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 #include <rte_bitops.h>
12
13 #include "rte_reciprocal.h"
14
rte_reciprocal_value(uint32_t d)15 struct rte_reciprocal rte_reciprocal_value(uint32_t d)
16 {
17 struct rte_reciprocal R;
18 uint64_t m;
19 int l;
20
21 l = rte_fls_u32(d - 1);
22 m = ((1ULL << 32) * ((1ULL << l) - d));
23 m /= d;
24
25 ++m;
26 R.m = m;
27 R.sh1 = RTE_MIN(l, 1);
28 R.sh2 = RTE_MAX(l - 1, 0);
29
30 return R;
31 }
32
33 /*
34 * Code taken from Hacker's Delight:
35 * http://www.hackersdelight.org/hdcodetxt/divlu.c.txt
36 * License permits inclusion here per:
37 * http://www.hackersdelight.org/permissions.htm
38 */
39 static uint64_t
divide_128_div_64_to_64(uint64_t u1,uint64_t u0,uint64_t v,uint64_t * r)40 divide_128_div_64_to_64(uint64_t u1, uint64_t u0, uint64_t v, uint64_t *r)
41 {
42 const uint64_t b = (1ULL << 32); /* Number base (16 bits). */
43 uint64_t un1, un0, /* Norm. dividend LSD's. */
44 vn1, vn0, /* Norm. divisor digits. */
45 q1, q0, /* Quotient digits. */
46 un64, un21, un10, /* Dividend digit pairs. */
47 rhat; /* A remainder. */
48 int s; /* Shift amount for norm. */
49
50 /* If overflow, set rem. to an impossible value. */
51 if (u1 >= v) {
52 if (r != NULL)
53 *r = (uint64_t) -1;
54 return (uint64_t) -1;
55 }
56
57 /* Count leading zeros. */
58 s = rte_clz64(v);
59 if (s > 0) {
60 v = v << s;
61 un64 = (u1 << s) | ((u0 >> (64 - s)) & (-s >> 31));
62 un10 = u0 << s;
63 } else {
64
65 un64 = u1 | u0;
66 un10 = u0;
67 }
68
69 vn1 = v >> 32;
70 vn0 = v & 0xFFFFFFFF;
71
72 un1 = un10 >> 32;
73 un0 = un10 & 0xFFFFFFFF;
74
75 q1 = un64/vn1;
76 rhat = un64 - q1*vn1;
77 again1:
78 if (q1 >= b || q1*vn0 > b*rhat + un1) {
79 q1 = q1 - 1;
80 rhat = rhat + vn1;
81 if (rhat < b)
82 goto again1;
83 }
84
85 un21 = un64*b + un1 - q1*v;
86
87 q0 = un21/vn1;
88 rhat = un21 - q0*vn1;
89 again2:
90 if (q0 >= b || q0*vn0 > b*rhat + un0) {
91 q0 = q0 - 1;
92 rhat = rhat + vn1;
93 if (rhat < b)
94 goto again2;
95 }
96
97 if (r != NULL)
98 *r = (un21*b + un0 - q0*v) >> s;
99 return q1*b + q0;
100 }
101
102 struct rte_reciprocal_u64
rte_reciprocal_value_u64(uint64_t d)103 rte_reciprocal_value_u64(uint64_t d)
104 {
105 struct rte_reciprocal_u64 R;
106 uint64_t m;
107 uint64_t r;
108 int l;
109
110 l = 63 - rte_clz64(d);
111
112 m = divide_128_div_64_to_64((1ULL << l), 0, d, &r) << 1;
113 if (r << 1 < r || r << 1 >= d)
114 m++;
115 m = (1ULL << l) - d ? m + 1 : 1;
116 R.m = m;
117
118 R.sh1 = l > 1 ? 1 : l;
119 R.sh2 = (l > 0) ? l : 0;
120 R.sh2 -= R.sh2 && (m == 1) ? 1 : 0;
121
122 return R;
123 }
124