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TOMOYO Linux Cross Reference
Linux/tools/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 _TOOLS_LINUX_LOG2_H
 13 #define _TOOLS_LINUX_LOG2_H
 14 
 15 /*
 16  * deal with unrepresentable constant logarithms
 17  */
 18 extern __attribute__((const, noreturn))
 19 int ____ilog2_NaN(void);
 20 
 21 /*
 22  * non-constant log of base 2 calculators
 23  * - the arch may override these in asm/bitops.h if they can be implemented
 24  *   more efficiently than using fls() and fls64()
 25  * - the arch is not required to handle n==0 if implementing the fallback
 26  */
 27 static inline __attribute__((const))
 28 int __ilog2_u32(u32 n)
 29 {
 30         return fls(n) - 1;
 31 }
 32 
 33 static inline __attribute__((const))
 34 int __ilog2_u64(u64 n)
 35 {
 36         return fls64(n) - 1;
 37 }
 38 
 39 /*
 40  *  Determine whether some value is a power of two, where zero is
 41  * *not* considered a power of two.
 42  */
 43 
 44 static inline __attribute__((const))
 45 bool is_power_of_2(unsigned long n)
 46 {
 47         return (n != 0 && ((n & (n - 1)) == 0));
 48 }
 49 
 50 /*
 51  * round up to nearest power of two
 52  */
 53 static inline __attribute__((const))
 54 unsigned long __roundup_pow_of_two(unsigned long n)
 55 {
 56         return 1UL << fls_long(n - 1);
 57 }
 58 
 59 /*
 60  * round down to nearest power of two
 61  */
 62 static inline __attribute__((const))
 63 unsigned long __rounddown_pow_of_two(unsigned long n)
 64 {
 65         return 1UL << (fls_long(n) - 1);
 66 }
 67 
 68 /**
 69  * ilog2 - log of base 2 of 32-bit or a 64-bit unsigned value
 70  * @n - parameter
 71  *
 72  * constant-capable log of base 2 calculation
 73  * - this can be used to initialise global variables from constant data, hence
 74  *   the massive ternary operator construction
 75  *
 76  * selects the appropriately-sized optimised version depending on sizeof(n)
 77  */
 78 #define ilog2(n)                                \
 79 (                                               \
 80         __builtin_constant_p(n) ? (             \
 81                 (n) < 1 ? ____ilog2_NaN() :     \
 82                 (n) & (1ULL << 63) ? 63 :       \
 83                 (n) & (1ULL << 62) ? 62 :       \
 84                 (n) & (1ULL << 61) ? 61 :       \
 85                 (n) & (1ULL << 60) ? 60 :       \
 86                 (n) & (1ULL << 59) ? 59 :       \
 87                 (n) & (1ULL << 58) ? 58 :       \
 88                 (n) & (1ULL << 57) ? 57 :       \
 89                 (n) & (1ULL << 56) ? 56 :       \
 90                 (n) & (1ULL << 55) ? 55 :       \
 91                 (n) & (1ULL << 54) ? 54 :       \
 92                 (n) & (1ULL << 53) ? 53 :       \
 93                 (n) & (1ULL << 52) ? 52 :       \
 94                 (n) & (1ULL << 51) ? 51 :       \
 95                 (n) & (1ULL << 50) ? 50 :       \
 96                 (n) & (1ULL << 49) ? 49 :       \
 97                 (n) & (1ULL << 48) ? 48 :       \
 98                 (n) & (1ULL << 47) ? 47 :       \
 99                 (n) & (1ULL << 46) ? 46 :       \
100                 (n) & (1ULL << 45) ? 45 :       \
101                 (n) & (1ULL << 44) ? 44 :       \
102                 (n) & (1ULL << 43) ? 43 :       \
103                 (n) & (1ULL << 42) ? 42 :       \
104                 (n) & (1ULL << 41) ? 41 :       \
105                 (n) & (1ULL << 40) ? 40 :       \
106                 (n) & (1ULL << 39) ? 39 :       \
107                 (n) & (1ULL << 38) ? 38 :       \
108                 (n) & (1ULL << 37) ? 37 :       \
