1 /*
2 * Copyright (C) 2017 The Android Open Source Project
3 *
4 * Licensed under the Apache License, Version 2.0 (the "License");
5 * you may not use this file except in compliance with the License.
6 * You may obtain a copy of the License at
7 *
8 * http://www.apache.org/licenses/LICENSE-2.0
9 *
10 * Unless required by applicable law or agreed to in writing, software
11 * distributed under the License is distributed on an "AS IS" BASIS,
12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
13 * See the License for the specific language governing permissions and
14 * limitations under the License.
15 */
16
17 #ifndef ART_LIBARTBASE_BASE_BIT_STRUCT_H_
18 #define ART_LIBARTBASE_BASE_BIT_STRUCT_H_
19
20 #include <type_traits>
21
22 #include "base/casts.h"
23 #include "bit_struct_detail.h"
24 #include "bit_utils.h"
25
26 //
27 // Zero-cost, type-safe, well-defined "structs" of bit fields.
28 //
29 // ---------------------------------------------
30 // Usage example:
31 // ---------------------------------------------
32 //
33 // // Definition for type 'Example'
34 // BITSTRUCT_DEFINE_START(Example, 10)
35 // BITSTRUCT_UINT(0, 2) u2; // Every field must be a BitStruct[*] with the same StorageType,
36 // BITSTRUCT_INT(2, 7) i7; // preferably using BITSTRUCT_{FIELD,UINT,INT}
37 // BITSTRUCT_UINT(9, 1) i1; // to fill in the StorageType parameter.
38 // BITSTRUCT_DEFINE_END(Example);
39 //
40 // Would define a bit struct with this layout:
41 // <- 1 -> <-- 7 --> <- 2 ->
42 // +--------+---------------+-----+
43 // | i1 | i7 | u2 +
44 // +--------+---------------+-----+
45 // 10 9 2 0
46 //
47 // // Read-write just like regular values.
48 // Example ex;
49 // ex.u2 = 3;
50 // ex.i7 = -25;
51 // ex.i1 = true;
52 // size_t u2 = ex.u2;
53 // int i7 = ex.i7;
54 // bool i1 = ex.i1;
55 //
56 // // It's packed down to the smallest # of machine words.
57 // assert(sizeof(Example) == 2);
58 // // The exact bit pattern is well-defined by the template parameters.
59 // uint16_t cast = *reinterpret_cast<uint16_t*>(ex);
60 // assert(cast == ((3) | (0b100111 << 2) | (true << 9);
61 //
62 // ---------------------------------------------
63 // Why not just use C++ bitfields?
64 // ---------------------------------------------
65 //
66 // The layout is implementation-defined.
67 // We do not know whether the fields are packed left-to-right or
68 // right-to-left, so it makes it useless when the memory layout needs to be
69 // precisely controlled.
70 //
71 // ---------------------------------------------
72 // More info:
73 // ---------------------------------------------
74 // Currently uintmax_t is the largest supported underlying storage type,
75 // all (kBitOffset + kBitWidth) must fit into BitSizeOf<uintmax_t>();
76 //
77 // Using BitStruct[U]int will automatically select an underlying type
78 // that's the smallest to fit your (offset + bitwidth).
79 //
80 // BitStructNumber can be used to manually select an underlying type.
81 //
82 // BitStructField can be used with custom standard-layout structs,
83 // thus allowing for arbitrary nesting of bit structs.
84 //
85 namespace art {
86 // Zero-cost wrapper around a struct 'T', allowing it to be stored as a bitfield
87 // at offset 'kBitOffset' and width 'kBitWidth'.
88 // The storage is plain unsigned int, whose size is the smallest required to fit
89 // 'kBitOffset + kBitWidth'. All operations to this become BitFieldExtract/BitFieldInsert
90 // operations to the underlying uint.
