-frame-pointer -mno-omit-leaf-frame-pointer
-mspecld-anomaly -mno-specld-anomaly -mcsync-anomaly
-mno-csync-anomaly -mlow-64k -mno-low64k -mstack-check-l1
-mid-shared-library -mno-id-shared-library -mshared-library-id=n
-mleaf-id-shared-library -mno-leaf-id-shared-library -msep-data
-mno-sep-data -mlong-calls -mno-long-calls -mfast-fp
-minline-plt -mmulticore -mcorea -mcoreb -msdram -micplb
C6X Options -mbig-endian -mlittle-endian -march=cpu -msim
-msdata=sdata-type
CRIS Options -mcpu=cpu -march=cpu -mtune=cpu -mmax-stack-frame=n
-melinux-stacksize=n -metrax4 -metrax100 -mpdebug -mcc-init
-mno-side-effects -mstack-align -mdata-align -mconst-align
-m32-bit -m16-bit -m8-bit -mno-prologue-epilogue -mno-gotplt
-melf -maout -melinux -mlinux -sim -sim2 -mmul-bug-workaround
-mno-mul-bug-workaround
CR16 Options -mmac -mcr16cplus -mcr16c -msim -mint32 -mbit-ops
-mdata-model=model
C-SKY Options -march=arch -mcpu=cpu -mbig-endian -EB
-mlittle-endian -EL -mhard-float -msoft-float -mfpu=fpu
-mdouble-float -mfdivdu -melrw -mistack -mmp -mcp -mcache
-msecurity -mtrust -mdsp -medsp -mvdsp -mdiv -msmart
-mhigh-registers -manchor -mpushpop -mmultiple-stld -mconstpool
-mstack-size -mccrt -mbranch-cost=n -mcse-cc -msched-prolog
Darwin Options -all_load -allowable_client -arch
-arch_errors_fatal -arch_only -bind_at_load -bundle
-bundle_loader -client_name -compatibility_version
-current_version -dead_strip -dependency-file -dylib_file
-dylinker_install_name -dynamic -dynamiclib
-exported_symbols_list -filelist -flat_namespace
-force_cpusubtype_ALL -force_flat_namespace
-headerpad_max_install_names -iframework -image_base -init
-install_name -keep_private_externs -multi_module
-multiply_defined -multiply_defined_unused -noall_load
-no_dead_strip_inits_and_terms -nofixprebinding -nomultidefs
-noprebind -noseglinkedit -pagezero_size -prebind
-prebind_all_twolevel_modules -private_bundle -read_only_relocs
-sectalign -sectobjectsymbols -whyload -seg1addr -sectcreate
-sectobjectsymbols -sectorder -segaddr -segs_read_only_addr
-segs_read_write_addr -seg_addr_table -seg_addr_table_filename
-seglinkedit -segprot -segs_read_only_addr -segs_read_write_addr
-single_module -static -sub_library -sub_umbrella
-twolevel_namespace -umbrella -undefined -unexported_symbols_list
-weak_reference_mismatches -whatsloaded -F -gused -gfull
-mmacosx-version-min=version -mkernel -mone-byte-bool
DEC Alpha Options -mno-fp-regs -msoft-float -mieee
-mieee-with-inexact -mieee-conformant -mfp-trap-mode=mode
-mfp-rounding-mode=mode -mtrap-precision=mode -mbuild-constants
-mcpu=cpu-type -mtune=cpu-type -mbwx -mmax -mfix -mcix
-mfloat-vax -mfloat-ieee -mexplicit-relocs -msmall-data
-mlarge-data -msmall-text -mlarge-text -mmemory-latency=time
eBPF Options -mbig-endian -mlittle-endian -mkernel=version
-mframe-limit=bytes -mxbpf
FR30 Options -msmall-model -mno-lsim
FT32 Options -msim -mlra -mnodiv -mft32b -mcompress -mnopm
FRV Options -mgpr-32 -mgpr-64 -mfpr-32 -mfpr-64 -mhard-float
-msoft-float -malloc-cc -mfixed-cc -mdword -mno-dword -mdouble
-mno-double -mmedia -mno-media -mmuladd -mno-muladd -mfdpic
-minline-plt -mgprel-ro -multilib-library-pic -mlinked-fp
-mlong-calls -malign-labels -mlibrary-pic -macc-4 -macc-8 -mpack
-mno-pack -mno-eflags -mcond-move -mno-cond-move
-moptimize-membar -mno-optimize-membar -mscc -mno-scc
-mcond-exec -mno-cond-exec -mvliw-branch -mno-vliw-branch
-mmulti-cond-exec -mno-multi-cond-exec -mnested-cond-exec
-mno-nested-cond-exec -mtomcat-stats -mTLS -mtls -mcpu=cpu
GNU/Linux Options -mglibc -muclibc -mmusl -mbionic -mandroid
-tno-android-cc -tno-android-ld
H8/300 Options -mrelax -mh -ms -mn -mexr -mno-exr -mint32
-malign-300
HPPA Options -march=architecture-type -mcaller-copies
-mdisable-fpregs -mdisable-indexing -mfast-indirect-calls -mgas
-mgnu-ld -mhp-ld -mfixed-range=register-range -mjump-in-delay
-mlinker-opt -mlong-calls -mlong-load-store -mno-disable-fpregs
-mno-disable-indexing -mno-fast-indirect-calls -mno-gas
-mno-jump-in-delay -mno-long-load-store -mno-portable-runtime
-mno-soft-float -mno-space-regs -msoft-float -mpa-risc-1-0
-mpa-risc-1-1 -mpa-risc-2-0 -mportable-runtime -mschedule=cpu-
type -mspace-regs -msio -mwsio -munix=unix-std -nolibdld
-static -threads
IA-64 Options -mbig-endian -mlittle-endian -mgnu-as -mgnu-ld
-mno-pic -mvolatile-asm-stop -mregister-names -msdata -mno-sdata
-mconstant-gp -mauto-pic -mfused-madd
-minline-float-divide-min-latency
-minline-float-divide-max-throughput -mno-inline-float-divide
-minline-int-divide-min-latency -minline-int-divide-max-throughput
-mno-inline-int-divide -minline-sqrt-min-latency
-minline-sqrt-max-throughput -mno-inline-sqrt -mdwarf2-asm
-mearly-stop-bits -mfixed-range=register-range -mtls-size=tls-size
-mtune=cpu-type -milp32 -mlp64 -msched-br-data-spec
-msched-ar-data-spec -msched-control-spec -msched-br-in-data-spec
-msched-ar-in-data-spec -msched-in-control-spec -msched-spec-ldc
-msched-spec-control-ldc -msched-prefer-non-data-spec-insns
-msched-prefer-non-control-spec-insns
-msched-stop-bits-after-every-cycle
-msched-count-spec-in-critical-path
-msel-sched-dont-check-control-spec -msched-fp-mem-deps-zero-cost
-msched-max-memory-insns-hard-limit -msched-max-memory-insns=max-
insns
LM32 Options -mbarrel-shift-enabled -mdivide-enabled
-mmultiply-enabled -msign-extend-enabled -muser-enabled
M32R/D Options -m32r2 -m32rx -m32r -mdebug -malign-loops
-mno-align-loops -missue-rate=number -mbranch-cost=number
-mmodel=code-size-model-type -msdata=sdata-type -mno-flush-func
-mflush-func=name -mno-flush-trap -mflush-trap=number -G num
M32C Options -mcpu=cpu -msim -memregs=number
M680x0 Options -march=arch -mcpu=cpu -mtune=tune -m68000 -m68020
-m68020-40 -m68020-60 -m68030 -m68040 -m68060 -mcpu32 -m5200
-m5206e -m528x -m5307 -m5407 -mcfv4e -mbitfield -mno-bitfield
-mc68000 -mc68020 -mnobitfield -mrtd -mno-rtd -mdiv -mno-div
-mshort -mno-short -mhard-float -m68881 -msoft-float -mpcrel
-malign-int -mstrict-align -msep-data -mno-sep-data
-mshared-library-id=n -mid-shared-library -mno-id-shared-library
-mxgot -mno-xgot -mlong-jump-table-offsets
MCore Options -mhardlit -mno-hardlit -mdiv -mno-div
-mrelax-immediates -mno-relax-immediates -mwide-bitfields
-mno-wide-bitfields -m4byte-functions -mno-4byte-functions
-mcallgraph-data -mno-callgraph-data -mslow-bytes -mno-slow-bytes
-mno-lsim -mlittle-endian -mbig-endian -m210 -m340
-mstack-increment
MeP Options -mabsdiff -mall-opts -maverage -mbased=n -mbitops
-mc=n -mclip -mconfig=name -mcop -mcop32 -mcop64 -mivc2 -mdc
-mdiv -meb -mel -mio-volatile -ml -mleadz -mm -mminmax
-mmult -mno-opts -mrepeat -ms -msatur -msdram -msim
-msimnovec -mtf -mtiny=n
MicroBlaze Options -msoft-float -mhard-float -msmall-divides
-mcpu=cpu -mmemcpy -mxl-soft-mul -mxl-soft-div -mxl-barrel-shift
-mxl-pattern-compare -mxl-stack-check -mxl-gp-opt -mno-clearbss
-mxl-multiply-high -mxl-float-convert -mxl-float-sqrt
-mbig-endian -mlittle-endian -mxl-reorder -mxl-mode-app-model
-mpic-data-is-text-relative
MIPS Options -EL -EB -march=arch -mtune=arch -mips1 -mips2
-mips3 -mips4 -mips32 -mips32r2 -mips32r3 -mips32r5 -mips32r6
-mips64 -mips64r2 -mips64r3 -mips64r5 -mips64r6 -mips16
-mno-mips16 -mflip-mips16 -minterlink-compressed
-mno-interlink-compressed -minterlink-mips16 -mno-interlink-mips16
-mabi=abi -mabicalls -mno-abicalls -mshared -mno-shared -mplt
-mno-plt -mxgot -mno-xgot -mgp32 -mgp64 -mfp32 -mfpxx -mfp64
-mhard-float -msoft-float -mno-float -msingle-float
-mdouble-float -modd-spreg -mno-odd-spreg -mabs=mode
-mnan=encoding -mdsp -mno-dsp -mdspr2 -mno-dspr2 -mmcu
-mmno-mcu -meva -mno-eva -mvirt -mno-virt -mxpa -mno-xpa -mcrc
-mno-crc -mginv -mno-ginv -mmicromips -mno-micromips -mmsa
-mno-msa -mloongson-mmi -mno-loongson-mmi -mloongson-ext
-mno-loongson-ext -mloongson-ext2 -mno-loongson-ext2 -mfpu=fpu-
type -msmartmips -mno-smartmips -mpaired-single
-mno-paired-single -mdmx -mno-mdmx -mips3d -mno-mips3d -mmt
-mno-mt -mllsc -mno-llsc -mlong64 -mlong32 -msym32 -mno-sym32
-Gnum -mlocal-sdata -mno-local-sdata -mextern-sdata
-mno-extern-sdata -mgpopt -mno-gopt -membedded-data
-mno-embedded-data -muninit-const-in-rodata
-mno-uninit-const-in-rodata -mcode-readable=setting
-msplit-addresses -mno-split-addresses -mexplicit-relocs
-mno-explicit-relocs -mcheck-zero-division
-mno-check-zero-division -mdivide-traps -mdivide-breaks
-mload-store-pairs -mno-load-store-pairs -mmemcpy -mno-memcpy
-mlong-calls -mno-long-calls -mmad -mno-mad -mimadd -mno-imadd
-mfused-madd -mno-fused-madd -nocpp -mfix-24k -mno-fix-24k
-mfix-r4000 -mno-fix-r4000 -mfix-r4400 -mno-fix-r4400
-mfix-r5900 -mno-fix-r5900 -mfix-r10000 -mno-fix-r10000
-mfix-rm7000 -mno-fix-rm7000 -mfix-vr4120 -mno-fix-vr4120
-mfix-vr4130 -mno-fix-vr4130 -mfix-sb1 -mno-fix-sb1
-mflush-func=func -mno-flush-func -mbranch-cost=num
-mbranch-likely -mno-branch-likely -mcompact-branches=policy
-mfp-exceptions -mno-fp-exceptions -mvr4130-align
-mno-vr4130-align -msynci -mno-synci -mlxc1-sxc1 -mno-lxc1-sxc1
-mmadd4 -mno-madd4 -mrelax-pic-calls -mno-relax-pic-calls
-mmcount-ra-address -mframe-header-opt -mno-frame-header-opt
MMIX Options -mlibfuncs -mno-libfuncs -mepsilon -mno-epsilon
-mabi=gnu -mabi=mmixware -mzero-extend -mknuthdiv
-mtoplevel-symbols -melf -mbranch-predict -mno-branch-predict
-mbase-addresses -mno-base-addresses -msingle-exit
-mno-single-exit
MN10300 Options -mmult-bug -mno-mult-bug -mno-am33 -mam33
-mam33-2 -mam34 -mtune=cpu-type -mreturn-pointer-on-d0 -mno-crt0
-mrelax -mliw -msetlb
Moxie Options -meb -mel -mmul.x -mno-crt0
MSP430 Options -msim -masm-hex -mmcu= -mcpu= -mlarge -msmall
-mrelax -mwarn-mcu -mcode-region= -mdata-region= -msilicon-errata=
-msilicon-errata-warn= -mhwmult= -minrt -mtiny-printf
NDS32 Options -mbig-endian -mlittle-endian -mreduced-regs
-mfull-regs -mcmov -mno-cmov -mext-perf -mno-ext-perf -mext-perf2
-mno-ext-perf2 -mext-string -mno-ext-string -mv3push -mno-v3push
-m16bit -mno-16bit -misr-vector-size=num -mcache-block-size=num
-march=arch -mcmodel=code-model -mctor-dtor -mrelax
Nios II Options -G num -mgpopt=option -mgpopt -mno-gpopt
-mgprel-sec=regexp -mr0rel-sec=regexp -mel -meb -mno-bypass-cache
-mbypass-cache -mno-cache-volatile -mcache-volatile
-mno-fast-sw-div -mfast-sw-div -mhw-mul -mno-hw-mul -mhw-mulx
-mno-hw-mulx -mno-hw-div -mhw-div -mcustom-insn=N
-mno-custom-insn -mcustom-fpu-cfg=name -mhal -msmallc
-msys-crt0=name -msys-lib=name -march=arch -mbmx -mno-bmx -mcdx
-mno-cdx
Nvidia PTX Options -m32 -m64 -mmainkernel -moptimize
OpenRISC Options -mboard=name -mnewlib -mhard-mul -mhard-div
-msoft-mul -msoft-div -msoft-float -mhard-float -mdouble-float
-munordered-float -mcmov -mror -mrori -msext -msfimm -mshftimm
PDP-11 Options -mfpu -msoft-float -mac0 -mno-ac0 -m40 -m45
-m10 -mint32 -mno-int16 -mint16 -mno-int32 -msplit -munix-asm
-mdec-asm -mgnu-asm -mlra
picoChip Options -mae=ae_type -mvliw-lookahead=N
-msymbol-as-address -mno-inefficient-warnings
PowerPC Options See RS/6000 and PowerPC Options.
PRU Options -mmcu=mcu -minrt -mno-relax -mloop -mabi=variant
RISC-V Options -mbranch-cost=N-instruction -mplt -mno-plt
-mabi=ABI-string -mfdiv -mno-fdiv -mdiv -mno-div -march=ISA-
string -mtune=processor-string -mpreferred-stack-boundary=num
-msmall-data-limit=N-bytes -msave-restore -mno-save-restore
-mstrict-align -mno-strict-align -mcmodel=medlow -mcmodel=medany
-mexplicit-relocs -mno-explicit-relocs -mrelax -mno-relax
-mriscv-attribute -mmo-riscv-attribute -malign-data=type
RL78 Options -msim -mmul=none -mmul=g13 -mmul=g14 -mallregs
-mcpu=g10 -mcpu=g13 -mcpu=g14 -mg10 -mg13 -mg14
-m64bit-doubles -m32bit-doubles -msave-mduc-in-interrupts
RS/6000 and PowerPC Options -mcpu=cpu-type -mtune=cpu-type
-mcmodel=code-model -mpowerpc64 -maltivec -mno-altivec
-mpowerpc-gpopt -mno-powerpc-gpopt -mpowerpc-gfxopt
-mno-powerpc-gfxopt -mmfcrf -mno-mfcrf -mpopcntb -mno-popcntb
-mpopcntd -mno-popcntd -mfprnd -mno-fprnd -mcmpb -mno-cmpb
-mhard-dfp -mno-hard-dfp -mfull-toc -mminimal-toc
-mno-fp-in-toc -mno-sum-in-toc -m64 -m32 -mxl-compat
-mno-xl-compat -mpe -malign-power -malign-natural -msoft-float
-mhard-float -mmultiple -mno-multiple -mupdate -mno-update
-mavoid-indexed-addresses -mno-avoid-indexed-addresses
-mfused-madd -mno-fused-madd -mbit-align -mno-bit-align
-mstrict-align -mno-strict-align -mrelocatable -mno-relocatable
-mrelocatable-lib -mno-relocatable-lib -mtoc -mno-toc -mlittle
-mlittle-endian -mbig -mbig-endian -mdynamic-no-pic -mswdiv
-msingle-pic-base -mprioritize-restricted-insns=priority
-msched-costly-dep=dependence_type -minsert-sched-nops=scheme
-mcall-aixdesc -mcall-eabi -mcall-freebsd -mcall-linux
-mcall-netbsd -mcall-openbsd -mcall-sysv -mcall-sysv-eabi
-mcall-sysv-noeabi -mtraceback=traceback_type -maix-struct-return
-msvr4-struct-return -mabi=abi-type -msecure-plt -mbss-plt
-mlongcall -mno-longcall -mpltseq -mno-pltseq
-mblock-move-inline-limit=num -mblock-compare-inline-limit=num
-mblock-compare-inline-loop-limit=num
-mstring-compare-inline-limit=num -misel -mno-isel -mvrsave
-mno-vrsave -mmulhw -mno-mulhw -mdlmzb -mno-dlmzb -mprototype
-mno-prototype -msim -mmvme -mads -myellowknife -memb -msdata
-msdata=opt -mreadonly-in-sdata -mvxworks -G num -mrecip
-mrecip=opt -mno-recip -mrecip-precision -mno-recip-precision
-mveclibabi=type -mfriz -mno-friz -mpointers-to-nested-functions
-mno-pointers-to-nested-functions -msave-toc-indirect
-mno-save-toc-indirect -mpower8-fusion -mno-mpower8-fusion
-mpower8-vector -mno-power8-vector -mcrypto -mno-crypto -mhtm
-mno-htm -mquad-memory -mno-quad-memory -mquad-memory-atomic
-mno-quad-memory-atomic -mcompat-align-parm -mno-compat-align-parm
-mfloat128 -mno-float128 -mfloat128-hardware
-mno-float128-hardware -mgnu-attribute -mno-gnu-attribute
-mstack-protector-guard=guard -mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset -mprefixed -mno-prefixed
-mpcrel -mno-pcrel -mmma -mno-mmma
RX Options -m64bit-doubles -m32bit-doubles -fpu -nofpu -mcpu=
-mbig-endian-data -mlittle-endian-data -msmall-data -msim
-mno-sim -mas100-syntax -mno-as100-syntax -mrelax
-mmax-constant-size= -mint-register= -mpid -mallow-string-insns
-mno-allow-string-insns -mjsr -mno-warn-multiple-fast-interrupts
-msave-acc-in-interrupts
S/390 and zSeries Options -mtune=cpu-type -march=cpu-type
-mhard-float -msoft-float -mhard-dfp -mno-hard-dfp
-mlong-double-64 -mlong-double-128 -mbackchain -mno-backchain
-mpacked-stack -mno-packed-stack -msmall-exec -mno-small-exec
-mmvcle -mno-mvcle -m64 -m31 -mdebug -mno-debug -mesa -mzarch
-mhtm -mvx -mzvector -mtpf-trace -mno-tpf-trace
-mtpf-trace-skip -mno-tpf-trace-skip -mfused-madd -mno-fused-madd
-mwarn-framesize -mwarn-dynamicstack -mstack-size -mstack-guard
-mhotpatch=halfwords,halfwords
Score Options -meb -mel -mnhwloop -muls -mmac -mscore5 -mscore5u
-mscore7 -mscore7d
SH Options -m1 -m2 -m2e -m2a-nofpu -m2a-single-only -m2a-single
-m2a -m3 -m3e -m4-nofpu -m4-single-only -m4-single -m4
-m4a-nofpu -m4a-single-only -m4a-single -m4a -m4al -mb -ml
-mdalign -mrelax -mbigtable -mfmovd -mrenesas -mno-renesas
-mnomacsave -mieee -mno-ieee -mbitops -misize
-minline-ic_invalidate -mpadstruct -mprefergot -musermode
-multcost=number -mdiv=strategy -mdivsi3_libfunc=name
-mfixed-range=register-range -maccumulate-outgoing-args
-matomic-model=atomic-model -mbranch-cost=num -mzdcbranch
-mno-zdcbranch -mcbranch-force-delay-slot -mfused-madd
-mno-fused-madd -mfsca -mno-fsca -mfsrra -mno-fsrra
-mpretend-cmove -mtas
Solaris 2 Options -mclear-hwcap -mno-clear-hwcap -mimpure-text
-mno-impure-text -pthreads
SPARC Options -mcpu=cpu-type -mtune=cpu-type -mcmodel=code-model
-mmemory-model=mem-model -m32 -m64 -mapp-regs -mno-app-regs
-mfaster-structs -mno-faster-structs -mflat -mno-flat -mfpu
-mno-fpu -mhard-float -msoft-float -mhard-quad-float
-msoft-quad-float -mstack-bias -mno-stack-bias -mstd-struct-return
-mno-std-struct-return -munaligned-doubles -mno-unaligned-doubles
-muser-mode -mno-user-mode -mv8plus -mno-v8plus -mvis -mno-vis
-mvis2 -mno-vis2 -mvis3 -mno-vis3 -mvis4 -mno-vis4 -mvis4b
-mno-vis4b -mcbcond -mno-cbcond -mfmaf -mno-fmaf -mfsmuld
-mno-fsmuld -mpopc -mno-popc -msubxc -mno-subxc -mfix-at697f
-mfix-ut699 -mfix-ut700 -mfix-gr712rc -mlra -mno-lra
System V Options -Qy -Qn -YP,paths -Ym,dir
TILE-Gx Options -mcpu=CPU -m32 -m64 -mbig-endian
-mlittle-endian -mcmodel=code-model
TILEPro Options -mcpu=cpu -m32
V850 Options -mlong-calls -mno-long-calls -mep -mno-ep
-mprolog-function -mno-prolog-function -mspace -mtda=n -msda=n
-mzda=n -mapp-regs -mno-app-regs -mdisable-callt
-mno-disable-callt -mv850e2v3 -mv850e2 -mv850e1 -mv850es -mv850e
-mv850 -mv850e3v5 -mloop -mrelax -mlong-jumps -msoft-float
-mhard-float -mgcc-abi -mrh850-abi -mbig-switch
VAX Options -mg -mgnu -munix
Visium Options -mdebug -msim -mfpu -mno-fpu -mhard-float
-msoft-float -mcpu=cpu-type -mtune=cpu-type -msv-mode
-muser-mode
VMS Options -mvms-return-codes -mdebug-main=prefix -mmalloc64
-mpointer-size=size
VxWorks Options -mrtp -non-static -Bstatic -Bdynamic -Xbind-lazy
-Xbind-now
x86 Options -mtune=cpu-type -march=cpu-type -mtune-ctrl=feature-
list -mdump-tune-features -mno-default -mfpmath=unit
-masm=dialect -mno-fancy-math-387 -mno-fp-ret-in-387 -m80387
-mhard-float -msoft-float -mno-wide-multiply -mrtd
-malign-double -mpreferred-stack-boundary=num
-mincoming-stack-boundary=num -mcld -mcx16 -msahf -mmovbe
-mcrc32 -mrecip -mrecip=opt -mvzeroupper -mprefer-avx128
-mprefer-vector-width=opt -mmmx -msse -msse2 -msse3 -mssse3
-msse4.1 -msse4.2 -msse4 -mavx -mavx2 -mavx512f -mavx512pf
-mavx512er -mavx512cd -mavx512vl -mavx512bw -mavx512dq
-mavx512ifma -mavx512vbmi -msha -maes -mpclmul -mfsgsbase
-mrdrnd -mf16c -mfma -mpconfig -mwbnoinvd -mptwrite
-mprefetchwt1 -mclflushopt -mclwb -mxsavec -mxsaves -msse4a
-m3dnow -m3dnowa -mpopcnt -mabm -mbmi -mtbm -mfma4 -mxop
-madx -mlzcnt -mbmi2 -mfxsr -mxsave -mxsaveopt -mrtm -mhle
-mlwp -mmwaitx -mclzero -mpku -mthreads -mgfni -mvaes
-mwaitpkg -mshstk -mmanual-endbr -mforce-indirect-call
-mavx512vbmi2 -mavx512bf16 -menqcmd -mvpclmulqdq -mavx512bitalg
-mmovdiri -mmovdir64b -mavx512vpopcntdq -mavx5124fmaps
-mavx512vnni -mavx5124vnniw -mprfchw -mrdpid -mrdseed -msgx
-mavx512vp2intersect -mcldemote -mms-bitfields
-mno-align-stringops -minline-all-stringops
-minline-stringops-dynamically -mstringop-strategy=alg
-mmemcpy-strategy=strategy -mmemset-strategy=strategy -mpush-args
-maccumulate-outgoing-args -m128bit-long-double
-m96bit-long-double -mlong-double-64 -mlong-double-80
-mlong-double-128 -mregparm=num -msseregparm -mveclibabi=type
-mvect8-ret-in-mem -mpc32 -mpc64 -mpc80 -mstackrealign
-momit-leaf-frame-pointer -mno-red-zone -mno-tls-direct-seg-refs
-mcmodel=code-model -mabi=name -maddress-mode=mode -m32 -m64
-mx32 -m16 -miamcu -mlarge-data-threshold=num -msse2avx
-mfentry -mrecord-mcount -mnop-mcount -m8bit-idiv
-minstrument-return=type -mfentry-name=name -mfentry-section=name
-mavx256-split-unaligned-load -mavx256-split-unaligned-store
-malign-data=type -mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
-mstack-protector-guard-symbol=symbol -mgeneral-regs-only
-mcall-ms2sysv-xlogues -mindirect-branch=choice
-mfunction-return=choice -mindirect-branch-register
x86 Windows Options -mconsole -mcygwin -mno-cygwin -mdll
-mnop-fun-dllimport -mthread -municode -mwin32 -mwindows
-fno-set-stack-executable
Xstormy16 Options -msim
Xtensa Options -mconst16 -mno-const16 -mfused-madd
-mno-fused-madd -mforce-no-pic -mserialize-volatile
-mno-serialize-volatile -mtext-section-literals
-mno-text-section-literals -mauto-litpools -mno-auto-litpools
-mtarget-align -mno-target-align -mlongcalls -mno-longcalls
zSeries Options See S/390 and zSeries Options.
Options Controlling the Kind of Output
Compilation can involve up to four stages: preprocessing, compilation
proper, assembly and linking, always in that order. GCC is capable of
preprocessing and compiling several files either into several assembler
input files, or into one assembler input file; then each assembler
input file produces an object file, and linking combines all the object
files (those newly compiled, and those specified as input) into an
executable file.
For any given input file, the file name suffix determines what kind of
compilation is done:
file.c
C source code that must be preprocessed.
file.i
C source code that should not be preprocessed.
file.ii
C++ source code that should not be preprocessed.
file.m
Objective-C source code. Note that you must link with the libobjc
library to make an Objective-C program work.
file.mi
Objective-C source code that should not be preprocessed.
file.mm
file.M
Objective-C++ source code. Note that you must link with the
libobjc library to make an Objective-C++ program work. Note that
.M refers to a literal capital M.
file.mii
Objective-C++ source code that should not be preprocessed.
file.h
C, C++, Objective-C or Objective-C++ header file to be turned into
a precompiled header (default), or C, C++ header file to be turned
into an Ada spec (via the -fdump-ada-spec switch).
file.cc
file.cp
file.cxx
file.cpp
file.CPP
file.c++
file.C
C++ source code that must be preprocessed. Note that in .cxx, the
last two letters must both be literally x. Likewise, .C refers to
a literal capital C.
file.mm
file.M
Objective-C++ source code that must be preprocessed.
file.mii
Objective-C++ source code that should not be preprocessed.
file.hh
file.H
file.hp
file.hxx
file.hpp
file.HPP
file.h++
file.tcc
C++ header file to be turned into a precompiled header or Ada spec.
file.f
file.for
file.ftn
Fixed form Fortran source code that should not be preprocessed.
file.F
file.FOR
file.fpp
file.FPP
file.FTN
Fixed form Fortran source code that must be preprocessed (with the
traditional preprocessor).
file.f90
file.f95
file.f03
file.f08
Free form Fortran source code that should not be preprocessed.
file.F90
file.F95
file.F03
file.F08
Free form Fortran source code that must be preprocessed (with the
traditional preprocessor).
file.go
Go source code.
file.brig
BRIG files (binary representation of HSAIL).
file.d
D source code.
file.di
D interface file.
file.dd
D documentation code (Ddoc).
file.ads
Ada source code file that contains a library unit declaration (a
declaration of a package, subprogram, or generic, or a generic
instantiation), or a library unit renaming declaration (a package,
generic, or subprogram renaming declaration). Such files are also
called specs.
file.adb
Ada source code file containing a library unit body (a subprogram
or package body). Such files are also called bodies.
file.s
Assembler code.
file.S
file.sx
Assembler code that must be preprocessed.
other
An object file to be fed straight into linking. Any file name with
no recognized suffix is treated this way.
You can specify the input language explicitly with the -x option:
-x language
Specify explicitly the language for the following input files
(rather than letting the compiler choose a default based on the
file name suffix). This option applies to all following input
files until the next -x option. Possible values for language are:
c c-header cpp-output
c++ c++-header c++-cpp-output
objective-c objective-c-header objective-c-cpp-output
objective-c++ objective-c++-header objective-c++-cpp-output
assembler assembler-with-cpp
ada
d
f77 f77-cpp-input f95 f95-cpp-input
go
brig
-x none
Turn off any specification of a language, so that subsequent files
are handled according to their file name suffixes (as they are if
-x has not been used at all).
If you only want some of the stages of compilation, you can use -x (or
filename suffixes) to tell gcc where to start, and one of the options
-c, -S, or -E to say where gcc is to stop. Note that some combinations
(for example, -x cpp-output -E) instruct gcc to do nothing at all.
-c Compile or assemble the source files, but do not link. The linking
stage simply is not done. The ultimate output is in the form of an
object file for each source file.
By default, the object file name for a source file is made by
replacing the suffix .c, .i, .s, etc., with .o.
Unrecognized input files, not requiring compilation or assembly,
are ignored.
-S Stop after the stage of compilation proper; do not assemble. The
output is in the form of an assembler code file for each non-
assembler input file specified.
By default, the assembler file name for a source file is made by
replacing the suffix .c, .i, etc., with .s.
Input files that don't require compilation are ignored.
-E Stop after the preprocessing stage; do not run the compiler proper.
The output is in the form of preprocessed source code, which is
sent to the standard output.
Input files that don't require preprocessing are ignored.
-o file
Place output in file file. This applies to whatever sort of output
is being produced, whether it be an executable file, an object
file, an assembler file or preprocessed C code.
If -o is not specified, the default is to put an executable file in
a.out, the object file for source.suffix in source.o, its assembler
file in source.s, a precompiled header file in source.suffix.gch,
and all preprocessed C source on standard output.
-v Print (on standard error output) the commands executed to run the
stages of compilation. Also print the version number of the
compiler driver program and of the preprocessor and the compiler
proper.
-###
Like -v except the commands are not executed and arguments are
quoted unless they contain only alphanumeric characters or "./-_".
This is useful for shell scripts to capture the driver-generated
command lines.
--help
Print (on the standard output) a description of the command-line
options understood by gcc. If the -v option is also specified then
--help is also passed on to the various processes invoked by gcc,
so that they can display the command-line options they accept. If
the -Wextra option has also been specified (prior to the --help
option), then command-line options that have no documentation
associated with them are also displayed.
--target-help
Print (on the standard output) a description of target-specific
command-line options for each tool. For some targets extra target-
specific information may also be printed.
--help={class|[^]qualifier}[,...]
Print (on the standard output) a description of the command-line
options understood by the compiler that fit into all specified
classes and qualifiers. These are the supported classes:
optimizers
Display all of the optimization options supported by the
compiler.
warnings
Display all of the options controlling warning messages
produced by the compiler.
target
Display target-specific options. Unlike the --target-help
option however, target-specific options of the linker and
assembler are not displayed. This is because those tools do
not currently support the extended --help= syntax.
params
Display the values recognized by the --param option.
language
Display the options supported for language, where language is
the name of one of the languages supported in this version of
GCC. If an option is supported by all languages, one needs to
select common class.
common
Display the options that are common to all languages.
These are the supported qualifiers:
undocumented
Display only those options that are undocumented.
joined
Display options taking an argument that appears after an equal
sign in the same continuous piece of text, such as:
--help=target.
separate
Display options taking an argument that appears as a separate
word following the original option, such as: -o output-file.
Thus for example to display all the undocumented target-specific
switches supported by the compiler, use:
--help=target,undocumented
The sense of a qualifier can be inverted by prefixing it with the ^
character, so for example to display all binary warning options
(i.e., ones that are either on or off and that do not take an
argument) that have a description, use:
--help=warnings,^joined,^undocumented
The argument to --help= should not consist solely of inverted
qualifiers.
Combining several classes is possible, although this usually
restricts the output so much that there is nothing to display. One
case where it does work, however, is when one of the classes is
target. For example, to display all the target-specific
optimization options, use:
--help=target,optimizers
The --help= option can be repeated on the command line. Each
successive use displays its requested class of options, skipping
those that have already been displayed. If --help is also
specified anywhere on the command line then this takes precedence
over any --help= option.
If the -Q option appears on the command line before the --help=
option, then the descriptive text displayed by --help= is changed.
Instead of describing the displayed options, an indication is given
as to whether the option is enabled, disabled or set to a specific
value (assuming that the compiler knows this at the point where the
--help= option is used).
Here is a truncated example from the ARM port of gcc:
% gcc -Q -mabi=2 --help=target -c
The following options are target specific:
-mabi= 2
-mabort-on-noreturn [disabled]
-mapcs [disabled]
The output is sensitive to the effects of previous command-line
options, so for example it is possible to find out which
optimizations are enabled at -O2 by using:
-Q -O2 --help=optimizers
Alternatively you can discover which binary optimizations are
enabled by -O3 by using:
gcc -c -Q -O3 --help=optimizers > /tmp/O3-opts
gcc -c -Q -O2 --help=optimizers > /tmp/O2-opts
diff /tmp/O2-opts /tmp/O3-opts | grep enabled
--version
Display the version number and copyrights of the invoked GCC.
-pass-exit-codes
Normally the gcc program exits with the code of 1 if any phase of
the compiler returns a non-success return code. If you specify
-pass-exit-codes, the gcc program instead returns with the
numerically highest error produced by any phase returning an error
indication. The C, C++, and Fortran front ends return 4 if an
internal compiler error is encountered.
-pipe
Use pipes rather than temporary files for communication between the
various stages of compilation. This fails to work on some systems
where the assembler is unable to read from a pipe; but the GNU
assembler has no trouble.
-specs=file
Process file after the compiler reads in the standard specs file,
in order to override the defaults which the gcc driver program uses
when determining what switches to pass to cc1, cc1plus, as, ld,
etc. More than one -specs=file can be specified on the command
line, and they are processed in order, from left to right.
-wrapper
Invoke all subcommands under a wrapper program. The name of the
wrapper program and its parameters are passed as a comma separated
list.
gcc -c t.c -wrapper gdb,--args
This invokes all subprograms of gcc under gdb --args, thus the
invocation of cc1 is gdb --args cc1 ....
-ffile-prefix-map=old=new
When compiling files residing in directory old, record any
references to them in the result of the compilation as if the files
resided in directory new instead. Specifying this option is
equivalent to specifying all the individual -f*-prefix-map options.
This can be used to make reproducible builds that are location
independent. See also -fmacro-prefix-map and -fdebug-prefix-map.
-fplugin=name.so
Load the plugin code in file name.so, assumed to be a shared object
to be dlopen'd by the compiler. The base name of the shared object
file is used to identify the plugin for the purposes of argument
parsing (See -fplugin-arg-name-key=value below). Each plugin
should define the callback functions specified in the Plugins API.
-fplugin-arg-name-key=value
Define an argument called key with a value of value for the plugin
called name.
-fdump-ada-spec[-slim]
For C and C++ source and include files, generate corresponding Ada
specs.
-fada-spec-parent=unit
In conjunction with -fdump-ada-spec[-slim] above, generate Ada
specs as child units of parent unit.
-fdump-go-spec=file
For input files in any language, generate corresponding Go
declarations in file. This generates Go "const", "type", "var",
and "func" declarations which may be a useful way to start writing
a Go interface to code written in some other language.
@file
Read command-line options from file. The options read are inserted
in place of the original @file option. If file does not exist, or
cannot be read, then the option will be treated literally, and not
removed.
Options in file are separated by whitespace. A whitespace
character may be included in an option by surrounding the entire
option in either single or double quotes. Any character (including
a backslash) may be included by prefixing the character to be
included with a backslash. The file may itself contain additional
@file options; any such options will be processed recursively.
Compiling C++ Programs
C++ source files conventionally use one of the suffixes .C, .cc, .cpp,
.CPP, .c++, .cp, or .cxx; C++ header files often use .hh, .hpp, .H, or
(for shared template code) .tcc; and preprocessed C++ files use the
suffix .ii. GCC recognizes files with these names and compiles them as
C++ programs even if you call the compiler the same way as for
compiling C programs (usually with the name gcc).
However, the use of gcc does not add the C++ library. g++ is a program
that calls GCC and automatically specifies linking against the C++
library. It treats .c, .h and .i files as C++ source files instead of
C source files unless -x is used. This program is also useful when
precompiling a C header file with a .h extension for use in C++
compilations. On many systems, g++ is also installed with the name
c++.
When you compile C++ programs, you may specify many of the same
command-line options that you use for compiling programs in any
language; or command-line options meaningful for C and related
languages; or options that are meaningful only for C++ programs.
Options Controlling C Dialect
The following options control the dialect of C (or languages derived
from C, such as C++, Objective-C and Objective-C++) that the compiler
accepts:
-ansi
In C mode, this is equivalent to -std=c90. In C++ mode, it is
equivalent to -std=c++98.
This turns off certain features of GCC that are incompatible with
ISO C90 (when compiling C code), or of standard C++ (when compiling
C++ code), such as the "asm" and "typeof" keywords, and predefined
macros such as "unix" and "vax" that identify the type of system
you are using. It also enables the undesirable and rarely used ISO
trigraph feature. For the C compiler, it disables recognition of
C++ style // comments as well as the "inline" keyword.
The alternate keywords "__asm__", "__extension__", "__inline__" and
"__typeof__" continue to work despite -ansi. You would not want to
use them in an ISO C program, of course, but it is useful to put
them in header files that might be included in compilations done
with -ansi. Alternate predefined macros such as "__unix__" and
"__vax__" are also available, with or without -ansi.
The -ansi option does not cause non-ISO programs to be rejected
gratuitously. For that, -Wpedantic is required in addition to
-ansi.
The macro "__STRICT_ANSI__" is predefined when the -ansi option is
used. Some header files may notice this macro and refrain from
declaring certain functions or defining certain macros that the ISO
standard doesn't call for; this is to avoid interfering with any
programs that might use these names for other things.
Functions that are normally built in but do not have semantics
defined by ISO C (such as "alloca" and "ffs") are not built-in
functions when -ansi is used.
-std=
Determine the language standard. This option is currently only
supported when compiling C or C++.
The compiler can accept several base standards, such as c90 or
c++98, and GNU dialects of those standards, such as gnu90 or
gnu++98. When a base standard is specified, the compiler accepts
all programs following that standard plus those using GNU
extensions that do not contradict it. For example, -std=c90 turns
off certain features of GCC that are incompatible with ISO C90,
such as the "asm" and "typeof" keywords, but not other GNU
extensions that do not have a meaning in ISO C90, such as omitting
the middle term of a "?:" expression. On the other hand, when a GNU
dialect of a standard is specified, all features supported by the
compiler are enabled, even when those features change the meaning
of the base standard. As a result, some strict-conforming programs
may be rejected. The particular standard is used by -Wpedantic to
identify which features are GNU extensions given that version of
the standard. For example -std=gnu90 -Wpedantic warns about C++
style // comments, while -std=gnu99 -Wpedantic does not.
A value for this option must be provided; possible values are
c90
c89
iso9899:1990
Support all ISO C90 programs (certain GNU extensions that
conflict with ISO C90 are disabled). Same as -ansi for C code.
iso9899:199409
ISO C90 as modified in amendment 1.
c99
c9x
iso9899:1999
iso9899:199x
ISO C99. This standard is substantially completely supported,
modulo bugs and floating-point issues (mainly but not entirely
relating to optional C99 features from Annexes F and G). See
<http://gcc.gnu.org/c99status.html> for more information. The
names c9x and iso9899:199x are deprecated.
c11
c1x
iso9899:2011
ISO C11, the 2011 revision of the ISO C standard. This
standard is substantially completely supported, modulo bugs,
floating-point issues (mainly but not entirely relating to
optional C11 features from Annexes F and G) and the optional
Annexes K (Bounds-checking interfaces) and L (Analyzability).
The name c1x is deprecated.
c17
c18
iso9899:2017
iso9899:2018
ISO C17, the 2017 revision of the ISO C standard (published in
2018). This standard is same as C11 except for corrections of
defects (all of which are also applied with -std=c11) and a new
value of "__STDC_VERSION__", and so is supported to the same
extent as C11.
c2x The next version of the ISO C standard, still under
development. The support for this version is experimental and
incomplete.
gnu90
gnu89
GNU dialect of ISO C90 (including some C99 features).
gnu99
gnu9x
GNU dialect of ISO C99. The name gnu9x is deprecated.
gnu11
gnu1x
GNU dialect of ISO C11. The name gnu1x is deprecated.
gnu17
gnu18
GNU dialect of ISO C17. This is the default for C code.
gnu2x
The next version of the ISO C standard, still under
development, plus GNU extensions. The support for this version
is experimental and incomplete.
c++98
c++03
The 1998 ISO C++ standard plus the 2003 technical corrigendum
and some additional defect reports. Same as -ansi for C++ code.
gnu++98
gnu++03
GNU dialect of -std=c++98.
c++11
c++0x
The 2011 ISO C++ standard plus amendments. The name c++0x is
deprecated.
gnu++11
gnu++0x
GNU dialect of -std=c++11. The name gnu++0x is deprecated.
c++14
c++1y
The 2014 ISO C++ standard plus amendments. The name c++1y is
deprecated.
gnu++14
gnu++1y
GNU dialect of -std=c++14. This is the default for C++ code.
The name gnu++1y is deprecated.
c++17
c++1z
The 2017 ISO C++ standard plus amendments. The name c++1z is
deprecated.
gnu++17
gnu++1z
GNU dialect of -std=c++17. The name gnu++1z is deprecated.
c++20
c++2a
The next revision of the ISO C++ standard, planned for 2020.
Support is highly experimental, and will almost certainly
change in incompatible ways in future releases.
gnu++20
gnu++2a
GNU dialect of -std=c++20. Support is highly experimental, and
will almost certainly change in incompatible ways in future
releases.
-fgnu89-inline
The option -fgnu89-inline tells GCC to use the traditional GNU
semantics for "inline" functions when in C99 mode.
Using this option is roughly equivalent to adding the "gnu_inline"
function attribute to all inline functions.
The option -fno-gnu89-inline explicitly tells GCC to use the C99
semantics for "inline" when in C99 or gnu99 mode (i.e., it
specifies the default behavior). This option is not supported in
-std=c90 or -std=gnu90 mode.
The preprocessor macros "__GNUC_GNU_INLINE__" and
"__GNUC_STDC_INLINE__" may be used to check which semantics are in
effect for "inline" functions.
-fpermitted-flt-eval-methods=style
ISO/IEC TS 18661-3 defines new permissible values for
"FLT_EVAL_METHOD" that indicate that operations and constants with
a semantic type that is an interchange or extended format should be
evaluated to the precision and range of that type. These new
values are a superset of those permitted under C99/C11, which does
not specify the meaning of other positive values of
"FLT_EVAL_METHOD". As such, code conforming to C11 may not have
been written expecting the possibility of the new values.
-fpermitted-flt-eval-methods specifies whether the compiler should
allow only the values of "FLT_EVAL_METHOD" specified in C99/C11, or
the extended set of values specified in ISO/IEC TS 18661-3.
style is either "c11" or "ts-18661-3" as appropriate.
The default when in a standards compliant mode (-std=c11 or
similar) is -fpermitted-flt-eval-methods=c11. The default when in
a GNU dialect (-std=gnu11 or similar) is
-fpermitted-flt-eval-methods=ts-18661-3.
-aux-info filename
Output to the given filename prototyped declarations for all
functions declared and/or defined in a translation unit, including
those in header files. This option is silently ignored in any
language other than C.
Besides declarations, the file indicates, in comments, the origin
of each declaration (source file and line), whether the declaration
was implicit, prototyped or unprototyped (I, N for new or O for
old, respectively, in the first character after the line number and
the colon), and whether it came from a declaration or a definition
(C or F, respectively, in the following character). In the case of
function definitions, a K&R-style list of arguments followed by
their declarations is also provided, inside comments, after the
declaration.
-fallow-parameterless-variadic-functions
Accept variadic functions without named parameters.
Although it is possible to define such a function, this is not very
useful as it is not possible to read the arguments. This is only
supported for C as this construct is allowed by C++.
-fno-asm
Do not recognize "asm", "inline" or "typeof" as a keyword, so that
code can use these words as identifiers. You can use the keywords
"__asm__", "__inline__" and "__typeof__" instead. -ansi implies
-fno-asm.
In C++, this switch only affects the "typeof" keyword, since "asm"
and "inline" are standard keywords. You may want to use the
-fno-gnu-keywords flag instead, which has the same effect. In C99
mode (-std=c99 or -std=gnu99), this switch only affects the "asm"
and "typeof" keywords, since "inline" is a standard keyword in ISO
C99.
-fno-builtin
-fno-builtin-function
Don't recognize built-in functions that do not begin with
__builtin_ as prefix.
GCC normally generates special code to handle certain built-in
functions more efficiently; for instance, calls to "alloca" may
become single instructions which adjust the stack directly, and
calls to "memcpy" may become inline copy loops. The resulting code
is often both smaller and faster, but since the function calls no
longer appear as such, you cannot set a breakpoint on those calls,
nor can you change the behavior of the functions by linking with a
different library. In addition, when a function is recognized as a
built-in function, GCC may use information about that function to
warn about problems with calls to that function, or to generate
more efficient code, even if the resulting code still contains
calls to that function. For example, warnings are given with
-Wformat for bad calls to "printf" when "printf" is built in and
"strlen" is known not to modify global memory.
With the -fno-builtin-function option only the built-in function
function is disabled. function must not begin with __builtin_. If
a function is named that is not built-in in this version of GCC,
this option is ignored. There is no corresponding
-fbuiltin-function option; if you wish to enable built-in functions
selectively when using -fno-builtin or -ffreestanding, you may
define macros such as:
#define abs(n) __builtin_abs ((n))
#define strcpy(d, s) __builtin_strcpy ((d), (s))
-fgimple
Enable parsing of function definitions marked with "__GIMPLE".
This is an experimental feature that allows unit testing of GIMPLE
passes.
-fhosted
Assert that compilation targets a hosted environment. This implies
-fbuiltin. A hosted environment is one in which the entire
standard library is available, and in which "main" has a return
type of "int". Examples are nearly everything except a kernel.
This is equivalent to -fno-freestanding.
-ffreestanding
Assert that compilation targets a freestanding environment. This
implies -fno-builtin. A freestanding environment is one in which
the standard library may not exist, and program startup may not
necessarily be at "main". The most obvious example is an OS
kernel. This is equivalent to -fno-hosted.
-fopenacc
Enable handling of OpenACC directives "#pragma acc" in C/C++ and
"!$acc" in Fortran. When -fopenacc is specified, the compiler
generates accelerated code according to the OpenACC Application
Programming Interface v2.6 <https://www.openacc.org>. This option
implies -pthread, and thus is only supported on targets that have
support for -pthread.
-fopenacc-dim=geom
Specify default compute dimensions for parallel offload regions
that do not explicitly specify. The geom value is a triple of
':'-separated sizes, in order 'gang', 'worker' and, 'vector'. A
size can be omitted, to use a target-specific default value.
-fopenmp
Enable handling of OpenMP directives "#pragma omp" in C/C++ and
"!$omp" in Fortran. When -fopenmp is specified, the compiler
generates parallel code according to the OpenMP Application Program
Interface v4.5 <https://www.openmp.org>. This option implies
-pthread, and thus is only supported on targets that have support
for -pthread. -fopenmp implies -fopenmp-simd.
-fopenmp-simd
Enable handling of OpenMP's SIMD directives with "#pragma omp" in
C/C++ and "!$omp" in Fortran. Other OpenMP directives are ignored.
-fgnu-tm
When the option -fgnu-tm is specified, the compiler generates code
for the Linux variant of Intel's current Transactional Memory ABI
specification document (Revision 1.1, May 6 2009). This is an
experimental feature whose interface may change in future versions
of GCC, as the official specification changes. Please note that
not all architectures are supported for this feature.
For more information on GCC's support for transactional memory,
Note that the transactional memory feature is not supported with
non-call exceptions (-fnon-call-exceptions).
-fms-extensions
Accept some non-standard constructs used in Microsoft header files.
In C++ code, this allows member names in structures to be similar
to previous types declarations.
typedef int UOW;
struct ABC {
UOW UOW;
};
Some cases of unnamed fields in structures and unions are only
accepted with this option.
Note that this option is off for all targets except for x86 targets
using ms-abi.
-fplan9-extensions
Accept some non-standard constructs used in Plan 9 code.
This enables -fms-extensions, permits passing pointers to
structures with anonymous fields to functions that expect pointers
to elements of the type of the field, and permits referring to
anonymous fields declared using a typedef. This is only
supported for C, not C++.
-fcond-mismatch
Allow conditional expressions with mismatched types in the second
and third arguments. The value of such an expression is void.
This option is not supported for C++.
-flax-vector-conversions
Allow implicit conversions between vectors with differing numbers
of elements and/or incompatible element types. This option should
not be used for new code.
-funsigned-char
Let the type "char" be unsigned, like "unsigned char".
Each kind of machine has a default for what "char" should be. It
is either like "unsigned char" by default or like "signed char" by
default.
Ideally, a portable program should always use "signed char" or
"unsigned char" when it depends on the signedness of an object.
But many programs have been written to use plain "char" and expect
it to be signed, or expect it to be unsigned, depending on the
machines they were written for. This option, and its inverse, let
you make such a program work with the opposite default.
The type "char" is always a distinct type from each of "signed
char" or "unsigned char", even though its behavior is always just
like one of those two.
-fsigned-char
Let the type "char" be signed, like "signed char".
Note that this is equivalent to -fno-unsigned-char, which is the
negative form of -funsigned-char. Likewise, the option
-fno-signed-char is equivalent to -funsigned-char.
-fsigned-bitfields
-funsigned-bitfields
-fno-signed-bitfields
-fno-unsigned-bitfields
These options control whether a bit-field is signed or unsigned,
when the declaration does not use either "signed" or "unsigned".
By default, such a bit-field is signed, because this is consistent:
the basic integer types such as "int" are signed types.
-fsso-struct=endianness
Set the default scalar storage order of structures and unions to
the specified endianness. The accepted values are big-endian,
little-endian and native for the native endianness of the target
(the default). This option is not supported for C++.
Warning: the -fsso-struct switch causes GCC to generate code that
is not binary compatible with code generated without it if the
specified endianness is not the native endianness of the target.
Options Controlling C++ Dialect
This section describes the command-line options that are only
meaningful for C++ programs. You can also use most of the GNU compiler
options regardless of what language your program is in. For example,
you might compile a file firstClass.C like this:
g++ -g -fstrict-enums -O -c firstClass.C
In this example, only -fstrict-enums is an option meant only for C++
programs; you can use the other options with any language supported by
GCC.
Some options for compiling C programs, such as -std, are also relevant
for C++ programs.
Here is a list of options that are only for compiling C++ programs:
-fabi-version=n
Use version n of the C++ ABI. The default is version 0.
Version 0 refers to the version conforming most closely to the C++
ABI specification. Therefore, the ABI obtained using version 0
will change in different versions of G++ as ABI bugs are fixed.
Version 1 is the version of the C++ ABI that first appeared in G++
3.2.
Version 2 is the version of the C++ ABI that first appeared in G++
3.4, and was the default through G++ 4.9.
Version 3 corrects an error in mangling a constant address as a
template argument.
Version 4, which first appeared in G++ 4.5, implements a standard
mangling for vector types.
Version 5, which first appeared in G++ 4.6, corrects the mangling
of attribute const/volatile on function pointer types, decltype of
a plain decl, and use of a function parameter in the declaration of
another parameter.
Version 6, which first appeared in G++ 4.7, corrects the promotion
behavior of C++11 scoped enums and the mangling of template
argument packs, const/static_cast, prefix ++ and --, and a class
scope function used as a template argument.
Version 7, which first appeared in G++ 4.8, that treats nullptr_t
as a builtin type and corrects the mangling of lambdas in default
argument scope.
Version 8, which first appeared in G++ 4.9, corrects the
substitution behavior of function types with function-cv-
qualifiers.
Version 9, which first appeared in G++ 5.2, corrects the alignment
of "nullptr_t".
Version 10, which first appeared in G++ 6.1, adds mangling of
attributes that affect type identity, such as ia32 calling
convention attributes (e.g. stdcall).
Version 11, which first appeared in G++ 7, corrects the mangling of
sizeof... expressions and operator names. For multiple entities
with the same name within a function, that are declared in
different scopes, the mangling now changes starting with the
twelfth occurrence. It also implies -fnew-inheriting-ctors.
Version 12, which first appeared in G++ 8, corrects the calling
conventions for empty classes on the x86_64 target and for classes
with only deleted copy/move constructors. It accidentally changes
the calling convention for classes with a deleted copy constructor
and a trivial move constructor.
Version 13, which first appeared in G++ 8.2, fixes the accidental
change in version 12.
Version 14, which first appeared in G++ 10, corrects the mangling
of the nullptr expression.
See also -Wabi.
-fabi-compat-version=n
On targets that support strong aliases, G++ works around mangling
changes by creating an alias with the correct mangled name when
defining a symbol with an incorrect mangled name. This switch
specifies which ABI version to use for the alias.
With -fabi-version=0 (the default), this defaults to 11 (GCC 7
compatibility). If another ABI version is explicitly selected,
this defaults to 0. For compatibility with GCC versions 3.2
through 4.9, use -fabi-compat-version=2.
If this option is not provided but -Wabi=n is, that version is used
for compatibility aliases. If this option is provided along with
-Wabi (without the version), the version from this option is used
for the warning.
-fno-access-control
Turn off all access checking. This switch is mainly useful for
working around bugs in the access control code.
-faligned-new
Enable support for C++17 "new" of types that require more alignment
than "void* ::operator new(std::size_t)" provides. A numeric
argument such as "-faligned-new=32" can be used to specify how much
alignment (in bytes) is provided by that function, but few users
will need to override the default of "alignof(std::max_align_t)".
This flag is enabled by default for -std=c++17.
-fchar8_t
-fno-char8_t
Enable support for "char8_t" as adopted for C++2a. This includes
the addition of a new "char8_t" fundamental type, changes to the
types of UTF-8 string and character literals, new signatures for
user-defined literals, associated standard library updates, and new
"__cpp_char8_t" and "__cpp_lib_char8_t" feature test macros.
This option enables functions to be overloaded for ordinary and
UTF-8 strings:
int f(const char *); // #1
int f(const char8_t *); // #2
int v1 = f("text"); // Calls #1
int v2 = f(u8"text"); // Calls #2
and introduces new signatures for user-defined literals:
int operator""_udl1(char8_t);
int v3 = u8'x'_udl1;
int operator""_udl2(const char8_t*, std::size_t);
int v4 = u8"text"_udl2;
template<typename T, T...> int operator""_udl3();
int v5 = u8"text"_udl3;
The change to the types of UTF-8 string and character literals
introduces incompatibilities with ISO C++11 and later standards.
For example, the following code is well-formed under ISO C++11, but
is ill-formed when -fchar8_t is specified.
char ca[] = u8"xx"; // error: char-array initialized from wide
// string
const char *cp = u8"xx";// error: invalid conversion from
// `const char8_t*' to `const char*'
int f(const char*);
auto v = f(u8"xx"); // error: invalid conversion from
// `const char8_t*' to `const char*'
std::string s{u8"xx"}; // error: no matching function for call to
// `std::basic_string<char>::basic_string()'
using namespace std::literals;
s = u8"xx"s; // error: conversion from
// `basic_string<char8_t>' to non-scalar
// type `basic_string<char>' requested
-fcheck-new
Check that the pointer returned by "operator new" is non-null
before attempting to modify the storage allocated. This check is
normally unnecessary because the C++ standard specifies that
"operator new" only returns 0 if it is declared "throw()", in which
case the compiler always checks the return value even without this
option. In all other cases, when "operator new" has a non-empty
exception specification, memory exhaustion is signalled by throwing
"std::bad_alloc". See also new (nothrow).
-fconcepts
-fconcepts-ts
Below -std=c++2a, -fconcepts enables support for the C++ Extensions
for Concepts Technical Specification, ISO 19217 (2015).
With -std=c++2a and above, Concepts are part of the language
standard, so -fconcepts defaults to on. But the standard
specification of Concepts differs significantly from the TS, so
some constructs that were allowed in the TS but didn't make it into
the standard can still be enabled by -fconcepts-ts.
-fconstexpr-depth=n
Set the maximum nested evaluation depth for C++11 constexpr
functions to n. A limit is needed to detect endless recursion
during constant expression evaluation. The minimum specified by
the standard is 512.
-fconstexpr-cache-depth=n
Set the maximum level of nested evaluation depth for C++11
constexpr functions that will be cached to n. This is a heuristic
that trades off compilation speed (when the cache avoids repeated
calculations) against memory consumption (when the cache grows very
large from highly recursive evaluations). The default is 8. Very
few users are likely to want to adjust it, but if your code does
heavy constexpr calculations you might want to experiment to find
which value works best for you.
-fconstexpr-loop-limit=n
Set the maximum number of iterations for a loop in C++14 constexpr
functions to n. A limit is needed to detect infinite loops during
constant expression evaluation. The default is 262144 (1<<18).
-fconstexpr-ops-limit=n
Set the maximum number of operations during a single constexpr
evaluation. Even when number of iterations of a single loop is
limited with the above limit, if there are several nested loops and
each of them has many iterations but still smaller than the above
limit, or if in a body of some loop or even outside of a loop too
many expressions need to be evaluated, the resulting constexpr
evaluation might take too long. The default is 33554432 (1<<25).
-fcoroutines
Enable support for the C++ coroutines extension (experimental).
-fno-elide-constructors
The C++ standard allows an implementation to omit creating a
temporary that is only used to initialize another object of the
same type. Specifying this option disables that optimization, and
forces G++ to call the copy constructor in all cases. This option
also causes G++ to call trivial member functions which otherwise
would be expanded inline.
In C++17, the compiler is required to omit these temporaries, but
this option still affects trivial member functions.
-fno-enforce-eh-specs
Don't generate code to check for violation of exception
specifications at run time. This option violates the C++ standard,
but may be useful for reducing code size in production builds, much
like defining "NDEBUG". This does not give user code permission to
throw exceptions in violation of the exception specifications; the
compiler still optimizes based on the specifications, so throwing
an unexpected exception results in undefined behavior at run time.
-fextern-tls-init
-fno-extern-tls-init
The C++11 and OpenMP standards allow "thread_local" and
"threadprivate" variables to have dynamic (runtime) initialization.
To support this, any use of such a variable goes through a wrapper
function that performs any necessary initialization. When the use
and definition of the variable are in the same translation unit,
this overhead can be optimized away, but when the use is in a
different translation unit there is significant overhead even if
the variable doesn't actually need dynamic initialization. If the
programmer can be sure that no use of the variable in a non-
defining TU needs to trigger dynamic initialization (either because
the variable is statically initialized, or a use of the variable in
the defining TU will be executed before any uses in another TU),
they can avoid this overhead with the -fno-extern-tls-init option.
On targets that support symbol aliases, the default is
-fextern-tls-init. On targets that do not support symbol aliases,
the default is -fno-extern-tls-init.
-fno-gnu-keywords
Do not recognize "typeof" as a keyword, so that code can use this
word as an identifier. You can use the keyword "__typeof__"
instead. This option is implied by the strict ISO C++ dialects:
-ansi, -std=c++98, -std=c++11, etc.
-fno-implicit-templates
Never emit code for non-inline templates that are instantiated
implicitly (i.e. by use); only emit code for explicit
instantiations. If you use this option, you must take care to
structure your code to include all the necessary explicit
instantiations to avoid getting undefined symbols at link time.
-fno-implicit-inline-templates
Don't emit code for implicit instantiations of inline templates,
either. The default is to handle inlines differently so that
compiles with and without optimization need the same set of
explicit instantiations.
-fno-implement-inlines
To save space, do not emit out-of-line copies of inline functions
controlled by "#pragma implementation". This causes linker errors
if these functions are not inlined everywhere they are called.
-fms-extensions
Disable Wpedantic warnings about constructs used in MFC, such as
implicit int and getting a pointer to member function via non-
standard syntax.
-fnew-inheriting-ctors
Enable the P0136 adjustment to the semantics of C++11 constructor
inheritance. This is part of C++17 but also considered to be a
Defect Report against C++11 and C++14. This flag is enabled by
default unless -fabi-version=10 or lower is specified.
-fnew-ttp-matching
Enable the P0522 resolution to Core issue 150, template template
parameters and default arguments: this allows a template with
default template arguments as an argument for a template template
parameter with fewer template parameters. This flag is enabled by
default for -std=c++17.
-fno-nonansi-builtins
Disable built-in declarations of functions that are not mandated by
ANSI/ISO C. These include "ffs", "alloca", "_exit", "index",
"bzero", "conjf", and other related functions.
-fnothrow-opt
Treat a "throw()" exception specification as if it were a
"noexcept" specification to reduce or eliminate the text size
overhead relative to a function with no exception specification.
If the function has local variables of types with non-trivial
destructors, the exception specification actually makes the
function smaller because the EH cleanups for those variables can be
optimized away. The semantic effect is that an exception thrown
out of a function with such an exception specification results in a
call to "terminate" rather than "unexpected".
-fno-operator-names
Do not treat the operator name keywords "and", "bitand", "bitor",
"compl", "not", "or" and "xor" as synonyms as keywords.
-fno-optional-diags
Disable diagnostics that the standard says a compiler does not need
to issue. Currently, the only such diagnostic issued by G++ is the
one for a name having multiple meanings within a class.
-fpermissive
Downgrade some diagnostics about nonconformant code from errors to
warnings. Thus, using -fpermissive allows some nonconforming code
to compile.
-fno-pretty-templates
When an error message refers to a specialization of a function
template, the compiler normally prints the signature of the
template followed by the template arguments and any typedefs or
typenames in the signature (e.g. "void f(T) [with T = int]" rather
than "void f(int)") so that it's clear which template is involved.
When an error message refers to a specialization of a class
template, the compiler omits any template arguments that match the
default template arguments for that template. If either of these
behaviors make it harder to understand the error message rather
than easier, you can use -fno-pretty-templates to disable them.
-fno-rtti
Disable generation of information about every class with virtual
functions for use by the C++ run-time type identification features
("dynamic_cast" and "typeid"). If you don't use those parts of the
language, you can save some space by using this flag. Note that
exception handling uses the same information, but G++ generates it
as needed. The "dynamic_cast" operator can still be used for casts
that do not require run-time type information, i.e. casts to "void
*" or to unambiguous base classes.
Mixing code compiled with -frtti with that compiled with -fno-rtti
may not work. For example, programs may fail to link if a class
compiled with -fno-rtti is used as a base for a class compiled with
-frtti.
-fsized-deallocation
Enable the built-in global declarations
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
as introduced in C++14. This is useful for user-defined
replacement deallocation functions that, for example, use the size
of the object to make deallocation faster. Enabled by default
under -std=c++14 and above. The flag -Wsized-deallocation warns
about places that might want to add a definition.
-fstrict-enums
Allow the compiler to optimize using the assumption that a value of
enumerated type can only be one of the values of the enumeration
(as defined in the C++ standard; basically, a value that can be
represented in the minimum number of bits needed to represent all
the enumerators). This assumption may not be valid if the program
uses a cast to convert an arbitrary integer value to the enumerated
type.
-fstrong-eval-order
Evaluate member access, array subscripting, and shift expressions
in left-to-right order, and evaluate assignment in right-to-left
order, as adopted for C++17. Enabled by default with -std=c++17.
-fstrong-eval-order=some enables just the ordering of member access
and shift expressions, and is the default without -std=c++17.
-ftemplate-backtrace-limit=n
Set the maximum number of template instantiation notes for a single
warning or error to n. The default value is 10.
-ftemplate-depth=n
Set the maximum instantiation depth for template classes to n. A
limit on the template instantiation depth is needed to detect
endless recursions during template class instantiation. ANSI/ISO
C++ conforming programs must not rely on a maximum depth greater
than 17 (changed to 1024 in C++11). The default value is 900, as
the compiler can run out of stack space before hitting 1024 in some
situations.
-fno-threadsafe-statics
Do not emit the extra code to use the routines specified in the C++
ABI for thread-safe initialization of local statics. You can use
this option to reduce code size slightly in code that doesn't need
to be thread-safe.
-fuse-cxa-atexit
Register destructors for objects with static storage duration with
the "__cxa_atexit" function rather than the "atexit" function.
This option is required for fully standards-compliant handling of
static destructors, but only works if your C library supports
"__cxa_atexit".
-fno-use-cxa-get-exception-ptr
Don't use the "__cxa_get_exception_ptr" runtime routine. This
causes "std::uncaught_exception" to be incorrect, but is necessary
if the runtime routine is not available.
-fvisibility-inlines-hidden
This switch declares that the user does not attempt to compare
pointers to inline functions or methods where the addresses of the
two functions are taken in different shared objects.
The effect of this is that GCC may, effectively, mark inline
methods with "__attribute__ ((visibility ("hidden")))" so that they
do not appear in the export table of a DSO and do not require a PLT
indirection when used within the DSO. Enabling this option can
have a dramatic effect on load and link times of a DSO as it
massively reduces the size of the dynamic export table when the
library makes heavy use of templates.
The behavior of this switch is not quite the same as marking the
methods as hidden directly, because it does not affect static
variables local to the function or cause the compiler to deduce
that the function is defined in only one shared object.
You may mark a method as having a visibility explicitly to negate
the effect of the switch for that method. For example, if you do
want to compare pointers to a particular inline method, you might
mark it as having default visibility. Marking the enclosing class
with explicit visibility has no effect.
Explicitly instantiated inline methods are unaffected by this
option as their linkage might otherwise cross a shared library
boundary.
-fvisibility-ms-compat
This flag attempts to use visibility settings to make GCC's C++
linkage model compatible with that of Microsoft Visual Studio.
The flag makes these changes to GCC's linkage model:
1. It sets the default visibility to "hidden", like
-fvisibility=hidden.
2. Types, but not their members, are not hidden by default.
3. The One Definition Rule is relaxed for types without explicit
visibility specifications that are defined in more than one
shared object: those declarations are permitted if they are
permitted when this option is not used.
In new code it is better to use -fvisibility=hidden and export
those classes that are intended to be externally visible.
Unfortunately it is possible for code to rely, perhaps
accidentally, on the Visual Studio behavior.
Among the consequences of these changes are that static data
members of the same type with the same name but defined in
different shared objects are different, so changing one does not
change the other; and that pointers to function members defined in
different shared objects may not compare equal. When this flag is
given, it is a violation of the ODR to define types with the same
name differently.
-fno-weak
Do not use weak symbol support, even if it is provided by the
linker. By default, G++ uses weak symbols if they are available.
This option exists only for testing, and should not be used by end-
users; it results in inferior code and has no benefits. This
option may be removed in a future release of G++.
-fext-numeric-literals (C++ and Objective-C++ only)
Accept imaginary, fixed-point, or machine-defined literal number
suffixes as GNU extensions. When this option is turned off these
suffixes are treated as C++11 user-defined literal numeric
suffixes. This is on by default for all pre-C++11 dialects and all
GNU dialects: -std=c++98, -std=gnu++98, -std=gnu++11, -std=gnu++14.
This option is off by default for ISO C++11 onwards (-std=c++11,
...).
-nostdinc++
Do not search for header files in the standard directories specific
to C++, but do still search the other standard directories. (This
option is used when building the C++ library.)
In addition, these warning options have meanings only for C++ programs:
-Wabi-tag (C++ and Objective-C++ only)
Warn when a type with an ABI tag is used in a context that does not
have that ABI tag. See C++ Attributes for more information about
ABI tags.
-Wcomma-subscript (C++ and Objective-C++ only)
Warn about uses of a comma expression within a subscripting
expression. This usage was deprecated in C++2a. However, a comma
expression wrapped in "( )" is not deprecated. Example:
void f(int *a, int b, int c) {
a[b,c]; // deprecated
a[(b,c)]; // OK
}
Enabled by default with -std=c++2a.
-Wctor-dtor-privacy (C++ and Objective-C++ only)
Warn when a class seems unusable because all the constructors or
destructors in that class are private, and it has neither friends
nor public static member functions. Also warn if there are no non-
private methods, and there's at least one private member function
that isn't a constructor or destructor.
-Wdelete-non-virtual-dtor (C++ and Objective-C++ only)
Warn when "delete" is used to destroy an instance of a class that
has virtual functions and non-virtual destructor. It is unsafe to
delete an instance of a derived class through a pointer to a base
class if the base class does not have a virtual destructor. This
warning is enabled by -Wall.
