__BUILTIN_PREFETCH(3) BSD Library Functions Manual __BUILTIN_PREFETCH(3)NAME__builtin_prefetch — GNU extension to prefetch memory
SYNOPSIS
void
__builtin_prefetch(const void *addr, ...);
DESCRIPTION
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 nec‐
essary 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).
EXAMPLES
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;
...
SEE ALSOgcc(1), attribute(3)
Ulrich Drepper, What Every Programmer Should Know About Memory,
http://www.akkadia.org/drepper/cpumemory.pdf, November 21, 2007.
CAVEATS
This is a non-standard, compiler-specific extension.
BSD December 22, 2010 BSD
