sysdeps/x86_64/ffs.c

   1 /* ffs -- find first set bit in a word, counted from least significant end.
   2    For AMD x86-64.
   3    This file is part of the GNU C Library.
   4    Copyright (C) 1991-2023 Free Software Foundation, Inc.
   5
   6    The GNU C Library is free software; you can redistribute it and/or
   7    modify it under the terms of the GNU Lesser General Public
   8    License as published by the Free Software Foundation; either
   9    version 2.1 of the License, or (at your option) any later version.
  10
  11    The GNU C Library is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  14    Lesser General Public License for more details.
  15
  16    You should have received a copy of the GNU Lesser General Public
  17    License along with the GNU C Library; if not, see
  18    <https://www.gnu.org/licenses/>.  */
  19
  20 #include <string.h>
  21
  22 #undef  ffs
  23
  24 int
  25 __ffs (int x)
  26 {
  27   int cnt;
  28   int tmp;
  29
  30   asm ("bsfl %2,%0\n"           /* Count low bits in X and store in %1.  */
  31        "cmovel %1,%0\n"         /* If number was zero, use -1 as result.  */
  32        : "=&r" (cnt), "=r" (tmp) : "rm" (x), "1" (-1));
  33
  34   return cnt + 1;
  35 }
  36 weak_alias (__ffs, ffs)
  37 libc_hidden_def (__ffs)
  38 libc_hidden_builtin_def (ffs)