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Linux/include/linux/log2.h

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  1 /* SPDX-License-Identifier: GPL-2.0-or-later */
  2 /* Integer base 2 logarithm calculation
  3  *
  4  * Copyright (C) 2006 Red Hat, Inc. All Rights Reserved.
  5  * Written by David Howells (dhowells@redhat.com)
  6  */
  7 
  8 #ifndef _LINUX_LOG2_H
  9 #define _LINUX_LOG2_H
 10 
 11 #include <linux/types.h>
 12 #include <linux/bitops.h>
 13 
 14 /*
 15  * non-constant log of base 2 calculators
 16  * - the arch may override these in asm/bitops.h if they can be implemented
 17  *   more efficiently than using fls() and fls64()
 18  * - the arch is not required to handle n==0 if implementing the fallback
 19  */
 20 #ifndef CONFIG_ARCH_HAS_ILOG2_U32
 21 static inline __attribute__((const))
 22 int __ilog2_u32(u32 n)
 23 {
 24         return fls(n) - 1;
 25 }
 26 #endif
 27 
 28 #ifndef CONFIG_ARCH_HAS_ILOG2_U64
 29 static inline __attribute__((const))
 30 int __ilog2_u64(u64 n)
 31 {
 32         return fls64(n) - 1;
 33 }
 34 #endif
 35 
 36 /**
 37  * is_power_of_2() - check if a value is a power of two
 38  * @n: the value to check
 39  *
 40  * Determine whether some value is a power of two, where zero is
 41  * *not* considered a power of two.
 42  * Return: true if @n is a power of 2, otherwise false.
 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  * __roundup_pow_of_two() - round up to nearest power of two
 52  * @n: value to round up
 53  */
 54 static inline __attribute__((const))
 55 unsigned long __roundup_pow_of_two(unsigned long n)
 56 {
 57         return 1UL << fls_long(n - 1);
 58 }
 59 
 60 /**
 61  * __rounddown_pow_of_two() - round down to nearest power of two
 62  * @n: value to round down
 63  */
 64 static inline __attribute__((const))
 65 unsigned long __rounddown_pow_of_two(unsigned long n)
 66 {
 67         return 1UL << (fls_long(n) - 1);
 68 }
 69 
 70 /**
 71  * const_ilog2 - log base 2 of 32-bit or a 64-bit constant unsigned value
 72  * @n: parameter
 73  *
 74  * Use this where sparse expects a true constant expression, e.g. for array
 75  * indices.
 76  */
 77 #define const_ilog2(n)                          \
 78 (                                               \
 79         __builtin_constant_p(n) ? (             \
 80                 (n) < 2 ? 0 :                   \
 81                 (n) & (1ULL << 63) ? 63 :       \
 82                 (n) & (1ULL << 62) ? 62 :       \
 83                 (n) & (1ULL << 61) ? 61 :       \
 84                 (n) & (1ULL << 60) ? 60 :       \
 85                 (n) & (1ULL << 59) ? 59 :       \
 86                 (n) & (1ULL << 58) ? 58 :       \
 87                 (n) & (1ULL << 57) ? 57 :       \
 88                 (n) & (1ULL << 56) ? 56 :       \
 89                 (n) & (1ULL << 55) ? 55 :       \
 90                 (n) & (1ULL << 54) ? 54 :       \
 91                 (n) & (1ULL << 53) ? 53 :       \
 92                 (n) & (1ULL << 52) ? 52 :       \
 93                 (n) & (1ULL << 51) ? 51 :       \
 94                 (n) & (1ULL << 50) ? 50 :       \
 95                 (n) & (1ULL << 49) ? 49 :       \
 96                 (n) & (1ULL << 48) ? 48 :       \
 97                 (n) & (1ULL << 47) ? 47 :       \
 98                 (n) & (1ULL << 46) ? 46 :       \
 99                 (n) & (1ULL << 45) ? 45 :       \
100                 (n) & (1ULL << 44) ? 44 :       \
101                 (n) & (1ULL << 43) ? 43 :       \
102                 (n) & (1ULL << 42) ? 42 :       \
103                 (n) & (1ULL << 41) ? 41 :       \
104                 (n) & (1ULL << 40) ? 40 :       \
105                 (n) & (1ULL << 39) ? 39 :       \
106                 (n) & (1ULL << 38) ? 38 :       \
107                 (n) & (1ULL << 37) ? 37 :       \
108                 (n) & (1ULL << 36) ? 36 :       \
109                 (n) & (1ULL << 35) ? 35 :       \
110                 (n) & (1ULL << 34) ? 34 :       \
111                 (n) & (1ULL << 33) ? 33 :       \
112                 (n) & (1ULL << 32) ? 32 :       \
113                 (n) & (1ULL << 31) ? 31 :       \
114                 (n) & (1ULL << 30) ? 30 :       \
115                 (n) & (1ULL << 29) ? 29 :       \
116                 (n) & (1ULL << 28) ? 28 :       \
