include/linux/log2.h

   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  * non-constant log of base 2 calculators
  20  * - the arch may override these in asm/bitops.h if they can be implemented
  21  *   more efficiently than using fls() and fls64()
  22  * - the arch is not required to handle n==0 if implementing the fallback
  23  */
  24 #ifndef CONFIG_ARCH_HAS_ILOG2_U32
  25 static inline __attribute__((const))
  26 int __ilog2_u32(u32 n)
  27 {
  28         return fls(n) - 1;
  29 }
  30 #endif
  31
  32 #ifndef CONFIG_ARCH_HAS_ILOG2_U64
  33 static inline __attribute__((const))
  34 int __ilog2_u64(u64 n)
  35 {
  36         return fls64(n) - 1;
  37 }
  38 #endif
  39
  40 /**
  41  * is_power_of_2() - check if a value is a power of two
  42  * @n: the value to check
  43  *
  44  * Determine whether some value is a power of two, where zero is
  45  * *not* considered a power of two.
  46  * Return: true if @n is a power of 2, otherwise false.
  47  */
  48 static inline __attribute__((const))
  49 bool is_power_of_2(unsigned long n)
  50 {
  51         return (n != 0 && ((n & (n - 1)) == 0));
  52 }
  53
  54 /**
  55  * __roundup_pow_of_two() - round up to nearest power of two
  56  * @n: value to round up
  57  */
  58 static inline __attribute__((const))
  59 unsigned long __roundup_pow_of_two(unsigned long n)
  60 {
  61         return 1UL << fls_long(n - 1);
  62 }
  63
  64 /**
  65  * __rounddown_pow_of_two() - round down to nearest power of two
  66  * @n: value to round down
  67  */
  68 static inline __attribute__((const))
  69 unsigned long __rounddown_pow_of_two(unsigned long n)
  70 {
  71         return 1UL << (fls_long(n) - 1);
  72 }
  73
  74 /**
  75  * const_ilog2 - log base 2 of 32-bit or a 64-bit constant unsigned value
  76  * @n: parameter
  77  *
  78  * Use this where sparse expects a true constant expression, e.g. for array
  79  * indices.
  80  */
  81 #define const_ilog2(n)                          \
  82 (                                               \
  83         __builtin_constant_p(n) ? (             \
  84                 (n) < 2 ? 0 :                   \
  85                 (n) & (1ULL << 63) ? 63 :       \
  86                 (n) & (1ULL << 62) ? 62 :       \
  87                 (n) & (1ULL << 61) ? 61 :       \
  88                 (n) & (1ULL << 60) ? 60 :       \
  89                 (n) & (1ULL << 59) ? 59 :       \
  90                 (n) & (1ULL << 58) ? 58 :       \
  91                 (n) & (1ULL << 57) ? 57 :       \
  92                 (n) & (1ULL << 56) ? 56 :       \
  93                 (n) & (1ULL << 55) ? 55 :       \
  94                 (n) & (1ULL << 54) ? 54 :       \
  95                 (n) & (1ULL << 53) ? 53 :       \
  96                 (n) & (1ULL << 52) ? 52 :       \
  97                 (n) & (1ULL << 51) ? 51 :       \
  98                 (n) & (1ULL << 50) ? 50 :       \
  99                 (n) & (1ULL << 49) ? 49 :       \
 100                 (n) & (1ULL << 48) ? 48 :       \
 101                 (n) & (1ULL << 47) ? 47 :       \
 102                 (n) & (1ULL << 46) ? 46 :       \
 103                 (n) & (1ULL << 45) ? 45 :       \
 104                 (n) & (1ULL << 44) ? 44 :       \
 105                 (n) & (1ULL << 43) ? 43 :       \
 106                 (n) & (1ULL << 42) ? 42 :       \
 107                 (n) & (1ULL << 41) ? 41 :       \
 108                 (n) & (1ULL << 40) ? 40 :       \
 109                 (n) & (1ULL << 39) ? 39 :       \
 110                 (n) & (1ULL << 38) ? 38 :       \
 111                 (n) & (1ULL << 37) ? 37 :       \
 112                 (n) & (1ULL << 36) ? 36 :       \
 113                 (n) & (1ULL << 35) ? 35 :       \
 114                 (n) & (1ULL << 34) ? 34 :       \
 115                 (n) & (1ULL << 33) ? 33 :       \
 116                 (n) & (1ULL << 32) ? 32 :       \
 117                 (n) & (1ULL << 31) ? 31 :       \
 118                 (n) & (1ULL << 30) ? 30 :       \
 119                 (n) & (1ULL << 29) ? 29 :       \
 120                 (n) & (1ULL << 28) ? 28 :       \
 121                 (n) & (1ULL << 27) ? 27 :       \
 122                 (n) & (1ULL << 26) ? 26 :       \
