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 // Denotes the beginning of a bit struct.
60 //
61 // This marker is required by the C++ standard in order to
62 // have a "common initial sequence".
63 //
64 // See C++ 9.5.1 [class.union]:
65 // If a standard-layout union contains several standard-layout structs that share a common
66 // initial sequence ... it is permitted to inspect the common initial sequence of any of
67 // standard-layout struct members.
68 template <size_t kSize>
69 struct DefineBitStructSize {
70 private:
71 typename MinimumTypeUnsignedHelper<kSize>::type _;
72 };
73
74 // Check if type "T" has a member called _ in it.
75 template <typename T>
76 struct HasUnderscoreField {
77 private:
78 using TrueT = std::integral_constant<bool, true>::type;
79 using FalseT = std::integral_constant<bool, false>::type;
80
81 template <typename C>
82 static constexpr auto Test(void*) -> decltype(std::declval<C>()._, TrueT{});
83
84 template <typename>
85 static constexpr FalseT Test(...);
86
87 public:
88 static constexpr bool value = decltype(Test<T>(nullptr))::value;
89 };
90
91 // Infer the type of the member of &T::M.
92 template <typename T, typename M>
93 M GetMemberType(M T:: *);
94
95 // Ensure the minimal type storage for 'T' matches its declared BitStructSizeOf.
96 // Nominally used by the BITSTRUCT_DEFINE_END macro.
97 template <typename T>
ValidateBitStructSize()98 static constexpr bool ValidateBitStructSize() {
99 static_assert(std::is_union<T>::value, "T must be union");
100 static_assert(std::is_standard_layout<T>::value, "T must be standard-layout");
101 static_assert(HasUnderscoreField<T>::value, "T must have the _ DefineBitStructSize");
102
103 const size_t kBitStructSizeOf = BitStructSizeOf<T>();
104 static_assert(std::is_same<decltype(GetMemberType(&T::_)),
105 DefineBitStructSize<kBitStructSizeOf>>::value,
106 "T::_ must be a DefineBitStructSize of the same size");
107
108 const size_t kExpectedSize = (BitStructSizeOf<T>() < kBitsPerByte)
109 ? kBitsPerByte
110 : RoundUpToPowerOfTwo(kBitStructSizeOf);
111
112 // Ensure no extra fields were added in between START/END.
113 const size_t kActualSize = sizeof(T) * kBitsPerByte;
114 return kExpectedSize == kActualSize;
115 }
116 } // namespace detail
117 } // namespace art
118
119 #endif // ART_LIBARTBASE_BASE_BIT_STRUCT_DETAIL_H_
120