1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
|
// Copyright (c) 2013 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.
// This file defines some bit utilities.
#ifndef BASE_BITS_H_
#define BASE_BITS_H_
#include <stddef.h>
#include <stdint.h>
#include <type_traits>
#include "base/compiler_specific.h"
#include "base/logging.h"
#include "build/build_config.h"
#if defined(COMPILER_MSVC)
#include <intrin.h>
#endif
namespace base {
namespace bits {
// Returns true iff |value| is a power of 2.
template <typename T,
typename = typename std::enable_if<std::is_integral<T>::value>>
constexpr inline bool IsPowerOfTwo(T value) {
// From "Hacker's Delight": Section 2.1 Manipulating Rightmost Bits.
//
// Only positive integers with a single bit set are powers of two. If only one
// bit is set in x (e.g. 0b00000100000000) then |x-1| will have that bit set
// to zero and all bits to its right set to 1 (e.g. 0b00000011111111). Hence
// |x & (x-1)| is 0 iff x is a power of two.
return value > 0 && (value & (value - 1)) == 0;
}
// Round up |size| to a multiple of alignment, which must be a power of two.
inline size_t Align(size_t size, size_t alignment) {
DCHECK(IsPowerOfTwo(alignment));
return (size + alignment - 1) & ~(alignment - 1);
}
// Round down |size| to a multiple of alignment, which must be a power of two.
inline size_t AlignDown(size_t size, size_t alignment) {
DCHECK(IsPowerOfTwo(alignment));
return size & ~(alignment - 1);
}
// CountLeadingZeroBits(value) returns the number of zero bits following the
// most significant 1 bit in |value| if |value| is non-zero, otherwise it
// returns {sizeof(T) * 8}.
// Example: 00100010 -> 2
//
// CountTrailingZeroBits(value) returns the number of zero bits preceding the
// least significant 1 bit in |value| if |value| is non-zero, otherwise it
// returns {sizeof(T) * 8}.
// Example: 00100010 -> 1
//
// C does not have an operator to do this, but fortunately the various
// compilers have built-ins that map to fast underlying processor instructions.
#if defined(COMPILER_MSVC)
template <typename T, unsigned bits = sizeof(T) * 8>
ALWAYS_INLINE
typename std::enable_if<std::is_unsigned<T>::value && sizeof(T) <= 4,
unsigned>::type
CountLeadingZeroBits(T x) {
static_assert(bits > 0, "invalid instantiation");
unsigned long index;
return LIKELY(_BitScanReverse(&index, static_cast<uint32_t>(x)))
? (31 - index - (32 - bits))
: bits;
}
template <typename T, unsigned bits = sizeof(T) * 8>
ALWAYS_INLINE
typename std::enable_if<std::is_unsigned<T>::value && sizeof(T) == 8,
unsigned>::type
CountLeadingZeroBits(T x) {
static_assert(bits > 0, "invalid instantiation");
unsigned long index;
// MSVC only supplies _BitScanReverse64 when building for a 64-bit target.
#if defined(ARCH_CPU_64_BITS)
return LIKELY(_BitScanReverse64(&index, static_cast<uint64_t>(x)))
? (63 - index)
: 64;
#else
uint32_t left = static_cast<uint32_t>(x >> 32);
if (LIKELY(_BitScanReverse(&index, left)))
return 31 - index;
uint32_t right = static_cast<uint32_t>(x);
if (LIKELY(_BitScanReverse(&index, right)))
return 63 - index;
return 64;
#endif
}
template <typename T, unsigned bits = sizeof(T) * 8>
ALWAYS_INLINE
typename std::enable_if<std::is_unsigned<T>::value && sizeof(T) <= 4,
unsigned>::type
CountTrailingZeroBits(T x) {
static_assert(bits > 0, "invalid instantiation");
unsigned long index;
return LIKELY(_BitScanForward(&index, static_cast<uint32_t>(x))) ? index
: bits;
}
template <typename T, unsigned bits = sizeof(T) * 8>
ALWAYS_INLINE
typename std::enable_if<std::is_unsigned<T>::value && sizeof(T) == 8,
unsigned>::type
CountTrailingZeroBits(T x) {
static_assert(bits > 0, "invalid instantiation");
unsigned long index;
// MSVC only supplies _BitScanForward64 when building for a 64-bit target.
#if defined(ARCH_CPU_64_BITS)
return LIKELY(_BitScanForward64(&index, static_cast<uint64_t>(x))) ? index
: 64;
#else
uint32_t right = static_cast<uint32_t>(x);
if (LIKELY(_BitScanForward(&index, right)))
return index;
uint32_t left = static_cast<uint32_t>(x >> 32);
if (LIKELY(_BitScanForward(&index, left)))
return 32 + index;
return 64;
#endif
}
ALWAYS_INLINE uint32_t CountLeadingZeroBits32(uint32_t x) {
return CountLeadingZeroBits(x);
}
ALWAYS_INLINE uint64_t CountLeadingZeroBits64(uint64_t x) {
return CountLeadingZeroBits(x);
}
#elif defined(COMPILER_GCC)
// __builtin_clz has undefined behaviour for an input of 0, even though there's
// clearly a return value that makes sense, and even though some processor clz
// instructions have defined behaviour for 0. We could drop to raw __asm__ to
// do better, but we'll avoid doing that unless we see proof that we need to.
template <typename T, unsigned bits = sizeof(T) * 8>
ALWAYS_INLINE
typename std::enable_if<std::is_unsigned<T>::value && sizeof(T) <= 8,
unsigned>::type
CountLeadingZeroBits(T value) {
static_assert(bits > 0, "invalid instantiation");
return LIKELY(value)
? bits == 64
? __builtin_clzll(static_cast<uint64_t>(value))
: __builtin_clz(static_cast<uint32_t>(value)) - (32 - bits)
: bits;
}
template <typename T, unsigned bits = sizeof(T) * 8>
ALWAYS_INLINE
typename std::enable_if<std::is_unsigned<T>::value && sizeof(T) <= 8,
unsigned>::type
CountTrailingZeroBits(T value) {
return LIKELY(value) ? bits == 64
? __builtin_ctzll(static_cast<uint64_t>(value))
: __builtin_ctz(static_cast<uint32_t>(value))
: bits;
}
ALWAYS_INLINE uint32_t CountLeadingZeroBits32(uint32_t x) {
return CountLeadingZeroBits(x);
}
ALWAYS_INLINE uint64_t CountLeadingZeroBits64(uint64_t x) {
return CountLeadingZeroBits(x);
}
#endif
ALWAYS_INLINE size_t CountLeadingZeroBitsSizeT(size_t x) {
return CountLeadingZeroBits(x);
}
ALWAYS_INLINE size_t CountTrailingZeroBitsSizeT(size_t x) {
return CountTrailingZeroBits(x);
}
// Returns the integer i such as 2^i <= n < 2^(i+1)
inline int Log2Floor(uint32_t n) {
return 31 - CountLeadingZeroBits(n);
}
// Returns the integer i such as 2^(i-1) < n <= 2^i
inline int Log2Ceiling(uint32_t n) {
// When n == 0, we want the function to return -1.
// When n == 0, (n - 1) will underflow to 0xFFFFFFFF, which is
// why the statement below starts with (n ? 32 : -1).
return (n ? 32 : -1) - CountLeadingZeroBits(n - 1);
}
} // namespace bits
} // namespace base
#endif // BASE_BITS_H_
|
