include/linux/reciprocal_div.h

   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
  23 struct reciprocal_value {
  24         u32 m;
  25         u8 sh1, sh2;
  26 };
  27
  28 struct reciprocal_value reciprocal_value(u32 d);
  29
  30 static 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 */