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Revision 1.8, Tue May 10 12:09:00 2011 UTC (12 years, 10 months ago) by jruoho
Branch: MAIN
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Branch point for: netbsd-8
Changes since 1.7: +5 -3 lines

Xref div(3) and remainder(3).

.\"	$NetBSD: fast_divide32.3,v 1.8 2011/05/10 12:09:00 jruoho Exp $
.\"
.\" Copyright (c) 2010 The NetBSD Foundation, Inc.
.\" All rights reserved.
.\"
.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Joerg Sonnenberger.
.\"
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.\" 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
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.\"
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\"
.Dd May 10, 2011
.Dt FAST_DIVIDE32 3
.Os
.Sh NAME
.Nm fast_divide32 ,
.Nm fast_divide32_prepare ,
.Nm fast_remainder32
.Nd fast 32bit division and remainder
.Sh SYNOPSIS
.In sys/bitops.h
.Ft uint32_t
.Fn fast_divide32 "uint32_t v" "uint32_t div" "uint32_t m" "uint8_t s1" \
"uint8_t s2"
.Ft uint32_t
.Fn fast_remainder32 "uint32_t v" "uint32_t div" "uint32_t m" "uint8_t s1" \
"uint8_t s2"
.Ft void
.Fn fast_divide32_prepare "uint32_t div" "uint32_t *m" "uint8_t *s1" \
"uint8_t *s2"
.Sh DESCRIPTION
The
.Nm fast_divide32
and
.Nm fast_remainder32
functions compute the equivalent of
.Fa v / Fa div
and
.Fa v % Fa div
using optimised
.Tn CPU
instructions.
The constants
.Fa m ,
.Fa s1 ,
and
.Fa s2
must first be preset for a given value of
.Fa div
with the
.Nm fast_divide32_prepare
function.
.Sh RATIONALE
These functions are useful for inner loops and other performance-sensitive
tasks.
The functions expand to code that is typically slightly larger than
a plain division instruction, but requires less time to execute.
The code for constant
.Fa div
arguments should be equivalent to the assembly created by
.Tn GCC .
.Sh EXAMPLES
The following example computes
.Va q = a / b
and
.Va r = a % b :
.Bd -literal -offset indent
uint32_t a, b, q, r, m;
uint8_t s1, s2;

fast_divide32_prepare(b, \*[Am]m, \*[Am]s1, \*[Am]s2);

q = fast_divide32(a, b, m, s1, s2);
r = fast_remainder32(a, b, m, s1, s2);
.Ed
.Sh SEE ALSO
.Xr bitops 3 ,
.Xr div 3 ,
.Xr remainder 3
.Rs
.%A Torbj\(:orn Granlund
.%A Peter L. Montgomery
.%T Division by Invariant Integers Using Multiplication
.%J ACM SIGPLAN Notices
.%D June 1994
.%N Issue 6
.%V Volume 29
.%P 61-72
.%U http://gmplib.org/~tege/divcnst-pldi94.pdf
.Re
.Sh HISTORY
The
.Nm
function appeared in
.Nx 6.0 .