string/ffs.c

   1 /* Copyright (C) 1991-2024 Free Software Foundation, Inc.
   2    This file is part of the GNU C Library.
   3
   4    The GNU C Library is free software; you can redistribute it and/or
   5    modify it under the terms of the GNU Lesser General Public
   6    License as published by the Free Software Foundation; either
   7    version 2.1 of the License, or (at your option) any later version.
   8
   9    The GNU C Library 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 GNU
  12    Lesser General Public License for more details.
  13
  14    You should have received a copy of the GNU Lesser General Public
  15    License along with the GNU C Library; if not, see
  16    <https://www.gnu.org/licenses/>.  */
  17
  18 #include <limits.h>
  19 #define ffsl __something_else
  20 #include <string.h>
  21 #undef  ffs
  22 #include <math-use-builtins.h>
  23
  24 /* Find the first bit set in I.  */
  25 int
  26 __ffs (int i)
  27 {
  28 #if USE_FFS_BUILTIN
  29   return __builtin_ffs (i);
  30 #else
  31   static const unsigned char table[] =
  32     {
  33       0,1,2,2,3,3,3,3,4,4,4,4,4,4,4,4,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,5,
  34       6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,6,
  35       7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
  36       7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,7,
  37       8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
  38       8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
  39       8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,
  40       8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8,8
  41     };
  42   unsigned int a;
  43   unsigned int x = i & -i;
  44
  45   a = x <= 0xffff ? (x <= 0xff ? 0 : 8) : (x <= 0xffffff ?  16 : 24);
  46
  47   return table[x >> a] + a;
  48 #endif
  49 }
  50 weak_alias (__ffs, ffs)
  51 libc_hidden_def (__ffs)
  52 libc_hidden_builtin_def (ffs)
  53
  54 #if ULONG_MAX == UINT_MAX
  55 #undef ffsl
  56 weak_alias (__ffs, ffsl)
  57 #endif