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) 1991,92,93,94,97,98,2001,2004 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, write to the Free
  19    Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA
  20    02111-1307 USA.  */
  21
  22 #include <string.h>
  23
  24 #undef  ffs
  25
  26 int
  27 __ffs (int x)
  28 {
  29   int cnt;
  30   int tmp;
  31
  32   asm ("bsfl %2,%0\n"           /* Count low bits in X and store in %1.  */
  33        "cmovel %1,%0\n"         /* If number was zero, use -1 as result.  */
  34        : "=&r" (cnt), "=r" (tmp) : "rm" (x), "1" (-1));
  35
  36   return cnt + 1;
  37 }
  38 weak_alias (__ffs, ffs)
  39 libc_hidden_builtin_def (ffs)