sysdeps/i386/ffs.c

   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) 1991-2021 Free Software Foundation, Inc.
   5    Contributed by Torbjorn Granlund (tege@sics.se).
   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    <https://www.gnu.org/licenses/>.  */
  20
  21 #define ffsl __something_else
  22 #include <string.h>
  23
  24 #undef  ffs
  25
  26 #ifdef  __GNUC__
  27
  28 int
  29 __ffs (int x)
  30 {
  31   int cnt;
  32   int tmp;
  33
  34   asm ("xorl %0,%0\n"           /* Set CNT to zero.  */
  35        "bsfl %2,%1\n"           /* Count low bits in X and store in %1.  */
  36        "jz 1f\n"                /* Jump if OK, i.e. X was non-zero.  */
  37        "leal 1(%1),%0\n"        /* Return bsfl-result plus one on %0.  */
  38        "1:" : "=&a" (cnt), "=r" (tmp) : "rm" (x));
  39
  40   return cnt;
  41 }
  42 weak_alias (__ffs, ffs)
  43 libc_hidden_def (__ffs)
  44 libc_hidden_builtin_def (ffs)
  45 #undef ffsl
  46 weak_alias (__ffs, ffsl)
  47
  48 #else
  49 #include <string/ffs.c>
  50 #endif