91 //
92 // Field memory representation:
93 //
94 // MSB <-- width --> LSB
95 // +--------+------------+--------+
96 // | ?????? | u bitfield | ?????? +
97 // +--------+------------+--------+
98 // offset 0
99 //
100 // Reading/writing the bitfield (un)packs it into a temporary T:
101 //
102 // MSB <-- width --> LSB
103 // +-----------------+------------+
104 // | 0.............0 | T bitfield |
105 // +-----------------+------------+
106 // 0
107 //
108 // It's the responsibility of the StorageType to ensure the bit representation
109 // of T can be represented by kBitWidth.
110 template <typename T,
111 size_t kBitOffset,
112 size_t kBitWidth,
113 typename StorageType>
114 struct BitStructField {
115 static_assert(std::is_standard_layout<T>::value, "T must be standard layout");
116
TBitStructField117 operator T() const {
118 return Get();
119 }
120
121 // Exclude overload when T==StorageType.
122 template <typename _ = void,
123 typename = std::enable_if_t<std::is_same<T, StorageType>::value, _>>
StorageTypeBitStructField124 explicit operator StorageType() const {
125 return BitFieldExtract(storage_, kBitOffset, kBitWidth);
126 }
127
128 BitStructField& operator=(T value) {
129 return Assign(*this, value);
130 }
131
BitStructSizeOfBitStructField132 static constexpr size_t BitStructSizeOf() {
133 return kBitWidth;
134 }
135
136 BitStructField& operator=(const BitStructField& other) {
137 // Warning. The default operator= will overwrite the entire storage!
138 return *this = static_cast<T>(other);
139 }
140
BitStructFieldBitStructField141 BitStructField(const BitStructField& other) {
142 Assign(*this, static_cast<T>(other));
143 }
144
145 BitStructField() = default;
146 ~BitStructField() = default;
147
148 protected:
149 template <typename T2>
AssignBitStructField150 T2& Assign(T2& what, T value) {
151 // Since C++ doesn't allow the type of operator= to change out
152 // in the subclass, reimplement operator= in each subclass
153 // manually and call this helper function.
154 static_assert(std::is_base_of<BitStructField, T2>::value, "T2 must inherit BitStructField");
155 what.Set(value);
156 return what;
157 }
158
GetBitStructField159 T Get() const {
160 ExtractionType storage = static_cast<ExtractionType>(storage_);
161 ExtractionType extracted = BitFieldExtract(storage, kBitOffset, kBitWidth);
162 ConversionType to_convert = dchecked_integral_cast<ConversionType>(extracted);
163 return ValueConverter::FromUnderlyingStorage(to_convert);
164 }
165
SetBitStructField166 void Set(T value) {
167 ConversionType converted = ValueConverter::ToUnderlyingStorage(value);
168 ExtractionType extracted = dchecked_integral_cast<ExtractionType>(converted);
169 storage_ = BitFieldInsert(storage_, extracted, kBitOffset, kBitWidth);
170 }
171
172 private:
173 using ValueConverter = detail::ValueConverter<T>;
174 using ConversionType = typename ValueConverter::StorageType;
175 using ExtractionType =
176 typename std::conditional<std::is_signed_v<ConversionType>,
177 std::make_signed_t<StorageType>,
178 StorageType>::type;
179
180 StorageType storage_;
181 };
182
183 // Base class for number-like BitStruct fields.
184 // T is the type to store in as a bit field.
185 // kBitOffset, kBitWidth define the position and length of the bitfield.
186 //
187 // (Common usage should be BitStructInt, BitStructUint -- this
188 // intermediate template allows a user-defined integer to be used.)