109                 (n) & (1ULL << 36) ? 36 :       \
110                 (n) & (1ULL << 35) ? 35 :       \
111                 (n) & (1ULL << 34) ? 34 :       \
112                 (n) & (1ULL << 33) ? 33 :       \
113                 (n) & (1ULL << 32) ? 32 :       \
114                 (n) & (1ULL << 31) ? 31 :       \
115                 (n) & (1ULL << 30) ? 30 :       \
116                 (n) & (1ULL << 29) ? 29 :       \
117                 (n) & (1ULL << 28) ? 28 :       \
118                 (n) & (1ULL << 27) ? 27 :       \
119                 (n) & (1ULL << 26) ? 26 :       \
120                 (n) & (1ULL << 25) ? 25 :       \
121                 (n) & (1ULL << 24) ? 24 :       \
122                 (n) & (1ULL << 23) ? 23 :       \
123                 (n) & (1ULL << 22) ? 22 :       \
124                 (n) & (1ULL << 21) ? 21 :       \
125                 (n) & (1ULL << 20) ? 20 :       \
126                 (n) & (1ULL << 19) ? 19 :       \
127                 (n) & (1ULL << 18) ? 18 :       \
128                 (n) & (1ULL << 17) ? 17 :       \
129                 (n) & (1ULL << 16) ? 16 :       \
130                 (n) & (1ULL << 15) ? 15 :       \
131                 (n) & (1ULL << 14) ? 14 :       \
132                 (n) & (1ULL << 13) ? 13 :       \
133                 (n) & (1ULL << 12) ? 12 :       \
134                 (n) & (1ULL << 11) ? 11 :       \
135                 (n) & (1ULL << 10) ? 10 :       \
136                 (n) & (1ULL <<  9) ?  9 :       \
137                 (n) & (1ULL <<  8) ?  8 :       \
138                 (n) & (1ULL <<  7) ?  7 :       \
139                 (n) & (1ULL <<  6) ?  6 :       \
140                 (n) & (1ULL <<  5) ?  5 :       \
141                 (n) & (1ULL <<  4) ?  4 :       \
142                 (n) & (1ULL <<  3) ?  3 :       \
143                 (n) & (1ULL <<  2) ?  2 :       \
144                 (n) & (1ULL <<  1) ?  1 :       \
145                 (n) & (1ULL <<  0) ?  0 :       \
146                 ____ilog2_NaN()                 \
147                                    ) :          \
148         (sizeof(n) <= 4) ?                      \
149         __ilog2_u32(n) :                        \
150         __ilog2_u64(n)                          \
151  )
152 
153 /**
154  * roundup_pow_of_two - round the given value up to nearest power of two
155  * @n - parameter
156  *
157  * round the given value up to the nearest power of two
158  * - the result is undefined when n == 0
159  * - this can be used to initialise global variables from constant data
160  */
161 #define roundup_pow_of_two(n)                   \
162 (                                               \
163         __builtin_constant_p(n) ? (             \
164                 (n == 1) ? 1 :                  \
165                 (1UL << (ilog2((n) - 1) + 1))   \
166                                    ) :          \
167         __roundup_pow_of_two(n)                 \
168  )
169 
170 /**
171  * rounddown_pow_of_two - round the given value down to nearest power of two
172  * @n - parameter
173  *
174  * round the given value down to the nearest power of two
175  * - the result is undefined when n == 0
176  * - this can be used to initialise global variables from constant data
177  */
178 #define rounddown_pow_of_two(n)                 \
179 (                                               \
180         __builtin_constant_p(n) ? (             \
181                 (1UL << ilog2(n))) :            \
182         __rounddown_pow_of_two(n)               \
183  )
184 
185 #endif /* _TOOLS_LINUX_LOG2_H */
186 

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