-Wdeprecated-copy (C++ and Objective-C++ only)
Warn that the implicit declaration of a copy constructor or copy
assignment operator is deprecated if the class has a user-provided
copy constructor or copy assignment operator, in C++11 and up.
This warning is enabled by -Wextra. With -Wdeprecated-copy-dtor,
also deprecate if the class has a user-provided destructor.
-Wno-init-list-lifetime (C++ and Objective-C++ only)
Do not warn about uses of "std::initializer_list" that are likely
to result in dangling pointers. Since the underlying array for an
"initializer_list" is handled like a normal C++ temporary object,
it is easy to inadvertently keep a pointer to the array past the
end of the array's lifetime. For example:
* If a function returns a temporary "initializer_list", or a
local "initializer_list" variable, the array's lifetime ends at
the end of the return statement, so the value returned has a
dangling pointer.
* If a new-expression creates an "initializer_list", the array
only lives until the end of the enclosing full-expression, so
the "initializer_list" in the heap has a dangling pointer.
* When an "initializer_list" variable is assigned from a brace-
enclosed initializer list, the temporary array created for the
right side of the assignment only lives until the end of the
full-expression, so at the next statement the
"initializer_list" variable has a dangling pointer.
// li's initial underlying array lives as long as li
std::initializer_list<int> li = { 1,2,3 };
// assignment changes li to point to a temporary array
li = { 4, 5 };
// now the temporary is gone and li has a dangling pointer
int i = li.begin()[0] // undefined behavior
* When a list constructor stores the "begin" pointer from the
"initializer_list" argument, this doesn't extend the lifetime
of the array, so if a class variable is constructed from a
temporary "initializer_list", the pointer is left dangling by
the end of the variable declaration statement.
-Wno-literal-suffix (C++ and Objective-C++ only)
Do not warn when a string or character literal is followed by a ud-
suffix which does not begin with an underscore. As a conforming
extension, GCC treats such suffixes as separate preprocessing
tokens in order to maintain backwards compatibility with code that
uses formatting macros from "<inttypes.h>". For example:
#define __STDC_FORMAT_MACROS
#include <inttypes.h>
#include <stdio.h>
int main() {
int64_t i64 = 123;
printf("My int64: %" PRId64"\n", i64);
}
In this case, "PRId64" is treated as a separate preprocessing
token.
This option also controls warnings when a user-defined literal
operator is declared with a literal suffix identifier that doesn't
begin with an underscore. Literal suffix identifiers that don't
begin with an underscore are reserved for future standardization.
These warnings are enabled by default.
-Wno-narrowing (C++ and Objective-C++ only)
For C++11 and later standards, narrowing conversions are diagnosed
by default, as required by the standard. A narrowing conversion
from a constant produces an error, and a narrowing conversion from
a non-constant produces a warning, but -Wno-narrowing suppresses
the diagnostic. Note that this does not affect the meaning of
well-formed code; narrowing conversions are still considered ill-
formed in SFINAE contexts.
With -Wnarrowing in C++98, warn when a narrowing conversion
prohibited by C++11 occurs within { }, e.g.
int i = { 2.2 }; // error: narrowing from double to int
This flag is included in -Wall and -Wc++11-compat.
-Wnoexcept (C++ and Objective-C++ only)
Warn when a noexcept-expression evaluates to false because of a
call to a function that does not have a non-throwing exception
specification (i.e. "throw()" or "noexcept") but is known by the
compiler to never throw an exception.
-Wnoexcept-type (C++ and Objective-C++ only)
Warn if the C++17 feature making "noexcept" part of a function type
changes the mangled name of a symbol relative to C++14. Enabled by
-Wabi and -Wc++17-compat.
As an example:
template <class T> void f(T t) { t(); };
void g() noexcept;
void h() { f(g); }
In C++14, "f" calls "f<void(*)()>", but in C++17 it calls
"f<void(*)()noexcept>".
-Wclass-memaccess (C++ and Objective-C++ only)
Warn when the destination of a call to a raw memory function such
as "memset" or "memcpy" is an object of class type, and when
writing into such an object might bypass the class non-trivial or
deleted constructor or copy assignment, violate const-correctness
or encapsulation, or corrupt virtual table pointers. Modifying the
representation of such objects may violate invariants maintained by
member functions of the class. For example, the call to "memset"
below is undefined because it modifies a non-trivial class object
and is, therefore, diagnosed. The safe way to either initialize or
clear the storage of objects of such types is by using the
appropriate constructor or assignment operator, if one is
available.
std::string str = "abc";
memset (&str, 0, sizeof str);
The -Wclass-memaccess option is enabled by -Wall. Explicitly
casting the pointer to the class object to "void *" or to a type
that can be safely accessed by the raw memory function suppresses
the warning.
-Wnon-virtual-dtor (C++ and Objective-C++ only)
Warn when a class has virtual functions and an accessible non-
virtual destructor itself or in an accessible polymorphic base
class, in which case it is possible but unsafe to delete an
instance of a derived class through a pointer to the class itself
or base class. This warning is automatically enabled if -Weffc++
is specified.
-Wregister (C++ and Objective-C++ only)
Warn on uses of the "register" storage class specifier, except when
it is part of the GNU Explicit Register Variables extension. The
use of the "register" keyword as storage class specifier has been
deprecated in C++11 and removed in C++17. Enabled by default with
-std=c++17.
-Wreorder (C++ and Objective-C++ only)
Warn when the order of member initializers given in the code does
not match the order in which they must be executed. For instance:
struct A {
int i;
int j;
A(): j (0), i (1) { }
};
The compiler rearranges the member initializers for "i" and "j" to
match the declaration order of the members, emitting a warning to
that effect. This warning is enabled by -Wall.
-Wno-pessimizing-move (C++ and Objective-C++ only)
This warning warns when a call to "std::move" prevents copy
elision. A typical scenario when copy elision can occur is when
returning in a function with a class return type, when the
expression being returned is the name of a non-volatile automatic
object, and is not a function parameter, and has the same type as
the function return type.
struct T {
...
};
T fn()
{
T t;
...
return std::move (t);
}
But in this example, the "std::move" call prevents copy elision.
This warning is enabled by -Wall.
-Wno-redundant-move (C++ and Objective-C++ only)
This warning warns about redundant calls to "std::move"; that is,
when a move operation would have been performed even without the
"std::move" call. This happens because the compiler is forced to
treat the object as if it were an rvalue in certain situations such
as returning a local variable, where copy elision isn't applicable.
Consider:
struct T {
...
};
T fn(T t)
{
...
return std::move (t);
}
Here, the "std::move" call is redundant. Because G++ implements
Core Issue 1579, another example is:
struct T { // convertible to U
...
};
struct U {
...
};
U fn()
{
T t;
...
return std::move (t);
}
In this example, copy elision isn't applicable because the type of
the expression being returned and the function return type differ,
yet G++ treats the return value as if it were designated by an
rvalue.
This warning is enabled by -Wextra.
-Wredundant-tags (C++ and Objective-C++ only)
Warn about redundant class-key and enum-key in references to class
types and enumerated types in contexts where the key can be
eliminated without causing an ambiguity. For example:
struct foo;
struct foo *p; // warn that keyword struct can be eliminated
On the other hand, in this example there is no warning:
struct foo;
void foo (); // "hides" struct foo
void bar (struct foo&); // no warning, keyword struct is necessary
-Wno-subobject-linkage (C++ and Objective-C++ only)
Do not warn if a class type has a base or a field whose type uses
the anonymous namespace or depends on a type with no linkage. If a
type A depends on a type B with no or internal linkage, defining it
in multiple translation units would be an ODR violation because the
meaning of B is different in each translation unit. If A only
appears in a single translation unit, the best way to silence the
warning is to give it internal linkage by putting it in an
anonymous namespace as well. The compiler doesn't give this
warning for types defined in the main .C file, as those are
unlikely to have multiple definitions. -Wsubobject-linkage is
enabled by default.
-Weffc++ (C++ and Objective-C++ only)
Warn about violations of the following style guidelines from Scott
Meyers' Effective C++ series of books:
* Define a copy constructor and an assignment operator for
classes with dynamically-allocated memory.
* Prefer initialization to assignment in constructors.
* Have "operator=" return a reference to *this.
* Don't try to return a reference when you must return an object.
* Distinguish between prefix and postfix forms of increment and
decrement operators.
* Never overload "&&", "||", or ",".
This option also enables -Wnon-virtual-dtor, which is also one of
the effective C++ recommendations. However, the check is extended
to warn about the lack of virtual destructor in accessible non-
polymorphic bases classes too.
When selecting this option, be aware that the standard library
headers do not obey all of these guidelines; use grep -v to filter
out those warnings.
-Wstrict-null-sentinel (C++ and Objective-C++ only)
Warn about the use of an uncasted "NULL" as sentinel. When
compiling only with GCC this is a valid sentinel, as "NULL" is
defined to "__null". Although it is a null pointer constant rather
than a null pointer, it is guaranteed to be of the same size as a
pointer. But this use is not portable across different compilers.
-Wno-non-template-friend (C++ and Objective-C++ only)
Disable warnings when non-template friend functions are declared
within a template. In very old versions of GCC that predate
implementation of the ISO standard, declarations such as friend int
foo(int), where the name of the friend is an unqualified-id, could
be interpreted as a particular specialization of a template
function; the warning exists to diagnose compatibility problems,
and is enabled by default.
-Wold-style-cast (C++ and Objective-C++ only)
Warn if an old-style (C-style) cast to a non-void type is used
within a C++ program. The new-style casts ("dynamic_cast",
"static_cast", "reinterpret_cast", and "const_cast") are less
vulnerable to unintended effects and much easier to search for.
-Woverloaded-virtual (C++ and Objective-C++ only)
Warn when a function declaration hides virtual functions from a
base class. For example, in:
struct A {
virtual void f();
};
struct B: public A {
void f(int);
};
the "A" class version of "f" is hidden in "B", and code like:
B* b;
b->f();
fails to compile.
-Wno-pmf-conversions (C++ and Objective-C++ only)
Disable the diagnostic for converting a bound pointer to member
function to a plain pointer.
-Wsign-promo (C++ and Objective-C++ only)
Warn when overload resolution chooses a promotion from unsigned or
enumerated type to a signed type, over a conversion to an unsigned
type of the same size. Previous versions of G++ tried to preserve
unsignedness, but the standard mandates the current behavior.
-Wtemplates (C++ and Objective-C++ only)
Warn when a primary template declaration is encountered. Some
coding rules disallow templates, and this may be used to enforce
that rule. The warning is inactive inside a system header file,
such as the STL, so one can still use the STL. One may also
instantiate or specialize templates.
-Wmismatched-tags (C++ and Objective-C++ only)
Warn for declarations of structs, classes, and class templates and
their specializations with a class-key that does not match either
the definition or the first declaration if no definition is
provided.
For example, the declaration of "struct Object" in the argument
list of "draw" triggers the warning. To avoid it, either remove
the redundant class-key "struct" or replace it with "class" to
match its definition.
class Object {
public:
virtual ~Object () = 0;
};
void draw (struct Object*);
It is not wrong to declare a class with the class-key "struct" as
the example above shows. The -Wmismatched-tags option is intended
to help achieve a consistent style of class declarations. In code
that is intended to be portable to Windows-based compilers the
warning helps prevent unresolved references due to the difference
in the mangling of symbols declared with different class-keys. The
option can be used either on its own or in conjunction with
-Wredundant-tags.
-Wmultiple-inheritance (C++ and Objective-C++ only)
Warn when a class is defined with multiple direct base classes.
Some coding rules disallow multiple inheritance, and this may be
used to enforce that rule. The warning is inactive inside a system
header file, such as the STL, so one can still use the STL. One
may also define classes that indirectly use multiple inheritance.
-Wvirtual-inheritance
Warn when a class is defined with a virtual direct base class.
Some coding rules disallow multiple inheritance, and this may be
used to enforce that rule. The warning is inactive inside a system
header file, such as the STL, so one can still use the STL. One
may also define classes that indirectly use virtual inheritance.
-Wno-virtual-move-assign
Suppress warnings about inheriting from a virtual base with a non-
trivial C++11 move assignment operator. This is dangerous because
if the virtual base is reachable along more than one path, it is
moved multiple times, which can mean both objects end up in the
moved-from state. If the move assignment operator is written to
avoid moving from a moved-from object, this warning can be
disabled.
-Wnamespaces
Warn when a namespace definition is opened. Some coding rules
disallow namespaces, and this may be used to enforce that rule.
The warning is inactive inside a system header file, such as the
STL, so one can still use the STL. One may also use using
directives and qualified names.
-Wno-terminate (C++ and Objective-C++ only)
Disable the warning about a throw-expression that will immediately
result in a call to "terminate".
-Wno-class-conversion (C++ and Objective-C++ only)
Do not warn when a conversion function converts an object to the
same type, to a base class of that type, or to void; such a
conversion function will never be called.
-Wvolatile (C++ and Objective-C++ only)
Warn about deprecated uses of the "volatile" qualifier. This
includes postfix and prefix "++" and "--" expressions of
"volatile"-qualified types, using simple assignments where the left
operand is a "volatile"-qualified non-class type for their value,
compound assignments where the left operand is a
"volatile"-qualified non-class type, "volatile"-qualified function
return type, "volatile"-qualified parameter type, and structured
bindings of a "volatile"-qualified type. This usage was deprecated
in C++20.
Enabled by default with -std=c++2a.
-Wzero-as-null-pointer-constant (C++ and Objective-C++ only)
Warn when a literal 0 is used as null pointer constant. This can
be useful to facilitate the conversion to "nullptr" in C++11.
-Waligned-new
Warn about a new-expression of a type that requires greater
alignment than the "alignof(std::max_align_t)" but uses an
allocation function without an explicit alignment parameter. This
option is enabled by -Wall.
Normally this only warns about global allocation functions, but
-Waligned-new=all also warns about class member allocation
functions.
-Wno-placement-new
-Wplacement-new=n
Warn about placement new expressions with undefined behavior, such
as constructing an object in a buffer that is smaller than the type
of the object. For example, the placement new expression below is
diagnosed because it attempts to construct an array of 64 integers
in a buffer only 64 bytes large.
char buf [64];
new (buf) int[64];
This warning is enabled by default.
-Wplacement-new=1
This is the default warning level of -Wplacement-new. At this
level the warning is not issued for some strictly undefined
constructs that GCC allows as extensions for compatibility with
legacy code. For example, the following "new" expression is
not diagnosed at this level even though it has undefined
behavior according to the C++ standard because it writes past
the end of the one-element array.
struct S { int n, a[1]; };
S *s = (S *)malloc (sizeof *s + 31 * sizeof s->a[0]);
new (s->a)int [32]();
-Wplacement-new=2
At this level, in addition to diagnosing all the same
constructs as at level 1, a diagnostic is also issued for
placement new expressions that construct an object in the last
member of structure whose type is an array of a single element
and whose size is less than the size of the object being
constructed. While the previous example would be diagnosed,
the following construct makes use of the flexible member array
extension to avoid the warning at level 2.
struct S { int n, a[]; };
S *s = (S *)malloc (sizeof *s + 32 * sizeof s->a[0]);
new (s->a)int [32]();
-Wcatch-value
-Wcatch-value=n (C++ and Objective-C++ only)
Warn about catch handlers that do not catch via reference. With
-Wcatch-value=1 (or -Wcatch-value for short) warn about polymorphic
class types that are caught by value. With -Wcatch-value=2 warn
about all class types that are caught by value. With
-Wcatch-value=3 warn about all types that are not caught by
reference. -Wcatch-value is enabled by -Wall.
-Wconditionally-supported (C++ and Objective-C++ only)
Warn for conditionally-supported (C++11 [intro.defs]) constructs.
-Wno-delete-incomplete (C++ and Objective-C++ only)
Do not warn when deleting a pointer to incomplete type, which may
cause undefined behavior at runtime. This warning is enabled by
default.
-Wextra-semi (C++, Objective-C++ only)
Warn about redundant semicolons after in-class function
definitions.
-Wno-inaccessible-base (C++, Objective-C++ only)
This option controls warnings when a base class is inaccessible in
a class derived from it due to ambiguity. The warning is enabled
by default. Note that the warning for ambiguous virtual bases is
enabled by the -Wextra option.
struct A { int a; };
struct B : A { };
struct C : B, A { };
-Wno-inherited-variadic-ctor
Suppress warnings about use of C++11 inheriting constructors when
the base class inherited from has a C variadic constructor; the
warning is on by default because the ellipsis is not inherited.
-Wno-invalid-offsetof (C++ and Objective-C++ only)
Suppress warnings from applying the "offsetof" macro to a non-POD
type. According to the 2014 ISO C++ standard, applying "offsetof"
to a non-standard-layout type is undefined. In existing C++
implementations, however, "offsetof" typically gives meaningful
results. This flag is for users who are aware that they are
writing nonportable code and who have deliberately chosen to ignore
the warning about it.
The restrictions on "offsetof" may be relaxed in a future version
of the C++ standard.
-Wsized-deallocation (C++ and Objective-C++ only)
Warn about a definition of an unsized deallocation function
void operator delete (void *) noexcept;
void operator delete[] (void *) noexcept;
without a definition of the corresponding sized deallocation
function
void operator delete (void *, std::size_t) noexcept;
void operator delete[] (void *, std::size_t) noexcept;
or vice versa. Enabled by -Wextra along with -fsized-deallocation.
-Wsuggest-final-types
Warn about types with virtual methods where code quality would be
improved if the type were declared with the C++11 "final"
specifier, or, if possible, declared in an anonymous namespace.
This allows GCC to more aggressively devirtualize the polymorphic
calls. This warning is more effective with link-time optimization,
where the information about the class hierarchy graph is more
complete.
-Wsuggest-final-methods
Warn about virtual methods where code quality would be improved if
the method were declared with the C++11 "final" specifier, or, if
possible, its type were declared in an anonymous namespace or with
the "final" specifier. This warning is more effective with link-
time optimization, where the information about the class hierarchy
graph is more complete. It is recommended to first consider
suggestions of -Wsuggest-final-types and then rebuild with new
annotations.
-Wsuggest-override
Warn about overriding virtual functions that are not marked with
the "override" keyword.
-Wuseless-cast (C++ and Objective-C++ only)
Warn when an expression is casted to its own type.
-Wno-conversion-null (C++ and Objective-C++ only)
Do not warn for conversions between "NULL" and non-pointer types.
-Wconversion-null is enabled by default.
Options Controlling Objective-C and Objective-C++ Dialects
(NOTE: This manual does not describe the Objective-C and Objective-C++
languages themselves.
This section describes the command-line options that are only
meaningful for Objective-C and Objective-C++ programs. You can also
use most of the language-independent GNU compiler options. For
example, you might compile a file some_class.m like this:
gcc -g -fgnu-runtime -O -c some_class.m
In this example, -fgnu-runtime is an option meant only for Objective-C
and Objective-C++ programs; you can use the other options with any
language supported by GCC.
Note that since Objective-C is an extension of the C language,
Objective-C compilations may also use options specific to the C front-
end (e.g., -Wtraditional). Similarly, Objective-C++ compilations may
use C++-specific options (e.g., -Wabi).
Here is a list of options that are only for compiling Objective-C and
Objective-C++ programs:
-fconstant-string-class=class-name
Use class-name as the name of the class to instantiate for each
literal string specified with the syntax "@"..."". The default
class name is "NXConstantString" if the GNU runtime is being used,
and "NSConstantString" if the NeXT runtime is being used (see
below). The -fconstant-cfstrings option, if also present,
overrides the -fconstant-string-class setting and cause "@"...""
literals to be laid out as constant CoreFoundation strings.
-fgnu-runtime
Generate object code compatible with the standard GNU Objective-C
runtime. This is the default for most types of systems.
-fnext-runtime
Generate output compatible with the NeXT runtime. This is the
default for NeXT-based systems, including Darwin and Mac OS X. The
macro "__NEXT_RUNTIME__" is predefined if (and only if) this option
is used.
-fno-nil-receivers
Assume that all Objective-C message dispatches ("[receiver
message:arg]") in this translation unit ensure that the receiver is
not "nil". This allows for more efficient entry points in the
runtime to be used. This option is only available in conjunction
with the NeXT runtime and ABI version 0 or 1.
-fobjc-abi-version=n
Use version n of the Objective-C ABI for the selected runtime.
This option is currently supported only for the NeXT runtime. In
that case, Version 0 is the traditional (32-bit) ABI without
support for properties and other Objective-C 2.0 additions.
Version 1 is the traditional (32-bit) ABI with support for
properties and other Objective-C 2.0 additions. Version 2 is the
modern (64-bit) ABI. If nothing is specified, the default is
Version 0 on 32-bit target machines, and Version 2 on 64-bit target
machines.
-fobjc-call-cxx-cdtors
For each Objective-C class, check if any of its instance variables
is a C++ object with a non-trivial default constructor. If so,
synthesize a special "- (id) .cxx_construct" instance method which
runs non-trivial default constructors on any such instance
variables, in order, and then return "self". Similarly, check if
any instance variable is a C++ object with a non-trivial
destructor, and if so, synthesize a special "- (void)
.cxx_destruct" method which runs all such default destructors, in
reverse order.
The "- (id) .cxx_construct" and "- (void) .cxx_destruct" methods
thusly generated only operate on instance variables declared in the
current Objective-C class, and not those inherited from
superclasses. It is the responsibility of the Objective-C runtime
to invoke all such methods in an object's inheritance hierarchy.
The "- (id) .cxx_construct" methods are invoked by the runtime
immediately after a new object instance is allocated; the "- (void)
.cxx_destruct" methods are invoked immediately before the runtime
deallocates an object instance.
As of this writing, only the NeXT runtime on Mac OS X 10.4 and
later has support for invoking the "- (id) .cxx_construct" and "-
(void) .cxx_destruct" methods.
-fobjc-direct-dispatch
Allow fast jumps to the message dispatcher. On Darwin this is
accomplished via the comm page.
-fobjc-exceptions
Enable syntactic support for structured exception handling in
Objective-C, similar to what is offered by C++. This option is
required to use the Objective-C keywords @try, @throw, @catch,
@finally and @synchronized. This option is available with both the
GNU runtime and the NeXT runtime (but not available in conjunction
with the NeXT runtime on Mac OS X 10.2 and earlier).
-fobjc-gc
Enable garbage collection (GC) in Objective-C and Objective-C++
programs. This option is only available with the NeXT runtime; the
GNU runtime has a different garbage collection implementation that
does not require special compiler flags.
-fobjc-nilcheck
For the NeXT runtime with version 2 of the ABI, check for a nil
receiver in method invocations before doing the actual method call.
This is the default and can be disabled using -fno-objc-nilcheck.
Class methods and super calls are never checked for nil in this way
no matter what this flag is set to. Currently this flag does
nothing when the GNU runtime, or an older version of the NeXT
runtime ABI, is used.
-fobjc-std=objc1
Conform to the language syntax of Objective-C 1.0, the language
recognized by GCC 4.0. This only affects the Objective-C additions
to the C/C++ language; it does not affect conformance to C/C++
standards, which is controlled by the separate C/C++ dialect option
flags. When this option is used with the Objective-C or
Objective-C++ compiler, any Objective-C syntax that is not
recognized by GCC 4.0 is rejected. This is useful if you need to
make sure that your Objective-C code can be compiled with older
versions of GCC.
-freplace-objc-classes
Emit a special marker instructing lldd(1) not to statically link in
the resulting object file, and allow ddyylldd(1) to load it in at run
time instead. This is used in conjunction with the Fix-and-
Continue debugging mode, where the object file in question may be
recompiled and dynamically reloaded in the course of program
execution, without the need to restart the program itself.
Currently, Fix-and-Continue functionality is only available in
conjunction with the NeXT runtime on Mac OS X 10.3 and later.
-fzero-link
When compiling for the NeXT runtime, the compiler ordinarily
replaces calls to "objc_getClass("...")" (when the name of the
class is known at compile time) with static class references that
get initialized at load time, which improves run-time performance.
Specifying the -fzero-link flag suppresses this behavior and causes
calls to "objc_getClass("...")" to be retained. This is useful in
Zero-Link debugging mode, since it allows for individual class
implementations to be modified during program execution. The GNU
runtime currently always retains calls to "objc_get_class("...")"
regardless of command-line options.
-fno-local-ivars
By default instance variables in Objective-C can be accessed as if
they were local variables from within the methods of the class
they're declared in. This can lead to shadowing between instance
variables and other variables declared either locally inside a
class method or globally with the same name. Specifying the
-fno-local-ivars flag disables this behavior thus avoiding variable
shadowing issues.
-fivar-visibility=[public|protected|private|package]
Set the default instance variable visibility to the specified
option so that instance variables declared outside the scope of any
access modifier directives default to the specified visibility.
-gen-decls
Dump interface declarations for all classes seen in the source file
to a file named sourcename.decl.
-Wassign-intercept (Objective-C and Objective-C++ only)
Warn whenever an Objective-C assignment is being intercepted by the
garbage collector.
-Wno-property-assign-default (Objective-C and Objective-C++ only)
Do not warn if a property for an Objective-C object has no assign
semantics specified.
-Wno-protocol (Objective-C and Objective-C++ only)
If a class is declared to implement a protocol, a warning is issued
for every method in the protocol that is not implemented by the
class. The default behavior is to issue a warning for every method
not explicitly implemented in the class, even if a method
implementation is inherited from the superclass. If you use the
-Wno-protocol option, then methods inherited from the superclass
are considered to be implemented, and no warning is issued for
them.
-Wselector (Objective-C and Objective-C++ only)
Warn if multiple methods of different types for the same selector
are found during compilation. The check is performed on the list
of methods in the final stage of compilation. Additionally, a
check is performed for each selector appearing in a
"@selector(...)" expression, and a corresponding method for that
selector has been found during compilation. Because these checks
scan the method table only at the end of compilation, these
warnings are not produced if the final stage of compilation is not
reached, for example because an error is found during compilation,
or because the -fsyntax-only option is being used.
-Wstrict-selector-match (Objective-C and Objective-C++ only)
Warn if multiple methods with differing argument and/or return
types are found for a given selector when attempting to send a
message using this selector to a receiver of type "id" or "Class".
When this flag is off (which is the default behavior), the compiler
omits such warnings if any differences found are confined to types
that share the same size and alignment.
-Wundeclared-selector (Objective-C and Objective-C++ only)
Warn if a "@selector(...)" expression referring to an undeclared
selector is found. A selector is considered undeclared if no
method with that name has been declared before the "@selector(...)"
expression, either explicitly in an @interface or @protocol
declaration, or implicitly in an @implementation section. This
option always performs its checks as soon as a "@selector(...)"
expression is found, while -Wselector only performs its checks in
the final stage of compilation. This also enforces the coding
style convention that methods and selectors must be declared before
being used.
-print-objc-runtime-info
Generate C header describing the largest structure that is passed
by value, if any.
Options to Control Diagnostic Messages Formatting
Traditionally, diagnostic messages have been formatted irrespective of
the output device's aspect (e.g. its width, ...). You can use the
options described below to control the formatting algorithm for
diagnostic messages, e.g. how many characters per line, how often
source location information should be reported. Note that some
language front ends may not honor these options.
-fmessage-length=n
Try to format error messages so that they fit on lines of about n
characters. If n is zero, then no line-wrapping is done; each
error message appears on a single line. This is the default for
all front ends.
Note - this option also affects the display of the #error and
#warning pre-processor directives, and the deprecated
function/type/variable attribute. It does not however affect the
pragma GCC warning and pragma GCC error pragmas.
-fdiagnostics-show-location=once
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit source location information once; that
is, in case the message is too long to fit on a single physical
line and has to be wrapped, the source location won't be emitted
(as prefix) again, over and over, in subsequent continuation lines.
This is the default behavior.
-fdiagnostics-show-location=every-line
Only meaningful in line-wrapping mode. Instructs the diagnostic
messages reporter to emit the same source location information (as
prefix) for physical lines that result from the process of breaking
a message which is too long to fit on a single line.
-fdiagnostics-color[=WHEN]
-fno-diagnostics-color
Use color in diagnostics. WHEN is never, always, or auto. The
default depends on how the compiler has been configured, it can be
any of the above WHEN options or also never if GCC_COLORS
environment variable isn't present in the environment, and auto
otherwise. auto makes GCC use color only when the standard error
is a terminal, and when not executing in an emacs shell. The forms
-fdiagnostics-color and -fno-diagnostics-color are aliases for
-fdiagnostics-color=always and -fdiagnostics-color=never,
respectively.
The colors are defined by the environment variable GCC_COLORS. Its
value is a colon-separated list of capabilities and Select Graphic
Rendition (SGR) substrings. SGR commands are interpreted by the
terminal or terminal emulator. (See the section in the
documentation of your text terminal for permitted values and their
meanings as character attributes.) These substring values are
integers in decimal representation and can be concatenated with
semicolons. Common values to concatenate include 1 for bold, 4 for
underline, 5 for blink, 7 for inverse, 39 for default foreground
color, 30 to 37 for foreground colors, 90 to 97 for 16-color mode
foreground colors, 38;5;0 to 38;5;255 for 88-color and 256-color
modes foreground colors, 49 for default background color, 40 to 47
for background colors, 100 to 107 for 16-color mode background
colors, and 48;5;0 to 48;5;255 for 88-color and 256-color modes
background colors.
The default GCC_COLORS is
error=01;31:warning=01;35:note=01;36:range1=32:range2=34:locus=01:\
quote=01:path=01;36:fixit-insert=32:fixit-delete=31:\
diff-filename=01:diff-hunk=32:diff-delete=31:diff-insert=32:\
type-diff=01;32
where 01;31 is bold red, 01;35 is bold magenta, 01;36 is bold cyan,
32 is green, 34 is blue, 01 is bold, and 31 is red. Setting
GCC_COLORS to the empty string disables colors. Supported
capabilities are as follows.
"error="
SGR substring for error: markers.
"warning="
SGR substring for warning: markers.
"note="
SGR substring for note: markers.
"path="
SGR substring for colorizing paths of control-flow events as
printed via -fdiagnostics-path-format=, such as the identifiers
of individual events and lines indicating interprocedural calls
and returns.
"range1="
SGR substring for first additional range.
"range2="
SGR substring for second additional range.
"locus="
SGR substring for location information, file:line or
file:line:column etc.
"quote="
SGR substring for information printed within quotes.
"fixit-insert="
SGR substring for fix-it hints suggesting text to be inserted
or replaced.
"fixit-delete="
SGR substring for fix-it hints suggesting text to be deleted.
"diff-filename="
SGR substring for filename headers within generated patches.
"diff-hunk="
SGR substring for the starts of hunks within generated patches.
"diff-delete="
SGR substring for deleted lines within generated patches.
"diff-insert="
SGR substring for inserted lines within generated patches.
"type-diff="
SGR substring for highlighting mismatching types within
template arguments in the C++ frontend.
-fdiagnostics-urls[=WHEN]
Use escape sequences to embed URLs in diagnostics. For example,
when -fdiagnostics-show-option emits text showing the command-line
option controlling a diagnostic, embed a URL for documentation of
that option.
WHEN is never, always, or auto. auto makes GCC use URL escape
sequences only when the standard error is a terminal, and when not
executing in an emacs shell or any graphical terminal which is
known to be incompatible with this feature, see below.
The default depends on how the compiler has been configured. It
can be any of the above WHEN options.
GCC can also be configured (via the
--with-diagnostics-urls=auto-if-env configure-time option) so that
the default is affected by environment variables. Under such a
configuration, GCC defaults to using auto if either GCC_URLS or
TERM_URLS environment variables are present and non-empty in the
environment of the compiler, or never if neither are.
However, even with -fdiagnostics-urls=always the behavior is
dependent on those environment variables: If GCC_URLS is set to
empty or no, do not embed URLs in diagnostics. If set to st, URLs
use ST escape sequences. If set to bel, the default, URLs use BEL
escape sequences. Any other non-empty value enables the feature.
If GCC_URLS is not set, use TERM_URLS as a fallback. Note: ST is
an ANSI escape sequence, string terminator ESC \, BEL is an ASCII
character, CTRL-G that usually sounds like a beep.
At this time GCC tries to detect also a few terminals that are
known to not implement the URL feature, and have bugs or at least
had bugs in some versions that are still in use, where the URL
escapes are likely to misbehave, i.e. print garbage on the screen.
That list is currently xfce4-terminal, certain known to be buggy
gnome-terminal versions, the linux console, and mingw. This check
can be skipped with the -fdiagnostics-urls=always.
-fno-diagnostics-show-option
By default, each diagnostic emitted includes text indicating the
command-line option that directly controls the diagnostic (if such
an option is known to the diagnostic machinery). Specifying the
-fno-diagnostics-show-option flag suppresses that behavior.
-fno-diagnostics-show-caret
By default, each diagnostic emitted includes the original source
line and a caret ^ indicating the column. This option suppresses
this information. The source line is truncated to n characters, if
the -fmessage-length=n option is given. When the output is done to
the terminal, the width is limited to the width given by the
COLUMNS environment variable or, if not set, to the terminal width.
-fno-diagnostics-show-labels
By default, when printing source code (via
-fdiagnostics-show-caret), diagnostics can label ranges of source
code with pertinent information, such as the types of expressions:
printf ("foo %s bar", long_i + long_j);
~^ ~~~~~~~~~~~~~~~
| |
char * long int
This option suppresses the printing of these labels (in the example
above, the vertical bars and the "char *" and "long int" text).
-fno-diagnostics-show-cwe
Diagnostic messages can optionally have an associated
@url{https://cwe.mitre.org/index.html, CWE} identifier. GCC itself
only provides such metadata for some of the -fanalyzer diagnostics.
GCC plugins may also provide diagnostics with such metadata. By
default, if this information is present, it will be printed with
the diagnostic. This option suppresses the printing of this
metadata.
-fno-diagnostics-show-line-numbers
By default, when printing source code (via
-fdiagnostics-show-caret), a left margin is printed, showing line
numbers. This option suppresses this left margin.
-fdiagnostics-minimum-margin-width=width
This option controls the minimum width of the left margin printed
by -fdiagnostics-show-line-numbers. It defaults to 6.
-fdiagnostics-parseable-fixits
Emit fix-it hints in a machine-parseable format, suitable for
consumption by IDEs. For each fix-it, a line will be printed after
the relevant diagnostic, starting with the string "fix-it:". For
example:
fix-it:"test.c":{45:3-45:21}:"gtk_widget_show_all"
The location is expressed as a half-open range, expressed as a
count of bytes, starting at byte 1 for the initial column. In the
above example, bytes 3 through 20 of line 45 of "test.c" are to be
replaced with the given string:
00000000011111111112222222222
12345678901234567890123456789
gtk_widget_showall (dlg);
^^^^^^^^^^^^^^^^^^
gtk_widget_show_all
The filename and replacement string escape backslash as "\\", tab
as "\t", newline as "\n", double quotes as "\"", non-printable
characters as octal (e.g. vertical tab as "\013").
An empty replacement string indicates that the given range is to be
removed. An empty range (e.g. "45:3-45:3") indicates that the
string is to be inserted at the given position.
-fdiagnostics-generate-patch
Print fix-it hints to stderr in unified diff format, after any
diagnostics are printed. For example:
--- test.c
+++ test.c
@ -42,5 +42,5 @
void show_cb(GtkDialog *dlg)
{
- gtk_widget_showall(dlg);
+ gtk_widget_show_all(dlg);
}
The diff may or may not be colorized, following the same rules as
for diagnostics (see -fdiagnostics-color).
-fdiagnostics-show-template-tree
In the C++ frontend, when printing diagnostics showing mismatching
template types, such as:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
the -fdiagnostics-show-template-tree flag enables printing a tree-
like structure showing the common and differing parts of the types,
such as:
map<
[...],
vector<
[double != float]>>
The parts that differ are highlighted with color ("double" and
"float" in this case).
-fno-elide-type
By default when the C++ frontend prints diagnostics showing
mismatching template types, common parts of the types are printed
as "[...]" to simplify the error message. For example:
could not convert 'std::map<int, std::vector<double> >()'
from 'map<[...],vector<double>>' to 'map<[...],vector<float>>
Specifying the -fno-elide-type flag suppresses that behavior. This
flag also affects the output of the
-fdiagnostics-show-template-tree flag.
-fdiagnostics-path-format=KIND
Specify how to print paths of control-flow events for diagnostics
that have such a path associated with them.
KIND is none, separate-events, or inline-events, the default.
none means to not print diagnostic paths.
separate-events means to print a separate "note" diagnostic for
each event within the diagnostic. For example:
test.c:29:5: error: passing NULL as argument 1 to 'PyList_Append' which requires a non-NULL parameter
test.c:25:10: note: (1) when 'PyList_New' fails, returning NULL
test.c:27:3: note: (2) when 'i < count'
test.c:29:5: note: (3) when calling 'PyList_Append', passing NULL from (1) as argument 1
inline-events means to print the events "inline" within the source
code. This view attempts to consolidate the events into runs of
sufficiently-close events, printing them as labelled ranges within
the source.
For example, the same events as above might be printed as:
'test': events 1-3
|
| 25 | list = PyList_New(0);
| | ^~~~~~~~~~~~~
| | |
| | (1) when 'PyList_New' fails, returning NULL
| 26 |
| 27 | for (i = 0; i < count; i++) {
| | ~~~
| | |
| | (2) when 'i < count'
| 28 | item = PyLong_FromLong(random());
| 29 | PyList_Append(list, item);
| | ~~~~~~~~~~~~~~~~~~~~~~~~~
| | |
| | (3) when calling 'PyList_Append', passing NULL from (1) as argument 1
|
Interprocedural control flow is shown by grouping the events by
stack frame, and using indentation to show how stack frames are
nested, pushed, and popped.
For example:
'test': events 1-2
|
| 133 | {
| | ^
| | |
| | (1) entering 'test'
| 134 | boxed_int *obj = make_boxed_int (i);
| | ~~~~~~~~~~~~~~~~~~
| | |
| | (2) calling 'make_boxed_int'
|
+--> 'make_boxed_int': events 3-4
|
| 120 | {
| | ^
| | |
| | (3) entering 'make_boxed_int'
| 121 | boxed_int *result = (boxed_int *)wrapped_malloc (sizeof (boxed_int));
| | ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
| | |
| | (4) calling 'wrapped_malloc'
|
+--> 'wrapped_malloc': events 5-6
|
| 7 | {
| | ^
| | |
| | (5) entering 'wrapped_malloc'
| 8 | return malloc (size);
| | ~~~~~~~~~~~~~
| | |
| | (6) calling 'malloc'
|
<-------------+
|
'test': event 7
|
| 138 | free_boxed_int (obj);
| | ^~~~~~~~~~~~~~~~~~~~
| | |
| | (7) calling 'free_boxed_int'
|
(etc)
-fdiagnostics-show-path-depths
This option provides additional information when printing control-
flow paths associated with a diagnostic.
If this is option is provided then the stack depth will be printed
for each run of events within
-fdiagnostics-path-format=separate-events.
This is intended for use by GCC developers and plugin developers
when debugging diagnostics that report interprocedural control
flow.
-fno-show-column
Do not print column numbers in diagnostics. This may be necessary
if diagnostics are being scanned by a program that does not
understand the column numbers, such as dejagnu.
-fdiagnostics-format=FORMAT
Select a different format for printing diagnostics. FORMAT is text
or json. The default is text.
The json format consists of a top-level JSON array containing JSON
objects representing the diagnostics.
The JSON is emitted as one line, without formatting; the examples
below have been formatted for clarity.
Diagnostics can have child diagnostics. For example, this error
and note:
misleading-indentation.c:15:3: warning: this 'if' clause does not
guard... [-Wmisleading-indentation]
15 | if (flag)
| ^~
misleading-indentation.c:17:5: note: ...this statement, but the latter
is misleadingly indented as if it were guarded by the 'if'
17 | y = 2;
| ^
might be printed in JSON form (after formatting) like this:
[
{
"kind": "warning",
"locations": [
{
"caret": {
"column": 3,
"file": "misleading-indentation.c",
"line": 15
},
"finish": {
"column": 4,
"file": "misleading-indentation.c",
"line": 15
}
}
],
"message": "this \u2018if\u2019 clause does not guard...",
"option": "-Wmisleading-indentation",
"option_url": "https://gcc.gnu.org/onlinedocs/gcc/Warning-Options.html#index-Wmisleading-indentation",
"children": [
{
"kind": "note",
"locations": [
{
"caret": {
"column": 5,
"file": "misleading-indentation.c",
"line": 17
}
}
],
"message": "...this statement, but the latter is ..."
}
]
},
...
]
where the "note" is a child of the "warning".
A diagnostic has a "kind". If this is "warning", then there is an
"option" key describing the command-line option controlling the
warning.
A diagnostic can contain zero or more locations. Each location has
up to three positions within it: a "caret" position and optional
"start" and "finish" positions. A location can also have an
optional "label" string. For example, this error:
bad-binary-ops.c:64:23: error: invalid operands to binary + (have 'S' {aka
'struct s'} and 'T' {aka 'struct t'})
64 | return callee_4a () + callee_4b ();
| ~~~~~~~~~~~~ ^ ~~~~~~~~~~~~
| | |
| | T {aka struct t}
| S {aka struct s}
has three locations. Its primary location is at the "+" token at
column 23. It has two secondary locations, describing the left and
right-hand sides of the expression, which have labels. It might be
printed in JSON form as:
{
"children": [],
"kind": "error",
"locations": [
{
"caret": {
"column": 23, "file": "bad-binary-ops.c", "line": 64
}
},
{
"caret": {
"column": 10, "file": "bad-binary-ops.c", "line": 64
},
"finish": {
"column": 21, "file": "bad-binary-ops.c", "line": 64
},
"label": "S {aka struct s}"
},
{
"caret": {
"column": 25, "file": "bad-binary-ops.c", "line": 64
},
"finish": {
"column": 36, "file": "bad-binary-ops.c", "line": 64
},
"label": "T {aka struct t}"
}
],
"message": "invalid operands to binary + ..."
}
If a diagnostic contains fix-it hints, it has a "fixits" array,
consisting of half-open intervals, similar to the output of
-fdiagnostics-parseable-fixits. For example, this diagnostic with
a replacement fix-it hint:
demo.c:8:15: error: 'struct s' has no member named 'colour'; did you
mean 'color'?
8 | return ptr->colour;
| ^~~~~~
| color
might be printed in JSON form as:
{
"children": [],
"fixits": [
{
"next": {
"column": 21,
"file": "demo.c",
"line": 8
},
"start": {
"column": 15,
"file": "demo.c",
"line": 8
},
"string": "color"
}
],
"kind": "error",
"locations": [
{
"caret": {
"column": 15,
"file": "demo.c",
"line": 8
},
"finish": {
"column": 20,
"file": "demo.c",
"line": 8
}
}
],
"message": "\u2018struct s\u2019 has no member named ..."
}
where the fix-it hint suggests replacing the text from "start" up
to but not including "next" with "string"'s value. Deletions are
expressed via an empty value for "string", insertions by having
"start" equal "next".
If the diagnostic has a path of control-flow events associated with
it, it has a "path" array of objects representing the events. Each
event object has a "description" string, a "location" object, along
with a "function" string and a "depth" number for representing
interprocedural paths. The "function" represents the current
function at that event, and the "depth" represents the stack depth
relative to some baseline: the higher, the more frames are within
the stack.
For example, the intraprocedural example shown for
-fdiagnostics-path-format= might have this JSON for its path:
"path": [
{
"depth": 0,
"description": "when 'PyList_New' fails, returning NULL",
"function": "test",
"location": {
"column": 10,
"file": "test.c",
"line": 25
}
},
{
"depth": 0,
"description": "when 'i < count'",
"function": "test",
"location": {
"column": 3,
"file": "test.c",
"line": 27
}
},
{
"depth": 0,
"description": "when calling 'PyList_Append', passing NULL from (1) as argument 1",
"function": "test",
"location": {
"column": 5,
"file": "test.c",
"line": 29
}
}
]
Options to Request or Suppress Warnings
Warnings are diagnostic messages that report constructions that are not
inherently erroneous but that are risky or suggest there may have been
an error.
The following language-independent options do not enable specific
warnings but control the kinds of diagnostics produced by GCC.
-fsyntax-only
Check the code for syntax errors, but don't do anything beyond
that.
-fmax-errors=n
Limits the maximum number of error messages to n, at which point
GCC bails out rather than attempting to continue processing the
source code. If n is 0 (the default), there is no limit on the
number of error messages produced. If -Wfatal-errors is also
specified, then -Wfatal-errors takes precedence over this option.
-w Inhibit all warning messages.
-Werror
Make all warnings into errors.
-Werror=
Make the specified warning into an error. The specifier for a
warning is appended; for example -Werror=switch turns the warnings
controlled by -Wswitch into errors. This switch takes a negative
form, to be used to negate -Werror for specific warnings; for
example -Wno-error=switch makes -Wswitch warnings not be errors,
even when -Werror is in effect.
The warning message for each controllable warning includes the
option that controls the warning. That option can then be used
with -Werror= and -Wno-error= as described above. (Printing of the
option in the warning message can be disabled using the
-fno-diagnostics-show-option flag.)
Note that specifying -Werror=foo automatically implies -Wfoo.
However, -Wno-error=foo does not imply anything.
-Wfatal-errors
This option causes the compiler to abort compilation on the first
error occurred rather than trying to keep going and printing
further error messages.
You can request many specific warnings with options beginning with -W,
for example -Wimplicit to request warnings on implicit declarations.
Each of these specific warning options also has a negative form
beginning -Wno- to turn off warnings; for example, -Wno-implicit. This
manual lists only one of the two forms, whichever is not the default.
For further language-specific options also refer to C++ Dialect Options
and Objective-C and Objective-C++ Dialect Options. Additional warnings
can be produced by enabling the static analyzer;
Some options, such as -Wall and -Wextra, turn on other options, such as
-Wunused, which may turn on further options, such as -Wunused-value.
The combined effect of positive and negative forms is that more
specific options have priority over less specific ones, independently
of their position in the command-line. For options of the same
specificity, the last one takes effect. Options enabled or disabled via
pragmas take effect as if they appeared at the end of the command-line.
When an unrecognized warning option is requested (e.g.,
-Wunknown-warning), GCC emits a diagnostic stating that the option is
not recognized. However, if the -Wno- form is used, the behavior is
slightly different: no diagnostic is produced for -Wno-unknown-warning
unless other diagnostics are being produced. This allows the use of
new -Wno- options with old compilers, but if something goes wrong, the
compiler warns that an unrecognized option is present.
The effectiveness of some warnings depends on optimizations also being
enabled. For example -Wsuggest-final-types is more effective with link-
time optimization and -Wmaybe-uninitialized does not warn at all unless
optimization is enabled.
-Wpedantic
-pedantic
Issue all the warnings demanded by strict ISO C and ISO C++; reject
all programs that use forbidden extensions, and some other programs
that do not follow ISO C and ISO C++. For ISO C, follows the
version of the ISO C standard specified by any -std option used.
Valid ISO C and ISO C++ programs should compile properly with or
without this option (though a rare few require -ansi or a -std
option specifying the required version of ISO C). However, without
this option, certain GNU extensions and traditional C and C++
features are supported as well. With this option, they are
rejected.
-Wpedantic does not cause warning messages for use of the alternate
keywords whose names begin and end with __. This alternate format
can also be used to disable warnings for non-ISO __intN types, i.e.
__intN__. Pedantic warnings are also disabled in the expression
that follows "__extension__". However, only system header files
should use these escape routes; application programs should avoid
them.
Some users try to use -Wpedantic to check programs for strict ISO C
conformance. They soon find that it does not do quite what they
want: it finds some non-ISO practices, but not all---only those for
which ISO C requires a diagnostic, and some others for which
diagnostics have been added.
A feature to report any failure to conform to ISO C might be useful
in some instances, but would require considerable additional work
and would be quite different from -Wpedantic. We don't have plans
to support such a feature in the near future.
Where the standard specified with -std represents a GNU extended
dialect of C, such as gnu90 or gnu99, there is a corresponding base
standard, the version of ISO C on which the GNU extended dialect is
based. Warnings from -Wpedantic are given where they are required
by the base standard. (It does not make sense for such warnings to
be given only for features not in the specified GNU C dialect,
since by definition the GNU dialects of C include all features the
compiler supports with the given option, and there would be nothing
to warn about.)
-pedantic-errors
Give an error whenever the base standard (see -Wpedantic) requires
a diagnostic, in some cases where there is undefined behavior at
compile-time and in some other cases that do not prevent
compilation of programs that are valid according to the standard.
This is not equivalent to -Werror=pedantic, since there are errors
enabled by this option and not enabled by the latter and vice
versa.
-Wall
This enables all the warnings about constructions that some users
consider questionable, and that are easy to avoid (or modify to
prevent the warning), even in conjunction with macros. This also
enables some language-specific warnings described in C++ Dialect
Options and Objective-C and Objective-C++ Dialect Options.
-Wall turns on the following warning flags:
-Waddress -Warray-bounds=1 (only with -O2) -Wbool-compare
-Wbool-operation -Wc++11-compat -Wc++14-compat -Wcatch-value (C++
and Objective-C++ only) -Wchar-subscripts -Wcomment
-Wduplicate-decl-specifier (C and Objective-C only) -Wenum-compare
(in C/ObjC; this is on by default in C++) -Wenum-conversion in
C/ObjC; -Wformat -Wformat-overflow -Wformat-truncation
-Wint-in-bool-context -Wimplicit (C and Objective-C only)
-Wimplicit-int (C and Objective-C only)
-Wimplicit-function-declaration (C and Objective-C only)
-Winit-self (only for C++) -Wlogical-not-parentheses -Wmain (only
for C/ObjC and unless -ffreestanding) -Wmaybe-uninitialized
-Wmemset-elt-size -Wmemset-transposed-args -Wmisleading-indentation
(only for C/C++) -Wmissing-attributes -Wmissing-braces (only for
C/ObjC) -Wmultistatement-macros -Wnarrowing (only for C++)
-Wnonnull -Wnonnull-compare -Wopenmp-simd -Wparentheses
-Wpessimizing-move (only for C++) -Wpointer-sign -Wreorder
-Wrestrict -Wreturn-type -Wsequence-point -Wsign-compare (only in
C++) -Wsizeof-pointer-div -Wsizeof-pointer-memaccess
-Wstrict-aliasing -Wstrict-overflow=1 -Wswitch
-Wtautological-compare -Wtrigraphs -Wuninitialized
-Wunknown-pragmas -Wunused-function -Wunused-label -Wunused-value
-Wunused-variable -Wvolatile-register-var -Wzero-length-bounds
Note that some warning flags are not implied by -Wall. Some of
them warn about constructions that users generally do not consider
questionable, but which occasionally you might wish to check for;
others warn about constructions that are necessary or hard to avoid
in some cases, and there is no simple way to modify the code to
suppress the warning. Some of them are enabled by -Wextra but many
of them must be enabled individually.
-Wextra
This enables some extra warning flags that are not enabled by
-Wall. (This option used to be called -W. The older name is still
supported, but the newer name is more descriptive.)
-Wclobbered -Wcast-function-type -Wdeprecated-copy (C++ only)
-Wempty-body -Wignored-qualifiers -Wimplicit-fallthrough=3
-Wmissing-field-initializers -Wmissing-parameter-type (C only)
-Wold-style-declaration (C only) -Woverride-init -Wsign-compare (C
only) -Wstring-compare -Wredundant-move (only for C++)
-Wtype-limits -Wuninitialized -Wshift-negative-value (in C++11 to
C++17 and in C99 and newer) -Wunused-parameter (only with -Wunused
or -Wall) -Wunused-but-set-parameter (only with -Wunused or -Wall)
The option -Wextra also prints warning messages for the following
cases:
* A pointer is compared against integer zero with "<", "<=", ">",
or ">=".
* (C++ only) An enumerator and a non-enumerator both appear in a
conditional expression.
* (C++ only) Ambiguous virtual bases.
* (C++ only) Subscripting an array that has been declared
"register".
* (C++ only) Taking the address of a variable that has been
declared "register".
* (C++ only) A base class is not initialized in the copy
constructor of a derived class.
-Wabi (C, Objective-C, C++ and Objective-C++ only)
Warn about code affected by ABI changes. This includes code that
may not be compatible with the vendor-neutral C++ ABI as well as
the psABI for the particular target.
Since G++ now defaults to updating the ABI with each major release,
normally -Wabi warns only about C++ ABI compatibility problems if
there is a check added later in a release series for an ABI issue
discovered since the initial release. -Wabi warns about more
things if an older ABI version is selected (with -fabi-version=n).
-Wabi can also be used with an explicit version number to warn
about C++ ABI compatibility with a particular -fabi-version level,
e.g. -Wabi=2 to warn about changes relative to -fabi-version=2.
If an explicit version number is provided and -fabi-compat-version
is not specified, the version number from this option is used for
compatibility aliases. If no explicit version number is provided
with this option, but -fabi-compat-version is specified, that
version number is used for C++ ABI warnings.
Although an effort has been made to warn about all such cases,
there are probably some cases that are not warned about, even
though G++ is generating incompatible code. There may also be
cases where warnings are emitted even though the code that is
generated is compatible.
You should rewrite your code to avoid these warnings if you are
concerned about the fact that code generated by G++ may not be
binary compatible with code generated by other compilers.
Known incompatibilities in -fabi-version=2 (which was the default
from GCC 3.4 to 4.9) include:
* A template with a non-type template parameter of reference type
was mangled incorrectly:
extern int N;
template <int &> struct S {};
void n (S<N>) {2}
This was fixed in -fabi-version=3.
* SIMD vector types declared using "__attribute ((vector_size))"
were mangled in a non-standard way that does not allow for
overloading of functions taking vectors of different sizes.
The mangling was changed in -fabi-version=4.
* "__attribute ((const))" and "noreturn" were mangled as type
qualifiers, and "decltype" of a plain declaration was folded
away.
These mangling issues were fixed in -fabi-version=5.
* Scoped enumerators passed as arguments to a variadic function
are promoted like unscoped enumerators, causing "va_arg" to
complain. On most targets this does not actually affect the
parameter passing ABI, as there is no way to pass an argument
smaller than "int".
Also, the ABI changed the mangling of template argument packs,
"const_cast", "static_cast", prefix increment/decrement, and a
class scope function used as a template argument.
These issues were corrected in -fabi-version=6.
* Lambdas in default argument scope were mangled incorrectly, and
the ABI changed the mangling of "nullptr_t".
These issues were corrected in -fabi-version=7.
* When mangling a function type with function-cv-qualifiers, the
un-qualified function type was incorrectly treated as a
substitution candidate.
This was fixed in -fabi-version=8, the default for GCC 5.1.
* "decltype(nullptr)" incorrectly had an alignment of 1, leading
to unaligned accesses. Note that this did not affect the ABI
of a function with a "nullptr_t" parameter, as parameters have
a minimum alignment.
This was fixed in -fabi-version=9, the default for GCC 5.2.
* Target-specific attributes that affect the identity of a type,
such as ia32 calling conventions on a function type (stdcall,
regparm, etc.), did not affect the mangled name, leading to
name collisions when function pointers were used as template
arguments.
This was fixed in -fabi-version=10, the default for GCC 6.1.
This option also enables warnings about psABI-related changes. The
known psABI changes at this point include:
* For SysV/x86-64, unions with "long double" members are passed
in memory as specified in psABI. Prior to GCC 4.4, this was
not the case. For example:
union U {
long double ld;
int i;
};
"union U" is now always passed in memory.
-Wchar-subscripts
Warn if an array subscript has type "char". This is a common cause
of error, as programmers often forget that this type is signed on
some machines. This warning is enabled by -Wall.
-Wno-coverage-mismatch
Warn if feedback profiles do not match when using the -fprofile-use
option. If a source file is changed between compiling with
-fprofile-generate and with -fprofile-use, the files with the
profile feedback can fail to match the source file and GCC cannot
use the profile feedback information. By default, this warning is
enabled and is treated as an error. -Wno-coverage-mismatch can be
used to disable the warning or -Wno-error=coverage-mismatch can be
used to disable the error. Disabling the error for this warning
can result in poorly optimized code and is useful only in the case
of very minor changes such as bug fixes to an existing code-base.
Completely disabling the warning is not recommended.
-Wno-cpp
(C, Objective-C, C++, Objective-C++ and Fortran only) Suppress
warning messages emitted by "#warning" directives.
-Wdouble-promotion (C, C++, Objective-C and Objective-C++ only)
Give a warning when a value of type "float" is implicitly promoted
to "double". CPUs with a 32-bit "single-precision" floating-point
unit implement "float" in hardware, but emulate "double" in
software. On such a machine, doing computations using "double"
values is much more expensive because of the overhead required for
software emulation.
It is easy to accidentally do computations with "double" because
floating-point literals are implicitly of type "double". For
example, in:
float area(float radius)
{
return 3.14159 * radius * radius;
}
the compiler performs the entire computation with "double" because
the floating-point literal is a "double".
-Wduplicate-decl-specifier (C and Objective-C only)
Warn if a declaration has duplicate "const", "volatile", "restrict"
or "_Atomic" specifier. This warning is enabled by -Wall.
-Wformat
-Wformat=n
Check calls to "printf" and "scanf", etc., to make sure that the
arguments supplied have types appropriate to the format string
specified, and that the conversions specified in the format string
make sense. This includes standard functions, and others specified
by format attributes, in the "printf", "scanf", "strftime" and
"strfmon" (an X/Open extension, not in the C standard) families (or
other target-specific families). Which functions are checked
without format attributes having been specified depends on the
standard version selected, and such checks of functions without the
attribute specified are disabled by -ffreestanding or -fno-builtin.
The formats are checked against the format features supported by
GNU libc version 2.2. These include all ISO C90 and C99 features,
as well as features from the Single Unix Specification and some BSD
and GNU extensions. Other library implementations may not support
all these features; GCC does not support warning about features
that go beyond a particular library's limitations. However, if
-Wpedantic is used with -Wformat, warnings are given about format
features not in the selected standard version (but not for
"strfmon" formats, since those are not in any version of the C
standard).
-Wformat=1
-Wformat
Option -Wformat is equivalent to -Wformat=1, and -Wno-format is
equivalent to -Wformat=0. Since -Wformat also checks for null
format arguments for several functions, -Wformat also implies
-Wnonnull. Some aspects of this level of format checking can
be disabled by the options: -Wno-format-contains-nul,
-Wno-format-extra-args, and -Wno-format-zero-length. -Wformat
is enabled by -Wall.
-Wformat=2
Enable -Wformat plus additional format checks. Currently
equivalent to -Wformat -Wformat-nonliteral -Wformat-security
-Wformat-y2k.
-Wno-format-contains-nul
If -Wformat is specified, do not warn about format strings that
contain NUL bytes.
-Wno-format-extra-args
If -Wformat is specified, do not warn about excess arguments to a
"printf" or "scanf" format function. The C standard specifies that
such arguments are ignored.
Where the unused arguments lie between used arguments that are
specified with $ operand number specifications, normally warnings
are still given, since the implementation could not know what type
to pass to "va_arg" to skip the unused arguments. However, in the
case of "scanf" formats, this option suppresses the warning if the
unused arguments are all pointers, since the Single Unix
Specification says that such unused arguments are allowed.
-Wformat-overflow
-Wformat-overflow=level
Warn about calls to formatted input/output functions such as
"sprintf" and "vsprintf" that might overflow the destination
buffer. When the exact number of bytes written by a format
directive cannot be determined at compile-time it is estimated
based on heuristics that depend on the level argument and on
optimization. While enabling optimization will in most cases
improve the accuracy of the warning, it may also result in false
positives.
-Wformat-overflow
-Wformat-overflow=1
Level 1 of -Wformat-overflow enabled by -Wformat employs a
conservative approach that warns only about calls that most
likely overflow the buffer. At this level, numeric arguments
to format directives with unknown values are assumed to have
the value of one, and strings of unknown length to be empty.
Numeric arguments that are known to be bounded to a subrange of
their type, or string arguments whose output is bounded either
by their directive's precision or by a finite set of string
literals, are assumed to take on the value within the range
that results in the most bytes on output. For example, the
call to "sprintf" below is diagnosed because even with both a
and b equal to zero, the terminating NUL character ('\0')
appended by the function to the destination buffer will be
written past its end. Increasing the size of the buffer by a
single byte is sufficient to avoid the warning, though it may
not be sufficient to avoid the overflow.
void f (int a, int b)
{
char buf [13];
sprintf (buf, "a = %i, b = %i\n", a, b);
}
-Wformat-overflow=2
Level 2 warns also about calls that might overflow the
destination buffer given an argument of sufficient length or
magnitude. At level 2, unknown numeric arguments are assumed
to have the minimum representable value for signed types with a
precision greater than 1, and the maximum representable value
otherwise. Unknown string arguments whose length cannot be
assumed to be bounded either by the directive's precision, or
by a finite set of string literals they may evaluate to, or the
character array they may point to, are assumed to be 1
character long.
At level 2, the call in the example above is again diagnosed,
but this time because with a equal to a 32-bit "INT_MIN" the
first %i directive will write some of its digits beyond the end
of the destination buffer. To make the call safe regardless of
the values of the two variables, the size of the destination
buffer must be increased to at least 34 bytes. GCC includes
the minimum size of the buffer in an informational note
following the warning.
An alternative to increasing the size of the destination buffer
is to constrain the range of formatted values. The maximum
length of string arguments can be bounded by specifying the
precision in the format directive. When numeric arguments of
format directives can be assumed to be bounded by less than the
precision of their type, choosing an appropriate length
modifier to the format specifier will reduce the required
buffer size. For example, if a and b in the example above can
be assumed to be within the precision of the "short int" type
then using either the %hi format directive or casting the
argument to "short" reduces the maximum required size of the
buffer to 24 bytes.
void f (int a, int b)
{
char buf [23];
sprintf (buf, "a = %hi, b = %i\n", a, (short)b);
}
-Wno-format-zero-length
If -Wformat is specified, do not warn about zero-length formats.
The C standard specifies that zero-length formats are allowed.
-Wformat-nonliteral
If -Wformat is specified, also warn if the format string is not a
string literal and so cannot be checked, unless the format function
takes its format arguments as a "va_list".
-Wformat-security
If -Wformat is specified, also warn about uses of format functions
that represent possible security problems. At present, this warns
about calls to "printf" and "scanf" functions where the format
string is not a string literal and there are no format arguments,
as in "printf (foo);". This may be a security hole if the format
string came from untrusted input and contains %n. (This is
currently a subset of what -Wformat-nonliteral warns about, but in
future warnings may be added to -Wformat-security that are not
included in -Wformat-nonliteral.)
-Wformat-signedness
If -Wformat is specified, also warn if the format string requires
an unsigned argument and the argument is signed and vice versa.
-Wformat-truncation
-Wformat-truncation=level
Warn about calls to formatted input/output functions such as
"snprintf" and "vsnprintf" that might result in output truncation.
When the exact number of bytes written by a format directive cannot
be determined at compile-time it is estimated based on heuristics
that depend on the level argument and on optimization. While
enabling optimization will in most cases improve the accuracy of
the warning, it may also result in false positives. Except as
noted otherwise, the option uses the same logic -Wformat-overflow.
-Wformat-truncation
-Wformat-truncation=1
Level 1 of -Wformat-truncation enabled by -Wformat employs a
conservative approach that warns only about calls to bounded
functions whose return value is unused and that will most
likely result in output truncation.
-Wformat-truncation=2
Level 2 warns also about calls to bounded functions whose
return value is used and that might result in truncation given
an argument of sufficient length or magnitude.
-Wformat-y2k
If -Wformat is specified, also warn about "strftime" formats that
may yield only a two-digit year.
-Wnonnull
Warn about passing a null pointer for arguments marked as requiring
a non-null value by the "nonnull" function attribute.
-Wnonnull is included in -Wall and -Wformat. It can be disabled
with the -Wno-nonnull option.
-Wnonnull-compare
Warn when comparing an argument marked with the "nonnull" function
attribute against null inside the function.
-Wnonnull-compare is included in -Wall. It can be disabled with
the -Wno-nonnull-compare option.
-Wnull-dereference
Warn if the compiler detects paths that trigger erroneous or
undefined behavior due to dereferencing a null pointer. This
option is only active when -fdelete-null-pointer-checks is active,
which is enabled by optimizations in most targets. The precision
of the warnings depends on the optimization options used.
-Winit-self (C, C++, Objective-C and Objective-C++ only)
Warn about uninitialized variables that are initialized with
themselves. Note this option can only be used with the
-Wuninitialized option.
For example, GCC warns about "i" being uninitialized in the
following snippet only when -Winit-self has been specified:
int f()
{
int i = i;
return i;
}
This warning is enabled by -Wall in C++.
-Wno-implicit-int (C and Objective-C only)
This option controls warnings when a declaration does not specify a
type. This warning is enabled by default in C99 and later dialects
of C, and also by -Wall.
-Wno-implicit-function-declaration (C and Objective-C only)
This option controls warnings when a function is used before being
declared. This warning is enabled by default in C99 and later
dialects of C, and also by -Wall. The warning is made into an
error by -pedantic-errors.
-Wimplicit (C and Objective-C only)
Same as -Wimplicit-int and -Wimplicit-function-declaration. This
warning is enabled by -Wall.
-Wimplicit-fallthrough
-Wimplicit-fallthrough is the same as -Wimplicit-fallthrough=3 and
-Wno-implicit-fallthrough is the same as -Wimplicit-fallthrough=0.
-Wimplicit-fallthrough=n
Warn when a switch case falls through. For example:
switch (cond)
{
case 1:
a = 1;
break;
case 2:
a = 2;
case 3:
a = 3;
break;
}
This warning does not warn when the last statement of a case cannot
fall through, e.g. when there is a return statement or a call to
function declared with the noreturn attribute.
-Wimplicit-fallthrough= also takes into account control flow
statements, such as ifs, and only warns when appropriate. E.g.
switch (cond)
{
case 1:
if (i > 3) {
bar (5);
break;
} else if (i < 1) {
bar (0);
} else
return;
default:
...
}
Since there are occasions where a switch case fall through is
desirable, GCC provides an attribute, "__attribute__
((fallthrough))", that is to be used along with a null statement to
suppress this warning that would normally occur:
switch (cond)
{
case 1:
bar (0);
__attribute__ ((fallthrough));
default:
...
}
C++17 provides a standard way to suppress the
-Wimplicit-fallthrough warning using "[[fallthrough]];" instead of
the GNU attribute. In C++11 or C++14 users can use
"[[gnu::fallthrough]];", which is a GNU extension. Instead of
these attributes, it is also possible to add a fallthrough comment
to silence the warning. The whole body of the C or C++ style
comment should match the given regular expressions listed below.
The option argument n specifies what kind of comments are accepted:
*<-Wimplicit-fallthrough=0 disables the warning altogether.>
*<-Wimplicit-fallthrough=1 matches ".*" regular>
expression, any comment is used as fallthrough comment.
*<-Wimplicit-fallthrough=2 case insensitively matches>
".*falls?[ \t-]*thr(ough|u).*" regular expression.
*<-Wimplicit-fallthrough=3 case sensitively matches one of the>
following regular expressions:
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t.!]*(ELSE,? |INTENTIONAL(LY)? )?FALL(S |
|-)?THR(OUGH|U)[ \t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*(Else,? |Intentional(ly)? )?Fall((s |
|-)[Tt]|t)hr(ough|u)[ \t.!]*(-[^\n\r]*)?">
*<"[ \t.!]*([Ee]lse,? |[Ii]ntentional(ly)? )?fall(s |
|-)?thr(ough|u)[ \t.!]*(-[^\n\r]*)?">
*<-Wimplicit-fallthrough=4 case sensitively matches one of the>
following regular expressions:
*<"-fallthrough">
*<"@fallthrough@">
*<"lint -fallthrough[ \t]*">
*<"[ \t]*FALLTHR(OUGH|U)[ \t]*">
*<-Wimplicit-fallthrough=5 doesn't recognize any comments as>
fallthrough comments, only attributes disable the warning.
The comment needs to be followed after optional whitespace and
other comments by "case" or "default" keywords or by a user label
that precedes some "case" or "default" label.
switch (cond)
{
case 1:
bar (0);
/* FALLTHRU */
default:
...
}
The -Wimplicit-fallthrough=3 warning is enabled by -Wextra.
-Wno-if-not-aligned (C, C++, Objective-C and Objective-C++ only)
Control if warnings triggered by the "warn_if_not_aligned"
attribute should be issued. These warnings are enabled by default.
-Wignored-qualifiers (C and C++ only)
Warn if the return type of a function has a type qualifier such as
"const". For ISO C such a type qualifier has no effect, since the
value returned by a function is not an lvalue. For C++, the
warning is only emitted for scalar types or "void". ISO C
prohibits qualified "void" return types on function definitions, so
such return types always receive a warning even without this
option.
This warning is also enabled by -Wextra.
-Wno-ignored-attributes (C and C++ only)
This option controls warnings when an attribute is ignored. This
is different from the -Wattributes option in that it warns whenever
the compiler decides to drop an attribute, not that the attribute
is either unknown, used in a wrong place, etc. This warning is
enabled by default.
-Wmain
Warn if the type of "main" is suspicious. "main" should be a
function with external linkage, returning int, taking either zero
arguments, two, or three arguments of appropriate types. This
warning is enabled by default in C++ and is enabled by either -Wall
or -Wpedantic.
-Wmisleading-indentation (C and C++ only)
Warn when the indentation of the code does not reflect the block
structure. Specifically, a warning is issued for "if", "else",
"while", and "for" clauses with a guarded statement that does not
use braces, followed by an unguarded statement with the same
indentation.
In the following example, the call to "bar" is misleadingly
indented as if it were guarded by the "if" conditional.
if (some_condition ())
foo ();
bar (); /* Gotcha: this is not guarded by the "if". */
In the case of mixed tabs and spaces, the warning uses the
-ftabstop= option to determine if the statements line up
(defaulting to 8).
The warning is not issued for code involving multiline preprocessor
logic such as the following example.
if (flagA)
foo (0);
#if SOME_CONDITION_THAT_DOES_NOT_HOLD
if (flagB)
#endif
foo (1);
The warning is not issued after a "#line" directive, since this
typically indicates autogenerated code, and no assumptions can be
made about the layout of the file that the directive references.
This warning is enabled by -Wall in C and C++.
