1 /* count-leading-zeros.h -- counts the number of leading 0 bits in a word.
2 Copyright (C) 2012-2023 Free Software Foundation, Inc.
3
4 This file is free software: you can redistribute it and/or modify
5 it under the terms of the GNU Lesser General Public License as
6 published by the Free Software Foundation; either version 2.1 of the
7 License, or (at your option) any later version.
8
9 This file is distributed in the hope that it will be useful,
10 but WITHOUT ANY WARRANTY; without even the implied warranty of
11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12 GNU Lesser General Public License for more details.
13
14 You should have received a copy of the GNU Lesser General Public License
15 along with this program. If not, see <https://www.gnu.org/licenses/>. */
16
17 /* Written by Eric Blake. */
18
19 #ifndef COUNT_LEADING_ZEROS_H
20 #define COUNT_LEADING_ZEROS_H 1
21
22 /* This file uses _GL_INLINE_HEADER_BEGIN, _GL_INLINE. */
23 #if !_GL_CONFIG_H_INCLUDED
24 #error "Please include config.h first."
25 #endif
26
27 #include <limits.h>
28 #include <stdlib.h>
29
30 _GL_INLINE_HEADER_BEGIN
31 #ifndef COUNT_LEADING_ZEROS_INLINE
32 # define COUNT_LEADING_ZEROS_INLINE _GL_INLINE
33 #endif
34
35 #ifdef __cplusplus
36 extern "C" {
37 #endif
38
39 /* Assuming the GCC builtin is BUILTIN and the MSC builtin is MSC_BUILTIN,
40 expand to code that computes the number of leading zeros of the local
41 variable 'x' of type TYPE (an unsigned integer type) and return it
42 from the current function. */
43 #if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) \
44 || (__clang_major__ >= 4)
45 # define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE) \
46 return x ? BUILTIN (x) : CHAR_BIT * sizeof x;
47 #elif _MSC_VER
48 # pragma intrinsic (_BitScanReverse)
49 # if defined _M_X64
50 # pragma intrinsic (_BitScanReverse64)
51 # endif
52 # define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE) \
53 do \
54 { \
55 unsigned long result; \
56 if (MSC_BUILTIN (&result, x)) \
57 return CHAR_BIT * sizeof x - 1 - result; \
58 return CHAR_BIT * sizeof x; \
59 } \
60 while (0)
61 #else
62 # define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE) \
63 do \
64 { \
65 int count; \
66 unsigned int leading_32; \
67 if (! x) \
68 return CHAR_BIT * sizeof x; \
69 for (count = 0; \
70 (leading_32 = ((x >> (sizeof (TYPE) * CHAR_BIT - 32)) \
71 & 0xffffffffU), \
72 count < CHAR_BIT * sizeof x - 32 && !leading_32); \
73 count += 32) \
74 x = x << 31 << 1; \
75 return count + count_leading_zeros_32 (leading_32); \
76 } \
77 while (0)
78
79 /* Compute and return the number of leading zeros in X,
80 where 0 < X < 2**32. */
81 COUNT_LEADING_ZEROS_INLINE int
82 count_leading_zeros_32 (unsigned int x)
83 {
84 /* <https://github.com/gibsjose/BitHacks>
85 <https://www.fit.vutbr.cz/~ibarina/pub/bithacks.pdf> */
86 static const char de_Bruijn_lookup[32] = {
87 31, 22, 30, 21, 18, 10, 29, 2, 20, 17, 15, 13, 9, 6, 28, 1,
88 23, 19, 11, 3, 16, 14, 7, 24, 12, 4, 8, 25, 5, 26, 27, 0
89 };
90
91 x |= x >> 1;
92 x |= x >> 2;
93 x |= x >> 4;
94 x |= x >> 8;
95 x |= x >> 16;
96 return de_Bruijn_lookup[((x * 0x07c4acddU) & 0xffffffffU) >> 27];
97 }
98 #endif
99
100 /* Compute and return the number of leading zeros in X. */
101 COUNT_LEADING_ZEROS_INLINE int
102 count_leading_zeros (unsigned int x)
103 {
104 COUNT_LEADING_ZEROS (__builtin_clz, _BitScanReverse, unsigned int);
105 }
106
107 /* Compute and return the number of leading zeros in X. */
108 COUNT_LEADING_ZEROS_INLINE int
109 count_leading_zeros_l (unsigned long int x)
110 {
111 COUNT_LEADING_ZEROS (__builtin_clzl, _BitScanReverse, unsigned long int);
112 }
113
114 /* Compute and return the number of leading zeros in X. */
115 COUNT_LEADING_ZEROS_INLINE int
116 count_leading_zeros_ll (unsigned long long int x)
117 {
118 #if (defined _MSC_VER && !defined __clang__) && !defined _M_X64
119 /* 32-bit MSVC does not have _BitScanReverse64, only _BitScanReverse. */
120 unsigned long result;
121 if (_BitScanReverse (&result, (unsigned long) (x >> 32)))
122 return CHAR_BIT * sizeof x - 1 - 32 - result;
123 if (_BitScanReverse (&result, (unsigned long) x))
124 return CHAR_BIT * sizeof x - 1 - result;
125 return CHAR_BIT * sizeof x;
126 #else
127 COUNT_LEADING_ZEROS (__builtin_clzll, _BitScanReverse64,
128 unsigned long long int);
129 #endif
130 }
131
132 #ifdef __cplusplus
133 }
134 #endif
135
136 _GL_INLINE_HEADER_END
137
138 #endif /* COUNT_LEADING_ZEROS_H */