117                 (n) & (1ULL << 27) ? 27 :       \
118                 (n) & (1ULL << 26) ? 26 :       \
119                 (n) & (1ULL << 25) ? 25 :       \
120                 (n) & (1ULL << 24) ? 24 :       \
121                 (n) & (1ULL << 23) ? 23 :       \
122                 (n) & (1ULL << 22) ? 22 :       \
123                 (n) & (1ULL << 21) ? 21 :       \
124                 (n) & (1ULL << 20) ? 20 :       \
125                 (n) & (1ULL << 19) ? 19 :       \
126                 (n) & (1ULL << 18) ? 18 :       \
127                 (n) & (1ULL << 17) ? 17 :       \
128                 (n) & (1ULL << 16) ? 16 :       \
129                 (n) & (1ULL << 15) ? 15 :       \
130                 (n) & (1ULL << 14) ? 14 :       \
131                 (n) & (1ULL << 13) ? 13 :       \
132                 (n) & (1ULL << 12) ? 12 :       \
133                 (n) & (1ULL << 11) ? 11 :       \
134                 (n) & (1ULL << 10) ? 10 :       \
135                 (n) & (1ULL <<  9) ?  9 :       \
136                 (n) & (1ULL <<  8) ?  8 :       \
137                 (n) & (1ULL <<  7) ?  7 :       \
138                 (n) & (1ULL <<  6) ?  6 :       \
139                 (n) & (1ULL <<  5) ?  5 :       \
140                 (n) & (1ULL <<  4) ?  4 :       \
141                 (n) & (1ULL <<  3) ?  3 :       \
142                 (n) & (1ULL <<  2) ?  2 :       \
143                 1) :                            \
144         -1)
145 
146 /**
147  * ilog2 - log base 2 of 32-bit or a 64-bit unsigned value
148  * @n: parameter
149  *
150  * constant-capable log of base 2 calculation
151  * - this can be used to initialise global variables from constant data, hence
152  * the massive ternary operator construction
153  *
154  * selects the appropriately-sized optimised version depending on sizeof(n)
155  */
156 #define ilog2(n) \
157 ( \
158         __builtin_constant_p(n) ?       \
159         const_ilog2(n) :                \
160         (sizeof(n) <= 4) ?              \
161         __ilog2_u32(n) :                \
162         __ilog2_u64(n)                  \
163  )
164 
165 /**
166  * roundup_pow_of_two - round the given value up to nearest power of two
167  * @n: parameter
168  *
169  * round the given value up to the nearest power of two
170  * - the result is undefined when n == 0
171  * - this can be used to initialise global variables from constant data
172  */
173 #define roundup_pow_of_two(n)                   \
174 (                                               \
175         __builtin_constant_p(n) ? (             \
176                 (n == 1) ? 1 :                  \
177                 (1UL << (ilog2((n) - 1) + 1))   \
178                                    ) :          \
179         __roundup_pow_of_two(n)                 \
180  )
181 
182 /**
183  * rounddown_pow_of_two - round the given value down to nearest power of two
184  * @n: parameter
185  *
186  * round the given value down to the nearest power of two
187  * - the result is undefined when n == 0
188  * - this can be used to initialise global variables from constant data
189  */
190 #define rounddown_pow_of_two(n)                 \
191 (                                               \
192         __builtin_constant_p(n) ? (             \
193                 (1UL << ilog2(n))) :            \
194         __rounddown_pow_of_two(n)               \
195  )
196 
197 static inline __attribute_const__
198 int __order_base_2(unsigned long n)
199 {
200         return n > 1 ? ilog2(n - 1) + 1 : 0;
201 }
202 
203 /**
204  * order_base_2 - calculate the (rounded up) base 2 order of the argument
205  * @n: parameter
206  *
207  * The first few values calculated by this routine:
208  *  ob2(0) = 0
209  *  ob2(1) = 0
210  *  ob2(2) = 1
211  *  ob2(3) = 2
212  *  ob2(4) = 2
213  *  ob2(5) = 3
214  *  ... and so on.
215  */
216 #define order_base_2(n)                         \
217 (                                               \
218         __builtin_constant_p(n) ? (             \
219                 ((n) == 0 || (n) == 1) ? 0 :    \
220                 ilog2((n) - 1) + 1) :           \
221         __order_base_2(n)                       \
222 )
223 #endif /* _LINUX_LOG2_H */
224 

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