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