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__BUILTIN_PREFETCH(3)Library Functions Manual__BUILTIN_PREFETCH(3)


__builtin_prefetch — GNU extension to prefetch memory


void __builtin_prefetch(const void *addr, ...);


The __builtin_prefetch() function prefetches memory from addr. The rationale is to minimize cache-miss latency by trying to move data into a cache before accessing the data. Possible use cases include frequently called sections of code in which it is known that the data in a given address is likely to be accessed soon.

In addition to addr, there are two optional stdarg(3) arguments, rw and locality. The value of the latter should be a compile-time constant integer between 0 and 3. The higher the value, the higher the temporal locality in the data. When locality is 0, it is assumed that there is little or no temporal locality in the data; after access, it is not necessary to leave the data in the cache. The default value is 3. The value of rw is either 0 or 1, corresponding with read and write prefetch, respectively. The default value of rw is 0. Also rw must be a compile-time constant integer.

The __builtin_prefetch() function translates into prefetch instructions only if the architecture has support for these. If there is no support, addr is evaluated only if it includes side effects, although no warnings are issued by gcc(1).


The following optimization appears in the heavily used cpu_in_cksum() function that calculates checksums for the inet(4) headers:

while (mlen >= 32) { __builtin_prefetch(data + 32); partial += *(uint16_t *)data; partial += *(uint16_t *)(data + 2); partial += *(uint16_t *)(data + 4);   ...   partial += *(uint16_t *)(data + 28); partial += *(uint16_t *)(data + 30);   data += 32; mlen -= 32;   ...


gcc(1), attribute(3)

Ulrich Drepper, What Every Programmer Should Know About Memory, November 21, 2007, http://www.akkadia.org/drepper/cpumemory.pdf.


This is a non-standard, compiler-specific extension.

NetBSDDecember 22, 2010NetBSD

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