1 // Copyright 2018 The Abseil Authors.
2 //
3 // Licensed under the Apache License, Version 2.0 (the "License");
4 // you may not use this file except in compliance with the License.
5 // You may obtain a copy of the License at
6 //
7 // https://www.apache.org/licenses/LICENSE-2.0
8 //
9 // Unless required by applicable law or agreed to in writing, software
10 // distributed under the License is distributed on an "AS IS" BASIS,
11 // WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
12 // See the License for the specific language governing permissions and
13 // limitations under the License.
14
15 #ifndef ABSL_BASE_INTERNAL_BITS_H_
16 #define ABSL_BASE_INTERNAL_BITS_H_
17
18 // This file contains bitwise ops which are implementation details of various
19 // absl libraries.
20
21 #include <cstdint>
22
23 #include "absl/base/config.h"
24
25 // Clang on Windows has __builtin_clzll; otherwise we need to use the
26 // windows intrinsic functions.
27 #if defined(_MSC_VER)
28 #include <intrin.h>
29 #if defined(_M_X64)
30 #pragma intrinsic(_BitScanReverse64)
31 #pragma intrinsic(_BitScanForward64)
32 #endif
33 #pragma intrinsic(_BitScanReverse)
34 #pragma intrinsic(_BitScanForward)
35 #endif
36
37 #include "absl/base/attributes.h"
38
39 #if defined(_MSC_VER)
40 // We can achieve something similar to attribute((always_inline)) with MSVC by
41 // using the __forceinline keyword, however this is not perfect. MSVC is
42 // much less aggressive about inlining, and even with the __forceinline keyword.
43 #define ABSL_BASE_INTERNAL_FORCEINLINE __forceinline
44 #else
45 // Use default attribute inline.
46 #define ABSL_BASE_INTERNAL_FORCEINLINE inline ABSL_ATTRIBUTE_ALWAYS_INLINE
47 #endif
48
49
50 namespace absl {
51 ABSL_NAMESPACE_BEGIN
52 namespace base_internal {
53
CountLeadingZeros64Slow(uint64_t n)54 ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros64Slow(uint64_t n) {
55 int zeroes = 60;
56 if (n >> 32) {
57 zeroes -= 32;
58 n >>= 32;
59 }
60 if (n >> 16) {
61 zeroes -= 16;
62 n >>= 16;
63 }
64 if (n >> 8) {
65 zeroes -= 8;
66 n >>= 8;
67 }
68 if (n >> 4) {
69 zeroes -= 4;
70 n >>= 4;
71 }
72 return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0"[n] + zeroes;
73 }
74
CountLeadingZeros64(uint64_t n)75 ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros64(uint64_t n) {
76 #if defined(_MSC_VER) && defined(_M_X64)
77 // MSVC does not have __buitin_clzll. Use _BitScanReverse64.
78 unsigned long result = 0; // NOLINT(runtime/int)
79 if (_BitScanReverse64(&result, n)) {
80 return 63 - result;
81 }
82 return 64;
83 #elif defined(_MSC_VER)
84 // MSVC does not have __buitin_clzll. Compose two calls to _BitScanReverse
85 unsigned long result = 0; // NOLINT(runtime/int)
86 if ((n >> 32) && _BitScanReverse(&result, n >> 32)) {
87 return 31 - result;
88 }
89 if (_BitScanReverse(&result, n)) {
90 return 63 - result;
91 }
92 return 64;
93 #elif defined(__GNUC__)
94 // Use __builtin_clzll, which uses the following instructions:
95 // x86: bsr
96 // ARM64: clz
97 // PPC: cntlzd
98 static_assert(sizeof(unsigned long long) == sizeof(n), // NOLINT(runtime/int)
99 "__builtin_clzll does not take 64-bit arg");
100
101 // Handle 0 as a special case because __builtin_clzll(0) is undefined.
