__builtin_prefetch(3) - NetBSD Manual Pages

__BUILTIN_PREFETCH(3)   NetBSD 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 ALSO

     gcc(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.

NetBSD 6.0                     December 22, 2010                    NetBSD 6.0