Updated: 2021/Apr/14

OPENSSL_ia32cap(3)                  OpenSSL                 OPENSSL_ia32cap(3)

       OPENSSL_ia32cap - the x86[_64] processor capabilities vector

       libcrypto, -lcrypto

        env OPENSSL_ia32cap=... <application>

       OpenSSL supports a range of x86[_64] instruction set extensions. These
       extensions are denoted by individual bits in capability vector returned
       by processor in EDX:ECX register pair after executing CPUID instruction
       with EAX=1 input value (see Intel Application Note #241618). This
       vector is copied to memory upon toolkit initialization and used to
       choose between different code paths to provide optimal performance
       across wide range of processors. For the moment of this writing
       following bits are significant:

       bit #4 denoting presence of Time-Stamp Counter.
       bit #19 denoting availability of CLFLUSH instruction;
       bit #20, reserved by Intel, is used to choose among RC4 code paths;
       bit #23 denoting MMX support;
       bit #24, FXSR bit, denoting availability of XMM registers;
       bit #25 denoting SSE support;
       bit #26 denoting SSE2 support;
       bit #28 denoting Hyperthreading, which is used to distinguish cores
       with shared cache;
       bit #30, reserved by Intel, denotes specifically Intel CPUs;
       bit #33 denoting availability of PCLMULQDQ instruction;
       bit #41 denoting SSSE3, Supplemental SSE3, support;
       bit #43 denoting AMD XOP support (forced to zero on non-AMD CPUs);
       bit #54 denoting availability of MOVBE instruction;
       bit #57 denoting AES-NI instruction set extension;
       bit #58, XSAVE bit, lack of which in combination with MOVBE is used to
       identify Atom Silvermont core;
       bit #59, OSXSAVE bit, denoting availability of YMM registers;
       bit #60 denoting AVX extension;
       bit #62 denoting availability of RDRAND instruction;

       For example, in 32-bit application context clearing bit #26 at run-time
       disables high-performance SSE2 code present in the crypto library,
       while clearing bit #24 disables SSE2 code operating on 128-bit XMM
       register bank. You might have to do the latter if target OpenSSL
       application is executed on SSE2 capable CPU, but under control of OS
       that does not enable XMM registers. Historically address of the
       capability vector copy was exposed to application through
       OPENSSL_ia32cap_loc(), but not anymore. Now the only way to affect the
       capability detection is to set OPENSSL_ia32cap environment variable
       prior target application start. To give a specific example, on Intel P4
       processor 'env OPENSSL_ia32cap=0x16980010 apps/openssl', or better yet
       'env OPENSSL_ia32cap=~0x1000000 apps/openssl' would achieve the desired
       effect. Alternatively you can reconfigure the toolkit with no-sse2
       option and recompile.

       Less intuitive is clearing bit #28, or ~0x10000000 in the "environment
       variable" terms. The truth is that it's not copied from CPUID output
       verbatim, but is adjusted to reflect whether or not the data cache is
       actually shared between logical cores. This in turn affects the
       decision on whether or not expensive countermeasures against cache-
       timing attacks are applied, most notably in AES assembler module.

       The capability vector is further extended with EBX value returned by
       CPUID with EAX=7 and ECX=0 as input. Following bits are significant:

       bit #64+3 denoting availability of BMI1 instructions, e.g. ANDN;
       bit #64+5 denoting availability of AVX2 instructions;
       bit #64+8 denoting availability of BMI2 instructions, e.g. MULX and
       bit #64+16 denoting availability of AVX512F extension;
       bit #64+18 denoting availability of RDSEED instruction;
       bit #64+19 denoting availability of ADCX and ADOX instructions;
       bit #64+21 denoting availability of VPMADD52[LH]UQ instructions, aka
       AVX512IFMA extension;
       bit #64+29 denoting availability of SHA extension;
       bit #64+30 denoting availability of AVX512BW extension;
       bit #64+31 denoting availability of AVX512VL extension;
       bit #64+41 denoting availability of VAES extension;
       bit #64+42 denoting availability of VPCLMULQDQ extension;

       To control this extended capability word use ':' as delimiter when
       setting up OPENSSL_ia32cap environment variable. For example assigning
       ':~0x20' would disable AVX2 code paths, and ':0' - all post-AVX

       It should be noted that whether or not some of the most "fancy"
       extension code paths are actually assembled depends on current
       assembler version. Base minimum of AES-NI/PCLMULQDQ, SSSE3 and SHA
       extension code paths are always assembled. Apart from that, minimum
       assembler version requirements are summarized in below table:

          Extension   | GNU as | nasm   | llvm
          AVX         | 2.19   | 2.09   | 3.0
          AVX2        | 2.22   | 2.10   | 3.1
          ADCX/ADOX   | 2.23   | 2.10   | 3.3
          AVX512      | 2.25   | 2.11.8 | see NOTES
          AVX512IFMA  | 2.26   | 2.11.8 | see NOTES
          VAES        | 2.30   | 2.13.3 |

       Even though AVX512 support was implemented in llvm 3.6, compilation of
       assembly modules apparently requires explicit -march flag. But then
       compiler generates processor-specific code, which in turn contradicts
       the mere idea of run-time switch execution facilitated by the variable
       in question. Till the limitation is lifted, it's possible to work
       around the problem by making build procedure use following script:

          exec clang -no-integrated-as "$@"

       instead of real clang. In which case it doesn't matter which clang
       version is used, as it is GNU assembler version that will be checked.

       Not available.

       Copyright 2004-2020 The OpenSSL Project Authors. All Rights Reserved.

       Licensed under the OpenSSL license (the "License").  You may not use
       this file except in compliance with the License.  You can obtain a copy
       in the file LICENSE in the source distribution or at

1.1.1i                            2020-12-10                OPENSSL_ia32cap(3)