1 /* ffs.c -- find the first set bit in a word.
2 Copyright (C) 2011-2020 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 <https://www.gnu.org/licenses/>. */
17 /* Written by Eric Blake. */
33 #if __GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)
34 return __builtin_ffs (i
);
35 #elif defined _MSC_VER
37 <https://docs.microsoft.com/en-us/cpp/intrinsics/bitscanforward-bitscanforward64> */
39 if (_BitScanForward (&bit
, i
))
44 /* <https://github.com/gibsjose/BitHacks>
45 gives this deBruijn constant for a branch-less computation, although
46 that table counted trailing zeros rather than bit position. This
47 requires 32-bit int, we fall back to a naive algorithm on the rare
48 platforms where that assumption is not true. */
49 if (CHAR_BIT
* sizeof i
== 32)
51 static unsigned int table
[] = {
52 1, 2, 29, 3, 30, 15, 25, 4, 31, 23, 21, 16, 26, 18, 5, 9,
53 32, 28, 14, 24, 22, 20, 17, 8, 27, 13, 19, 7, 12, 6, 11, 10
56 unsigned int bit
= u
& -u
;
57 return table
[(bit
* 0x077cb531U
) >> 27] - !i
;
62 for (j
= 0; j
< CHAR_BIT
* sizeof i
; j
++)