1 /* SPDX-License-Identifier: BSD-3-Clause
2 * Copyright(c) 2017 Cavium, Inc
3 * Copyright(c) Hannes Frederic Sowa
4 * All rights reserved.
5 */
6
7 #include <stdio.h>
8 #include <stdint.h>
9
10 #include <rte_common.h>
11
12 #include "rte_reciprocal.h"
13
rte_reciprocal_value(uint32_t d)14 struct rte_reciprocal rte_reciprocal_value(uint32_t d)
15 {
16 struct rte_reciprocal R;
17 uint64_t m;
18 int l;
19
20 l = rte_fls_u32(d - 1);
21 m = ((1ULL << 32) * ((1ULL << l) - d));
22 m /= d;
23
24 ++m;
25 R.m = m;
26 R.sh1 = RTE_MIN(l, 1);
27 R.sh2 = RTE_MAX(l - 1, 0);
28
29 return R;
30 }
31
32 /*
33 * Code taken from Hacker's Delight:
34 * http://www.hackersdelight.org/hdcodetxt/divlu.c.txt
35 * License permits inclusion here per:
36 * http://www.hackersdelight.org/permissions.htm
37 */
38 static uint64_t
divide_128_div_64_to_64(uint64_t u1,uint64_t u0,uint64_t v,uint64_t * r)39 divide_128_div_64_to_64(uint64_t u1, uint64_t u0, uint64_t v, uint64_t *r)
40 {
41 const uint64_t b = (1ULL << 32); /* Number base (16 bits). */
42 uint64_t un1, un0, /* Norm. dividend LSD's. */
43 vn1, vn0, /* Norm. divisor digits. */
44 q1, q0, /* Quotient digits. */
45 un64, un21, un10, /* Dividend digit pairs. */
46 rhat; /* A remainder. */
47 int s; /* Shift amount for norm. */
48
49 /* If overflow, set rem. to an impossible value. */
50 if (u1 >= v) {
51 if (r != NULL)
52 *r = (uint64_t) -1;
53 return (uint64_t) -1;
54 }
55
56 /* Count leading zeros. */
57 s = __builtin_clzll(v);
58 if (s > 0) {
59 v = v << s;
60 un64 = (u1 << s) | ((u0 >> (64 - s)) & (-s >> 31));
61 un10 = u0 << s;
62 } else {
63
64 un64 = u1 | u0;
65 un10 = u0;
66 }
67
68 vn1 = v >> 32;
69 vn0 = v & 0xFFFFFFFF;
70
71 un1 = un10 >> 32;
72 un0 = un10 & 0xFFFFFFFF;
73
74 q1 = un64/vn1;
75 rhat = un64 - q1*vn1;
76 again1:
77 if (q1 >= b || q1*vn0 > b*rhat + un1) {
78 q1 = q1 - 1;
79 rhat = rhat + vn1;
80 if (rhat < b)
81 goto again1;
82 }
83
84 un21 = un64*b + un1 - q1*v;
85
86 q0 = un21/vn1;
87 rhat = un21 - q0*vn1;
88 again2:
89 if (q0 >= b || q0*vn0 > b*rhat + un0) {
90 q0 = q0 - 1;
91 rhat = rhat + vn1;
92 if (rhat < b)
93 goto again2;
94 }
95
96 if (r != NULL)
97 *r = (un21*b + un0 - q0*v) >> s;
98 return q1*b + q0;
99 }
100
101 struct rte_reciprocal_u64
rte_reciprocal_value_u64(uint64_t d)102 rte_reciprocal_value_u64(uint64_t d)
103 {
104 struct rte_reciprocal_u64 R;
105 uint64_t m;
106 uint64_t r;
107 int l;
108
109 l = 63 - __builtin_clzll(d);
110
111 m = divide_128_div_64_to_64((1ULL << l), 0, d, &r) << 1;
112 if (r << 1 < r || r << 1 >= d)
113 m++;
114 m = (1ULL << l) - d ? m + 1 : 1;
115 R.m = m;
116
117 R.sh1 = l > 1 ? 1 : l;
118 R.sh2 = (l > 0) ? l : 0;
119 R.sh2 -= R.sh2 && (m == 1) ? 1 : 0;
120
121 return R;
122 }
123