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 file describes reciprocical division.
   8  *
   9  * This optimizes the (A/B) problem, when A and B are two u32
  10  * and B is a known value (but not known at compile time)
  11  *
  12  * The math principle used is :
  13  *   Let RECIPROCAL_VALUE(B) be (((1LL << 32) + (B - 1))/ B)
  14  *   Then A / B = (u32)(((u64)(A) * (R)) >> 32)
  15  *
  16  * This replaces a divide by a multiply (and a shift), and
  17  * is generally less expensive in CPU cycles.
  18  */
  19
  20 /*
  21  * Computes the reciprocal value (R) for the value B of the divisor.
  22  * Should not be called before each reciprocal_divide(),
  23  * or else the performance is slower than a normal divide.
  24  */
  25 extern u32 reciprocal_value(u32 B);
  26
  27
  28 static inline u32 reciprocal_divide(u32 A, u32 R)
  29 {
  30         return (u32)(((u64)A * R) >> 32);
  31 }
  32 #endif