ffs.c

一个C源代码分析器

/* ffs -- find first set bit in a word, counted from least significant end.
   For Intel 80x86, x>=3.
   Copyright (C) 1991, 1992, 1993, 1994 Free Software Foundation, Inc.
   Contributed by Torbjorn Granlund ([e-mail]).

The GNU C Library is free software; you can redistribute it and/or
modify it under the terms of the GNU Library General Public License as
published by the Free Software Foundation; either version 2 of the
License, or (at your option) any later version.

The GNU C Library is distributed in the hope that it will be useful,
but WITHOUT ANY WARRANTY; without even the implied warranty of
MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
Library General Public License for more details.

You should have received a copy of the GNU Library General Public
License along with the GNU C Library; see the file COPYING.LIB.  If
not, write to the Free Software Foundation, Inc., 675 Mass Ave,
Cambridge, MA 02139, USA.  */

#include <ansidecl.h>
#include <string.h>

#undef	ffs

#ifdef	__GNUC__

int
DEFUN(ffs, (x), int x)
{
  int cnt;
  int tmp;

  asm ("xorl %0,%0\n"		/* Set CNT to zero.  */
       "bsfl %2,%1\n"		/* Count low bits in X and store in %1.  */
       "jz nonzero\n"		/* Jump if OK, i.e. X was non-zero.  */
       "leal 1(%1),%0\n"	/* Return bsfl-result plus one on %0.  */
       "nonzero:" : "=&a" (cnt), "=r" (tmp) : "rm" (x));

  return cnt;
}

#else
#include <sysdeps/generic/ffs.c>
#endif