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/* ffs.c -- find the first set bit in a word.
Copyright (C) 2011-2021 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. */
#include <config.h>
/* Specification. */
#include <strings.h>
#include <limits.h>
#if defined _MSC_VER && !(__clang_major__ >= 4)
# include <intrin.h>
#endif
int
ffs (int i)
{
#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || (__clang_major__ >= 4)
return __builtin_ffs (i);
#elif defined _MSC_VER
/* _BitScanForward
<https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanforward-bitscanforward64> */
unsigned long bit;
if (_BitScanForward (&bit, i))
return bit + 1;
else
return 0;
#else
/* <https://github.com/gibsjose/BitHacks>
gives this deBruijn constant for a branch-less computation, although
that table counted trailing zeros rather than bit position. This
requires 32-bit int, we fall back to a naive algorithm on the rare
platforms where that assumption is not true. */
if (CHAR_BIT * sizeof i == 32)
{
static unsigned int table[] = {
1, 2, 29, 3, 30, 15, 25, 4, 31, 23, 21, 16, 26, 18, 5, 9,
32, 28, 14, 24, 22, 20, 17, 8, 27, 13, 19, 7, 12, 6, 11, 10
};
unsigned int u = i;
unsigned int bit = u & -u;
return table[(bit * 0x077cb531U) >> 27] - !i;
}
else
{
unsigned int j;
for (j = 0; j < CHAR_BIT * sizeof i; j++)
if (i & (1U << j))
return j + 1;
return 0;
}
#endif
}
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