summaryrefslogtreecommitdiff
path: root/absl/base/internal/bits.h
blob: bc7faaee3b257edc37cd9090b7777d7035d3c3b8 (plain)
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
// Copyright 2018 The Abseil Authors.
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//      http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

#ifndef ABSL_BASE_INTERNAL_BITS_H_
#define ABSL_BASE_INTERNAL_BITS_H_

// This file contains bitwise ops which are implementation details of various
// absl libraries.

#include <cstdint>

// Clang on Windows has __builtin_clzll; otherwise we need to use the
// windows intrinsic functions.
#if defined(_MSC_VER)
#include <intrin.h>
#if defined(_M_X64)
#pragma intrinsic(_BitScanReverse64)
#pragma intrinsic(_BitScanForward64)
#endif
#pragma intrinsic(_BitScanReverse)
#pragma intrinsic(_BitScanForward)
#endif

#include "absl/base/attributes.h"

#if defined(_MSC_VER)
// We can achieve something similar to attribute((always_inline)) with MSVC by
// using the __forceinline keyword, however this is not perfect. MSVC is
// much less aggressive about inlining, and even with the __forceinline keyword.
#define ABSL_BASE_INTERNAL_FORCEINLINE __forceinline
#else
// Use default attribute inline.
#define ABSL_BASE_INTERNAL_FORCEINLINE inline ABSL_ATTRIBUTE_ALWAYS_INLINE
#endif


namespace absl {
namespace base_internal {

ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros64Slow(uint64_t n) {
  int zeroes = 60;
  if (n >> 32) zeroes -= 32, n >>= 32;
  if (n >> 16) zeroes -= 16, n >>= 16;
  if (n >> 8) zeroes -= 8, n >>= 8;
  if (n >> 4) zeroes -= 4, n >>= 4;
  return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0"[n] + zeroes;
}

ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros64(uint64_t n) {
#if defined(_MSC_VER) && defined(_M_X64)
  // MSVC does not have __buitin_clzll. Use _BitScanReverse64.
  unsigned long result = 0;  // NOLINT(runtime/int)
  if (_BitScanReverse64(&result, n)) {
    return 63 - result;
  }
  return 64;
#elif defined(_MSC_VER)
  // MSVC does not have __buitin_clzll. Compose two calls to _BitScanReverse
  unsigned long result = 0;  // NOLINT(runtime/int)
  if ((n >> 32) && _BitScanReverse(&result, n >> 32)) {
    return 31 - result;
  }
  if (_BitScanReverse(&result, n)) {
    return 63 - result;
  }
  return 64;
#elif defined(__GNUC__)
  // Use __builtin_clzll, which uses the following instructions:
  //  x86: bsr
  //  ARM64: clz
  //  PPC: cntlzd
  static_assert(sizeof(unsigned long long) == sizeof(n),  // NOLINT(runtime/int)
                "__builtin_clzll does not take 64-bit arg");

  // Handle 0 as a special case because __builtin_clzll(0) is undefined.
  if (n == 0) {
    return 64;
  }
  return __builtin_clzll(n);
#else
  return CountLeadingZeros64Slow(n);
#endif
}

ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros32Slow(uint64_t n) {
  int zeroes = 28;
  if (n >> 16) zeroes -= 16, n >>= 16;
  if (n >> 8) zeroes -= 8, n >>= 8;
  if (n >> 4) zeroes -= 4, n >>= 4;
  return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0"[n] + zeroes;
}

ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros32(uint32_t n) {
#if defined(_MSC_VER)
  unsigned long result = 0;  // NOLINT(runtime/int)
  if (_BitScanReverse(&result, n)) {
    return 31 - result;
  }
  return 32;
#elif defined(__GNUC__)
  // Use __builtin_clz, which uses the following instructions:
  //  x86: bsr
  //  ARM64: clz
  //  PPC: cntlzd
  static_assert(sizeof(int) == sizeof(n),
                "__builtin_clz does not take 32-bit arg");

  // Handle 0 as a special case because __builtin_clz(0) is undefined.
  if (n == 0) {
    return 32;
  }
  return __builtin_clz(n);
#else
  return CountLeadingZeros32Slow(n);
#endif
}

ABSL_BASE_INTERNAL_FORCEINLINE int CountTrailingZerosNonZero64Slow(uint64_t n) {
  int c = 63;
  n &= ~n + 1;
  if (n & 0x00000000FFFFFFFF) c -= 32;
  if (n & 0x0000FFFF0000FFFF) c -= 16;
  if (n & 0x00FF00FF00FF00FF) c -= 8;
  if (n & 0x0F0F0F0F0F0F0F0F) c -= 4;
  if (n & 0x3333333333333333) c -= 2;
  if (n & 0x5555555555555555) c -= 1;
  return c;
}

ABSL_BASE_INTERNAL_FORCEINLINE int CountTrailingZerosNonZero64(uint64_t n) {
#if defined(_MSC_VER) && defined(_M_X64)
  unsigned long result = 0;  // NOLINT(runtime/int)
  _BitScanForward64(&result, n);
  return result;
#elif defined(_MSC_VER)
  unsigned long result = 0;  // NOLINT(runtime/int)
  if (static_cast<uint32_t>(n) == 0) {
    _BitScanForward(&result, n >> 32);
    return result + 32;
  }
  _BitScanForward(&result, n);
  return result;
#elif defined(__GNUC__)
  static_assert(sizeof(unsigned long long) == sizeof(n),  // NOLINT(runtime/int)
                "__builtin_ctzll does not take 64-bit arg");
  return __builtin_ctzll(n);
#else
  return CountTrailingZerosNonZero64Slow(n);
#endif
}

ABSL_BASE_INTERNAL_FORCEINLINE int CountTrailingZerosNonZero32Slow(uint32_t n) {
  int c = 31;
  n &= ~n + 1;
  if (n & 0x0000FFFF) c -= 16;
  if (n & 0x00FF00FF) c -= 8;
  if (n & 0x0F0F0F0F) c -= 4;
  if (n & 0x33333333) c -= 2;
  if (n & 0x55555555) c -= 1;
  return c;
}

ABSL_BASE_INTERNAL_FORCEINLINE int CountTrailingZerosNonZero32(uint32_t n) {
#if defined(_MSC_VER)
  unsigned long result = 0;  // NOLINT(runtime/int)
  _BitScanForward(&result, n);
  return result;
#elif defined(__GNUC__)
  static_assert(sizeof(int) == sizeof(n),
                "__builtin_ctz does not take 32-bit arg");
  return __builtin_ctz(n);
#else
  return CountTrailingZerosNonZero32Slow(n);
#endif
}

#undef ABSL_BASE_INTERNAL_FORCEINLINE

}  // namespace base_internal
}  // namespace absl

#endif  // ABSL_BASE_INTERNAL_BITS_H_