fast_divide32(3) - NetBSD Manual Pages

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FAST_DIVIDE32(3)        NetBSD Library Functions Manual       FAST_DIVIDE32(3)


NAME

     fast_divide32, fast_divide32_prepare, fast_remainder32 -- fast 32bit
     division and remainder


SYNOPSIS

     #include <sys/bitops.h>

     uint32_t
     fast_divide32(uint32_t v, uint32_t div, uint32_t m, uint8_t s1,
         uint8_t s2);

     uint32_t
     fast_remainder32(uint32_t v, uint32_t div, uint32_t m, uint8_t s1,
         uint8_t s2);

     void
     fast_divide32_prepare(uint32_t div, uint32_t *m, uint8_t *s1,
         uint8_t *s2);


DESCRIPTION

     The fast_divide32 and fast_remainder32 functions compute the equivalent
     of v / div and v % div using optimised CPU instructions.  The constants
     m, s1, and s2 must first be preset for a given value of div with the
     fast_divide32_prepare function.


RATIONALE

     These functions are useful for inner loops and other performance-sensi-
     tive tasks.  The functions expand to code that is typically slightly
     larger than a plain division instruction, but requires less time to exe-
     cute.  The code for constant div arguments should be equivalent to the
     assembly created by GCC.


EXAMPLES

     The following example computes q = a / b and r = a % b:

           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);


SEE ALSO

     bitops(3), div(3), remainder(3)

     Torbjörn Granlund and Peter L. Montgomery, "Division by Invariant
     Integers Using Multiplication", ACM SIGPLAN Notices, Issue 6, Volume 29,
     https://gmplib.org/~tege/divcnst-pldi94.pdf, 61-72, June 1994.


HISTORY

     The fast_divide32 function appeared in NetBSD 6.0.

NetBSD 10.99                     May 10, 2011                     NetBSD 10.99