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authorDavid Howells <dhowells@redhat.com>2006-12-08 05:37:49 -0500
committerLinus Torvalds <torvalds@woody.osdl.org>2006-12-08 11:28:51 -0500
commitf0d1b0b30d250a07627ad8b9fbbb5c7cc08422e8 (patch)
tree0aa5379150574374351fb92af7881a48dbfcf2ce /include/linux/log2.h
parentb3d7ae5f47a58a9f7b152deeaf7daa1fc558a8f1 (diff)
[PATCH] LOG2: Implement a general integer log2 facility in the kernel
This facility provides three entry points: ilog2() Log base 2 of unsigned long ilog2_u32() Log base 2 of u32 ilog2_u64() Log base 2 of u64 These facilities can either be used inside functions on dynamic data: int do_something(long q) { ...; y = ilog2(x) ...; } Or can be used to statically initialise global variables with constant values: unsigned n = ilog2(27); When performing static initialisation, the compiler will report "error: initializer element is not constant" if asked to take a log of zero or of something not reducible to a constant. They treat negative numbers as unsigned. When not dealing with a constant, they fall back to using fls() which permits them to use arch-specific log calculation instructions - such as BSR on x86/x86_64 or SCAN on FRV - if available. [akpm@osdl.org: MMC fix] Signed-off-by: David Howells <dhowells@redhat.com> Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org> Cc: Paul Mackerras <paulus@samba.org> Cc: Herbert Xu <herbert@gondor.apana.org.au> Cc: David Howells <dhowells@redhat.com> Cc: Wojtek Kaniewski <wojtekka@toxygen.net> Signed-off-by: Andrew Morton <akpm@osdl.org> Signed-off-by: Linus Torvalds <torvalds@osdl.org>
Diffstat (limited to 'include/linux/log2.h')
-rw-r--r--include/linux/log2.h131
1 files changed, 131 insertions, 0 deletions
diff --git a/include/linux/log2.h b/include/linux/log2.h
new file mode 100644
index 000000000000..3979c60325ff
--- /dev/null
+++ b/include/linux/log2.h
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1/* Integer base 2 logarithm calculation
2 *
3 * Copyright (C) 2006 Red Hat, Inc. All Rights Reserved.
4 * Written by David Howells (dhowells@redhat.com)
5 *
6 * This program is free software; you can redistribute it and/or
7 * modify it under the terms of the GNU General Public License
8 * as published by the Free Software Foundation; either version
9 * 2 of the License, or (at your option) any later version.
10 */
11
12#ifndef _LINUX_LOG2_H
13#define _LINUX_LOG2_H
14
15#include <linux/types.h>
16#include <linux/bitops.h>
17
18/*
19 * deal with unrepresentable constant logarithms
20 */
21extern __attribute__((const, noreturn))
22int ____ilog2_NaN(void);
23
24/*
25 * non-constant log of base 2 calculators
26 * - the arch may override these in asm/bitops.h if they can be implemented
27 * more efficiently than using fls() and fls64()
28 * - the arch is not required to handle n==0 if implementing the fallback
