NetBSD/share/man/man3/fast_divide32.3

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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, &m, &s1, &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 .