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)