include/linux/reciprocal_div.h

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