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_DETAIL_H_
18 #define ART_LIBARTBASE_BASE_BIT_STRUCT_DETAIL_H_
19
20 #include "bit_utils.h"
21 #include "globals.h"
22
23 #include <type_traits>
24
25 // Implementation details for bit_struct.h
26 // Not intended to be used stand-alone.
27
28 namespace art {
29
30 template <typename T>
31 static constexpr size_t BitStructSizeOf();
32
33 namespace detail {
34 // Select the smallest uintX_t that will fit kBitSize bits.
35 template <size_t kBitSize>
36 struct MinimumTypeUnsignedHelper {
37 using type =
38 typename std::conditional<kBitSize == 0, void, // NOLINT [whitespace/operators] [3]
39 typename std::conditional<kBitSize <= 8, uint8_t, // NOLINT [whitespace/operators] [3]
40 typename std::conditional<kBitSize <= 16, uint16_t, // NOLINT [whitespace/operators] [3]
41 typename std::conditional<kBitSize <= 32, uint32_t,
42 typename std::conditional<kBitSize <= 64, uint64_t,
43 typename std::conditional<kBitSize <= BitSizeOf<uintmax_t>(), uintmax_t,
44 void>::type>::type>::type>::type>::type>::type;
45 };
46
47 // Select the smallest [u]intX_t that will fit kBitSize bits.
48 // Automatically picks intX_t or uintX_t based on the sign-ness of T.
49 template <typename T, size_t kBitSize>
50 struct MinimumTypeHelper {
51 using type_unsigned = typename MinimumTypeUnsignedHelper<kBitSize>::type;
52
53 using type =
54 typename std::conditional</* if */ std::is_signed<T>::value,
55 /* then */ typename std::make_signed<type_unsigned>::type,
56 /* else */ type_unsigned>::type;
57 };
58
59 // Helper for converting to and from T to an integral type.
60 template <typename T>
61 union ValueConverter {
62 using StorageType = typename MinimumTypeHelper<T, sizeof(T) * kBitsPerByte>::type;
63
ToUnderlyingStorage(T value)64 static constexpr StorageType ToUnderlyingStorage(T value) {
65 ValueConverter converter;
66 converter.value_.val_ = value;
67 return converter.storage_.val_;
68 }
69
FromUnderlyingStorage(StorageType storage)70 static constexpr T FromUnderlyingStorage(StorageType storage) {
71 ValueConverter converter;
72 converter.storage_.val_ = storage;
73 return converter.value_.val_;
74 }
75
76 // Underlying values must be wrapped in separate standard-layout structs.
77 // See below for more details.
78 struct StorageWrapper {
79 StorageType val_;
80 };
81 struct ValueWrapper {
82 T val_;
83 };
84
85 // Safely alias storage_ and value_ together.
86 //
87 // See C++ 9.5.1 [class.union]:
88 // If a standard-layout union contains several standard-layout structs that share a common
89 // initial sequence ... it is permitted to inspect the common initial sequence of any of
90 // standard-layout struct members.
91 StorageWrapper storage_;
92 ValueWrapper value_;
93 #if __cplusplus >= 202000L
94 #error "When upgrading to C++20, remove this error and check that this is OK for all use cases."
95 static_assert(std::is_layout_compatible_v<StorageWrapper, ValueWrapper>);
96 #endif
97
98 // Future work: In theory almost non-standard layout can be supported here,
99 // assuming they don't rely on the address of (this).
100 // We just have to use memcpy since the union-aliasing would not work.
101 };
102
103 // Denotes the beginning of a bit struct.
104 //
105 // This marker is required by the C++ standard in order to
106 // have a "common initial sequence".
107 //
108 // See C++ 9.5.1 [class.union]:
109 // If a standard-layout union contains several standard-layout structs that share a common
110 // initial sequence ... it is permitted to inspect the common initial sequence of any of
111 // standard-layout struct members.
112 template <size_t kSize>
113 struct DefineBitStructSize {
114 private:
115 typename MinimumTypeUnsignedHelper<kSize>::type _;
116 };
117
118 // Check if type "T" has a member called _ in it.
119 template <typename T>
120 struct HasUnderscoreField {
121 private:
122 using TrueT = std::integral_constant<bool, true>::type;
123 using FalseT = std::integral_constant<bool, false>::type;
124
125 template <typename C>
126 static constexpr auto Test(void*) -> decltype(std::declval<C>()._, TrueT{});
127
128 template <typename>
129 static constexpr FalseT Test(...);
130
131 public:
132 static constexpr bool value = decltype(Test<T>(nullptr))::value;
133 };
134
135 // Infer the type of the member of &T::M.
136 template <typename T, typename M>
137 M GetMemberType(M T:: *);
138
139 // Ensure the minimal type storage for 'T' matches its declared BitStructSizeOf.
140 // Nominally used by the BITSTRUCT_DEFINE_END macro.
141 template <typename T>
ValidateBitStructSize()142 static constexpr bool ValidateBitStructSize() {
143 static_assert(std::is_union<T>::value, "T must be union");
144 static_assert(std::is_standard_layout<T>::value, "T must be standard-layout");
145 static_assert(HasUnderscoreField<T>::value, "T must have the _ DefineBitStructSize");
146
147 const size_t kBitStructSizeOf = BitStructSizeOf<T>();
148 static_assert(std::is_same<decltype(GetMemberType(&T::_)),
149 DefineBitStructSize<kBitStructSizeOf>>::value,
150 "T::_ must be a DefineBitStructSize of the same size");
151
152 const size_t kExpectedSize = (BitStructSizeOf<T>() < kBitsPerByte)
153 ? kBitsPerByte
154 : RoundUpToPowerOfTwo(kBitStructSizeOf);
155
156 // Ensure no extra fields were added in between START/END.
157 const size_t kActualSize = sizeof(T) * kBitsPerByte;
158 return kExpectedSize == kActualSize;
159 }
160 } // namespace detail
161 } // namespace art
162
163 #endif // ART_LIBARTBASE_BASE_BIT_STRUCT_DETAIL_H_
164