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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
https://gmplib.org/~tege/divcnst-pldi94.pdf
Re
Sh HISTORY
The
Nm
function appeared in
Nx 6.0 .
