93 lines
2.8 KiB
C
Executable File
93 lines
2.8 KiB
C
Executable File
/* ffsl.h -- find the first set bit in a word.
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Copyright (C) 2011-2024 Free Software Foundation, Inc.
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This file is free software: you can redistribute it and/or modify
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it under the terms of the GNU Lesser General Public License as
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published by the Free Software Foundation; either version 2.1 of the
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License, or (at your option) any later version.
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This file 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 Lesser General Public License for more details.
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You should have received a copy of the GNU Lesser 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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/* This file is meant to be included by ffsl.c and ffsll.c, after
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they have defined FUNC and TYPE. */
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#include <config.h>
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/* Specification. */
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#include <string.h>
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#include <limits.h>
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#include <strings.h>
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#if defined _MSC_VER && !(__clang_major__ >= 4)
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# include <intrin.h>
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/* Copied from ffs.c. */
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static inline int
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ffs (int i)
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{
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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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}
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#endif
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#if !defined FUNC || !defined TYPE
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# error
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#endif
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int
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FUNC (TYPE i)
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{
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#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) \
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|| (__clang_major__ >= 4)
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return GCC_BUILTIN (i);
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#elif defined _MSC_VER && defined MSVC_BUILTIN
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/* _BitScanForward, _BitScanForward64
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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 (MSVC_BUILTIN (&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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unsigned TYPE j = i;
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/* Split j into chunks, and look at one chunk after the other. */
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enum { chunk_bits = CHAR_BIT * sizeof (unsigned int) };
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/* The number of chunks is ceil (sizeof (TYPE) / sizeof (unsigned int))
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= (sizeof (TYPE) - 1) / sizeof (unsigned int) + 1. */
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enum { chunk_count = (sizeof (TYPE) - 1) / sizeof (unsigned int) + 1 };
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if (chunk_count > 1)
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{
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size_t k;
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/* It is tempting to write if (!j) here, but if we do this,
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Solaris 10/x86 "cc -O" miscompiles the code. */
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if (!i)
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return 0;
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/* Unroll the first loop round. k = 0. */
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if ((unsigned int) j)
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return ffs ((unsigned int) j);
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/* Generic loop. */
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for (k = 1; k < chunk_count - 1; k++)
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if ((unsigned int) (j >> (k * chunk_bits)) != 0)
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return k * chunk_bits + ffs ((unsigned int) (j >> (k * chunk_bits)));
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}
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/* Last loop round. k = chunk_count - 1. */
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return (chunk_count - 1) * chunk_bits
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+ ffs ((unsigned int) (j >> ((chunk_count - 1) * chunk_bits)));
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#endif
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}
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