1  /* ffs -- find first set bit in a word, counted from least significant end.
       2     For Intel 80x86, x>=3.
       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  #define ffsl __something_else
      21  #include <string.h>
      22  
      23  #undef	ffs
      24  
      25  #ifdef	__GNUC__
      26  
      27  int
      28  __ffs (int x)
      29  {
      30    int cnt;
      31    int tmp;
      32  
      33    asm ("xorl %0,%0\n"		/* Set CNT to zero.  */
      34         "bsfl %2,%1\n"		/* Count low bits in X and store in %1.  */
      35         "jz 1f\n"		/* Jump if OK, i.e. X was non-zero.  */
      36         "leal 1(%1),%0\n"	/* Return bsfl-result plus one on %0.  */
      37         "1:" : "=&a" (cnt), "=r" (tmp) : "rm" (x));
      38  
      39    return cnt;
      40  }
      41  weak_alias (__ffs, ffs)
      42  libc_hidden_def (__ffs)
      43  libc_hidden_builtin_def (ffs)
      44  #undef ffsl
      45  weak_alias (__ffs, ffsl)
      46  
      47  #else
      48  #include <string/ffs.c>
      49  #endif