 123                 (n) & (1ULL << 25) ? 25 :       \
 124                 (n) & (1ULL << 24) ? 24 :       \
 125                 (n) & (1ULL << 23) ? 23 :       \
 126                 (n) & (1ULL << 22) ? 22 :       \
 127                 (n) & (1ULL << 21) ? 21 :       \
 128                 (n) & (1ULL << 20) ? 20 :       \
 129                 (n) & (1ULL << 19) ? 19 :       \
 130                 (n) & (1ULL << 18) ? 18 :       \
 131                 (n) & (1ULL << 17) ? 17 :       \
 132                 (n) & (1ULL << 16) ? 16 :       \
 133                 (n) & (1ULL << 15) ? 15 :       \
 134                 (n) & (1ULL << 14) ? 14 :       \
 135                 (n) & (1ULL << 13) ? 13 :       \
 136                 (n) & (1ULL << 12) ? 12 :       \
 137                 (n) & (1ULL << 11) ? 11 :       \
 138                 (n) & (1ULL << 10) ? 10 :       \
 139                 (n) & (1ULL <<  9) ?  9 :       \
 140                 (n) & (1ULL <<  8) ?  8 :       \
 141                 (n) & (1ULL <<  7) ?  7 :       \
 142                 (n) & (1ULL <<  6) ?  6 :       \
 143                 (n) & (1ULL <<  5) ?  5 :       \
 144                 (n) & (1ULL <<  4) ?  4 :       \
 145                 (n) & (1ULL <<  3) ?  3 :       \
 146                 (n) & (1ULL <<  2) ?  2 :       \
 147                 1) :                            \
 148         -1)
 149
 150 /**
 151  * ilog2 - log base 2 of 32-bit or a 64-bit unsigned value
 152  * @n: parameter
 153  *
 154  * constant-capable log of base 2 calculation
 155  * - this can be used to initialise global variables from constant data, hence
 156  * the massive ternary operator construction
 157  *
 158  * selects the appropriately-sized optimised version depending on sizeof(n)
 159  */
 160 #define ilog2(n) \
 161 ( \
 162         __builtin_constant_p(n) ?       \
 163         const_ilog2(n) :                \
 164         (sizeof(n) <= 4) ?              \
 165         __ilog2_u32(n) :                \
 166         __ilog2_u64(n)                  \
 167  )
 168
 169 /**
 170  * roundup_pow_of_two - round the given value up to nearest power of two
 171  * @n: parameter
 172  *
 173  * round the given value up to the nearest power of two
 174  * - the result is undefined when n == 0
 175  * - this can be used to initialise global variables from constant data
 176  */
 177 #define roundup_pow_of_two(n)                   \
 178 (                                               \
 179         __builtin_constant_p(n) ? (             \
 180                 (n == 1) ? 1 :                  \
 181                 (1UL << (ilog2((n) - 1) + 1))   \
 182                                    ) :          \
 183         __roundup_pow_of_two(n)                 \
 184  )
 185
 186 /**
 187  * rounddown_pow_of_two - round the given value down to nearest power of two
 188  * @n: parameter
 189  *
 190  * round the given value down to the nearest power of two
 191  * - the result is undefined when n == 0
 192  * - this can be used to initialise global variables from constant data
 193  */
 194 #define rounddown_pow_of_two(n)                 \
 195 (                                               \
 196         __builtin_constant_p(n) ? (             \
 197                 (1UL << ilog2(n))) :            \
 198         __rounddown_pow_of_two(n)               \
 199  )
 200
 201 static inline __attribute_const__
 202 int __order_base_2(unsigned long n)
 203 {
 204         return n > 1 ? ilog2(n - 1) + 1 : 0;
 205 }
 206
 207 /**
 208  * order_base_2 - calculate the (rounded up) base 2 order of the argument
 209  * @n: parameter
 210  *
 211  * The first few values calculated by this routine:
 212  *  ob2(0) = 0
 213  *  ob2(1) = 0
 214  *  ob2(2) = 1
 215  *  ob2(3) = 2
 216  *  ob2(4) = 2
 217  *  ob2(5) = 3
 218  *  ... and so on.
 219  */
 220 #define order_base_2(n)                         \
 221 (                                               \
 222         __builtin_constant_p(n) ? (             \
 223                 ((n) == 0 || (n) == 1) ? 0 :    \
 224                 ilog2((n) - 1) + 1) :           \
 225         __order_base_2(n)                       \
 226 )
 227 #endif /* _LINUX_LOG2_H */