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NetBSD 6.1.5 - man page for __builtin_prefetch (netbsd section 3)

__BUILTIN_PREFETCH(3)		   BSD Library Functions Manual 	    __BUILTIN_PREFETCH(3)

     __builtin_prefetch -- GNU extension to prefetch memory

     __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 archi-
     tecture 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 calcu-
     lates 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,
     http://www.akkadia.org/drepper/cpumemory.pdf, November 21, 2007.

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

BSD					December 22, 2010				      BSD

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