1 /* ffsl.h -- 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. */
19 /* This file is meant to be included by ffsl.c and ffsll.c, after
20 they have defined FUNC and TYPE. */
27 #if !defined FUNC || !defined TYPE
34 #if (__GNUC__ > 3 || (__GNUC__ == 3 && __GNUC_MINOR__ >= 4)) && defined GCC_BUILTIN
35 return GCC_BUILTIN (i);
38 /* Split j into chunks, and look at one chunk after the other. */
39 enum { chunk_bits = CHAR_BIT * sizeof (unsigned int) };
40 /* The number of chunks is ceil (sizeof (TYPE) / sizeof (unsigned int))
41 = (sizeof (TYPE) - 1) / sizeof (unsigned int) + 1. */
42 enum { chunk_count = (sizeof (TYPE) - 1) / sizeof (unsigned int) + 1 };
48 /* It is tempting to write if (!j) here, but if we do this,
49 Solaris 10/x86 "cc -O" miscompiles the code. */
52 /* Unroll the first loop round. k = 0. */
54 return ffs ((unsigned int) j);
56 for (k = 1; k < chunk_count - 1; k++)
57 if ((unsigned int) (j >> (k * chunk_bits)) != 0)
58 return k * chunk_bits + ffs ((unsigned int) (j >> (k * chunk_bits)));
60 /* Last loop round. k = chunk_count - 1. */
61 return (chunk_count - 1) * chunk_bits
62 + ffs ((unsigned int) (j >> ((chunk_count - 1) * chunk_bits)));