-Wmissing-attributes
Warn when a declaration of a function is missing one or more
attributes that a related function is declared with and whose
absence may adversely affect the correctness or efficiency of
generated code. For example, the warning is issued for
declarations of aliases that use attributes to specify less
restrictive requirements than those of their targets. This
typically represents a potential optimization opportunity. By
contrast, the -Wattribute-alias=2 option controls warnings issued
when the alias is more restrictive than the target, which could
lead to incorrect code generation. Attributes considered include
"alloc_align", "alloc_size", "cold", "const", "hot", "leaf",
"malloc", "nonnull", "noreturn", "nothrow", "pure",
"returns_nonnull", and "returns_twice".
In C++, the warning is issued when an explicit specialization of a
primary template declared with attribute "alloc_align",
"alloc_size", "assume_aligned", "format", "format_arg", "malloc",
or "nonnull" is declared without it. Attributes "deprecated",
"error", and "warning" suppress the warning..
You can use the "copy" attribute to apply the same set of
attributes to a declaration as that on another declaration without
explicitly enumerating the attributes. This attribute can be
applied to declarations of functions, variables, or types.
-Wmissing-attributes is enabled by -Wall.
For example, since the declaration of the primary function template
below makes use of both attribute "malloc" and "alloc_size" the
declaration of the explicit specialization of the template is
diagnosed because it is missing one of the attributes.
template <class T>
T* __attribute__ ((malloc, alloc_size (1)))
allocate (size_t);
template <>
void* __attribute__ ((malloc)) // missing alloc_size
allocate<void> (size_t);
-Wmissing-braces
Warn if an aggregate or union initializer is not fully bracketed.
In the following example, the initializer for "a" is not fully
bracketed, but that for "b" is fully bracketed.
int a[2][2] = { 0, 1, 2, 3 };
int b[2][2] = { { 0, 1 }, { 2, 3 } };
This warning is enabled by -Wall.
-Wmissing-include-dirs (C, C++, Objective-C and Objective-C++ only)
Warn if a user-supplied include directory does not exist.
-Wno-missing-profile
This option controls warnings if feedback profiles are missing when
using the -fprofile-use option. This option diagnoses those cases
where a new function or a new file is added between compiling with
-fprofile-generate and with -fprofile-use, without regenerating the
profiles. In these cases, the profile feedback data files do not
contain any profile feedback information for the newly added
function or file respectively. Also, in the case when profile
count data (.gcda) files are removed, GCC cannot use any profile
feedback information. In all these cases, warnings are issued to
inform you that a profile generation step is due. Ignoring the
warning can result in poorly optimized code. -Wno-missing-profile
can be used to disable the warning, but this is not recommended and
should be done only when non-existent profile data is justified.
-Wmultistatement-macros
Warn about unsafe multiple statement macros that appear to be
guarded by a clause such as "if", "else", "for", "switch", or
"while", in which only the first statement is actually guarded
after the macro is expanded.
For example:
#define DOIT x++; y++
if (c)
DOIT;
will increment "y" unconditionally, not just when "c" holds. The
can usually be fixed by wrapping the macro in a do-while loop:
#define DOIT do { x++; y++; } while (0)
if (c)
DOIT;
This warning is enabled by -Wall in C and C++.
-Wparentheses
Warn if parentheses are omitted in certain contexts, such as when
there is an assignment in a context where a truth value is
expected, or when operators are nested whose precedence people
often get confused about.
Also warn if a comparison like "x<=y<=z" appears; this is
equivalent to "(x<=y ? 1 : 0) <= z", which is a different
interpretation from that of ordinary mathematical notation.
Also warn for dangerous uses of the GNU extension to "?:" with
omitted middle operand. When the condition in the "?": operator is
a boolean expression, the omitted value is always 1. Often
programmers expect it to be a value computed inside the conditional
expression instead.
For C++ this also warns for some cases of unnecessary parentheses
in declarations, which can indicate an attempt at a function call
instead of a declaration:
{
// Declares a local variable called mymutex.
std::unique_lock<std::mutex> (mymutex);
// User meant std::unique_lock<std::mutex> lock (mymutex);
}
This warning is enabled by -Wall.
-Wsequence-point
Warn about code that may have undefined semantics because of
violations of sequence point rules in the C and C++ standards.
The C and C++ standards define the order in which expressions in a
C/C++ program are evaluated in terms of sequence points, which
represent a partial ordering between the execution of parts of the
program: those executed before the sequence point, and those
executed after it. These occur after the evaluation of a full
expression (one which is not part of a larger expression), after
the evaluation of the first operand of a "&&", "||", "? :" or ","
(comma) operator, before a function is called (but after the
evaluation of its arguments and the expression denoting the called
function), and in certain other places. Other than as expressed by
the sequence point rules, the order of evaluation of subexpressions
of an expression is not specified. All these rules describe only a
partial order rather than a total order, since, for example, if two
functions are called within one expression with no sequence point
between them, the order in which the functions are called is not
specified. However, the standards committee have ruled that
function calls do not overlap.
It is not specified when between sequence points modifications to
the values of objects take effect. Programs whose behavior depends
on this have undefined behavior; the C and C++ standards specify
that "Between the previous and next sequence point an object shall
have its stored value modified at most once by the evaluation of an
expression. Furthermore, the prior value shall be read only to
determine the value to be stored.". If a program breaks these
rules, the results on any particular implementation are entirely
unpredictable.
Examples of code with undefined behavior are "a = a++;", "a[n] =
b[n++]" and "a[i++] = i;". Some more complicated cases are not
diagnosed by this option, and it may give an occasional false
positive result, but in general it has been found fairly effective
at detecting this sort of problem in programs.
The C++17 standard will define the order of evaluation of operands
in more cases: in particular it requires that the right-hand side
of an assignment be evaluated before the left-hand side, so the
above examples are no longer undefined. But this option will still
warn about them, to help people avoid writing code that is
undefined in C and earlier revisions of C++.
The standard is worded confusingly, therefore there is some debate
over the precise meaning of the sequence point rules in subtle
cases. Links to discussions of the problem, including proposed
formal definitions, may be found on the GCC readings page, at
<http://gcc.gnu.org/readings.html>.
This warning is enabled by -Wall for C and C++.
-Wno-return-local-addr
Do not warn about returning a pointer (or in C++, a reference) to a
variable that goes out of scope after the function returns.
-Wreturn-type
Warn whenever a function is defined with a return type that
defaults to "int". Also warn about any "return" statement with no
return value in a function whose return type is not "void" (falling
off the end of the function body is considered returning without a
value).
For C only, warn about a "return" statement with an expression in a
function whose return type is "void", unless the expression type is
also "void". As a GNU extension, the latter case is accepted
without a warning unless -Wpedantic is used. Attempting to use the
return value of a non-"void" function other than "main" that flows
off the end by reaching the closing curly brace that terminates the
function is undefined.
Unlike in C, in C++, flowing off the end of a non-"void" function
other than "main" results in undefined behavior even when the value
of the function is not used.
This warning is enabled by default in C++ and by -Wall otherwise.
-Wno-shift-count-negative
Controls warnings if a shift count is negative. This warning is
enabled by default.
-Wno-shift-count-overflow
Controls warnings if a shift count is greater than or equal to the
bit width of the type. This warning is enabled by default.
-Wshift-negative-value
Warn if left shifting a negative value. This warning is enabled by
-Wextra in C99 (and newer) and C++11 to C++17 modes.
-Wno-shift-overflow
-Wshift-overflow=n
These options control warnings about left shift overflows.
-Wshift-overflow=1
This is the warning level of -Wshift-overflow and is enabled by
default in C99 and C++11 modes (and newer). This warning level
does not warn about left-shifting 1 into the sign bit.
(However, in C, such an overflow is still rejected in contexts
where an integer constant expression is required.) No warning
is emitted in C++2A mode (and newer), as signed left shifts
always wrap.
-Wshift-overflow=2
This warning level also warns about left-shifting 1 into the
sign bit, unless C++14 mode (or newer) is active.
-Wswitch
Warn whenever a "switch" statement has an index of enumerated type
and lacks a "case" for one or more of the named codes of that
enumeration. (The presence of a "default" label prevents this
warning.) "case" labels outside the enumeration range also provoke
warnings when this option is used (even if there is a "default"
label). This warning is enabled by -Wall.
-Wswitch-default
Warn whenever a "switch" statement does not have a "default" case.
-Wswitch-enum
Warn whenever a "switch" statement has an index of enumerated type
and lacks a "case" for one or more of the named codes of that
enumeration. "case" labels outside the enumeration range also
provoke warnings when this option is used. The only difference
between -Wswitch and this option is that this option gives a
warning about an omitted enumeration code even if there is a
"default" label.
-Wno-switch-bool
Do not warn when a "switch" statement has an index of boolean type
and the case values are outside the range of a boolean type. It is
possible to suppress this warning by casting the controlling
expression to a type other than "bool". For example:
switch ((int) (a == 4))
{
...
}
This warning is enabled by default for C and C++ programs.
-Wno-switch-outside-range
This option controls warnings when a "switch" case has a value that
is outside of its respective type range. This warning is enabled
by default for C and C++ programs.
-Wno-switch-unreachable
Do not warn when a "switch" statement contains statements between
the controlling expression and the first case label, which will
never be executed. For example:
switch (cond)
{
i = 15;
...
case 5:
...
}
-Wswitch-unreachable does not warn if the statement between the
controlling expression and the first case label is just a
declaration:
switch (cond)
{
int i;
...
case 5:
i = 5;
...
}
This warning is enabled by default for C and C++ programs.
-Wsync-nand (C and C++ only)
Warn when "__sync_fetch_and_nand" and "__sync_nand_and_fetch"
built-in functions are used. These functions changed semantics in
GCC 4.4.
-Wunused-but-set-parameter
Warn whenever a function parameter is assigned to, but otherwise
unused (aside from its declaration).
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused together with -Wextra.
-Wunused-but-set-variable
Warn whenever a local variable is assigned to, but otherwise unused
(aside from its declaration). This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
This warning is also enabled by -Wunused, which is enabled by
-Wall.
-Wunused-function
Warn whenever a static function is declared but not defined or a
non-inline static function is unused. This warning is enabled by
-Wall.
-Wunused-label
Warn whenever a label is declared but not used. This warning is
enabled by -Wall.
To suppress this warning use the "unused" attribute.
-Wunused-local-typedefs (C, Objective-C, C++ and Objective-C++ only)
Warn when a typedef locally defined in a function is not used.
This warning is enabled by -Wall.
-Wunused-parameter
Warn whenever a function parameter is unused aside from its
declaration.
To suppress this warning use the "unused" attribute.
-Wno-unused-result
Do not warn if a caller of a function marked with attribute
"warn_unused_result" does not use its return value. The default is
-Wunused-result.
-Wunused-variable
Warn whenever a local or static variable is unused aside from its
declaration. This option implies -Wunused-const-variable=1 for C,
but not for C++. This warning is enabled by -Wall.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable
-Wunused-const-variable=n
Warn whenever a constant static variable is unused aside from its
declaration. -Wunused-const-variable=1 is enabled by
-Wunused-variable for C, but not for C++. In C this declares
variable storage, but in C++ this is not an error since const
variables take the place of "#define"s.
To suppress this warning use the "unused" attribute.
-Wunused-const-variable=1
This is the warning level that is enabled by -Wunused-variable
for C. It warns only about unused static const variables
defined in the main compilation unit, but not about static
const variables declared in any header included.
-Wunused-const-variable=2
This warning level also warns for unused constant static
variables in headers (excluding system headers). This is the
warning level of -Wunused-const-variable and must be explicitly
requested since in C++ this isn't an error and in C it might be
harder to clean up all headers included.
-Wunused-value
Warn whenever a statement computes a result that is explicitly not
used. To suppress this warning cast the unused expression to
"void". This includes an expression-statement or the left-hand side
of a comma expression that contains no side effects. For example,
an expression such as "x[i,j]" causes a warning, while
"x[(void)i,j]" does not.
This warning is enabled by -Wall.
-Wunused
All the above -Wunused options combined.
In order to get a warning about an unused function parameter, you
must either specify -Wextra -Wunused (note that -Wall implies
-Wunused), or separately specify -Wunused-parameter.
-Wuninitialized
Warn if an automatic variable is used without first being
initialized. In C++, warn if a non-static reference or non-static
"const" member appears in a class without constructors.
If you want to warn about code that uses the uninitialized value of
the variable in its own initializer, use the -Winit-self option.
These warnings occur for individual uninitialized elements of
structure, union or array variables as well as for variables that
are uninitialized as a whole. They do not occur for variables or
elements declared "volatile". Because these warnings depend on
optimization, the exact variables or elements for which there are
warnings depend on the precise optimization options and version of
GCC used.
Note that there may be no warning about a variable that is used
only to compute a value that itself is never used, because such
computations may be deleted by data flow analysis before the
warnings are printed.
-Wno-invalid-memory-model
This option controls warnings for invocations of __atomic Builtins,
__sync Builtins, and the C11 atomic generic functions with a memory
consistency argument that is either invalid for the operation or
outside the range of values of the "memory_order" enumeration. For
example, since the "__atomic_store" and "__atomic_store_n" built-
ins are only defined for the relaxed, release, and sequentially
consistent memory orders the following code is diagnosed:
void store (int *i)
{
__atomic_store_n (i, 0, memory_order_consume);
}
-Winvalid-memory-model is enabled by default.
-Wmaybe-uninitialized
For an automatic (i.e. local) variable, if there exists a path from
the function entry to a use of the variable that is initialized,
but there exist some other paths for which the variable is not
initialized, the compiler emits a warning if it cannot prove the
uninitialized paths are not executed at run time.
These warnings are only possible in optimizing compilation, because
otherwise GCC does not keep track of the state of variables.
These warnings are made optional because GCC may not be able to
determine when the code is correct in spite of appearing to have an
error. Here is one example of how this can happen:
{
int x;
switch (y)
{
case 1: x = 1;
break;
case 2: x = 4;
break;
case 3: x = 5;
}
foo (x);
}
If the value of "y" is always 1, 2 or 3, then "x" is always
initialized, but GCC doesn't know this. To suppress the warning,
you need to provide a default case with assert(0) or similar code.
This option also warns when a non-volatile automatic variable might
be changed by a call to "longjmp". The compiler sees only the
calls to "setjmp". It cannot know where "longjmp" will be called;
in fact, a signal handler could call it at any point in the code.
As a result, you may get a warning even when there is in fact no
problem because "longjmp" cannot in fact be called at the place
that would cause a problem.
Some spurious warnings can be avoided if you declare all the
functions you use that never return as "noreturn".
This warning is enabled by -Wall or -Wextra.
-Wunknown-pragmas
Warn when a "#pragma" directive is encountered that is not
understood by GCC. If this command-line option is used, warnings
are even issued for unknown pragmas in system header files. This
is not the case if the warnings are only enabled by the -Wall
command-line option.
-Wno-pragmas
Do not warn about misuses of pragmas, such as incorrect parameters,
invalid syntax, or conflicts between pragmas. See also
-Wunknown-pragmas.
-Wno-prio-ctor-dtor
Do not warn if a priority from 0 to 100 is used for constructor or
destructor. The use of constructor and destructor attributes allow
you to assign a priority to the constructor/destructor to control
its order of execution before "main" is called or after it returns.
The priority values must be greater than 100 as the compiler
reserves priority values between 0--100 for the implementation.
-Wstrict-aliasing
This option is only active when -fstrict-aliasing is active. It
warns about code that might break the strict aliasing rules that
the compiler is using for optimization. The warning does not catch
all cases, but does attempt to catch the more common pitfalls. It
is included in -Wall. It is equivalent to -Wstrict-aliasing=3
-Wstrict-aliasing=n
This option is only active when -fstrict-aliasing is active. It
warns about code that might break the strict aliasing rules that
the compiler is using for optimization. Higher levels correspond
to higher accuracy (fewer false positives). Higher levels also
correspond to more effort, similar to the way -O works.
-Wstrict-aliasing is equivalent to -Wstrict-aliasing=3.
Level 1: Most aggressive, quick, least accurate. Possibly useful
when higher levels do not warn but -fstrict-aliasing still breaks
the code, as it has very few false negatives. However, it has many
false positives. Warns for all pointer conversions between
possibly incompatible types, even if never dereferenced. Runs in
the front end only.
Level 2: Aggressive, quick, not too precise. May still have many
false positives (not as many as level 1 though), and few false
negatives (but possibly more than level 1). Unlike level 1, it
only warns when an address is taken. Warns about incomplete types.
Runs in the front end only.
Level 3 (default for -Wstrict-aliasing): Should have very few false
positives and few false negatives. Slightly slower than levels 1
or 2 when optimization is enabled. Takes care of the common
pun+dereference pattern in the front end: "*(int*)&some_float". If
optimization is enabled, it also runs in the back end, where it
deals with multiple statement cases using flow-sensitive points-to
information. Only warns when the converted pointer is
dereferenced. Does not warn about incomplete types.
-Wstrict-overflow
-Wstrict-overflow=n
This option is only active when signed overflow is undefined. It
warns about cases where the compiler optimizes based on the
assumption that signed overflow does not occur. Note that it does
not warn about all cases where the code might overflow: it only
warns about cases where the compiler implements some optimization.
Thus this warning depends on the optimization level.
An optimization that assumes that signed overflow does not occur is
perfectly safe if the values of the variables involved are such
that overflow never does, in fact, occur. Therefore this warning
can easily give a false positive: a warning about code that is not
actually a problem. To help focus on important issues, several
warning levels are defined. No warnings are issued for the use of
undefined signed overflow when estimating how many iterations a
loop requires, in particular when determining whether a loop will
be executed at all.
-Wstrict-overflow=1
Warn about cases that are both questionable and easy to avoid.
For example the compiler simplifies "x + 1 > x" to 1. This
level of -Wstrict-overflow is enabled by -Wall; higher levels
are not, and must be explicitly requested.
-Wstrict-overflow=2
Also warn about other cases where a comparison is simplified to
a constant. For example: "abs (x) >= 0". This can only be
simplified when signed integer overflow is undefined, because
"abs (INT_MIN)" overflows to "INT_MIN", which is less than
zero. -Wstrict-overflow (with no level) is the same as
-Wstrict-overflow=2.
-Wstrict-overflow=3
Also warn about other cases where a comparison is simplified.
For example: "x + 1 > 1" is simplified to "x > 0".
-Wstrict-overflow=4
Also warn about other simplifications not covered by the above
cases. For example: "(x * 10) / 5" is simplified to "x * 2".
-Wstrict-overflow=5
Also warn about cases where the compiler reduces the magnitude
of a constant involved in a comparison. For example: "x + 2 >
y" is simplified to "x + 1 >= y". This is reported only at the
highest warning level because this simplification applies to
many comparisons, so this warning level gives a very large
number of false positives.
-Wstring-compare
Warn for calls to "strcmp" and "strncmp" whose result is determined
to be either zero or non-zero in tests for such equality owing to
the length of one argument being greater than the size of the array
the other argument is stored in (or the bound in the case of
"strncmp"). Such calls could be mistakes. For example, the call
to "strcmp" below is diagnosed because its result is necessarily
non-zero irrespective of the contents of the array "a".
extern char a[4];
void f (char *d)
{
strcpy (d, "string");
...
if (0 == strcmp (a, d)) // cannot be true
puts ("a and d are the same");
}
-Wstring-compare is enabled by -Wextra.
-Wstringop-overflow
-Wstringop-overflow=type
Warn for calls to string manipulation functions such as "memcpy"
and "strcpy" that are determined to overflow the destination
buffer. The optional argument is one greater than the type of
Object Size Checking to perform to determine the size of the
destination. The argument is meaningful only for functions that
operate on character arrays but not for raw memory functions like
"memcpy" which always make use of Object Size type-0. The option
also warns for calls that specify a size in excess of the largest
possible object or at most "SIZE_MAX / 2" bytes. The option
produces the best results with optimization enabled but can detect
a small subset of simple buffer overflows even without optimization
in calls to the GCC built-in functions like "__builtin_memcpy" that
correspond to the standard functions. In any case, the option
warns about just a subset of buffer overflows detected by the
corresponding overflow checking built-ins. For example, the option
issues a warning for the "strcpy" call below because it copies at
least 5 characters (the string "blue" including the terminating
NUL) into the buffer of size 4.
enum Color { blue, purple, yellow };
const char* f (enum Color clr)
{
static char buf [4];
const char *str;
switch (clr)
{
case blue: str = "blue"; break;
case purple: str = "purple"; break;
case yellow: str = "yellow"; break;
}
return strcpy (buf, str); // warning here
}
Option -Wstringop-overflow=2 is enabled by default.
-Wstringop-overflow
-Wstringop-overflow=1
The -Wstringop-overflow=1 option uses type-zero Object Size
Checking to determine the sizes of destination objects. This
is the default setting of the option. At this setting the
option does not warn for writes past the end of subobjects of
larger objects accessed by pointers unless the size of the
largest surrounding object is known. When the destination may
be one of several objects it is assumed to be the largest one
of them. On Linux systems, when optimization is enabled at
this setting the option warns for the same code as when the
"_FORTIFY_SOURCE" macro is defined to a non-zero value.
-Wstringop-overflow=2
The -Wstringop-overflow=2 option uses type-one Object Size
Checking to determine the sizes of destination objects. At
this setting the option warna about overflows when writing to
members of the largest complete objects whose exact size is
known. However, it does not warn for excessive writes to the
same members of unknown objects referenced by pointers since
they may point to arrays containing unknown numbers of
elements.
-Wstringop-overflow=3
The -Wstringop-overflow=3 option uses type-two Object Size
Checking to determine the sizes of destination objects. At
this setting the option warns about overflowing the smallest
object or data member. This is the most restrictive setting of
the option that may result in warnings for safe code.
-Wstringop-overflow=4
The -Wstringop-overflow=4 option uses type-three Object Size
Checking to determine the sizes of destination objects. At
this setting the option warns about overflowing any data
members, and when the destination is one of several objects it
uses the size of the largest of them to decide whether to issue
a warning. Similarly to -Wstringop-overflow=3 this setting of
the option may result in warnings for benign code.
-Wno-stringop-truncation
Do not warn for calls to bounded string manipulation functions such
as "strncat", "strncpy", and "stpncpy" that may either truncate the
copied string or leave the destination unchanged.
In the following example, the call to "strncat" specifies a bound
that is less than the length of the source string. As a result,
the copy of the source will be truncated and so the call is
diagnosed. To avoid the warning use "bufsize - strlen (buf) - 1)"
as the bound.
void append (char *buf, size_t bufsize)
{
strncat (buf, ".txt", 3);
}
As another example, the following call to "strncpy" results in
copying to "d" just the characters preceding the terminating NUL,
without appending the NUL to the end. Assuming the result of
"strncpy" is necessarily a NUL-terminated string is a common
mistake, and so the call is diagnosed. To avoid the warning when
the result is not expected to be NUL-terminated, call "memcpy"
instead.
void copy (char *d, const char *s)
{
strncpy (d, s, strlen (s));
}
In the following example, the call to "strncpy" specifies the size
of the destination buffer as the bound. If the length of the
source string is equal to or greater than this size the result of
the copy will not be NUL-terminated. Therefore, the call is also
diagnosed. To avoid the warning, specify "sizeof buf - 1" as the
bound and set the last element of the buffer to "NUL".
void copy (const char *s)
{
char buf[80];
strncpy (buf, s, sizeof buf);
...
}
In situations where a character array is intended to store a
sequence of bytes with no terminating "NUL" such an array may be
annotated with attribute "nonstring" to avoid this warning. Such
arrays, however, are not suitable arguments to functions that
expect "NUL"-terminated strings. To help detect accidental misuses
of such arrays GCC issues warnings unless it can prove that the use
is safe.
-Wsuggest-attribute=[pure|const|noreturn|format|cold|malloc]
Warn for cases where adding an attribute may be beneficial. The
attributes currently supported are listed below.
-Wsuggest-attribute=pure
-Wsuggest-attribute=const
-Wsuggest-attribute=noreturn
-Wmissing-noreturn
-Wsuggest-attribute=malloc
Warn about functions that might be candidates for attributes
"pure", "const" or "noreturn" or "malloc". The compiler only
warns for functions visible in other compilation units or (in
the case of "pure" and "const") if it cannot prove that the
function returns normally. A function returns normally if it
doesn't contain an infinite loop or return abnormally by
throwing, calling "abort" or trapping. This analysis requires
option -fipa-pure-const, which is enabled by default at -O and
higher. Higher optimization levels improve the accuracy of the
analysis.
-Wsuggest-attribute=format
-Wmissing-format-attribute
Warn about function pointers that might be candidates for
"format" attributes. Note these are only possible candidates,
not absolute ones. GCC guesses that function pointers with
"format" attributes that are used in assignment,
initialization, parameter passing or return statements should
have a corresponding "format" attribute in the resulting type.
I.e. the left-hand side of the assignment or initialization,
the type of the parameter variable, or the return type of the
containing function respectively should also have a "format"
attribute to avoid the warning.
GCC also warns about function definitions that might be
candidates for "format" attributes. Again, these are only
possible candidates. GCC guesses that "format" attributes
might be appropriate for any function that calls a function
like "vprintf" or "vscanf", but this might not always be the
case, and some functions for which "format" attributes are
appropriate may not be detected.
-Wsuggest-attribute=cold
Warn about functions that might be candidates for "cold"
attribute. This is based on static detection and generally
only warns about functions which always leads to a call to
another "cold" function such as wrappers of C++ "throw" or
fatal error reporting functions leading to "abort".
-Walloc-zero
Warn about calls to allocation functions decorated with attribute
"alloc_size" that specify zero bytes, including those to the built-
in forms of the functions "aligned_alloc", "alloca", "calloc",
"malloc", and "realloc". Because the behavior of these functions
when called with a zero size differs among implementations (and in
the case of "realloc" has been deprecated) relying on it may result
in subtle portability bugs and should be avoided.
-Walloc-size-larger-than=byte-size
Warn about calls to functions decorated with attribute "alloc_size"
that attempt to allocate objects larger than the specified number
of bytes, or where the result of the size computation in an integer
type with infinite precision would exceed the value of PTRDIFF_MAX
on the target. -Walloc-size-larger-than=PTRDIFF_MAX is enabled by
default. Warnings controlled by the option can be disabled either
by specifying byte-size of SIZE_MAX or more or by
-Wno-alloc-size-larger-than.
-Wno-alloc-size-larger-than
Disable -Walloc-size-larger-than= warnings. The option is
equivalent to -Walloc-size-larger-than=SIZE_MAX or larger.
-Walloca
This option warns on all uses of "alloca" in the source.
-Walloca-larger-than=byte-size
This option warns on calls to "alloca" with an integer argument
whose value is either zero, or that is not bounded by a controlling
predicate that limits its value to at most byte-size. It also
warns for calls to "alloca" where the bound value is unknown.
Arguments of non-integer types are considered unbounded even if
they appear to be constrained to the expected range.
For example, a bounded case of "alloca" could be:
void func (size_t n)
{
void *p;
if (n <= 1000)
p = alloca (n);
else
p = malloc (n);
f (p);
}
In the above example, passing "-Walloca-larger-than=1000" would not
issue a warning because the call to "alloca" is known to be at most
1000 bytes. However, if "-Walloca-larger-than=500" were passed,
the compiler would emit a warning.
Unbounded uses, on the other hand, are uses of "alloca" with no
controlling predicate constraining its integer argument. For
example:
void func ()
{
void *p = alloca (n);
f (p);
}
If "-Walloca-larger-than=500" were passed, the above would trigger
a warning, but this time because of the lack of bounds checking.
Note, that even seemingly correct code involving signed integers
could cause a warning:
void func (signed int n)
{
if (n < 500)
{
p = alloca (n);
f (p);
}
}
In the above example, n could be negative, causing a larger than
expected argument to be implicitly cast into the "alloca" call.
This option also warns when "alloca" is used in a loop.
-Walloca-larger-than=PTRDIFF_MAX is enabled by default but is
usually only effective when -ftree-vrp is active (default for -O2
and above).
See also -Wvla-larger-than=byte-size.
-Wno-alloca-larger-than
Disable -Walloca-larger-than= warnings. The option is equivalent
to -Walloca-larger-than=SIZE_MAX or larger.
-Warith-conversion
Do warn about implicit conversions from arithmetic operations even
when conversion of the operands to the same type cannot change
their values. This affects warnings from -Wconversion,
-Wfloat-conversion, and -Wsign-conversion.
void f (char c, int i)
{
c = c + i; // warns with B<-Wconversion>
c = c + 1; // only warns with B<-Warith-conversion>
}
-Warray-bounds
-Warray-bounds=n
This option is only active when -ftree-vrp is active (default for
-O2 and above). It warns about subscripts to arrays that are always
out of bounds. This warning is enabled by -Wall.
-Warray-bounds=1
This is the warning level of -Warray-bounds and is enabled by
-Wall; higher levels are not, and must be explicitly requested.
-Warray-bounds=2
This warning level also warns about out of bounds access for
arrays at the end of a struct and for arrays accessed through
pointers. This warning level may give a larger number of false
positives and is deactivated by default.
-Wattribute-alias=n
-Wno-attribute-alias
Warn about declarations using the "alias" and similar attributes
whose target is incompatible with the type of the alias.
-Wattribute-alias=1
The default warning level of the -Wattribute-alias option
diagnoses incompatibilities between the type of the alias
declaration and that of its target. Such incompatibilities are
typically indicative of bugs.
-Wattribute-alias=2
At this level -Wattribute-alias also diagnoses cases where the
attributes of the alias declaration are more restrictive than
the attributes applied to its target. These mismatches can
potentially result in incorrect code generation. In other
cases they may be benign and could be resolved simply by adding
the missing attribute to the target. For comparison, see the
-Wmissing-attributes option, which controls diagnostics when
the alias declaration is less restrictive than the target,
rather than more restrictive.
Attributes considered include "alloc_align", "alloc_size",
"cold", "const", "hot", "leaf", "malloc", "nonnull",
"noreturn", "nothrow", "pure", "returns_nonnull", and
"returns_twice".
-Wattribute-alias is equivalent to -Wattribute-alias=1. This is
the default. You can disable these warnings with either
-Wno-attribute-alias or -Wattribute-alias=0.
-Wbool-compare
Warn about boolean expression compared with an integer value
different from "true"/"false". For instance, the following
comparison is always false:
int n = 5;
...
if ((n > 1) == 2) { ... }
This warning is enabled by -Wall.
-Wbool-operation
Warn about suspicious operations on expressions of a boolean type.
For instance, bitwise negation of a boolean is very likely a bug in
the program. For C, this warning also warns about incrementing or
decrementing a boolean, which rarely makes sense. (In C++,
decrementing a boolean is always invalid. Incrementing a boolean
is invalid in C++17, and deprecated otherwise.)
This warning is enabled by -Wall.
-Wduplicated-branches
Warn when an if-else has identical branches. This warning detects
cases like
if (p != NULL)
return 0;
else
return 0;
It doesn't warn when both branches contain just a null statement.
This warning also warn for conditional operators:
int i = x ? *p : *p;
-Wduplicated-cond
Warn about duplicated conditions in an if-else-if chain. For
instance, warn for the following code:
if (p->q != NULL) { ... }
else if (p->q != NULL) { ... }
-Wframe-address
Warn when the __builtin_frame_address or __builtin_return_address
is called with an argument greater than 0. Such calls may return
indeterminate values or crash the program. The warning is included
in -Wall.
-Wno-discarded-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on pointers are being discarded.
Typically, the compiler warns if a "const char *" variable is
passed to a function that takes a "char *" parameter. This option
can be used to suppress such a warning.
-Wno-discarded-array-qualifiers (C and Objective-C only)
Do not warn if type qualifiers on arrays which are pointer targets
are being discarded. Typically, the compiler warns if a "const int
(*)[]" variable is passed to a function that takes a "int (*)[]"
parameter. This option can be used to suppress such a warning.
-Wno-incompatible-pointer-types (C and Objective-C only)
Do not warn when there is a conversion between pointers that have
incompatible types. This warning is for cases not covered by
-Wno-pointer-sign, which warns for pointer argument passing or
assignment with different signedness.
-Wno-int-conversion (C and Objective-C only)
Do not warn about incompatible integer to pointer and pointer to
integer conversions. This warning is about implicit conversions;
for explicit conversions the warnings -Wno-int-to-pointer-cast and
-Wno-pointer-to-int-cast may be used.
-Wzero-length-bounds
Warn about accesses to elements of zero-length array members that
might overlap other members of the same object. Declaring interior
zero-length arrays is discouraged because accesses to them are
undefined. See
For example, the first two stores in function "bad" are diagnosed
because the array elements overlap the subsequent members "b" and
"c". The third store is diagnosed by -Warray-bounds because it is
beyond the bounds of the enclosing object.
struct X { int a[0]; int b, c; };
struct X x;
void bad (void)
{
x.a[0] = 0; // -Wzero-length-bounds
x.a[1] = 1; // -Wzero-length-bounds
x.a[2] = 2; // -Warray-bounds
}
Option -Wzero-length-bounds is enabled by -Warray-bounds.
-Wno-div-by-zero
Do not warn about compile-time integer division by zero. Floating-
point division by zero is not warned about, as it can be a
legitimate way of obtaining infinities and NaNs.
-Wsystem-headers
Print warning messages for constructs found in system header files.
Warnings from system headers are normally suppressed, on the
assumption that they usually do not indicate real problems and
would only make the compiler output harder to read. Using this
command-line option tells GCC to emit warnings from system headers
as if they occurred in user code. However, note that using -Wall
in conjunction with this option does not warn about unknown pragmas
in system headers---for that, -Wunknown-pragmas must also be used.
-Wtautological-compare
Warn if a self-comparison always evaluates to true or false. This
warning detects various mistakes such as:
int i = 1;
...
if (i > i) { ... }
This warning also warns about bitwise comparisons that always
evaluate to true or false, for instance:
if ((a & 16) == 10) { ... }
will always be false.
This warning is enabled by -Wall.
-Wtrampolines
Warn about trampolines generated for pointers to nested functions.
A trampoline is a small piece of data or code that is created at
run time on the stack when the address of a nested function is
taken, and is used to call the nested function indirectly. For
some targets, it is made up of data only and thus requires no
special treatment. But, for most targets, it is made up of code
and thus requires the stack to be made executable in order for the
program to work properly.
-Wfloat-equal
Warn if floating-point values are used in equality comparisons.
The idea behind this is that sometimes it is convenient (for the
programmer) to consider floating-point values as approximations to
infinitely precise real numbers. If you are doing this, then you
need to compute (by analyzing the code, or in some other way) the
maximum or likely maximum error that the computation introduces,
and allow for it when performing comparisons (and when producing
output, but that's a different problem). In particular, instead of
testing for equality, you should check to see whether the two
values have ranges that overlap; and this is done with the
relational operators, so equality comparisons are probably
mistaken.
-Wtraditional (C and Objective-C only)
Warn about certain constructs that behave differently in
traditional and ISO C. Also warn about ISO C constructs that have
no traditional C equivalent, and/or problematic constructs that
should be avoided.
* Macro parameters that appear within string literals in the
macro body. In traditional C macro replacement takes place
within string literals, but in ISO C it does not.
* In traditional C, some preprocessor directives did not exist.
Traditional preprocessors only considered a line to be a
directive if the # appeared in column 1 on the line. Therefore
-Wtraditional warns about directives that traditional C
understands but ignores because the # does not appear as the
first character on the line. It also suggests you hide
directives like "#pragma" not understood by traditional C by
indenting them. Some traditional implementations do not
recognize "#elif", so this option suggests avoiding it
altogether.
* A function-like macro that appears without arguments.
* The unary plus operator.
* The U integer constant suffix, or the F or L floating-point
constant suffixes. (Traditional C does support the L suffix on
integer constants.) Note, these suffixes appear in macros
defined in the system headers of most modern systems, e.g. the
_MIN/_MAX macros in "<limits.h>". Use of these macros in user
code might normally lead to spurious warnings, however GCC's
integrated preprocessor has enough context to avoid warning in
these cases.
* A function declared external in one block and then used after
the end of the block.
* A "switch" statement has an operand of type "long".
* A non-"static" function declaration follows a "static" one.
This construct is not accepted by some traditional C compilers.
* The ISO type of an integer constant has a different width or
signedness from its traditional type. This warning is only
issued if the base of the constant is ten. I.e. hexadecimal or
octal values, which typically represent bit patterns, are not
warned about.
* Usage of ISO string concatenation is detected.
* Initialization of automatic aggregates.
* Identifier conflicts with labels. Traditional C lacks a
separate namespace for labels.
* Initialization of unions. If the initializer is zero, the
warning is omitted. This is done under the assumption that the
zero initializer in user code appears conditioned on e.g.
"__STDC__" to avoid missing initializer warnings and relies on
default initialization to zero in the traditional C case.
* Conversions by prototypes between fixed/floating-point values
and vice versa. The absence of these prototypes when compiling
with traditional C causes serious problems. This is a subset
of the possible conversion warnings; for the full set use
-Wtraditional-conversion.
* Use of ISO C style function definitions. This warning
intentionally is not issued for prototype declarations or
variadic functions because these ISO C features appear in your
code when using libiberty's traditional C compatibility macros,
"PARAMS" and "VPARAMS". This warning is also bypassed for
nested functions because that feature is already a GCC
extension and thus not relevant to traditional C compatibility.
-Wtraditional-conversion (C and Objective-C only)
Warn if a prototype causes a type conversion that is different from
what would happen to the same argument in the absence of a
prototype. This includes conversions of fixed point to floating
and vice versa, and conversions changing the width or signedness of
a fixed-point argument except when the same as the default
promotion.
-Wdeclaration-after-statement (C and Objective-C only)
Warn when a declaration is found after a statement in a block.
This construct, known from C++, was introduced with ISO C99 and is
by default allowed in GCC. It is not supported by ISO C90.
-Wshadow
Warn whenever a local variable or type declaration shadows another
variable, parameter, type, class member (in C++), or instance
variable (in Objective-C) or whenever a built-in function is
shadowed. Note that in C++, the compiler warns if a local variable
shadows an explicit typedef, but not if it shadows a
struct/class/enum. If this warning is enabled, it includes also
all instances of local shadowing. This means that
-Wno-shadow=local and -Wno-shadow=compatible-local are ignored when
-Wshadow is used. Same as -Wshadow=global.
-Wno-shadow-ivar (Objective-C only)
Do not warn whenever a local variable shadows an instance variable
in an Objective-C method.
-Wshadow=global
Warn for any shadowing. Same as -Wshadow.
-Wshadow=local
Warn when a local variable shadows another local variable or
parameter.
-Wshadow=compatible-local
Warn when a local variable shadows another local variable or
parameter whose type is compatible with that of the shadowing
variable. In C++, type compatibility here means the type of the
shadowing variable can be converted to that of the shadowed
variable. The creation of this flag (in addition to
-Wshadow=local) is based on the idea that when a local variable
shadows another one of incompatible type, it is most likely
intentional, not a bug or typo, as shown in the following example:
for (SomeIterator i = SomeObj.begin(); i != SomeObj.end(); ++i)
{
for (int i = 0; i < N; ++i)
{
...
}
...
}
Since the two variable "i" in the example above have incompatible
types, enabling only -Wshadow=compatible-local does not emit a
warning. Because their types are incompatible, if a programmer
accidentally uses one in place of the other, type checking is
expected to catch that and emit an error or warning. Use of this
flag instead of -Wshadow=local can possibly reduce the number of
warnings triggered by intentional shadowing. Note that this also
means that shadowing "const char *i" by "char *i" does not emit a
warning.
This warning is also enabled by -Wshadow=local.
-Wlarger-than=byte-size
Warn whenever an object is defined whose size exceeds byte-size.
-Wlarger-than=PTRDIFF_MAX is enabled by default. Warnings
controlled by the option can be disabled either by specifying byte-
size of SIZE_MAX or more or by -Wno-larger-than.
-Wno-larger-than
Disable -Wlarger-than= warnings. The option is equivalent to
-Wlarger-than=SIZE_MAX or larger.
-Wframe-larger-than=byte-size
Warn if the size of a function frame exceeds byte-size. The
computation done to determine the stack frame size is approximate
and not conservative. The actual requirements may be somewhat
greater than byte-size even if you do not get a warning. In
addition, any space allocated via "alloca", variable-length arrays,
or related constructs is not included by the compiler when
determining whether or not to issue a warning.
-Wframe-larger-than=PTRDIFF_MAX is enabled by default. Warnings
controlled by the option can be disabled either by specifying byte-
size of SIZE_MAX or more or by -Wno-frame-larger-than.
-Wno-frame-larger-than
Disable -Wframe-larger-than= warnings. The option is equivalent to
-Wframe-larger-than=SIZE_MAX or larger.
-Wno-free-nonheap-object
Do not warn when attempting to free an object that was not
allocated on the heap.
-Wstack-usage=byte-size
Warn if the stack usage of a function might exceed byte-size. The
computation done to determine the stack usage is conservative. Any
space allocated via "alloca", variable-length arrays, or related
constructs is included by the compiler when determining whether or
not to issue a warning.
The message is in keeping with the output of -fstack-usage.
* If the stack usage is fully static but exceeds the specified
amount, it's:
warning: stack usage is 1120 bytes
* If the stack usage is (partly) dynamic but bounded, it's:
warning: stack usage might be 1648 bytes
* If the stack usage is (partly) dynamic and not bounded, it's:
warning: stack usage might be unbounded
-Wstack-usage=PTRDIFF_MAX is enabled by default. Warnings
controlled by the option can be disabled either by specifying byte-
size of SIZE_MAX or more or by -Wno-stack-usage.
-Wno-stack-usage
Disable -Wstack-usage= warnings. The option is equivalent to
-Wstack-usage=SIZE_MAX or larger.
-Wunsafe-loop-optimizations
Warn if the loop cannot be optimized because the compiler cannot
assume anything on the bounds of the loop indices. With
-funsafe-loop-optimizations warn if the compiler makes such
assumptions.
-Wno-pedantic-ms-format (MinGW targets only)
When used in combination with -Wformat and -pedantic without GNU
extensions, this option disables the warnings about non-ISO
"printf" / "scanf" format width specifiers "I32", "I64", and "I"
used on Windows targets, which depend on the MS runtime.
-Wpointer-arith
Warn about anything that depends on the "size of" a function type
or of "void". GNU C assigns these types a size of 1, for
convenience in calculations with "void *" pointers and pointers to
functions. In C++, warn also when an arithmetic operation involves
"NULL". This warning is also enabled by -Wpedantic.
-Wno-pointer-compare
Do not warn if a pointer is compared with a zero character
constant. This usually means that the pointer was meant to be
dereferenced. For example:
const char *p = foo ();
if (p == '\0')
return 42;
Note that the code above is invalid in C++11.
This warning is enabled by default.
-Wtype-limits
Warn if a comparison is always true or always false due to the
limited range of the data type, but do not warn for constant
expressions. For example, warn if an unsigned variable is compared
against zero with "<" or ">=". This warning is also enabled by
-Wextra.
-Wabsolute-value (C and Objective-C only)
Warn for calls to standard functions that compute the absolute
value of an argument when a more appropriate standard function is
available. For example, calling "abs(3.14)" triggers the warning
because the appropriate function to call to compute the absolute
value of a double argument is "fabs". The option also triggers
warnings when the argument in a call to such a function has an
unsigned type. This warning can be suppressed with an explicit
type cast and it is also enabled by -Wextra.
-Wcomment
-Wcomments
Warn whenever a comment-start sequence /* appears in a /* comment,
or whenever a backslash-newline appears in a // comment. This
warning is enabled by -Wall.
-Wtrigraphs
Warn if any trigraphs are encountered that might change the meaning
of the program. Trigraphs within comments are not warned about,
except those that would form escaped newlines.
This option is implied by -Wall. If -Wall is not given, this
option is still enabled unless trigraphs are enabled. To get
trigraph conversion without warnings, but get the other -Wall
warnings, use -trigraphs -Wall -Wno-trigraphs.
-Wundef
Warn if an undefined identifier is evaluated in an "#if" directive.
Such identifiers are replaced with zero.
-Wexpansion-to-defined
Warn whenever defined is encountered in the expansion of a macro
(including the case where the macro is expanded by an #if
directive). Such usage is not portable. This warning is also
enabled by -Wpedantic and -Wextra.
-Wunused-macros
Warn about macros defined in the main file that are unused. A
macro is used if it is expanded or tested for existence at least
once. The preprocessor also warns if the macro has not been used
at the time it is redefined or undefined.
Built-in macros, macros defined on the command line, and macros
defined in include files are not warned about.
Note: If a macro is actually used, but only used in skipped
conditional blocks, then the preprocessor reports it as unused. To
avoid the warning in such a case, you might improve the scope of
the macro's definition by, for example, moving it into the first
skipped block. Alternatively, you could provide a dummy use with
something like:
#if defined the_macro_causing_the_warning
#endif
-Wno-endif-labels
Do not warn whenever an "#else" or an "#endif" are followed by
text. This sometimes happens in older programs with code of the
form
#if FOO
...
#else FOO
...
#endif FOO
The second and third "FOO" should be in comments. This warning is
on by default.
-Wbad-function-cast (C and Objective-C only)
Warn when a function call is cast to a non-matching type. For
example, warn if a call to a function returning an integer type is
cast to a pointer type.
-Wc90-c99-compat (C and Objective-C only)
Warn about features not present in ISO C90, but present in ISO C99.
For instance, warn about use of variable length arrays, "long long"
type, "bool" type, compound literals, designated initializers, and
so on. This option is independent of the standards mode. Warnings
are disabled in the expression that follows "__extension__".
-Wc99-c11-compat (C and Objective-C only)
Warn about features not present in ISO C99, but present in ISO C11.
For instance, warn about use of anonymous structures and unions,
"_Atomic" type qualifier, "_Thread_local" storage-class specifier,
"_Alignas" specifier, "Alignof" operator, "_Generic" keyword, and
so on. This option is independent of the standards mode. Warnings
are disabled in the expression that follows "__extension__".
-Wc11-c2x-compat (C and Objective-C only)
Warn about features not present in ISO C11, but present in ISO C2X.
For instance, warn about omitting the string in "_Static_assert",
use of [[]] syntax for attributes, use of decimal floating-point
types, and so on. This option is independent of the standards
mode. Warnings are disabled in the expression that follows
"__extension__".
-Wc++-compat (C and Objective-C only)
Warn about ISO C constructs that are outside of the common subset
of ISO C and ISO C++, e.g. request for implicit conversion from
"void *" to a pointer to non-"void" type.
-Wc++11-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++
1998 and ISO C++ 2011, e.g., identifiers in ISO C++ 1998 that are
keywords in ISO C++ 2011. This warning turns on -Wnarrowing and is
enabled by -Wall.
-Wc++14-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++
2011 and ISO C++ 2014. This warning is enabled by -Wall.
-Wc++17-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++
2014 and ISO C++ 2017. This warning is enabled by -Wall.
-Wc++20-compat (C++ and Objective-C++ only)
Warn about C++ constructs whose meaning differs between ISO C++
2017 and ISO C++ 2020. This warning is enabled by -Wall.
-Wcast-qual
Warn whenever a pointer is cast so as to remove a type qualifier
from the target type. For example, warn if a "const char *" is
cast to an ordinary "char *".
Also warn when making a cast that introduces a type qualifier in an
unsafe way. For example, casting "char **" to "const char **" is
unsafe, as in this example:
/* p is char ** value. */
const char **q = (const char **) p;
/* Assignment of readonly string to const char * is OK. */
*q = "string";
/* Now char** pointer points to read-only memory. */
**p = 'b';
-Wcast-align
Warn whenever a pointer is cast such that the required alignment of
the target is increased. For example, warn if a "char *" is cast
to an "int *" on machines where integers can only be accessed at
two- or four-byte boundaries.
-Wcast-align=strict
Warn whenever a pointer is cast such that the required alignment of
the target is increased. For example, warn if a "char *" is cast
to an "int *" regardless of the target machine.
-Wcast-function-type
Warn when a function pointer is cast to an incompatible function
pointer. In a cast involving function types with a variable
argument list only the types of initial arguments that are provided
are considered. Any parameter of pointer-type matches any other
pointer-type. Any benign differences in integral types are
ignored, like "int" vs. "long" on ILP32 targets. Likewise type
qualifiers are ignored. The function type "void (*) (void)" is
special and matches everything, which can be used to suppress this
warning. In a cast involving pointer to member types this warning
warns whenever the type cast is changing the pointer to member
type. This warning is enabled by -Wextra.
-Wwrite-strings
When compiling C, give string constants the type "const
char[length]" so that copying the address of one into a non-"const"
"char *" pointer produces a warning. These warnings help you find
at compile time code that can try to write into a string constant,
but only if you have been very careful about using "const" in
declarations and prototypes. Otherwise, it is just a nuisance.
This is why we did not make -Wall request these warnings.
When compiling C++, warn about the deprecated conversion from
string literals to "char *". This warning is enabled by default
for C++ programs.
-Wclobbered
Warn for variables that might be changed by "longjmp" or "vfork".
This warning is also enabled by -Wextra.
-Wconversion
Warn for implicit conversions that may alter a value. This includes
conversions between real and integer, like "abs (x)" when "x" is
"double"; conversions between signed and unsigned, like "unsigned
ui = -1"; and conversions to smaller types, like "sqrtf (M_PI)". Do
not warn for explicit casts like "abs ((int) x)" and "ui =
(unsigned) -1", or if the value is not changed by the conversion
like in "abs (2.0)". Warnings about conversions between signed and
unsigned integers can be disabled by using -Wno-sign-conversion.
For C++, also warn for confusing overload resolution for user-
defined conversions; and conversions that never use a type
conversion operator: conversions to "void", the same type, a base
class or a reference to them. Warnings about conversions between
signed and unsigned integers are disabled by default in C++ unless
-Wsign-conversion is explicitly enabled.
Warnings about conversion from arithmetic on a small type back to
that type are only given with -Warith-conversion.
-Wdangling-else
Warn about constructions where there may be confusion to which "if"
statement an "else" branch belongs. Here is an example of such a
case:
{
if (a)
if (b)
foo ();
else
bar ();
}
In C/C++, every "else" branch belongs to the innermost possible
"if" statement, which in this example is "if (b)". This is often
not what the programmer expected, as illustrated in the above
example by indentation the programmer chose. When there is the
potential for this confusion, GCC issues a warning when this flag
is specified. To eliminate the warning, add explicit braces around
the innermost "if" statement so there is no way the "else" can
belong to the enclosing "if". The resulting code looks like this:
{
if (a)
{
if (b)
foo ();
else
bar ();
}
}
This warning is enabled by -Wparentheses.
-Wdate-time
Warn when macros "__TIME__", "__DATE__" or "__TIMESTAMP__" are
encountered as they might prevent bit-wise-identical reproducible
compilations.
-Wempty-body
Warn if an empty body occurs in an "if", "else" or "do while"
statement. This warning is also enabled by -Wextra.
-Wno-endif-labels
Do not warn about stray tokens after "#else" and "#endif".
-Wenum-compare
Warn about a comparison between values of different enumerated
types. In C++ enumerated type mismatches in conditional
expressions are also diagnosed and the warning is enabled by
default. In C this warning is enabled by -Wall.
-Wenum-conversion (C, Objective-C only)
Warn when a value of enumerated type is implicitly converted to a
different enumerated type. This warning is enabled by -Wextra.
-Wjump-misses-init (C, Objective-C only)
Warn if a "goto" statement or a "switch" statement jumps forward
across the initialization of a variable, or jumps backward to a
label after the variable has been initialized. This only warns
about variables that are initialized when they are declared. This
warning is only supported for C and Objective-C; in C++ this sort
of branch is an error in any case.
-Wjump-misses-init is included in -Wc++-compat. It can be disabled
with the -Wno-jump-misses-init option.
-Wsign-compare
Warn when a comparison between signed and unsigned values could
produce an incorrect result when the signed value is converted to
unsigned. In C++, this warning is also enabled by -Wall. In C, it
is also enabled by -Wextra.
-Wsign-conversion
Warn for implicit conversions that may change the sign of an
integer value, like assigning a signed integer expression to an
unsigned integer variable. An explicit cast silences the warning.
In C, this option is enabled also by -Wconversion.
-Wfloat-conversion
Warn for implicit conversions that reduce the precision of a real
value. This includes conversions from real to integer, and from
higher precision real to lower precision real values. This option
is also enabled by -Wconversion.
-Wno-scalar-storage-order
Do not warn on suspicious constructs involving reverse scalar
storage order.
-Wsizeof-pointer-div
Warn for suspicious divisions of two sizeof expressions that divide
the pointer size by the element size, which is the usual way to
compute the array size but won't work out correctly with pointers.
This warning warns e.g. about "sizeof (ptr) / sizeof (ptr[0])" if
"ptr" is not an array, but a pointer. This warning is enabled by
-Wall.
-Wsizeof-pointer-memaccess
Warn for suspicious length parameters to certain string and memory
built-in functions if the argument uses "sizeof". This warning
triggers for example for "memset (ptr, 0, sizeof (ptr));" if "ptr"
is not an array, but a pointer, and suggests a possible fix, or
about "memcpy (&foo, ptr, sizeof (&foo));".
-Wsizeof-pointer-memaccess also warns about calls to bounded string
copy functions like "strncat" or "strncpy" that specify as the
bound a "sizeof" expression of the source array. For example, in
the following function the call to "strncat" specifies the size of
the source string as the bound. That is almost certainly a mistake
and so the call is diagnosed.
void make_file (const char *name)
{
char path[PATH_MAX];
strncpy (path, name, sizeof path - 1);
strncat (path, ".text", sizeof ".text");
...
}
The -Wsizeof-pointer-memaccess option is enabled by -Wall.
-Wno-sizeof-array-argument
Do not warn when the "sizeof" operator is applied to a parameter
that is declared as an array in a function definition. This
warning is enabled by default for C and C++ programs.
-Wmemset-elt-size
Warn for suspicious calls to the "memset" built-in function, if the
first argument references an array, and the third argument is a
number equal to the number of elements, but not equal to the size
of the array in memory. This indicates that the user has omitted a
multiplication by the element size. This warning is enabled by
-Wall.
-Wmemset-transposed-args
Warn for suspicious calls to the "memset" built-in function where
the second argument is not zero and the third argument is zero.
For example, the call "memset (buf, sizeof buf, 0)" is diagnosed
because "memset (buf, 0, sizeof buf)" was meant instead. The
diagnostic is only emitted if the third argument is a literal zero.
Otherwise, if it is an expression that is folded to zero, or a cast
of zero to some type, it is far less likely that the arguments have
been mistakenly transposed and no warning is emitted. This warning
is enabled by -Wall.
-Waddress
Warn about suspicious uses of memory addresses. These include using
the address of a function in a conditional expression, such as
"void func(void); if (func)", and comparisons against the memory
address of a string literal, such as "if (x == "abc")". Such uses
typically indicate a programmer error: the address of a function
always evaluates to true, so their use in a conditional usually
indicate that the programmer forgot the parentheses in a function
call; and comparisons against string literals result in unspecified
behavior and are not portable in C, so they usually indicate that
the programmer intended to use "strcmp". This warning is enabled
by -Wall.
-Wno-address-of-packed-member
Do not warn when the address of packed member of struct or union is
taken, which usually results in an unaligned pointer value. This
is enabled by default.
-Wlogical-op
Warn about suspicious uses of logical operators in expressions.
This includes using logical operators in contexts where a bit-wise
operator is likely to be expected. Also warns when the operands of
a logical operator are the same:
extern int a;
if (a < 0 && a < 0) { ... }
-Wlogical-not-parentheses
Warn about logical not used on the left hand side operand of a
comparison. This option does not warn if the right operand is
considered to be a boolean expression. Its purpose is to detect
suspicious code like the following:
int a;
...
if (!a > 1) { ... }
It is possible to suppress the warning by wrapping the LHS into
parentheses:
if ((!a) > 1) { ... }
This warning is enabled by -Wall.
-Waggregate-return
Warn if any functions that return structures or unions are defined
or called. (In languages where you can return an array, this also
elicits a warning.)
-Wno-aggressive-loop-optimizations
Warn if in a loop with constant number of iterations the compiler
detects undefined behavior in some statement during one or more of
the iterations.
-Wno-attributes
Do not warn if an unexpected "__attribute__" is used, such as
unrecognized attributes, function attributes applied to variables,
etc. This does not stop errors for incorrect use of supported
attributes.
-Wno-builtin-declaration-mismatch
Warn if a built-in function is declared with an incompatible
signature or as a non-function, or when a built-in function
declared with a type that does not include a prototype is called
with arguments whose promoted types do not match those expected by
the function. When -Wextra is specified, also warn when a built-in
function that takes arguments is declared without a prototype. The
-Wbuiltin-declaration-mismatch warning is enabled by default. To
avoid the warning include the appropriate header to bring the
prototypes of built-in functions into scope.
For example, the call to "memset" below is diagnosed by the warning
because the function expects a value of type "size_t" as its
argument but the type of 32 is "int". With -Wextra, the
declaration of the function is diagnosed as well.
extern void* memset ();
void f (void *d)
{
memset (d, '\0', 32);
}
-Wno-builtin-macro-redefined
Do not warn if certain built-in macros are redefined. This
suppresses warnings for redefinition of "__TIMESTAMP__",
"__TIME__", "__DATE__", "__FILE__", and "__BASE_FILE__".
-Wstrict-prototypes (C and Objective-C only)
Warn if a function is declared or defined without specifying the
argument types. (An old-style function definition is permitted
without a warning if preceded by a declaration that specifies the
argument types.)
-Wold-style-declaration (C and Objective-C only)
Warn for obsolescent usages, according to the C Standard, in a
declaration. For example, warn if storage-class specifiers like
"static" are not the first things in a declaration. This warning
is also enabled by -Wextra.
-Wold-style-definition (C and Objective-C only)
Warn if an old-style function definition is used. A warning is
given even if there is a previous prototype. A definition using ()
is not considered an old-style definition in C2X mode, because it
is equivalent to (void) in that case, but is considered an old-
style definition for older standards.
-Wmissing-parameter-type (C and Objective-C only)
A function parameter is declared without a type specifier in
K&R-style functions:
void foo(bar) { }
This warning is also enabled by -Wextra.
-Wmissing-prototypes (C and Objective-C only)
Warn if a global function is defined without a previous prototype
declaration. This warning is issued even if the definition itself
provides a prototype. Use this option to detect global functions
that do not have a matching prototype declaration in a header file.
This option is not valid for C++ because all function declarations
provide prototypes and a non-matching declaration declares an
overload rather than conflict with an earlier declaration. Use
-Wmissing-declarations to detect missing declarations in C++.
-Wmissing-declarations
Warn if a global function is defined without a previous
declaration. Do so even if the definition itself provides a
prototype. Use this option to detect global functions that are not
declared in header files. In C, no warnings are issued for
functions with previous non-prototype declarations; use
-Wmissing-prototypes to detect missing prototypes. In C++, no
warnings are issued for function templates, or for inline
functions, or for functions in anonymous namespaces.
-Wmissing-field-initializers
Warn if a structure's initializer has some fields missing. For
example, the following code causes such a warning, because "x.h" is
implicitly zero:
struct s { int f, g, h; };
struct s x = { 3, 4 };
This option does not warn about designated initializers, so the
following modification does not trigger a warning:
struct s { int f, g, h; };
struct s x = { .f = 3, .g = 4 };
In C this option does not warn about the universal zero initializer
{ 0 }:
struct s { int f, g, h; };
struct s x = { 0 };
Likewise, in C++ this option does not warn about the empty { }
initializer, for example:
struct s { int f, g, h; };
s x = { };
This warning is included in -Wextra. To get other -Wextra warnings
without this one, use -Wextra -Wno-missing-field-initializers.
-Wno-multichar
Do not warn if a multicharacter constant ('FOOF') is used. Usually
they indicate a typo in the user's code, as they have
implementation-defined values, and should not be used in portable
code.
-Wnormalized=[none|id|nfc|nfkc]
In ISO C and ISO C++, two identifiers are different if they are
different sequences of characters. However, sometimes when
characters outside the basic ASCII character set are used, you can
have two different character sequences that look the same. To
avoid confusion, the ISO 10646 standard sets out some normalization
rules which when applied ensure that two sequences that look the
same are turned into the same sequence. GCC can warn you if you
are using identifiers that have not been normalized; this option
controls that warning.
There are four levels of warning supported by GCC. The default is
-Wnormalized=nfc, which warns about any identifier that is not in
the ISO 10646 "C" normalized form, NFC. NFC is the recommended
form for most uses. It is equivalent to -Wnormalized.
Unfortunately, there are some characters allowed in identifiers by
ISO C and ISO C++ that, when turned into NFC, are not allowed in
identifiers. That is, there's no way to use these symbols in
portable ISO C or C++ and have all your identifiers in NFC.
-Wnormalized=id suppresses the warning for these characters. It is
hoped that future versions of the standards involved will correct
this, which is why this option is not the default.
You can switch the warning off for all characters by writing
-Wnormalized=none or -Wno-normalized. You should only do this if
you are using some other normalization scheme (like "D"), because
otherwise you can easily create bugs that are literally impossible
to see.
Some characters in ISO 10646 have distinct meanings but look
identical in some fonts or display methodologies, especially once
formatting has been applied. For instance "\u207F", "SUPERSCRIPT
LATIN SMALL LETTER N", displays just like a regular "n" that has
been placed in a superscript. ISO 10646 defines the NFKC
normalization scheme to convert all these into a standard form as
well, and GCC warns if your code is not in NFKC if you use
-Wnormalized=nfkc. This warning is comparable to warning about
every identifier that contains the letter O because it might be
confused with the digit 0, and so is not the default, but may be
useful as a local coding convention if the programming environment
cannot be fixed to display these characters distinctly.
-Wno-attribute-warning
Do not warn about usage of functions declared with "warning"
attribute. By default, this warning is enabled.
-Wno-attribute-warning can be used to disable the warning or
-Wno-error=attribute-warning can be used to disable the error when
compiled with -Werror flag.
-Wno-deprecated
Do not warn about usage of deprecated features.
-Wno-deprecated-declarations
Do not warn about uses of functions, variables, and types marked as
deprecated by using the "deprecated" attribute.
-Wno-overflow
Do not warn about compile-time overflow in constant expressions.
-Wno-odr
Warn about One Definition Rule violations during link-time
optimization. Enabled by default.
-Wopenmp-simd
Warn if the vectorizer cost model overrides the OpenMP simd
directive set by user. The -fsimd-cost-model=unlimited option can
be used to relax the cost model.
-Woverride-init (C and Objective-C only)
Warn if an initialized field without side effects is overridden
when using designated initializers.
This warning is included in -Wextra. To get other -Wextra warnings
without this one, use -Wextra -Wno-override-init.
-Wno-override-init-side-effects (C and Objective-C only)
Do not warn if an initialized field with side effects is overridden
when using designated initializers. This warning is enabled by
default.
-Wpacked
Warn if a structure is given the packed attribute, but the packed
attribute has no effect on the layout or size of the structure.
Such structures may be mis-aligned for little benefit. For
instance, in this code, the variable "f.x" in "struct bar" is
misaligned even though "struct bar" does not itself have the packed
attribute:
struct foo {
int x;
char a, b, c, d;
} __attribute__((packed));
struct bar {
char z;
struct foo f;
};
-Wnopacked-bitfield-compat
The 4.1, 4.2 and 4.3 series of GCC ignore the "packed" attribute on
bit-fields of type "char". This was fixed in GCC 4.4 but the
change can lead to differences in the structure layout. GCC
informs you when the offset of such a field has changed in GCC 4.4.
For example there is no longer a 4-bit padding between field "a"
and "b" in this structure:
struct foo
{
char a:4;
char b:8;
} __attribute__ ((packed));
This warning is enabled by default. Use
-Wno-packed-bitfield-compat to disable this warning.
-Wpacked-not-aligned (C, C++, Objective-C and Objective-C++ only)
Warn if a structure field with explicitly specified alignment in a
packed struct or union is misaligned. For example, a warning will
be issued on "struct S", like, "warning: alignment 1 of 'struct S'
is less than 8", in this code:
struct __attribute__ ((aligned (8))) S8 { char a[8]; };
struct __attribute__ ((packed)) S {
struct S8 s8;
};
This warning is enabled by -Wall.
-Wpadded
Warn if padding is included in a structure, either to align an
element of the structure or to align the whole structure.
Sometimes when this happens it is possible to rearrange the fields
of the structure to reduce the padding and so make the structure
smaller.
-Wredundant-decls
Warn if anything is declared more than once in the same scope, even
in cases where multiple declaration is valid and changes nothing.
-Wrestrict
Warn when an object referenced by a "restrict"-qualified parameter
(or, in C++, a "__restrict"-qualified parameter) is aliased by
another argument, or when copies between such objects overlap. For
example, the call to the "strcpy" function below attempts to
truncate the string by replacing its initial characters with the
last four. However, because the call writes the terminating NUL
into "a[4]", the copies overlap and the call is diagnosed.
void foo (void)
{
char a[] = "abcd1234";
strcpy (a, a + 4);
...
}
The -Wrestrict option detects some instances of simple overlap even
without optimization but works best at -O2 and above. It is
included in -Wall.
-Wnested-externs (C and Objective-C only)
Warn if an "extern" declaration is encountered within a function.
-Winline
Warn if a function that is declared as inline cannot be inlined.
Even with this option, the compiler does not warn about failures to
inline functions declared in system headers.
The compiler uses a variety of heuristics to determine whether or
not to inline a function. For example, the compiler takes into
account the size of the function being inlined and the amount of
inlining that has already been done in the current function.
Therefore, seemingly insignificant changes in the source program
can cause the warnings produced by -Winline to appear or disappear.
-Wint-in-bool-context
Warn for suspicious use of integer values where boolean values are
expected, such as conditional expressions (?:) using non-boolean
integer constants in boolean context, like "if (a <= b ? 2 : 3)".
Or left shifting of signed integers in boolean context, like "for
(a = 0; 1 << a; a++);". Likewise for all kinds of multiplications
regardless of the data type. This warning is enabled by -Wall.
-Wno-int-to-pointer-cast
Suppress warnings from casts to pointer type of an integer of a
different size. In C++, casting to a pointer type of smaller size
is an error. Wint-to-pointer-cast is enabled by default.
-Wno-pointer-to-int-cast (C and Objective-C only)
Suppress warnings from casts from a pointer to an integer type of a
different size.
-Winvalid-pch
Warn if a precompiled header is found in the search path but cannot
be used.
-Wlong-long
Warn if "long long" type is used. This is enabled by either
-Wpedantic or -Wtraditional in ISO C90 and C++98 modes. To inhibit
the warning messages, use -Wno-long-long.
-Wvariadic-macros
Warn if variadic macros are used in ISO C90 mode, or if the GNU
alternate syntax is used in ISO C99 mode. This is enabled by
either -Wpedantic or -Wtraditional. To inhibit the warning
messages, use -Wno-variadic-macros.
-Wno-varargs
Do not warn upon questionable usage of the macros used to handle
variable arguments like "va_start". These warnings are enabled by
default.
-Wvector-operation-performance
Warn if vector operation is not implemented via SIMD capabilities
of the architecture. Mainly useful for the performance tuning.
Vector operation can be implemented "piecewise", which means that
the scalar operation is performed on every vector element; "in
parallel", which means that the vector operation is implemented
using scalars of wider type, which normally is more performance
efficient; and "as a single scalar", which means that vector fits
into a scalar type.
-Wvla
Warn if a variable-length array is used in the code. -Wno-vla
prevents the -Wpedantic warning of the variable-length array.
-Wvla-larger-than=byte-size
If this option is used, the compiler warns for declarations of
variable-length arrays whose size is either unbounded, or bounded
by an argument that allows the array size to exceed byte-size
bytes. This is similar to how -Walloca-larger-than=byte-size
works, but with variable-length arrays.
Note that GCC may optimize small variable-length arrays of a known
value into plain arrays, so this warning may not get triggered for
such arrays.
-Wvla-larger-than=PTRDIFF_MAX is enabled by default but is
typically only effective when -ftree-vrp is active (default for -O2
and above).
See also -Walloca-larger-than=byte-size.
-Wno-vla-larger-than
Disable -Wvla-larger-than= warnings. The option is equivalent to
-Wvla-larger-than=SIZE_MAX or larger.
-Wvolatile-register-var
Warn if a register variable is declared volatile. The volatile
modifier does not inhibit all optimizations that may eliminate
reads and/or writes to register variables. This warning is enabled
by -Wall.
-Wdisabled-optimization
Warn if a requested optimization pass is disabled. This warning
does not generally indicate that there is anything wrong with your
code; it merely indicates that GCC's optimizers are unable to
handle the code effectively. Often, the problem is that your code
is too big or too complex; GCC refuses to optimize programs when
the optimization itself is likely to take inordinate amounts of
time.
-Wpointer-sign (C and Objective-C only)
Warn for pointer argument passing or assignment with different
signedness. This option is only supported for C and Objective-C.
It is implied by -Wall and by -Wpedantic, which can be disabled
with -Wno-pointer-sign.
-Wstack-protector
This option is only active when -fstack-protector is active. It
warns about functions that are not protected against stack
smashing.
-Woverlength-strings
Warn about string constants that are longer than the "minimum
maximum" length specified in the C standard. Modern compilers
generally allow string constants that are much longer than the
standard's minimum limit, but very portable programs should avoid
using longer strings.
The limit applies after string constant concatenation, and does not
count the trailing NUL. In C90, the limit was 509 characters; in
C99, it was raised to 4095. C++98 does not specify a normative
minimum maximum, so we do not diagnose overlength strings in C++.
This option is implied by -Wpedantic, and can be disabled with
-Wno-overlength-strings.
-Wunsuffixed-float-constants (C and Objective-C only)
Issue a warning for any floating constant that does not have a
suffix. When used together with -Wsystem-headers it warns about
such constants in system header files. This can be useful when
preparing code to use with the "FLOAT_CONST_DECIMAL64" pragma from
the decimal floating-point extension to C99.
-Wno-lto-type-mismatch
During the link-time optimization, do not warn about type
mismatches in global declarations from different compilation units.
Requires -flto to be enabled. Enabled by default.
-Wno-designated-init (C and Objective-C only)
Suppress warnings when a positional initializer is used to
initialize a structure that has been marked with the
"designated_init" attribute.
-Wno-hsa
Do not warn when HSAIL cannot be emitted for the compiled function
or OpenMP construct. These warnings are enabled by default.
Options That Control Static Analysis
-fanalyzer
This option enables an static analysis of program flow which looks
for "interesting" interprocedural paths through the code, and
issues warnings for problems found on them.
This analysis is much more expensive than other GCC warnings.
