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) 1991-2023 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