189 template <typename T, size_t kBitOffset, size_t kBitWidth, typename StorageType>
190 struct BitStructNumber : public BitStructField<T, kBitOffset, kBitWidth, StorageType> {
191 BitStructNumber& operator=(T value) {
192 return BaseType::Assign(*this, value);
193 }
194
TBitStructNumber195 /*implicit*/ operator T() const {
196 return Get();
197 }
198
199 explicit operator bool() const {
200 return static_cast<bool>(Get());
201 }
202
203 BitStructNumber& operator++() {
204 *this = Get() + 1u;
205 return *this;
206 }
207
208 StorageType operator++(int) {
209 return Get() + 1u;
210 }
211
212 BitStructNumber& operator--() {
213 *this = Get() - 1u;
214 return *this;
215 }
216
217 StorageType operator--(int) {
218 return Get() - 1u;
219 }
220
221 private:
222 using BaseType = BitStructField<T, kBitOffset, kBitWidth, StorageType>;
223 using BaseType::Get;
224 };
225
226 // Create a BitStruct field which uses the smallest underlying int storage type,
227 // in order to be large enough to fit (kBitOffset + kBitWidth).
228 //
229 // Values are sign-extended when they are read out.
230 template <size_t kBitOffset, size_t kBitWidth, typename StorageType>
231 using BitStructInt =
232 BitStructNumber<typename detail::MinimumTypeHelper<int, kBitOffset + kBitWidth>::type,
233 kBitOffset,
234 kBitWidth,
235 StorageType>;
236
237 // Create a BitStruct field which uses the smallest underlying uint storage type,
238 // in order to be large enough to fit (kBitOffset + kBitWidth).
239 //
240 // Values are zero-extended when they are read out.
241 template <size_t kBitOffset, size_t kBitWidth, typename StorageType>
242 using BitStructUint =
243 BitStructNumber<typename detail::MinimumTypeHelper<unsigned int, kBitOffset + kBitWidth>::type,
244 kBitOffset,
245 kBitWidth,
246 StorageType>;
247
248 // Start a definition for a bitstruct.
249 // A bitstruct is defined to be a union with a common initial subsequence
250 // that we call 'DefineBitStructSize<bitwidth>'.
251 //
252 // See top of file for usage example.
253 //
254 // This marker is required by the C++ standard in order to
255 // have a "common initial sequence".
256 //
257 // See C++ 9.5.1 [class.union]:
258 // If a standard-layout union contains several standard-layout structs that share a common
259 // initial sequence ... it is permitted to inspect the common initial sequence of any of
260 // standard-layout struct members.
261 #define BITSTRUCT_DEFINE_START(name, bitwidth) \
262 union name { /* NOLINT */ \
263 using StorageType = \
264 typename detail::MinimumTypeUnsignedHelper<(bitwidth)>::type; \
265 art::detail::DefineBitStructSize<(bitwidth)> _; \
266 static constexpr size_t BitStructSizeOf() { return (bitwidth); } \
267 name& operator=(const name& other) { _ = other._; return *this; } /* NOLINT */ \
268 name(const name& other) : _(other._) {} \
269 name() = default; \
270 ~name() = default;
271
272 // Define a field. See top of file for usage example.
273 #define BITSTRUCT_FIELD(type, bit_offset, bit_width) \
274 BitStructField<type, (bit_offset), (bit_width), StorageType>
275 #define BITSTRUCT_INT(bit_offset, bit_width) \
276 BitStructInt<(bit_offset), (bit_width), StorageType>
277 #define BITSTRUCT_UINT(bit_offset, bit_width) \
278 BitStructUint<(bit_offset), (bit_width), StorageType>
279
280 // End the definition of a bitstruct, and insert a sanity check
281 // to ensure that the bitstruct did not exceed the specified size.
282 //
283 // See top of file for usage example.
284 #define BITSTRUCT_DEFINE_END(name) \
285 }; \
286 static_assert(art::detail::ValidateBitStructSize<name>(), \
287 #name "bitsize incorrect: " \
288 "did you insert extra fields that weren't BitStructX, " \
289 "and does the size match the sum of the field widths?")
290
291 // Determine the minimal bit size for a user-defined type T.
292 // Used by BitStructField to determine how small a custom type is.
293 template <typename T>
BitStructSizeOf()294 static constexpr size_t BitStructSizeOf() {
295 return T::BitStructSizeOf();
296 }
297
298 } // namespace art
299
300 #endif // ART_LIBARTBASE_BASE_BIT_STRUCT_H_
301