Enabling this option effectively enables the following warnings:
-Wanalyzer-double-fclose -Wanalyzer-double-free
-Wanalyzer-exposure-through-output-file -Wanalyzer-file-leak
-Wanalyzer-free-of-non-heap -Wanalyzer-malloc-leak
-Wanalyzer-possible-null-argument
-Wanalyzer-possible-null-dereference -Wanalyzer-null-argument
-Wanalyzer-null-dereference -Wanalyzer-stale-setjmp-buffer
-Wanalyzer-tainted-array-index
-Wanalyzer-unsafe-call-within-signal-handler
-Wanalyzer-use-after-free
-Wanalyzer-use-of-pointer-in-stale-stack-frame
This option is only available if GCC was configured with analyzer
support enabled.
-Wanalyzer-too-complex
If -fanalyzer is enabled, the analyzer uses various heuristics to
attempt to explore the control flow and data flow in the program,
but these can be defeated by sufficiently complicated code.
By default, the analysis silently stops if the code is too
complicated for the analyzer to fully explore and it reaches an
internal limit. The -Wanalyzer-too-complex option warns if this
occurs.
-Wno-analyzer-double-fclose
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-double-fclose to disable it.
This diagnostic warns for paths through the code in which a "FILE
*" can have "fclose" called on it more than once.
-Wno-analyzer-double-free
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-double-free to disable it.
This diagnostic warns for paths through the code in which a pointer
can have "free" called on it more than once.
-Wno-analyzer-exposure-through-output-file
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-exposure-through-output-file to disable it.
This diagnostic warns for paths through the code in which a
security-sensitive value is written to an output file (such as
writing a password to a log file).
-Wno-analyzer-file-leak
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-file-leak to disable it.
This diagnostic warns for paths through the code in which a
"<stdio.h>" "FILE *" stream object is leaked.
-Wno-analyzer-free-of-non-heap
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-free-of-non-heap to disable it.
This diagnostic warns for paths through the code in which "free" is
called on a non-heap pointer (e.g. an on-stack buffer, or a
global).
-Wno-analyzer-malloc-leak
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-malloc-leak to disable it.
This diagnostic warns for paths through the code in which a pointer
allocated via "malloc" is leaked.
-Wno-analyzer-possible-null-argument
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-possible-null-argument to disable it.
This diagnostic warns for paths through the code in which a
possibly-NULL value is passed to a function argument marked with
"__attribute__((nonnull))" as requiring a non-NULL value.
-Wno-analyzer-possible-null-dereference
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-possible-null-dereference to disable it.
This diagnostic warns for paths through the code in which a
possibly-NULL value is dereferenced.
-Wno-analyzer-null-argument
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-null-argument to disable it.
This diagnostic warns for paths through the code in which a value
known to be NULL is passed to a function argument marked with
"__attribute__((nonnull))" as requiring a non-NULL value.
-Wno-analyzer-null-dereference
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-null-dereference to disable it.
This diagnostic warns for paths through the code in which a value
known to be NULL is dereferenced.
-Wno-analyzer-stale-setjmp-buffer
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-stale-setjmp-buffer to disable it.
This diagnostic warns for paths through the code in which "longjmp"
is called to rewind to a "jmp_buf" relating to a "setjmp" call in a
function that has returned.
When "setjmp" is called on a "jmp_buf" to record a rewind location,
it records the stack frame. The stack frame becomes invalid when
the function containing the "setjmp" call returns. Attempting to
rewind to it via "longjmp" would reference a stack frame that no
longer exists, and likely lead to a crash (or worse).
-Wno-analyzer-tainted-array-index
This warning requires both -fanalyzer and -fanalyzer-checker=taint
to enable it; use -Wno-analyzer-tainted-array-index to disable it.
This diagnostic warns for paths through the code in which a value
that could be under an attacker's control is used as the index of
an array access without being sanitized.
-Wno-analyzer-unsafe-call-within-signal-handler
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-unsafe-call-within-signal-handler to disable it.
This diagnostic warns for paths through the code in which a
function known to be async-signal-unsafe (such as "fprintf") is
called from a signal handler.
-Wno-analyzer-use-after-free
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-use-after-free to disable it.
This diagnostic warns for paths through the code in which a pointer
is used after "free" is called on it.
-Wno-analyzer-use-of-pointer-in-stale-stack-frame
This warning requires -fanalyzer, which enables it; use
-Wno-analyzer-use-of-pointer-in-stale-stack-frame to disable it.
This diagnostic warns for paths through the code in which a pointer
is dereferenced that points to a variable in a stale stack frame.
Pertinent parameters for controlling the exploration are: --param
analyzer-bb-explosion-factor=value, --param
analyzer-max-enodes-per-program-point=value, --param
analyzer-max-recursion-depth=value, and --param
analyzer-min-snodes-for-call-summary=value.
The following options control the analyzer.
-fanalyzer-call-summaries
Simplify interprocedural analysis by computing the effect of
certain calls, rather than exploring all paths through the function
from callsite to each possible return.
If enabled, call summaries are only used for functions with more
than one call site, and that are sufficiently complicated (as per
--param analyzer-min-snodes-for-call-summary=value).
-fanalyzer-checker=name
Restrict the analyzer to run just the named checker, and enable it.
Some checkers are disabled by default (even with -fanalyzer), such
as the "taint" checker that implements
-Wanalyzer-tainted-array-index, and this option is required to
enable them.
-fanalyzer-fine-grained
This option is intended for analyzer developers.
Internally the analyzer builds an "exploded graph" that combines
control flow graphs with data flow information.
By default, an edge in this graph can contain the effects of a run
of multiple statements within a basic block. With
-fanalyzer-fine-grained, each statement gets its own edge.
-fanalyzer-show-duplicate-count
This option is intended for analyzer developers: if multiple
diagnostics have been detected as being duplicates of each other,
it emits a note when reporting the best diagnostic, giving the
number of additional diagnostics that were suppressed by the
deduplication logic.
-fno-analyzer-state-merge
This option is intended for analyzer developers.
By default the analyzer attempts to simplify analysis by merging
sufficiently similar states at each program point as it builds its
"exploded graph". With -fno-analyzer-state-merge this merging can
be suppressed, for debugging state-handling issues.
-fno-analyzer-state-purge
This option is intended for analyzer developers.
By default the analyzer attempts to simplify analysis by purging
aspects of state at a program point that appear to no longer be
relevant e.g. the values of locals that aren't accessed later in
the function and which aren't relevant to leak analysis.
With -fno-analyzer-state-purge this purging of state can be
suppressed, for debugging state-handling issues.
-fanalyzer-transitivity
This option enables transitivity of constraints within the
analyzer.
-fanalyzer-verbose-edges
This option is intended for analyzer developers. It enables more
verbose, lower-level detail in the descriptions of control flow
within diagnostic paths.
-fanalyzer-verbose-state-changes
This option is intended for analyzer developers. It enables more
verbose, lower-level detail in the descriptions of events relating
to state machines within diagnostic paths.
-fanalyzer-verbosity=level
This option controls the complexity of the control flow paths that
are emitted for analyzer diagnostics.
The level can be one of:
0 At this level, interprocedural call and return events are
displayed, along with the most pertinent state-change events
relating to a diagnostic. For example, for a double-"free"
diagnostic, both calls to "free" will be shown.
1 As per the previous level, but also show events for the entry
to each function.
2 As per the previous level, but also show events relating to
control flow that are significant to triggering the issue (e.g.
"true path taken" at a conditional).
This level is the default.
3 As per the previous level, but show all control flow events,
not just significant ones.
4 This level is intended for analyzer developers; it adds various
other events intended for debugging the analyzer.
-fdump-analyzer
Dump internal details about what the analyzer is doing to
file.analyzer.txt. This option is overridden by
-fdump-analyzer-stderr.
-fdump-analyzer-stderr
Dump internal details about what the analyzer is doing to stderr.
This option overrides -fdump-analyzer.
-fdump-analyzer-callgraph
Dump a representation of the call graph suitable for viewing with
GraphViz to file.callgraph.dot.
-fdump-analyzer-exploded-graph
Dump a representation of the "exploded graph" suitable for viewing
with GraphViz to file.eg.dot. Nodes are color-coded based on
state-machine states to emphasize state changes.
-fdump-analyzer-exploded-nodes
Emit diagnostics showing where nodes in the "exploded graph" are in
relation to the program source.
-fdump-analyzer-exploded-nodes-2
Dump a textual representation of the "exploded graph" to
file.eg.txt.
-fdump-analyzer-exploded-nodes-3
Dump a textual representation of the "exploded graph" to one dump
file per node, to file.eg-id.txt. This is typically a large number
of dump files.
-fdump-analyzer-state-purge
As per -fdump-analyzer-supergraph, dump a representation of the
"supergraph" suitable for viewing with GraphViz, but annotate the
graph with information on what state will be purged at each node.
The graph is written to file.state-purge.dot.
-fdump-analyzer-supergraph
Dump representations of the "supergraph" suitable for viewing with
GraphViz to file.supergraph.dot and to file.supergraph-eg.dot.
These show all of the control flow graphs in the program, with
interprocedural edges for calls and returns. The second dump
contains annotations showing nodes in the "exploded graph" and
diagnostics associated with them.
Options for Debugging Your Program
To tell GCC to emit extra information for use by a debugger, in almost
all cases you need only to add -g to your other options.
GCC allows you to use -g with -O. The shortcuts taken by optimized
code may occasionally be surprising: some variables you declared may
not exist at all; flow of control may briefly move where you did not
expect it; some statements may not be executed because they compute
constant results or their values are already at hand; some statements
may execute in different places because they have been moved out of
loops. Nevertheless it is possible to debug optimized output. This
makes it reasonable to use the optimizer for programs that might have
bugs.
If you are not using some other optimization option, consider using -Og
with -g. With no -O option at all, some compiler passes that collect
information useful for debugging do not run at all, so that -Og may
result in a better debugging experience.
-g Produce debugging information in the operating system's native
format (stabs, COFF, XCOFF, or DWARF). GDB can work with this
debugging information.
On most systems that use stabs format, -g enables use of extra
debugging information that only GDB can use; this extra information
makes debugging work better in GDB but probably makes other
debuggers crash or refuse to read the program. If you want to
control for certain whether to generate the extra information, use
-gstabs+, -gstabs, -gxcoff+, -gxcoff, or -gvms (see below).
-ggdb
Produce debugging information for use by GDB. This means to use
the most expressive format available (DWARF, stabs, or the native
format if neither of those are supported), including GDB extensions
if at all possible.
-gdwarf
-gdwarf-version
Produce debugging information in DWARF format (if that is
supported). The value of version may be either 2, 3, 4 or 5; the
default version for most targets is 4. DWARF Version 5 is only
experimental.
Note that with DWARF Version 2, some ports require and always use
some non-conflicting DWARF 3 extensions in the unwind tables.
Version 4 may require GDB 7.0 and -fvar-tracking-assignments for
maximum benefit.
GCC no longer supports DWARF Version 1, which is substantially
different than Version 2 and later. For historical reasons, some
other DWARF-related options such as -fno-dwarf2-cfi-asm) retain a
reference to DWARF Version 2 in their names, but apply to all
currently-supported versions of DWARF.
-gstabs
Produce debugging information in stabs format (if that is
supported), without GDB extensions. This is the format used by DBX
on most BSD systems. On MIPS, Alpha and System V Release 4 systems
this option produces stabs debugging output that is not understood
by DBX. On System V Release 4 systems this option requires the GNU
assembler.
-gstabs+
Produce debugging information in stabs format (if that is
supported), using GNU extensions understood only by the GNU
debugger (GDB). The use of these extensions is likely to make
other debuggers crash or refuse to read the program.
-gxcoff
Produce debugging information in XCOFF format (if that is
supported). This is the format used by the DBX debugger on IBM
RS/6000 systems.
-gxcoff+
Produce debugging information in XCOFF format (if that is
supported), using GNU extensions understood only by the GNU
debugger (GDB). The use of these extensions is likely to make
other debuggers crash or refuse to read the program, and may cause
assemblers other than the GNU assembler (GAS) to fail with an
error.
-gvms
Produce debugging information in Alpha/VMS debug format (if that is
supported). This is the format used by DEBUG on Alpha/VMS systems.
-glevel
-ggdblevel
-gstabslevel
-gxcofflevel
-gvmslevel
Request debugging information and also use level to specify how
much information. The default level is 2.
Level 0 produces no debug information at all. Thus, -g0 negates
-g.
Level 1 produces minimal information, enough for making backtraces
in parts of the program that you don't plan to debug. This
includes descriptions of functions and external variables, and line
number tables, but no information about local variables.
Level 3 includes extra information, such as all the macro
definitions present in the program. Some debuggers support macro
expansion when you use -g3.
If you use multiple -g options, with or without level numbers, the
last such option is the one that is effective.
-gdwarf does not accept a concatenated debug level, to avoid
confusion with -gdwarf-level. Instead use an additional -glevel
option to change the debug level for DWARF.
-fno-eliminate-unused-debug-symbols
By default, no debug information is produced for symbols that are
not actually used. Use this option if you want debug information
for all symbols.
-femit-class-debug-always
Instead of emitting debugging information for a C++ class in only
one object file, emit it in all object files using the class. This
option should be used only with debuggers that are unable to handle
the way GCC normally emits debugging information for classes
because using this option increases the size of debugging
information by as much as a factor of two.
-fno-merge-debug-strings
Direct the linker to not merge together strings in the debugging
information that are identical in different object files. Merging
is not supported by all assemblers or linkers. Merging decreases
the size of the debug information in the output file at the cost of
increasing link processing time. Merging is enabled by default.
-fdebug-prefix-map=old=new
When compiling files residing in directory old, record debugging
information describing them as if the files resided in directory
new instead. This can be used to replace a build-time path with an
install-time path in the debug info. It can also be used to change
an absolute path to a relative path by using . for new. This can
give more reproducible builds, which are location independent, but
may require an extra command to tell GDB where to find the source
files. See also -ffile-prefix-map.
-fvar-tracking
Run variable tracking pass. It computes where variables are stored
at each position in code. Better debugging information is then
generated (if the debugging information format supports this
information).
It is enabled by default when compiling with optimization (-Os, -O,
-O2, ...), debugging information (-g) and the debug info format
supports it.
-fvar-tracking-assignments
Annotate assignments to user variables early in the compilation and
attempt to carry the annotations over throughout the compilation
all the way to the end, in an attempt to improve debug information
while optimizing. Use of -gdwarf-4 is recommended along with it.
It can be enabled even if var-tracking is disabled, in which case
annotations are created and maintained, but discarded at the end.
By default, this flag is enabled together with -fvar-tracking,
except when selective scheduling is enabled.
-gsplit-dwarf
Separate as much DWARF debugging information as possible into a
separate output file with the extension .dwo. This option allows
the build system to avoid linking files with debug information. To
be useful, this option requires a debugger capable of reading .dwo
files.
-gdescribe-dies
Add description attributes to some DWARF DIEs that have no name
attribute, such as artificial variables, external references and
call site parameter DIEs.
-gpubnames
Generate DWARF ".debug_pubnames" and ".debug_pubtypes" sections.
-ggnu-pubnames
Generate ".debug_pubnames" and ".debug_pubtypes" sections in a
format suitable for conversion into a GDB index. This option is
only useful with a linker that can produce GDB index version 7.
-fdebug-types-section
When using DWARF Version 4 or higher, type DIEs can be put into
their own ".debug_types" section instead of making them part of the
".debug_info" section. It is more efficient to put them in a
separate comdat section since the linker can then remove
duplicates. But not all DWARF consumers support ".debug_types"
sections yet and on some objects ".debug_types" produces larger
instead of smaller debugging information.
-grecord-gcc-switches
-gno-record-gcc-switches
This switch causes the command-line options used to invoke the
compiler that may affect code generation to be appended to the
DW_AT_producer attribute in DWARF debugging information. The
options are concatenated with spaces separating them from each
other and from the compiler version. It is enabled by default.
See also -frecord-gcc-switches for another way of storing compiler
options into the object file.
-gstrict-dwarf
Disallow using extensions of later DWARF standard version than
selected with -gdwarf-version. On most targets using non-
conflicting DWARF extensions from later standard versions is
allowed.
-gno-strict-dwarf
Allow using extensions of later DWARF standard version than
selected with -gdwarf-version.
-gas-loc-support
Inform the compiler that the assembler supports ".loc" directives.
It may then use them for the assembler to generate DWARF2+ line
number tables.
This is generally desirable, because assembler-generated line-
number tables are a lot more compact than those the compiler can
generate itself.
This option will be enabled by default if, at GCC configure time,
the assembler was found to support such directives.
-gno-as-loc-support
Force GCC to generate DWARF2+ line number tables internally, if
DWARF2+ line number tables are to be generated.
-gas-locview-support
Inform the compiler that the assembler supports "view" assignment
and reset assertion checking in ".loc" directives.
This option will be enabled by default if, at GCC configure time,
the assembler was found to support them.
-gno-as-locview-support
Force GCC to assign view numbers internally, if
-gvariable-location-views are explicitly requested.
-gcolumn-info
-gno-column-info
Emit location column information into DWARF debugging information,
rather than just file and line. This option is enabled by default.
-gstatement-frontiers
-gno-statement-frontiers
This option causes GCC to create markers in the internal
representation at the beginning of statements, and to keep them
roughly in place throughout compilation, using them to guide the
output of "is_stmt" markers in the line number table. This is
enabled by default when compiling with optimization (-Os, -O, -O2,
...), and outputting DWARF 2 debug information at the normal level.
-gvariable-location-views
-gvariable-location-views=incompat5
-gno-variable-location-views
Augment variable location lists with progressive view numbers
implied from the line number table. This enables debug information
consumers to inspect state at certain points of the program, even
if no instructions associated with the corresponding source
locations are present at that point. If the assembler lacks
support for view numbers in line number tables, this will cause the
compiler to emit the line number table, which generally makes them
somewhat less compact. The augmented line number tables and
location lists are fully backward-compatible, so they can be
consumed by debug information consumers that are not aware of these
augmentations, but they won't derive any benefit from them either.
This is enabled by default when outputting DWARF 2 debug
information at the normal level, as long as there is assembler
support, -fvar-tracking-assignments is enabled and -gstrict-dwarf
is not. When assembler support is not available, this may still be
enabled, but it will force GCC to output internal line number
tables, and if -ginternal-reset-location-views is not enabled, that
will most certainly lead to silently mismatching location views.
There is a proposed representation for view numbers that is not
backward compatible with the location list format introduced in
DWARF 5, that can be enabled with
-gvariable-location-views=incompat5. This option may be removed in
the future, is only provided as a reference implementation of the
proposed representation. Debug information consumers are not
expected to support this extended format, and they would be
rendered unable to decode location lists using it.
-ginternal-reset-location-views
-gno-internal-reset-location-views
Attempt to determine location views that can be omitted from
location view lists. This requires the compiler to have very
accurate insn length estimates, which isn't always the case, and it
may cause incorrect view lists to be generated silently when using
an assembler that does not support location view lists. The GNU
assembler will flag any such error as a "view number mismatch".
This is only enabled on ports that define a reliable estimation
function.
-ginline-points
-gno-inline-points
Generate extended debug information for inlined functions.
Location view tracking markers are inserted at inlined entry
points, so that address and view numbers can be computed and output
in debug information. This can be enabled independently of
location views, in which case the view numbers won't be output, but
it can only be enabled along with statement frontiers, and it is
only enabled by default if location views are enabled.
-gz[=type]
Produce compressed debug sections in DWARF format, if that is
supported. If type is not given, the default type depends on the
capabilities of the assembler and linker used. type may be one of
none (don't compress debug sections), zlib (use zlib compression in
ELF gABI format), or zlib-gnu (use zlib compression in traditional
GNU format). If the linker doesn't support writing compressed
debug sections, the option is rejected. Otherwise, if the
assembler does not support them, -gz is silently ignored when
producing object files.
-femit-struct-debug-baseonly
Emit debug information for struct-like types only when the base
name of the compilation source file matches the base name of file
in which the struct is defined.
This option substantially reduces the size of debugging
information, but at significant potential loss in type information
to the debugger. See -femit-struct-debug-reduced for a less
aggressive option. See -femit-struct-debug-detailed for more
detailed control.
This option works only with DWARF debug output.
-femit-struct-debug-reduced
Emit debug information for struct-like types only when the base
name of the compilation source file matches the base name of file
in which the type is defined, unless the struct is a template or
defined in a system header.
This option significantly reduces the size of debugging
information, with some potential loss in type information to the
debugger. See -femit-struct-debug-baseonly for a more aggressive
option. See -femit-struct-debug-detailed for more detailed
control.
This option works only with DWARF debug output.
-femit-struct-debug-detailed[=spec-list]
Specify the struct-like types for which the compiler generates
debug information. The intent is to reduce duplicate struct debug
information between different object files within the same program.
This option is a detailed version of -femit-struct-debug-reduced
and -femit-struct-debug-baseonly, which serves for most needs.
A specification has the
syntax[dir:|ind:][ord:|gen:](any|sys|base|none)
The optional first word limits the specification to structs that
are used directly (dir:) or used indirectly (ind:). A struct type
is used directly when it is the type of a variable, member.
Indirect uses arise through pointers to structs. That is, when use
of an incomplete struct is valid, the use is indirect. An example
is struct one direct; struct two * indirect;.
The optional second word limits the specification to ordinary
structs (ord:) or generic structs (gen:). Generic structs are a
bit complicated to explain. For C++, these are non-explicit
specializations of template classes, or non-template classes within
the above. Other programming languages have generics, but
-femit-struct-debug-detailed does not yet implement them.
The third word specifies the source files for those structs for
which the compiler should emit debug information. The values none
and any have the normal meaning. The value base means that the
base of name of the file in which the type declaration appears must
match the base of the name of the main compilation file. In
practice, this means that when compiling foo.c, debug information
is generated for types declared in that file and foo.h, but not
other header files. The value sys means those types satisfying
base or declared in system or compiler headers.
You may need to experiment to determine the best settings for your
application.
The default is -femit-struct-debug-detailed=all.
This option works only with DWARF debug output.
-fno-dwarf2-cfi-asm
Emit DWARF unwind info as compiler generated ".eh_frame" section
instead of using GAS ".cfi_*" directives.
-fno-eliminate-unused-debug-types
Normally, when producing DWARF output, GCC avoids producing debug
symbol output for types that are nowhere used in the source file
being compiled. Sometimes it is useful to have GCC emit debugging
information for all types declared in a compilation unit,
regardless of whether or not they are actually used in that
compilation unit, for example if, in the debugger, you want to cast
a value to a type that is not actually used in your program (but is
declared). More often, however, this results in a significant
amount of wasted space.
Options That Control Optimization
These options control various sorts of optimizations.
Without any optimization option, the compiler's goal is to reduce the
cost of compilation and to make debugging produce the expected results.
Statements are independent: if you stop the program with a breakpoint
between statements, you can then assign a new value to any variable or
change the program counter to any other statement in the function and
get exactly the results you expect from the source code.
Turning on optimization flags makes the compiler attempt to improve the
performance and/or code size at the expense of compilation time and
possibly the ability to debug the program.
The compiler performs optimization based on the knowledge it has of the
program. Compiling multiple files at once to a single output file mode
allows the compiler to use information gained from all of the files
when compiling each of them.
Not all optimizations are controlled directly by a flag. Only
optimizations that have a flag are listed in this section.
Most optimizations are completely disabled at -O0 or if an -O level is
not set on the command line, even if individual optimization flags are
specified. Similarly, -Og suppresses many optimization passes.
Depending on the target and how GCC was configured, a slightly
different set of optimizations may be enabled at each -O level than
those listed here. You can invoke GCC with -Q --help=optimizers to
find out the exact set of optimizations that are enabled at each level.
-O
-O1 Optimize. Optimizing compilation takes somewhat more time, and a
lot more memory for a large function.
With -O, the compiler tries to reduce code size and execution time,
without performing any optimizations that take a great deal of
compilation time.
-O turns on the following optimization flags:
-fauto-inc-dec -fbranch-count-reg -fcombine-stack-adjustments
-fcompare-elim -fcprop-registers -fdce -fdefer-pop -fdelayed-branch
-fdse -fforward-propagate -fguess-branch-probability
-fif-conversion -fif-conversion2 -finline-functions-called-once
-fipa-profile -fipa-pure-const -fipa-reference
-fipa-reference-addressable -fmerge-constants
-fmove-loop-invariants -fomit-frame-pointer -freorder-blocks
-fshrink-wrap -fshrink-wrap-separate -fsplit-wide-types
-fssa-backprop -fssa-phiopt -ftree-bit-ccp -ftree-ccp -ftree-ch
-ftree-coalesce-vars -ftree-copy-prop -ftree-dce
-ftree-dominator-opts -ftree-dse -ftree-forwprop -ftree-fre
-ftree-phiprop -ftree-pta -ftree-scev-cprop -ftree-sink -ftree-slsr
-ftree-sra -ftree-ter -funit-at-a-time
-O2 Optimize even more. GCC performs nearly all supported
optimizations that do not involve a space-speed tradeoff. As
compared to -O, this option increases both compilation time and the
performance of the generated code.
-O2 turns on all optimization flags specified by -O. It also turns
on the following optimization flags:
-falign-functions -falign-jumps -falign-labels -falign-loops
-fcaller-saves -fcode-hoisting -fcrossjumping -fcse-follow-jumps
-fcse-skip-blocks -fdelete-null-pointer-checks -fdevirtualize
-fdevirtualize-speculatively -fexpensive-optimizations
-ffinite-loops -fgcse -fgcse-lm -fhoist-adjacent-loads
-finline-functions -finline-small-functions -findirect-inlining
-fipa-bit-cp -fipa-cp -fipa-icf -fipa-ra -fipa-sra -fipa-vrp
-fisolate-erroneous-paths-dereference -flra-remat
-foptimize-sibling-calls -foptimize-strlen -fpartial-inlining
-fpeephole2 -freorder-blocks-algorithm=stc
-freorder-blocks-and-partition -freorder-functions
-frerun-cse-after-loop -fschedule-insns -fschedule-insns2
-fsched-interblock -fsched-spec -fstore-merging -fstrict-aliasing
-fthread-jumps -ftree-builtin-call-dce -ftree-pre
-ftree-switch-conversion -ftree-tail-merge -ftree-vrp
Please note the warning under -fgcse about invoking -O2 on programs
that use computed gotos.
-O3 Optimize yet more. -O3 turns on all optimizations specified by -O2
and also turns on the following optimization flags:
-fgcse-after-reload -fipa-cp-clone -floop-interchange
-floop-unroll-and-jam -fpeel-loops -fpredictive-commoning
-fsplit-loops -fsplit-paths -ftree-loop-distribution
-ftree-loop-vectorize -ftree-partial-pre -ftree-slp-vectorize
-funswitch-loops -fvect-cost-model -fvect-cost-model=dynamic
-fversion-loops-for-strides
-O0 Reduce compilation time and make debugging produce the expected
results. This is the default.
-Os Optimize for size. -Os enables all -O2 optimizations except those
that often increase code size:
-falign-functions -falign-jumps -falign-labels -falign-loops
-fprefetch-loop-arrays -freorder-blocks-algorithm=stc
It also enables -finline-functions, causes the compiler to tune for
code size rather than execution speed, and performs further
optimizations designed to reduce code size.
-Ofast
Disregard strict standards compliance. -Ofast enables all -O3
optimizations. It also enables optimizations that are not valid
for all standard-compliant programs. It turns on -ffast-math,
-fallow-store-data-races and the Fortran-specific -fstack-arrays,
unless -fmax-stack-var-size is specified, and -fno-protect-parens.
-Og Optimize debugging experience. -Og should be the optimization
level of choice for the standard edit-compile-debug cycle, offering
a reasonable level of optimization while maintaining fast
compilation and a good debugging experience. It is a better choice
than -O0 for producing debuggable code because some compiler passes
that collect debug information are disabled at -O0.
Like -O0, -Og completely disables a number of optimization passes
so that individual options controlling them have no effect.
Otherwise -Og enables all -O1 optimization flags except for those
that may interfere with debugging:
-fbranch-count-reg -fdelayed-branch -fdse -fif-conversion
-fif-conversion2 -finline-functions-called-once
-fmove-loop-invariants -fssa-phiopt -ftree-bit-ccp -ftree-dse
-ftree-pta -ftree-sra
If you use multiple -O options, with or without level numbers, the last
such option is the one that is effective.
Options of the form -fflag specify machine-independent flags. Most
flags have both positive and negative forms; the negative form of -ffoo
is -fno-foo. In the table below, only one of the forms is listed---the
one you typically use. You can figure out the other form by either
removing no- or adding it.
The following options control specific optimizations. They are either
activated by -O options or are related to ones that are. You can use
the following flags in the rare cases when "fine-tuning" of
optimizations to be performed is desired.
-fno-defer-pop
For machines that must pop arguments after a function call, always
pop the arguments as soon as each function returns. At levels -O1
and higher, -fdefer-pop is the default; this allows the compiler to
let arguments accumulate on the stack for several function calls
and pop them all at once.
-fforward-propagate
Perform a forward propagation pass on RTL. The pass tries to
combine two instructions and checks if the result can be
simplified. If loop unrolling is active, two passes are performed
and the second is scheduled after loop unrolling.
This option is enabled by default at optimization levels -O, -O2,
-O3, -Os.
-ffp-contract=style
-ffp-contract=off disables floating-point expression contraction.
-ffp-contract=fast enables floating-point expression contraction
such as forming of fused multiply-add operations if the target has
native support for them. -ffp-contract=on enables floating-point
expression contraction if allowed by the language standard. This
is currently not implemented and treated equal to
-ffp-contract=off.
The default is -ffp-contract=fast.
-fomit-frame-pointer
Omit the frame pointer in functions that don't need one. This
avoids the instructions to save, set up and restore the frame
pointer; on many targets it also makes an extra register available.
On some targets this flag has no effect because the standard
calling sequence always uses a frame pointer, so it cannot be
omitted.
Note that -fno-omit-frame-pointer doesn't guarantee the frame
pointer is used in all functions. Several targets always omit the
frame pointer in leaf functions.
Enabled by default at -O and higher.
-foptimize-sibling-calls
Optimize sibling and tail recursive calls.
Enabled at levels -O2, -O3, -Os.
-foptimize-strlen
Optimize various standard C string functions (e.g. "strlen",
"strchr" or "strcpy") and their "_FORTIFY_SOURCE" counterparts into
faster alternatives.
Enabled at levels -O2, -O3.
-fno-inline
Do not expand any functions inline apart from those marked with the
"always_inline" attribute. This is the default when not
optimizing.
Single functions can be exempted from inlining by marking them with
the "noinline" attribute.
-finline-small-functions
Integrate functions into their callers when their body is smaller
than expected function call code (so overall size of program gets
smaller). The compiler heuristically decides which functions are
simple enough to be worth integrating in this way. This inlining
applies to all functions, even those not declared inline.
Enabled at levels -O2, -O3, -Os.
-findirect-inlining
Inline also indirect calls that are discovered to be known at
compile time thanks to previous inlining. This option has any
effect only when inlining itself is turned on by the
-finline-functions or -finline-small-functions options.
Enabled at levels -O2, -O3, -Os.
-finline-functions
Consider all functions for inlining, even if they are not declared
inline. The compiler heuristically decides which functions are
worth integrating in this way.
If all calls to a given function are integrated, and the function
is declared "static", then the function is normally not output as
assembler code in its own right.
Enabled at levels -O2, -O3, -Os. Also enabled by -fprofile-use and
-fauto-profile.
-finline-functions-called-once
Consider all "static" functions called once for inlining into their
caller even if they are not marked "inline". If a call to a given
function is integrated, then the function is not output as
assembler code in its own right.
Enabled at levels -O1, -O2, -O3 and -Os, but not -Og.
-fearly-inlining
Inline functions marked by "always_inline" and functions whose body
seems smaller than the function call overhead early before doing
-fprofile-generate instrumentation and real inlining pass. Doing
so makes profiling significantly cheaper and usually inlining
faster on programs having large chains of nested wrapper functions.
Enabled by default.
-fipa-sra
Perform interprocedural scalar replacement of aggregates, removal
of unused parameters and replacement of parameters passed by
reference by parameters passed by value.
Enabled at levels -O2, -O3 and -Os.
-finline-limit=n
By default, GCC limits the size of functions that can be inlined.
This flag allows coarse control of this limit. n is the size of
functions that can be inlined in number of pseudo instructions.
Inlining is actually controlled by a number of parameters, which
may be specified individually by using --param name=value. The
-finline-limit=n option sets some of these parameters as follows:
max-inline-insns-single
is set to n/2.
max-inline-insns-auto
is set to n/2.
See below for a documentation of the individual parameters
controlling inlining and for the defaults of these parameters.
Note: there may be no value to -finline-limit that results in
default behavior.
Note: pseudo instruction represents, in this particular context, an
abstract measurement of function's size. In no way does it
represent a count of assembly instructions and as such its exact
meaning might change from one release to an another.
-fno-keep-inline-dllexport
This is a more fine-grained version of -fkeep-inline-functions,
which applies only to functions that are declared using the
"dllexport" attribute or declspec.
-fkeep-inline-functions
In C, emit "static" functions that are declared "inline" into the
object file, even if the function has been inlined into all of its
callers. This switch does not affect functions using the "extern
inline" extension in GNU C90. In C++, emit any and all inline
functions into the object file.
-fkeep-static-functions
Emit "static" functions into the object file, even if the function
is never used.
-fkeep-static-consts
Emit variables declared "static const" when optimization isn't
turned on, even if the variables aren't referenced.
GCC enables this option by default. If you want to force the
compiler to check if a variable is referenced, regardless of
whether or not optimization is turned on, use the
-fno-keep-static-consts option.
-fmerge-constants
Attempt to merge identical constants (string constants and
floating-point constants) across compilation units.
This option is the default for optimized compilation if the
assembler and linker support it. Use -fno-merge-constants to
inhibit this behavior.
Enabled at levels -O, -O2, -O3, -Os.
-fmerge-all-constants
Attempt to merge identical constants and identical variables.
This option implies -fmerge-constants. In addition to
-fmerge-constants this considers e.g. even constant initialized
arrays or initialized constant variables with integral or floating-
point types. Languages like C or C++ require each variable,
including multiple instances of the same variable in recursive
calls, to have distinct locations, so using this option results in
non-conforming behavior.
-fmodulo-sched
Perform swing modulo scheduling immediately before the first
scheduling pass. This pass looks at innermost loops and reorders
their instructions by overlapping different iterations.
-fmodulo-sched-allow-regmoves
Perform more aggressive SMS-based modulo scheduling with register
moves allowed. By setting this flag certain anti-dependences edges
are deleted, which triggers the generation of reg-moves based on
the life-range analysis. This option is effective only with
-fmodulo-sched enabled.
-fno-branch-count-reg
Disable the optimization pass that scans for opportunities to use
"decrement and branch" instructions on a count register instead of
instruction sequences that decrement a register, compare it against
zero, and then branch based upon the result. This option is only
meaningful on architectures that support such instructions, which
include x86, PowerPC, IA-64 and S/390. Note that the
-fno-branch-count-reg option doesn't remove the decrement and
branch instructions from the generated instruction stream
introduced by other optimization passes.
The default is -fbranch-count-reg at -O1 and higher, except for
-Og.
-fno-function-cse
Do not put function addresses in registers; make each instruction
that calls a constant function contain the function's address
explicitly.
This option results in less efficient code, but some strange hacks
that alter the assembler output may be confused by the
optimizations performed when this option is not used.
The default is -ffunction-cse
-fno-zero-initialized-in-bss
If the target supports a BSS section, GCC by default puts variables
that are initialized to zero into BSS. This can save space in the
resulting code.
This option turns off this behavior because some programs
explicitly rely on variables going to the data section---e.g., so
that the resulting executable can find the beginning of that
section and/or make assumptions based on that.
The default is -fzero-initialized-in-bss.
-fthread-jumps
Perform optimizations that check to see if a jump branches to a
location where another comparison subsumed by the first is found.
If so, the first branch is redirected to either the destination of
the second branch or a point immediately following it, depending on
whether the condition is known to be true or false.
Enabled at levels -O2, -O3, -Os.
-fsplit-wide-types
When using a type that occupies multiple registers, such as "long
long" on a 32-bit system, split the registers apart and allocate
them independently. This normally generates better code for those
types, but may make debugging more difficult.
Enabled at levels -O, -O2, -O3, -Os.
-fsplit-wide-types-early
Fully split wide types early, instead of very late. This option
has no effect unless -fsplit-wide-types is turned on.
This is the default on some targets.
-fcse-follow-jumps
In common subexpression elimination (CSE), scan through jump
instructions when the target of the jump is not reached by any
other path. For example, when CSE encounters an "if" statement
with an "else" clause, CSE follows the jump when the condition
tested is false.
Enabled at levels -O2, -O3, -Os.
-fcse-skip-blocks
This is similar to -fcse-follow-jumps, but causes CSE to follow
jumps that conditionally skip over blocks. When CSE encounters a
simple "if" statement with no else clause, -fcse-skip-blocks causes
CSE to follow the jump around the body of the "if".
Enabled at levels -O2, -O3, -Os.
-frerun-cse-after-loop
Re-run common subexpression elimination after loop optimizations
are performed.
Enabled at levels -O2, -O3, -Os.
-fgcse
Perform a global common subexpression elimination pass. This pass
also performs global constant and copy propagation.
Note: When compiling a program using computed gotos, a GCC
extension, you may get better run-time performance if you disable
the global common subexpression elimination pass by adding
-fno-gcse to the command line.
Enabled at levels -O2, -O3, -Os.
-fgcse-lm
When -fgcse-lm is enabled, global common subexpression elimination
attempts to move loads that are only killed by stores into
themselves. This allows a loop containing a load/store sequence to
be changed to a load outside the loop, and a copy/store within the
loop.
Enabled by default when -fgcse is enabled.
-fgcse-sm
When -fgcse-sm is enabled, a store motion pass is run after global
common subexpression elimination. This pass attempts to move
stores out of loops. When used in conjunction with -fgcse-lm,
loops containing a load/store sequence can be changed to a load
before the loop and a store after the loop.
Not enabled at any optimization level.
-fgcse-las
When -fgcse-las is enabled, the global common subexpression
elimination pass eliminates redundant loads that come after stores
to the same memory location (both partial and full redundancies).
Not enabled at any optimization level.
-fgcse-after-reload
When -fgcse-after-reload is enabled, a redundant load elimination
pass is performed after reload. The purpose of this pass is to
clean up redundant spilling.
Enabled by -fprofile-use and -fauto-profile.
-faggressive-loop-optimizations
This option tells the loop optimizer to use language constraints to
derive bounds for the number of iterations of a loop. This assumes
that loop code does not invoke undefined behavior by for example
causing signed integer overflows or out-of-bound array accesses.
The bounds for the number of iterations of a loop are used to guide
loop unrolling and peeling and loop exit test optimizations. This
option is enabled by default.
-funconstrained-commons
This option tells the compiler that variables declared in common
blocks (e.g. Fortran) may later be overridden with longer trailing
arrays. This prevents certain optimizations that depend on knowing
the array bounds.
-fcrossjumping
Perform cross-jumping transformation. This transformation unifies
equivalent code and saves code size. The resulting code may or may
not perform better than without cross-jumping.
Enabled at levels -O2, -O3, -Os.
-fauto-inc-dec
Combine increments or decrements of addresses with memory accesses.
This pass is always skipped on architectures that do not have
instructions to support this. Enabled by default at -O and higher
on architectures that support this.
-fdce
Perform dead code elimination (DCE) on RTL. Enabled by default at
-O and higher.
-fdse
Perform dead store elimination (DSE) on RTL. Enabled by default at
-O and higher.
-fif-conversion
Attempt to transform conditional jumps into branch-less
equivalents. This includes use of conditional moves, min, max, set
flags and abs instructions, and some tricks doable by standard
arithmetics. The use of conditional execution on chips where it is
available is controlled by -fif-conversion2.
Enabled at levels -O, -O2, -O3, -Os, but not with -Og.
-fif-conversion2
Use conditional execution (where available) to transform
conditional jumps into branch-less equivalents.
Enabled at levels -O, -O2, -O3, -Os, but not with -Og.
-fdeclone-ctor-dtor
The C++ ABI requires multiple entry points for constructors and
destructors: one for a base subobject, one for a complete object,
and one for a virtual destructor that calls operator delete
afterwards. For a hierarchy with virtual bases, the base and
complete variants are clones, which means two copies of the
function. With this option, the base and complete variants are
changed to be thunks that call a common implementation.
Enabled by -Os.
-fdelete-null-pointer-checks
Assume that programs cannot safely dereference null pointers, and
that no code or data element resides at address zero. This option
enables simple constant folding optimizations at all optimization
levels. In addition, other optimization passes in GCC use this
flag to control global dataflow analyses that eliminate useless
checks for null pointers; these assume that a memory access to
address zero always results in a trap, so that if a pointer is
checked after it has already been dereferenced, it cannot be null.
Note however that in some environments this assumption is not true.
Use -fno-delete-null-pointer-checks to disable this optimization
for programs that depend on that behavior.
This option is enabled by default on most targets. On Nios II ELF,
it defaults to off. On AVR, CR16, and MSP430, this option is
completely disabled.
Passes that use the dataflow information are enabled independently
at different optimization levels.
-fdevirtualize
Attempt to convert calls to virtual functions to direct calls.
This is done both within a procedure and interprocedurally as part
of indirect inlining (-findirect-inlining) and interprocedural
constant propagation (-fipa-cp). Enabled at levels -O2, -O3, -Os.
-fdevirtualize-speculatively
Attempt to convert calls to virtual functions to speculative direct
calls. Based on the analysis of the type inheritance graph,
determine for a given call the set of likely targets. If the set is
small, preferably of size 1, change the call into a conditional
deciding between direct and indirect calls. The speculative calls
enable more optimizations, such as inlining. When they seem
useless after further optimization, they are converted back into
original form.
-fdevirtualize-at-ltrans
Stream extra information needed for aggressive devirtualization
when running the link-time optimizer in local transformation mode.
This option enables more devirtualization but significantly
increases the size of streamed data. For this reason it is disabled
by default.
-fexpensive-optimizations
Perform a number of minor optimizations that are relatively
expensive.
Enabled at levels -O2, -O3, -Os.
-free
Attempt to remove redundant extension instructions. This is
especially helpful for the x86-64 architecture, which implicitly
zero-extends in 64-bit registers after writing to their lower
32-bit half.
Enabled for Alpha, AArch64 and x86 at levels -O2, -O3, -Os.
-fno-lifetime-dse
In C++ the value of an object is only affected by changes within
its lifetime: when the constructor begins, the object has an
indeterminate value, and any changes during the lifetime of the
object are dead when the object is destroyed. Normally dead store
elimination will take advantage of this; if your code relies on the
value of the object storage persisting beyond the lifetime of the
object, you can use this flag to disable this optimization. To
preserve stores before the constructor starts (e.g. because your
operator new clears the object storage) but still treat the object
as dead after the destructor, you can use -flifetime-dse=1. The
default behavior can be explicitly selected with -flifetime-dse=2.
-flifetime-dse=0 is equivalent to -fno-lifetime-dse.
-flive-range-shrinkage
Attempt to decrease register pressure through register live range
shrinkage. This is helpful for fast processors with small or
moderate size register sets.
-fira-algorithm=algorithm
Use the specified coloring algorithm for the integrated register
allocator. The algorithm argument can be priority, which specifies
Chow's priority coloring, or CB, which specifies Chaitin-Briggs
coloring. Chaitin-Briggs coloring is not implemented for all
architectures, but for those targets that do support it, it is the
default because it generates better code.
-fira-region=region
Use specified regions for the integrated register allocator. The
region argument should be one of the following:
all Use all loops as register allocation regions. This can give
the best results for machines with a small and/or irregular
register set.
mixed
Use all loops except for loops with small register pressure as
the regions. This value usually gives the best results in most
cases and for most architectures, and is enabled by default
when compiling with optimization for speed (-O, -O2, ...).
one Use all functions as a single region. This typically results
in the smallest code size, and is enabled by default for -Os or
-O0.
-fira-hoist-pressure
Use IRA to evaluate register pressure in the code hoisting pass for
decisions to hoist expressions. This option usually results in
smaller code, but it can slow the compiler down.
This option is enabled at level -Os for all targets.
-fira-loop-pressure
Use IRA to evaluate register pressure in loops for decisions to
move loop invariants. This option usually results in generation of
faster and smaller code on machines with large register files (>=
32 registers), but it can slow the compiler down.
This option is enabled at level -O3 for some targets.
-fno-ira-share-save-slots
Disable sharing of stack slots used for saving call-used hard
registers living through a call. Each hard register gets a
separate stack slot, and as a result function stack frames are
larger.
-fno-ira-share-spill-slots
Disable sharing of stack slots allocated for pseudo-registers.
Each pseudo-register that does not get a hard register gets a
separate stack slot, and as a result function stack frames are
larger.
-flra-remat
Enable CFG-sensitive rematerialization in LRA. Instead of loading
values of spilled pseudos, LRA tries to rematerialize (recalculate)
values if it is profitable.
Enabled at levels -O2, -O3, -Os.
-fdelayed-branch
If supported for the target machine, attempt to reorder
instructions to exploit instruction slots available after delayed
branch instructions.
Enabled at levels -O, -O2, -O3, -Os, but not at -Og.
-fschedule-insns
If supported for the target machine, attempt to reorder
instructions to eliminate execution stalls due to required data
being unavailable. This helps machines that have slow floating
point or memory load instructions by allowing other instructions to
be issued until the result of the load or floating-point
instruction is required.
Enabled at levels -O2, -O3.
-fschedule-insns2
Similar to -fschedule-insns, but requests an additional pass of
instruction scheduling after register allocation has been done.
This is especially useful on machines with a relatively small
number of registers and where memory load instructions take more
than one cycle.
Enabled at levels -O2, -O3, -Os.
-fno-sched-interblock
Disable instruction scheduling across basic blocks, which is
normally enabled when scheduling before register allocation, i.e.
with -fschedule-insns or at -O2 or higher.
-fno-sched-spec
Disable speculative motion of non-load instructions, which is
normally enabled when scheduling before register allocation, i.e.
with -fschedule-insns or at -O2 or higher.
-fsched-pressure
Enable register pressure sensitive insn scheduling before register
allocation. This only makes sense when scheduling before register
allocation is enabled, i.e. with -fschedule-insns or at -O2 or
higher. Usage of this option can improve the generated code and
decrease its size by preventing register pressure increase above
the number of available hard registers and subsequent spills in
register allocation.
-fsched-spec-load
Allow speculative motion of some load instructions. This only
makes sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
-fsched-spec-load-dangerous
Allow speculative motion of more load instructions. This only
makes sense when scheduling before register allocation, i.e. with
-fschedule-insns or at -O2 or higher.
-fsched-stalled-insns
-fsched-stalled-insns=n
Define how many insns (if any) can be moved prematurely from the
queue of stalled insns into the ready list during the second
scheduling pass. -fno-sched-stalled-insns means that no insns are
moved prematurely, -fsched-stalled-insns=0 means there is no limit
on how many queued insns can be moved prematurely.
-fsched-stalled-insns without a value is equivalent to
-fsched-stalled-insns=1.
-fsched-stalled-insns-dep
-fsched-stalled-insns-dep=n
Define how many insn groups (cycles) are examined for a dependency
on a stalled insn that is a candidate for premature removal from
the queue of stalled insns. This has an effect only during the
second scheduling pass, and only if -fsched-stalled-insns is used.
-fno-sched-stalled-insns-dep is equivalent to
-fsched-stalled-insns-dep=0. -fsched-stalled-insns-dep without a
value is equivalent to -fsched-stalled-insns-dep=1.
-fsched2-use-superblocks
When scheduling after register allocation, use superblock
scheduling. This allows motion across basic block boundaries,
resulting in faster schedules. This option is experimental, as not
all machine descriptions used by GCC model the CPU closely enough
to avoid unreliable results from the algorithm.
This only makes sense when scheduling after register allocation,
i.e. with -fschedule-insns2 or at -O2 or higher.
-fsched-group-heuristic
Enable the group heuristic in the scheduler. This heuristic favors
the instruction that belongs to a schedule group. This is enabled
by default when scheduling is enabled, i.e. with -fschedule-insns
or -fschedule-insns2 or at -O2 or higher.
-fsched-critical-path-heuristic
Enable the critical-path heuristic in the scheduler. This
heuristic favors instructions on the critical path. This is
enabled by default when scheduling is enabled, i.e. with
-fschedule-insns or -fschedule-insns2 or at -O2 or higher.
-fsched-spec-insn-heuristic
Enable the speculative instruction heuristic in the scheduler.
This heuristic favors speculative instructions with greater
dependency weakness. This is enabled by default when scheduling is
enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2
or higher.
-fsched-rank-heuristic
Enable the rank heuristic in the scheduler. This heuristic favors
the instruction belonging to a basic block with greater size or
frequency. This is enabled by default when scheduling is enabled,
i.e. with -fschedule-insns or -fschedule-insns2 or at -O2 or
higher.
-fsched-last-insn-heuristic
Enable the last-instruction heuristic in the scheduler. This
heuristic favors the instruction that is less dependent on the last
instruction scheduled. This is enabled by default when scheduling
is enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at
-O2 or higher.
-fsched-dep-count-heuristic
Enable the dependent-count heuristic in the scheduler. This
heuristic favors the instruction that has more instructions
depending on it. This is enabled by default when scheduling is
enabled, i.e. with -fschedule-insns or -fschedule-insns2 or at -O2
or higher.
-freschedule-modulo-scheduled-loops
Modulo scheduling is performed before traditional scheduling. If a
loop is modulo scheduled, later scheduling passes may change its
schedule. Use this option to control that behavior.
-fselective-scheduling
Schedule instructions using selective scheduling algorithm.
Selective scheduling runs instead of the first scheduler pass.
-fselective-scheduling2
Schedule instructions using selective scheduling algorithm.
Selective scheduling runs instead of the second scheduler pass.
-fsel-sched-pipelining
Enable software pipelining of innermost loops during selective
scheduling. This option has no effect unless one of
-fselective-scheduling or -fselective-scheduling2 is turned on.
-fsel-sched-pipelining-outer-loops
When pipelining loops during selective scheduling, also pipeline
outer loops. This option has no effect unless
-fsel-sched-pipelining is turned on.
-fsemantic-interposition
Some object formats, like ELF, allow interposing of symbols by the
dynamic linker. This means that for symbols exported from the DSO,
the compiler cannot perform interprocedural propagation, inlining
and other optimizations in anticipation that the function or
variable in question may change. While this feature is useful, for
example, to rewrite memory allocation functions by a debugging
implementation, it is expensive in the terms of code quality. With
-fno-semantic-interposition the compiler assumes that if
interposition happens for functions the overwriting function will
have precisely the same semantics (and side effects). Similarly if
interposition happens for variables, the constructor of the
variable will be the same. The flag has no effect for functions
explicitly declared inline (where it is never allowed for
interposition to change semantics) and for symbols explicitly
declared weak.
-fshrink-wrap
Emit function prologues only before parts of the function that need
it, rather than at the top of the function. This flag is enabled
by default at -O and higher.
-fshrink-wrap-separate
Shrink-wrap separate parts of the prologue and epilogue separately,
so that those parts are only executed when needed. This option is
on by default, but has no effect unless -fshrink-wrap is also
turned on and the target supports this.
-fcaller-saves
Enable allocation of values to registers that are clobbered by
function calls, by emitting extra instructions to save and restore
the registers around such calls. Such allocation is done only when
it seems to result in better code.
This option is always enabled by default on certain machines,
usually those which have no call-preserved registers to use
instead.
Enabled at levels -O2, -O3, -Os.
-fcombine-stack-adjustments
Tracks stack adjustments (pushes and pops) and stack memory
references and then tries to find ways to combine them.
Enabled by default at -O1 and higher.
-fipa-ra
Use caller save registers for allocation if those registers are not
used by any called function. In that case it is not necessary to
save and restore them around calls. This is only possible if
called functions are part of same compilation unit as current
function and they are compiled before it.
Enabled at levels -O2, -O3, -Os, however the option is disabled if
generated code will be instrumented for profiling (-p, or -pg) or
if callee's register usage cannot be known exactly (this happens on
targets that do not expose prologues and epilogues in RTL).
-fconserve-stack
Attempt to minimize stack usage. The compiler attempts to use less
stack space, even if that makes the program slower. This option
implies setting the large-stack-frame parameter to 100 and the
large-stack-frame-growth parameter to 400.
-ftree-reassoc
Perform reassociation on trees. This flag is enabled by default at
-O and higher.
-fcode-hoisting
Perform code hoisting. Code hoisting tries to move the evaluation
of expressions executed on all paths to the function exit as early
as possible. This is especially useful as a code size
optimization, but it often helps for code speed as well. This flag
is enabled by default at -O2 and higher.
-ftree-pre
Perform partial redundancy elimination (PRE) on trees. This flag
is enabled by default at -O2 and -O3.
-ftree-partial-pre
Make partial redundancy elimination (PRE) more aggressive. This
flag is enabled by default at -O3.
-ftree-forwprop
Perform forward propagation on trees. This flag is enabled by
default at -O and higher.
-ftree-fre
Perform full redundancy elimination (FRE) on trees. The difference
between FRE and PRE is that FRE only considers expressions that are
computed on all paths leading to the redundant computation. This
analysis is faster than PRE, though it exposes fewer redundancies.
This flag is enabled by default at -O and higher.
-ftree-phiprop
Perform hoisting of loads from conditional pointers on trees. This
pass is enabled by default at -O and higher.
-fhoist-adjacent-loads
Speculatively hoist loads from both branches of an if-then-else if
the loads are from adjacent locations in the same structure and the
target architecture has a conditional move instruction. This flag
is enabled by default at -O2 and higher.
-ftree-copy-prop
Perform copy propagation on trees. This pass eliminates
unnecessary copy operations. This flag is enabled by default at -O
and higher.
-fipa-pure-const
Discover which functions are pure or constant. Enabled by default
at -O and higher.
-fipa-reference
Discover which static variables do not escape the compilation unit.
Enabled by default at -O and higher.
-fipa-reference-addressable
Discover read-only, write-only and non-addressable static
variables. Enabled by default at -O and higher.
-fipa-stack-alignment
Reduce stack alignment on call sites if possible. Enabled by
default.
-fipa-pta
Perform interprocedural pointer analysis and interprocedural
modification and reference analysis. This option can cause
excessive memory and compile-time usage on large compilation units.
It is not enabled by default at any optimization level.
-fipa-profile
Perform interprocedural profile propagation. The functions called
only from cold functions are marked as cold. Also functions
executed once (such as "cold", "noreturn", static constructors or
destructors) are identified. Cold functions and loop less parts of
functions executed once are then optimized for size. Enabled by
default at -O and higher.
-fipa-cp
Perform interprocedural constant propagation. This optimization
analyzes the program to determine when values passed to functions
are constants and then optimizes accordingly. This optimization
can substantially increase performance if the application has
constants passed to functions. This flag is enabled by default at
-O2, -Os and -O3. It is also enabled by -fprofile-use and
-fauto-profile.
-fipa-cp-clone
Perform function cloning to make interprocedural constant
propagation stronger. When enabled, interprocedural constant
propagation performs function cloning when externally visible
function can be called with constant arguments. Because this
optimization can create multiple copies of functions, it may
significantly increase code size (see --param
ipa-cp-unit-growth=value). This flag is enabled by default at -O3.
It is also enabled by -fprofile-use and -fauto-profile.
-fipa-bit-cp
When enabled, perform interprocedural bitwise constant propagation.
This flag is enabled by default at -O2 and by -fprofile-use and
-fauto-profile. It requires that -fipa-cp is enabled.
-fipa-vrp
When enabled, perform interprocedural propagation of value ranges.
This flag is enabled by default at -O2. It requires that -fipa-cp
is enabled.
-fipa-icf
Perform Identical Code Folding for functions and read-only
variables. The optimization reduces code size and may disturb
unwind stacks by replacing a function by equivalent one with a
different name. The optimization works more effectively with link-
time optimization enabled.
Although the behavior is similar to the Gold Linker's ICF
optimization, GCC ICF works on different levels and thus the
optimizations are not same - there are equivalences that are found
only by GCC and equivalences found only by Gold.
This flag is enabled by default at -O2 and -Os.
-flive-patching=level
Control GCC's optimizations to produce output suitable for live-
patching.
If the compiler's optimization uses a function's body or
information extracted from its body to optimize/change another
function, the latter is called an impacted function of the former.
If a function is patched, its impacted functions should be patched
too.
The impacted functions are determined by the compiler's
interprocedural optimizations. For example, a caller is impacted
when inlining a function into its caller, cloning a function and
changing its caller to call this new clone, or extracting a
function's pureness/constness information to optimize its direct or
indirect callers, etc.
Usually, the more IPA optimizations enabled, the larger the number
of impacted functions for each function. In order to control the
number of impacted functions and more easily compute the list of
impacted function, IPA optimizations can be partially enabled at
two different levels.
The level argument should be one of the following:
inline-clone
Only enable inlining and cloning optimizations, which includes
inlining, cloning, interprocedural scalar replacement of
aggregates and partial inlining. As a result, when patching a
function, all its callers and its clones' callers are impacted,
therefore need to be patched as well.
-flive-patching=inline-clone disables the following
optimization flags: -fwhole-program -fipa-pta -fipa-reference
-fipa-ra -fipa-icf -fipa-icf-functions -fipa-icf-variables
-fipa-bit-cp -fipa-vrp -fipa-pure-const
-fipa-reference-addressable -fipa-stack-alignment
inline-only-static
Only enable inlining of static functions. As a result, when
patching a static function, all its callers are impacted and so
need to be patched as well.
In addition to all the flags that -flive-patching=inline-clone
disables, -flive-patching=inline-only-static disables the
following additional optimization flags: -fipa-cp-clone
-fipa-sra -fpartial-inlining -fipa-cp
When -flive-patching is specified without any value, the default
value is inline-clone.
This flag is disabled by default.
Note that -flive-patching is not supported with link-time
optimization (-flto).
-fisolate-erroneous-paths-dereference
Detect paths that trigger erroneous or undefined behavior due to
dereferencing a null pointer. Isolate those paths from the main
control flow and turn the statement with erroneous or undefined
behavior into a trap. This flag is enabled by default at -O2 and
higher and depends on -fdelete-null-pointer-checks also being
enabled.
-fisolate-erroneous-paths-attribute
Detect paths that trigger erroneous or undefined behavior due to a
null value being used in a way forbidden by a "returns_nonnull" or
"nonnull" attribute. Isolate those paths from the main control
flow and turn the statement with erroneous or undefined behavior
into a trap. This is not currently enabled, but may be enabled by
-O2 in the future.
-ftree-sink
Perform forward store motion on trees. This flag is enabled by
default at -O and higher.
-ftree-bit-ccp
Perform sparse conditional bit constant propagation on trees and
propagate pointer alignment information. This pass only operates
on local scalar variables and is enabled by default at -O1 and
higher, except for -Og. It requires that -ftree-ccp is enabled.
-ftree-ccp
Perform sparse conditional constant propagation (CCP) on trees.
This pass only operates on local scalar variables and is enabled by
default at -O and higher.
-fssa-backprop
Propagate information about uses of a value up the definition chain
in order to simplify the definitions. For example, this pass
strips sign operations if the sign of a value never matters. The
flag is enabled by default at -O and higher.
-fssa-phiopt
Perform pattern matching on SSA PHI nodes to optimize conditional
code. This pass is enabled by default at -O1 and higher, except
for -Og.
-ftree-switch-conversion
Perform conversion of simple initializations in a switch to
initializations from a scalar array. This flag is enabled by
default at -O2 and higher.
-ftree-tail-merge
Look for identical code sequences. When found, replace one with a
jump to the other. This optimization is known as tail merging or
cross jumping. This flag is enabled by default at -O2 and higher.
The compilation time in this pass can be limited using max-tail-
merge-comparisons parameter and max-tail-merge-iterations
parameter.
-ftree-dce
Perform dead code elimination (DCE) on trees. This flag is enabled
by default at -O and higher.
-ftree-builtin-call-dce
Perform conditional dead code elimination (DCE) for calls to built-
in functions that may set "errno" but are otherwise free of side
effects. This flag is enabled by default at -O2 and higher if -Os
is not also specified.
-ffinite-loops
Assume that a loop with an exit will eventually take the exit and
not loop indefinitely. This allows the compiler to remove loops
that otherwise have no side-effects, not considering eventual
endless looping as such.
This option is enabled by default at -O2 for C++ with -std=c++11 or
higher.
-ftree-dominator-opts
Perform a variety of simple scalar cleanups (constant/copy
propagation, redundancy elimination, range propagation and
expression simplification) based on a dominator tree traversal.
This also performs jump threading (to reduce jumps to jumps). This
flag is enabled by default at -O and higher.
-ftree-dse
Perform dead store elimination (DSE) on trees. A dead store is a
store into a memory location that is later overwritten by another
store without any intervening loads. In this case the earlier
store can be deleted. This flag is enabled by default at -O and
higher.
-ftree-ch
Perform loop header copying on trees. This is beneficial since it
increases effectiveness of code motion optimizations. It also
saves one jump. This flag is enabled by default at -O and higher.
It is not enabled for -Os, since it usually increases code size.
-ftree-loop-optimize
Perform loop optimizations on trees. This flag is enabled by
default at -O and higher.
-ftree-loop-linear
-floop-strip-mine
-floop-block
Perform loop nest optimizations. Same as -floop-nest-optimize. To
use this code transformation, GCC has to be configured with
--with-isl to enable the Graphite loop transformation
infrastructure.
-fgraphite-identity
Enable the identity transformation for graphite. For every SCoP we
generate the polyhedral representation and transform it back to
gimple. Using -fgraphite-identity we can check the costs or
benefits of the GIMPLE -> GRAPHITE -> GIMPLE transformation. Some
minimal optimizations are also performed by the code generator isl,
like index splitting and dead code elimination in loops.
-floop-nest-optimize
Enable the isl based loop nest optimizer. This is a generic loop
nest optimizer based on the Pluto optimization algorithms. It
calculates a loop structure optimized for data-locality and
parallelism. This option is experimental.
-floop-parallelize-all
Use the Graphite data dependence analysis to identify loops that
can be parallelized. Parallelize all the loops that can be
analyzed to not contain loop carried dependences without checking
that it is profitable to parallelize the loops.
-ftree-coalesce-vars
While transforming the program out of the SSA representation,
attempt to reduce copying by coalescing versions of different user-
defined variables, instead of just compiler temporaries. This may
severely limit the ability to debug an optimized program compiled
with -fno-var-tracking-assignments. In the negated form, this flag
prevents SSA coalescing of user variables. This option is enabled
by default if optimization is enabled, and it does very little
otherwise.
-ftree-loop-if-convert
Attempt to transform conditional jumps in the innermost loops to
branch-less equivalents. The intent is to remove control-flow from
the innermost loops in order to improve the ability of the
vectorization pass to handle these loops. This is enabled by
default if vectorization is enabled.
-ftree-loop-distribution
Perform loop distribution. This flag can improve cache performance
on big loop bodies and allow further loop optimizations, like
parallelization or vectorization, to take place. For example, the
loop
DO I = 1, N
A(I) = B(I) + C
D(I) = E(I) * F
ENDDO
is transformed to
DO I = 1, N
A(I) = B(I) + C
ENDDO
DO I = 1, N
D(I) = E(I) * F
ENDDO
This flag is enabled by default at -O3. It is also enabled by
-fprofile-use and -fauto-profile.
-ftree-loop-distribute-patterns
Perform loop distribution of patterns that can be code generated
with calls to a library. This flag is enabled by default at -O2
and higher, and by -fprofile-use and -fauto-profile.
This pass distributes the initialization loops and generates a call
to memset zero. For example, the loop
DO I = 1, N
A(I) = 0
B(I) = A(I) + I
ENDDO
is transformed to
DO I = 1, N
A(I) = 0
ENDDO
DO I = 1, N
B(I) = A(I) + I
ENDDO
and the initialization loop is transformed into a call to memset
zero. This flag is enabled by default at -O3. It is also enabled
by -fprofile-use and -fauto-profile.
-floop-interchange
Perform loop interchange outside of graphite. This flag can
improve cache performance on loop nest and allow further loop
optimizations, like vectorization, to take place. For example, the
loop
for (int i = 0; i < N; i++)
for (int j = 0; j < N; j++)
for (int k = 0; k < N; k++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];
is transformed to
for (int i = 0; i < N; i++)
for (int k = 0; k < N; k++)
for (int j = 0; j < N; j++)
c[i][j] = c[i][j] + a[i][k]*b[k][j];
This flag is enabled by default at -O3. It is also enabled by
-fprofile-use and -fauto-profile.
-floop-unroll-and-jam
Apply unroll and jam transformations on feasible loops. In a loop
nest this unrolls the outer loop by some factor and fuses the
resulting multiple inner loops. This flag is enabled by default at
-O3. It is also enabled by -fprofile-use and -fauto-profile.
-ftree-loop-im
Perform loop invariant motion on trees. This pass moves only
invariants that are hard to handle at RTL level (function calls,
operations that expand to nontrivial sequences of insns). With
-funswitch-loops it also moves operands of conditions that are
invariant out of the loop, so that we can use just trivial
invariantness analysis in loop unswitching. The pass also includes
store motion.
-ftree-loop-ivcanon
Create a canonical counter for number of iterations in loops for
which determining number of iterations requires complicated
analysis. Later optimizations then may determine the number
easily. Useful especially in connection with unrolling.
-ftree-scev-cprop
Perform final value replacement. If a variable is modified in a
loop in such a way that its value when exiting the loop can be
determined using only its initial value and the number of loop
iterations, replace uses of the final value by such a computation,
provided it is sufficiently cheap. This reduces data dependencies
and may allow further simplifications. Enabled by default at -O
and higher.
-fivopts
Perform induction variable optimizations (strength reduction,
induction variable merging and induction variable elimination) on
trees.
-ftree-parallelize-loops=n
Parallelize loops, i.e., split their iteration space to run in n
threads. This is only possible for loops whose iterations are
independent and can be arbitrarily reordered. The optimization is
only profitable on multiprocessor machines, for loops that are CPU-
intensive, rather than constrained e.g. by memory bandwidth. This
option implies -pthread, and thus is only supported on targets that
have support for -pthread.
-ftree-pta
Perform function-local points-to analysis on trees. This flag is
enabled by default at -O1 and higher, except for -Og.
-ftree-sra
Perform scalar replacement of aggregates. This pass replaces
structure references with scalars to prevent committing structures
to memory too early. This flag is enabled by default at -O1 and
higher, except for -Og.
-fstore-merging
Perform merging of narrow stores to consecutive memory addresses.
This pass merges contiguous stores of immediate values narrower
than a word into fewer wider stores to reduce the number of
instructions. This is enabled by default at -O2 and higher as well
as -Os.
-ftree-ter
Perform temporary expression replacement during the SSA->normal
phase. Single use/single def temporaries are replaced at their use
location with their defining expression. This results in non-
GIMPLE code, but gives the expanders much more complex trees to
work on resulting in better RTL generation. This is enabled by
default at -O and higher.
-ftree-slsr
Perform straight-line strength reduction on trees. This recognizes
related expressions involving multiplications and replaces them by
less expensive calculations when possible. This is enabled by
default at -O and higher.
-ftree-vectorize
Perform vectorization on trees. This flag enables
-ftree-loop-vectorize and -ftree-slp-vectorize if not explicitly
specified.
-ftree-loop-vectorize
Perform loop vectorization on trees. This flag is enabled by
default at -O3 and by -ftree-vectorize, -fprofile-use, and
-fauto-profile.
-ftree-slp-vectorize
Perform basic block vectorization on trees. This flag is enabled by
default at -O3 and by -ftree-vectorize, -fprofile-use, and
-fauto-profile.
-fvect-cost-model=model
Alter the cost model used for vectorization. The model argument
should be one of unlimited, dynamic or cheap. With the unlimited
model the vectorized code-path is assumed to be profitable while
with the dynamic model a runtime check guards the vectorized code-
path to enable it only for iteration counts that will likely
execute faster than when executing the original scalar loop. The
cheap model disables vectorization of loops where doing so would be
cost prohibitive for example due to required runtime checks for
data dependence or alignment but otherwise is equal to the dynamic
model. The default cost model depends on other optimization flags
and is either dynamic or cheap.
-fsimd-cost-model=model
Alter the cost model used for vectorization of loops marked with
the OpenMP simd directive. The model argument should be one of
unlimited, dynamic, cheap. All values of model have the same
meaning as described in -fvect-cost-model and by default a cost
model defined with -fvect-cost-model is used.
-ftree-vrp
Perform Value Range Propagation on trees. This is similar to the
constant propagation pass, but instead of values, ranges of values
are propagated. This allows the optimizers to remove unnecessary
range checks like array bound checks and null pointer checks. This
is enabled by default at -O2 and higher. Null pointer check
elimination is only done if -fdelete-null-pointer-checks is
enabled.
-fsplit-paths
Split paths leading to loop backedges. This can improve dead code
elimination and common subexpression elimination. This is enabled
by default at -O3 and above.
-fsplit-ivs-in-unroller
Enables expression of values of induction variables in later
iterations of the unrolled loop using the value in the first
iteration. This breaks long dependency chains, thus improving
efficiency of the scheduling passes.
A combination of -fweb and CSE is often sufficient to obtain the
same effect. However, that is not reliable in cases where the loop
body is more complicated than a single basic block. It also does
not work at all on some architectures due to restrictions in the
CSE pass.
This optimization is enabled by default.
-fvariable-expansion-in-unroller
With this option, the compiler creates multiple copies of some
local variables when unrolling a loop, which can result in superior
code.
This optimization is enabled by default for PowerPC targets, but
disabled by default otherwise.
-fpartial-inlining
Inline parts of functions. This option has any effect only when
inlining itself is turned on by the -finline-functions or
-finline-small-functions options.
Enabled at levels -O2, -O3, -Os.
-fpredictive-commoning
Perform predictive commoning optimization, i.e., reusing
computations (especially memory loads and stores) performed in
previous iterations of loops.
This option is enabled at level -O3. It is also enabled by
-fprofile-use and -fauto-profile.
-fprefetch-loop-arrays
If supported by the target machine, generate instructions to
prefetch memory to improve the performance of loops that access
large arrays.
