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/* count-leading-zeros.h -- counts the number of leading 0 bits in a word.
Copyright (C) 2012 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 <http://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>
#include "verify.h"
_GL_INLINE_HEADER_BEGIN
#ifndef COUNT_LEADING_ZEROS_INLINE
# define COUNT_LEADING_ZEROS_INLINE _GL_INLINE
#endif
/* Expand the code which computes the number of leading zeros of the local
variable 'x' of type TYPE (an unsigned integer type) and returns it
from the current function. */
#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)
# define COUNT_LEADING_ZEROS(BUILTIN, TYPE) \
return x ? BUILTIN (x) : CHAR_BIT * sizeof x;
#else
# define COUNT_LEADING_ZEROS(BUILTIN, TYPE) \
/* This condition is written so as to avoid shifting by more than \
31 bits at once, and also avoids a random HP-UX cc bug. */ \
verify (((TYPE) -1 >> 31 >> 31 >> 2) == 0); /* TYPE has at most 64 bits */ \
int count = 0; \
if (1 < (TYPE) -1 >> 31) { /* TYPE has more than 32 bits? */ \
count = count_leading_zeros_32 (x >> 31 >> 1); \
if (count < 32) \
return count; \
} \
return count + count_leading_zeros_32 (x);
/* Compute and return the number of leading zeros in the least
significant 32 bits of X. */
COUNT_LEADING_ZEROS_INLINE int
count_leading_zeros_32 (unsigned int x)
{
/* http://graphics.stanford.edu/~seander/bithacks.html */
static const char deBruijnLookup[32] = {
0, 9, 1, 10, 13, 21, 2, 29, 11, 14, 16, 18, 22, 25, 3, 30,
8, 12, 20, 28, 15, 17, 24, 7, 19, 27, 23, 6, 26, 5, 4, 31
};
x &= 0xffffffffU;
if (!x)
return 32;
x |= x >> 1;
x |= x >> 2;
x |= x >> 4;
x |= x >> 8;
x |= x >> 16;
return 31 - deBruijnLookup[(x * 0x07c4acddU) >> 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, 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, unsigned long int);
}
#if HAVE_UNSIGNED_LONG_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, unsigned long long int);
}
#endif
_GL_INLINE_HEADER_END
#endif /* COUNT_LEADING_ZEROS_H */
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