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 struct rte_reciprocal {
31 uint32_t m;
32 uint8_t sh1, sh2;
33 };
34
35 struct rte_reciprocal_u64 {
36 uint64_t m;
37 uint8_t sh1, sh2;
38 };
39
rte_reciprocal_divide(uint32_t a,struct rte_reciprocal R)40 static inline uint32_t rte_reciprocal_divide(uint32_t a, struct rte_reciprocal R)
41 {
42 uint32_t t = (uint32_t)(((uint64_t)a * R.m) >> 32);
43
44 return (t + ((a - t) >> R.sh1)) >> R.sh2;
45 }
46
47 static __rte_always_inline uint64_t
mullhi_u64(uint64_t x,uint64_t y)48 mullhi_u64(uint64_t x, uint64_t y)
49 {
50 #ifdef __SIZEOF_INT128__
51 __uint128_t xl = x;
52 __uint128_t rl = xl * y;
53
54 return (rl >> 64);
55 #else
56 uint64_t u0, u1, v0, v1, k, t;
57 uint64_t w1, w2;
58 uint64_t whi;
59
60 u1 = x >> 32; u0 = x & 0xFFFFFFFF;
61 v1 = y >> 32; v0 = y & 0xFFFFFFFF;
62
63 t = u0*v0;
64 k = t >> 32;
65
66 t = u1*v0 + k;
67 w1 = t & 0xFFFFFFFF;
68 w2 = t >> 32;
69
70 t = u0*v1 + w1;
71 k = t >> 32;
72
73 whi = u1*v1 + w2 + k;
74
75 return whi;
76 #endif
77 }
78
79 static __rte_always_inline uint64_t
rte_reciprocal_divide_u64(uint64_t a,const struct rte_reciprocal_u64 * R)80 rte_reciprocal_divide_u64(uint64_t a, const struct rte_reciprocal_u64 *R)
81 {
82 uint64_t t = mullhi_u64(a, R->m);
83
84 return (t + ((a - t) >> R->sh1)) >> R->sh2;
85 }
86
87 struct rte_reciprocal rte_reciprocal_value(uint32_t d);
88 struct rte_reciprocal_u64 rte_reciprocal_value_u64(uint64_t d);
89
90 #endif /* _RTE_RECIPROCAL_H_ */
91