29 */
30#ifndef CONFIG_ARCH_HAS_ILOG2_U32
31static inline __attribute__((const))
32int __ilog2_u32(u32 n)
33{
34 return fls(n) - 1;
35}
36#endif
37
38#ifndef CONFIG_ARCH_HAS_ILOG2_U64
39static inline __attribute__((const))
40int __ilog2_u64(u64 n)
41{
42 return fls64(n) - 1;
43}
44#endif
45
46/**
47 * ilog2 - log of base 2 of 32-bit or a 64-bit unsigned value
48 * @n - parameter
49 *
50 * constant-capable log of base 2 calculation
51 * - this can be used to initialise global variables from constant data, hence
52 * the massive ternary operator construction
53 *
54 * selects the appropriately-sized optimised version depending on sizeof(n)
55 */
56#define ilog2(n) \
57( \
58 __builtin_constant_p(n) ? ( \
59 (n) < 1 ? ____ilog2_NaN() : \
60 (n) & (1ULL << 63) ? 63 : \
61 (n) & (1ULL << 62) ? 62 : \
62 (n) & (1ULL << 61) ? 61 : \
63 (n) & (1ULL << 60) ? 60 : \
64 (n) & (1ULL << 59) ? 59 : \
65 (n) & (1ULL << 58) ? 58 : \
66 (n) & (1ULL << 57) ? 57 : \
67 (n) & (1ULL << 56) ? 56 : \
68 (n) & (1ULL << 55) ? 55 : \
69 (n) & (1ULL << 54) ? 54 : \
70 (n) & (1ULL << 53) ? 53 : \
71 (n) & (1ULL << 52) ? 52 : \
72 (n) & (1ULL << 51) ? 51 : \
73 (n) & (1ULL << 50) ? 50 : \
74 (n) & (1ULL << 49) ? 49 : \
75 (n) & (1ULL << 48) ? 48 : \
76 (n) & (1ULL << 47) ? 47 : \
77 (n) & (1ULL << 46) ? 46 : \
78 (n) & (1ULL << 45) ? 45 : \
79 (n) & (1ULL << 44) ? 44 : \
80 (n) & (1ULL << 43) ? 43 : \
81 (n) & (1ULL << 42) ? 42 : \
82 (n) & (1ULL << 41) ? 41 : \
83 (n) & (1ULL << 40) ? 40 : \
84 (n) & (1ULL << 39) ? 39 : \
85 (n) & (1ULL << 38) ? 38 : \
86 (n) & (1ULL << 37) ? 37 : \
87 (n) & (1ULL << 36) ? 36 : \
88 (n) & (1ULL << 35) ? 35 : \
89 (n) & (1ULL << 34) ? 34 : \
90 (n) & (1ULL << 33) ? 33 : \
91 (n) & (1ULL << 32) ? 32 : \
92 (n) & (1ULL << 31) ? 31 : \
93 (n) & (1ULL << 30) ? 30 : \
94 (n) & (1ULL << 29) ? 29 : \
95 (n) & (1ULL << 28) ? 28 : \
96 (n) & (1ULL << 27) ? 27 : \
97 (n) & (1ULL << 26) ? 26 : \
98 (n) & (1ULL << 25) ? 25 : \
99 (n) & (1ULL << 24) ? 24 : \
100 (n) & (1ULL << 23) ? 23 : \
101 (n) & (1ULL << 22) ? 22 : \
102 (n) & (1ULL << 21) ? 21 : \
103 (n) & (1ULL << 20) ? 20 : \
104 (n) & (1ULL << 19) ? 19 : \
105 (n) & (1ULL << 18) ? 18 : \
106 (n) & (1ULL << 17) ? 17 : \
107 (n) & (1ULL << 16) ? 16 : \
108 (n) & (1ULL << 15) ? 15 : \
109 (n) & (1ULL << 14) ? 14 : \
110 (n) & (1ULL << 13) ? 13 : \
111 (n) & (1ULL << 12) ? 12 : \
112 (n) & (1ULL << 11) ? 11 : \
113 (n) & (1ULL << 10) ? 10 : \
114 (n) & (1ULL << 9) ? 9 : \
115 (n) & (1ULL << 8) ? 8 : \
116 (n) & (1ULL << 7) ? 7 : \
117 (n) & (1ULL << 6) ? 6 : \
118 (n) & (1ULL << 5) ? 5 : \
119 (n) & (1ULL << 4) ? 4 : \
120 (n) & (1ULL << 3) ? 3 : \
121 (n) & (1ULL << 2) ? 2 : \
122 (n) & (1ULL << 1) ? 1 : \
123 (n) & (1ULL << 0) ? 0 : \
124 ____ilog2_NaN() \
125 ) : \
126 (sizeof(n) <= 4) ? \
127 __ilog2_u32(n) : \
128 __ilog2_u64(n) \
129 )
130
131#endif /* _LINUX_LOG2_H */