114 lines
3.2 KiB
Groff
114 lines
3.2 KiB
Groff
.\" $NetBSD: fast_divide32.3,v 1.9 2017/07/03 21:30:58 wiz Exp $
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.\"
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.\" Copyright (c) 2010 The NetBSD Foundation, Inc.
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.\" All rights reserved.
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.\"
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.\" This code is derived from software contributed to The NetBSD Foundation
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.\" by Joerg Sonnenberger.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE NETBSD FOUNDATION, INC. AND CONTRIBUTORS
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.\" ``AS IS'' AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED
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.\" TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
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.\" PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE FOUNDATION OR CONTRIBUTORS
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.\" BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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.\" CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
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.\" SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
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.\" INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
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.\" CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
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.\" ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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.\" POSSIBILITY OF SUCH DAMAGE.
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.\"
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.Dd May 10, 2011
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.Dt FAST_DIVIDE32 3
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.Os
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.Sh NAME
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.Nm fast_divide32 ,
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.Nm fast_divide32_prepare ,
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.Nm fast_remainder32
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.Nd fast 32bit division and remainder
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.Sh SYNOPSIS
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.In sys/bitops.h
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.Ft uint32_t
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.Fn fast_divide32 "uint32_t v" "uint32_t div" "uint32_t m" "uint8_t s1" \
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"uint8_t s2"
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.Ft uint32_t
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.Fn fast_remainder32 "uint32_t v" "uint32_t div" "uint32_t m" "uint8_t s1" \
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"uint8_t s2"
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.Ft void
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.Fn fast_divide32_prepare "uint32_t div" "uint32_t *m" "uint8_t *s1" \
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"uint8_t *s2"
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.Sh DESCRIPTION
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The
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.Nm fast_divide32
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and
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.Nm fast_remainder32
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functions compute the equivalent of
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.Fa v / Fa div
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and
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.Fa v % Fa div
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using optimised
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.Tn CPU
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instructions.
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The constants
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.Fa m ,
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.Fa s1 ,
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and
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.Fa s2
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must first be preset for a given value of
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.Fa div
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with the
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.Nm fast_divide32_prepare
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function.
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.Sh RATIONALE
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These functions are useful for inner loops and other performance-sensitive
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tasks.
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The functions expand to code that is typically slightly larger than
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a plain division instruction, but requires less time to execute.
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The code for constant
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.Fa div
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arguments should be equivalent to the assembly created by
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.Tn GCC .
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.Sh EXAMPLES
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The following example computes
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.Va q = a / b
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and
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.Va r = a % b :
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.Bd -literal -offset indent
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uint32_t a, b, q, r, m;
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uint8_t s1, s2;
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fast_divide32_prepare(b, &m, &s1, &s2);
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q = fast_divide32(a, b, m, s1, s2);
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r = fast_remainder32(a, b, m, s1, s2);
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.Ed
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.Sh SEE ALSO
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.Xr bitops 3 ,
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.Xr div 3 ,
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.Xr remainder 3
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.Rs
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.%A Torbj\(:orn Granlund
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.%A Peter L. Montgomery
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.%T Division by Invariant Integers Using Multiplication
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.%J ACM SIGPLAN Notices
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.%D June 1994
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.%N Issue 6
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.%V Volume 29
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.%P 61-72
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.%U http://gmplib.org/~tege/divcnst-pldi94.pdf
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.Re
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.Sh HISTORY
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The
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.Nm
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function appeared in
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.Nx 6.0 .
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