This option may generate better or worse code; results are highly
dependent on the structure of loops within the source code.
Disabled at level -Os.
-fno-printf-return-value
Do not substitute constants for known return value of formatted
output functions such as "sprintf", "snprintf", "vsprintf", and
"vsnprintf" (but not "printf" of "fprintf"). This transformation
allows GCC to optimize or even eliminate branches based on the
known return value of these functions called with arguments that
are either constant, or whose values are known to be in a range
that makes determining the exact return value possible. For
example, when -fprintf-return-value is in effect, both the branch
and the body of the "if" statement (but not the call to "snprint")
can be optimized away when "i" is a 32-bit or smaller integer
because the return value is guaranteed to be at most 8.
char buf[9];
if (snprintf (buf, "%08x", i) >= sizeof buf)
...
The -fprintf-return-value option relies on other optimizations and
yields best results with -O2 and above. It works in tandem with
the -Wformat-overflow and -Wformat-truncation options. The
-fprintf-return-value option is enabled by default.
-fno-peephole
-fno-peephole2
Disable any machine-specific peephole optimizations. The
difference between -fno-peephole and -fno-peephole2 is in how they
are implemented in the compiler; some targets use one, some use the
other, a few use both.
-fpeephole is enabled by default. -fpeephole2 enabled at levels
-O2, -O3, -Os.
-fno-guess-branch-probability
Do not guess branch probabilities using heuristics.
GCC uses heuristics to guess branch probabilities if they are not
provided by profiling feedback (-fprofile-arcs). These heuristics
are based on the control flow graph. If some branch probabilities
are specified by "__builtin_expect", then the heuristics are used
to guess branch probabilities for the rest of the control flow
graph, taking the "__builtin_expect" info into account. The
interactions between the heuristics and "__builtin_expect" can be
complex, and in some cases, it may be useful to disable the
heuristics so that the effects of "__builtin_expect" are easier to
understand.
It is also possible to specify expected probability of the
expression with "__builtin_expect_with_probability" built-in
function.
The default is -fguess-branch-probability at levels -O, -O2, -O3,
-Os.
-freorder-blocks
Reorder basic blocks in the compiled function in order to reduce
number of taken branches and improve code locality.
Enabled at levels -O, -O2, -O3, -Os.
-freorder-blocks-algorithm=algorithm
Use the specified algorithm for basic block reordering. The
algorithm argument can be simple, which does not increase code size
(except sometimes due to secondary effects like alignment), or stc,
the "software trace cache" algorithm, which tries to put all often
executed code together, minimizing the number of branches executed
by making extra copies of code.
The default is simple at levels -O, -Os, and stc at levels -O2,
-O3.
-freorder-blocks-and-partition
In addition to reordering basic blocks in the compiled function, in
order to reduce number of taken branches, partitions hot and cold
basic blocks into separate sections of the assembly and .o files,
to improve paging and cache locality performance.
This optimization is automatically turned off in the presence of
exception handling or unwind tables (on targets using
setjump/longjump or target specific scheme), for linkonce sections,
for functions with a user-defined section attribute and on any
architecture that does not support named sections. When
-fsplit-stack is used this option is not enabled by default (to
avoid linker errors), but may be enabled explicitly (if using a
working linker).
Enabled for x86 at levels -O2, -O3, -Os.
-freorder-functions
Reorder functions in the object file in order to improve code
locality. This is implemented by using special subsections
".text.hot" for most frequently executed functions and
".text.unlikely" for unlikely executed functions. Reordering is
done by the linker so object file format must support named
sections and linker must place them in a reasonable way.
This option isn't effective unless you either provide profile
feedback (see -fprofile-arcs for details) or manually annotate
functions with "hot" or "cold" attributes.
Enabled at levels -O2, -O3, -Os.
-fstrict-aliasing
Allow the compiler to assume the strictest aliasing rules
applicable to the language being compiled. For C (and C++), this
activates optimizations based on the type of expressions. In
particular, an object of one type is assumed never to reside at the
same address as an object of a different type, unless the types are
almost the same. For example, an "unsigned int" can alias an
"int", but not a "void*" or a "double". A character type may alias
any other type.
Pay special attention to code like this:
union a_union {
int i;
double d;
};
int f() {
union a_union t;
t.d = 3.0;
return t.i;
}
The practice of reading from a different union member than the one
most recently written to (called "type-punning") is common. Even
with -fstrict-aliasing, type-punning is allowed, provided the
memory is accessed through the union type. So, the code above
works as expected. However, this code might not:
int f() {
union a_union t;
int* ip;
t.d = 3.0;
ip = &t.i;
return *ip;
}
Similarly, access by taking the address, casting the resulting
pointer and dereferencing the result has undefined behavior, even
if the cast uses a union type, e.g.:
int f() {
double d = 3.0;
return ((union a_union *) &d)->i;
}
The -fstrict-aliasing option is enabled at levels -O2, -O3, -Os.
-falign-functions
-falign-functions=n
-falign-functions=n:m
-falign-functions=n:m:n2
-falign-functions=n:m:n2:m2
Align the start of functions to the next power-of-two greater than
or equal to n, skipping up to m-1 bytes. This ensures that at
least the first m bytes of the function can be fetched by the CPU
without crossing an n-byte alignment boundary.
If m is not specified, it defaults to n.
Examples: -falign-functions=32 aligns functions to the next 32-byte
boundary, -falign-functions=24 aligns to the next 32-byte boundary
only if this can be done by skipping 23 bytes or less,
-falign-functions=32:7 aligns to the next 32-byte boundary only if
this can be done by skipping 6 bytes or less.
The second pair of n2:m2 values allows you to specify a secondary
alignment: -falign-functions=64:7:32:3 aligns to the next 64-byte
boundary if this can be done by skipping 6 bytes or less, otherwise
aligns to the next 32-byte boundary if this can be done by skipping
2 bytes or less. If m2 is not specified, it defaults to n2.
Some assemblers only support this flag when n is a power of two; in
that case, it is rounded up.
-fno-align-functions and -falign-functions=1 are equivalent and
mean that functions are not aligned.
If n is not specified or is zero, use a machine-dependent default.
The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-flimit-function-alignment
If this option is enabled, the compiler tries to avoid
unnecessarily overaligning functions. It attempts to instruct the
assembler to align by the amount specified by -falign-functions,
but not to skip more bytes than the size of the function.
-falign-labels
-falign-labels=n
-falign-labels=n:m
-falign-labels=n:m:n2
-falign-labels=n:m:n2:m2
Align all branch targets to a power-of-two boundary.
Parameters of this option are analogous to the -falign-functions
option. -fno-align-labels and -falign-labels=1 are equivalent and
mean that labels are not aligned.
If -falign-loops or -falign-jumps are applicable and are greater
than this value, then their values are used instead.
If n is not specified or is zero, use a machine-dependent default
which is very likely to be 1, meaning no alignment. The maximum
allowed n option value is 65536.
Enabled at levels -O2, -O3.
-falign-loops
-falign-loops=n
-falign-loops=n:m
-falign-loops=n:m:n2
-falign-loops=n:m:n2:m2
Align loops to a power-of-two boundary. If the loops are executed
many times, this makes up for any execution of the dummy padding
instructions.
If -falign-labels is greater than this value, then its value is
used instead.
Parameters of this option are analogous to the -falign-functions
option. -fno-align-loops and -falign-loops=1 are equivalent and
mean that loops are not aligned. The maximum allowed n option
value is 65536.
If n is not specified or is zero, use a machine-dependent default.
Enabled at levels -O2, -O3.
-falign-jumps
-falign-jumps=n
-falign-jumps=n:m
-falign-jumps=n:m:n2
-falign-jumps=n:m:n2:m2
Align branch targets to a power-of-two boundary, for branch targets
where the targets can only be reached by jumping. In this case, no
dummy operations need be executed.
If -falign-labels is greater than this value, then its value is
used instead.
Parameters of this option are analogous to the -falign-functions
option. -fno-align-jumps and -falign-jumps=1 are equivalent and
mean that loops are not aligned.
If n is not specified or is zero, use a machine-dependent default.
The maximum allowed n option value is 65536.
Enabled at levels -O2, -O3.
-fno-allocation-dce
Do not remove unused C++ allocations in dead code elimination.
-fallow-store-data-races
Allow the compiler to introduce new data races on stores.
Enabled at level -Ofast.
-funit-at-a-time
This option is left for compatibility reasons. -funit-at-a-time has
no effect, while -fno-unit-at-a-time implies -fno-toplevel-reorder
and -fno-section-anchors.
Enabled by default.
-fno-toplevel-reorder
Do not reorder top-level functions, variables, and "asm"
statements. Output them in the same order that they appear in the
input file. When this option is used, unreferenced static
variables are not removed. This option is intended to support
existing code that relies on a particular ordering. For new code,
it is better to use attributes when possible.
-ftoplevel-reorder is the default at -O1 and higher, and also at
-O0 if -fsection-anchors is explicitly requested. Additionally
-fno-toplevel-reorder implies -fno-section-anchors.
-fweb
Constructs webs as commonly used for register allocation purposes
and assign each web individual pseudo register. This allows the
register allocation pass to operate on pseudos directly, but also
strengthens several other optimization passes, such as CSE, loop
optimizer and trivial dead code remover. It can, however, make
debugging impossible, since variables no longer stay in a "home
register".
Enabled by default with -funroll-loops.
-fwhole-program
Assume that the current compilation unit represents the whole
program being compiled. All public functions and variables with
the exception of "main" and those merged by attribute
"externally_visible" become static functions and in effect are
optimized more aggressively by interprocedural optimizers.
This option should not be used in combination with -flto. Instead
relying on a linker plugin should provide safer and more precise
information.
-flto[=n]
This option runs the standard link-time optimizer. When invoked
with source code, it generates GIMPLE (one of GCC's internal
representations) and writes it to special ELF sections in the
object file. When the object files are linked together, all the
function bodies are read from these ELF sections and instantiated
as if they had been part of the same translation unit.
To use the link-time optimizer, -flto and optimization options
should be specified at compile time and during the final link. It
is recommended that you compile all the files participating in the
same link with the same options and also specify those options at
link time. For example:
gcc -c -O2 -flto foo.c
gcc -c -O2 -flto bar.c
gcc -o myprog -flto -O2 foo.o bar.o
The first two invocations to GCC save a bytecode representation of
GIMPLE into special ELF sections inside foo.o and bar.o. The final
invocation reads the GIMPLE bytecode from foo.o and bar.o, merges
the two files into a single internal image, and compiles the result
as usual. Since both foo.o and bar.o are merged into a single
image, this causes all the interprocedural analyses and
optimizations in GCC to work across the two files as if they were a
single one. This means, for example, that the inliner is able to
inline functions in bar.o into functions in foo.o and vice-versa.
Another (simpler) way to enable link-time optimization is:
gcc -o myprog -flto -O2 foo.c bar.c
The above generates bytecode for foo.c and bar.c, merges them
together into a single GIMPLE representation and optimizes them as
usual to produce myprog.
The important thing to keep in mind is that to enable link-time
optimizations you need to use the GCC driver to perform the link
step. GCC automatically performs link-time optimization if any of
the objects involved were compiled with the -flto command-line
option. You can always override the automatic decision to do link-
time optimization by passing -fno-lto to the link command.
To make whole program optimization effective, it is necessary to
make certain whole program assumptions. The compiler needs to know
what functions and variables can be accessed by libraries and
runtime outside of the link-time optimized unit. When supported by
the linker, the linker plugin (see -fuse-linker-plugin) passes
information to the compiler about used and externally visible
symbols. When the linker plugin is not available, -fwhole-program
should be used to allow the compiler to make these assumptions,
which leads to more aggressive optimization decisions.
When a file is compiled with -flto without -fuse-linker-plugin, the
generated object file is larger than a regular object file because
it contains GIMPLE bytecodes and the usual final code (see
-ffat-lto-objects. This means that object files with LTO
information can be linked as normal object files; if -fno-lto is
passed to the linker, no interprocedural optimizations are applied.
Note that when -fno-fat-lto-objects is enabled the compile stage is
faster but you cannot perform a regular, non-LTO link on them.
When producing the final binary, GCC only applies link-time
optimizations to those files that contain bytecode. Therefore, you
can mix and match object files and libraries with GIMPLE bytecodes
and final object code. GCC automatically selects which files to
optimize in LTO mode and which files to link without further
processing.
Generally, options specified at link time override those specified
at compile time, although in some cases GCC attempts to infer link-
time options from the settings used to compile the input files.
If you do not specify an optimization level option -O at link time,
then GCC uses the highest optimization level used when compiling
the object files. Note that it is generally ineffective to specify
an optimization level option only at link time and not at compile
time, for two reasons. First, compiling without optimization
suppresses compiler passes that gather information needed for
effective optimization at link time. Second, some early
optimization passes can be performed only at compile time and not
at link time.
There are some code generation flags preserved by GCC when
generating bytecodes, as they need to be used during the final
link. Currently, the following options and their settings are
taken from the first object file that explicitly specifies them:
-fPIC, -fpic, -fpie, -fcommon, -fexceptions, -fnon-call-exceptions,
-fgnu-tm and all the -m target flags.
Certain ABI-changing flags are required to match in all compilation
units, and trying to override this at link time with a conflicting
value is ignored. This includes options such as
-freg-struct-return and -fpcc-struct-return.
Other options such as -ffp-contract, -fno-strict-overflow, -fwrapv,
-fno-trapv or -fno-strict-aliasing are passed through to the link
stage and merged conservatively for conflicting translation units.
Specifically -fno-strict-overflow, -fwrapv and -fno-trapv take
precedence; and for example -ffp-contract=off takes precedence over
-ffp-contract=fast. You can override them at link time.
Diagnostic options such as -Wstringop-overflow are passed through
to the link stage and their setting matches that of the compile-
step at function granularity. Note that this matters only for
diagnostics emitted during optimization. Note that code transforms
such as inlining can lead to warnings being enabled or disabled for
regions if code not consistent with the setting at compile time.
When you need to pass options to the assembler via -Wa or
-Xassembler make sure to either compile such translation units with
-fno-lto or consistently use the same assembler options on all
translation units. You can alternatively also specify assembler
options at LTO link time.
To enable debug info generation you need to supply -g at compile
time. If any of the input files at link time were built with debug
info generation enabled the link will enable debug info generation
as well. Any elaborate debug info settings like the dwarf level
-gdwarf-5 need to be explicitly repeated at the linker command line
and mixing different settings in different translation units is
discouraged.
If LTO encounters objects with C linkage declared with incompatible
types in separate translation units to be linked together
(undefined behavior according to ISO C99 6.2.7), a non-fatal
diagnostic may be issued. The behavior is still undefined at run
time. Similar diagnostics may be raised for other languages.
Another feature of LTO is that it is possible to apply
interprocedural optimizations on files written in different
languages:
gcc -c -flto foo.c
g++ -c -flto bar.cc
gfortran -c -flto baz.f90
g++ -o myprog -flto -O3 foo.o bar.o baz.o -lgfortran
Notice that the final link is done with g++ to get the C++ runtime
libraries and -lgfortran is added to get the Fortran runtime
libraries. In general, when mixing languages in LTO mode, you
should use the same link command options as when mixing languages
in a regular (non-LTO) compilation.
If object files containing GIMPLE bytecode are stored in a library
archive, say libfoo.a, it is possible to extract and use them in an
LTO link if you are using a linker with plugin support. To create
static libraries suitable for LTO, use gcc-ar and gcc-ranlib
instead of ar and ranlib; to show the symbols of object files with
GIMPLE bytecode, use gcc-nm. Those commands require that ar,
ranlib and nm have been compiled with plugin support. At link
time, use the flag -fuse-linker-plugin to ensure that the library
participates in the LTO optimization process:
gcc -o myprog -O2 -flto -fuse-linker-plugin a.o b.o -lfoo
With the linker plugin enabled, the linker extracts the needed
GIMPLE files from libfoo.a and passes them on to the running GCC to
make them part of the aggregated GIMPLE image to be optimized.
If you are not using a linker with plugin support and/or do not
enable the linker plugin, then the objects inside libfoo.a are
extracted and linked as usual, but they do not participate in the
LTO optimization process. In order to make a static library
suitable for both LTO optimization and usual linkage, compile its
object files with -flto -ffat-lto-objects.
Link-time optimizations do not require the presence of the whole
program to operate. If the program does not require any symbols to
be exported, it is possible to combine -flto and -fwhole-program to
allow the interprocedural optimizers to use more aggressive
assumptions which may lead to improved optimization opportunities.
Use of -fwhole-program is not needed when linker plugin is active
(see -fuse-linker-plugin).
The current implementation of LTO makes no attempt to generate
bytecode that is portable between different types of hosts. The
bytecode files are versioned and there is a strict version check,
so bytecode files generated in one version of GCC do not work with
an older or newer version of GCC.
Link-time optimization does not work well with generation of
debugging information on systems other than those using a
combination of ELF and DWARF.
If you specify the optional n, the optimization and code generation
done at link time is executed in parallel using n parallel jobs by
utilizing an installed make program. The environment variable MAKE
may be used to override the program used.
You can also specify -flto=jobserver to use GNU make's job server
mode to determine the number of parallel jobs. This is useful when
the Makefile calling GCC is already executing in parallel. You
must prepend a + to the command recipe in the parent Makefile for
this to work. This option likely only works if MAKE is GNU make.
Even without the option value, GCC tries to automatically detect a
running GNU make's job server.
Use -flto=auto to use GNU make's job server, if available, or
otherwise fall back to autodetection of the number of CPU threads
present in your system.
-flto-partition=alg
Specify the partitioning algorithm used by the link-time optimizer.
The value is either 1to1 to specify a partitioning mirroring the
original source files or balanced to specify partitioning into
equally sized chunks (whenever possible) or max to create new
partition for every symbol where possible. Specifying none as an
algorithm disables partitioning and streaming completely. The
default value is balanced. While 1to1 can be used as an workaround
for various code ordering issues, the max partitioning is intended
for internal testing only. The value one specifies that exactly
one partition should be used while the value none bypasses
partitioning and executes the link-time optimization step directly
from the WPA phase.
-flto-compression-level=n
This option specifies the level of compression used for
intermediate language written to LTO object files, and is only
meaningful in conjunction with LTO mode (-flto). Valid values are
0 (no compression) to 9 (maximum compression). Values outside this
range are clamped to either 0 or 9. If the option is not given, a
default balanced compression setting is used.
-fuse-linker-plugin
Enables the use of a linker plugin during link-time optimization.
This option relies on plugin support in the linker, which is
available in gold or in GNU ld 2.21 or newer.
This option enables the extraction of object files with GIMPLE
bytecode out of library archives. This improves the quality of
optimization by exposing more code to the link-time optimizer.
This information specifies what symbols can be accessed externally
(by non-LTO object or during dynamic linking). Resulting code
quality improvements on binaries (and shared libraries that use
hidden visibility) are similar to -fwhole-program. See -flto for a
description of the effect of this flag and how to use it.
This option is enabled by default when LTO support in GCC is
enabled and GCC was configured for use with a linker supporting
plugins (GNU ld 2.21 or newer or gold).
-ffat-lto-objects
Fat LTO objects are object files that contain both the intermediate
language and the object code. This makes them usable for both LTO
linking and normal linking. This option is effective only when
compiling with -flto and is ignored at link time.
-fno-fat-lto-objects improves compilation time over plain LTO, but
requires the complete toolchain to be aware of LTO. It requires a
linker with linker plugin support for basic functionality.
Additionally, nm, ar and ranlib need to support linker plugins to
allow a full-featured build environment (capable of building static
libraries etc). GCC provides the gcc-ar, gcc-nm, gcc-ranlib
wrappers to pass the right options to these tools. With non fat LTO
makefiles need to be modified to use them.
Note that modern binutils provide plugin auto-load mechanism.
Installing the linker plugin into $libdir/bfd-plugins has the same
effect as usage of the command wrappers (gcc-ar, gcc-nm and gcc-
ranlib).
The default is -fno-fat-lto-objects on targets with linker plugin
support.
-fcompare-elim
After register allocation and post-register allocation instruction
splitting, identify arithmetic instructions that compute processor
flags similar to a comparison operation based on that arithmetic.
If possible, eliminate the explicit comparison operation.
This pass only applies to certain targets that cannot explicitly
represent the comparison operation before register allocation is
complete.
Enabled at levels -O, -O2, -O3, -Os.
-fcprop-registers
After register allocation and post-register allocation instruction
splitting, perform a copy-propagation pass to try to reduce
scheduling dependencies and occasionally eliminate the copy.
Enabled at levels -O, -O2, -O3, -Os.
-fprofile-correction
Profiles collected using an instrumented binary for multi-threaded
programs may be inconsistent due to missed counter updates. When
this option is specified, GCC uses heuristics to correct or smooth
out such inconsistencies. By default, GCC emits an error message
when an inconsistent profile is detected.
This option is enabled by -fauto-profile.
-fprofile-partial-training
With "-fprofile-use" all portions of programs not executed during
train run are optimized agressively for size rather than speed. In
some cases it is not practical to train all possible hot paths in
the program. (For example, program may contain functions specific
for a given hardware and trianing may not cover all hardware
configurations program is run on.) With
"-fprofile-partial-training" profile feedback will be ignored for
all functions not executed during the train run leading them to be
optimized as if they were compiled without profile feedback. This
leads to better performance when train run is not representative
but also leads to significantly bigger code.
-fprofile-use
-fprofile-use=path
Enable profile feedback-directed optimizations, and the following
optimizations, many of which are generally profitable only with
profile feedback available:
-fbranch-probabilities -fprofile-values -funroll-loops
-fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops
-funswitch-loops -fgcse-after-reload -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-reorder-functions
Before you can use this option, you must first generate profiling
information.
By default, GCC emits an error message if the feedback profiles do
not match the source code. This error can be turned into a warning
by using -Wno-error=coverage-mismatch. Note this may result in
poorly optimized code. Additionally, by default, GCC also emits a
warning message if the feedback profiles do not exist (see
-Wmissing-profile).
If path is specified, GCC looks at the path to find the profile
feedback data files. See -fprofile-dir.
-fauto-profile
-fauto-profile=path
Enable sampling-based feedback-directed optimizations, and the
following optimizations, many of which are generally profitable
only with profile feedback available:
-fbranch-probabilities -fprofile-values -funroll-loops
-fpeel-loops -ftracer -fvpt -finline-functions -fipa-cp
-fipa-cp-clone -fipa-bit-cp -fpredictive-commoning -fsplit-loops
-funswitch-loops -fgcse-after-reload -ftree-loop-vectorize
-ftree-slp-vectorize -fvect-cost-model=dynamic
-ftree-loop-distribute-patterns -fprofile-correction
path is the name of a file containing AutoFDO profile information.
If omitted, it defaults to fbdata.afdo in the current directory.
Producing an AutoFDO profile data file requires running your
program with the perf utility on a supported GNU/Linux target
system. For more information, see <https://perf.wiki.kernel.org/>.
E.g.
perf record -e br_inst_retired:near_taken -b -o perf.data \
-- your_program
Then use the create_gcov tool to convert the raw profile data to a
format that can be used by GCC. You must also supply the
unstripped binary for your program to this tool. See
<https://github.com/google/autofdo>.
E.g.
create_gcov --binary=your_program.unstripped --profile=perf.data \
--gcov=profile.afdo
The following options control compiler behavior regarding floating-
point arithmetic. These options trade off between speed and
correctness. All must be specifically enabled.
-ffloat-store
Do not store floating-point variables in registers, and inhibit
other options that might change whether a floating-point value is
taken from a register or memory.
This option prevents undesirable excess precision on machines such
as the 68000 where the floating registers (of the 68881) keep more
precision than a "double" is supposed to have. Similarly for the
x86 architecture. For most programs, the excess precision does
only good, but a few programs rely on the precise definition of
IEEE floating point. Use -ffloat-store for such programs, after
modifying them to store all pertinent intermediate computations
into variables.
-fexcess-precision=style
This option allows further control over excess precision on
machines where floating-point operations occur in a format with
more precision or range than the IEEE standard and interchange
floating-point types. By default, -fexcess-precision=fast is in
effect; this means that operations may be carried out in a wider
precision than the types specified in the source if that would
result in faster code, and it is unpredictable when rounding to the
types specified in the source code takes place. When compiling C,
if -fexcess-precision=standard is specified then excess precision
follows the rules specified in ISO C99; in particular, both casts
and assignments cause values to be rounded to their semantic types
(whereas -ffloat-store only affects assignments). This option is
enabled by default for C if a strict conformance option such as
-std=c99 is used. -ffast-math enables -fexcess-precision=fast by
default regardless of whether a strict conformance option is used.
-fexcess-precision=standard is not implemented for languages other
than C. On the x86, it has no effect if -mfpmath=sse or
-mfpmath=sse+387 is specified; in the former case, IEEE semantics
apply without excess precision, and in the latter, rounding is
unpredictable.
-ffast-math
Sets the options -fno-math-errno, -funsafe-math-optimizations,
-ffinite-math-only, -fno-rounding-math, -fno-signaling-nans,
-fcx-limited-range and -fexcess-precision=fast.
This option causes the preprocessor macro "__FAST_MATH__" to be
defined.
This option is not turned on by any -O option besides -Ofast since
it can result in incorrect output for programs that depend on an
exact implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do
not require the guarantees of these specifications.
-fno-math-errno
Do not set "errno" after calling math functions that are executed
with a single instruction, e.g., "sqrt". A program that relies on
IEEE exceptions for math error handling may want to use this flag
for speed while maintaining IEEE arithmetic compatibility.
This option is not turned on by any -O option since it can result
in incorrect output for programs that depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do
not require the guarantees of these specifications.
The default is -fmath-errno.
On Darwin systems, the math library never sets "errno". There is
therefore no reason for the compiler to consider the possibility
that it might, and -fno-math-errno is the default.
-funsafe-math-optimizations
Allow optimizations for floating-point arithmetic that (a) assume
that arguments and results are valid and (b) may violate IEEE or
ANSI standards. When used at link time, it may include libraries
or startup files that change the default FPU control word or other
similar optimizations.
This option is not turned on by any -O option since it can result
in incorrect output for programs that depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do
not require the guarantees of these specifications. Enables
-fno-signed-zeros, -fno-trapping-math, -fassociative-math and
-freciprocal-math.
The default is -fno-unsafe-math-optimizations.
-fassociative-math
Allow re-association of operands in series of floating-point
operations. This violates the ISO C and C++ language standard by
possibly changing computation result. NOTE: re-ordering may change
the sign of zero as well as ignore NaNs and inhibit or create
underflow or overflow (and thus cannot be used on code that relies
on rounding behavior like "(x + 2**52) - 2**52". May also reorder
floating-point comparisons and thus may not be used when ordered
comparisons are required. This option requires that both
-fno-signed-zeros and -fno-trapping-math be in effect. Moreover,
it doesn't make much sense with -frounding-math. For Fortran the
option is automatically enabled when both -fno-signed-zeros and
-fno-trapping-math are in effect.
The default is -fno-associative-math.
-freciprocal-math
Allow the reciprocal of a value to be used instead of dividing by
the value if this enables optimizations. For example "x / y" can
be replaced with "x * (1/y)", which is useful if "(1/y)" is subject
to common subexpression elimination. Note that this loses
precision and increases the number of flops operating on the value.
The default is -fno-reciprocal-math.
-ffinite-math-only
Allow optimizations for floating-point arithmetic that assume that
arguments and results are not NaNs or +-Infs.
This option is not turned on by any -O option since it can result
in incorrect output for programs that depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions. It may, however, yield faster code for programs that do
not require the guarantees of these specifications.
The default is -fno-finite-math-only.
-fno-signed-zeros
Allow optimizations for floating-point arithmetic that ignore the
signedness of zero. IEEE arithmetic specifies the behavior of
distinct +0.0 and -0.0 values, which then prohibits simplification
of expressions such as x+0.0 or 0.0*x (even with
-ffinite-math-only). This option implies that the sign of a zero
result isn't significant.
The default is -fsigned-zeros.
-fno-trapping-math
Compile code assuming that floating-point operations cannot
generate user-visible traps. These traps include division by zero,
overflow, underflow, inexact result and invalid operation. This
option requires that -fno-signaling-nans be in effect. Setting
this option may allow faster code if one relies on "non-stop" IEEE
arithmetic, for example.
This option should never be turned on by any -O option since it can
result in incorrect output for programs that depend on an exact
implementation of IEEE or ISO rules/specifications for math
functions.
The default is -ftrapping-math.
-frounding-math
Disable transformations and optimizations that assume default
floating-point rounding behavior. This is round-to-zero for all
floating point to integer conversions, and round-to-nearest for all
other arithmetic truncations. This option should be specified for
programs that change the FP rounding mode dynamically, or that may
be executed with a non-default rounding mode. This option disables
constant folding of floating-point expressions at compile time
(which may be affected by rounding mode) and arithmetic
transformations that are unsafe in the presence of sign-dependent
rounding modes.
The default is -fno-rounding-math.
This option is experimental and does not currently guarantee to
disable all GCC optimizations that are affected by rounding mode.
Future versions of GCC may provide finer control of this setting
using C99's "FENV_ACCESS" pragma. This command-line option will be
used to specify the default state for "FENV_ACCESS".
-fsignaling-nans
Compile code assuming that IEEE signaling NaNs may generate user-
visible traps during floating-point operations. Setting this
option disables optimizations that may change the number of
exceptions visible with signaling NaNs. This option implies
-ftrapping-math.
This option causes the preprocessor macro "__SUPPORT_SNAN__" to be
defined.
The default is -fno-signaling-nans.
This option is experimental and does not currently guarantee to
disable all GCC optimizations that affect signaling NaN behavior.
-fno-fp-int-builtin-inexact
Do not allow the built-in functions "ceil", "floor", "round" and
"trunc", and their "float" and "long double" variants, to generate
code that raises the "inexact" floating-point exception for
noninteger arguments. ISO C99 and C11 allow these functions to
raise the "inexact" exception, but ISO/IEC TS 18661-1:2014, the C
bindings to IEEE 754-2008, as integrated into ISO C2X, does not
allow these functions to do so.
The default is -ffp-int-builtin-inexact, allowing the exception to
be raised, unless C2X or a later C standard is selected. This
option does nothing unless -ftrapping-math is in effect.
Even if -fno-fp-int-builtin-inexact is used, if the functions
generate a call to a library function then the "inexact" exception
may be raised if the library implementation does not follow TS
18661.
-fsingle-precision-constant
Treat floating-point constants as single precision instead of
implicitly converting them to double-precision constants.
-fcx-limited-range
When enabled, this option states that a range reduction step is not
needed when performing complex division. Also, there is no
checking whether the result of a complex multiplication or division
is "NaN + I*NaN", with an attempt to rescue the situation in that
case. The default is -fno-cx-limited-range, but is enabled by
-ffast-math.
This option controls the default setting of the ISO C99
"CX_LIMITED_RANGE" pragma. Nevertheless, the option applies to all
languages.
-fcx-fortran-rules
Complex multiplication and division follow Fortran rules. Range
reduction is done as part of complex division, but there is no
checking whether the result of a complex multiplication or division
is "NaN + I*NaN", with an attempt to rescue the situation in that
case.
The default is -fno-cx-fortran-rules.
The following options control optimizations that may improve
performance, but are not enabled by any -O options. This section
includes experimental options that may produce broken code.
-fbranch-probabilities
After running a program compiled with -fprofile-arcs, you can
compile it a second time using -fbranch-probabilities, to improve
optimizations based on the number of times each branch was taken.
When a program compiled with -fprofile-arcs exits, it saves arc
execution counts to a file called sourcename.gcda for each source
file. The information in this data file is very dependent on the
structure of the generated code, so you must use the same source
code and the same optimization options for both compilations.
With -fbranch-probabilities, GCC puts a REG_BR_PROB note on each
JUMP_INSN and CALL_INSN. These can be used to improve
optimization. Currently, they are only used in one place: in
reorg.c, instead of guessing which path a branch is most likely to
take, the REG_BR_PROB values are used to exactly determine which
path is taken more often.
Enabled by -fprofile-use and -fauto-profile.
-fprofile-values
If combined with -fprofile-arcs, it adds code so that some data
about values of expressions in the program is gathered.
With -fbranch-probabilities, it reads back the data gathered from
profiling values of expressions for usage in optimizations.
Enabled by -fprofile-generate, -fprofile-use, and -fauto-profile.
-fprofile-reorder-functions
Function reordering based on profile instrumentation collects first
time of execution of a function and orders these functions in
ascending order.
Enabled with -fprofile-use.
-fvpt
If combined with -fprofile-arcs, this option instructs the compiler
to add code to gather information about values of expressions.
With -fbranch-probabilities, it reads back the data gathered and
actually performs the optimizations based on them. Currently the
optimizations include specialization of division operations using
the knowledge about the value of the denominator.
Enabled with -fprofile-use and -fauto-profile.
-frename-registers
Attempt to avoid false dependencies in scheduled code by making use
of registers left over after register allocation. This
optimization most benefits processors with lots of registers.
Depending on the debug information format adopted by the target,
however, it can make debugging impossible, since variables no
longer stay in a "home register".
Enabled by default with -funroll-loops.
-fschedule-fusion
Performs a target dependent pass over the instruction stream to
schedule instructions of same type together because target machine
can execute them more efficiently if they are adjacent to each
other in the instruction flow.
Enabled at levels -O2, -O3, -Os.
-ftracer
Perform tail duplication to enlarge superblock size. This
transformation simplifies the control flow of the function allowing
other optimizations to do a better job.
Enabled by -fprofile-use and -fauto-profile.
-funroll-loops
Unroll loops whose number of iterations can be determined at
compile time or upon entry to the loop. -funroll-loops implies
-frerun-cse-after-loop, -fweb and -frename-registers. It also
turns on complete loop peeling (i.e. complete removal of loops with
a small constant number of iterations). This option makes code
larger, and may or may not make it run faster.
Enabled by -fprofile-use and -fauto-profile.
-funroll-all-loops
Unroll all loops, even if their number of iterations is uncertain
when the loop is entered. This usually makes programs run more
slowly. -funroll-all-loops implies the same options as
-funroll-loops.
-fpeel-loops
Peels loops for which there is enough information that they do not
roll much (from profile feedback or static analysis). It also
turns on complete loop peeling (i.e. complete removal of loops with
small constant number of iterations).
Enabled by -O3, -fprofile-use, and -fauto-profile.
-fmove-loop-invariants
Enables the loop invariant motion pass in the RTL loop optimizer.
Enabled at level -O1 and higher, except for -Og.
-fsplit-loops
Split a loop into two if it contains a condition that's always true
for one side of the iteration space and false for the other.
Enabled by -fprofile-use and -fauto-profile.
-funswitch-loops
Move branches with loop invariant conditions out of the loop, with
duplicates of the loop on both branches (modified according to
result of the condition).
Enabled by -fprofile-use and -fauto-profile.
-fversion-loops-for-strides
If a loop iterates over an array with a variable stride, create
another version of the loop that assumes the stride is always one.
For example:
for (int i = 0; i < n; ++i)
x[i * stride] = ...;
becomes:
if (stride == 1)
for (int i = 0; i < n; ++i)
x[i] = ...;
else
for (int i = 0; i < n; ++i)
x[i * stride] = ...;
This is particularly useful for assumed-shape arrays in Fortran
where (for example) it allows better vectorization assuming
contiguous accesses. This flag is enabled by default at -O3. It
is also enabled by -fprofile-use and -fauto-profile.
-ffunction-sections
-fdata-sections
Place each function or data item into its own section in the output
file if the target supports arbitrary sections. The name of the
function or the name of the data item determines the section's name
in the output file.
Use these options on systems where the linker can perform
optimizations to improve locality of reference in the instruction
space. Most systems using the ELF object format have linkers with
such optimizations. On AIX, the linker rearranges sections
(CSECTs) based on the call graph. The performance impact varies.
Together with a linker garbage collection (linker --gc-sections
option) these options may lead to smaller statically-linked
executables (after stripping).
On ELF/DWARF systems these options do not degenerate the quality of
the debug information. There could be issues with other object
files/debug info formats.
Only use these options when there are significant benefits from
doing so. When you specify these options, the assembler and linker
create larger object and executable files and are also slower.
These options affect code generation. They prevent optimizations
by the compiler and assembler using relative locations inside a
translation unit since the locations are unknown until link time.
An example of such an optimization is relaxing calls to short call
instructions.
-fstdarg-opt
Optimize the prologue of variadic argument functions with respect
to usage of those arguments.
-fsection-anchors
Try to reduce the number of symbolic address calculations by using
shared "anchor" symbols to address nearby objects. This
transformation can help to reduce the number of GOT entries and GOT
accesses on some targets.
For example, the implementation of the following function "foo":
static int a, b, c;
int foo (void) { return a + b + c; }
usually calculates the addresses of all three variables, but if you
compile it with -fsection-anchors, it accesses the variables from a
common anchor point instead. The effect is similar to the
following pseudocode (which isn't valid C):
int foo (void)
{
register int *xr = &x;
return xr[&a - &x] + xr[&b - &x] + xr[&c - &x];
}
Not all targets support this option.
--param name=value
In some places, GCC uses various constants to control the amount of
optimization that is done. For example, GCC does not inline
functions that contain more than a certain number of instructions.
You can control some of these constants on the command line using
the --param option.
The names of specific parameters, and the meaning of the values,
are tied to the internals of the compiler, and are subject to
change without notice in future releases.
In order to get minimal, maximal and default value of a parameter,
one can use --help=param -Q options.
In each case, the value is an integer. The following choices of
name are recognized for all targets:
predictable-branch-outcome
When branch is predicted to be taken with probability lower
than this threshold (in percent), then it is considered well
predictable.
max-rtl-if-conversion-insns
RTL if-conversion tries to remove conditional branches around a
block and replace them with conditionally executed
instructions. This parameter gives the maximum number of
instructions in a block which should be considered for if-
conversion. The compiler will also use other heuristics to
decide whether if-conversion is likely to be profitable.
max-rtl-if-conversion-predictable-cost
max-rtl-if-conversion-unpredictable-cost
RTL if-conversion will try to remove conditional branches
around a block and replace them with conditionally executed
instructions. These parameters give the maximum permissible
cost for the sequence that would be generated by if-conversion
depending on whether the branch is statically determined to be
predictable or not. The units for this parameter are the same
as those for the GCC internal seq_cost metric. The compiler
will try to provide a reasonable default for this parameter
using the BRANCH_COST target macro.
max-crossjump-edges
The maximum number of incoming edges to consider for cross-
jumping. The algorithm used by -fcrossjumping is O(N^2) in the
number of edges incoming to each block. Increasing values mean
more aggressive optimization, making the compilation time
increase with probably small improvement in executable size.
min-crossjump-insns
The minimum number of instructions that must be matched at the
end of two blocks before cross-jumping is performed on them.
This value is ignored in the case where all instructions in the
block being cross-jumped from are matched.
max-grow-copy-bb-insns
The maximum code size expansion factor when copying basic
blocks instead of jumping. The expansion is relative to a jump
instruction.
max-goto-duplication-insns
The maximum number of instructions to duplicate to a block that
jumps to a computed goto. To avoid O(N^2) behavior in a number
of passes, GCC factors computed gotos early in the compilation
process, and unfactors them as late as possible. Only computed
jumps at the end of a basic blocks with no more than max-goto-
duplication-insns are unfactored.
max-delay-slot-insn-search
The maximum number of instructions to consider when looking for
an instruction to fill a delay slot. If more than this
arbitrary number of instructions are searched, the time savings
from filling the delay slot are minimal, so stop searching.
Increasing values mean more aggressive optimization, making the
compilation time increase with probably small improvement in
execution time.
max-delay-slot-live-search
When trying to fill delay slots, the maximum number of
instructions to consider when searching for a block with valid
live register information. Increasing this arbitrarily chosen
value means more aggressive optimization, increasing the
compilation time. This parameter should be removed when the
delay slot code is rewritten to maintain the control-flow
graph.
max-gcse-memory
The approximate maximum amount of memory that can be allocated
in order to perform the global common subexpression elimination
optimization. If more memory than specified is required, the
optimization is not done.
max-gcse-insertion-ratio
If the ratio of expression insertions to deletions is larger
than this value for any expression, then RTL PRE inserts or
removes the expression and thus leaves partially redundant
computations in the instruction stream.
max-pending-list-length
The maximum number of pending dependencies scheduling allows
before flushing the current state and starting over. Large
functions with few branches or calls can create excessively
large lists which needlessly consume memory and resources.
max-modulo-backtrack-attempts
The maximum number of backtrack attempts the scheduler should
make when modulo scheduling a loop. Larger values can
exponentially increase compilation time.
max-inline-insns-single
Several parameters control the tree inliner used in GCC. This
number sets the maximum number of instructions (counted in
GCC's internal representation) in a single function that the
tree inliner considers for inlining. This only affects
functions declared inline and methods implemented in a class
declaration (C++).
max-inline-insns-auto
When you use -finline-functions (included in -O3), a lot of
functions that would otherwise not be considered for inlining
by the compiler are investigated. To those functions, a
different (more restrictive) limit compared to functions
declared inline can be applied (--param max-inline-insns-auto).
max-inline-insns-small
This is bound applied to calls which are considered relevant
with -finline-small-functions.
max-inline-insns-size
This is bound applied to calls which are optimized for size.
Small growth may be desirable to anticipate optimization
oppurtunities exposed by inlining.
uninlined-function-insns
Number of instructions accounted by inliner for function
overhead such as function prologue and epilogue.
uninlined-function-time
Extra time accounted by inliner for function overhead such as
time needed to execute function prologue and epilogue
inline-heuristics-hint-percent
The scale (in percents) applied to inline-insns-single,
inline-insns-single-O2, inline-insns-auto when inline
heuristics hints that inlining is very profitable (will enable
later optimizations).
uninlined-thunk-insns
uninlined-thunk-time
Same as --param uninlined-function-insns and --param uninlined-
function-time but applied to function thunks
inline-min-speedup
When estimated performance improvement of caller + callee
runtime exceeds this threshold (in percent), the function can
be inlined regardless of the limit on --param max-inline-insns-
single and --param max-inline-insns-auto.
large-function-insns
The limit specifying really large functions. For functions
larger than this limit after inlining, inlining is constrained
by --param large-function-growth. This parameter is useful
primarily to avoid extreme compilation time caused by non-
linear algorithms used by the back end.
large-function-growth
Specifies maximal growth of large function caused by inlining
in percents. For example, parameter value 100 limits large
function growth to 2.0 times the original size.
large-unit-insns
The limit specifying large translation unit. Growth caused by
inlining of units larger than this limit is limited by --param
inline-unit-growth. For small units this might be too tight.
For example, consider a unit consisting of function A that is
inline and B that just calls A three times. If B is small
relative to A, the growth of unit is 300\% and yet such
inlining is very sane. For very large units consisting of
small inlineable functions, however, the overall unit growth
limit is needed to avoid exponential explosion of code size.
Thus for smaller units, the size is increased to --param large-
unit-insns before applying --param inline-unit-growth.
inline-unit-growth
Specifies maximal overall growth of the compilation unit caused
by inlining. For example, parameter value 20 limits unit
growth to 1.2 times the original size. Cold functions (either
marked cold via an attribute or by profile feedback) are not
accounted into the unit size.
ipa-cp-unit-growth
Specifies maximal overall growth of the compilation unit caused
by interprocedural constant propagation. For example,
parameter value 10 limits unit growth to 1.1 times the original
size.
large-stack-frame
The limit specifying large stack frames. While inlining the
algorithm is trying to not grow past this limit too much.
large-stack-frame-growth
Specifies maximal growth of large stack frames caused by
inlining in percents. For example, parameter value 1000 limits
large stack frame growth to 11 times the original size.
max-inline-insns-recursive
max-inline-insns-recursive-auto
Specifies the maximum number of instructions an out-of-line
copy of a self-recursive inline function can grow into by
performing recursive inlining.
--param max-inline-insns-recursive applies to functions
declared inline. For functions not declared inline, recursive
inlining happens only when -finline-functions (included in -O3)
is enabled; --param max-inline-insns-recursive-auto applies
instead.
max-inline-recursive-depth
max-inline-recursive-depth-auto
Specifies the maximum recursion depth used for recursive
inlining.
--param max-inline-recursive-depth applies to functions
declared inline. For functions not declared inline, recursive
inlining happens only when -finline-functions (included in -O3)
is enabled; --param max-inline-recursive-depth-auto applies
instead.
min-inline-recursive-probability
Recursive inlining is profitable only for function having deep
recursion in average and can hurt for function having little
recursion depth by increasing the prologue size or complexity
of function body to other optimizers.
When profile feedback is available (see -fprofile-generate) the
actual recursion depth can be guessed from the probability that
function recurses via a given call expression. This parameter
limits inlining only to call expressions whose probability
exceeds the given threshold (in percents).
early-inlining-insns
Specify growth that the early inliner can make. In effect it
increases the amount of inlining for code having a large
abstraction penalty.
max-early-inliner-iterations
Limit of iterations of the early inliner. This basically
bounds the number of nested indirect calls the early inliner
can resolve. Deeper chains are still handled by late inlining.
comdat-sharing-probability
Probability (in percent) that C++ inline function with comdat
visibility are shared across multiple compilation units.
profile-func-internal-id
A parameter to control whether to use function internal id in
profile database lookup. If the value is 0, the compiler uses
an id that is based on function assembler name and filename,
which makes old profile data more tolerant to source changes
such as function reordering etc.
min-vect-loop-bound
The minimum number of iterations under which loops are not
vectorized when -ftree-vectorize is used. The number of
iterations after vectorization needs to be greater than the
value specified by this option to allow vectorization.
gcse-cost-distance-ratio
Scaling factor in calculation of maximum distance an expression
can be moved by GCSE optimizations. This is currently
supported only in the code hoisting pass. The bigger the
ratio, the more aggressive code hoisting is with simple
expressions, i.e., the expressions that have cost less than
gcse-unrestricted-cost. Specifying 0 disables hoisting of
simple expressions.
gcse-unrestricted-cost
Cost, roughly measured as the cost of a single typical machine
instruction, at which GCSE optimizations do not constrain the
distance an expression can travel. This is currently supported
only in the code hoisting pass. The lesser the cost, the more
aggressive code hoisting is. Specifying 0 allows all
expressions to travel unrestricted distances.
max-hoist-depth
The depth of search in the dominator tree for expressions to
hoist. This is used to avoid quadratic behavior in hoisting
algorithm. The value of 0 does not limit on the search, but
may slow down compilation of huge functions.
max-tail-merge-comparisons
The maximum amount of similar bbs to compare a bb with. This
is used to avoid quadratic behavior in tree tail merging.
max-tail-merge-iterations
The maximum amount of iterations of the pass over the function.
This is used to limit compilation time in tree tail merging.
store-merging-allow-unaligned
Allow the store merging pass to introduce unaligned stores if
it is legal to do so.
max-stores-to-merge
The maximum number of stores to attempt to merge into wider
stores in the store merging pass.
max-unrolled-insns
The maximum number of instructions that a loop may have to be
unrolled. If a loop is unrolled, this parameter also
determines how many times the loop code is unrolled.
max-average-unrolled-insns
The maximum number of instructions biased by probabilities of
their execution that a loop may have to be unrolled. If a loop
is unrolled, this parameter also determines how many times the
loop code is unrolled.
max-unroll-times
The maximum number of unrollings of a single loop.
max-peeled-insns
The maximum number of instructions that a loop may have to be
peeled. If a loop is peeled, this parameter also determines
how many times the loop code is peeled.
max-peel-times
The maximum number of peelings of a single loop.
max-peel-branches
The maximum number of branches on the hot path through the
peeled sequence.
max-completely-peeled-insns
The maximum number of insns of a completely peeled loop.
max-completely-peel-times
The maximum number of iterations of a loop to be suitable for
complete peeling.
max-completely-peel-loop-nest-depth
The maximum depth of a loop nest suitable for complete peeling.
max-unswitch-insns
The maximum number of insns of an unswitched loop.
max-unswitch-level
The maximum number of branches unswitched in a single loop.
lim-expensive
The minimum cost of an expensive expression in the loop
invariant motion.
min-loop-cond-split-prob
When FDO profile information is available, min-loop-cond-split-
prob specifies minimum threshold for probability of semi-
invariant condition statement to trigger loop split.
iv-consider-all-candidates-bound
Bound on number of candidates for induction variables, below
which all candidates are considered for each use in induction
variable optimizations. If there are more candidates than
this, only the most relevant ones are considered to avoid
quadratic time complexity.
iv-max-considered-uses
The induction variable optimizations give up on loops that
contain more induction variable uses.
iv-always-prune-cand-set-bound
If the number of candidates in the set is smaller than this
value, always try to remove unnecessary ivs from the set when
adding a new one.
avg-loop-niter
Average number of iterations of a loop.
dse-max-object-size
Maximum size (in bytes) of objects tracked bytewise by dead
store elimination. Larger values may result in larger
compilation times.
dse-max-alias-queries-per-store
Maximum number of queries into the alias oracle per store.
Larger values result in larger compilation times and may result
in more removed dead stores.
scev-max-expr-size
Bound on size of expressions used in the scalar evolutions
analyzer. Large expressions slow the analyzer.
scev-max-expr-complexity
Bound on the complexity of the expressions in the scalar
evolutions analyzer. Complex expressions slow the analyzer.
max-tree-if-conversion-phi-args
Maximum number of arguments in a PHI supported by TREE if
conversion unless the loop is marked with simd pragma.
vect-max-version-for-alignment-checks
The maximum number of run-time checks that can be performed
when doing loop versioning for alignment in the vectorizer.
vect-max-version-for-alias-checks
The maximum number of run-time checks that can be performed
when doing loop versioning for alias in the vectorizer.
vect-max-peeling-for-alignment
The maximum number of loop peels to enhance access alignment
for vectorizer. Value -1 means no limit.
max-iterations-to-track
The maximum number of iterations of a loop the brute-force
algorithm for analysis of the number of iterations of the loop
tries to evaluate.
hot-bb-count-fraction
The denominator n of fraction 1/n of the maximal execution
count of a basic block in the entire program that a basic block
needs to at least have in order to be considered hot. The
default is 10000, which means that a basic block is considered
hot if its execution count is greater than 1/10000 of the
maximal execution count. 0 means that it is never considered
hot. Used in non-LTO mode.
hot-bb-count-ws-permille
The number of most executed permilles, ranging from 0 to 1000,
of the profiled execution of the entire program to which the
execution count of a basic block must be part of in order to be
considered hot. The default is 990, which means that a basic
block is considered hot if its execution count contributes to
the upper 990 permilles, or 99.0%, of the profiled execution of
the entire program. 0 means that it is never considered hot.
Used in LTO mode.
hot-bb-frequency-fraction
The denominator n of fraction 1/n of the execution frequency of
the entry block of a function that a basic block of this
function needs to at least have in order to be considered hot.
The default is 1000, which means that a basic block is
considered hot in a function if it is executed more frequently
than 1/1000 of the frequency of the entry block of the
function. 0 means that it is never considered hot.
unlikely-bb-count-fraction
The denominator n of fraction 1/n of the number of profiled
runs of the entire program below which the execution count of a
basic block must be in order for the basic block to be
considered unlikely executed. The default is 20, which means
that a basic block is considered unlikely executed if it is
executed in fewer than 1/20, or 5%, of the runs of the program.
0 means that it is always considered unlikely executed.
max-predicted-iterations
The maximum number of loop iterations we predict statically.
This is useful in cases where a function contains a single loop
with known bound and another loop with unknown bound. The
known number of iterations is predicted correctly, while the
unknown number of iterations average to roughly 10. This means
that the loop without bounds appears artificially cold relative
to the other one.
builtin-expect-probability
Control the probability of the expression having the specified
value. This parameter takes a percentage (i.e. 0 ... 100) as
input.
builtin-string-cmp-inline-length
The maximum length of a constant string for a builtin string
cmp call eligible for inlining.
align-threshold
Select fraction of the maximal frequency of executions of a
basic block in a function to align the basic block.
align-loop-iterations
A loop expected to iterate at least the selected number of
iterations is aligned.
tracer-dynamic-coverage
tracer-dynamic-coverage-feedback
This value is used to limit superblock formation once the given
percentage of executed instructions is covered. This limits
unnecessary code size expansion.
The tracer-dynamic-coverage-feedback parameter is used only
when profile feedback is available. The real profiles (as
opposed to statically estimated ones) are much less balanced
allowing the threshold to be larger value.
tracer-max-code-growth
Stop tail duplication once code growth has reached given
percentage. This is a rather artificial limit, as most of the
duplicates are eliminated later in cross jumping, so it may be
set to much higher values than is the desired code growth.
tracer-min-branch-ratio
Stop reverse growth when the reverse probability of best edge
is less than this threshold (in percent).
tracer-min-branch-probability
tracer-min-branch-probability-feedback
Stop forward growth if the best edge has probability lower than
this threshold.
Similarly to tracer-dynamic-coverage two parameters are
provided. tracer-min-branch-probability-feedback is used for
compilation with profile feedback and tracer-min-branch-
probability compilation without. The value for compilation
with profile feedback needs to be more conservative (higher) in
order to make tracer effective.
stack-clash-protection-guard-size
Specify the size of the operating system provided stack guard
as 2 raised to num bytes. Higher values may reduce the number
of explicit probes, but a value larger than the operating
system provided guard will leave code vulnerable to stack clash
style attacks.
stack-clash-protection-probe-interval
Stack clash protection involves probing stack space as it is
allocated. This param controls the maximum distance between
probes into the stack as 2 raised to num bytes. Higher values
may reduce the number of explicit probes, but a value larger
than the operating system provided guard will leave code
vulnerable to stack clash style attacks.
max-cse-path-length
The maximum number of basic blocks on path that CSE considers.
max-cse-insns
The maximum number of instructions CSE processes before
flushing.
ggc-min-expand
GCC uses a garbage collector to manage its own memory
allocation. This parameter specifies the minimum percentage by
which the garbage collector's heap should be allowed to expand
between collections. Tuning this may improve compilation
speed; it has no effect on code generation.
The default is 30% + 70% * (RAM/1GB) with an upper bound of
100% when RAM >= 1GB. If "getrlimit" is available, the notion
of "RAM" is the smallest of actual RAM and "RLIMIT_DATA" or
"RLIMIT_AS". If GCC is not able to calculate RAM on a
particular platform, the lower bound of 30% is used. Setting
this parameter and ggc-min-heapsize to zero causes a full
collection to occur at every opportunity. This is extremely
slow, but can be useful for debugging.
ggc-min-heapsize
Minimum size of the garbage collector's heap before it begins
bothering to collect garbage. The first collection occurs
after the heap expands by ggc-min-expand% beyond ggc-min-
heapsize. Again, tuning this may improve compilation speed,
and has no effect on code generation.
The default is the smaller of RAM/8, RLIMIT_RSS, or a limit
that tries to ensure that RLIMIT_DATA or RLIMIT_AS are not
exceeded, but with a lower bound of 4096 (four megabytes) and
an upper bound of 131072 (128 megabytes). If GCC is not able
to calculate RAM on a particular platform, the lower bound is
used. Setting this parameter very large effectively disables
garbage collection. Setting this parameter and ggc-min-expand
to zero causes a full collection to occur at every opportunity.
max-reload-search-insns
The maximum number of instruction reload should look backward
for equivalent register. Increasing values mean more
aggressive optimization, making the compilation time increase
with probably slightly better performance.
max-cselib-memory-locations
The maximum number of memory locations cselib should take into
account. Increasing values mean more aggressive optimization,
making the compilation time increase with probably slightly
better performance.
max-sched-ready-insns
The maximum number of instructions ready to be issued the
scheduler should consider at any given time during the first
scheduling pass. Increasing values mean more thorough
searches, making the compilation time increase with probably
little benefit.
max-sched-region-blocks
The maximum number of blocks in a region to be considered for
interblock scheduling.
max-pipeline-region-blocks
The maximum number of blocks in a region to be considered for
pipelining in the selective scheduler.
max-sched-region-insns
The maximum number of insns in a region to be considered for
interblock scheduling.
max-pipeline-region-insns
The maximum number of insns in a region to be considered for
pipelining in the selective scheduler.
min-spec-prob
The minimum probability (in percents) of reaching a source
block for interblock speculative scheduling.
max-sched-extend-regions-iters
The maximum number of iterations through CFG to extend regions.
A value of 0 disables region extensions.
max-sched-insn-conflict-delay
The maximum conflict delay for an insn to be considered for
speculative motion.
sched-spec-prob-cutoff
The minimal probability of speculation success (in percents),
so that speculative insns are scheduled.
sched-state-edge-prob-cutoff
The minimum probability an edge must have for the scheduler to
save its state across it.
sched-mem-true-dep-cost
Minimal distance (in CPU cycles) between store and load
targeting same memory locations.
selsched-max-lookahead
The maximum size of the lookahead window of selective
scheduling. It is a depth of search for available
instructions.
selsched-max-sched-times
The maximum number of times that an instruction is scheduled
during selective scheduling. This is the limit on the number
of iterations through which the instruction may be pipelined.
selsched-insns-to-rename
The maximum number of best instructions in the ready list that
are considered for renaming in the selective scheduler.
sms-min-sc
The minimum value of stage count that swing modulo scheduler
generates.
max-last-value-rtl
The maximum size measured as number of RTLs that can be
recorded in an expression in combiner for a pseudo register as
last known value of that register.
max-combine-insns
The maximum number of instructions the RTL combiner tries to
combine.
integer-share-limit
Small integer constants can use a shared data structure,
reducing the compiler's memory usage and increasing its speed.
This sets the maximum value of a shared integer constant.
ssp-buffer-size
The minimum size of buffers (i.e. arrays) that receive stack
smashing protection when -fstack-protection is used.
min-size-for-stack-sharing
The minimum size of variables taking part in stack slot sharing
when not optimizing.
max-jump-thread-duplication-stmts
Maximum number of statements allowed in a block that needs to
be duplicated when threading jumps.
max-fields-for-field-sensitive
Maximum number of fields in a structure treated in a field
sensitive manner during pointer analysis.
prefetch-latency
Estimate on average number of instructions that are executed
before prefetch finishes. The distance prefetched ahead is
proportional to this constant. Increasing this number may also
lead to less streams being prefetched (see simultaneous-
prefetches).
simultaneous-prefetches
Maximum number of prefetches that can run at the same time.
l1-cache-line-size
The size of cache line in L1 data cache, in bytes.
l1-cache-size
The size of L1 data cache, in kilobytes.
l2-cache-size
The size of L2 data cache, in kilobytes.
prefetch-dynamic-strides
Whether the loop array prefetch pass should issue software
prefetch hints for strides that are non-constant. In some
cases this may be beneficial, though the fact the stride is
non-constant may make it hard to predict when there is clear
benefit to issuing these hints.
Set to 1 if the prefetch hints should be issued for non-
constant strides. Set to 0 if prefetch hints should be issued
only for strides that are known to be constant and below
prefetch-minimum-stride.
prefetch-minimum-stride
Minimum constant stride, in bytes, to start using prefetch
hints for. If the stride is less than this threshold, prefetch
hints will not be issued.
This setting is useful for processors that have hardware
prefetchers, in which case there may be conflicts between the
hardware prefetchers and the software prefetchers. If the
hardware prefetchers have a maximum stride they can handle, it
should be used here to improve the use of software prefetchers.
A value of -1 means we don't have a threshold and therefore
prefetch hints can be issued for any constant stride.
This setting is only useful for strides that are known and
constant.
loop-interchange-max-num-stmts
The maximum number of stmts in a loop to be interchanged.
loop-interchange-stride-ratio
The minimum ratio between stride of two loops for interchange
to be profitable.
min-insn-to-prefetch-ratio
The minimum ratio between the number of instructions and the
number of prefetches to enable prefetching in a loop.
prefetch-min-insn-to-mem-ratio
The minimum ratio between the number of instructions and the
number of memory references to enable prefetching in a loop.
use-canonical-types
Whether the compiler should use the "canonical" type system.
Should always be 1, which uses a more efficient internal
mechanism for comparing types in C++ and Objective-C++.
However, if bugs in the canonical type system are causing
compilation failures, set this value to 0 to disable canonical
types.
switch-conversion-max-branch-ratio
Switch initialization conversion refuses to create arrays that
are bigger than switch-conversion-max-branch-ratio times the
number of branches in the switch.
max-partial-antic-length
Maximum length of the partial antic set computed during the
tree partial redundancy elimination optimization (-ftree-pre)
when optimizing at -O3 and above. For some sorts of source
code the enhanced partial redundancy elimination optimization
can run away, consuming all of the memory available on the host
machine. This parameter sets a limit on the length of the sets
that are computed, which prevents the runaway behavior.
Setting a value of 0 for this parameter allows an unlimited set
length.
rpo-vn-max-loop-depth
Maximum loop depth that is value-numbered optimistically. When
the limit hits the innermost rpo-vn-max-loop-depth loops and
the outermost loop in the loop nest are value-numbered
optimistically and the remaining ones not.
sccvn-max-alias-queries-per-access
Maximum number of alias-oracle queries we perform when looking
for redundancies for loads and stores. If this limit is hit
the search is aborted and the load or store is not considered
redundant. The number of queries is algorithmically limited to
the number of stores on all paths from the load to the function
entry.
ira-max-loops-num
IRA uses regional register allocation by default. If a
function contains more loops than the number given by this
parameter, only at most the given number of the most
frequently-executed loops form regions for regional register
allocation.
ira-max-conflict-table-size
Although IRA uses a sophisticated algorithm to compress the
conflict table, the table can still require excessive amounts
of memory for huge functions. If the conflict table for a
function could be more than the size in MB given by this
parameter, the register allocator instead uses a faster,
simpler, and lower-quality algorithm that does not require
building a pseudo-register conflict table.
ira-loop-reserved-regs
IRA can be used to evaluate more accurate register pressure in
loops for decisions to move loop invariants (see -O3). The
number of available registers reserved for some other purposes
is given by this parameter. Default of the parameter is the
best found from numerous experiments.
lra-inheritance-ebb-probability-cutoff
LRA tries to reuse values reloaded in registers in subsequent
insns. This optimization is called inheritance. EBB is used
as a region to do this optimization. The parameter defines a
minimal fall-through edge probability in percentage used to add
BB to inheritance EBB in LRA. The default value was chosen
from numerous runs of SPEC2000 on x86-64.
loop-invariant-max-bbs-in-loop
Loop invariant motion can be very expensive, both in
compilation time and in amount of needed compile-time memory,
with very large loops. Loops with more basic blocks than this
parameter won't have loop invariant motion optimization
performed on them.
loop-max-datarefs-for-datadeps
Building data dependencies is expensive for very large loops.
This parameter limits the number of data references in loops
that are considered for data dependence analysis. These large
loops are no handled by the optimizations using loop data
dependencies.
max-vartrack-size
Sets a maximum number of hash table slots to use during
variable tracking dataflow analysis of any function. If this
limit is exceeded with variable tracking at assignments
enabled, analysis for that function is retried without it,
after removing all debug insns from the function. If the limit
is exceeded even without debug insns, var tracking analysis is
completely disabled for the function. Setting the parameter to
zero makes it unlimited.
max-vartrack-expr-depth
Sets a maximum number of recursion levels when attempting to
map variable names or debug temporaries to value expressions.
This trades compilation time for more complete debug
information. If this is set too low, value expressions that
are available and could be represented in debug information may
end up not being used; setting this higher may enable the
compiler to find more complex debug expressions, but compile
time and memory use may grow.
max-debug-marker-count
Sets a threshold on the number of debug markers (e.g. begin
stmt markers) to avoid complexity explosion at inlining or
expanding to RTL. If a function has more such gimple stmts than
the set limit, such stmts will be dropped from the inlined copy
of a function, and from its RTL expansion.
min-nondebug-insn-uid
Use uids starting at this parameter for nondebug insns. The
range below the parameter is reserved exclusively for debug
insns created by -fvar-tracking-assignments, but debug insns
may get (non-overlapping) uids above it if the reserved range
is exhausted.
ipa-sra-ptr-growth-factor
IPA-SRA replaces a pointer to an aggregate with one or more new
parameters only when their cumulative size is less or equal to
ipa-sra-ptr-growth-factor times the size of the original
pointer parameter.
ipa-sra-max-replacements
Maximum pieces of an aggregate that IPA-SRA tracks. As a
consequence, it is also the maximum number of replacements of a
formal parameter.
sra-max-scalarization-size-Ospeed
sra-max-scalarization-size-Osize
The two Scalar Reduction of Aggregates passes (SRA and IPA-SRA)
aim to replace scalar parts of aggregates with uses of
independent scalar variables. These parameters control the
maximum size, in storage units, of aggregate which is
considered for replacement when compiling for speed (sra-max-
scalarization-size-Ospeed) or size (sra-max-scalarization-size-
Osize) respectively.
sra-max-propagations
The maximum number of artificial accesses that Scalar
Replacement of Aggregates (SRA) will track, per one local
variable, in order to facilitate copy propagation.
tm-max-aggregate-size
When making copies of thread-local variables in a transaction,
this parameter specifies the size in bytes after which
variables are saved with the logging functions as opposed to
save/restore code sequence pairs. This option only applies
when using -fgnu-tm.
graphite-max-nb-scop-params
To avoid exponential effects in the Graphite loop transforms,
the number of parameters in a Static Control Part (SCoP) is
bounded. A value of zero can be used to lift the bound. A
variable whose value is unknown at compilation time and defined
outside a SCoP is a parameter of the SCoP.
loop-block-tile-size
Loop blocking or strip mining transforms, enabled with
-floop-block or -floop-strip-mine, strip mine each loop in the
loop nest by a given number of iterations. The strip length
can be changed using the loop-block-tile-size parameter.
ipa-cp-value-list-size
IPA-CP attempts to track all possible values and types passed
to a function's parameter in order to propagate them and
perform devirtualization. ipa-cp-value-list-size is the
maximum number of values and types it stores per one formal
parameter of a function.
ipa-cp-eval-threshold
IPA-CP calculates its own score of cloning profitability
heuristics and performs those cloning opportunities with scores
that exceed ipa-cp-eval-threshold.
ipa-cp-max-recursive-depth
Maximum depth of recursive cloning for self-recursive function.
ipa-cp-min-recursive-probability
Recursive cloning only when the probability of call being
executed exceeds the parameter.
ipa-cp-recursion-penalty
Percentage penalty the recursive functions will receive when
they are evaluated for cloning.
ipa-cp-single-call-penalty
Percentage penalty functions containing a single call to
another function will receive when they are evaluated for
cloning.
ipa-max-agg-items
IPA-CP is also capable to propagate a number of scalar values
passed in an aggregate. ipa-max-agg-items controls the maximum
number of such values per one parameter.
ipa-cp-loop-hint-bonus
When IPA-CP determines that a cloning candidate would make the
number of iterations of a loop known, it adds a bonus of ipa-
cp-loop-hint-bonus to the profitability score of the candidate.
ipa-max-aa-steps
During its analysis of function bodies, IPA-CP employs alias
analysis in order to track values pointed to by function
parameters. In order not spend too much time analyzing huge
functions, it gives up and consider all memory clobbered after
examining ipa-max-aa-steps statements modifying memory.
ipa-max-switch-predicate-bounds
Maximal number of boundary endpoints of case ranges of switch
statement. For switch exceeding this limit, IPA-CP will not
construct cloning cost predicate, which is used to estimate
cloning benefit, for default case of the switch statement.
ipa-max-param-expr-ops
IPA-CP will analyze conditional statement that references some
function parameter to estimate benefit for cloning upon certain
constant value. But if number of operations in a parameter
expression exceeds ipa-max-param-expr-ops, the expression is
treated as complicated one, and is not handled by IPA analysis.
lto-partitions
Specify desired number of partitions produced during WHOPR
compilation. The number of partitions should exceed the number
of CPUs used for compilation.
lto-min-partition
Size of minimal partition for WHOPR (in estimated
instructions). This prevents expenses of splitting very small
programs into too many partitions.
lto-max-partition
Size of max partition for WHOPR (in estimated instructions).
to provide an upper bound for individual size of partition.
Meant to be used only with balanced partitioning.
lto-max-streaming-parallelism
Maximal number of parallel processes used for LTO streaming.
cxx-max-namespaces-for-diagnostic-help
The maximum number of namespaces to consult for suggestions
when C++ name lookup fails for an identifier.
sink-frequency-threshold
The maximum relative execution frequency (in percents) of the
target block relative to a statement's original block to allow
statement sinking of a statement. Larger numbers result in
more aggressive statement sinking. A small positive adjustment
is applied for statements with memory operands as those are
even more profitable so sink.
max-stores-to-sink
The maximum number of conditional store pairs that can be sunk.
Set to 0 if either vectorization (-ftree-vectorize) or if-
conversion (-ftree-loop-if-convert) is disabled.
case-values-threshold
The smallest number of different values for which it is best to
use a jump-table instead of a tree of conditional branches. If
the value is 0, use the default for the machine.
jump-table-max-growth-ratio-for-size
The maximum code size growth ratio when expanding into a jump
table (in percent). The parameter is used when optimizing for
size.
jump-table-max-growth-ratio-for-speed
The maximum code size growth ratio when expanding into a jump
table (in percent). The parameter is used when optimizing for
speed.
tree-reassoc-width
Set the maximum number of instructions executed in parallel in
reassociated tree. This parameter overrides target dependent
heuristics used by default if has non zero value.
sched-pressure-algorithm
Choose between the two available implementations of
-fsched-pressure. Algorithm 1 is the original implementation
and is the more likely to prevent instructions from being
reordered. Algorithm 2 was designed to be a compromise between
the relatively conservative approach taken by algorithm 1 and
the rather aggressive approach taken by the default scheduler.
It relies more heavily on having a regular register file and
accurate register pressure classes. See haifa-sched.c in the
GCC sources for more details.
The default choice depends on the target.
max-slsr-cand-scan
Set the maximum number of existing candidates that are
considered when seeking a basis for a new straight-line
strength reduction candidate.
asan-globals
Enable buffer overflow detection for global objects. This kind
of protection is enabled by default if you are using
-fsanitize=address option. To disable global objects
protection use --param asan-globals=0.
asan-stack
Enable buffer overflow detection for stack objects. This kind
of protection is enabled by default when using
-fsanitize=address. To disable stack protection use --param
asan-stack=0 option.
asan-instrument-reads
Enable buffer overflow detection for memory reads. This kind
of protection is enabled by default when using
-fsanitize=address. To disable memory reads protection use
--param asan-instrument-reads=0.
asan-instrument-writes
Enable buffer overflow detection for memory writes. This kind
of protection is enabled by default when using
-fsanitize=address. To disable memory writes protection use
--param asan-instrument-writes=0 option.
asan-memintrin
Enable detection for built-in functions. This kind of
protection is enabled by default when using -fsanitize=address.
