LXR linux/include/linux/reciprocal

   1/* SPDX-License-Identifier: GPL-2.0 */
   2#ifndef _LINUX_RECIPROCAL_DIV_H
   3#define _LINUX_RECIPROCAL_DIV_H
   4
   5#include <linux/types.h>
   6
   7/*
   8 * This algorithm is based on the paper "Division by Invariant
   9 * Integers Using Multiplication" by Torbjörn Granlund and Peter
  10 * L. Montgomery.
  11 *
  12 * The assembler implementation from Agner Fog, which this code is
  13 * based on, can be found here:
  14 * http://www.agner.org/optimize/asmlib.zip
  15 *
  16 * This optimization for A/B is helpful if the divisor B is mostly
  17 * runtime invariant. The reciprocal of B is calculated in the
  18 * slow-path with reciprocal_value(). The fast-path can then just use
  19 * a much faster multiplication operation with a variable dividend A
  20 * to calculate the division A/B.
  21 */
  22
  23struct reciprocal_value {
  24        u32 m;
  25        u8 sh1, sh2;
  26};
  27
  28struct reciprocal_value reciprocal_value(u32 d);
  29
  30static inline u32 reciprocal_divide(u32 a, struct reciprocal_value R)
  31{
  32        u32 t = (u32)(((u64)a * R.m) >> 32);
  33        return (t + ((a - t) >> R.sh1)) >> R.sh2;
  34}
  35
  36#endif /* _LINUX_RECIPROCAL_DIV_H */
  37