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TOMOYO Linux Cross Reference
Linux/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  */
 21 extern __attribute__((const, noreturn))
 22 int ____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
 31 static inline __attribute__((const))
 32 int __ilog2_u32(u32 n)
 33 {
 34         return fls(n) - 1;
 35 }
 36 #endif
 37 
 38 #ifndef CONFIG_ARCH_HAS_ILOG2_U64
 39 static inline __attribute__((const))
 40 int __ilog2_u64(u64 n)
 41 {
 42         return fls64(n) - 1;
 43 }
 44 #endif
 45 
 46 /*
 47  *  Determine whether some value is a power of two, where zero is
 48  * *not* considered a power of two.
 49  */
 50 
 51 static inline __attribute__((const))
 52 bool is_power_of_2(unsigned long n)
 53 {
 54         return (n != 0 && ((n & (n - 1)) == 0));
 55 }
 56 
 57 /*
 58  * round up to nearest power of two
 59  */
 60 static inline __attribute__((const))
 61 unsigned long __roundup_pow_of_two(unsigned long n)
 62 {
 63         return 1UL << fls_long(n - 1);
 64 }
 65 
 66 /*
 67  * round down to nearest power of two
 68  */
 69 static inline __attribute__((const))
 70 unsigned long __rounddown_pow_of_two(unsigned long n)
 71 {
 72         return 1UL << (fls_long(n) - 1);
 73 }
 74 
 75 /**
 76  * ilog2 - log of base 2 of 32-bit or a 64-bit unsigned value
 77  * @n - parameter
 78  *
 79  * constant-capable log of base 2 calculation
 80  * - this can be used to initialise global variables from constant data, hence
 81  *   the massive ternary operator construction
 82  *
 83  * selects the appropriately-sized optimised version depending on sizeof(n)
 84  */
 85 #define ilog2(n)                                \
 86 (                                               \
 87         __builtin_constant_p(n) ? (             \
 88                 (n) < 1 ? ____ilog2_NaN() :     \
 89                 (n) & (1ULL << 63) ? 63 :       \
 90                 (n) & (1ULL << 62) ? 62 :       \
 91                 (n) & (1ULL << 61) ? 61 :       \
 92                 (n) & (1ULL << 60) ? 60 :       \
 93                 (n) & (1ULL << 59) ? 59 :       \
 94                 (n) & (1ULL << 58) ? 58 :       \
 95                 (n) & (1ULL << 57) ? 57 :       \
 96                 (n) & (1ULL << 56) ? 56 :       \
 97                 (n) & (1ULL << 55) ? 55 :       \
 98                 (n) & (1ULL << 54) ? 54 :       \
 99                 (n) & (1ULL << 53) ? 53 :       \
100                 (n) & (1ULL << 52) ? 52 :       \
101                 (n) & (1ULL << 51) ? 51 :       \
102                 (n) & (1ULL << 50) ? 50 :       \
103                 (n) & (1ULL << 49) ? 49 :       \
104                 (n) & (1ULL << 48) ? 48 :       \
105                 (n) & (1ULL << 47) ? 47 :       \
106                 (n) & (1ULL << 46) ? 46 :       \
107                 (n) & (1ULL << 45) ? 45 :       \
108                 (n) & (1ULL << 44) ? 44 :       \
109                 (n) & (1ULL << 43) ? 43 :       \
110                 (n) & (1ULL << 42) ? 42 :       \
111                 (n) & (1ULL << 41) ? 41 :       \
112                 (n) & (1ULL << 40) ? 40 :       \
113                 (n) & (1ULL << 39) ? 39 :       \
114                 (n) & (1ULL << 38) ? 38 :       \
115                 (n) & (1ULL << 37) ? 37 :       \
116                 (n) & (1ULL << 36) ? 36 :       \
117                 (n) & (1ULL << 35) ? 35 :       \
118                 (n) & (1ULL << 34) ? 34 :       \
119                 (n) & (1ULL << 33) ? 33 :       \
120                 (n) & (1ULL << 32) ? 32 :       \
121                 (n) & (1ULL << 31) ? 31 :       \
122                 (n) & (1ULL << 30) ? 30 :       \