To disable built-in functions protection use --param
asan-memintrin=0.
asan-use-after-return
Enable detection of use-after-return. This kind of protection
is enabled by default when using the -fsanitize=address option.
To disable it use --param asan-use-after-return=0.
Note: By default the check is disabled at run time. To enable
it, add "detect_stack_use_after_return=1" to the environment
variable ASAN_OPTIONS.
asan-instrumentation-with-call-threshold
If number of memory accesses in function being instrumented is
greater or equal to this number, use callbacks instead of
inline checks. E.g. to disable inline code use --param
asan-instrumentation-with-call-threshold=0.
use-after-scope-direct-emission-threshold
If the size of a local variable in bytes is smaller or equal to
this number, directly poison (or unpoison) shadow memory
instead of using run-time callbacks.
max-fsm-thread-path-insns
Maximum number of instructions to copy when duplicating blocks
on a finite state automaton jump thread path.
max-fsm-thread-length
Maximum number of basic blocks on a finite state automaton jump
thread path.
max-fsm-thread-paths
Maximum number of new jump thread paths to create for a finite
state automaton.
parloops-chunk-size
Chunk size of omp schedule for loops parallelized by parloops.
parloops-schedule
Schedule type of omp schedule for loops parallelized by
parloops (static, dynamic, guided, auto, runtime).
parloops-min-per-thread
The minimum number of iterations per thread of an innermost
parallelized loop for which the parallelized variant is
preferred over the single threaded one. Note that for a
parallelized loop nest the minimum number of iterations of the
outermost loop per thread is two.
max-ssa-name-query-depth
Maximum depth of recursion when querying properties of SSA
names in things like fold routines. One level of recursion
corresponds to following a use-def chain.
hsa-gen-debug-stores
Enable emission of special debug stores within HSA kernels
which are then read and reported by libgomp plugin. Generation
of these stores is disabled by default, use --param
hsa-gen-debug-stores=1 to enable it.
max-speculative-devirt-maydefs
The maximum number of may-defs we analyze when looking for a
must-def specifying the dynamic type of an object that invokes
a virtual call we may be able to devirtualize speculatively.
max-vrp-switch-assertions
The maximum number of assertions to add along the default edge
of a switch statement during VRP.
unroll-jam-min-percent
The minimum percentage of memory references that must be
optimized away for the unroll-and-jam transformation to be
considered profitable.
unroll-jam-max-unroll
The maximum number of times the outer loop should be unrolled
by the unroll-and-jam transformation.
max-rtl-if-conversion-unpredictable-cost
Maximum permissible cost for the sequence that would be
generated by the RTL if-conversion pass for a branch that is
considered unpredictable.
max-variable-expansions-in-unroller
If -fvariable-expansion-in-unroller is used, the maximum number
of times that an individual variable will be expanded during
loop unrolling.
tracer-min-branch-probability-feedback
Stop forward growth if the probability of best edge is less
than this threshold (in percent). Used when profile feedback is
available.
partial-inlining-entry-probability
Maximum probability of the entry BB of split region (in percent
relative to entry BB of the function) to make partial inlining
happen.
max-tracked-strlens
Maximum number of strings for which strlen optimization pass
will track string lengths.
gcse-after-reload-partial-fraction
The threshold ratio for performing partial redundancy
elimination after reload.
gcse-after-reload-critical-fraction
The threshold ratio of critical edges execution count that
permit performing redundancy elimination after reload.
max-loop-header-insns
The maximum number of insns in loop header duplicated by the
copy loop headers pass.
vect-epilogues-nomask
Enable loop epilogue vectorization using smaller vector size.
slp-max-insns-in-bb
Maximum number of instructions in basic block to be considered
for SLP vectorization.
avoid-fma-max-bits
Maximum number of bits for which we avoid creating FMAs.
sms-loop-average-count-threshold
A threshold on the average loop count considered by the swing
modulo scheduler.
sms-dfa-history
The number of cycles the swing modulo scheduler considers when
checking conflicts using DFA.
max-inline-insns-recursive-auto
The maximum number of instructions non-inline function can grow
to via recursive inlining.
graphite-allow-codegen-errors
Whether codegen errors should be ICEs when -fchecking.
sms-max-ii-factor
A factor for tuning the upper bound that swing modulo scheduler
uses for scheduling a loop.
lra-max-considered-reload-pseudos
The max number of reload pseudos which are considered during
spilling a non-reload pseudo.
max-pow-sqrt-depth
Maximum depth of sqrt chains to use when synthesizing
exponentiation by a real constant.
max-dse-active-local-stores
Maximum number of active local stores in RTL dead store
elimination.
asan-instrument-allocas
Enable asan allocas/VLAs protection.
max-iterations-computation-cost
Bound on the cost of an expression to compute the number of
iterations.
max-isl-operations
Maximum number of isl operations, 0 means unlimited.
graphite-max-arrays-per-scop
Maximum number of arrays per scop.
max-vartrack-reverse-op-size
Max. size of loc list for which reverse ops should be added.
tracer-dynamic-coverage-feedback
The percentage of function, weighted by execution frequency,
that must be covered by trace formation. Used when profile
feedback is available.
max-inline-recursive-depth-auto
The maximum depth of recursive inlining for non-inline
functions.
fsm-scale-path-stmts
Scale factor to apply to the number of statements in a
threading path when comparing to the number of (scaled) blocks.
fsm-maximum-phi-arguments
Maximum number of arguments a PHI may have before the FSM
threader will not try to thread through its block.
uninit-control-dep-attempts
Maximum number of nested calls to search for control
dependencies during uninitialized variable analysis.
sra-max-scalarization-size-Osize
Maximum size, in storage units, of an aggregate which should be
considered for scalarization when compiling for size.
fsm-scale-path-blocks
Scale factor to apply to the number of blocks in a threading
path when comparing to the number of (scaled) statements.
sched-autopref-queue-depth
Hardware autoprefetcher scheduler model control flag. Number
of lookahead cycles the model looks into; at ' ' only enable
instruction sorting heuristic.
loop-versioning-max-inner-insns
The maximum number of instructions that an inner loop can have
before the loop versioning pass considers it too big to copy.
loop-versioning-max-outer-insns
The maximum number of instructions that an outer loop can have
before the loop versioning pass considers it too big to copy,
discounting any instructions in inner loops that directly
benefit from versioning.
ssa-name-def-chain-limit
The maximum number of SSA_NAME assignments to follow in
determining a property of a variable such as its value. This
limits the number of iterations or recursive calls GCC performs
when optimizing certain statements or when determining their
validity prior to issuing diagnostics.
store-merging-max-size
Maximum size of a single store merging region in bytes.
hash-table-verification-limit
The number of elements for which hash table verification is
done for each searched element.
max-find-base-term-values
Maximum number of VALUEs handled during a single find_base_term
call.
analyzer-max-enodes-per-program-point
The maximum number of exploded nodes per program point within
the analyzer, before terminating analysis of that point.
analyzer-min-snodes-for-call-summary
The minimum number of supernodes within a function for the
analyzer to consider summarizing its effects at call sites.
analyzer-max-recursion-depth
The maximum number of times a callsite can appear in a call
stack within the analyzer, before terminating analysis of a
call that would recurse deeper.
gimple-fe-computed-hot-bb-threshold
The number of executions of a basic block which is considered
hot. The parameter is used only in GIMPLE FE.
analyzer-bb-explosion-factor
The maximum number of 'after supernode' exploded nodes within
the analyzer per supernode, before terminating analysis.
The following choices of name are available on AArch64 targets:
aarch64-sve-compare-costs
When vectorizing for SVE, consider using "unpacked" vectors for
smaller elements and use the cost model to pick the cheapest
approach. Also use the cost model to choose between SVE and
Advanced SIMD vectorization.
Using unpacked vectors includes storing smaller elements in
larger containers and accessing elements with extending loads
and truncating stores.
aarch64-float-recp-precision
The number of Newton iterations for calculating the reciprocal
for float type. The precision of division is proportional to
this param when division approximation is enabled. The default
value is 1.
aarch64-double-recp-precision
The number of Newton iterations for calculating the reciprocal
for double type. The precision of division is propotional to
this param when division approximation is enabled. The default
value is 2.
aarch64-autovec-preference
Force an ISA selection strategy for auto-vectorization.
Accepts values from 0 to 4, inclusive.
0 Use the default heuristics.
1 Use only Advanced SIMD for auto-vectorization.
2 Use only SVE for auto-vectorization.
3 Use both Advanced SIMD and SVE. Prefer Advanced SIMD when
the costs are deemed equal.
4 Use both Advanced SIMD and SVE. Prefer SVE when the costs
are deemed equal.
The default value is 0.
Program Instrumentation Options
GCC supports a number of command-line options that control adding run-
time instrumentation to the code it normally generates. For example,
one purpose of instrumentation is collect profiling statistics for use
in finding program hot spots, code coverage analysis, or profile-guided
optimizations. Another class of program instrumentation is adding run-
time checking to detect programming errors like invalid pointer
dereferences or out-of-bounds array accesses, as well as deliberately
hostile attacks such as stack smashing or C++ vtable hijacking. There
is also a general hook which can be used to implement other forms of
tracing or function-level instrumentation for debug or program analysis
purposes.
-p
-pg Generate extra code to write profile information suitable for the
analysis program prof (for -p) or gprof (for -pg). You must use
this option when compiling the source files you want data about,
and you must also use it when linking.
You can use the function attribute "no_instrument_function" to
suppress profiling of individual functions when compiling with
these options.
-fprofile-arcs
Add code so that program flow arcs are instrumented. During
execution the program records how many times each branch and call
is executed and how many times it is taken or returns. On targets
that support constructors with priority support, profiling properly
handles constructors, destructors and C++ constructors (and
destructors) of classes which are used as a type of a global
variable.
When the compiled program exits it saves this data to a file called
auxname.gcda for each source file. The data may be used for
profile-directed optimizations (-fbranch-probabilities), or for
test coverage analysis (-ftest-coverage). Each object file's
auxname is generated from the name of the output file, if
explicitly specified and it is not the final executable, otherwise
it is the basename of the source file. In both cases any suffix is
removed (e.g. foo.gcda for input file dir/foo.c, or dir/foo.gcda
for output file specified as -o dir/foo.o).
--coverage
This option is used to compile and link code instrumented for
coverage analysis. The option is a synonym for -fprofile-arcs
-ftest-coverage (when compiling) and -lgcov (when linking). See
the documentation for those options for more details.
* Compile the source files with -fprofile-arcs plus optimization
and code generation options. For test coverage analysis, use
the additional -ftest-coverage option. You do not need to
profile every source file in a program.
* Compile the source files additionally with -fprofile-abs-path
to create absolute path names in the .gcno files. This allows
gcov to find the correct sources in projects where compilations
occur with different working directories.
* Link your object files with -lgcov or -fprofile-arcs (the
latter implies the former).
* Run the program on a representative workload to generate the
arc profile information. This may be repeated any number of
times. You can run concurrent instances of your program, and
provided that the file system supports locking, the data files
will be correctly updated. Unless a strict ISO C dialect
option is in effect, "fork" calls are detected and correctly
handled without double counting.
* For profile-directed optimizations, compile the source files
again with the same optimization and code generation options
plus -fbranch-probabilities.
* For test coverage analysis, use gcov to produce human readable
information from the .gcno and .gcda files. Refer to the gcov
documentation for further information.
With -fprofile-arcs, for each function of your program GCC creates
a program flow graph, then finds a spanning tree for the graph.
Only arcs that are not on the spanning tree have to be
instrumented: the compiler adds code to count the number of times
that these arcs are executed. When an arc is the only exit or only
entrance to a block, the instrumentation code can be added to the
block; otherwise, a new basic block must be created to hold the
instrumentation code.
-ftest-coverage
Produce a notes file that the gcov code-coverage utility can use to
show program coverage. Each source file's note file is called
auxname.gcno. Refer to the -fprofile-arcs option above for a
description of auxname and instructions on how to generate test
coverage data. Coverage data matches the source files more closely
if you do not optimize.
-fprofile-abs-path
Automatically convert relative source file names to absolute path
names in the .gcno files. This allows gcov to find the correct
sources in projects where compilations occur with different working
directories.
-fprofile-dir=path
Set the directory to search for the profile data files in to path.
This option affects only the profile data generated by
-fprofile-generate, -ftest-coverage, -fprofile-arcs and used by
-fprofile-use and -fbranch-probabilities and its related options.
Both absolute and relative paths can be used. By default, GCC uses
the current directory as path, thus the profile data file appears
in the same directory as the object file. In order to prevent the
file name clashing, if the object file name is not an absolute
path, we mangle the absolute path of the sourcename.gcda file and
use it as the file name of a .gcda file. See similar option
-fprofile-note.
When an executable is run in a massive parallel environment, it is
recommended to save profile to different folders. That can be done
with variables in path that are exported during run-time:
%p process ID.
%q{VAR}
value of environment variable VAR
-fprofile-generate
-fprofile-generate=path
Enable options usually used for instrumenting application to
produce profile useful for later recompilation with profile
feedback based optimization. You must use -fprofile-generate both
when compiling and when linking your program.
The following options are enabled: -fprofile-arcs,
-fprofile-values, -finline-functions, and -fipa-bit-cp.
If path is specified, GCC looks at the path to find the profile
feedback data files. See -fprofile-dir.
To optimize the program based on the collected profile information,
use -fprofile-use.
-fprofile-note=path
If path is specified, GCC saves .gcno file into path location. If
you combine the option with multiple source files, the .gcno file
will be overwritten.
-fprofile-prefix-path=path
This option can be used in combination with
profile-generate=profile_dir and profile-use=profile_dir to inform
GCC where is the base directory of built source tree. By default
profile_dir will contain files with mangled absolute paths of all
object files in the built project. This is not desirable when
directory used to build the instrumented binary differs from the
directory used to build the binary optimized with profile feedback
because the profile data will not be found during the optimized
build. In such setups -fprofile-prefix-path=path with path
pointing to the base directory of the build can be used to strip
the irrelevant part of the path and keep all file names relative to
the main build directory.
-fprofile-update=method
Alter the update method for an application instrumented for profile
feedback based optimization. The method argument should be one of
single, atomic or prefer-atomic. The first one is useful for
single-threaded applications, while the second one prevents profile
corruption by emitting thread-safe code.
Warning: When an application does not properly join all threads (or
creates an detached thread), a profile file can be still corrupted.
Using prefer-atomic would be transformed either to atomic, when
supported by a target, or to single otherwise. The GCC driver
automatically selects prefer-atomic when -pthread is present in the
command line.
-fprofile-filter-files=regex
Instrument only functions from files where names match any regular
expression (separated by a semi-colon).
For example, -fprofile-filter-files=main.c;module.*.c will
instrument only main.c and all C files starting with 'module'.
-fprofile-exclude-files=regex
Instrument only functions from files where names do not match all
the regular expressions (separated by a semi-colon).
For example, -fprofile-exclude-files=/usr/* will prevent
instrumentation of all files that are located in /usr/ folder.
-fprofile-reproducible=[multithreaded|parallel-runs|serial]
Control level of reproducibility of profile gathered by
"-fprofile-generate". This makes it possible to rebuild program
with same outcome which is useful, for example, for distribution
packages.
With -fprofile-reproducible=serial the profile gathered by
-fprofile-generate is reproducible provided the trained program
behaves the same at each invocation of the train run, it is not
multi-threaded and profile data streaming is always done in the
same order. Note that profile streaming happens at the end of
program run but also before "fork" function is invoked.
Note that it is quite common that execution counts of some part of
programs depends, for example, on length of temporary file names or
memory space randomization (that may affect hash-table collision
rate). Such non-reproducible part of programs may be annotated by
"no_instrument_function" function attribute. "gcov-dump" with -l
can be used to dump gathered data and verify that they are indeed
reproducible.
With -fprofile-reproducible=parallel-runs collected profile stays
reproducible regardless the order of streaming of the data into
gcda files. This setting makes it possible to run multiple
instances of instrumented program in parallel (such as with "make
-j"). This reduces quality of gathered data, in particular of
indirect call profiling.
-fsanitize=address
Enable AddressSanitizer, a fast memory error detector. Memory
access instructions are instrumented to detect out-of-bounds and
use-after-free bugs. The option enables
-fsanitize-address-use-after-scope. See
<https://github.com/google/sanitizers/wiki/AddressSanitizer> for
more details. The run-time behavior can be influenced using the
ASAN_OPTIONS environment variable. When set to "help=1", the
available options are shown at startup of the instrumented program.
See
<https://github.com/google/sanitizers/wiki/AddressSanitizerFlags#run-time-flags>
for a list of supported options. The option cannot be combined
with -fsanitize=thread.
-fsanitize=kernel-address
Enable AddressSanitizer for Linux kernel. See
<https://github.com/google/kasan/wiki> for more details.
-fsanitize=pointer-compare
Instrument comparison operation (<, <=, >, >=) with pointer
operands. The option must be combined with either
-fsanitize=kernel-address or -fsanitize=address The option cannot
be combined with -fsanitize=thread. Note: By default the check is
disabled at run time. To enable it, add
"detect_invalid_pointer_pairs=2" to the environment variable
ASAN_OPTIONS. Using "detect_invalid_pointer_pairs=1" detects
invalid operation only when both pointers are non-null.
-fsanitize=pointer-subtract
Instrument subtraction with pointer operands. The option must be
combined with either -fsanitize=kernel-address or
-fsanitize=address The option cannot be combined with
-fsanitize=thread. Note: By default the check is disabled at run
time. To enable it, add "detect_invalid_pointer_pairs=2" to the
environment variable ASAN_OPTIONS. Using
"detect_invalid_pointer_pairs=1" detects invalid operation only
when both pointers are non-null.
-fsanitize=thread
Enable ThreadSanitizer, a fast data race detector. Memory access
instructions are instrumented to detect data race bugs. See
<https://github.com/google/sanitizers/wiki#threadsanitizer> for
more details. The run-time behavior can be influenced using the
TSAN_OPTIONS environment variable; see
<https://github.com/google/sanitizers/wiki/ThreadSanitizerFlags>
for a list of supported options. The option cannot be combined
with -fsanitize=address, -fsanitize=leak.
Note that sanitized atomic builtins cannot throw exceptions when
operating on invalid memory addresses with non-call exceptions
(-fnon-call-exceptions).
-fsanitize=leak
Enable LeakSanitizer, a memory leak detector. This option only
matters for linking of executables and the executable is linked
against a library that overrides "malloc" and other allocator
functions. See
<https://github.com/google/sanitizers/wiki/AddressSanitizerLeakSanitizer>
for more details. The run-time behavior can be influenced using
the LSAN_OPTIONS environment variable. The option cannot be
combined with -fsanitize=thread.
-fsanitize=undefined
Enable UndefinedBehaviorSanitizer, a fast undefined behavior
detector. Various computations are instrumented to detect
undefined behavior at runtime. See
<https://clang.llvm.org/docs/UndefinedBehaviorSanitizer.html> for
more details. The run-time behavior can be influenced using the
UBSAN_OPTIONS environment variable. Current suboptions are:
-fsanitize=shift
This option enables checking that the result of a shift
operation is not undefined. Note that what exactly is
considered undefined differs slightly between C and C++, as
well as between ISO C90 and C99, etc. This option has two
suboptions, -fsanitize=shift-base and
-fsanitize=shift-exponent.
-fsanitize=shift-exponent
This option enables checking that the second argument of a
shift operation is not negative and is smaller than the
precision of the promoted first argument.
-fsanitize=shift-base
If the second argument of a shift operation is within range,
check that the result of a shift operation is not undefined.
Note that what exactly is considered undefined differs slightly
between C and C++, as well as between ISO C90 and C99, etc.
-fsanitize=integer-divide-by-zero
Detect integer division by zero as well as "INT_MIN / -1"
division.
-fsanitize=unreachable
With this option, the compiler turns the
"__builtin_unreachable" call into a diagnostics message call
instead. When reaching the "__builtin_unreachable" call, the
behavior is undefined.
-fsanitize=vla-bound
This option instructs the compiler to check that the size of a
variable length array is positive.
-fsanitize=null
This option enables pointer checking. Particularly, the
application built with this option turned on will issue an
error message when it tries to dereference a NULL pointer, or
if a reference (possibly an rvalue reference) is bound to a
NULL pointer, or if a method is invoked on an object pointed by
a NULL pointer.
-fsanitize=return
This option enables return statement checking. Programs built
with this option turned on will issue an error message when the
end of a non-void function is reached without actually
returning a value. This option works in C++ only.
-fsanitize=signed-integer-overflow
This option enables signed integer overflow checking. We check
that the result of "+", "*", and both unary and binary "-" does
not overflow in the signed arithmetics. Note, integer
promotion rules must be taken into account. That is, the
following is not an overflow:
signed char a = SCHAR_MAX;
a++;
-fsanitize=bounds
This option enables instrumentation of array bounds. Various
out of bounds accesses are detected. Flexible array members,
flexible array member-like arrays, and initializers of
variables with static storage are not instrumented.
-fsanitize=bounds-strict
This option enables strict instrumentation of array bounds.
Most out of bounds accesses are detected, including flexible
array members and flexible array member-like arrays.
Initializers of variables with static storage are not
instrumented.
-fsanitize=alignment
This option enables checking of alignment of pointers when they
are dereferenced, or when a reference is bound to
insufficiently aligned target, or when a method or constructor
is invoked on insufficiently aligned object.
-fsanitize=object-size
This option enables instrumentation of memory references using
the "__builtin_object_size" function. Various out of bounds
pointer accesses are detected.
-fsanitize=float-divide-by-zero
Detect floating-point division by zero. Unlike other similar
options, -fsanitize=float-divide-by-zero is not enabled by
-fsanitize=undefined, since floating-point division by zero can
be a legitimate way of obtaining infinities and NaNs.
-fsanitize=float-cast-overflow
This option enables floating-point type to integer conversion
checking. We check that the result of the conversion does not
overflow. Unlike other similar options,
-fsanitize=float-cast-overflow is not enabled by
-fsanitize=undefined. This option does not work well with
"FE_INVALID" exceptions enabled.
-fsanitize=nonnull-attribute
This option enables instrumentation of calls, checking whether
null values are not passed to arguments marked as requiring a
non-null value by the "nonnull" function attribute.
-fsanitize=returns-nonnull-attribute
This option enables instrumentation of return statements in
functions marked with "returns_nonnull" function attribute, to
detect returning of null values from such functions.
-fsanitize=bool
This option enables instrumentation of loads from bool. If a
value other than 0/1 is loaded, a run-time error is issued.
-fsanitize=enum
This option enables instrumentation of loads from an enum type.
If a value outside the range of values for the enum type is
loaded, a run-time error is issued.
-fsanitize=vptr
This option enables instrumentation of C++ member function
calls, member accesses and some conversions between pointers to
base and derived classes, to verify the referenced object has
the correct dynamic type.
-fsanitize=pointer-overflow
This option enables instrumentation of pointer arithmetics. If
the pointer arithmetics overflows, a run-time error is issued.
-fsanitize=builtin
This option enables instrumentation of arguments to selected
builtin functions. If an invalid value is passed to such
arguments, a run-time error is issued. E.g. passing 0 as the
argument to "__builtin_ctz" or "__builtin_clz" invokes
undefined behavior and is diagnosed by this option.
While -ftrapv causes traps for signed overflows to be emitted,
-fsanitize=undefined gives a diagnostic message. This currently
works only for the C family of languages.
-fno-sanitize=all
This option disables all previously enabled sanitizers.
-fsanitize=all is not allowed, as some sanitizers cannot be used
together.
-fasan-shadow-offset=number
This option forces GCC to use custom shadow offset in
AddressSanitizer checks. It is useful for experimenting with
different shadow memory layouts in Kernel AddressSanitizer.
-fsanitize-sections=s1,s2,...
Sanitize global variables in selected user-defined sections. si
may contain wildcards.
-fsanitize-recover[=opts]
-fsanitize-recover= controls error recovery mode for sanitizers
mentioned in comma-separated list of opts. Enabling this option
for a sanitizer component causes it to attempt to continue running
the program as if no error happened. This means multiple runtime
errors can be reported in a single program run, and the exit code
of the program may indicate success even when errors have been
reported. The -fno-sanitize-recover= option can be used to alter
this behavior: only the first detected error is reported and
program then exits with a non-zero exit code.
Currently this feature only works for -fsanitize=undefined (and its
suboptions except for -fsanitize=unreachable and
-fsanitize=return), -fsanitize=float-cast-overflow,
-fsanitize=float-divide-by-zero, -fsanitize=bounds-strict,
-fsanitize=kernel-address and -fsanitize=address. For these
sanitizers error recovery is turned on by default, except
-fsanitize=address, for which this feature is experimental.
-fsanitize-recover=all and -fno-sanitize-recover=all is also
accepted, the former enables recovery for all sanitizers that
support it, the latter disables recovery for all sanitizers that
support it.
Even if a recovery mode is turned on the compiler side, it needs to
be also enabled on the runtime library side, otherwise the failures
are still fatal. The runtime library defaults to "halt_on_error=0"
for ThreadSanitizer and UndefinedBehaviorSanitizer, while default
value for AddressSanitizer is "halt_on_error=1". This can be
overridden through setting the "halt_on_error" flag in the
corresponding environment variable.
Syntax without an explicit opts parameter is deprecated. It is
equivalent to specifying an opts list of:
undefined,float-cast-overflow,float-divide-by-zero,bounds-strict
-fsanitize-address-use-after-scope
Enable sanitization of local variables to detect use-after-scope
bugs. The option sets -fstack-reuse to none.
-fsanitize-undefined-trap-on-error
The -fsanitize-undefined-trap-on-error option instructs the
compiler to report undefined behavior using "__builtin_trap" rather
than a "libubsan" library routine. The advantage of this is that
the "libubsan" library is not needed and is not linked in, so this
is usable even in freestanding environments.
-fsanitize-coverage=trace-pc
Enable coverage-guided fuzzing code instrumentation. Inserts a
call to "__sanitizer_cov_trace_pc" into every basic block.
-fsanitize-coverage=trace-cmp
Enable dataflow guided fuzzing code instrumentation. Inserts a
call to "__sanitizer_cov_trace_cmp1", "__sanitizer_cov_trace_cmp2",
"__sanitizer_cov_trace_cmp4" or "__sanitizer_cov_trace_cmp8" for
integral comparison with both operands variable or
"__sanitizer_cov_trace_const_cmp1",
"__sanitizer_cov_trace_const_cmp2",
"__sanitizer_cov_trace_const_cmp4" or
"__sanitizer_cov_trace_const_cmp8" for integral comparison with one
operand constant, "__sanitizer_cov_trace_cmpf" or
"__sanitizer_cov_trace_cmpd" for float or double comparisons and
"__sanitizer_cov_trace_switch" for switch statements.
-fcf-protection=[full|branch|return|none|check]
Enable code instrumentation of control-flow transfers to increase
program security by checking that target addresses of control-flow
transfer instructions (such as indirect function call, function
return, indirect jump) are valid. This prevents diverting the flow
of control to an unexpected target. This is intended to protect
against such threats as Return-oriented Programming (ROP), and
similarly call/jmp-oriented programming (COP/JOP).
The value "branch" tells the compiler to implement checking of
validity of control-flow transfer at the point of indirect branch
instructions, i.e. call/jmp instructions. The value "return"
implements checking of validity at the point of returning from a
function. The value "full" is an alias for specifying both
"branch" and "return". The value "none" turns off instrumentation.
The value "check" is used for the final link with link-time
optimization (LTO). An error is issued if LTO object files are
compiled with different -fcf-protection values. The value "check"
is ignored at the compile time.
The macro "__CET__" is defined when -fcf-protection is used. The
first bit of "__CET__" is set to 1 for the value "branch" and the
second bit of "__CET__" is set to 1 for the "return".
You can also use the "nocf_check" attribute to identify which
functions and calls should be skipped from instrumentation.
Currently the x86 GNU/Linux target provides an implementation based
on Intel Control-flow Enforcement Technology (CET) which works for
i686 processor or newer.
-fstack-protector
Emit extra code to check for buffer overflows, such as stack
smashing attacks. This is done by adding a guard variable to
functions with vulnerable objects. This includes functions that
call "alloca", and functions with buffers larger than or equal to 8
bytes. The guards are initialized when a function is entered and
then checked when the function exits. If a guard check fails, an
error message is printed and the program exits. Only variables
that are actually allocated on the stack are considered, optimized
away variables or variables allocated in registers don't count.
-fstack-protector-all
Like -fstack-protector except that all functions are protected.
-fstack-protector-strong
Like -fstack-protector but includes additional functions to be
protected --- those that have local array definitions, or have
references to local frame addresses. Only variables that are
actually allocated on the stack are considered, optimized away
variables or variables allocated in registers don't count.
-fstack-protector-explicit
Like -fstack-protector but only protects those functions which have
the "stack_protect" attribute.
-fstack-check
Generate code to verify that you do not go beyond the boundary of
the stack. You should specify this flag if you are running in an
environment with multiple threads, but you only rarely need to
specify it in a single-threaded environment since stack overflow is
automatically detected on nearly all systems if there is only one
stack.
Note that this switch does not actually cause checking to be done;
the operating system or the language runtime must do that. The
switch causes generation of code to ensure that they see the stack
being extended.
You can additionally specify a string parameter: no means no
checking, generic means force the use of old-style checking,
specific means use the best checking method and is equivalent to
bare -fstack-check.
Old-style checking is a generic mechanism that requires no specific
target support in the compiler but comes with the following
drawbacks:
1. Modified allocation strategy for large objects: they are always
allocated dynamically if their size exceeds a fixed threshold.
Note this may change the semantics of some code.
2. Fixed limit on the size of the static frame of functions: when
it is topped by a particular function, stack checking is not
reliable and a warning is issued by the compiler.
3. Inefficiency: because of both the modified allocation strategy
and the generic implementation, code performance is hampered.
Note that old-style stack checking is also the fallback method for
specific if no target support has been added in the compiler.
-fstack-check= is designed for Ada's needs to detect infinite
recursion and stack overflows. specific is an excellent choice
when compiling Ada code. It is not generally sufficient to protect
against stack-clash attacks. To protect against those you want
-fstack-clash-protection.
-fstack-clash-protection
Generate code to prevent stack clash style attacks. When this
option is enabled, the compiler will only allocate one page of
stack space at a time and each page is accessed immediately after
allocation. Thus, it prevents allocations from jumping over any
stack guard page provided by the operating system.
Most targets do not fully support stack clash protection. However,
on those targets -fstack-clash-protection will protect dynamic
stack allocations. -fstack-clash-protection may also provide
limited protection for static stack allocations if the target
supports -fstack-check=specific.
-fstack-limit-register=reg
-fstack-limit-symbol=sym
-fno-stack-limit
Generate code to ensure that the stack does not grow beyond a
certain value, either the value of a register or the address of a
symbol. If a larger stack is required, a signal is raised at run
time. For most targets, the signal is raised before the stack
overruns the boundary, so it is possible to catch the signal
without taking special precautions.
For instance, if the stack starts at absolute address 0x80000000
and grows downwards, you can use the flags
-fstack-limit-symbol=__stack_limit and
-Wl,--defsym,__stack_limit=0x7ffe0000 to enforce a stack limit of
128KB. Note that this may only work with the GNU linker.
You can locally override stack limit checking by using the
"no_stack_limit" function attribute.
-fsplit-stack
Generate code to automatically split the stack before it overflows.
The resulting program has a discontiguous stack which can only
overflow if the program is unable to allocate any more memory.
This is most useful when running threaded programs, as it is no
longer necessary to calculate a good stack size to use for each
thread. This is currently only implemented for the x86 targets
running GNU/Linux.
When code compiled with -fsplit-stack calls code compiled without
-fsplit-stack, there may not be much stack space available for the
latter code to run. If compiling all code, including library code,
with -fsplit-stack is not an option, then the linker can fix up
these calls so that the code compiled without -fsplit-stack always
has a large stack. Support for this is implemented in the gold
linker in GNU binutils release 2.21 and later.
-fvtable-verify=[std|preinit|none]
This option is only available when compiling C++ code. It turns on
(or off, if using -fvtable-verify=none) the security feature that
verifies at run time, for every virtual call, that the vtable
pointer through which the call is made is valid for the type of the
object, and has not been corrupted or overwritten. If an invalid
vtable pointer is detected at run time, an error is reported and
execution of the program is immediately halted.
This option causes run-time data structures to be built at program
startup, which are used for verifying the vtable pointers. The
options std and preinit control the timing of when these data
structures are built. In both cases the data structures are built
before execution reaches "main". Using -fvtable-verify=std causes
the data structures to be built after shared libraries have been
loaded and initialized. -fvtable-verify=preinit causes them to be
built before shared libraries have been loaded and initialized.
If this option appears multiple times in the command line with
different values specified, none takes highest priority over both
std and preinit; preinit takes priority over std.
-fvtv-debug
When used in conjunction with -fvtable-verify=std or
-fvtable-verify=preinit, causes debug versions of the runtime
functions for the vtable verification feature to be called. This
flag also causes the compiler to log information about which vtable
pointers it finds for each class. This information is written to a
file named vtv_set_ptr_data.log in the directory named by the
environment variable VTV_LOGS_DIR if that is defined or the current
working directory otherwise.
Note: This feature appends data to the log file. If you want a
fresh log file, be sure to delete any existing one.
-fvtv-counts
This is a debugging flag. When used in conjunction with
-fvtable-verify=std or -fvtable-verify=preinit, this causes the
compiler to keep track of the total number of virtual calls it
encounters and the number of verifications it inserts. It also
counts the number of calls to certain run-time library functions
that it inserts and logs this information for each compilation
unit. The compiler writes this information to a file named
vtv_count_data.log in the directory named by the environment
variable VTV_LOGS_DIR if that is defined or the current working
directory otherwise. It also counts the size of the vtable pointer
sets for each class, and writes this information to
vtv_class_set_sizes.log in the same directory.
Note: This feature appends data to the log files. To get fresh
log files, be sure to delete any existing ones.
-finstrument-functions
Generate instrumentation calls for entry and exit to functions.
Just after function entry and just before function exit, the
following profiling functions are called with the address of the
current function and its call site. (On some platforms,
"__builtin_return_address" does not work beyond the current
function, so the call site information may not be available to the
profiling functions otherwise.)
void __cyg_profile_func_enter (void *this_fn,
void *call_site);
void __cyg_profile_func_exit (void *this_fn,
void *call_site);
The first argument is the address of the start of the current
function, which may be looked up exactly in the symbol table.
This instrumentation is also done for functions expanded inline in
other functions. The profiling calls indicate where, conceptually,
the inline function is entered and exited. This means that
addressable versions of such functions must be available. If all
your uses of a function are expanded inline, this may mean an
additional expansion of code size. If you use "extern inline" in
your C code, an addressable version of such functions must be
provided. (This is normally the case anyway, but if you get lucky
and the optimizer always expands the functions inline, you might
have gotten away without providing static copies.)
A function may be given the attribute "no_instrument_function", in
which case this instrumentation is not done. This can be used, for
example, for the profiling functions listed above, high-priority
interrupt routines, and any functions from which the profiling
functions cannot safely be called (perhaps signal handlers, if the
profiling routines generate output or allocate memory).
-finstrument-functions-exclude-file-list=file,file,...
Set the list of functions that are excluded from instrumentation
(see the description of -finstrument-functions). If the file that
contains a function definition matches with one of file, then that
function is not instrumented. The match is done on substrings: if
the file parameter is a substring of the file name, it is
considered to be a match.
For example:
-finstrument-functions-exclude-file-list=/bits/stl,include/sys
excludes any inline function defined in files whose pathnames
contain /bits/stl or include/sys.
If, for some reason, you want to include letter , in one of sym,
write ,. For example,
-finstrument-functions-exclude-file-list=',,tmp' (note the single
quote surrounding the option).
-finstrument-functions-exclude-function-list=sym,sym,...
This is similar to -finstrument-functions-exclude-file-list, but
this option sets the list of function names to be excluded from
instrumentation. The function name to be matched is its user-
visible name, such as "vector<int> blah(const vector<int> &)", not
the internal mangled name (e.g., "_Z4blahRSt6vectorIiSaIiEE"). The
match is done on substrings: if the sym parameter is a substring of
the function name, it is considered to be a match. For C99 and C++
extended identifiers, the function name must be given in UTF-8, not
using universal character names.
-fpatchable-function-entry=N[,M]
Generate N NOPs right at the beginning of each function, with the
function entry point before the Mth NOP. If M is omitted, it
defaults to 0 so the function entry points to the address just at
the first NOP. The NOP instructions reserve extra space which can
be used to patch in any desired instrumentation at run time,
provided that the code segment is writable. The amount of space is
controllable indirectly via the number of NOPs; the NOP instruction
used corresponds to the instruction emitted by the internal GCC
back-end interface "gen_nop". This behavior is target-specific and
may also depend on the architecture variant and/or other
compilation options.
For run-time identification, the starting addresses of these areas,
which correspond to their respective function entries minus M, are
additionally collected in the "__patchable_function_entries"
section of the resulting binary.
Note that the value of "__attribute__ ((patchable_function_entry
(N,M)))" takes precedence over command-line option
-fpatchable-function-entry=N,M. This can be used to increase the
area size or to remove it completely on a single function. If
"N=0", no pad location is recorded.
The NOP instructions are inserted at---and maybe before, depending
on M---the function entry address, even before the prologue.
Options Controlling the Preprocessor
These options control the C preprocessor, which is run on each C source
file before actual compilation.
If you use the -E option, nothing is done except preprocessing. Some
of these options make sense only together with -E because they cause
the preprocessor output to be unsuitable for actual compilation.
In addition to the options listed here, there are a number of options
to control search paths for include files documented in Directory
Options. Options to control preprocessor diagnostics are listed in
Warning Options.
-D name
Predefine name as a macro, with definition 1.
-D name=definition
The contents of definition are tokenized and processed as if they
appeared during translation phase three in a #define directive. In
particular, the definition is truncated by embedded newline
characters.
If you are invoking the preprocessor from a shell or shell-like
program you may need to use the shell's quoting syntax to protect
characters such as spaces that have a meaning in the shell syntax.
If you wish to define a function-like macro on the command line,
write its argument list with surrounding parentheses before the
equals sign (if any). Parentheses are meaningful to most shells,
so you should quote the option. With sh and csh,
-D'name(args...)=definition' works.
-D and -U options are processed in the order they are given on the
command line. All -imacros file and -include file options are
processed after all -D and -U options.
-U name
Cancel any previous definition of name, either built in or provided
with a -D option.
-include file
Process file as if "#include "file"" appeared as the first line of
the primary source file. However, the first directory searched for
file is the preprocessor's working directory instead of the
directory containing the main source file. If not found there, it
is searched for in the remainder of the "#include "..."" search
chain as normal.
If multiple -include options are given, the files are included in
the order they appear on the command line.
-imacros file
Exactly like -include, except that any output produced by scanning
file is thrown away. Macros it defines remain defined. This
allows you to acquire all the macros from a header without also
processing its declarations.
All files specified by -imacros are processed before all files
specified by -include.
-undef
Do not predefine any system-specific or GCC-specific macros. The
standard predefined macros remain defined.
-pthread
Define additional macros required for using the POSIX threads
library. You should use this option consistently for both
compilation and linking. This option is supported on GNU/Linux
targets, most other Unix derivatives, and also on x86 Cygwin and
MinGW targets.
-M Instead of outputting the result of preprocessing, output a rule
suitable for make describing the dependencies of the main source
file. The preprocessor outputs one make rule containing the object
file name for that source file, a colon, and the names of all the
included files, including those coming from -include or -imacros
command-line options.
Unless specified explicitly (with -MT or -MQ), the object file name
consists of the name of the source file with any suffix replaced
with object file suffix and with any leading directory parts
removed. If there are many included files then the rule is split
into several lines using \-newline. The rule has no commands.
This option does not suppress the preprocessor's debug output, such
as -dM. To avoid mixing such debug output with the dependency
rules you should explicitly specify the dependency output file with
-MF, or use an environment variable like DEPENDENCIES_OUTPUT.
Debug output is still sent to the regular output stream as normal.
Passing -M to the driver implies -E, and suppresses warnings with
an implicit -w.
-MM Like -M but do not mention header files that are found in system
header directories, nor header files that are included, directly or
indirectly, from such a header.
This implies that the choice of angle brackets or double quotes in
an #include directive does not in itself determine whether that
header appears in -MM dependency output.
-MF file
When used with -M or -MM, specifies a file to write the
dependencies to. If no -MF switch is given the preprocessor sends
the rules to the same place it would send preprocessed output.
When used with the driver options -MD or -MMD, -MF overrides the
default dependency output file.
If file is -, then the dependencies are written to stdout.
-MG In conjunction with an option such as -M requesting dependency
generation, -MG assumes missing header files are generated files
and adds them to the dependency list without raising an error. The
dependency filename is taken directly from the "#include" directive
without prepending any path. -MG also suppresses preprocessed
output, as a missing header file renders this useless.
This feature is used in automatic updating of makefiles.
-MP This option instructs CPP to add a phony target for each dependency
other than the main file, causing each to depend on nothing. These
dummy rules work around errors make gives if you remove header
files without updating the Makefile to match.
This is typical output:
test.o: test.c test.h
test.h:
-MT target
Change the target of the rule emitted by dependency generation. By
default CPP takes the name of the main input file, deletes any
directory components and any file suffix such as .c, and appends
the platform's usual object suffix. The result is the target.
An -MT option sets the target to be exactly the string you specify.
If you want multiple targets, you can specify them as a single
argument to -MT, or use multiple -MT options.
For example, -MT '$(objpfx)foo.o' might give
$(objpfx)foo.o: foo.c
-MQ target
Same as -MT, but it quotes any characters which are special to
Make. -MQ '$(objpfx)foo.o' gives
$$(objpfx)foo.o: foo.c
The default target is automatically quoted, as if it were given
with -MQ.
-MD -MD is equivalent to -M -MF file, except that -E is not implied.
The driver determines file based on whether an -o option is given.
If it is, the driver uses its argument but with a suffix of .d,
otherwise it takes the name of the input file, removes any
directory components and suffix, and applies a .d suffix.
If -MD is used in conjunction with -E, any -o switch is understood
to specify the dependency output file, but if used without -E, each
-o is understood to specify a target object file.
Since -E is not implied, -MD can be used to generate a dependency
output file as a side effect of the compilation process.
-MMD
Like -MD except mention only user header files, not system header
files.
-fpreprocessed
Indicate to the preprocessor that the input file has already been
preprocessed. This suppresses things like macro expansion,
trigraph conversion, escaped newline splicing, and processing of
most directives. The preprocessor still recognizes and removes
comments, so that you can pass a file preprocessed with -C to the
compiler without problems. In this mode the integrated
preprocessor is little more than a tokenizer for the front ends.
-fpreprocessed is implicit if the input file has one of the
extensions .i, .ii or .mi. These are the extensions that GCC uses
for preprocessed files created by -save-temps.
-cxx-isystem dir
Search dir for C++ header files, after all directories specified by
-I but before the standard system directories. Mark it as a system
directory, so that it gets the same special treatment as is applied
to the standard system directories.
-fdirectives-only
When preprocessing, handle directives, but do not expand macros.
The option's behavior depends on the -E and -fpreprocessed options.
With -E, preprocessing is limited to the handling of directives
such as "#define", "#ifdef", and "#error". Other preprocessor
operations, such as macro expansion and trigraph conversion are not
performed. In addition, the -dD option is implicitly enabled.
With -fpreprocessed, predefinition of command line and most builtin
macros is disabled. Macros such as "__LINE__", which are
contextually dependent, are handled normally. This enables
compilation of files previously preprocessed with "-E
-fdirectives-only".
With both -E and -fpreprocessed, the rules for -fpreprocessed take
precedence. This enables full preprocessing of files previously
preprocessed with "-E -fdirectives-only".
-iremap src:dst
Replace the prefix src in __FILE__ with dst at expansion time.
This option can be specified more than once. Processing stops at
the first match.
-fdollars-in-identifiers
Accept $ in identifiers.
-fextended-identifiers
Accept universal character names and extended characters in
identifiers. This option is enabled by default for C99 (and later
C standard versions) and C++.
-fno-canonical-system-headers
When preprocessing, do not shorten system header paths with
canonicalization.
-fmax-include-depth=depth
Set the maximum depth of the nested #include. The default is 200.
-ftabstop=width
Set the distance between tab stops. This helps the preprocessor
report correct column numbers in warnings or errors, even if tabs
appear on the line. If the value is less than 1 or greater than
100, the option is ignored. The default is 8.
-ftrack-macro-expansion[=level]
Track locations of tokens across macro expansions. This allows the
compiler to emit diagnostic about the current macro expansion stack
when a compilation error occurs in a macro expansion. Using this
option makes the preprocessor and the compiler consume more memory.
The level parameter can be used to choose the level of precision of
token location tracking thus decreasing the memory consumption if
necessary. Value 0 of level de-activates this option. Value 1
tracks tokens locations in a degraded mode for the sake of minimal
memory overhead. In this mode all tokens resulting from the
expansion of an argument of a function-like macro have the same
location. Value 2 tracks tokens locations completely. This value is
the most memory hungry. When this option is given no argument, the
default parameter value is 2.
Note that "-ftrack-macro-expansion=2" is activated by default.
-fmacro-prefix-map=old=new
When preprocessing files residing in directory old, expand the
"__FILE__" and "__BASE_FILE__" macros as if the files resided in
directory new instead. This can be used to change an absolute path
to a relative path by using . for new which can result in more
reproducible builds that are location independent. This option
also affects "__builtin_FILE()" during compilation. See also
-ffile-prefix-map.
-fexec-charset=charset
Set the execution character set, used for string and character
constants. The default is UTF-8. charset can be any encoding
supported by the system's "iconv" library routine.
-fwide-exec-charset=charset
Set the wide execution character set, used for wide string and
character constants. The default is one of UTF-32BE, UTF-32LE,
UTF-16BE, or UTF-16LE, whichever corresponds to the width of
"wchar_t" and the big-endian or little-endian byte order being used
for code generation. As with -fexec-charset, charset can be any
encoding supported by the system's "iconv" library routine;
however, you will have problems with encodings that do not fit
exactly in "wchar_t".
-finput-charset=charset
Set the input character set, used for translation from the
character set of the input file to the source character set used by
GCC. If the locale does not specify, or GCC cannot get this
information from the locale, the default is UTF-8. This can be
overridden by either the locale or this command-line option.
Currently the command-line option takes precedence if there's a
conflict. charset can be any encoding supported by the system's
"iconv" library routine.
-fpch-deps
When using precompiled headers, this flag causes the dependency-
output flags to also list the files from the precompiled header's
dependencies. If not specified, only the precompiled header are
listed and not the files that were used to create it, because those
files are not consulted when a precompiled header is used.
-fpch-preprocess
This option allows use of a precompiled header together with -E.
It inserts a special "#pragma", "#pragma GCC pch_preprocess
"filename"" in the output to mark the place where the precompiled
header was found, and its filename. When -fpreprocessed is in use,
GCC recognizes this "#pragma" and loads the PCH.
This option is off by default, because the resulting preprocessed
output is only really suitable as input to GCC. It is switched on
by -save-temps.
You should not write this "#pragma" in your own code, but it is
safe to edit the filename if the PCH file is available in a
different location. The filename may be absolute or it may be
relative to GCC's current directory.
-fworking-directory
Enable generation of linemarkers in the preprocessor output that
let the compiler know the current working directory at the time of
preprocessing. When this option is enabled, the preprocessor
emits, after the initial linemarker, a second linemarker with the
current working directory followed by two slashes. GCC uses this
directory, when it's present in the preprocessed input, as the
directory emitted as the current working directory in some
debugging information formats. This option is implicitly enabled
if debugging information is enabled, but this can be inhibited with
the negated form -fno-working-directory. If the -P flag is present
in the command line, this option has no effect, since no "#line"
directives are emitted whatsoever.
-A predicate=answer
Make an assertion with the predicate predicate and answer answer.
This form is preferred to the older form -A predicate(answer),
which is still supported, because it does not use shell special
characters.
-A -predicate=answer
Cancel an assertion with the predicate predicate and answer answer.
-C Do not discard comments. All comments are passed through to the
output file, except for comments in processed directives, which are
deleted along with the directive.
You should be prepared for side effects when using -C; it causes
the preprocessor to treat comments as tokens in their own right.
For example, comments appearing at the start of what would be a
directive line have the effect of turning that line into an
ordinary source line, since the first token on the line is no
longer a #.
-CC Do not discard comments, including during macro expansion. This is
like -C, except that comments contained within macros are also
passed through to the output file where the macro is expanded.
In addition to the side effects of the -C option, the -CC option
causes all C++-style comments inside a macro to be converted to
C-style comments. This is to prevent later use of that macro from
inadvertently commenting out the remainder of the source line.
The -CC option is generally used to support lint comments.
-P Inhibit generation of linemarkers in the output from the
preprocessor. This might be useful when running the preprocessor
on something that is not C code, and will be sent to a program
which might be confused by the linemarkers.
-traditional
-traditional-cpp
Try to imitate the behavior of pre-standard C preprocessors, as
opposed to ISO C preprocessors. See the GNU CPP manual for
details.
Note that GCC does not otherwise attempt to emulate a pre-standard
C compiler, and these options are only supported with the -E
switch, or when invoking CPP explicitly.
-trigraphs
Support ISO C trigraphs. These are three-character sequences, all
starting with ??, that are defined by ISO C to stand for single
characters. For example, ??/ stands for \, so '??/n' is a
character constant for a newline.
The nine trigraphs and their replacements are
Trigraph: ??( ??) ??< ??> ??= ??/ ??' ??! ??-
Replacement: [ ] { } # \ ^ | ~
By default, GCC ignores trigraphs, but in standard-conforming modes
it converts them. See the -std and -ansi options.
-remap
Enable special code to work around file systems which only permit
very short file names, such as MS-DOS.
-H Print the name of each header file used, in addition to other
normal activities. Each name is indented to show how deep in the
#include stack it is. Precompiled header files are also printed,
even if they are found to be invalid; an invalid precompiled header
file is printed with ...x and a valid one with ...! .
-dletters
Says to make debugging dumps during compilation as specified by
letters. The flags documented here are those relevant to the
preprocessor. Other letters are interpreted by the compiler
proper, or reserved for future versions of GCC, and so are silently
ignored. If you specify letters whose behavior conflicts, the
result is undefined.
-dM Instead of the normal output, generate a list of #define
directives for all the macros defined during the execution of
the preprocessor, including predefined macros. This gives you
a way of finding out what is predefined in your version of the
preprocessor. Assuming you have no file foo.h, the command
touch foo.h; cpp -dM foo.h
shows all the predefined macros.
If you use -dM without the -E option, -dM is interpreted as a
synonym for -fdump-rtl-mach.
-dD Like -dM except in two respects: it does not include the
predefined macros, and it outputs both the #define directives
and the result of preprocessing. Both kinds of output go to
the standard output file.
-dN Like -dD, but emit only the macro names, not their expansions.
-dI Output #include directives in addition to the result of
preprocessing.
-dU Like -dD except that only macros that are expanded, or whose
definedness is tested in preprocessor directives, are output;
the output is delayed until the use or test of the macro; and
#undef directives are also output for macros tested but
undefined at the time.
-fdebug-cpp
This option is only useful for debugging GCC. When used from CPP
or with -E, it dumps debugging information about location maps.
Every token in the output is preceded by the dump of the map its
location belongs to.
When used from GCC without -E, this option has no effect.
-Wp,option
You can use -Wp,option to bypass the compiler driver and pass
option directly through to the preprocessor. If option contains
commas, it is split into multiple options at the commas. However,
many options are modified, translated or interpreted by the
compiler driver before being passed to the preprocessor, and -Wp
forcibly bypasses this phase. The preprocessor's direct interface
is undocumented and subject to change, so whenever possible you
should avoid using -Wp and let the driver handle the options
instead.
-Xpreprocessor option
Pass option as an option to the preprocessor. You can use this to
supply system-specific preprocessor options that GCC does not
recognize.
If you want to pass an option that takes an argument, you must use
-Xpreprocessor twice, once for the option and once for the
argument.
-no-integrated-cpp
Perform preprocessing as a separate pass before compilation. By
default, GCC performs preprocessing as an integrated part of input
tokenization and parsing. If this option is provided, the
appropriate language front end (cc1, cc1plus, or cc1obj for C, C++,
and Objective-C, respectively) is instead invoked twice, once for
preprocessing only and once for actual compilation of the
preprocessed input. This option may be useful in conjunction with
the -B or -wrapper options to specify an alternate preprocessor or
perform additional processing of the program source between normal
preprocessing and compilation.
Passing Options to the Assembler
You can pass options to the assembler.
-Wa,option
Pass option as an option to the assembler. If option contains
commas, it is split into multiple options at the commas.
-Xassembler option
Pass option as an option to the assembler. You can use this to
supply system-specific assembler options that GCC does not
recognize.
If you want to pass an option that takes an argument, you must use
-Xassembler twice, once for the option and once for the argument.
Options for Linking
These options come into play when the compiler links object files into
an executable output file. They are meaningless if the compiler is not
doing a link step.
object-file-name
A file name that does not end in a special recognized suffix is
considered to name an object file or library. (Object files are
distinguished from libraries by the linker according to the file
contents.) If linking is done, these object files are used as
input to the linker.
-c
-S
-E If any of these options is used, then the linker is not run, and
object file names should not be used as arguments.
-flinker-output=type
This option controls code generation of the link-time optimizer.
By default the linker output is automatically determined by the
linker plugin. For debugging the compiler and if incremental
linking with a non-LTO object file is desired, it may be useful to
control the type manually.
If type is exec, code generation produces a static binary. In this
case -fpic and -fpie are both disabled.
If type is dyn, code generation produces a shared library. In this
case -fpic or -fPIC is preserved, but not enabled automatically.
This allows to build shared libraries without position-independent
code on architectures where this is possible, i.e. on x86.
If type is pie, code generation produces an -fpie executable. This
results in similar optimizations as exec except that -fpie is not
disabled if specified at compilation time.
If type is rel, the compiler assumes that incremental linking is
done. The sections containing intermediate code for link-time
optimization are merged, pre-optimized, and output to the resulting
object file. In addition, if -ffat-lto-objects is specified, binary
code is produced for future non-LTO linking. The object file
produced by incremental linking is smaller than a static library
produced from the same object files. At link time the result of
incremental linking also loads faster than a static library
assuming that the majority of objects in the library are used.
Finally nolto-rel configures the compiler for incremental linking
where code generation is forced, a final binary is produced, and
the intermediate code for later link-time optimization is stripped.
When multiple object files are linked together the resulting code
is better optimized than with link-time optimizations disabled (for
example, cross-module inlining happens), but most of benefits of
whole program optimizations are lost.
During the incremental link (by -r) the linker plugin defaults to
rel. With current interfaces to GNU Binutils it is however not
possible to incrementally link LTO objects and non-LTO objects into
a single mixed object file. If any of object files in incremental
link cannot be used for link-time optimization, the linker plugin
issues a warning and uses nolto-rel. To maintain whole program
optimization, it is recommended to link such objects into static
library instead. Alternatively it is possible to use H.J. Lu's
binutils with support for mixed objects.
-fuse-ld=bfd
Use the bfd linker instead of the default linker.
-fuse-ld=gold
Use the gold linker instead of the default linker.
-fuse-ld=lld
Use the LLVM lld linker instead of the default linker.
-llibrary
-l library
Search the library named library when linking. (The second
alternative with the library as a separate argument is only for
POSIX compliance and is not recommended.)
The -l option is passed directly to the linker by GCC. Refer to
your linker documentation for exact details. The general
description below applies to the GNU linker.
The linker searches a standard list of directories for the library.
The directories searched include several standard system
directories plus any that you specify with -L.
Static libraries are archives of object files, and have file names
like liblibrary.a. Some targets also support shared libraries,
which typically have names like liblibrary.so. If both static and
shared libraries are found, the linker gives preference to linking
with the shared library unless the -static option is used.
It makes a difference where in the command you write this option;
the linker searches and processes libraries and object files in the
order they are specified. Thus, foo.o -lz bar.o searches library z
after file foo.o but before bar.o. If bar.o refers to functions in
z, those functions may not be loaded.
-lobjc
You need this special case of the -l option in order to link an
Objective-C or Objective-C++ program.
-nostartfiles
Do not use the standard system startup files when linking. The
standard system libraries are used normally, unless -nostdlib,
-nolibc, or -nodefaultlibs is used.
-nodefaultlibs
Do not use the standard system libraries when linking. Only the
libraries you specify are passed to the linker, and options
specifying linkage of the system libraries, such as -static-libgcc
or -shared-libgcc, are ignored. The standard startup files are
used normally, unless -nostartfiles is used.
The compiler may generate calls to "memcmp", "memset", "memcpy" and
"memmove". These entries are usually resolved by entries in libc.
These entry points should be supplied through some other mechanism
when this option is specified.
-nolibc
Do not use the C library or system libraries tightly coupled with
it when linking. Still link with the startup files, libgcc or
toolchain provided language support libraries such as libgnat,
libgfortran or libstdc++ unless options preventing their inclusion
are used as well. This typically removes -lc from the link command
line, as well as system libraries that normally go with it and
become meaningless when absence of a C library is assumed, for
example -lpthread or -lm in some configurations. This is intended
for bare-board targets when there is indeed no C library available.
-nostdlib
Do not use the standard system startup files or libraries when
linking. No startup files and only the libraries you specify are
passed to the linker, and options specifying linkage of the system
libraries, such as -static-libgcc or -shared-libgcc, are ignored.
The compiler may generate calls to "memcmp", "memset", "memcpy" and
"memmove". These entries are usually resolved by entries in libc.
These entry points should be supplied through some other mechanism
when this option is specified.
One of the standard libraries bypassed by -nostdlib and
-nodefaultlibs is libgcc.a, a library of internal subroutines which
GCC uses to overcome shortcomings of particular machines, or
special needs for some languages.
In most cases, you need libgcc.a even when you want to avoid other
standard libraries. In other words, when you specify -nostdlib or
-nodefaultlibs you should usually specify -lgcc as well. This
ensures that you have no unresolved references to internal GCC
library subroutines. (An example of such an internal subroutine is
"__main", used to ensure C++ constructors are called.)
-e entry
--entry=entry
Specify that the program entry point is entry. The argument is
interpreted by the linker; the GNU linker accepts either a symbol
name or an address.
-pie
Produce a dynamically linked position independent executable on
targets that support it. For predictable results, you must also
specify the same set of options used for compilation (-fpie, -fPIE,
or model suboptions) when you specify this linker option.
-no-pie
Don't produce a dynamically linked position independent executable.
-static-pie
Produce a static position independent executable on targets that
support it. A static position independent executable is similar to
a static executable, but can be loaded at any address without a
dynamic linker. For predictable results, you must also specify the
same set of options used for compilation (-fpie, -fPIE, or model
suboptions) when you specify this linker option.
-pthread
Link with the POSIX threads library. This option is supported on
GNU/Linux targets, most other Unix derivatives, and also on x86
Cygwin and MinGW targets. On some targets this option also sets
flags for the preprocessor, so it should be used consistently for
both compilation and linking.
-r Produce a relocatable object as output. This is also known as
partial linking.
-rdynamic
Pass the flag -export-dynamic to the ELF linker, on targets that
support it. This instructs the linker to add all symbols, not only
used ones, to the dynamic symbol table. This option is needed for
some uses of "dlopen" or to allow obtaining backtraces from within
a program.
-s Remove all symbol table and relocation information from the
executable.
-static
On systems that support dynamic linking, this overrides -pie and
prevents linking with the shared libraries. On other systems, this
option has no effect.
-shared
Produce a shared object which can then be linked with other objects
to form an executable. Not all systems support this option. For
predictable results, you must also specify the same set of options
used for compilation (-fpic, -fPIC, or model suboptions) when you
specify this linker option.[1]
-shared-libgcc
-static-libgcc
On systems that provide libgcc as a shared library, these options
force the use of either the shared or static version, respectively.
If no shared version of libgcc was built when the compiler was
configured, these options have no effect.
There are several situations in which an application should use the
shared libgcc instead of the static version. The most common of
these is when the application wishes to throw and catch exceptions
across different shared libraries. In that case, each of the
libraries as well as the application itself should use the shared
libgcc.
Therefore, the G++ driver automatically adds -shared-libgcc
whenever you build a shared library or a main executable, because
C++ programs typically use exceptions, so this is the right thing
to do.
If, instead, you use the GCC driver to create shared libraries, you
may find that they are not always linked with the shared libgcc.
If GCC finds, at its configuration time, that you have a non-GNU
linker or a GNU linker that does not support option --eh-frame-hdr,
it links the shared version of libgcc into shared libraries by
default. Otherwise, it takes advantage of the linker and optimizes
away the linking with the shared version of libgcc, linking with
the static version of libgcc by default. This allows exceptions to
propagate through such shared libraries, without incurring
relocation costs at library load time.
However, if a library or main executable is supposed to throw or
catch exceptions, you must link it using the G++ driver, or using
the option -shared-libgcc, such that it is linked with the shared
libgcc.
-static-libasan
When the -fsanitize=address option is used to link a program, the
GCC driver automatically links against libasan. If libasan is
available as a shared library, and the -static option is not used,
then this links against the shared version of libasan. The
-static-libasan option directs the GCC driver to link libasan
statically, without necessarily linking other libraries statically.
-static-libtsan
When the -fsanitize=thread option is used to link a program, the
GCC driver automatically links against libtsan. If libtsan is
available as a shared library, and the -static option is not used,
then this links against the shared version of libtsan. The
-static-libtsan option directs the GCC driver to link libtsan
statically, without necessarily linking other libraries statically.
-static-liblsan
When the -fsanitize=leak option is used to link a program, the GCC
driver automatically links against liblsan. If liblsan is
available as a shared library, and the -static option is not used,
then this links against the shared version of liblsan. The
-static-liblsan option directs the GCC driver to link liblsan
statically, without necessarily linking other libraries statically.
-static-libubsan
When the -fsanitize=undefined option is used to link a program, the
GCC driver automatically links against libubsan. If libubsan is
available as a shared library, and the -static option is not used,
then this links against the shared version of libubsan. The
-static-libubsan option directs the GCC driver to link libubsan
statically, without necessarily linking other libraries statically.
-static-libstdc++
When the g++ program is used to link a C++ program, it normally
automatically links against libstdc++. If libstdc++ is available
as a shared library, and the -static option is not used, then this
links against the shared version of libstdc++. That is normally
fine. However, it is sometimes useful to freeze the version of
libstdc++ used by the program without going all the way to a fully
static link. The -static-libstdc++ option directs the g++ driver
to link libstdc++ statically, without necessarily linking other
libraries statically.
-symbolic
Bind references to global symbols when building a shared object.
Warn about any unresolved references (unless overridden by the link
editor option -Xlinker -z -Xlinker defs). Only a few systems
support this option.
-T script
Use script as the linker script. This option is supported by most
systems using the GNU linker. On some targets, such as bare-board
targets without an operating system, the -T option may be required
when linking to avoid references to undefined symbols.
-Xlinker option
Pass option as an option to the linker. You can use this to supply
system-specific linker options that GCC does not recognize.
If you want to pass an option that takes a separate argument, you
must use -Xlinker twice, once for the option and once for the
argument. For example, to pass -assert definitions, you must write
-Xlinker -assert -Xlinker definitions. It does not work to write
-Xlinker "-assert definitions", because this passes the entire
string as a single argument, which is not what the linker expects.
When using the GNU linker, it is usually more convenient to pass
arguments to linker options using the option=value syntax than as
separate arguments. For example, you can specify -Xlinker
-Map=output.map rather than -Xlinker -Map -Xlinker output.map.
Other linkers may not support this syntax for command-line options.
-Wl,option
Pass option as an option to the linker. If option contains commas,
it is split into multiple options at the commas. You can use this
syntax to pass an argument to the option. For example,
-Wl,-Map,output.map passes -Map output.map to the linker. When
using the GNU linker, you can also get the same effect with
-Wl,-Map=output.map.
-u symbol
Pretend the symbol symbol is undefined, to force linking of library
modules to define it. You can use -u multiple times with different
symbols to force loading of additional library modules.
-z keyword
-z is passed directly on to the linker along with the keyword
keyword. See the section in the documentation of your linker for
permitted values and their meanings.
Options for Directory Search
These options specify directories to search for header files, for
libraries and for parts of the compiler:
-I dir
-iquote dir
-isystem dir
-idirafter dir
Add the directory dir to the list of directories to be searched for
header files during preprocessing. If dir begins with = or
$SYSROOT, then the = or $SYSROOT is replaced by the sysroot prefix;
see --sysroot and -isysroot.
Directories specified with -iquote apply only to the quote form of
the directive, "#include "file"". Directories specified with -I,
-isystem, or -idirafter apply to lookup for both the
"#include "file"" and "#include <file>" directives.
You can specify any number or combination of these options on the
command line to search for header files in several directories.
The lookup order is as follows:
1. For the quote form of the include directive, the directory of
the current file is searched first.
2. For the quote form of the include directive, the directories
specified by -iquote options are searched in left-to-right
order, as they appear on the command line.
3. Directories specified with -I options are scanned in left-to-
right order.
4. Directories specified with -isystem options are scanned in
left-to-right order.
5. Standard system directories are scanned.
6. Directories specified with -idirafter options are scanned in
left-to-right order.
You can use -I to override a system header file, substituting your
own version, since these directories are searched before the
standard system header file directories. However, you should not
use this option to add directories that contain vendor-supplied
system header files; use -isystem for that.
The -isystem and -idirafter options also mark the directory as a
system directory, so that it gets the same special treatment that
is applied to the standard system directories.
If a standard system include directory, or a directory specified
with -isystem, is also specified with -I, the -I option is ignored.
The directory is still searched but as a system directory at its
normal position in the system include chain. This is to ensure
that GCC's procedure to fix buggy system headers and the ordering
for the "#include_next" directive are not inadvertently changed.
If you really need to change the search order for system
directories, use the -nostdinc and/or -isystem options.
-I- Split the include path. This option has been deprecated. Please
use -iquote instead for -I directories before the -I- and remove
the -I- option.
Any directories specified with -I options before -I- are searched
only for headers requested with "#include "file""; they are not
searched for "#include <file>". If additional directories are
specified with -I options after the -I-, those directories are
searched for all #include directives.
In addition, -I- inhibits the use of the directory of the current
file directory as the first search directory for "#include "file"".
There is no way to override this effect of -I-.
-iprefix prefix
Specify prefix as the prefix for subsequent -iwithprefix options.
If the prefix represents a directory, you should include the final
/.
-iwithprefix dir
-iwithprefixbefore dir
Append dir to the prefix specified previously with -iprefix, and
add the resulting directory to the include search path.
-iwithprefixbefore puts it in the same place -I would; -iwithprefix
puts it where -idirafter would.
-isysroot dir
This option is like the --sysroot option, but applies only to
header files (except for Darwin targets, where it applies to both
header files and libraries). See the --sysroot option for more
information.
-imultilib dir
Use dir as a subdirectory of the directory containing target-
specific C++ headers.
-nostdinc
Do not search the standard system directories for header files.
Only the directories explicitly specified with -I, -iquote,
-isystem, and/or -idirafter options (and the directory of the
current file, if appropriate) are searched.
-nostdinc++
Do not search for header files in the C++-specific standard
directories, but do still search the other standard directories.
(This option is used when building the C++ library.)
-iplugindir=dir
Set the directory to search for plugins that are passed by
-fplugin=name instead of -fplugin=path/name.so. This option is not
meant to be used by the user, but only passed by the driver.
-Ldir
Add directory dir to the list of directories to be searched for -l.
-Bprefix
This option specifies where to find the executables, libraries,
include files, and data files of the compiler itself.
The compiler driver program runs one or more of the subprograms
cpp, cc1, as and ld. It tries prefix as a prefix for each program
it tries to run, both with and without machine/version/ for the
corresponding target machine and compiler version.
For each subprogram to be run, the compiler driver first tries the
-B prefix, if any. If that name is not found, or if -B is not
specified, the driver tries two standard prefixes, /usr/lib/gcc/
and /usr/local/lib/gcc/. If neither of those results in a file
name that is found, the unmodified program name is searched for
using the directories specified in your PATH environment variable.
The compiler checks to see if the path provided by -B refers to a
directory, and if necessary it adds a directory separator character
at the end of the path.
-B prefixes that effectively specify directory names also apply to
libraries in the linker, because the compiler translates these
options into -L options for the linker. They also apply to include
files in the preprocessor, because the compiler translates these
options into -isystem options for the preprocessor. In this case,
the compiler appends include to the prefix.
The runtime support file libgcc.a can also be searched for using
the -B prefix, if needed. If it is not found there, the two
standard prefixes above are tried, and that is all. The file is
left out of the link if it is not found by those means.
Another way to specify a prefix much like the -B prefix is to use
the environment variable GCC_EXEC_PREFIX.
As a special kludge, if the path provided by -B is [dir/]stageN/,
where N is a number in the range 0 to 9, then it is replaced by
[dir/]include. This is to help with boot-strapping the compiler.
-no-canonical-prefixes
Do not expand any symbolic links, resolve references to /../ or
/./, or make the path absolute when generating a relative prefix.
--sysroot=dir
Use dir as the logical root directory for headers and libraries.
For example, if the compiler normally searches for headers in
/usr/include and libraries in /usr/lib, it instead searches
dir/usr/include and dir/usr/lib.
If you use both this option and the -isysroot option, then the
--sysroot option applies to libraries, but the -isysroot option
applies to header files.
The GNU linker (beginning with version 2.16) has the necessary
support for this option. If your linker does not support this
option, the header file aspect of --sysroot still works, but the
library aspect does not.
--no-sysroot-suffix
For some targets, a suffix is added to the root directory specified
with --sysroot, depending on the other options used, so that
headers may for example be found in dir/suffix/usr/include instead
of dir/usr/include. This option disables the addition of such a
suffix.
Options for Code Generation Conventions
These machine-independent options control the interface conventions
used in code generation.
