1  /* ffs.c -- find the first set bit in a word.
       2     Copyright (C) 2011-2021 Free Software Foundation, Inc.
       3  
       4     This file is free software: you can redistribute it and/or modify
       5     it under the terms of the GNU Lesser General Public License as
       6     published by the Free Software Foundation; either version 2.1 of the
       7     License, or (at your option) any later version.
       8  
       9     This file is distributed in the hope that it will be useful,
      10     but WITHOUT ANY WARRANTY; without even the implied warranty of
      11     MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
      12     GNU Lesser General Public License for more details.
      13  
      14     You should have received a copy of the GNU Lesser General Public License
      15     along with this program.  If not, see <https://www.gnu.org/licenses/>.  */
      16  
      17  /* Written by Eric Blake.  */
      18  
      19  #include <config.h>
      20  
      21  /* Specification.  */
      22  #include <strings.h>
      23  
      24  #include <limits.h>
      25  
      26  #if defined _MSC_VER && !(__clang_major__ >= 4)
      27  # include <intrin.h>
      28  #endif
      29  
      30  int
      31  ffs (int i)
      32  {
      33  #if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4) || (__clang_major__ >= 4)
      34    return __builtin_ffs (i);
      35  #elif defined _MSC_VER
      36    /* _BitScanForward
      37       <https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanforward-bitscanforward64> */
      38    unsigned long bit;
      39    if (_BitScanForward (&bit, i))
      40      return bit + 1;
      41    else
      42      return 0;
      43  #else
      44    /* <https://github.com/gibsjose/BitHacks>
      45       gives this deBruijn constant for a branch-less computation, although
      46       that table counted trailing zeros rather than bit position.  This
      47       requires 32-bit int, we fall back to a naive algorithm on the rare
      48       platforms where that assumption is not true.  */
      49    if (CHAR_BIT * sizeof i == 32)
      50      {
      51        static unsigned int table[] = {
      52          1, 2, 29, 3, 30, 15, 25, 4, 31, 23, 21, 16, 26, 18, 5, 9,
      53          32, 28, 14, 24, 22, 20, 17, 8, 27, 13, 19, 7, 12, 6, 11, 10
      54        };
      55        unsigned int u = i;
      56        unsigned int bit = u & -u;
      57        return table[(bit * 0x077cb531U) >> 27] - !i;
      58      }
      59    else
      60      {
      61        unsigned int j;
      62        for (j = 0; j < CHAR_BIT * sizeof i; j++)
      63          if (i & (1U << j))
      64            return j + 1;
      65        return 0;
      66      }
      67  #endif
      68  }