110 lines
3.1 KiB
C
110 lines
3.1 KiB
C
/* integer_length_l - find most significant bit in an 'unsigned long'.
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Copyright (C) 2011-2021 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 Bruno Haible <bruno@clisp.org>, 2011. */
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#include <config.h>
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/* Specification. */
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#include "integer_length.h"
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#include <limits.h>
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#include <stddef.h>
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#undef MIN
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#define MIN(a,b) ((a) < (b) ? (a) : (b))
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#ifdef USE_LONG_LONG
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# define FUNC integer_length_ll
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# define TYPE unsigned long long
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# define GCC_BUILTIN __builtin_clzll
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# ifdef _WIN64
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# define MSVC_BUILTIN _BitScanReverse64
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# endif
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#else
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# define FUNC integer_length_l
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# define TYPE unsigned long
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# define GCC_BUILTIN __builtin_clzl
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# define MSVC_BUILTIN _BitScanReverse
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#endif
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#if defined _MSC_VER && !(__clang_major__ >= 4)
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# include <intrin.h>
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/* Copied from integer_length.c. */
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static inline int
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integer_length (unsigned int x)
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{
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/* _BitScanReverse
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<https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanreverse-bitscanreverse64> */
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unsigned long bit;
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if (_BitScanReverse (&bit, x))
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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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#define NBITS (sizeof (TYPE) * CHAR_BIT)
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/* Modify shift count to be in bounds, even in dead code, otherwise GCC
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emits warnings "right shift count >= width of type". */
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#define MSC(i) MIN (i, NBITS - 1)
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int
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FUNC (TYPE x)
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{
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#if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || (__clang_major__ >= 4)
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if (x == 0)
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return 0;
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else
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return NBITS - GCC_BUILTIN (x);
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#elif defined _MSC_VER && defined MSVC_BUILTIN
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/* _BitScanReverse, _BitScanReverse64
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<https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanreverse-bitscanreverse64> */
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unsigned long bit;
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if (MSVC_BUILTIN (&bit, x))
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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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/* Split x into chunks, and look at one chunk after the other. */
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if (sizeof (TYPE) > 2 * sizeof (unsigned int))
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{
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/* Generic loop. */
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size_t i;
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for (i = (sizeof (TYPE) - 1) / sizeof (unsigned int); i >= 2; i--)
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{
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unsigned int y = x >> (i * sizeof (unsigned int) * CHAR_BIT);
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if (y != 0)
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return i * sizeof (unsigned int) * CHAR_BIT + integer_length (y);
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}
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}
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/* Unrolled the last 2 loop rounds. */
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/* i = 1 */
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if (sizeof (TYPE) > 1 * sizeof (unsigned int))
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{
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unsigned int y = x >> MSC (1 * sizeof (unsigned int) * CHAR_BIT);
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if (y != 0)
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return 1 * sizeof (unsigned int) * CHAR_BIT + integer_length (y);
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}
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/* i = 0 */
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return integer_length ((unsigned int) x);
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#endif
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}
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