Most of them have both positive and negative forms; the negative form
of -ffoo is -fno-foo. In the table below, only one of the forms is
listed---the one that is not the default. You can figure out the other
form by either removing no- or adding it.
-fstack-reuse=reuse-level
This option controls stack space reuse for user declared local/auto
variables and compiler generated temporaries. reuse_level can be
all, named_vars, or none. all enables stack reuse for all local
variables and temporaries, named_vars enables the reuse only for
user defined local variables with names, and none disables stack
reuse completely. The default value is all. The option is needed
when the program extends the lifetime of a scoped local variable or
a compiler generated temporary beyond the end point defined by the
language. When a lifetime of a variable ends, and if the variable
lives in memory, the optimizing compiler has the freedom to reuse
its stack space with other temporaries or scoped local variables
whose live range does not overlap with it. Legacy code extending
local lifetime is likely to break with the stack reuse
optimization.
For example,
int *p;
{
int local1;
p = &local1;
local1 = 10;
....
}
{
int local2;
local2 = 20;
...
}
if (*p == 10) // out of scope use of local1
{
}
Another example:
struct A
{
A(int k) : i(k), j(k) { }
int i;
int j;
};
A *ap;
void foo(const A& ar)
{
ap = &ar;
}
void bar()
{
foo(A(10)); // temp object's lifetime ends when foo returns
{
A a(20);
....
}
ap->i+= 10; // ap references out of scope temp whose space
// is reused with a. What is the value of ap->i?
}
The lifetime of a compiler generated temporary is well defined by
the C++ standard. When a lifetime of a temporary ends, and if the
temporary lives in memory, the optimizing compiler has the freedom
to reuse its stack space with other temporaries or scoped local
variables whose live range does not overlap with it. However some
of the legacy code relies on the behavior of older compilers in
which temporaries' stack space is not reused, the aggressive stack
reuse can lead to runtime errors. This option is used to control
the temporary stack reuse optimization.
-ftrapv
This option generates traps for signed overflow on addition,
subtraction, multiplication operations. The options -ftrapv and
-fwrapv override each other, so using -ftrapv -fwrapv on the
command-line results in -fwrapv being effective. Note that only
active options override, so using -ftrapv -fwrapv -fno-wrapv on the
command-line results in -ftrapv being effective.
-fwrapv
This option instructs the compiler to assume that signed arithmetic
overflow of addition, subtraction and multiplication wraps around
using twos-complement representation. This flag enables some
optimizations and disables others. The options -ftrapv and -fwrapv
override each other, so using -ftrapv -fwrapv on the command-line
results in -fwrapv being effective. Note that only active options
override, so using -ftrapv -fwrapv -fno-wrapv on the command-line
results in -ftrapv being effective.
-fwrapv-pointer
This option instructs the compiler to assume that pointer
arithmetic overflow on addition and subtraction wraps around using
twos-complement representation. This flag disables some
optimizations which assume pointer overflow is invalid.
-fstrict-overflow
This option implies -fno-wrapv -fno-wrapv-pointer and when negated
implies -fwrapv -fwrapv-pointer.
-fexceptions
Enable exception handling. Generates extra code needed to
propagate exceptions. For some targets, this implies GCC generates
frame unwind information for all functions, which can produce
significant data size overhead, although it does not affect
execution. If you do not specify this option, GCC enables it by
default for languages like C++ that normally require exception
handling, and disables it for languages like C that do not normally
require it. However, you may need to enable this option when
compiling C code that needs to interoperate properly with exception
handlers written in C++. You may also wish to disable this option
if you are compiling older C++ programs that don't use exception
handling.
-fnon-call-exceptions
Generate code that allows trapping instructions to throw
exceptions. Note that this requires platform-specific runtime
support that does not exist everywhere. Moreover, it only allows
trapping instructions to throw exceptions, i.e. memory references
or floating-point instructions. It does not allow exceptions to be
thrown from arbitrary signal handlers such as "SIGALRM".
-fdelete-dead-exceptions
Consider that instructions that may throw exceptions but don't
otherwise contribute to the execution of the program can be
optimized away. This option is enabled by default for the Ada
front end, as permitted by the Ada language specification.
Optimization passes that cause dead exceptions to be removed are
enabled independently at different optimization levels.
-funwind-tables
Similar to -fexceptions, except that it just generates any needed
static data, but does not affect the generated code in any other
way. You normally do not need to enable this option; instead, a
language processor that needs this handling enables it on your
behalf.
-fasynchronous-unwind-tables
Generate unwind table in DWARF format, if supported by target
machine. The table is exact at each instruction boundary, so it
can be used for stack unwinding from asynchronous events (such as
debugger or garbage collector).
-fno-gnu-unique
On systems with recent GNU assembler and C library, the C++
compiler uses the "STB_GNU_UNIQUE" binding to make sure that
definitions of template static data members and static local
variables in inline functions are unique even in the presence of
"RTLD_LOCAL"; this is necessary to avoid problems with a library
used by two different "RTLD_LOCAL" plugins depending on a
definition in one of them and therefore disagreeing with the other
one about the binding of the symbol. But this causes "dlclose" to
be ignored for affected DSOs; if your program relies on
reinitialization of a DSO via "dlclose" and "dlopen", you can use
-fno-gnu-unique.
-fpcc-struct-return
Return "short" "struct" and "union" values in memory like longer
ones, rather than in registers. This convention is less efficient,
but it has the advantage of allowing intercallability between GCC-
compiled files and files compiled with other compilers,
particularly the Portable C Compiler (pcc).
The precise convention for returning structures in memory depends
on the target configuration macros.
Short structures and unions are those whose size and alignment
match that of some integer type.
Warning: code compiled with the -fpcc-struct-return switch is not
binary compatible with code compiled with the -freg-struct-return
switch. Use it to conform to a non-default application binary
interface.
-freg-struct-return
Return "struct" and "union" values in registers when possible.
This is more efficient for small structures than
-fpcc-struct-return.
If you specify neither -fpcc-struct-return nor -freg-struct-return,
GCC defaults to whichever convention is standard for the target.
If there is no standard convention, GCC defaults to
-fpcc-struct-return, except on targets where GCC is the principal
compiler. In those cases, we can choose the standard, and we chose
the more efficient register return alternative.
Warning: code compiled with the -freg-struct-return switch is not
binary compatible with code compiled with the -fpcc-struct-return
switch. Use it to conform to a non-default application binary
interface.
-fshort-enums
Allocate to an "enum" type only as many bytes as it needs for the
declared range of possible values. Specifically, the "enum" type
is equivalent to the smallest integer type that has enough room.
Warning: the -fshort-enums switch causes GCC to generate code that
is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
-fshort-wchar
Override the underlying type for "wchar_t" to be "short unsigned
int" instead of the default for the target. This option is useful
for building programs to run under WINE.
Warning: the -fshort-wchar switch causes GCC to generate code that
is not binary compatible with code generated without that switch.
Use it to conform to a non-default application binary interface.
-fcommon
In C code, this option controls the placement of global variables
defined without an initializer, known as tentative definitions in
the C standard. Tentative definitions are distinct from
declarations of a variable with the "extern" keyword, which do not
allocate storage.
The default is -fno-common, which specifies that the compiler
places uninitialized global variables in the BSS section of the
object file. This inhibits the merging of tentative definitions by
the linker so you get a multiple-definition error if the same
variable is accidentally defined in more than one compilation unit.
The -fcommon places uninitialized global variables in a common
block. This allows the linker to resolve all tentative definitions
of the same variable in different compilation units to the same
object, or to a non-tentative definition. This behavior is
inconsistent with C++, and on many targets implies a speed and code
size penalty on global variable references. It is mainly useful to
enable legacy code to link without errors.
-fno-ident
Ignore the "#ident" directive.
-finhibit-size-directive
Don't output a ".size" assembler directive, or anything else that
would cause trouble if the function is split in the middle, and the
two halves are placed at locations far apart in memory. This
option is used when compiling crtstuff.c; you should not need to
use it for anything else.
-fverbose-asm
Put extra commentary information in the generated assembly code to
make it more readable. This option is generally only of use to
those who actually need to read the generated assembly code
(perhaps while debugging the compiler itself).
-fno-verbose-asm, the default, causes the extra information to be
omitted and is useful when comparing two assembler files.
The added comments include:
* information on the compiler version and command-line options,
* the source code lines associated with the assembly
instructions, in the form FILENAME:LINENUMBER:CONTENT OF LINE,
* hints on which high-level expressions correspond to the various
assembly instruction operands.
For example, given this C source file:
int test (int n)
{
int i;
int total = 0;
for (i = 0; i < n; i++)
total += i * i;
return total;
}
compiling to (x86_64) assembly via -S and emitting the result
direct to stdout via -o -
gcc -S test.c -fverbose-asm -Os -o -
gives output similar to this:
.file "test.c"
# GNU C11 (GCC) version 7.0.0 20160809 (experimental) (x86_64-pc-linux-gnu)
[...snip...]
# options passed:
[...snip...]
.text
.globl test
.type test, @function
test:
.LFB0:
.cfi_startproc
# test.c:4: int total = 0;
xorl %eax, %eax # <retval>
# test.c:6: for (i = 0; i < n; i++)
xorl %edx, %edx # i
.L2:
# test.c:6: for (i = 0; i < n; i++)
cmpl %edi, %edx # n, i
jge .L5 #,
# test.c:7: total += i * i;
movl %edx, %ecx # i, tmp92
imull %edx, %ecx # i, tmp92
# test.c:6: for (i = 0; i < n; i++)
incl %edx # i
# test.c:7: total += i * i;
addl %ecx, %eax # tmp92, <retval>
jmp .L2 #
.L5:
# test.c:10: }
ret
.cfi_endproc
.LFE0:
.size test, .-test
.ident "GCC: (GNU) 7.0.0 20160809 (experimental)"
.section .note.GNU-stack,"",@progbits
The comments are intended for humans rather than machines and hence
the precise format of the comments is subject to change.
-frecord-gcc-switches
This switch causes the command line used to invoke the compiler to
be recorded into the object file that is being created. This
switch is only implemented on some targets and the exact format of
the recording is target and binary file format dependent, but it
usually takes the form of a section containing ASCII text. This
switch is related to the -fverbose-asm switch, but that switch only
records information in the assembler output file as comments, so it
never reaches the object file. See also -grecord-gcc-switches for
another way of storing compiler options into the object file.
-fpic
Generate position-independent code (PIC) suitable for use in a
shared library, if supported for the target machine. Such code
accesses all constant addresses through a global offset table
(GOT). The dynamic loader resolves the GOT entries when the
program starts (the dynamic loader is not part of GCC; it is part
of the operating system). If the GOT size for the linked
executable exceeds a machine-specific maximum size, you get an
error message from the linker indicating that -fpic does not work;
in that case, recompile with -fPIC instead. (These maximums are 8k
on the SPARC, 28k on AArch64 and 32k on the m68k and RS/6000. The
x86 has no such limit.)
Position-independent code requires special support, and therefore
works only on certain machines. For the x86, GCC supports PIC for
System V but not for the Sun 386i. Code generated for the IBM
RS/6000 is always position-independent.
When this flag is set, the macros "__pic__" and "__PIC__" are
defined to 1.
-fPIC
If supported for the target machine, emit position-independent
code, suitable for dynamic linking and avoiding any limit on the
size of the global offset table. This option makes a difference on
AArch64, m68k, PowerPC and SPARC.
Position-independent code requires special support, and therefore
works only on certain machines.
When this flag is set, the macros "__pic__" and "__PIC__" are
defined to 2.
-fpie
-fPIE
These options are similar to -fpic and -fPIC, but the generated
position-independent code can be only linked into executables.
Usually these options are used to compile code that will be linked
using the -pie GCC option.
-fpie and -fPIE both define the macros "__pie__" and "__PIE__".
The macros have the value 1 for -fpie and 2 for -fPIE.
-fno-plt
Do not use the PLT for external function calls in position-
independent code. Instead, load the callee address at call sites
from the GOT and branch to it. This leads to more efficient code
by eliminating PLT stubs and exposing GOT loads to optimizations.
On architectures such as 32-bit x86 where PLT stubs expect the GOT
pointer in a specific register, this gives more register allocation
freedom to the compiler. Lazy binding requires use of the PLT;
with -fno-plt all external symbols are resolved at load time.
Alternatively, the function attribute "noplt" can be used to avoid
calls through the PLT for specific external functions.
In position-dependent code, a few targets also convert calls to
functions that are marked to not use the PLT to use the GOT
instead.
-fno-jump-tables
Do not use jump tables for switch statements even where it would be
more efficient than other code generation strategies. This option
is of use in conjunction with -fpic or -fPIC for building code that
forms part of a dynamic linker and cannot reference the address of
a jump table. On some targets, jump tables do not require a GOT
and this option is not needed.
-ffixed-reg
Treat the register named reg as a fixed register; generated code
should never refer to it (except perhaps as a stack pointer, frame
pointer or in some other fixed role).
reg must be the name of a register. The register names accepted
are machine-specific and are defined in the "REGISTER_NAMES" macro
in the machine description macro file.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-used-reg
Treat the register named reg as an allocable register that is
clobbered by function calls. It may be allocated for temporaries
or variables that do not live across a call. Functions compiled
this way do not save and restore the register reg.
It is an error to use this flag with the frame pointer or stack
pointer. Use of this flag for other registers that have fixed
pervasive roles in the machine's execution model produces
disastrous results.
This flag does not have a negative form, because it specifies a
three-way choice.
-fcall-saved-reg
Treat the register named reg as an allocable register saved by
functions. It may be allocated even for temporaries or variables
that live across a call. Functions compiled this way save and
restore the register reg if they use it.
It is an error to use this flag with the frame pointer or stack
pointer. Use of this flag for other registers that have fixed
pervasive roles in the machine's execution model produces
disastrous results.
A different sort of disaster results from the use of this flag for
a register in which function values may be returned.
This flag does not have a negative form, because it specifies a
three-way choice.
-fpack-struct[=n]
Without a value specified, pack all structure members together
without holes. When a value is specified (which must be a small
power of two), pack structure members according to this value,
representing the maximum alignment (that is, objects with default
alignment requirements larger than this are output potentially
unaligned at the next fitting location.
Warning: the -fpack-struct switch causes GCC to generate code that
is not binary compatible with code generated without that switch.
Additionally, it makes the code suboptimal. Use it to conform to a
non-default application binary interface.
-fleading-underscore
This option and its counterpart, -fno-leading-underscore, forcibly
change the way C symbols are represented in the object file. One
use is to help link with legacy assembly code.
Warning: the -fleading-underscore switch causes GCC to generate
code that is not binary compatible with code generated without that
switch. Use it to conform to a non-default application binary
interface. Not all targets provide complete support for this
switch.
-ftls-model=model
Alter the thread-local storage model to be used. The model
argument should be one of global-dynamic, local-dynamic, initial-
exec or local-exec. Note that the choice is subject to
optimization: the compiler may use a more efficient model for
symbols not visible outside of the translation unit, or if -fpic is
not given on the command line.
The default without -fpic is initial-exec; with -fpic the default
is global-dynamic.
-ftrampolines
For targets that normally need trampolines for nested functions,
always generate them instead of using descriptors. Otherwise, for
targets that do not need them, like for example HP-PA or IA-64, do
nothing.
A trampoline is a small piece of code that is created at run time
on the stack when the address of a nested function is taken, and is
used to call the nested function indirectly. Therefore, it
requires the stack to be made executable in order for the program
to work properly.
-fno-trampolines is enabled by default on a language by language
basis to let the compiler avoid generating them, if it computes
that this is safe, and replace them with descriptors. Descriptors
are made up of data only, but the generated code must be prepared
to deal with them. As of this writing, -fno-trampolines is enabled
by default only for Ada.
Moreover, code compiled with -ftrampolines and code compiled with
-fno-trampolines are not binary compatible if nested functions are
present. This option must therefore be used on a program-wide
basis and be manipulated with extreme care.
-fvisibility=[default|internal|hidden|protected]
Set the default ELF image symbol visibility to the specified
option---all symbols are marked with this unless overridden within
the code. Using this feature can very substantially improve
linking and load times of shared object libraries, produce more
optimized code, provide near-perfect API export and prevent symbol
clashes. It is strongly recommended that you use this in any
shared objects you distribute.
Despite the nomenclature, default always means public; i.e.,
available to be linked against from outside the shared object.
protected and internal are pretty useless in real-world usage so
the only other commonly used option is hidden. The default if
-fvisibility isn't specified is default, i.e., make every symbol
public.
A good explanation of the benefits offered by ensuring ELF symbols
have the correct visibility is given by "How To Write Shared
Libraries" by Ulrich Drepper (which can be found at
<https://www.akkadia.org/drepper/>)---however a superior solution
made possible by this option to marking things hidden when the
default is public is to make the default hidden and mark things
public. This is the norm with DLLs on Windows and with
-fvisibility=hidden and "__attribute__ ((visibility("default")))"
instead of "__declspec(dllexport)" you get almost identical
semantics with identical syntax. This is a great boon to those
working with cross-platform projects.
For those adding visibility support to existing code, you may find
"#pragma GCC visibility" of use. This works by you enclosing the
declarations you wish to set visibility for with (for example)
"#pragma GCC visibility push(hidden)" and "#pragma GCC visibility
pop". Bear in mind that symbol visibility should be viewed as part
of the API interface contract and thus all new code should always
specify visibility when it is not the default; i.e., declarations
only for use within the local DSO should always be marked
explicitly as hidden as so to avoid PLT indirection
overheads---making this abundantly clear also aids readability and
self-documentation of the code. Note that due to ISO C++
specification requirements, "operator new" and "operator delete"
must always be of default visibility.
Be aware that headers from outside your project, in particular
system headers and headers from any other library you use, may not
be expecting to be compiled with visibility other than the default.
You may need to explicitly say "#pragma GCC visibility
push(default)" before including any such headers.
"extern" declarations are not affected by -fvisibility, so a lot of
code can be recompiled with -fvisibility=hidden with no
modifications. However, this means that calls to "extern"
functions with no explicit visibility use the PLT, so it is more
effective to use "__attribute ((visibility))" and/or "#pragma GCC
visibility" to tell the compiler which "extern" declarations should
be treated as hidden.
Note that -fvisibility does affect C++ vague linkage entities. This
means that, for instance, an exception class that is be thrown
between DSOs must be explicitly marked with default visibility so
that the type_info nodes are unified between the DSOs.
An overview of these techniques, their benefits and how to use them
is at <http://gcc.gnu.org/wiki/Visibility>.
-fstrict-volatile-bitfields
This option should be used if accesses to volatile bit-fields (or
other structure fields, although the compiler usually honors those
types anyway) should use a single access of the width of the
field's type, aligned to a natural alignment if possible. For
example, targets with memory-mapped peripheral registers might
require all such accesses to be 16 bits wide; with this flag you
can declare all peripheral bit-fields as "unsigned short" (assuming
short is 16 bits on these targets) to force GCC to use 16-bit
accesses instead of, perhaps, a more efficient 32-bit access.
If this option is disabled, the compiler uses the most efficient
instruction. In the previous example, that might be a 32-bit load
instruction, even though that accesses bytes that do not contain
any portion of the bit-field, or memory-mapped registers unrelated
to the one being updated.
In some cases, such as when the "packed" attribute is applied to a
structure field, it may not be possible to access the field with a
single read or write that is correctly aligned for the target
machine. In this case GCC falls back to generating multiple
accesses rather than code that will fault or truncate the result at
run time.
Note: Due to restrictions of the C/C++11 memory model, write
accesses are not allowed to touch non bit-field members. It is
therefore recommended to define all bits of the field's type as
bit-field members.
The default value of this option is determined by the application
binary interface for the target processor.
-fsync-libcalls
This option controls whether any out-of-line instance of the
"__sync" family of functions may be used to implement the C++11
"__atomic" family of functions.
The default value of this option is enabled, thus the only useful
form of the option is -fno-sync-libcalls. This option is used in
the implementation of the libatomic runtime library.
GCC Developer Options
This section describes command-line options that are primarily of
interest to GCC developers, including options to support compiler
testing and investigation of compiler bugs and compile-time performance
problems. This includes options that produce debug dumps at various
points in the compilation; that print statistics such as memory use and
execution time; and that print information about GCC's configuration,
such as where it searches for libraries. You should rarely need to use
any of these options for ordinary compilation and linking tasks.
Many developer options that cause GCC to dump output to a file take an
optional =filename suffix. You can specify stdout or - to dump to
standard output, and stderr for standard error.
If =filename is omitted, a default dump file name is constructed by
concatenating the base dump file name, a pass number, phase letter, and
pass name. The base dump file name is the name of output file produced
by the compiler if explicitly specified and not an executable;
otherwise it is the source file name. The pass number is determined by
the order passes are registered with the compiler's pass manager. This
is generally the same as the order of execution, but passes registered
by plugins, target-specific passes, or passes that are otherwise
registered late are numbered higher than the pass named final, even if
they are executed earlier. The phase letter is one of i (inter-
procedural analysis), l (language-specific), r (RTL), or t (tree). The
files are created in the directory of the output file.
-fcallgraph-info
-fcallgraph-info=MARKERS
Makes the compiler output callgraph information for the program, on
a per-object-file basis. The information is generated in the
common VCG format. It can be decorated with additional, per-node
and/or per-edge information, if a list of comma-separated markers
is additionally specified. When the "su" marker is specified, the
callgraph is decorated with stack usage information; it is
equivalent to -fstack-usage. When the "da" marker is specified,
the callgraph is decorated with information about dynamically
allocated objects.
When compiling with -flto, no callgraph information is output along
with the object file. At LTO link time, -fcallgraph-info may
generate multiple callgraph information files next to intermediate
LTO output files.
-dletters
-fdump-rtl-pass
-fdump-rtl-pass=filename
Says to make debugging dumps during compilation at times specified
by letters. This is used for debugging the RTL-based passes of the
compiler.
Some -dletters switches have different meaning when -E is used for
preprocessing.
Debug dumps can be enabled with a -fdump-rtl switch or some -d
option letters. Here are the possible letters for use in pass and
letters, and their meanings:
-fdump-rtl-alignments
Dump after branch alignments have been computed.
-fdump-rtl-asmcons
Dump after fixing rtl statements that have unsatisfied in/out
constraints.
-fdump-rtl-auto_inc_dec
Dump after auto-inc-dec discovery. This pass is only run on
architectures that have auto inc or auto dec instructions.
-fdump-rtl-barriers
Dump after cleaning up the barrier instructions.
-fdump-rtl-bbpart
Dump after partitioning hot and cold basic blocks.
-fdump-rtl-bbro
Dump after block reordering.
-fdump-rtl-btl1
-fdump-rtl-btl2
-fdump-rtl-btl1 and -fdump-rtl-btl2 enable dumping after the
two branch target load optimization passes.
-fdump-rtl-bypass
Dump after jump bypassing and control flow optimizations.
-fdump-rtl-combine
Dump after the RTL instruction combination pass.
-fdump-rtl-compgotos
Dump after duplicating the computed gotos.
-fdump-rtl-ce1
-fdump-rtl-ce2
-fdump-rtl-ce3
-fdump-rtl-ce1, -fdump-rtl-ce2, and -fdump-rtl-ce3 enable
dumping after the three if conversion passes.
-fdump-rtl-cprop_hardreg
Dump after hard register copy propagation.
-fdump-rtl-csa
Dump after combining stack adjustments.
-fdump-rtl-cse1
-fdump-rtl-cse2
-fdump-rtl-cse1 and -fdump-rtl-cse2 enable dumping after the
two common subexpression elimination passes.
-fdump-rtl-dce
Dump after the standalone dead code elimination passes.
-fdump-rtl-dbr
Dump after delayed branch scheduling.
-fdump-rtl-dce1
-fdump-rtl-dce2
-fdump-rtl-dce1 and -fdump-rtl-dce2 enable dumping after the
two dead store elimination passes.
-fdump-rtl-eh
Dump after finalization of EH handling code.
-fdump-rtl-eh_ranges
Dump after conversion of EH handling range regions.
-fdump-rtl-expand
Dump after RTL generation.
-fdump-rtl-fwprop1
-fdump-rtl-fwprop2
-fdump-rtl-fwprop1 and -fdump-rtl-fwprop2 enable dumping after
the two forward propagation passes.
-fdump-rtl-gcse1
-fdump-rtl-gcse2
-fdump-rtl-gcse1 and -fdump-rtl-gcse2 enable dumping after
global common subexpression elimination.
-fdump-rtl-init-regs
Dump after the initialization of the registers.
-fdump-rtl-initvals
Dump after the computation of the initial value sets.
-fdump-rtl-into_cfglayout
Dump after converting to cfglayout mode.
-fdump-rtl-ira
Dump after iterated register allocation.
-fdump-rtl-jump
Dump after the second jump optimization.
-fdump-rtl-loop2
-fdump-rtl-loop2 enables dumping after the rtl loop
optimization passes.
-fdump-rtl-mach
Dump after performing the machine dependent reorganization
pass, if that pass exists.
-fdump-rtl-mode_sw
Dump after removing redundant mode switches.
-fdump-rtl-rnreg
Dump after register renumbering.
-fdump-rtl-outof_cfglayout
Dump after converting from cfglayout mode.
-fdump-rtl-peephole2
Dump after the peephole pass.
-fdump-rtl-postreload
Dump after post-reload optimizations.
-fdump-rtl-pro_and_epilogue
Dump after generating the function prologues and epilogues.
-fdump-rtl-sched1
-fdump-rtl-sched2
-fdump-rtl-sched1 and -fdump-rtl-sched2 enable dumping after
the basic block scheduling passes.
-fdump-rtl-ree
Dump after sign/zero extension elimination.
-fdump-rtl-seqabstr
Dump after common sequence discovery.
-fdump-rtl-shorten
Dump after shortening branches.
-fdump-rtl-sibling
Dump after sibling call optimizations.
-fdump-rtl-split1
-fdump-rtl-split2
-fdump-rtl-split3
-fdump-rtl-split4
-fdump-rtl-split5
These options enable dumping after five rounds of instruction
splitting.
-fdump-rtl-sms
Dump after modulo scheduling. This pass is only run on some
architectures.
-fdump-rtl-stack
Dump after conversion from GCC's "flat register file" registers
to the x87's stack-like registers. This pass is only run on
x86 variants.
-fdump-rtl-subreg1
-fdump-rtl-subreg2
-fdump-rtl-subreg1 and -fdump-rtl-subreg2 enable dumping after
the two subreg expansion passes.
-fdump-rtl-unshare
Dump after all rtl has been unshared.
-fdump-rtl-vartrack
Dump after variable tracking.
-fdump-rtl-vregs
Dump after converting virtual registers to hard registers.
-fdump-rtl-web
Dump after live range splitting.
-fdump-rtl-regclass
-fdump-rtl-subregs_of_mode_init
-fdump-rtl-subregs_of_mode_finish
-fdump-rtl-dfinit
-fdump-rtl-dfinish
These dumps are defined but always produce empty files.
-da
-fdump-rtl-all
Produce all the dumps listed above.
-dA Annotate the assembler output with miscellaneous debugging
information.
-dD Dump all macro definitions, at the end of preprocessing, in
addition to normal output.
-dH Produce a core dump whenever an error occurs.
-dp Annotate the assembler output with a comment indicating which
pattern and alternative is used. The length and cost of each
instruction are also printed.
-dP Dump the RTL in the assembler output as a comment before each
instruction. Also turns on -dp annotation.
-dx Just generate RTL for a function instead of compiling it.
Usually used with -fdump-rtl-expand.
-fdump-debug
Dump debugging information generated during the debug generation
phase.
-fdump-earlydebug
Dump debugging information generated during the early debug
generation phase.
-fdump-noaddr
When doing debugging dumps, suppress address output. This makes it
more feasible to use diff on debugging dumps for compiler
invocations with different compiler binaries and/or different text
/ bss / data / heap / stack / dso start locations.
-freport-bug
Collect and dump debug information into a temporary file if an
internal compiler error (ICE) occurs.
-fdump-unnumbered
When doing debugging dumps, suppress instruction numbers and
address output. This makes it more feasible to use diff on
debugging dumps for compiler invocations with different options, in
particular with and without -g.
-fdump-unnumbered-links
When doing debugging dumps (see -d option above), suppress
instruction numbers for the links to the previous and next
instructions in a sequence.
-fdump-ipa-switch
-fdump-ipa-switch-options
Control the dumping at various stages of inter-procedural analysis
language tree to a file. The file name is generated by appending a
switch specific suffix to the source file name, and the file is
created in the same directory as the output file. The following
dumps are possible:
all Enables all inter-procedural analysis dumps.
cgraph
Dumps information about call-graph optimization, unused
function removal, and inlining decisions.
inline
Dump after function inlining.
Additionally, the options -optimized, -missed, -note, and -all can
be provided, with the same meaning as for -fopt-info, defaulting to
-optimized.
For example, -fdump-ipa-inline-optimized-missed will emit
information on callsites that were inlined, along with callsites
that were not inlined.
By default, the dump will contain messages about successful
optimizations (equivalent to -optimized) together with low-level
details about the analysis.
-fdump-lang-all
-fdump-lang-switch
-fdump-lang-switch-options
-fdump-lang-switch-options=filename
Control the dumping of language-specific information. The options
and filename portions behave as described in the -fdump-tree
option. The following switch values are accepted:
all Enable all language-specific dumps.
class
Dump class hierarchy information. Virtual table information is
emitted unless 'slim' is specified. This option is applicable
to C++ only.
raw Dump the raw internal tree data. This option is applicable to
C++ only.
-fdump-passes
Print on stderr the list of optimization passes that are turned on
and off by the current command-line options.
-fdump-statistics-option
Enable and control dumping of pass statistics in a separate file.
The file name is generated by appending a suffix ending in
.statistics to the source file name, and the file is created in the
same directory as the output file. If the -option form is used,
-stats causes counters to be summed over the whole compilation unit
while -details dumps every event as the passes generate them. The
default with no option is to sum counters for each function
compiled.
-fdump-tree-all
-fdump-tree-switch
-fdump-tree-switch-options
-fdump-tree-switch-options=filename
Control the dumping at various stages of processing the
intermediate language tree to a file. If the -options form is
used, options is a list of - separated options which control the
details of the dump. Not all options are applicable to all dumps;
those that are not meaningful are ignored. The following options
are available
address
Print the address of each node. Usually this is not meaningful
as it changes according to the environment and source file.
Its primary use is for tying up a dump file with a debug
environment.
asmname
If "DECL_ASSEMBLER_NAME" has been set for a given decl, use
that in the dump instead of "DECL_NAME". Its primary use is
ease of use working backward from mangled names in the assembly
file.
slim
When dumping front-end intermediate representations, inhibit
dumping of members of a scope or body of a function merely
because that scope has been reached. Only dump such items when
they are directly reachable by some other path.
When dumping pretty-printed trees, this option inhibits dumping
the bodies of control structures.
When dumping RTL, print the RTL in slim (condensed) form
instead of the default LISP-like representation.
raw Print a raw representation of the tree. By default, trees are
pretty-printed into a C-like representation.
details
Enable more detailed dumps (not honored by every dump option).
Also include information from the optimization passes.
stats
Enable dumping various statistics about the pass (not honored
by every dump option).
blocks
Enable showing basic block boundaries (disabled in raw dumps).
graph
For each of the other indicated dump files (-fdump-rtl-pass),
dump a representation of the control flow graph suitable for
viewing with GraphViz to file.passid.pass.dot. Each function
in the file is pretty-printed as a subgraph, so that GraphViz
can render them all in a single plot.
This option currently only works for RTL dumps, and the RTL is
always dumped in slim form.
vops
Enable showing virtual operands for every statement.
lineno
Enable showing line numbers for statements.
uid Enable showing the unique ID ("DECL_UID") for each variable.
verbose
Enable showing the tree dump for each statement.
eh Enable showing the EH region number holding each statement.
scev
Enable showing scalar evolution analysis details.
optimized
Enable showing optimization information (only available in
certain passes).
missed
Enable showing missed optimization information (only available
in certain passes).
note
Enable other detailed optimization information (only available
in certain passes).
all Turn on all options, except raw, slim, verbose and lineno.
optall
Turn on all optimization options, i.e., optimized, missed, and
note.
To determine what tree dumps are available or find the dump for a
pass of interest follow the steps below.
1. Invoke GCC with -fdump-passes and in the stderr output look for
a code that corresponds to the pass you are interested in. For
example, the codes "tree-evrp", "tree-vrp1", and "tree-vrp2"
correspond to the three Value Range Propagation passes. The
number at the end distinguishes distinct invocations of the
same pass.
2. To enable the creation of the dump file, append the pass code
to the -fdump- option prefix and invoke GCC with it. For
example, to enable the dump from the Early Value Range
Propagation pass, invoke GCC with the -fdump-tree-evrp option.
Optionally, you may specify the name of the dump file. If you
don't specify one, GCC creates as described below.
3. Find the pass dump in a file whose name is composed of three
components separated by a period: the name of the source file
GCC was invoked to compile, a numeric suffix indicating the
pass number followed by the letter t for tree passes (and the
letter r for RTL passes), and finally the pass code. For
example, the Early VRP pass dump might be in a file named
myfile.c.038t.evrp in the current working directory. Note that
the numeric codes are not stable and may change from one
version of GCC to another.
-fopt-info
-fopt-info-options
-fopt-info-options=filename
Controls optimization dumps from various optimization passes. If
the -options form is used, options is a list of - separated option
keywords to select the dump details and optimizations.
The options can be divided into three groups:
1. options describing what kinds of messages should be emitted,
2. options describing the verbosity of the dump, and
3. options describing which optimizations should be included.
The options from each group can be freely mixed as they are non-
overlapping. However, in case of any conflicts, the later options
override the earlier options on the command line.
The following options control which kinds of messages should be
emitted:
optimized
Print information when an optimization is successfully applied.
It is up to a pass to decide which information is relevant. For
example, the vectorizer passes print the source location of
loops which are successfully vectorized.
missed
Print information about missed optimizations. Individual passes
control which information to include in the output.
note
Print verbose information about optimizations, such as certain
transformations, more detailed messages about decisions etc.
all Print detailed optimization information. This includes
optimized, missed, and note.
The following option controls the dump verbosity:
internals
By default, only "high-level" messages are emitted. This option
enables additional, more detailed, messages, which are likely
to only be of interest to GCC developers.
One or more of the following option keywords can be used to
describe a group of optimizations:
ipa Enable dumps from all interprocedural optimizations.
loop
Enable dumps from all loop optimizations.
inline
Enable dumps from all inlining optimizations.
omp Enable dumps from all OMP (Offloading and Multi Processing)
optimizations.
vec Enable dumps from all vectorization optimizations.
optall
Enable dumps from all optimizations. This is a superset of the
optimization groups listed above.
If options is omitted, it defaults to optimized-optall, which means
to dump messages about successful optimizations from all the
passes, omitting messages that are treated as "internals".
If the filename is provided, then the dumps from all the applicable
optimizations are concatenated into the filename. Otherwise the
dump is output onto stderr. Though multiple -fopt-info options are
accepted, only one of them can include a filename. If other
filenames are provided then all but the first such option are
ignored.
Note that the output filename is overwritten in case of multiple
translation units. If a combined output from multiple translation
units is desired, stderr should be used instead.
In the following example, the optimization info is output to
stderr:
gcc -O3 -fopt-info
This example:
gcc -O3 -fopt-info-missed=missed.all
outputs missed optimization report from all the passes into
missed.all, and this one:
gcc -O2 -ftree-vectorize -fopt-info-vec-missed
prints information about missed optimization opportunities from
vectorization passes on stderr. Note that -fopt-info-vec-missed is
equivalent to -fopt-info-missed-vec. The order of the optimization
group names and message types listed after -fopt-info does not
matter.
As another example,
gcc -O3 -fopt-info-inline-optimized-missed=inline.txt
outputs information about missed optimizations as well as optimized
locations from all the inlining passes into inline.txt.
Finally, consider:
gcc -fopt-info-vec-missed=vec.miss -fopt-info-loop-optimized=loop.opt
Here the two output filenames vec.miss and loop.opt are in conflict
since only one output file is allowed. In this case, only the first
option takes effect and the subsequent options are ignored. Thus
only vec.miss is produced which contains dumps from the vectorizer
about missed opportunities.
-fsave-optimization-record
Write a SRCFILE.opt-record.json.gz file detailing what
optimizations were performed, for those optimizations that support
-fopt-info.
This option is experimental and the format of the data within the
compressed JSON file is subject to change.
It is roughly equivalent to a machine-readable version of
-fopt-info-all, as a collection of messages with source file, line
number and column number, with the following additional data for
each message:
* the execution count of the code being optimized, along with
metadata about whether this was from actual profile data, or
just an estimate, allowing consumers to prioritize messages by
code hotness,
* the function name of the code being optimized, where
applicable,
* the "inlining chain" for the code being optimized, so that when
a function is inlined into several different places (which
might themselves be inlined), the reader can distinguish
between the copies,
* objects identifying those parts of the message that refer to
expressions, statements or symbol-table nodes, which of these
categories they are, and, when available, their source code
location,
* the GCC pass that emitted the message, and
* the location in GCC's own code from which the message was
emitted
Additionally, some messages are logically nested within other
messages, reflecting implementation details of the optimization
passes.
-fsched-verbose=n
On targets that use instruction scheduling, this option controls
the amount of debugging output the scheduler prints to the dump
files.
For n greater than zero, -fsched-verbose outputs the same
information as -fdump-rtl-sched1 and -fdump-rtl-sched2. For n
greater than one, it also output basic block probabilities,
detailed ready list information and unit/insn info. For n greater
than two, it includes RTL at abort point, control-flow and regions
info. And for n over four, -fsched-verbose also includes
dependence info.
-fenable-kind-pass
-fdisable-kind-pass=range-list
This is a set of options that are used to explicitly disable/enable
optimization passes. These options are intended for use for
debugging GCC. Compiler users should use regular options for
enabling/disabling passes instead.
-fdisable-ipa-pass
Disable IPA pass pass. pass is the pass name. If the same pass
is statically invoked in the compiler multiple times, the pass
name should be appended with a sequential number starting from
1.
-fdisable-rtl-pass
-fdisable-rtl-pass=range-list
Disable RTL pass pass. pass is the pass name. If the same
pass is statically invoked in the compiler multiple times, the
pass name should be appended with a sequential number starting
from 1. range-list is a comma-separated list of function
ranges or assembler names. Each range is a number pair
separated by a colon. The range is inclusive in both ends. If
the range is trivial, the number pair can be simplified as a
single number. If the function's call graph node's uid falls
within one of the specified ranges, the pass is disabled for
that function. The uid is shown in the function header of a
dump file, and the pass names can be dumped by using option
-fdump-passes.
-fdisable-tree-pass
-fdisable-tree-pass=range-list
Disable tree pass pass. See -fdisable-rtl for the description
of option arguments.
-fenable-ipa-pass
Enable IPA pass pass. pass is the pass name. If the same pass
is statically invoked in the compiler multiple times, the pass
name should be appended with a sequential number starting from
1.
-fenable-rtl-pass
-fenable-rtl-pass=range-list
Enable RTL pass pass. See -fdisable-rtl for option argument
description and examples.
-fenable-tree-pass
-fenable-tree-pass=range-list
Enable tree pass pass. See -fdisable-rtl for the description
of option arguments.
Here are some examples showing uses of these options.
# disable ccp1 for all functions
-fdisable-tree-ccp1
# disable complete unroll for function whose cgraph node uid is 1
-fenable-tree-cunroll=1
# disable gcse2 for functions at the following ranges [1,1],
# [300,400], and [400,1000]
# disable gcse2 for functions foo and foo2
-fdisable-rtl-gcse2=foo,foo2
# disable early inlining
-fdisable-tree-einline
# disable ipa inlining
-fdisable-ipa-inline
# enable tree full unroll
-fenable-tree-unroll
-fchecking
-fchecking=n
Enable internal consistency checking. The default depends on the
compiler configuration. -fchecking=2 enables further internal
consistency checking that might affect code generation.
-frandom-seed=string
This option provides a seed that GCC uses in place of random
numbers in generating certain symbol names that have to be
different in every compiled file. It is also used to place unique
stamps in coverage data files and the object files that produce
them. You can use the -frandom-seed option to produce reproducibly
identical object files.
The string can either be a number (decimal, octal or hex) or an
arbitrary string (in which case it's converted to a number by
computing CRC32).
The string should be different for every file you compile.
-save-temps
-save-temps=cwd
Store the usual "temporary" intermediate files permanently; place
them in the current directory and name them based on the source
file. Thus, compiling foo.c with -c -save-temps produces files
foo.i and foo.s, as well as foo.o. This creates a preprocessed
foo.i output file even though the compiler now normally uses an
integrated preprocessor.
When used in combination with the -x command-line option,
-save-temps is sensible enough to avoid over writing an input
source file with the same extension as an intermediate file. The
corresponding intermediate file may be obtained by renaming the
source file before using -save-temps.
If you invoke GCC in parallel, compiling several different source
files that share a common base name in different subdirectories or
the same source file compiled for multiple output destinations, it
is likely that the different parallel compilers will interfere with
each other, and overwrite the temporary files. For instance:
gcc -save-temps -o outdir1/foo.o indir1/foo.c&
gcc -save-temps -o outdir2/foo.o indir2/foo.c&
may result in foo.i and foo.o being written to simultaneously by
both compilers.
-save-temps=obj
Store the usual "temporary" intermediate files permanently. If the
-o option is used, the temporary files are based on the object
file. If the -o option is not used, the -save-temps=obj switch
behaves like -save-temps.
For example:
gcc -save-temps=obj -c foo.c
gcc -save-temps=obj -c bar.c -o dir/xbar.o
gcc -save-temps=obj foobar.c -o dir2/yfoobar
creates foo.i, foo.s, dir/xbar.i, dir/xbar.s, dir2/yfoobar.i,
dir2/yfoobar.s, and dir2/yfoobar.o.
-time[=file]
Report the CPU time taken by each subprocess in the compilation
sequence. For C source files, this is the compiler proper and
assembler (plus the linker if linking is done).
Without the specification of an output file, the output looks like
this:
# cc1 0.12 0.01
# as 0.00 0.01
The first number on each line is the "user time", that is time
spent executing the program itself. The second number is "system
time", time spent executing operating system routines on behalf of
the program. Both numbers are in seconds.
With the specification of an output file, the output is appended to
the named file, and it looks like this:
0.12 0.01 cc1 <options>
0.00 0.01 as <options>
The "user time" and the "system time" are moved before the program
name, and the options passed to the program are displayed, so that
one can later tell what file was being compiled, and with which
options.
-fdump-final-insns[=file]
Dump the final internal representation (RTL) to file. If the
optional argument is omitted (or if file is "."), the name of the
dump file is determined by appending ".gkd" to the compilation
output file name.
-fcompare-debug[=opts]
If no error occurs during compilation, run the compiler a second
time, adding opts and -fcompare-debug-second to the arguments
passed to the second compilation. Dump the final internal
representation in both compilations, and print an error if they
differ.
If the equal sign is omitted, the default -gtoggle is used.
The environment variable GCC_COMPARE_DEBUG, if defined, non-empty
and nonzero, implicitly enables -fcompare-debug. If
GCC_COMPARE_DEBUG is defined to a string starting with a dash, then
it is used for opts, otherwise the default -gtoggle is used.
-fcompare-debug=, with the equal sign but without opts, is
equivalent to -fno-compare-debug, which disables the dumping of the
final representation and the second compilation, preventing even
GCC_COMPARE_DEBUG from taking effect.
To verify full coverage during -fcompare-debug testing, set
GCC_COMPARE_DEBUG to say -fcompare-debug-not-overridden, which GCC
rejects as an invalid option in any actual compilation (rather than
preprocessing, assembly or linking). To get just a warning,
setting GCC_COMPARE_DEBUG to -w%n-fcompare-debug not overridden
will do.
-fcompare-debug-second
This option is implicitly passed to the compiler for the second
compilation requested by -fcompare-debug, along with options to
silence warnings, and omitting other options that would cause the
compiler to produce output to files or to standard output as a side
effect. Dump files and preserved temporary files are renamed so as
to contain the ".gk" additional extension during the second
compilation, to avoid overwriting those generated by the first.
When this option is passed to the compiler driver, it causes the
first compilation to be skipped, which makes it useful for little
other than debugging the compiler proper.
-gtoggle
Turn off generation of debug info, if leaving out this option
generates it, or turn it on at level 2 otherwise. The position of
this argument in the command line does not matter; it takes effect
after all other options are processed, and it does so only once, no
matter how many times it is given. This is mainly intended to be
used with -fcompare-debug.
-fvar-tracking-assignments-toggle
Toggle -fvar-tracking-assignments, in the same way that -gtoggle
toggles -g.
-Q Makes the compiler print out each function name as it is compiled,
and print some statistics about each pass when it finishes.
-ftime-report
Makes the compiler print some statistics about the time consumed by
each pass when it finishes.
-ftime-report-details
Record the time consumed by infrastructure parts separately for
each pass.
-fira-verbose=n
Control the verbosity of the dump file for the integrated register
allocator. The default value is 5. If the value n is greater or
equal to 10, the dump output is sent to stderr using the same
format as n minus 10.
-flto-report
Prints a report with internal details on the workings of the link-
time optimizer. The contents of this report vary from version to
version. It is meant to be useful to GCC developers when
processing object files in LTO mode (via -flto).
Disabled by default.
-flto-report-wpa
Like -flto-report, but only print for the WPA phase of link-time
optimization.
-fmem-report
Makes the compiler print some statistics about permanent memory
allocation when it finishes.
-fmem-report-wpa
Makes the compiler print some statistics about permanent memory
allocation for the WPA phase only.
-fpre-ipa-mem-report
-fpost-ipa-mem-report
Makes the compiler print some statistics about permanent memory
allocation before or after interprocedural optimization.
-fprofile-report
Makes the compiler print some statistics about consistency of the
(estimated) profile and effect of individual passes.
-fstack-usage
Makes the compiler output stack usage information for the program,
on a per-function basis. The filename for the dump is made by
appending .su to the auxname. auxname is generated from the name
of the output file, if explicitly specified and it is not an
executable, otherwise it is the basename of the source file. An
entry is made up of three fields:
* The name of the function.
* A number of bytes.
* One or more qualifiers: "static", "dynamic", "bounded".
The qualifier "static" means that the function manipulates the
stack statically: a fixed number of bytes are allocated for the
frame on function entry and released on function exit; no stack
adjustments are otherwise made in the function. The second field
is this fixed number of bytes.
The qualifier "dynamic" means that the function manipulates the
stack dynamically: in addition to the static allocation described
above, stack adjustments are made in the body of the function, for
example to push/pop arguments around function calls. If the
qualifier "bounded" is also present, the amount of these
adjustments is bounded at compile time and the second field is an
upper bound of the total amount of stack used by the function. If
it is not present, the amount of these adjustments is not bounded
at compile time and the second field only represents the bounded
part.
-fstats
Emit statistics about front-end processing at the end of the
compilation. This option is supported only by the C++ front end,
and the information is generally only useful to the G++ development
team.
-fdbg-cnt-list
Print the name and the counter upper bound for all debug counters.
-fdbg-cnt=counter-value-list
Set the internal debug counter lower and upper bound. counter-
value-list is a comma-separated list of
name:lower_bound1-upper_bound1 [:lower_bound2-upper_bound2...]
tuples which sets the name of the counter and list of closed
intervals. The lower_bound is optional and is zero initialized if
not set. For example, with -fdbg-cnt=dce:2-4:10-11,tail_call:10,
"dbg_cnt(dce)" returns true only for second, third, fourth, tenth
and eleventh invocation. For "dbg_cnt(tail_call)" true is returned
for first 10 invocations.
-print-file-name=library
Print the full absolute name of the library file library that would
be used when linking---and don't do anything else. With this
option, GCC does not compile or link anything; it just prints the
file name.
-print-multi-directory
Print the directory name corresponding to the multilib selected by
any other switches present in the command line. This directory is
supposed to exist in GCC_EXEC_PREFIX.
-print-multi-lib
Print the mapping from multilib directory names to compiler
switches that enable them. The directory name is separated from
the switches by ;, and each switch starts with an @ instead of the
-, without spaces between multiple switches. This is supposed to
ease shell processing.
-print-multi-os-directory
Print the path to OS libraries for the selected multilib, relative
to some lib subdirectory. If OS libraries are present in the lib
subdirectory and no multilibs are used, this is usually just ., if
OS libraries are present in libsuffix sibling directories this
prints e.g. ../lib64, ../lib or ../lib32, or if OS libraries are
present in lib/subdir subdirectories it prints e.g. amd64, sparcv9
or ev6.
-print-multiarch
Print the path to OS libraries for the selected multiarch, relative
to some lib subdirectory.
-print-prog-name=program
Like -print-file-name, but searches for a program such as cpp.
-print-libgcc-file-name
Same as -print-file-name=libgcc.a.
This is useful when you use -nostdlib or -nodefaultlibs but you do
want to link with libgcc.a. You can do:
gcc -nostdlib <files>... `gcc -print-libgcc-file-name`
-print-search-dirs
Print the name of the configured installation directory and a list
of program and library directories gcc searches---and don't do
anything else.
This is useful when gcc prints the error message installation
problem, cannot exec cpp0: No such file or directory. To resolve
this you either need to put cpp0 and the other compiler components
where gcc expects to find them, or you can set the environment
variable GCC_EXEC_PREFIX to the directory where you installed them.
Don't forget the trailing /.
-print-sysroot
Print the target sysroot directory that is used during compilation.
This is the target sysroot specified either at configure time or
using the --sysroot option, possibly with an extra suffix that
depends on compilation options. If no target sysroot is specified,
the option prints nothing.
-print-sysroot-headers-suffix
Print the suffix added to the target sysroot when searching for
headers, or give an error if the compiler is not configured with
such a suffix---and don't do anything else.
-dumpmachine
Print the compiler's target machine (for example,
i686-pc-linux-gnu)---and don't do anything else.
-dumpversion
Print the compiler version (for example, 3.0, 6.3.0 or 7)---and
don't do anything else. This is the compiler version used in
filesystem paths and specs. Depending on how the compiler has been
configured it can be just a single number (major version), two
numbers separated by a dot (major and minor version) or three
numbers separated by dots (major, minor and patchlevel version).
-dumpfullversion
Print the full compiler version---and don't do anything else. The
output is always three numbers separated by dots, major, minor and
patchlevel version.
-dumpspecs
Print the compiler's built-in specs---and don't do anything else.
(This is used when GCC itself is being built.)
Machine-Dependent Options
Each target machine supported by GCC can have its own options---for
example, to allow you to compile for a particular processor variant or
ABI, or to control optimizations specific to that machine. By
convention, the names of machine-specific options start with -m.
Some configurations of the compiler also support additional target-
specific options, usually for compatibility with other compilers on the
same platform.
AArch64 Options
These options are defined for AArch64 implementations:
-mabi=name
Generate code for the specified data model. Permissible values are
ilp32 for SysV-like data model where int, long int and pointers are
32 bits, and lp64 for SysV-like data model where int is 32 bits,
but long int and pointers are 64 bits.
The default depends on the specific target configuration. Note
that the LP64 and ILP32 ABIs are not link-compatible; you must
compile your entire program with the same ABI, and link with a
compatible set of libraries.
-mbig-endian
Generate big-endian code. This is the default when GCC is
configured for an aarch64_be-*-* target.
-mgeneral-regs-only
Generate code which uses only the general-purpose registers. This
will prevent the compiler from using floating-point and Advanced
SIMD registers but will not impose any restrictions on the
assembler.
-mlittle-endian
Generate little-endian code. This is the default when GCC is
configured for an aarch64-*-* but not an aarch64_be-*-* target.
-mcmodel=tiny
Generate code for the tiny code model. The program and its
statically defined symbols must be within 1MB of each other.
Programs can be statically or dynamically linked.
-mcmodel=small
Generate code for the small code model. The program and its
statically defined symbols must be within 4GB of each other.
Programs can be statically or dynamically linked. This is the
default code model.
-mcmodel=large
Generate code for the large code model. This makes no assumptions
about addresses and sizes of sections. Programs can be statically
linked only. The -mcmodel=large option is incompatible with
-mabi=ilp32, -fpic and -fPIC.
-mstrict-align
-mno-strict-align
Avoid or allow generating memory accesses that may not be aligned
on a natural object boundary as described in the architecture
specification.
-momit-leaf-frame-pointer
-mno-omit-leaf-frame-pointer
Omit or keep the frame pointer in leaf functions. The former
behavior is the default.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported
locations are global for a global canary or sysreg for a canary in
an appropriate system register.
With the latter choice the options -mstack-protector-guard-reg=reg
and -mstack-protector-guard-offset=offset furthermore specify which
system register to use as base register for reading the canary, and
from what offset from that base register. There is no default
register or offset as this is entirely for use within the Linux
kernel.
-mstack-protector-guard=guard
-mstack-protector-guard-reg=reg
-mstack-protector-guard-offset=offset
Generate stack protection code using canary at guard. Supported
locations are global for a global canary or sysreg for a canary in
an appropriate system register.
With the latter choice the options -mstack-protector-guard-reg=reg
and -mstack-protector-guard-offset=offset furthermore specify which
system register to use as base register for reading the canary, and
from what offset from that base register. There is no default
register or offset as this is entirely for use within the Linux
kernel.
-mtls-dialect=desc
Use TLS descriptors as the thread-local storage mechanism for
dynamic accesses of TLS variables. This is the default.
-mtls-dialect=traditional
Use traditional TLS as the thread-local storage mechanism for
dynamic accesses of TLS variables.
-mtls-size=size
Specify bit size of immediate TLS offsets. Valid values are 12,
24, 32, 48. This option requires binutils 2.26 or newer.
-mfix-cortex-a53-835769
-mno-fix-cortex-a53-835769
Enable or disable the workaround for the ARM Cortex-A53 erratum
number 835769. This involves inserting a NOP instruction between
memory instructions and 64-bit integer multiply-accumulate
instructions.
-mfix-cortex-a53-843419
-mno-fix-cortex-a53-843419
Enable or disable the workaround for the ARM Cortex-A53 erratum
number 843419. This erratum workaround is made at link time and
this will only pass the corresponding flag to the linker.
-mlow-precision-recip-sqrt
-mno-low-precision-recip-sqrt
Enable or disable the reciprocal square root approximation. This
option only has an effect if -ffast-math or
-funsafe-math-optimizations is used as well. Enabling this reduces
precision of reciprocal square root results to about 16 bits for
single precision and to 32 bits for double precision.
-mlow-precision-sqrt
-mno-low-precision-sqrt
Enable or disable the square root approximation. This option only
has an effect if -ffast-math or -funsafe-math-optimizations is used
as well. Enabling this reduces precision of square root results to
about 16 bits for single precision and to 32 bits for double
precision. If enabled, it implies -mlow-precision-recip-sqrt.
-mlow-precision-div
-mno-low-precision-div
Enable or disable the division approximation. This option only has
an effect if -ffast-math or -funsafe-math-optimizations is used as
well. Enabling this reduces precision of division results to about
16 bits for single precision and to 32 bits for double precision.
-mtrack-speculation
-mno-track-speculation
Enable or disable generation of additional code to track
speculative execution through conditional branches. The tracking
state can then be used by the compiler when expanding calls to
"__builtin_speculation_safe_copy" to permit a more efficient code
sequence to be generated.
-moutline-atomics
-mno-outline-atomics
Enable or disable calls to out-of-line helpers to implement atomic
operations. These helpers will, at runtime, determine if the LSE
instructions from ARMv8.1-A can be used; if not, they will use the
load/store-exclusive instructions that are present in the base
ARMv8.0 ISA.
This option is only applicable when compiling for the base ARMv8.0
instruction set. If using a later revision, e.g. -march=armv8.1-a
or -march=armv8-a+lse, the ARMv8.1-Atomics instructions will be
used directly. The same applies when using -mcpu= when the
selected cpu supports the lse feature. This option is on by
default.
-march=name
Specify the name of the target architecture and, optionally, one or
more feature modifiers. This option has the form
-march=arch{+[no]feature}*.
The table below summarizes the permissible values for arch and the
features that they enable by default:
arch value : Architecture : Includes by default
armv8-a : Armv8-A : +fp, +simd
armv8.1-a : Armv8.1-A : armv8-a, +crc, +lse, +rdma
armv8.2-a : Armv8.2-A : armv8.1-a
armv8.3-a : Armv8.3-A : armv8.2-a
armv8.4-a : Armv8.4-A : armv8.3-a, +fp16fml, +dotprod
armv8.5-a : Armv8.5-A : armv8.4-a, +sb, +ssbs, +predres
armv8.6-a : Armv8.6-A : armv8.5-a, +bf16, +i8mm
The value native is available on native AArch64 GNU/Linux and
causes the compiler to pick the architecture of the host system.
This option has no effect if the compiler is unable to recognize
the architecture of the host system,
The permissible values for feature are listed in the sub-section on
aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers.
Where conflicting feature modifiers are specified, the right-most
feature is used.
GCC uses name to determine what kind of instructions it can emit
when generating assembly code. If -march is specified without
either of -mtune or -mcpu also being specified, the code is tuned
to perform well across a range of target processors implementing
the target architecture.
-mtune=name
Specify the name of the target processor for which GCC should tune
the performance of the code. Permissible values for this option
are: generic, cortex-a35, cortex-a53, cortex-a55, cortex-a57,
cortex-a72, cortex-a73, cortex-a75, cortex-a76, cortex-a76ae,
cortex-a77, cortex-a65, cortex-a65ae, cortex-a34, ares, exynos-m1,
emag, falkor, neoverse-e1, neoverse-n1, neoverse-n2, neoverse-v1,
neoverse-v2, neoverse-512tvb, qdf24xx, saphira, phecda, xgene1,
vulcan, octeontx, octeontx81, octeontx83, octeontx2, octeontx2t98,
octeontx2t96 octeontx2t93, octeontx2f95, octeontx2f95n,
octeontx2f95mm, a64fx, thunderx, thunderxt88, thunderxt88p1,
thunderxt81, tsv110, thunderxt83, thunderx2t99, thunderx3t110,
zeus, cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a73.cortex-a35, cortex-a73.cortex-a53,
cortex-a75.cortex-a55, cortex-a76.cortex-a55, ampere1, ampere1a,
native.
The values cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a73.cortex-a35, cortex-a73.cortex-a53,
cortex-a75.cortex-a55, cortex-a76.cortex-a55 specify that GCC
should tune for a big.LITTLE system.
The value neoverse-512tvb specifies that GCC should tune for
Neoverse cores that (a) implement SVE and (b) have a total vector
bandwidth of 512 bits per cycle. In other words, the option tells
GCC to tune for Neoverse cores that can execute 4 128-bit Advanced
SIMD arithmetic instructions a cycle and that can execute an
equivalent number of SVE arithmetic instructions per cycle (2 for
256-bit SVE, 4 for 128-bit SVE). This is more general than tuning
for a specific core like Neoverse V1 but is more specific than the
default tuning described below.
Additionally on native AArch64 GNU/Linux systems the value native
tunes performance to the host system. This option has no effect if
the compiler is unable to recognize the processor of the host
system.
Where none of -mtune=, -mcpu= or -march= are specified, the code is
tuned to perform well across a range of target processors.
This option cannot be suffixed by feature modifiers.
-mcpu=name
Specify the name of the target processor, optionally suffixed by
one or more feature modifiers. This option has the form
-mcpu=cpu{+[no]feature}*, where the permissible values for cpu are
the same as those available for -mtune. The permissible values for
feature are documented in the sub-section on
aarch64-feature-modifiers,,-march and -mcpu Feature Modifiers.
Where conflicting feature modifiers are specified, the right-most
feature is used.
GCC uses name to determine what kind of instructions it can emit
when generating assembly code (as if by -march) and to determine
the target processor for which to tune for performance (as if by
-mtune). Where this option is used in conjunction with -march or
-mtune, those options take precedence over the appropriate part of
this option.
-mcpu=neoverse-512tvb is special in that it does not refer to a
specific core, but instead refers to all Neoverse cores that (a)
implement SVE and (b) have a total vector bandwidth of 512 bits a
cycle. Unless overridden by -march, -mcpu=neoverse-512tvb
generates code that can run on a Neoverse V1 core, since Neoverse
V1 is the first Neoverse core with these properties. Unless
overridden by -mtune, -mcpu=neoverse-512tvb tunes code in the same
way as for -mtune=neoverse-512tvb.
-moverride=string
Override tuning decisions made by the back-end in response to a
-mtune= switch. The syntax, semantics, and accepted values for
string in this option are not guaranteed to be consistent across
releases.
This option is only intended to be useful when developing GCC.
-mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files. This
option is provided for use in debugging the compiler.
-mpc-relative-literal-loads
-mno-pc-relative-literal-loads
Enable or disable PC-relative literal loads. With this option
literal pools are accessed using a single instruction and emitted
after each function. This limits the maximum size of functions to
1MB. This is enabled by default for -mcmodel=tiny.
-msign-return-address=scope
Select the function scope on which return address signing will be
applied. Permissible values are none, which disables return
address signing, non-leaf, which enables pointer signing for
functions which are not leaf functions, and all, which enables
pointer signing for all functions. The default value is none. This
option has been deprecated by -mbranch-protection.
-mbranch-protection=none|standard|pac-ret[+leaf+b-key]|bti
Select the branch protection features to use. none is the default
and turns off all types of branch protection. standard turns on
all types of branch protection features. If a feature has
additional tuning options, then standard sets it to its standard
level. pac-ret[+leaf] turns on return address signing to its
standard level: signing functions that save the return address to
memory (non-leaf functions will practically always do this) using
the a-key. The optional argument leaf can be used to extend the
signing to include leaf functions. The optional argument b-key can
be used to sign the functions with the B-key instead of the A-key.
bti turns on branch target identification mechanism.
-mharden-sls=opts
Enable compiler hardening against straight line speculation (SLS).
opts is a comma-separated list of the following options:
retbr
blr
In addition, -mharden-sls=all enables all SLS hardening while
-mharden-sls=none disables all SLS hardening.
-msve-vector-bits=bits
Specify the number of bits in an SVE vector register. This option
only has an effect when SVE is enabled.
GCC supports two forms of SVE code generation: "vector-length
agnostic" output that works with any size of vector register and
"vector-length specific" output that allows GCC to make assumptions
about the vector length when it is useful for optimization reasons.
The possible values of bits are: scalable, 128, 256, 512, 1024 and
2048. Specifying scalable selects vector-length agnostic output.
At present -msve-vector-bits=128 also generates vector-length
agnostic output for big-endian targets. All other values generate
vector-length specific code. The behavior of these values may
change in future releases and no value except scalable should be
relied on for producing code that is portable across different
hardware SVE vector lengths.
The default is -msve-vector-bits=scalable, which produces vector-
length agnostic code.
-march and -mcpu Feature Modifiers
Feature modifiers used with -march and -mcpu can be any of the
following and their inverses nofeature:
crc Enable CRC extension. This is on by default for -march=armv8.1-a.
crypto
Enable Crypto extension. This also enables Advanced SIMD and
floating-point instructions.
fp Enable floating-point instructions. This is on by default for all
possible values for options -march and -mcpu.
simd
Enable Advanced SIMD instructions. This also enables floating-
point instructions. This is on by default for all possible values
for options -march and -mcpu.
sve Enable Scalable Vector Extension instructions. This also enables
Advanced SIMD and floating-point instructions.
lse Enable Large System Extension instructions. This is on by default
for -march=armv8.1-a.
rdma
Enable Round Double Multiply Accumulate instructions. This is on
by default for -march=armv8.1-a.
fp16
Enable FP16 extension. This also enables floating-point
instructions.
fp16fml
Enable FP16 fmla extension. This also enables FP16 extensions and
floating-point instructions. This option is enabled by default for
-march=armv8.4-a. Use of this option with architectures prior to
Armv8.2-A is not supported.
rcpc
Enable the RcPc extension. This does not change code generation
from GCC, but is passed on to the assembler, enabling inline asm
statements to use instructions from the RcPc extension.
dotprod
Enable the Dot Product extension. This also enables Advanced SIMD
instructions.
aes Enable the Armv8-a aes and pmull crypto extension. This also
enables Advanced SIMD instructions.
sha2
Enable the Armv8-a sha2 crypto extension. This also enables
Advanced SIMD instructions.
sha3
Enable the sha512 and sha3 crypto extension. This also enables
Advanced SIMD instructions. Use of this option with architectures
prior to Armv8.2-A is not supported.
sm4 Enable the sm3 and sm4 crypto extension. This also enables
Advanced SIMD instructions. Use of this option with architectures
prior to Armv8.2-A is not supported.
profile
Enable the Statistical Profiling extension. This option is only to
enable the extension at the assembler level and does not affect
code generation.
rng Enable the Armv8.5-a Random Number instructions. This option is
only to enable the extension at the assembler level and does not
affect code generation.
memtag
Enable the Armv8.5-a Memory Tagging Extensions. Use of this option
with architectures prior to Armv8.5-A is not supported.
sb Enable the Armv8-a Speculation Barrier instruction. This option is
only to enable the extension at the assembler level and does not
affect code generation. This option is enabled by default for
-march=armv8.5-a.
ssbs
Enable the Armv8-a Speculative Store Bypass Safe instruction. This
option is only to enable the extension at the assembler level and
does not affect code generation. This option is enabled by default
for -march=armv8.5-a.
predres
Enable the Armv8-a Execution and Data Prediction Restriction
instructions. This option is only to enable the extension at the
assembler level and does not affect code generation. This option
is enabled by default for -march=armv8.5-a.
sve2
Enable the Armv8-a Scalable Vector Extension 2. This also enables
SVE instructions.
sve2-bitperm
Enable SVE2 bitperm instructions. This also enables SVE2
instructions.
sve2-sm4
Enable SVE2 sm4 instructions. This also enables SVE2 instructions.
sve2-aes
Enable SVE2 aes instructions. This also enables SVE2 instructions.
sve2-sha3
Enable SVE2 sha3 instructions. This also enables SVE2
instructions.
tme Enable the Transactional Memory Extension.
i8mm
Enable 8-bit Integer Matrix Multiply instructions. This also
enables Advanced SIMD and floating-point instructions. This option
is enabled by default for -march=armv8.6-a. Use of this option
with architectures prior to Armv8.2-A is not supported.
f32mm
Enable 32-bit Floating point Matrix Multiply instructions. This
also enables SVE instructions. Use of this option with
architectures prior to Armv8.2-A is not supported.
f64mm
Enable 64-bit Floating point Matrix Multiply instructions. This
also enables SVE instructions. Use of this option with
architectures prior to Armv8.2-A is not supported.
bf16
Enable brain half-precision floating-point instructions. This also
enables Advanced SIMD and floating-point instructions. This option
is enabled by default for -march=armv8.6-a. Use of this option
with architectures prior to Armv8.2-A is not supported.
Feature crypto implies aes, sha2, and simd, which implies fp.
Conversely, nofp implies nosimd, which implies nocrypto, noaes and
nosha2.
Adapteva Epiphany Options
These -m options are defined for Adapteva Epiphany:
-mhalf-reg-file
Don't allocate any register in the range "r32"..."r63". That
allows code to run on hardware variants that lack these registers.
-mprefer-short-insn-regs
Preferentially allocate registers that allow short instruction
generation. This can result in increased instruction count, so
this may either reduce or increase overall code size.
-mbranch-cost=num
Set the cost of branches to roughly num "simple" instructions.
This cost is only a heuristic and is not guaranteed to produce
consistent results across releases.
-mcmove
Enable the generation of conditional moves.
-mnops=num
Emit num NOPs before every other generated instruction.
-mno-soft-cmpsf
For single-precision floating-point comparisons, emit an "fsub"
instruction and test the flags. This is faster than a software
comparison, but can get incorrect results in the presence of NaNs,
or when two different small numbers are compared such that their
difference is calculated as zero. The default is -msoft-cmpsf,
which uses slower, but IEEE-compliant, software comparisons.
-mstack-offset=num
Set the offset between the top of the stack and the stack pointer.
E.g., a value of 8 means that the eight bytes in the range
"sp+0...sp+7" can be used by leaf functions without stack
allocation. Values other than 8 or 16 are untested and unlikely to
work. Note also that this option changes the ABI; compiling a
program with a different stack offset than the libraries have been
compiled with generally does not work. This option can be useful
if you want to evaluate if a different stack offset would give you
better code, but to actually use a different stack offset to build
working programs, it is recommended to configure the toolchain with
the appropriate --with-stack-offset=num option.
-mno-round-nearest
Make the scheduler assume that the rounding mode has been set to
truncating. The default is -mround-nearest.
-mlong-calls
If not otherwise specified by an attribute, assume all calls might
be beyond the offset range of the "b" / "bl" instructions, and
therefore load the function address into a register before
performing a (otherwise direct) call. This is the default.
-mshort-calls
If not otherwise specified by an attribute, assume all direct calls
are in the range of the "b" / "bl" instructions, so use these
instructions for direct calls. The default is -mlong-calls.
-msmall16
Assume addresses can be loaded as 16-bit unsigned values. This
does not apply to function addresses for which -mlong-calls
semantics are in effect.
-mfp-mode=mode
Set the prevailing mode of the floating-point unit. This
determines the floating-point mode that is provided and expected at
function call and return time. Making this mode match the mode you
predominantly need at function start can make your programs smaller
and faster by avoiding unnecessary mode switches.
mode can be set to one the following values:
caller
Any mode at function entry is valid, and retained or restored
when the function returns, and when it calls other functions.
This mode is useful for compiling libraries or other
compilation units you might want to incorporate into different
programs with different prevailing FPU modes, and the
convenience of being able to use a single object file outweighs
the size and speed overhead for any extra mode switching that
might be needed, compared with what would be needed with a more
specific choice of prevailing FPU mode.
truncate
This is the mode used for floating-point calculations with
truncating (i.e. round towards zero) rounding mode. That
includes conversion from floating point to integer.
round-nearest
This is the mode used for floating-point calculations with
round-to-nearest-or-even rounding mode.
int This is the mode used to perform integer calculations in the
FPU, e.g. integer multiply, or integer multiply-and-
accumulate.
The default is -mfp-mode=caller
-mno-split-lohi
-mno-postinc
-mno-postmodify
Code generation tweaks that disable, respectively, splitting of
32-bit loads, generation of post-increment addresses, and
generation of post-modify addresses. The defaults are msplit-lohi,
-mpost-inc, and -mpost-modify.
