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) 1991-2016 Free Software Foundation, Inc.
   5    Contributed by Ulrich Drepper <drepper@cygnus.com>.
   6
   7    The GNU C Library is free software; you can redistribute it and/or
   8    modify it under the terms of the GNU Lesser General Public
   9    License as published by the Free Software Foundation; either
  10    version 2.1 of the License, or (at your option) any later version.
  11
  12    The GNU C Library is distributed in the hope that it will be useful,
  13    but WITHOUT ANY WARRANTY; without even the implied warranty of
  14    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
  15    Lesser General Public License for more details.
  16
  17    You should have received a copy of the GNU Lesser General Public
  18    License along with the GNU C Library; if not, see
  19    <http://www.gnu.org/licenses/>.  */
  20
  21 #include <string.h>
  22
  23 #undef  ffs
  24
  25 int
  26 __ffs (int x)
  27 {
  28   int cnt;
  29   int tmp;
  30
  31   asm ("bsfl %2,%0\n"           /* Count low bits in X and store in %1.  */
  32        "cmovel %1,%0\n"         /* If number was zero, use -1 as result.  */
  33        : "=&r" (cnt), "=r" (tmp) : "rm" (x), "1" (-1));
  34
  35   return cnt + 1;
  36 }
  37 weak_alias (__ffs, ffs)
  38 libc_hidden_def (__ffs)
  39 libc_hidden_builtin_def (ffs)