DPDK  24.03.0
rte_reciprocal.h
1 /* SPDX-License-Identifier: BSD-3-Clause
2  * Copyright(c) 2017 Cavium, Inc
3  */
4 /*
5  * Reciprocal divide
6  *
7  * Used with permission from original authors
8  * Hannes Frederic Sowa and Daniel Borkmann
9  *
10  * This algorithm is based on the paper "Division by Invariant
11  * Integers Using Multiplication" by Torbjörn Granlund and Peter
12  * L. Montgomery.
13  *
14  * The assembler implementation from Agner Fog, which this code is
15  * based on, can be found here:
16  * http://www.agner.org/optimize/asmlib.zip
17  *
18  * This optimization for A/B is helpful if the divisor B is mostly
19  * runtime invariant. The reciprocal of B is calculated in the
20  * slow-path with reciprocal_value(). The fast-path can then just use
21  * a much faster multiplication operation with a variable dividend A
22  * to calculate the division A/B.
23  */
24 
25 #ifndef _RTE_RECIPROCAL_H_
26 #define _RTE_RECIPROCAL_H_
27 
28 #include <stdint.h>
29 
30 #include <rte_common.h>
31 
32 #ifdef __cplusplus
33 extern "C" {
34 #endif
35 
36 struct rte_reciprocal {
37  uint32_t m;
38  uint8_t sh1, sh2;
39 };
40 
41 struct rte_reciprocal_u64 {
42  uint64_t m;
43  uint8_t sh1, sh2;
44 };
45 
46 static inline uint32_t rte_reciprocal_divide(uint32_t a, struct rte_reciprocal R)
47 {
48  uint32_t t = (uint32_t)(((uint64_t)a * R.m) >> 32);
49 
50  return (t + ((a - t) >> R.sh1)) >> R.sh2;
51 }
52 
53 static __rte_always_inline uint64_t
54 mullhi_u64(uint64_t x, uint64_t y)
55 {
56 #ifdef __SIZEOF_INT128__
57  __uint128_t xl = x;
58  __uint128_t rl = xl * y;
59 
60  return (rl >> 64);
61 #else
62  uint64_t u0, u1, v0, v1, k, t;
63  uint64_t w1, w2;
64  uint64_t whi;
65 
66  u1 = x >> 32; u0 = x & 0xFFFFFFFF;
67  v1 = y >> 32; v0 = y & 0xFFFFFFFF;
68 
69  t = u0*v0;
70  k = t >> 32;
71 
72  t = u1*v0 + k;
73  w1 = t & 0xFFFFFFFF;
74  w2 = t >> 32;
75 
76  t = u0*v1 + w1;
77  k = t >> 32;
78 
79  whi = u1*v1 + w2 + k;
80 
81  return whi;
82 #endif
83 }
84 
85 static __rte_always_inline uint64_t
86 rte_reciprocal_divide_u64(uint64_t a, const struct rte_reciprocal_u64 *R)
87 {
88  uint64_t t = mullhi_u64(a, R->m);
89 
90  return (t + ((a - t) >> R->sh1)) >> R->sh2;
91 }
92 
93 struct rte_reciprocal rte_reciprocal_value(uint32_t d);
94 struct rte_reciprocal_u64 rte_reciprocal_value_u64(uint64_t d);
95 
96 #ifdef __cplusplus
97 }
98 #endif
99 
100 #endif /* _RTE_RECIPROCAL_H_ */
#define __rte_always_inline
Definition: rte_common.h:355