-mnovect-double
Change the preferred SIMD mode to SImode. The default is
-mvect-double, which uses DImode as preferred SIMD mode.
-max-vect-align=num
The maximum alignment for SIMD vector mode types. num may be 4 or
8. The default is 8. Note that this is an ABI change, even though
many library function interfaces are unaffected if they don't use
SIMD vector modes in places that affect size and/or alignment of
relevant types.
-msplit-vecmove-early
Split vector moves into single word moves before reload. In theory
this can give better register allocation, but so far the reverse
seems to be generally the case.
-m1reg-reg
Specify a register to hold the constant -1, which makes loading
small negative constants and certain bitmasks faster. Allowable
values for reg are r43 and r63, which specify use of that register
as a fixed register, and none, which means that no register is used
for this purpose. The default is -m1reg-none.
AMD GCN Options
These options are defined specifically for the AMD GCN port.
-march=gpu
-mtune=gpu
Set architecture type or tuning for gpu. Supported values for gpu
are
fiji
Compile for GCN3 Fiji devices (gfx803).
gfx900
Compile for GCN5 Vega 10 devices (gfx900).
gfx906
Compile for GCN5 Vega 20 devices (gfx906).
-mstack-size=bytes
Specify how many bytes of stack space will be requested for each
GPU thread (wave-front). Beware that there may be many threads and
limited memory available. The size of the stack allocation may
also have an impact on run-time performance. The default is 32KB
when using OpenACC or OpenMP, and 1MB otherwise.
ARC Options
The following options control the architecture variant for which code
is being compiled:
-mbarrel-shifter
Generate instructions supported by barrel shifter. This is the
default unless -mcpu=ARC601 or -mcpu=ARCEM is in effect.
-mjli-always
Force to call a function using jli_s instruction. This option is
valid only for ARCv2 architecture.
-mcpu=cpu
Set architecture type, register usage, and instruction scheduling
parameters for cpu. There are also shortcut alias options
available for backward compatibility and convenience. Supported
values for cpu are
arc600
Compile for ARC600. Aliases: -mA6, -mARC600.
arc601
Compile for ARC601. Alias: -mARC601.
arc700
Compile for ARC700. Aliases: -mA7, -mARC700. This is the
default when configured with --with-cpu=arc700.
arcem
Compile for ARC EM.
archs
Compile for ARC HS.
em Compile for ARC EM CPU with no hardware extensions.
em4 Compile for ARC EM4 CPU.
em4_dmips
Compile for ARC EM4 DMIPS CPU.
em4_fpus
Compile for ARC EM4 DMIPS CPU with the single-precision
floating-point extension.
em4_fpuda
Compile for ARC EM4 DMIPS CPU with single-precision floating-
point and double assist instructions.
hs Compile for ARC HS CPU with no hardware extensions except the
atomic instructions.
hs34
Compile for ARC HS34 CPU.
hs38
Compile for ARC HS38 CPU.
hs38_linux
Compile for ARC HS38 CPU with all hardware extensions on.
arc600_norm
Compile for ARC 600 CPU with "norm" instructions enabled.
arc600_mul32x16
Compile for ARC 600 CPU with "norm" and 32x16-bit multiply
instructions enabled.
arc600_mul64
Compile for ARC 600 CPU with "norm" and "mul64"-family
instructions enabled.
arc601_norm
Compile for ARC 601 CPU with "norm" instructions enabled.
arc601_mul32x16
Compile for ARC 601 CPU with "norm" and 32x16-bit multiply
instructions enabled.
arc601_mul64
Compile for ARC 601 CPU with "norm" and "mul64"-family
instructions enabled.
nps400
Compile for ARC 700 on NPS400 chip.
em_mini
Compile for ARC EM minimalist configuration featuring reduced
register set.
-mdpfp
-mdpfp-compact
Generate double-precision FPX instructions, tuned for the compact
implementation.
-mdpfp-fast
Generate double-precision FPX instructions, tuned for the fast
implementation.
-mno-dpfp-lrsr
Disable "lr" and "sr" instructions from using FPX extension aux
registers.
-mea
Generate extended arithmetic instructions. Currently only "divaw",
"adds", "subs", and "sat16" are supported. Only valid for
-mcpu=ARC700.
-mno-mpy
Do not generate "mpy"-family instructions for ARC700. This option
is deprecated.
-mmul32x16
Generate 32x16-bit multiply and multiply-accumulate instructions.
-mmul64
Generate "mul64" and "mulu64" instructions. Only valid for
-mcpu=ARC600.
-mnorm
Generate "norm" instructions. This is the default if -mcpu=ARC700
is in effect.
-mspfp
-mspfp-compact
Generate single-precision FPX instructions, tuned for the compact
implementation.
-mspfp-fast
Generate single-precision FPX instructions, tuned for the fast
implementation.
-msimd
Enable generation of ARC SIMD instructions via target-specific
builtins. Only valid for -mcpu=ARC700.
-msoft-float
This option ignored; it is provided for compatibility purposes
only. Software floating-point code is emitted by default, and this
default can overridden by FPX options; -mspfp, -mspfp-compact, or
-mspfp-fast for single precision, and -mdpfp, -mdpfp-compact, or
-mdpfp-fast for double precision.
-mswap
Generate "swap" instructions.
-matomic
This enables use of the locked load/store conditional extension to
implement atomic memory built-in functions. Not available for ARC
6xx or ARC EM cores.
-mdiv-rem
Enable "div" and "rem" instructions for ARCv2 cores.
-mcode-density
Enable code density instructions for ARC EM. This option is on by
default for ARC HS.
-mll64
Enable double load/store operations for ARC HS cores.
-mtp-regno=regno
Specify thread pointer register number.
-mmpy-option=multo
Compile ARCv2 code with a multiplier design option. You can
specify the option using either a string or numeric value for
multo. wlh1 is the default value. The recognized values are:
0
none
No multiplier available.
1
w 16x16 multiplier, fully pipelined. The following instructions
are enabled: "mpyw" and "mpyuw".
2
wlh1
32x32 multiplier, fully pipelined (1 stage). The following
instructions are additionally enabled: "mpy", "mpyu", "mpym",
"mpymu", and "mpy_s".
3
wlh2
32x32 multiplier, fully pipelined (2 stages). The following
instructions are additionally enabled: "mpy", "mpyu", "mpym",
"mpymu", and "mpy_s".
4
wlh3
Two 16x16 multipliers, blocking, sequential. The following
instructions are additionally enabled: "mpy", "mpyu", "mpym",
"mpymu", and "mpy_s".
5
wlh4
One 16x16 multiplier, blocking, sequential. The following
instructions are additionally enabled: "mpy", "mpyu", "mpym",
"mpymu", and "mpy_s".
6
wlh5
One 32x4 multiplier, blocking, sequential. The following
instructions are additionally enabled: "mpy", "mpyu", "mpym",
"mpymu", and "mpy_s".
7
plus_dmpy
ARC HS SIMD support.
8
plus_macd
ARC HS SIMD support.
9
plus_qmacw
ARC HS SIMD support.
This option is only available for ARCv2 cores.
-mfpu=fpu
Enables support for specific floating-point hardware extensions for
ARCv2 cores. Supported values for fpu are:
fpus
Enables support for single-precision floating-point hardware
extensions.
fpud
Enables support for double-precision floating-point hardware
extensions. The single-precision floating-point extension is
also enabled. Not available for ARC EM.
fpuda
Enables support for double-precision floating-point hardware
extensions using double-precision assist instructions. The
single-precision floating-point extension is also enabled.
This option is only available for ARC EM.
fpuda_div
Enables support for double-precision floating-point hardware
extensions using double-precision assist instructions. The
single-precision floating-point, square-root, and divide
extensions are also enabled. This option is only available for
ARC EM.
fpuda_fma
Enables support for double-precision floating-point hardware
extensions using double-precision assist instructions. The
single-precision floating-point and fused multiply and add
hardware extensions are also enabled. This option is only
available for ARC EM.
fpuda_all
Enables support for double-precision floating-point hardware
extensions using double-precision assist instructions. All
single-precision floating-point hardware extensions are also
enabled. This option is only available for ARC EM.
fpus_div
Enables support for single-precision floating-point, square-
root and divide hardware extensions.
fpud_div
Enables support for double-precision floating-point, square-
root and divide hardware extensions. This option includes
option fpus_div. Not available for ARC EM.
fpus_fma
Enables support for single-precision floating-point and fused
multiply and add hardware extensions.
fpud_fma
Enables support for double-precision floating-point and fused
multiply and add hardware extensions. This option includes
option fpus_fma. Not available for ARC EM.
fpus_all
Enables support for all single-precision floating-point
hardware extensions.
fpud_all
Enables support for all single- and double-precision floating-
point hardware extensions. Not available for ARC EM.
-mirq-ctrl-saved=register-range, blink, lp_count
Specifies general-purposes registers that the processor
automatically saves/restores on interrupt entry and exit.
register-range is specified as two registers separated by a dash.
The register range always starts with "r0", the upper limit is "fp"
register. blink and lp_count are optional. This option is only
valid for ARC EM and ARC HS cores.
-mrgf-banked-regs=number
Specifies the number of registers replicated in second register
bank on entry to fast interrupt. Fast interrupts are interrupts
with the highest priority level P0. These interrupts save only PC
and STATUS32 registers to avoid memory transactions during
interrupt entry and exit sequences. Use this option when you are
using fast interrupts in an ARC V2 family processor. Permitted
values are 4, 8, 16, and 32.
-mlpc-width=width
Specify the width of the "lp_count" register. Valid values for
width are 8, 16, 20, 24, 28 and 32 bits. The default width is
fixed to 32 bits. If the width is less than 32, the compiler does
not attempt to transform loops in your program to use the zero-
delay loop mechanism unless it is known that the "lp_count"
register can hold the required loop-counter value. Depending on
the width specified, the compiler and run-time library might
continue to use the loop mechanism for various needs. This option
defines macro "__ARC_LPC_WIDTH__" with the value of width.
-mrf16
This option instructs the compiler to generate code for a 16-entry
register file. This option defines the "__ARC_RF16__" preprocessor
macro.
-mbranch-index
Enable use of "bi" or "bih" instructions to implement jump tables.
The following options are passed through to the assembler, and also
define preprocessor macro symbols.
-mdsp-packa
Passed down to the assembler to enable the DSP Pack A extensions.
Also sets the preprocessor symbol "__Xdsp_packa". This option is
deprecated.
-mdvbf
Passed down to the assembler to enable the dual Viterbi butterfly
extension. Also sets the preprocessor symbol "__Xdvbf". This
option is deprecated.
-mlock
Passed down to the assembler to enable the locked load/store
conditional extension. Also sets the preprocessor symbol
"__Xlock".
-mmac-d16
Passed down to the assembler. Also sets the preprocessor symbol
"__Xxmac_d16". This option is deprecated.
-mmac-24
Passed down to the assembler. Also sets the preprocessor symbol
"__Xxmac_24". This option is deprecated.
-mrtsc
Passed down to the assembler to enable the 64-bit time-stamp
counter extension instruction. Also sets the preprocessor symbol
"__Xrtsc". This option is deprecated.
-mswape
Passed down to the assembler to enable the swap byte ordering
extension instruction. Also sets the preprocessor symbol
"__Xswape".
-mtelephony
Passed down to the assembler to enable dual- and single-operand
instructions for telephony. Also sets the preprocessor symbol
"__Xtelephony". This option is deprecated.
-mxy
Passed down to the assembler to enable the XY memory extension.
Also sets the preprocessor symbol "__Xxy".
The following options control how the assembly code is annotated:
-misize
Annotate assembler instructions with estimated addresses.
-mannotate-align
Explain what alignment considerations lead to the decision to make
an instruction short or long.
The following options are passed through to the linker:
-marclinux
Passed through to the linker, to specify use of the "arclinux"
emulation. This option is enabled by default in tool chains built
for "arc-linux-uclibc" and "arceb-linux-uclibc" targets when
profiling is not requested.
-marclinux_prof
Passed through to the linker, to specify use of the "arclinux_prof"
emulation. This option is enabled by default in tool chains built
for "arc-linux-uclibc" and "arceb-linux-uclibc" targets when
profiling is requested.
The following options control the semantics of generated code:
-mlong-calls
Generate calls as register indirect calls, thus providing access to
the full 32-bit address range.
-mmedium-calls
Don't use less than 25-bit addressing range for calls, which is the
offset available for an unconditional branch-and-link instruction.
Conditional execution of function calls is suppressed, to allow use
of the 25-bit range, rather than the 21-bit range with conditional
branch-and-link. This is the default for tool chains built for
"arc-linux-uclibc" and "arceb-linux-uclibc" targets.
-G num
Put definitions of externally-visible data in a small data section
if that data is no bigger than num bytes. The default value of num
is 4 for any ARC configuration, or 8 when we have double load/store
operations.
-mno-sdata
Do not generate sdata references. This is the default for tool
chains built for "arc-linux-uclibc" and "arceb-linux-uclibc"
targets.
-mvolatile-cache
Use ordinarily cached memory accesses for volatile references.
This is the default.
-mno-volatile-cache
Enable cache bypass for volatile references.
The following options fine tune code generation:
-malign-call
Do alignment optimizations for call instructions.
-mauto-modify-reg
Enable the use of pre/post modify with register displacement.
-mbbit-peephole
Enable bbit peephole2.
-mno-brcc
This option disables a target-specific pass in arc_reorg to
generate compare-and-branch ("brcc") instructions. It has no
effect on generation of these instructions driven by the combiner
pass.
-mcase-vector-pcrel
Use PC-relative switch case tables to enable case table shortening.
This is the default for -Os.
-mcompact-casesi
Enable compact "casesi" pattern. This is the default for -Os, and
only available for ARCv1 cores. This option is deprecated.
-mno-cond-exec
Disable the ARCompact-specific pass to generate conditional
execution instructions.
Due to delay slot scheduling and interactions between operand
numbers, literal sizes, instruction lengths, and the support for
conditional execution, the target-independent pass to generate
conditional execution is often lacking, so the ARC port has kept a
special pass around that tries to find more conditional execution
generation opportunities after register allocation, branch
shortening, and delay slot scheduling have been done. This pass
generally, but not always, improves performance and code size, at
the cost of extra compilation time, which is why there is an option
to switch it off. If you have a problem with call instructions
exceeding their allowable offset range because they are
conditionalized, you should consider using -mmedium-calls instead.
-mearly-cbranchsi
Enable pre-reload use of the "cbranchsi" pattern.
-mexpand-adddi
Expand "adddi3" and "subdi3" at RTL generation time into "add.f",
"adc" etc. This option is deprecated.
-mindexed-loads
Enable the use of indexed loads. This can be problematic because
some optimizers then assume that indexed stores exist, which is not
the case.
-mlra
Enable Local Register Allocation. This is still experimental for
ARC, so by default the compiler uses standard reload (i.e.
-mno-lra).
-mlra-priority-none
Don't indicate any priority for target registers.
-mlra-priority-compact
Indicate target register priority for r0..r3 / r12..r15.
-mlra-priority-noncompact
Reduce target register priority for r0..r3 / r12..r15.
-mmillicode
When optimizing for size (using -Os), prologues and epilogues that
have to save or restore a large number of registers are often
shortened by using call to a special function in libgcc; this is
referred to as a millicode call. As these calls can pose
performance issues, and/or cause linking issues when linking in a
nonstandard way, this option is provided to turn on or off
millicode call generation.
-mcode-density-frame
This option enable the compiler to emit "enter" and "leave"
instructions. These instructions are only valid for CPUs with
code-density feature.
-mmixed-code
Tweak register allocation to help 16-bit instruction generation.
This generally has the effect of decreasing the average instruction
size while increasing the instruction count.
-mq-class
Ths option is deprecated. Enable q instruction alternatives. This
is the default for -Os.
-mRcq
Enable Rcq constraint handling. Most short code generation depends
on this. This is the default.
-mRcw
Enable Rcw constraint handling. Most ccfsm condexec mostly depends
on this. This is the default.
-msize-level=level
Fine-tune size optimization with regards to instruction lengths and
alignment. The recognized values for level are:
0 No size optimization. This level is deprecated and treated
like 1.
1 Short instructions are used opportunistically.
2 In addition, alignment of loops and of code after barriers are
dropped.
3 In addition, optional data alignment is dropped, and the option
Os is enabled.
This defaults to 3 when -Os is in effect. Otherwise, the behavior
when this is not set is equivalent to level 1.
-mtune=cpu
Set instruction scheduling parameters for cpu, overriding any
implied by -mcpu=.
Supported values for cpu are
ARC600
Tune for ARC600 CPU.
ARC601
Tune for ARC601 CPU.
ARC700
Tune for ARC700 CPU with standard multiplier block.
ARC700-xmac
Tune for ARC700 CPU with XMAC block.
ARC725D
Tune for ARC725D CPU.
ARC750D
Tune for ARC750D CPU.
-mmultcost=num
Cost to assume for a multiply instruction, with 4 being equal to a
normal instruction.
-munalign-prob-threshold=probability
Set probability threshold for unaligning branches. When tuning for
ARC700 and optimizing for speed, branches without filled delay slot
are preferably emitted unaligned and long, unless profiling
indicates that the probability for the branch to be taken is below
probability. The default is (REG_BR_PROB_BASE/2), i.e. 5000.
The following options are maintained for backward compatibility, but
are now deprecated and will be removed in a future release:
-margonaut
Obsolete FPX.
-mbig-endian
-EB Compile code for big-endian targets. Use of these options is now
deprecated. Big-endian code is supported by configuring GCC to
build "arceb-elf32" and "arceb-linux-uclibc" targets, for which big
endian is the default.
-mlittle-endian
-EL Compile code for little-endian targets. Use of these options is
now deprecated. Little-endian code is supported by configuring GCC
to build "arc-elf32" and "arc-linux-uclibc" targets, for which
little endian is the default.
-mbarrel_shifter
Replaced by -mbarrel-shifter.
-mdpfp_compact
Replaced by -mdpfp-compact.
-mdpfp_fast
Replaced by -mdpfp-fast.
-mdsp_packa
Replaced by -mdsp-packa.
-mEA
Replaced by -mea.
-mmac_24
Replaced by -mmac-24.
-mmac_d16
Replaced by -mmac-d16.
-mspfp_compact
Replaced by -mspfp-compact.
-mspfp_fast
Replaced by -mspfp-fast.
-mtune=cpu
Values arc600, arc601, arc700 and arc700-xmac for cpu are replaced
by ARC600, ARC601, ARC700 and ARC700-xmac respectively.
-multcost=num
Replaced by -mmultcost.
ARM Options
These -m options are defined for the ARM port:
-mabi=name
Generate code for the specified ABI. Permissible values are: apcs-
gnu, atpcs, aapcs, aapcs-linux and iwmmxt.
-mapcs-frame
Generate a stack frame that is compliant with the ARM Procedure
Call Standard for all functions, even if this is not strictly
necessary for correct execution of the code. Specifying
-fomit-frame-pointer with this option causes the stack frames not
to be generated for leaf functions. The default is
-mno-apcs-frame. This option is deprecated.
-mapcs
This is a synonym for -mapcs-frame and is deprecated.
-mthumb-interwork
Generate code that supports calling between the ARM and Thumb
instruction sets. Without this option, on pre-v5 architectures,
the two instruction sets cannot be reliably used inside one
program. The default is -mno-thumb-interwork, since slightly
larger code is generated when -mthumb-interwork is specified. In
AAPCS configurations this option is meaningless.
-mno-sched-prolog
Prevent the reordering of instructions in the function prologue, or
the merging of those instruction with the instructions in the
function's body. This means that all functions start with a
recognizable set of instructions (or in fact one of a choice from a
small set of different function prologues), and this information
can be used to locate the start of functions inside an executable
piece of code. The default is -msched-prolog.
-mfloat-abi=name
Specifies which floating-point ABI to use. Permissible values are:
soft, softfp and hard.
Specifying soft causes GCC to generate output containing library
calls for floating-point operations. softfp allows the generation
of code using hardware floating-point instructions, but still uses
the soft-float calling conventions. hard allows generation of
floating-point instructions and uses FPU-specific calling
conventions.
The default depends on the specific target configuration. Note
that the hard-float and soft-float ABIs are not link-compatible;
you must compile your entire program with the same ABI, and link
with a compatible set of libraries.
-mgeneral-regs-only
Generate code which uses only the general-purpose registers. This
will prevent the compiler from using floating-point and Advanced
SIMD registers but will not impose any restrictions on the
assembler.
-mlittle-endian
Generate code for a processor running in little-endian mode. This
is the default for all standard configurations.
-mbig-endian
Generate code for a processor running in big-endian mode; the
default is to compile code for a little-endian processor.
-mbe8
-mbe32
When linking a big-endian image select between BE8 and BE32
formats. The option has no effect for little-endian images and is
ignored. The default is dependent on the selected target
architecture. For ARMv6 and later architectures the default is
BE8, for older architectures the default is BE32. BE32 format has
been deprecated by ARM.
-march=name[+extension...]
This specifies the name of the target ARM architecture. GCC uses
this name to determine what kind of instructions it can emit when
generating assembly code. This option can be used in conjunction
with or instead of the -mcpu= option.
Permissible names are: armv4t, armv5t, armv5te, armv6, armv6j,
armv6k, armv6kz, armv6t2, armv6z, armv6zk, armv7, armv7-a, armv7ve,
armv8-a, armv8.1-a, armv8.2-a, armv8.3-a, armv8.4-a, armv8.5-a,
armv8.6-a, armv7-r, armv8-r, armv6-m, armv6s-m, armv7-m, armv7e-m,
armv8-m.base, armv8-m.main, armv8.1-m.main, iwmmxt and iwmmxt2.
Additionally, the following architectures, which lack support for
the Thumb execution state, are recognized but support is
deprecated: armv4.
Many of the architectures support extensions. These can be added
by appending +extension to the architecture name. Extension
options are processed in order and capabilities accumulate. An
extension will also enable any necessary base extensions upon which
it depends. For example, the +crypto extension will always enable
the +simd extension. The exception to the additive construction is
for extensions that are prefixed with +no...: these extensions
disable the specified option and any other extensions that may
depend on the presence of that extension.
For example, -march=armv7-a+simd+nofp+vfpv4 is equivalent to
writing -march=armv7-a+vfpv4 since the +simd option is entirely
disabled by the +nofp option that follows it.
Most extension names are generically named, but have an effect that
is dependent upon the architecture to which it is applied. For
example, the +simd option can be applied to both armv7-a and
armv8-a architectures, but will enable the original ARMv7-A
Advanced SIMD (Neon) extensions for armv7-a and the ARMv8-A variant
for armv8-a.
The table below lists the supported extensions for each
architecture. Architectures not mentioned do not support any
extensions.
armv5te
armv6
armv6j
armv6k
armv6kz
armv6t2
armv6z
armv6zk
+fp The VFPv2 floating-point instructions. The extension
+vfpv2 can be used as an alias for this extension.
+nofp
Disable the floating-point instructions.
armv7
The common subset of the ARMv7-A, ARMv7-R and ARMv7-M
architectures.
+fp The VFPv3 floating-point instructions, with 16 double-
precision registers. The extension +vfpv3-d16 can be used
as an alias for this extension. Note that floating-point
is not supported by the base ARMv7-M architecture, but is
compatible with both the ARMv7-A and ARMv7-R architectures.
+nofp
Disable the floating-point instructions.
armv7-a
+mp The multiprocessing extension.
+sec
The security extension.
+fp The VFPv3 floating-point instructions, with 16 double-
precision registers. The extension +vfpv3-d16 can be used
as an alias for this extension.
+simd
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point
instructions. The extensions +neon and +neon-vfpv3 can be
used as aliases for this extension.
+vfpv3
The VFPv3 floating-point instructions, with 32 double-
precision registers.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions, with 16 double-
precision registers and the half-precision floating-point
conversion operations.
+vfpv3-fp16
The VFPv3 floating-point instructions, with 32 double-
precision registers and the half-precision floating-point
conversion operations.
+vfpv4-d16
The VFPv4 floating-point instructions, with 16 double-
precision registers.
+vfpv4
The VFPv4 floating-point instructions, with 32 double-
precision registers.
+neon-fp16
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point
instructions, with the half-precision floating-point
conversion operations.
+neon-vfpv4
The Advanced SIMD (Neon) v2 and the VFPv4 floating-point
instructions.
+nosimd
Disable the Advanced SIMD instructions (does not disable
floating point).
+nofp
Disable the floating-point and Advanced SIMD instructions.
armv7ve
The extended version of the ARMv7-A architecture with support
for virtualization.
+fp The VFPv4 floating-point instructions, with 16 double-
precision registers. The extension +vfpv4-d16 can be used
as an alias for this extension.
+simd
The Advanced SIMD (Neon) v2 and the VFPv4 floating-point
instructions. The extension +neon-vfpv4 can be used as an
alias for this extension.
+vfpv3-d16
The VFPv3 floating-point instructions, with 16 double-
precision registers.
+vfpv3
The VFPv3 floating-point instructions, with 32 double-
precision registers.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions, with 16 double-
precision registers and the half-precision floating-point
conversion operations.
+vfpv3-fp16
The VFPv3 floating-point instructions, with 32 double-
precision registers and the half-precision floating-point
conversion operations.
+vfpv4-d16
The VFPv4 floating-point instructions, with 16 double-
precision registers.
+vfpv4
The VFPv4 floating-point instructions, with 32 double-
precision registers.
+neon
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point
instructions. The extension +neon-vfpv3 can be used as an
alias for this extension.
+neon-fp16
The Advanced SIMD (Neon) v1 and the VFPv3 floating-point
instructions, with the half-precision floating-point
conversion operations.
+nosimd
Disable the Advanced SIMD instructions (does not disable
floating point).
+nofp
Disable the floating-point and Advanced SIMD instructions.
armv8-a
+crc
The Cyclic Redundancy Check (CRC) instructions.
+simd
The ARMv8-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction Instructions.
armv8.1-a
+simd
The ARMv8.1-A Advanced SIMD and floating-point
instructions.
+crypto
The cryptographic instructions. This also enables the
Advanced SIMD and floating-point instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction Instructions.
armv8.2-a
armv8.3-a
+fp16
The half-precision floating-point data processing
instructions. This also enables the Advanced SIMD and
floating-point instructions.
+fp16fml
The half-precision floating-point fmla extension. This
also enables the half-precision floating-point extension
and Advanced SIMD and floating-point instructions.
+simd
The ARMv8.1-A Advanced SIMD and floating-point
instructions.
+crypto
The cryptographic instructions. This also enables the
Advanced SIMD and floating-point instructions.
+dotprod
Enable the Dot Product extension. This also enables
Advanced SIMD instructions.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction Instructions.
+i8mm
8-bit Integer Matrix Multiply instructions. This also
enables Advanced SIMD and floating-point instructions.
+bf16
Brain half-precision floating-point instructions. This
also enables Advanced SIMD and floating-point instructions.
armv8.4-a
+fp16
The half-precision floating-point data processing
instructions. This also enables the Advanced SIMD and
floating-point instructions as well as the Dot Product
extension and the half-precision floating-point fmla
extension.
+simd
The ARMv8.3-A Advanced SIMD and floating-point instructions
as well as the Dot Product extension.
+crypto
The cryptographic instructions. This also enables the
Advanced SIMD and floating-point instructions as well as
the Dot Product extension.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
+sb Speculation Barrier Instruction.
+predres
Execution and Data Prediction Restriction Instructions.
+i8mm
8-bit Integer Matrix Multiply instructions. This also
enables Advanced SIMD and floating-point instructions.
+bf16
Brain half-precision floating-point instructions. This
also enables Advanced SIMD and floating-point instructions.
armv8.5-a
+fp16
The half-precision floating-point data processing
instructions. This also enables the Advanced SIMD and
floating-point instructions as well as the Dot Product
extension and the half-precision floating-point fmla
extension.
+simd
The ARMv8.3-A Advanced SIMD and floating-point instructions
as well as the Dot Product extension.
+crypto
The cryptographic instructions. This also enables the
Advanced SIMD and floating-point instructions as well as
the Dot Product extension.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
+i8mm
8-bit Integer Matrix Multiply instructions. This also
enables Advanced SIMD and floating-point instructions.
+bf16
Brain half-precision floating-point instructions. This
also enables Advanced SIMD and floating-point instructions.
armv8.6-a
+fp16
The half-precision floating-point data processing
instructions. This also enables the Advanced SIMD and
floating-point instructions as well as the Dot Product
extension and the half-precision floating-point fmla
extension.
+simd
The ARMv8.3-A Advanced SIMD and floating-point instructions
as well as the Dot Product extension.
+crypto
The cryptographic instructions. This also enables the
Advanced SIMD and floating-point instructions as well as
the Dot Product extension.
+nocrypto
Disable the cryptographic extension.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
+i8mm
8-bit Integer Matrix Multiply instructions. This also
enables Advanced SIMD and floating-point instructions.
+bf16
Brain half-precision floating-point instructions. This
also enables Advanced SIMD and floating-point instructions.
armv7-r
+fp.sp
The single-precision VFPv3 floating-point instructions.
The extension +vfpv3xd can be used as an alias for this
extension.
+fp The VFPv3 floating-point instructions with 16 double-
precision registers. The extension +vfpv3-d16 can be used
as an alias for this extension.
+vfpv3xd-d16-fp16
The single-precision VFPv3 floating-point instructions with
16 double-precision registers and the half-precision
floating-point conversion operations.
+vfpv3-d16-fp16
The VFPv3 floating-point instructions with 16 double-
precision registers and the half-precision floating-point
conversion operations.
+nofp
Disable the floating-point extension.
+idiv
The ARM-state integer division instructions.
+noidiv
Disable the ARM-state integer division extension.
armv7e-m
+fp The single-precision VFPv4 floating-point instructions.
+fpv5
The single-precision FPv5 floating-point instructions.
+fp.dp
The single- and double-precision FPv5 floating-point
instructions.
+nofp
Disable the floating-point extensions.
armv8.1-m.main
+dsp
The DSP instructions.
+mve
The M-Profile Vector Extension (MVE) integer instructions.
+mve.fp
The M-Profile Vector Extension (MVE) integer and single
precision floating-point instructions.
+fp The single-precision floating-point instructions.
+fp.dp
The single- and double-precision floating-point
instructions.
+nofp
Disable the floating-point extension.
+cdecp0, +cdecp1, ... , +cdecp7
Enable the Custom Datapath Extension (CDE) on selected
coprocessors according to the numbers given in the options
in the range 0 to 7.
armv8-m.main
+dsp
The DSP instructions.
+nodsp
Disable the DSP extension.
+fp The single-precision floating-point instructions.
+fp.dp
The single- and double-precision floating-point
instructions.
+nofp
Disable the floating-point extension.
+cdecp0, +cdecp1, ... , +cdecp7
Enable the Custom Datapath Extension (CDE) on selected
coprocessors according to the numbers given in the options
in the range 0 to 7.
armv8-r
+crc
The Cyclic Redundancy Check (CRC) instructions.
+fp.sp
The single-precision FPv5 floating-point instructions.
+simd
The ARMv8-A Advanced SIMD and floating-point instructions.
+crypto
The cryptographic instructions.
+nocrypto
Disable the cryptographic instructions.
+nofp
Disable the floating-point, Advanced SIMD and cryptographic
instructions.
-march=native causes the compiler to auto-detect the architecture
of the build computer. At present, this feature is only supported
on GNU/Linux, and not all architectures are recognized. If the
auto-detect is unsuccessful the option has no effect.
-mtune=name
This option specifies the name of the target ARM processor for
which GCC should tune the performance of the code. For some ARM
implementations better performance can be obtained by using this
option. Permissible names are: arm7tdmi, arm7tdmi-s, arm710t,
arm720t, arm740t, strongarm, strongarm110, strongarm1100,
0strongarm1110, arm8, arm810, arm9, arm9e, arm920, arm920t,
arm922t, arm946e-s, arm966e-s, arm968e-s, arm926ej-s, arm940t,
arm9tdmi, arm10tdmi, arm1020t, arm1026ej-s, arm10e, arm1020e,
arm1022e, arm1136j-s, arm1136jf-s, mpcore, mpcorenovfp,
arm1156t2-s, arm1156t2f-s, arm1176jz-s, arm1176jzf-s,
generic-armv7-a, cortex-a5, cortex-a7, cortex-a8, cortex-a9,
cortex-a12, cortex-a15, cortex-a17, cortex-a32, cortex-a35,
cortex-a53, cortex-a55, cortex-a57, cortex-a72, cortex-a73,
cortex-a75, cortex-a76, cortex-a76ae, cortex-a77, ares, cortex-r4,
cortex-r4f, cortex-r5, cortex-r7, cortex-r8, cortex-r52, cortex-m0,
cortex-m0plus, cortex-m1, cortex-m3, cortex-m4, cortex-m7,
cortex-m23, cortex-m33, cortex-m35p, cortex-m55,
cortex-m1.small-multiply, cortex-m0.small-multiply,
cortex-m0plus.small-multiply, exynos-m1, marvell-pj4, neoverse-n1,
neoverse-n2, neoverse-v1, xscale, iwmmxt, iwmmxt2, ep9312, fa526,
fa626, fa606te, fa626te, fmp626, fa726te, xgene1.
Additionally, this option can specify that GCC should tune the
performance of the code for a big.LITTLE system. Permissible names
are: cortex-a15.cortex-a7, cortex-a17.cortex-a7,
cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a72.cortex-a35, cortex-a73.cortex-a53,
cortex-a75.cortex-a55, cortex-a76.cortex-a55.
-mtune=generic-arch specifies that GCC should tune the performance
for a blend of processors within architecture arch. The aim is to
generate code that run well on the current most popular processors,
balancing between optimizations that benefit some CPUs in the
range, and avoiding performance pitfalls of other CPUs. The
effects of this option may change in future GCC versions as CPU
models come and go.
-mtune permits the same extension options as -mcpu, but the
extension options do not affect the tuning of the generated code.
-mtune=native causes the compiler to auto-detect the CPU of the
build computer. At present, this feature is only supported on
GNU/Linux, and not all architectures are recognized. If the auto-
detect is unsuccessful the option has no effect.
-mcpu=name[+extension...]
This specifies the name of the target ARM processor. GCC uses this
name to derive the name of the target ARM architecture (as if
specified by -march) and the ARM processor type for which to tune
for performance (as if specified by -mtune). Where this option is
used in conjunction with -march or -mtune, those options take
precedence over the appropriate part of this option.
Many of the supported CPUs implement optional architectural
extensions. Where this is so the architectural extensions are
normally enabled by default. If implementations that lack the
extension exist, then the extension syntax can be used to disable
those extensions that have been omitted. For floating-point and
Advanced SIMD (Neon) instructions, the settings of the options
-mfloat-abi and -mfpu must also be considered: floating-point and
Advanced SIMD instructions will only be used if -mfloat-abi is not
set to soft; and any setting of -mfpu other than auto will override
the available floating-point and SIMD extension instructions.
For example, cortex-a9 can be found in three major configurations:
integer only, with just a floating-point unit or with floating-
point and Advanced SIMD. The default is to enable all the
instructions, but the extensions +nosimd and +nofp can be used to
disable just the SIMD or both the SIMD and floating-point
instructions respectively.
Permissible names for this option are the same as those for -mtune.
The following extension options are common to the listed CPUs:
+nodsp
Disable the DSP instructions on cortex-m33, cortex-m35p.
+nofp
Disables the floating-point instructions on arm9e, arm946e-s,
arm966e-s, arm968e-s, arm10e, arm1020e, arm1022e, arm926ej-s,
arm1026ej-s, cortex-r5, cortex-r7, cortex-r8, cortex-m4,
cortex-m7, cortex-m33 and cortex-m35p. Disables the floating-
point and SIMD instructions on generic-armv7-a, cortex-a5,
cortex-a7, cortex-a8, cortex-a9, cortex-a12, cortex-a15,
cortex-a17, cortex-a15.cortex-a7, cortex-a17.cortex-a7,
cortex-a32, cortex-a35, cortex-a53 and cortex-a55.
+nofp.dp
Disables the double-precision component of the floating-point
instructions on cortex-r5, cortex-r7, cortex-r8, cortex-r52 and
cortex-m7.
+nosimd
Disables the SIMD (but not floating-point) instructions on
generic-armv7-a, cortex-a5, cortex-a7 and cortex-a9.
+crypto
Enables the cryptographic instructions on cortex-a32,
cortex-a35, cortex-a53, cortex-a55, cortex-a57, cortex-a72,
cortex-a73, cortex-a75, exynos-m1, xgene1,
cortex-a57.cortex-a53, cortex-a72.cortex-a53,
cortex-a73.cortex-a35, cortex-a73.cortex-a53 and
cortex-a75.cortex-a55.
Additionally the generic-armv7-a pseudo target defaults to VFPv3
with 16 double-precision registers. It supports the following
extension options: mp, sec, vfpv3-d16, vfpv3, vfpv3-d16-fp16,
vfpv3-fp16, vfpv4-d16, vfpv4, neon, neon-vfpv3, neon-fp16,
neon-vfpv4. The meanings are the same as for the extensions to
-march=armv7-a.
-mcpu=generic-arch is also permissible, and is equivalent to
-march=arch -mtune=generic-arch. See -mtune for more information.
-mcpu=native causes the compiler to auto-detect the CPU of the
build computer. At present, this feature is only supported on
GNU/Linux, and not all architectures are recognized. If the auto-
detect is unsuccessful the option has no effect.
-mfpu=name
This specifies what floating-point hardware (or hardware emulation)
is available on the target. Permissible names are: auto, vfpv2,
vfpv3, vfpv3-fp16, vfpv3-d16, vfpv3-d16-fp16, vfpv3xd,
vfpv3xd-fp16, neon-vfpv3, neon-fp16, vfpv4, vfpv4-d16, fpv4-sp-d16,
neon-vfpv4, fpv5-d16, fpv5-sp-d16, fp-armv8, neon-fp-armv8 and
crypto-neon-fp-armv8. Note that neon is an alias for neon-vfpv3
and vfp is an alias for vfpv2.
The setting auto is the default and is special. It causes the
compiler to select the floating-point and Advanced SIMD
instructions based on the settings of -mcpu and -march.
If the selected floating-point hardware includes the NEON extension
(e.g. -mfpu=neon), note that floating-point operations are not
generated by GCC's auto-vectorization pass unless
-funsafe-math-optimizations is also specified. This is because
NEON hardware does not fully implement the IEEE 754 standard for
floating-point arithmetic (in particular denormal values are
treated as zero), so the use of NEON instructions may lead to a
loss of precision.
You can also set the fpu name at function level by using the
"target("fpu=")" function attributes or pragmas.
-mfp16-format=name
Specify the format of the "__fp16" half-precision floating-point
type. Permissible names are none, ieee, and alternative; the
default is none, in which case the "__fp16" type is not defined.
-mstructure-size-boundary=n
The sizes of all structures and unions are rounded up to a multiple
of the number of bits set by this option. Permissible values are
8, 32 and 64. The default value varies for different toolchains.
For the COFF targeted toolchain the default value is 8. A value of
64 is only allowed if the underlying ABI supports it.
Specifying a larger number can produce faster, more efficient code,
but can also increase the size of the program. Different values
are potentially incompatible. Code compiled with one value cannot
necessarily expect to work with code or libraries compiled with
another value, if they exchange information using structures or
unions.
This option is deprecated.
-mabort-on-noreturn
Generate a call to the function "abort" at the end of a "noreturn"
function. It is executed if the function tries to return.
-mlong-calls
-mno-long-calls
Tells the compiler to perform function calls by first loading the
address of the function into a register and then performing a
subroutine call on this register. This switch is needed if the
target function lies outside of the 64-megabyte addressing range of
the offset-based version of subroutine call instruction.
Even if this switch is enabled, not all function calls are turned
into long calls. The heuristic is that static functions, functions
that have the "short_call" attribute, functions that are inside the
scope of a "#pragma no_long_calls" directive, and functions whose
definitions have already been compiled within the current
compilation unit are not turned into long calls. The exceptions to
this rule are that weak function definitions, functions with the
"long_call" attribute or the "section" attribute, and functions
that are within the scope of a "#pragma long_calls" directive are
always turned into long calls.
This feature is not enabled by default. Specifying -mno-long-calls
restores the default behavior, as does placing the function calls
within the scope of a "#pragma long_calls_off" directive. Note
these switches have no effect on how the compiler generates code to
handle function calls via function pointers.
-msingle-pic-base
Treat the register used for PIC addressing as read-only, rather
than loading it in the prologue for each function. The runtime
system is responsible for initializing this register with an
appropriate value before execution begins.
-mpic-register=reg
Specify the register to be used for PIC addressing. For standard
PIC base case, the default is any suitable register determined by
compiler. For single PIC base case, the default is R9 if target is
EABI based or stack-checking is enabled, otherwise the default is
R10.
-mpic-data-is-text-relative
Assume that the displacement between the text and data segments is
fixed at static link time. This permits using PC-relative
addressing operations to access data known to be in the data
segment. For non-VxWorks RTP targets, this option is enabled by
default. When disabled on such targets, it will enable
-msingle-pic-base by default.
-mpoke-function-name
Write the name of each function into the text section, directly
preceding the function prologue. The generated code is similar to
this:
t0
.ascii "arm_poke_function_name", 0
.align
t1
.word 0xff000000 + (t1 - t0)
arm_poke_function_name
mov ip, sp
stmfd sp!, {fp, ip, lr, pc}
sub fp, ip, #4
When performing a stack backtrace, code can inspect the value of
"pc" stored at "fp + 0". If the trace function then looks at
location "pc - 12" and the top 8 bits are set, then we know that
there is a function name embedded immediately preceding this
location and has length "((pc[-3]) & 0xff000000)".
-mthumb
-marm
Select between generating code that executes in ARM and Thumb
states. The default for most configurations is to generate code
that executes in ARM state, but the default can be changed by
configuring GCC with the --with-mode=state configure option.
You can also override the ARM and Thumb mode for each function by
using the "target("thumb")" and "target("arm")" function attributes
or pragmas.
-mflip-thumb
Switch ARM/Thumb modes on alternating functions. This option is
provided for regression testing of mixed Thumb/ARM code generation,
and is not intended for ordinary use in compiling code.
-mtpcs-frame
Generate a stack frame that is compliant with the Thumb Procedure
Call Standard for all non-leaf functions. (A leaf function is one
that does not call any other functions.) The default is
-mno-tpcs-frame.
-mtpcs-leaf-frame
Generate a stack frame that is compliant with the Thumb Procedure
Call Standard for all leaf functions. (A leaf function is one that
does not call any other functions.) The default is
-mno-apcs-leaf-frame.
-mcallee-super-interworking
Gives all externally visible functions in the file being compiled
an ARM instruction set header which switches to Thumb mode before
executing the rest of the function. This allows these functions to
be called from non-interworking code. This option is not valid in
AAPCS configurations because interworking is enabled by default.
-mcaller-super-interworking
Allows calls via function pointers (including virtual functions) to
execute correctly regardless of whether the target code has been
compiled for interworking or not. There is a small overhead in the
cost of executing a function pointer if this option is enabled.
This option is not valid in AAPCS configurations because
interworking is enabled by default.
-mtp=name
Specify the access model for the thread local storage pointer. The
valid models are soft, which generates calls to "__aeabi_read_tp",
cp15, which fetches the thread pointer from "cp15" directly
(supported in the arm6k architecture), and auto, which uses the
best available method for the selected processor. The default
setting is auto.
-mtls-dialect=dialect
Specify the dialect to use for accessing thread local storage. Two
dialects are supported---gnu and gnu2. The gnu dialect selects the
original GNU scheme for supporting local and global dynamic TLS
models. The gnu2 dialect selects the GNU descriptor scheme, which
provides better performance for shared libraries. The GNU
descriptor scheme is compatible with the original scheme, but does
require new assembler, linker and library support. Initial and
local exec TLS models are unaffected by this option and always use
the original scheme.
-mword-relocations
Only generate absolute relocations on word-sized values (i.e.
R_ARM_ABS32). This is enabled by default on targets (uClinux,
SymbianOS) where the runtime loader imposes this restriction, and
when -fpic or -fPIC is specified. This option conflicts with
-mslow-flash-data.
-mfix-cortex-m3-ldrd
Some Cortex-M3 cores can cause data corruption when "ldrd"
instructions with overlapping destination and base registers are
used. This option avoids generating these instructions. This
option is enabled by default when -mcpu=cortex-m3 is specified.
-munaligned-access
-mno-unaligned-access
Enables (or disables) reading and writing of 16- and 32- bit values
from addresses that are not 16- or 32- bit aligned. By default
unaligned access is disabled for all pre-ARMv6, all ARMv6-M and for
ARMv8-M Baseline architectures, and enabled for all other
architectures. If unaligned access is not enabled then words in
packed data structures are accessed a byte at a time.
The ARM attribute "Tag_CPU_unaligned_access" is set in the
generated object file to either true or false, depending upon the
setting of this option. If unaligned access is enabled then the
preprocessor symbol "__ARM_FEATURE_UNALIGNED" is also defined.
-mneon-for-64bits
This option is deprecated and has no effect.
-mslow-flash-data
Assume loading data from flash is slower than fetching instruction.
Therefore literal load is minimized for better performance. This
option is only supported when compiling for ARMv7 M-profile and off
by default. It conflicts with -mword-relocations.
-masm-syntax-unified
Assume inline assembler is using unified asm syntax. The default
is currently off which implies divided syntax. This option has no
impact on Thumb2. However, this may change in future releases of
GCC. Divided syntax should be considered deprecated.
-mrestrict-it
Restricts generation of IT blocks to conform to the rules of
ARMv8-A. IT blocks can only contain a single 16-bit instruction
from a select set of instructions. This option is on by default for
ARMv8-A Thumb mode.
-mprint-tune-info
Print CPU tuning information as comment in assembler file. This is
an option used only for regression testing of the compiler and not
intended for ordinary use in compiling code. This option is
disabled by default.
-mverbose-cost-dump
Enable verbose cost model dumping in the debug dump files. This
option is provided for use in debugging the compiler.
-mpure-code
Do not allow constant data to be placed in code sections.
Additionally, when compiling for ELF object format give all text
sections the ELF processor-specific section attribute
"SHF_ARM_PURECODE". This option is only available when generating
non-pic code for M-profile targets.
-mcmse
Generate secure code as per the "ARMv8-M Security Extensions:
Requirements on Development Tools Engineering Specification", which
can be found on
<https://developer.arm.com/documentation/ecm0359818/latest/>.
-mfix-cmse-cve-2021-35465
Mitigate against a potential security issue with the "VLLDM"
instruction in some M-profile devices when using CMSE
(CVE-2021-365465). This option is enabled by default when the
option -mcpu= is used with "cortex-m33", "cortex-m35p" or
"cortex-m55". The option -mno-fix-cmse-cve-2021-35465 can be used
to disable the mitigation.
-mfdpic
-mno-fdpic
Select the FDPIC ABI, which uses 64-bit function descriptors to
represent pointers to functions. When the compiler is configured
for "arm-*-uclinuxfdpiceabi" targets, this option is on by default
and implies -fPIE if none of the PIC/PIE-related options is
provided. On other targets, it only enables the FDPIC-specific
code generation features, and the user should explicitly provide
the PIC/PIE-related options as needed.
Note that static linking is not supported because it would still
involve the dynamic linker when the program self-relocates. If
such behavior is acceptable, use -static and -Wl,-dynamic-linker
options.
The opposite -mno-fdpic option is useful (and required) to build
the Linux kernel using the same ("arm-*-uclinuxfdpiceabi")
toolchain as the one used to build the userland programs.
AVR Options
These options are defined for AVR implementations:
-mmcu=mcu
Specify Atmel AVR instruction set architectures (ISA) or MCU type.
The default for this option is avr2.
GCC supports the following AVR devices and ISAs:
"avr2"
"Classic" devices with up to 8 KiB of program memory. mcu =
"attiny22", "attiny26", "at90s2313", "at90s2323", "at90s2333",
"at90s2343", "at90s4414", "at90s4433", "at90s4434",
"at90c8534", "at90s8515", "at90s8535".
"avr25"
"Classic" devices with up to 8 KiB of program memory and with
the "MOVW" instruction. mcu = "attiny13", "attiny13a",
"attiny24", "attiny24a", "attiny25", "attiny261", "attiny261a",
"attiny2313", "attiny2313a", "attiny43u", "attiny44",
"attiny44a", "attiny45", "attiny48", "attiny441", "attiny461",
"attiny461a", "attiny4313", "attiny84", "attiny84a",
"attiny85", "attiny87", "attiny88", "attiny828", "attiny841",
"attiny861", "attiny861a", "ata5272", "ata6616c", "at86rf401".
"avr3"
"Classic" devices with 16 KiB up to 64 KiB of program memory.
mcu = "at76c711", "at43usb355".
"avr31"
"Classic" devices with 128 KiB of program memory. mcu =
"atmega103", "at43usb320".
"avr35"
"Classic" devices with 16 KiB up to 64 KiB of program memory
and with the "MOVW" instruction. mcu = "attiny167",
"attiny1634", "atmega8u2", "atmega16u2", "atmega32u2",
"ata5505", "ata6617c", "ata664251", "at90usb82", "at90usb162".
"avr4"
"Enhanced" devices with up to 8 KiB of program memory. mcu =
"atmega48", "atmega48a", "atmega48p", "atmega48pa",
"atmega48pb", "atmega8", "atmega8a", "atmega8hva", "atmega88",
"atmega88a", "atmega88p", "atmega88pa", "atmega88pb",
"atmega8515", "atmega8535", "ata6285", "ata6286", "ata6289",
"ata6612c", "at90pwm1", "at90pwm2", "at90pwm2b", "at90pwm3",
"at90pwm3b", "at90pwm81".
"avr5"
"Enhanced" devices with 16 KiB up to 64 KiB of program memory.
mcu = "atmega16", "atmega16a", "atmega16hva", "atmega16hva2",
"atmega16hvb", "atmega16hvbrevb", "atmega16m1", "atmega16u4",
"atmega161", "atmega162", "atmega163", "atmega164a",
"atmega164p", "atmega164pa", "atmega165", "atmega165a",
"atmega165p", "atmega165pa", "atmega168", "atmega168a",
"atmega168p", "atmega168pa", "atmega168pb", "atmega169",
"atmega169a", "atmega169p", "atmega169pa", "atmega32",
"atmega32a", "atmega32c1", "atmega32hvb", "atmega32hvbrevb",
"atmega32m1", "atmega32u4", "atmega32u6", "atmega323",
"atmega324a", "atmega324p", "atmega324pa", "atmega325",
"atmega325a", "atmega325p", "atmega325pa", "atmega328",
"atmega328p", "atmega328pb", "atmega329", "atmega329a",
"atmega329p", "atmega329pa", "atmega3250", "atmega3250a",
"atmega3250p", "atmega3250pa", "atmega3290", "atmega3290a",
"atmega3290p", "atmega3290pa", "atmega406", "atmega64",
"atmega64a", "atmega64c1", "atmega64hve", "atmega64hve2",
"atmega64m1", "atmega64rfr2", "atmega640", "atmega644",
"atmega644a", "atmega644p", "atmega644pa", "atmega644rfr2",
"atmega645", "atmega645a", "atmega645p", "atmega649",
"atmega649a", "atmega649p", "atmega6450", "atmega6450a",
"atmega6450p", "atmega6490", "atmega6490a", "atmega6490p",
"ata5795", "ata5790", "ata5790n", "ata5791", "ata6613c",
"ata6614q", "ata5782", "ata5831", "ata8210", "ata8510",
"ata5702m322", "at90pwm161", "at90pwm216", "at90pwm316",
"at90can32", "at90can64", "at90scr100", "at90usb646",
"at90usb647", "at94k", "m3000".
"avr51"
"Enhanced" devices with 128 KiB of program memory. mcu =
"atmega128", "atmega128a", "atmega128rfa1", "atmega128rfr2",
"atmega1280", "atmega1281", "atmega1284", "atmega1284p",
"atmega1284rfr2", "at90can128", "at90usb1286", "at90usb1287".
"avr6"
"Enhanced" devices with 3-byte PC, i.e. with more than 128 KiB
of program memory. mcu = "atmega256rfr2", "atmega2560",
"atmega2561", "atmega2564rfr2".
"avrxmega2"
"XMEGA" devices with more than 8 KiB and up to 64 KiB of
program memory. mcu = "atxmega8e5", "atxmega16a4",
"atxmega16a4u", "atxmega16c4", "atxmega16d4", "atxmega16e5",
"atxmega32a4", "atxmega32a4u", "atxmega32c3", "atxmega32c4",
"atxmega32d3", "atxmega32d4", "atxmega32e5".
"avrxmega3"
"XMEGA" devices with up to 64 KiB of combined program memory
and RAM, and with program memory visible in the RAM address
space. mcu = "attiny202", "attiny204", "attiny212",
"attiny214", "attiny402", "attiny404", "attiny406",
"attiny412", "attiny414", "attiny416", "attiny417",
"attiny804", "attiny806", "attiny807", "attiny814",
"attiny816", "attiny817", "attiny1604", "attiny1606",
"attiny1607", "attiny1614", "attiny1616", "attiny1617",
"attiny3214", "attiny3216", "attiny3217", "atmega808",
"atmega809", "atmega1608", "atmega1609", "atmega3208",
"atmega3209", "atmega4808", "atmega4809".
"avrxmega4"
"XMEGA" devices with more than 64 KiB and up to 128 KiB of
program memory. mcu = "atxmega64a3", "atxmega64a3u",
"atxmega64a4u", "atxmega64b1", "atxmega64b3", "atxmega64c3",
"atxmega64d3", "atxmega64d4".
"avrxmega5"
"XMEGA" devices with more than 64 KiB and up to 128 KiB of
program memory and more than 64 KiB of RAM. mcu =
"atxmega64a1", "atxmega64a1u".
"avrxmega6"
"XMEGA" devices with more than 128 KiB of program memory. mcu
= "atxmega128a3", "atxmega128a3u", "atxmega128b1",
"atxmega128b3", "atxmega128c3", "atxmega128d3", "atxmega128d4",
"atxmega192a3", "atxmega192a3u", "atxmega192c3",
"atxmega192d3", "atxmega256a3", "atxmega256a3b",
"atxmega256a3bu", "atxmega256a3u", "atxmega256c3",
"atxmega256d3", "atxmega384c3", "atxmega384d3".
"avrxmega7"
"XMEGA" devices with more than 128 KiB of program memory and
more than 64 KiB of RAM. mcu = "atxmega128a1", "atxmega128a1u",
"atxmega128a4u".
"avrtiny"
"TINY" Tiny core devices with 512 B up to 4 KiB of program
memory. mcu = "attiny4", "attiny5", "attiny9", "attiny10",
"attiny20", "attiny40".
"avr1"
This ISA is implemented by the minimal AVR core and supported
for assembler only. mcu = "attiny11", "attiny12", "attiny15",
"attiny28", "at90s1200".
-mabsdata
Assume that all data in static storage can be accessed by LDS / STS
instructions. This option has only an effect on reduced Tiny
devices like ATtiny40. See also the "absdata" AVR Variable
Attributes,variable attribute.
-maccumulate-args
Accumulate outgoing function arguments and acquire/release the
needed stack space for outgoing function arguments once in function
prologue/epilogue. Without this option, outgoing arguments are
pushed before calling a function and popped afterwards.
Popping the arguments after the function call can be expensive on
AVR so that accumulating the stack space might lead to smaller
executables because arguments need not be removed from the stack
after such a function call.
This option can lead to reduced code size for functions that
perform several calls to functions that get their arguments on the
stack like calls to printf-like functions.
-mbranch-cost=cost
Set the branch costs for conditional branch instructions to cost.
Reasonable values for cost are small, non-negative integers. The
default branch cost is 0.
-mcall-prologues
Functions prologues/epilogues are expanded as calls to appropriate
subroutines. Code size is smaller.
-mdouble=bits
-mlong-double=bits
Set the size (in bits) of the "double" or "long double" type,
respectively. Possible values for bits are 32 and 64. Whether or
not a specific value for bits is allowed depends on the
"--with-double=" and "--with-long-double=" configure options
("https://gcc.gnu.org/install/configure.html#avr"), and the same
applies for the default values of the options.
-mgas-isr-prologues
Interrupt service routines (ISRs) may use the "__gcc_isr" pseudo
instruction supported by GNU Binutils. If this option is on, the
feature can still be disabled for individual ISRs by means of the
AVR Function Attributes,,"no_gccisr" function attribute. This
feature is activated per default if optimization is on (but not
with -Og, @pxref{Optimize Options}), and if GNU Binutils support
PR21683 ("https://sourceware.org/PR21683").
-mint8
Assume "int" to be 8-bit integer. This affects the sizes of all
types: a "char" is 1 byte, an "int" is 1 byte, a "long" is 2 bytes,
and "long long" is 4 bytes. Please note that this option does not
conform to the C standards, but it results in smaller code size.
-mmain-is-OS_task
Do not save registers in "main". The effect is the same like
attaching attribute AVR Function Attributes,,"OS_task" to "main".
It is activated per default if optimization is on.
-mn-flash=num
Assume that the flash memory has a size of num times 64 KiB.
-mno-interrupts
Generated code is not compatible with hardware interrupts. Code
size is smaller.
-mrelax
Try to replace "CALL" resp. "JMP" instruction by the shorter
"RCALL" resp. "RJMP" instruction if applicable. Setting -mrelax
just adds the --mlink-relax option to the assembler's command line
and the --relax option to the linker's command line.
Jump relaxing is performed by the linker because jump offsets are
not known before code is located. Therefore, the assembler code
generated by the compiler is the same, but the instructions in the
executable may differ from instructions in the assembler code.
Relaxing must be turned on if linker stubs are needed, see the
section on "EIND" and linker stubs below.
-mrmw
Assume that the device supports the Read-Modify-Write instructions
"XCH", "LAC", "LAS" and "LAT".
-mshort-calls
Assume that "RJMP" and "RCALL" can target the whole program memory.
This option is used internally for multilib selection. It is not
an optimization option, and you don't need to set it by hand.
-msp8
Treat the stack pointer register as an 8-bit register, i.e. assume
the high byte of the stack pointer is zero. In general, you don't
need to set this option by hand.
This option is used internally by the compiler to select and build
multilibs for architectures "avr2" and "avr25". These
architectures mix devices with and without "SPH". For any setting
other than -mmcu=avr2 or -mmcu=avr25 the compiler driver adds or
removes this option from the compiler proper's command line,
because the compiler then knows if the device or architecture has
an 8-bit stack pointer and thus no "SPH" register or not.
-mstrict-X
Use address register "X" in a way proposed by the hardware. This
means that "X" is only used in indirect, post-increment or pre-
decrement addressing.
Without this option, the "X" register may be used in the same way
as "Y" or "Z" which then is emulated by additional instructions.
For example, loading a value with "X+const" addressing with a small
non-negative "const < 64" to a register Rn is performed as
adiw r26, const ; X += const
ld <Rn>, X ; <Rn> = *X
sbiw r26, const ; X -= const
-mtiny-stack
Only change the lower 8 bits of the stack pointer.
-mfract-convert-truncate
Allow to use truncation instead of rounding towards zero for
fractional fixed-point types.
-nodevicelib
Don't link against AVR-LibC's device specific library "lib<mcu>.a".
-nodevicespecs
Don't add -specs=device-specs/specs-mcu to the compiler driver's
command line. The user takes responsibility for supplying the sub-
processes like compiler proper, assembler and linker with
appropriate command line options. This means that the user has to
supply her private device specs file by means of -specs=path-to-
specs-file. There is no more need for option -mmcu=mcu.
This option can also serve as a replacement for the older way of
specifying custom device-specs files that needed -B some-path to
point to a directory which contains a folder named "device-specs"
which contains a specs file named "specs-mcu", where mcu was
specified by -mmcu=mcu.
-Waddr-space-convert
Warn about conversions between address spaces in the case where the
resulting address space is not contained in the incoming address
space.
-Wmisspelled-isr
Warn if the ISR is misspelled, i.e. without __vector prefix.
Enabled by default.
"EIND" and Devices with More Than 128 Ki Bytes of Flash
Pointers in the implementation are 16 bits wide. The address of a
function or label is represented as word address so that indirect jumps
and calls can target any code address in the range of 64 Ki words.
In order to facilitate indirect jump on devices with more than 128 Ki
bytes of program memory space, there is a special function register
called "EIND" that serves as most significant part of the target
address when "EICALL" or "EIJMP" instructions are used.
Indirect jumps and calls on these devices are handled as follows by the
compiler and are subject to some limitations:
* The compiler never sets "EIND".
* The compiler uses "EIND" implicitly in "EICALL"/"EIJMP"
instructions or might read "EIND" directly in order to emulate an
indirect call/jump by means of a "RET" instruction.
* The compiler assumes that "EIND" never changes during the startup
code or during the application. In particular, "EIND" is not
saved/restored in function or interrupt service routine
prologue/epilogue.
* For indirect calls to functions and computed goto, the linker
generates stubs. Stubs are jump pads sometimes also called
trampolines. Thus, the indirect call/jump jumps to such a stub.
The stub contains a direct jump to the desired address.
* Linker relaxation must be turned on so that the linker generates
the stubs correctly in all situations. See the compiler option
-mrelax and the linker option --relax. There are corner cases
where the linker is supposed to generate stubs but aborts without
relaxation and without a helpful error message.
* The default linker script is arranged for code with "EIND = 0". If
code is supposed to work for a setup with "EIND != 0", a custom
linker script has to be used in order to place the sections whose
name start with ".trampolines" into the segment where "EIND" points
to.
* The startup code from libgcc never sets "EIND". Notice that
startup code is a blend of code from libgcc and AVR-LibC. For the
impact of AVR-LibC on "EIND", see the AVR-LibC user manual
("http://nongnu.org/avr-libc/user-manual/").
* It is legitimate for user-specific startup code to set up "EIND"
early, for example by means of initialization code located in
section ".init3". Such code runs prior to general startup code that
initializes RAM and calls constructors, but after the bit of
startup code from AVR-LibC that sets "EIND" to the segment where
the vector table is located.
#include <avr/io.h>
static void
__attribute__((section(".init3"),naked,used,no_instrument_function))
init3_set_eind (void)
{
__asm volatile ("ldi r24,pm_hh8(__trampolines_start)\n\t"
"out %i0,r24" :: "n" (&EIND) : "r24","memory");
}
The "__trampolines_start" symbol is defined in the linker script.
* Stubs are generated automatically by the linker if the following
two conditions are met:
-<The address of a label is taken by means of the "gs" modifier>
(short for generate stubs) like so:
LDI r24, lo8(gs(<func>))
LDI r25, hi8(gs(<func>))
-<The final location of that label is in a code segment>
outside the segment where the stubs are located.
* The compiler emits such "gs" modifiers for code labels in the
following situations:
-<Taking address of a function or code label.>
-<Computed goto.>
-<If prologue-save function is used, see -mcall-prologues>
command-line option.
-<Switch/case dispatch tables. If you do not want such dispatch>
tables you can specify the -fno-jump-tables command-line
option.
-<C and C++ constructors/destructors called during
startup/shutdown.>
-<If the tools hit a "gs()" modifier explained above.>
* Jumping to non-symbolic addresses like so is not supported:
int main (void)
{
/* Call function at word address 0x2 */
return ((int(*)(void)) 0x2)();
}
Instead, a stub has to be set up, i.e. the function has to be
called through a symbol ("func_4" in the example):
int main (void)
{
extern int func_4 (void);
/* Call function at byte address 0x4 */
return func_4();
}
and the application be linked with -Wl,--defsym,func_4=0x4.
Alternatively, "func_4" can be defined in the linker script.
Handling of the "RAMPD", "RAMPX", "RAMPY" and "RAMPZ" Special Function
Registers
Some AVR devices support memories larger than the 64 KiB range that can
be accessed with 16-bit pointers. To access memory locations outside
this 64 KiB range, the content of a "RAMP" register is used as high
part of the address: The "X", "Y", "Z" address register is concatenated
with the "RAMPX", "RAMPY", "RAMPZ" special function register,
respectively, to get a wide address. Similarly, "RAMPD" is used
together with direct addressing.
* The startup code initializes the "RAMP" special function registers
with zero.
* If a AVR Named Address Spaces,named address space other than
generic or "__flash" is used, then "RAMPZ" is set as needed before
the operation.
* If the device supports RAM larger than 64 KiB and the compiler
needs to change "RAMPZ" to accomplish an operation, "RAMPZ" is
reset to zero after the operation.
* If the device comes with a specific "RAMP" register, the ISR
prologue/epilogue saves/restores that SFR and initializes it with
zero in case the ISR code might (implicitly) use it.
* RAM larger than 64 KiB is not supported by GCC for AVR targets. If
you use inline assembler to read from locations outside the 16-bit
address range and change one of the "RAMP" registers, you must
reset it to zero after the access.
AVR Built-in Macros
GCC defines several built-in macros so that the user code can test for
the presence or absence of features. Almost any of the following
built-in macros are deduced from device capabilities and thus triggered
by the -mmcu= command-line option.
For even more AVR-specific built-in macros see AVR Named Address Spaces
and AVR Built-in Functions.
"__AVR_ARCH__"
Build-in macro that resolves to a decimal number that identifies
the architecture and depends on the -mmcu=mcu option. Possible
values are:
2, 25, 3, 31, 35, 4, 5, 51, 6
for mcu="avr2", "avr25", "avr3", "avr31", "avr35", "avr4", "avr5",
"avr51", "avr6",
respectively and
100, 102, 103, 104, 105, 106, 107
for mcu="avrtiny", "avrxmega2", "avrxmega3", "avrxmega4",
"avrxmega5", "avrxmega6", "avrxmega7", respectively. If mcu
specifies a device, this built-in macro is set accordingly. For
example, with -mmcu=atmega8 the macro is defined to 4.
"__AVR_Device__"
Setting -mmcu=device defines this built-in macro which reflects the
device's name. For example, -mmcu=atmega8 defines the built-in
macro "__AVR_ATmega8__", -mmcu=attiny261a defines
"__AVR_ATtiny261A__", etc.
The built-in macros' names follow the scheme "__AVR_Device__" where
Device is the device name as from the AVR user manual. The
difference between Device in the built-in macro and device in
-mmcu=device is that the latter is always lowercase.
If device is not a device but only a core architecture like avr51,
this macro is not defined.
"__AVR_DEVICE_NAME__"
Setting -mmcu=device defines this built-in macro to the device's
name. For example, with -mmcu=atmega8 the macro is defined to
"atmega8".
If device is not a device but only a core architecture like avr51,
this macro is not defined.
"__AVR_XMEGA__"
The device / architecture belongs to the XMEGA family of devices.
"__AVR_HAVE_ELPM__"
The device has the "ELPM" instruction.
"__AVR_HAVE_ELPMX__"
The device has the "ELPM Rn,Z" and "ELPM Rn,Z+" instructions.
"__AVR_HAVE_MOVW__"
The device has the "MOVW" instruction to perform 16-bit register-
register moves.
"__AVR_HAVE_LPMX__"
The device has the "LPM Rn,Z" and "LPM Rn,Z+" instructions.
"__AVR_HAVE_MUL__"
The device has a hardware multiplier.
"__AVR_HAVE_JMP_CALL__"
The device has the "JMP" and "CALL" instructions. This is the case
for devices with more than 8 KiB of program memory.
"__AVR_HAVE_EIJMP_EICALL__"
"__AVR_3_BYTE_PC__"
The device has the "EIJMP" and "EICALL" instructions. This is the
case for devices with more than 128 KiB of program memory. This
also means that the program counter (PC) is 3 bytes wide.
"__AVR_2_BYTE_PC__"
The program counter (PC) is 2 bytes wide. This is the case for
devices with up to 128 KiB of program memory.
"__AVR_HAVE_8BIT_SP__"
"__AVR_HAVE_16BIT_SP__"
The stack pointer (SP) register is treated as 8-bit respectively
16-bit register by the compiler. The definition of these macros is
affected by -mtiny-stack.
"__AVR_HAVE_SPH__"
"__AVR_SP8__"
The device has the SPH (high part of stack pointer) special
function register or has an 8-bit stack pointer, respectively. The
definition of these macros is affected by -mmcu= and in the cases
of -mmcu=avr2 and -mmcu=avr25 also by -msp8.
"__AVR_HAVE_RAMPD__"
"__AVR_HAVE_RAMPX__"
"__AVR_HAVE_RAMPY__"
"__AVR_HAVE_RAMPZ__"
The device has the "RAMPD", "RAMPX", "RAMPY", "RAMPZ" special
function register, respectively.
"__NO_INTERRUPTS__"
This macro reflects the -mno-interrupts command-line option.
"__AVR_ERRATA_SKIP__"
"__AVR_ERRATA_SKIP_JMP_CALL__"
Some AVR devices (AT90S8515, ATmega103) must not skip 32-bit
instructions because of a hardware erratum. Skip instructions are
"SBRS", "SBRC", "SBIS", "SBIC" and "CPSE". The second macro is
only defined if "__AVR_HAVE_JMP_CALL__" is also set.
"__AVR_ISA_RMW__"
The device has Read-Modify-Write instructions (XCH, LAC, LAS and
LAT).
"__AVR_SFR_OFFSET__=offset"
Instructions that can address I/O special function registers
directly like "IN", "OUT", "SBI", etc. may use a different address
as if addressed by an instruction to access RAM like "LD" or "STS".
This offset depends on the device architecture and has to be
subtracted from the RAM address in order to get the respective I/O
address.
"__AVR_SHORT_CALLS__"
The -mshort-calls command line option is set.
"__AVR_PM_BASE_ADDRESS__=addr"
Some devices support reading from flash memory by means of "LD*"
instructions. The flash memory is seen in the data address space
at an offset of "__AVR_PM_BASE_ADDRESS__". If this macro is not
defined, this feature is not available. If defined, the address
space is linear and there is no need to put ".rodata" into RAM.
This is handled by the default linker description file, and is
currently available for "avrtiny" and "avrxmega3". Even more
convenient, there is no need to use address spaces like "__flash"
or features like attribute "progmem" and "pgm_read_*".
"__WITH_AVRLIBC__"
The compiler is configured to be used together with AVR-Libc. See
the --with-avrlibc configure option.
"__HAVE_DOUBLE_MULTILIB__"
Defined if -mdouble= acts as a multilib option.
"__HAVE_DOUBLE32__"
"__HAVE_DOUBLE64__"
Defined if the compiler supports 32-bit double resp. 64-bit double.
The actual layout is specified by option -md