LXR linux/include/linux/reciprocal

   1#ifndef _LINUX_RECIPROCAL_DIV_H
   2#define _LINUX_RECIPROCAL_DIV_H
   3
   4#include <linux/types.h>
   5
   6/*
   7 * This algorithm is based on the paper "Division by Invariant
   8 * Integers Using Multiplication" by Torbjörn Granlund and Peter
   9 * L. Montgomery.
  10 *
  11 * The assembler implementation from Agner Fog, which this code is
  12 * based on, can be found here:
  13 * http://www.agner.org/optimize/asmlib.zip
  14 *
  15 * This optimization for A/B is helpful if the divisor B is mostly
  16 * runtime invariant. The reciprocal of B is calculated in the
  17 * slow-path with reciprocal_value(). The fast-path can then just use
  18 * a much faster multiplication operation with a variable dividend A
  19 * to calculate the division A/B.
  20 */
  21
  22struct reciprocal_value {
  23        u32 m;
  24        u8 sh1, sh2;
  25};
  26
  27struct reciprocal_value reciprocal_value(u32 d);
  28
  29static inline u32 reciprocal_divide(u32 a, struct reciprocal_value R)
  30{
  31        u32 t = (u32)(((u64)a * R.m) >> 32);
  32        return (t + ((a - t) >> R.sh1)) >> R.sh2;
  33}
  34
  35#endif /* _LINUX_RECIPROCAL_DIV_H */
  36