123                 (n) & (1ULL << 29) ? 29 :       \
124                 (n) & (1ULL << 28) ? 28 :       \
125                 (n) & (1ULL << 27) ? 27 :       \
126                 (n) & (1ULL << 26) ? 26 :       \
127                 (n) & (1ULL << 25) ? 25 :       \
128                 (n) & (1ULL << 24) ? 24 :       \
129                 (n) & (1ULL << 23) ? 23 :       \
130                 (n) & (1ULL << 22) ? 22 :       \
131                 (n) & (1ULL << 21) ? 21 :       \
132                 (n) & (1ULL << 20) ? 20 :       \
133                 (n) & (1ULL << 19) ? 19 :       \
134                 (n) & (1ULL << 18) ? 18 :       \
135                 (n) & (1ULL << 17) ? 17 :       \
136                 (n) & (1ULL << 16) ? 16 :       \
137                 (n) & (1ULL << 15) ? 15 :       \
138                 (n) & (1ULL << 14) ? 14 :       \
139                 (n) & (1ULL << 13) ? 13 :       \
140                 (n) & (1ULL << 12) ? 12 :       \
141                 (n) & (1ULL << 11) ? 11 :       \
142                 (n) & (1ULL << 10) ? 10 :       \
143                 (n) & (1ULL <<  9) ?  9 :       \
144                 (n) & (1ULL <<  8) ?  8 :       \
145                 (n) & (1ULL <<  7) ?  7 :       \
146                 (n) & (1ULL <<  6) ?  6 :       \
147                 (n) & (1ULL <<  5) ?  5 :       \
148                 (n) & (1ULL <<  4) ?  4 :       \
149                 (n) & (1ULL <<  3) ?  3 :       \
150                 (n) & (1ULL <<  2) ?  2 :       \
151                 (n) & (1ULL <<  1) ?  1 :       \
152                 (n) & (1ULL <<  0) ?  0 :       \
153                 ____ilog2_NaN()                 \
154                                    ) :          \
155         (sizeof(n) <= 4) ?                      \
156         __ilog2_u32(n) :                        \
157         __ilog2_u64(n)                          \
158  )
159 
160 /**
161  * roundup_pow_of_two - round the given value up to nearest power of two
162  * @n - parameter
163  *
164  * round the given value up to the nearest power of two
165  * - the result is undefined when n == 0
166  * - this can be used to initialise global variables from constant data
167  */
168 #define roundup_pow_of_two(n)                   \
169 (                                               \
170         __builtin_constant_p(n) ? (             \
171                 (n == 1) ? 1 :                  \
172                 (1UL << (ilog2((n) - 1) + 1))   \
173                                    ) :          \
174         __roundup_pow_of_two(n)                 \
175  )
176 
177 /**
178  * rounddown_pow_of_two - round the given value down to nearest power of two
179  * @n - parameter
180  *
181  * round the given value down to the nearest power of two
182  * - the result is undefined when n == 0
183  * - this can be used to initialise global variables from constant data
184  */
185 #define rounddown_pow_of_two(n)                 \
186 (                                               \
187         __builtin_constant_p(n) ? (             \
188                 (1UL << ilog2(n))) :            \
189         __rounddown_pow_of_two(n)               \
190  )
191 
192 /**
193  * order_base_2 - calculate the (rounded up) base 2 order of the argument
194  * @n: parameter
195  *
196  * The first few values calculated by this routine:
197  *  ob2(0) = 0
198  *  ob2(1) = 0
199  *  ob2(2) = 1
200  *  ob2(3) = 2
201  *  ob2(4) = 2
202  *  ob2(5) = 3
203  *  ... and so on.
204  */
205 
206 #define order_base_2(n) ilog2(roundup_pow_of_two(n))
207 
208 #endif /* _LINUX_LOG2_H */
209 

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