lib/ffs.c

   1 /* ffs.c -- find the first set bit in a word.
   2    Copyright (C) 2011-2024 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 }