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/* count-leading-zeros.h -- counts the number of leading 0 bits in a word.
Copyright (C) 2012-2020 Free Software Foundation, Inc.
This program is free software: you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
the Free Software Foundation; either version 3 of the License, or
(at your option) any later version.
This program is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
GNU General Public License for more details.
You should have received a copy of the GNU General Public License
along with this program. If not, see <https://www.gnu.org/licenses/>. */
/* Written by Eric Blake. */
#ifndef COUNT_LEADING_ZEROS_H
#define COUNT_LEADING_ZEROS_H 1
#include <limits.h>
#include <stdlib.h>
#ifndef _GL_INLINE_HEADER_BEGIN
#error "Please include config.h first."
#endif
_GL_INLINE_HEADER_BEGIN
#ifndef COUNT_LEADING_ZEROS_INLINE
# define COUNT_LEADING_ZEROS_INLINE _GL_INLINE
#endif
/* Assuming the GCC builtin is BUILTIN and the MSC builtin is MSC_BUILTIN,
expand to code that computes the number of leading zeros of the local
variable 'x' of type TYPE (an unsigned integer type) and return it
from the current function. */
#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)
# define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE) \
return x ? BUILTIN (x) : CHAR_BIT * sizeof x;
#elif _MSC_VER
# pragma intrinsic _BitScanReverse
# pragma intrinsic _BitScanReverse64
# define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE) \
do \
{ \
unsigned long result; \
return MSC_BUILTIN (&result, x) ? result : CHAR_BIT * sizeof x; \
} \
while (0)
#else
# define COUNT_LEADING_ZEROS(BUILTIN, MSC_BUILTIN, TYPE) \
do \
{ \
int count; \
unsigned int leading_32; \
if (! x) \
return CHAR_BIT * sizeof x; \
for (count = 0; \
(leading_32 = ((x >> (sizeof (TYPE) * CHAR_BIT - 32)) \
& 0xffffffffU), \
count < CHAR_BIT * sizeof x - 32 && !leading_32); \
count += 32) \
x = x << 31 << 1; \
return count + count_leading_zeros_32 (leading_32); \
} \
while (0)
/* Compute and return the number of leading zeros in X,
where 0 < X < 2**32. */
COUNT_LEADING_ZEROS_INLINE int
count_leading_zeros_32 (unsigned int x)
{
/* <https://github.com/gibsjose/BitHacks>
<https://www.fit.vutbr.cz/~ibarina/pub/bithacks.pdf> */
static const char de_Bruijn_lookup[32] = {
31, 22, 30, 21, 18, 10, 29, 2, 20, 17, 15, 13, 9, 6, 28, 1,
23, 19, 11, 3, 16, 14, 7, 24, 12, 4, 8, 25, 5, 26, 27, 0
};
x |= x >> 1;
x |= x >> 2;
x |= x >> 4;
x |= x >> 8;
x |= x >> 16;
return de_Bruijn_lookup[((x * 0x07c4acddU) & 0xffffffffU) >> 27];
}
#endif
/* Compute and return the number of leading zeros in X. */
COUNT_LEADING_ZEROS_INLINE int
count_leading_zeros (unsigned int x)
{
COUNT_LEADING_ZEROS (__builtin_clz, _BitScanReverse, unsigned int);
}
/* Compute and return the number of leading zeros in X. */
COUNT_LEADING_ZEROS_INLINE int
count_leading_zeros_l (unsigned long int x)
{
COUNT_LEADING_ZEROS (__builtin_clzl, _BitScanReverse, unsigned long int);
}
/* Compute and return the number of leading zeros in X. */
COUNT_LEADING_ZEROS_INLINE int
count_leading_zeros_ll (unsigned long long int x)
{
COUNT_LEADING_ZEROS (__builtin_clzll, _BitScanReverse64,
unsigned long long int);
}
_GL_INLINE_HEADER_END
#endif /* COUNT_LEADING_ZEROS_H */
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