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 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 */
  25extern u32 reciprocal_value(u32 B);
  26
  27
  28static inline u32 reciprocal_divide(u32 A, u32 R)
  29{
  30        return (u32)(((u64)A * R) >> 32);
  31}
  32#endif
  33