102 if (n == 0) {
103 return 64;
104 }
105 return __builtin_clzll(n);
106 #else
107 return CountLeadingZeros64Slow(n);
108 #endif
109 }
110
CountLeadingZeros32Slow(uint64_t n)111 ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros32Slow(uint64_t n) {
112 int zeroes = 28;
113 if (n >> 16) {
114 zeroes -= 16;
115 n >>= 16;
116 }
117 if (n >> 8) {
118 zeroes -= 8;
119 n >>= 8;
120 }
121 if (n >> 4) {
122 zeroes -= 4;
123 n >>= 4;
124 }
125 return "\4\3\2\2\1\1\1\1\0\0\0\0\0\0\0"[n] + zeroes;
126 }
127
CountLeadingZeros32(uint32_t n)128 ABSL_BASE_INTERNAL_FORCEINLINE int CountLeadingZeros32(uint32_t n) {
129 #if defined(_MSC_VER)
130 unsigned long result = 0; // NOLINT(runtime/int)
131 if (_BitScanReverse(&result, n)) {
132 return 31 - result;
133 }
134 return 32;
135 #elif defined(__GNUC__)
136 // Use __builtin_clz, which uses the following instructions:
137 // x86: bsr
138 // ARM64: clz
139 // PPC: cntlzd
140 static_assert(sizeof(int) == sizeof(n),
141 "__builtin_clz does not take 32-bit arg");
142
143 // Handle 0 as a special case because __builtin_clz(0) is undefined.
144 if (n == 0) {
145 return 32;
146 }
147 return __builtin_clz(n);
148 #else
149 return CountLeadingZeros32Slow(n);
150 #endif
151 }
152
CountTrailingZerosNonZero64Slow(uint64_t n)153 ABSL_BASE_INTERNAL_FORCEINLINE int CountTrailingZerosNonZero64Slow(uint64_t n) {
154 int c = 63;
155 n &= ~n + 1;
156 if (n & 0x00000000FFFFFFFF) c -= 32;
157 if (n & 0x0000FFFF0000FFFF) c -= 16;
158 if (n & 0x00FF00FF00FF00FF) c -= 8;
159 if (n & 0x0F0F0F0F0F0F0F0F) c -= 4;
160 if (n & 0x3333333333333333) c -= 2;
161 if (n & 0x5555555555555555) c -= 1;
162 return c;
163 }
164
CountTrailingZerosNonZero64(uint64_t n)165 ABSL_BASE_INTERNAL_FORCEINLINE int CountTrailingZerosNonZero64(uint64_t n) {
166 #if defined(_MSC_VER) && defined(_M_X64)
167 unsigned long result = 0; // NOLINT(runtime/int)
168 _BitScanForward64(&result, n);
169 return result;
170 #elif defined(_MSC_VER)
171 unsigned long result = 0; // NOLINT(runtime/int)
172 if (static_cast<uint32_t>(n) == 0) {
173 _BitScanForward(&result, n >> 32);
174 return result + 32;
175 }
176 _BitScanForward(&result, n);
177 return result;
178 #elif defined(__GNUC__)
179 static_assert(sizeof(unsigned long long) == sizeof(n), // NOLINT(runtime/int)
180 "__builtin_ctzll does not take 64-bit arg");
181 return __builtin_ctzll(n);
182 #else
183 return CountTrailingZerosNonZero64Slow(n);
184 #endif
185 }
186
CountTrailingZerosNonZero32Slow(uint32_t n)187 ABSL_BASE_INTERNAL_FORCEINLINE int CountTrailingZerosNonZero32Slow(uint32_t n) {
188 int c = 31;
189 n &= ~n + 1;
190 if (n & 0x0000FFFF) c -= 16;
191 if (n & 0x00FF00FF) c -= 8;
192 if (n & 0x0F0F0F0F) c -= 4;
193 if (n & 0x33333333) c -= 2;
194 if (n & 0x55555555) c -= 1;
195 return c;
196 }
197
CountTrailingZerosNonZero32(uint32_t n)198 ABSL_BASE_INTERNAL_FORCEINLINE int CountTrailingZerosNonZero32(uint32_t n) {
199 #if defined(_MSC_VER)
200 unsigned long result = 0; // NOLINT(runtime/int)
201 _BitScanForward(&result, n);
202 return result;
203 #elif defined(__GNUC__)
204 static_assert(sizeof(int) == sizeof(n),
205 "__builtin_ctz does not take 32-bit arg");
206 return __builtin_ctz(n);
207 #else
208 return CountTrailingZerosNonZero32Slow(n);
209 #endif
210 }
211
212 #undef ABSL_BASE_INTERNAL_FORCEINLINE
213
214 } // namespace base_internal
215 ABSL_NAMESPACE_END
216 } // namespace absl
217
218 #endif // ABSL_BASE_INTERNAL_BITS_H_
219