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123 lines
4.5 KiB
C
123 lines
4.5 KiB
C
/* count-leading-zeros.h -- counts the number of leading 0 bits in a word.
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Copyright (C) 2012-2021 Free Software Foundation, Inc.
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This program is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation; either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <https://www.gnu.org/licenses/>. */
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/* Written by Eric Blake. */
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#ifndef COUNT_LEADING_ZEROS_H
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#define COUNT_LEADING_ZEROS_H 1
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#include <limits.h>
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#include <stdlib.h>
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#ifndef _GL_INLINE_HEADER_BEGIN
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#error "Please include config.h first."
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#endif
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_GL_INLINE_HEADER_BEGIN
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#ifndef COUNT_LEADING_ZEROS_INLINE
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# define COUNT_LEADING_ZEROS_INLINE _GL_INLINE
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#endif
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#ifdef __cplusplus
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extern "C" {
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#endif
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/* Assuming the GCC builtin is BUILTIN and the MSC builtin is MSC_BUILTIN,
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expand to code that computes the number of leading zeros of the local
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variable 'x' of type TYPE (an unsigned integer type) and return it
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from the current function. */
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#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) \
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|| (__clang_major__ >= 4)
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# define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE) \
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return x ? BUILTIN (x) : CHAR_BIT * sizeof x;
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#elif _MSC_VER
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# pragma intrinsic _BitScanReverse
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# pragma intrinsic _BitScanReverse64
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# define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE) \
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do \
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{ \
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unsigned long result; \
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return MSC_BUILTIN (&result, x) ? result : CHAR_BIT * sizeof x; \
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} \
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while (0)
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#else
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# define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE) \
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do \
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{ \
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int count; \
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unsigned int leading_32; \
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if (! x) \
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return CHAR_BIT * sizeof x; \
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for (count = 0; \
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(leading_32 = ((x >> (sizeof (TYPE) * CHAR_BIT - 32)) \
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& 0xffffffffU), \
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count < CHAR_BIT * sizeof x - 32 && !leading_32); \
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count += 32) \
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x = x << 31 << 1; \
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return count + count_leading_zeros_32 (leading_32); \
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} \
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while (0)
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/* Compute and return the number of leading zeros in X,
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where 0 < X < 2**32. */
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COUNT_LEADING_ZEROS_INLINE int
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count_leading_zeros_32 (unsigned int x)
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{
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/* <https://github.com/gibsjose/BitHacks>
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<https://www.fit.vutbr.cz/~ibarina/pub/bithacks.pdf> */
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static const char de_Bruijn_lookup[32] = {
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31, 22, 30, 21, 18, 10, 29, 2, 20, 17, 15, 13, 9, 6, 28, 1,
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23, 19, 11, 3, 16, 14, 7, 24, 12, 4, 8, 25, 5, 26, 27, 0
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};
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x |= x >> 1;
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x |= x >> 2;
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x |= x >> 4;
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x |= x >> 8;
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x |= x >> 16;
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return de_Bruijn_lookup[((x * 0x07c4acddU) & 0xffffffffU) >> 27];
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}
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#endif
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/* Compute and return the number of leading zeros in X. */
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COUNT_LEADING_ZEROS_INLINE int
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count_leading_zeros (unsigned int x)
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{
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COUNT_LEADING_ZEROS (__builtin_clz, _BitScanReverse, unsigned int);
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}
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/* Compute and return the number of leading zeros in X. */
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COUNT_LEADING_ZEROS_INLINE int
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count_leading_zeros_l (unsigned long int x)
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{
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COUNT_LEADING_ZEROS (__builtin_clzl, _BitScanReverse, unsigned long int);
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}
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/* Compute and return the number of leading zeros in X. */
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COUNT_LEADING_ZEROS_INLINE int
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count_leading_zeros_ll (unsigned long long int x)
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{
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COUNT_LEADING_ZEROS (__builtin_clzll, _BitScanReverse64,
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unsigned long long int);
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}
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#ifdef __cplusplus
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}
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#endif
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_GL_INLINE_HEADER_END
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#endif /* COUNT_LEADING_ZEROS_H */
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