1 /* ffsl.h -- find the first set bit in a word.
2 Copyright (C) 2011-2017 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. */
19 /* This file is meant to be included by ffsl.c and ffsll.c, after
20 they have defined FUNC and TYPE. */
30 #if !defined FUNC || !defined TYPE
37 #if (__GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)) && defined GCC_BUILTIN
38 return GCC_BUILTIN (i
);
41 /* Split j into chunks, and look at one chunk after the other. */
42 enum { chunk_bits
= CHAR_BIT
* sizeof (unsigned int) };
43 /* The number of chunks is ceil (sizeof (TYPE) / sizeof (unsigned int))
44 = (sizeof (TYPE) - 1) / sizeof (unsigned int) + 1. */
45 enum { chunk_count
= (sizeof (TYPE
) - 1) / sizeof (unsigned int) + 1 };
51 /* It is tempting to write if (!j) here, but if we do this,
52 Solaris 10/x86 "cc -O" miscompiles the code. */
55 /* Unroll the first loop round. k = 0. */
57 return ffs ((unsigned int) j
);
59 for (k
= 1; k
< chunk_count
- 1; k
++)
60 if ((unsigned int) (j
>> (k
* chunk_bits
)) != 0)
61 return k
* chunk_bits
+ ffs ((unsigned int) (j
>> (k
* chunk_bits
)));
63 /* Last loop round. k = chunk_count - 1. */
64 return (chunk_count
- 1) * chunk_bits
65 + ffs ((unsigned int) (j
>> ((chunk_count
- 1) * chunk_bits
)));