1 /*
2 Copyright (c) 1981, 1993
3 The Regents of the University of California. All rights reserved.
4
5 Redistribution and use in source and binary forms, with or without
6 modification, are permitted provided that the following conditions
7 are met:
8 1. Redistributions of source code must retain the above copyright
9 notice, this list of conditions and the following disclaimer.
10 2. Redistributions in binary form must reproduce the above copyright
11 notice, this list of conditions and the following disclaimer in the
12 documentation and/or other materials provided with the distribution.
13 3. Neither the name of the University nor the names of its contributors
14 may be used to endorse or promote products derived from this software
15 without specific prior written permission.
16
17 THIS SOFTWARE IS PROVIDED BY THE REGENTS AND CONTRIBUTORS ``AS IS'' AND
18 ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
19 IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
20 ARE DISCLAIMED. IN NO EVENT SHALL THE REGENTS OR CONTRIBUTORS BE LIABLE
21 FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
22 DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
23 OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
24 HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
25 LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
26 OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
27 SUCH DAMAGE.
28 */
29 /*
30 FUNCTION
31 <<ffs>>---find first bit set in a word
32
33 INDEX
34 ffs
35
36 SYNOPSIS
37 #include <strings.h>
38 int ffs(int <[word]>);
39
40 DESCRIPTION
41
42 <<ffs>> returns the first bit set in a word.
43
44 RETURNS
45 <<ffs>> returns 0 if <[c]> is 0, 1 if <[c]> is odd, 2 if <[c]> is a multiple of
46 2, etc.
47
48 PORTABILITY
49 <<ffs>> is not ANSI C.
50
51 No supporting OS subroutines are required. */
52
53 #include <strings.h>
54
55 /*
56 * GCC calls "ffs" for __builtin_ffs when the target doesn't have
57 * custom code and INT_TYPE_SIZE < BITS_PER_WORD so we can't use that
58 * builtin to implement ffs here. Instead, fall back to the ctz code
59 * instead.
60 */
61 #if defined(_HAVE_BUILTIN_FFS) && defined(__GNUC__) && __INT_WIDTH__ != __LONG_WIDTH__
62 #undef _HAVE_BUILTIN_FFS
63 #endif
64
65 int
ffs(int i)66 ffs(int i)
67 {
68 #ifdef _HAVE_BUILTIN_FFS
69 return (__builtin_ffs(i));
70 #elif defined(_HAVE_BUILTIN_CTZ)
71 if (i == 0)
72 return 0;
73 return __builtin_ctz((unsigned int)i) + 1;
74 #else
75 int r;
76
77 if (!i)
78 return 0;
79
80 r = 0;
81 for (;;)
82 {
83 if (((1 << r++) & i) != 0)
84 break;
85 }
86 return r;
87 #endif
88 }
89
