ceph/src/dpdk/lib/librte_sched/rte_reciprocal.h

   1 /*
   2  * Reciprocal divide
   3  *
   4  * Used with permission from original authors
   5  *  Hannes Frederic Sowa and Daniel Borkmann
   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 #ifndef _RTE_RECIPROCAL_H_
  23 #define _RTE_RECIPROCAL_H_
  24
  25 #include <stdint.h>
  26
  27 struct rte_reciprocal {
  28         uint32_t m;
  29         uint8_t sh1, sh2;
  30 };
  31
  32 static inline uint32_t rte_reciprocal_divide(uint32_t a, struct rte_reciprocal R)
  33 {
  34         uint32_t t = (uint32_t)(((uint64_t)a * R.m) >> 32);
  35
  36         return (t + ((a - t) >> R.sh1)) >> R.sh2;
  37 }
  38
  39 struct rte_reciprocal rte_reciprocal_value(uint32_t d);
  40
  41 #endif /* _RTE_RECIPROCAL_H_ */