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       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                 :   10019770 :         u32 t = (u32)(((u64)a * R.m) >> 32);
      32                 :   10019770 :         return (t + ((a - t) >> R.sh1)) >> R.sh2;
      33                 :            : }
      34                 :            : 
      35                 :            : #endif /* _LINUX_RECIPROCAL_DIV_H */