lib/ffs.c

   1 /* ffs.c -- find the first set bit in a word.
   2    Copyright (C) 2011-2019 Free Software Foundation, Inc.
   3
   4    This program is free software: you can redistribute it and/or modify
   5    it under the terms of the GNU General Public License as published by
   6    the Free Software Foundation; either version 3 of the License, or
   7    (at your option) any later version.
   8
   9    This program 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 General Public License for more details.
  13
  14    You should have received a copy of the GNU 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 int
  27 ffs (int i)
  28 {
  29 #if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)
  30   return __builtin_ffs (i);
  31 #else
  32   /* <https://github.com/gibsjose/BitHacks>
  33      gives this deBruijn constant for a branch-less computation, although
  34      that table counted trailing zeros rather than bit position.  This
  35      requires 32-bit int, we fall back to a naive algorithm on the rare
  36      platforms where that assumption is not true.  */
  37   if (CHAR_BIT * sizeof i == 32)
  38     {
  39       static unsigned int table[] = {
  40         1, 2, 29, 3, 30, 15, 25, 4, 31, 23, 21, 16, 26, 18, 5, 9,
  41         32, 28, 14, 24, 22, 20, 17, 8, 27, 13, 19, 7, 12, 6, 11, 10
  42       };
  43       unsigned int u = i;
  44       unsigned int bit = u & -u;
  45       return table[(bit * 0x077cb531U) >> 27] - !i;
  46     }
  47   else
  48     {
  49       unsigned int j;
  50       for (j = 0; j < CHAR_BIT * sizeof i; j++)
  51         if (i & (1U << j))
  52           return j + 1;
  53       return 0;
  54     }
  55 #endif
  56 }