1 /* ffs.c -- find the first set bit in a word.
2 Copyright (C) 2011 Free Software Foundation, Inc.
4 This program is free software: you can redistribute it and/or modify
5 it under the terms of the GNU General Public License as published by
6 the Free Software Foundation; either version 3 of the License, or
7 (at your option) any later version.
9 This program is distributed in the hope that it will be useful,
10 but WITHOUT ANY WARRANTY; without even the implied warranty of
11 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
12 GNU General Public License for more details.
14 You should have received a copy of the GNU General Public License
15 along with this program. If not, see <http://www.gnu.org/licenses/>. */
17 /* Written by Eric Blake. */
24 #if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)
25 return __builtin_ffs (i);
27 /* http://graphics.stanford.edu/~seander/bithacks.html#ZerosOnRightMultLookup
28 gives this deBruijn constant for a branch-less computation, although
29 that table counted trailing zeros rather than bit position. */
30 static unsigned int table[] = {
31 1, 2, 29, 3, 30, 15, 25, 4, 31, 23, 21, 16, 26, 18, 5, 9,
32 32, 28, 14, 24, 22, 20, 17, 8, 27, 13, 19, 7, 12, 6, 11, 10
35 unsigned int bit = u & -u;
36 return table[(bit * 0x077cb531U) >> 27] - !i;