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+.\" $NetBSD: __builtin_prefetch.3,v 1.4 2024/09/07 20:33:53 rillig Exp $
+.\"
+.\" Copyright (c) 2010 Jukka Ruohonen <jruohonen@iki.fi>
+.\" All rights reserved.
+.\"
+.\" Redistribution and use in source and binary forms, with or without
+.\" modification, are permitted provided that the following conditions
+.\" are met:
+.\" 1. Redistributions of source code must retain the above copyright
+.\"    notice, this list of conditions and the following disclaimer.
+.\" 2. Redistributions in binary form must reproduce the above copyright
+.\"    notice, this list of conditions and the following disclaimer in the
+.\"    documentation and/or other materials provided with the distribution.
+.\"
+.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
+.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
+.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+.\" PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
+.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
+.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
+.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
+.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
+.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
+.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+.\" POSSIBILITY OF SUCH DAMAGE.
+.\"
+.Dd December 22, 2010
+.Dt __BUILTIN_PREFETCH 3
+.Os
+.Sh NAME
+.Nm __builtin_prefetch
+.Nd GNU extension to prefetch memory
+.Sh SYNOPSIS
+.Ft void
+.Fn __builtin_prefetch "const void *addr" "..."
+.Sh DESCRIPTION
+The
+.Fn __builtin_prefetch
+function prefetches memory from
+.Fa 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.
+.Pp
+In addition to
+.Fa addr ,
+there are two optional
+.Xr stdarg 3
+arguments,
+.Fa rw
+and
+.Fa 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
+.Fa 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
+.Fa rw
+is either 0 or 1, corresponding with read and write prefetch, respectively.
+The default value of
+.Fa rw
+is 0.
+Also
+.Fa rw
+must be a compile-time constant integer.
+.Pp
+The
+.Fn __builtin_prefetch
+function translates into prefetch instructions
+only if the architecture has support for these.
+If there is no support,
+.Fa addr
+is evaluated only if it includes side effects,
+although no warnings are issued by
+.Xr gcc 1 .
+.Sh EXAMPLES
+The following optimization appears in the heavily used
+.Fn cpu_in_cksum
+function that calculates checksums for the
+.Xr inet 4
+headers:
+.Bd -literal -offset indent
+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;
+
+	\&...
+.Ed
+.Sh SEE ALSO
+.Xr gcc 1 ,
+.Xr attribute 3
+.Rs
+.%A Ulrich Drepper
+.%T What Every Programmer Should Know About Memory
+.%D November 21, 2007
+.%U https://www.akkadia.org/drepper/cpumemory.pdf
+.Re
+.Sh CAVEATS
+This is a non-standard, compiler-specific extension.