1.\" $NetBSD: __builtin_prefetch.3,v 1.3 2010/12/22 22:02:54 wiz Exp $ 2.\" 3.\" Copyright (c) 2010 Jukka Ruohonen <jruohonen@iki.fi> 4.\" All rights reserved. 5.\" 6.\" Redistribution and use in source and binary forms, with or without 7.\" modification, are permitted provided that the following conditions 8.\" are met: 9.\" 1. Redistributions of source code must retain the above copyright 10.\" notice, this list of conditions and the following disclaimer. 11.\" 2. Redistributions in binary form must reproduce the above copyright 12.\" notice, this list of conditions and the following disclaimer in the 13.\" documentation and/or other materials provided with the distribution. 14.\" 15.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS 16.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED 17.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR 18.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS 19.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 20.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 21.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 22.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 23.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 24.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 25.\" POSSIBILITY OF SUCH DAMAGE. 26.\" 27.Dd December 22, 2010 28.Dt __BUILTIN_PREFETCH 3 29.Os 30.Sh NAME 31.Nm __builtin_prefetch 32.Nd GNU extension to prefetch memory 33.Sh SYNOPSIS 34.Ft void 35.Fn __builtin_prefetch "const void *addr" "..." 36.Sh DESCRIPTION 37The 38.Fn __builtin_prefetch 39function prefetches memory from 40.Fa addr . 41The rationale is to minimize cache-miss latency by 42trying to move data into a cache before accessing the data. 43Possible use cases include frequently called sections of code 44in which it is known that the data in a given address is likely 45to be accessed soon. 46.Pp 47In addition to 48.Fa addr , 49there are two optional 50.Xr stdarg 3 51arguments, 52.Fa rw 53and 54.Fa locality . 55The value of the latter should be a compile-time 56constant integer between 0 and 3. 57The higher the value, the higher the temporal locality in the data. 58When 59.Fa locality 60is 0, it is assumed that there is little or no temporal locality in the data; 61after access, it is not necessary to leave the data in the cache. 62The default value is 3. 63The value of 64.Fa rw 65is either 0 or 1, corresponding with read and write prefetch, respectively. 66The default value of 67.Fa rw 68is 0. 69Also 70.Fa rw 71must be a compile-time constant integer. 72.Pp 73The 74.Fn __builtin_prefetch 75function translates into prefetch instructions 76only if the architecture has support for these. 77If there is no support, 78.Fa addr 79is evaluated only if it includes side effects, 80although no warnings are issued by 81.Xr gcc 1 . 82.Sh EXAMPLES 83The following optimization appears in the heavily used 84.Fn cpu_in_cksum 85function that calculates checksums for the 86.Xr inet 4 87headers: 88.Bd -literal -offset indent 89while (mlen >= 32) { 90 __builtin_prefetch(data + 32); 91 partial += *(uint16_t *)data; 92 partial += *(uint16_t *)(data + 2); 93 partial += *(uint16_t *)(data + 4); 94 95 \&... 96 97 partial += *(uint16_t *)(data + 28); 98 partial += *(uint16_t *)(data + 30); 99 100 data += 32; 101 mlen -= 32; 102 103 \&... 104.Ed 105.Sh SEE ALSO 106.Xr gcc 1 , 107.Xr attribute 3 108.Rs 109.%A Ulrich Drepper 110.%T What Every Programmer Should Know About Memory 111.%D November 21, 2007 112.%U http://www.akkadia.org/drepper/cpumemory.pdf 113.Re 114.Sh CAVEATS 115This is a non-standard, compiler-specific extension. 116