1 //===--- AlignOf.h - Portable calculation of type alignment -----*- C++ -*-===//
2 //
3 // The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 //
10 // This file defines the AlignOf function that computes alignments for
11 // arbitrary types.
12 //
13 //===----------------------------------------------------------------------===//
14
15 #ifndef LLVM_SUPPORT_ALIGNOF_H
16 #define LLVM_SUPPORT_ALIGNOF_H
17
18 #include "llvm/Support/Compiler.h"
19 #include <cstddef>
20
21 namespace llvm {
22 template <typename T>
23 struct AlignmentCalcImpl {
24 char x;
25 T t;
26 private:
AlignmentCalcImplAlignmentCalcImpl27 AlignmentCalcImpl() {} // Never instantiate.
28 };
29
30 /// AlignOf - A templated class that contains an enum value representing
31 /// the alignment of the template argument. For example,
32 /// AlignOf<int>::Alignment represents the alignment of type "int". The
33 /// alignment calculated is the minimum alignment, and not necessarily
34 /// the "desired" alignment returned by GCC's __alignof__ (for example). Note
35 /// that because the alignment is an enum value, it can be used as a
36 /// compile-time constant (e.g., for template instantiation).
37 template <typename T>
38 struct AlignOf {
39 enum { Alignment =
40 static_cast<unsigned int>(sizeof(AlignmentCalcImpl<T>) - sizeof(T)) };
41
42 enum { Alignment_GreaterEqual_2Bytes = Alignment >= 2 ? 1 : 0 };
43 enum { Alignment_GreaterEqual_4Bytes = Alignment >= 4 ? 1 : 0 };
44 enum { Alignment_GreaterEqual_8Bytes = Alignment >= 8 ? 1 : 0 };
45 enum { Alignment_GreaterEqual_16Bytes = Alignment >= 16 ? 1 : 0 };
46
47 enum { Alignment_LessEqual_2Bytes = Alignment <= 2 ? 1 : 0 };
48 enum { Alignment_LessEqual_4Bytes = Alignment <= 4 ? 1 : 0 };
49 enum { Alignment_LessEqual_8Bytes = Alignment <= 8 ? 1 : 0 };
50 enum { Alignment_LessEqual_16Bytes = Alignment <= 16 ? 1 : 0 };
51 };
52
53 /// alignOf - A templated function that returns the minimum alignment of
54 /// of a type. This provides no extra functionality beyond the AlignOf
55 /// class besides some cosmetic cleanliness. Example usage:
56 /// alignOf<int>() returns the alignment of an int.
57 template <typename T>
alignOf()58 inline unsigned alignOf() { return AlignOf<T>::Alignment; }
59
60 /// \struct AlignedCharArray
61 /// \brief Helper for building an aligned character array type.
62 ///
63 /// This template is used to explicitly build up a collection of aligned
64 /// character array types. We have to build these up using a macro and explicit
65 /// specialization to cope with old versions of MSVC and GCC where only an
66 /// integer literal can be used to specify an alignment constraint. Once built
67 /// up here, we can then begin to indirect between these using normal C++
68 /// template parameters.
69
70 // MSVC requires special handling here.
71 #ifndef _MSC_VER
72
73 #if __has_feature(cxx_alignas)
74 template<std::size_t Alignment, std::size_t Size>
75 struct AlignedCharArray {
76 alignas(Alignment) char buffer[Size];
77 };
78
79 #elif defined(__GNUC__) || defined(__IBM_ATTRIBUTES)
80 /// \brief Create a type with an aligned char buffer.
81 template<std::size_t Alignment, std::size_t Size>
82 struct AlignedCharArray;
83
84 #define LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(x) \
85 template<std::size_t Size> \
86 struct AlignedCharArray<x, Size> { \
87 __attribute__((aligned(x))) char buffer[Size]; \
88 };
89
90 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(1)
91 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(2)
92 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(4)
93 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(8)
94 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(16)
95 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(32)
96 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(64)
97 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(128)
98
99 #undef LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT
100
101 #else
102 # error No supported align as directive.
103 #endif
104
105 #else // _MSC_VER
106
107 /// \brief Create a type with an aligned char buffer.
108 template<std::size_t Alignment, std::size_t Size>
109 struct AlignedCharArray;
110
111 // We provide special variations of this template for the most common
112 // alignments because __declspec(align(...)) doesn't actually work when it is
113 // a member of a by-value function argument in MSVC, even if the alignment
114 // request is something reasonably like 8-byte or 16-byte. Note that we can't
115 // even include the declspec with the union that forces the alignment because
116 // MSVC warns on the existence of the declspec despite the union member forcing
117 // proper alignment.
118
119 template<std::size_t Size>
120 struct AlignedCharArray<1, Size> {
121 union {
122 char aligned;
123 char buffer[Size];
124 };
125 };
126
127 template<std::size_t Size>
128 struct AlignedCharArray<2, Size> {
129 union {
130 short aligned;
131 char buffer[Size];
132 };
133 };
134
135 template<std::size_t Size>
136 struct AlignedCharArray<4, Size> {
137 union {
138 int aligned;
139 char buffer[Size];
140 };
141 };
142
143 template<std::size_t Size>
144 struct AlignedCharArray<8, Size> {
145 union {
146 double aligned;
147 char buffer[Size];
148 };
149 };
150
151
152 // The rest of these are provided with a __declspec(align(...)) and we simply
153 // can't pass them by-value as function arguments on MSVC.
154
155 #define LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(x) \
156 template<std::size_t Size> \
157 struct AlignedCharArray<x, Size> { \
158 __declspec(align(x)) char buffer[Size]; \
159 };
160
161 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(16)
162 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(32)
163 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(64)
164 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(128)
165
166 #undef LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT
167
168 #endif // _MSC_VER
169
170 namespace detail {
171 template <typename T1,
172 typename T2 = char, typename T3 = char, typename T4 = char,
173 typename T5 = char, typename T6 = char, typename T7 = char>
174 class AlignerImpl {
175 T1 t1; T2 t2; T3 t3; T4 t4; T5 t5; T6 t6; T7 t7;
176
177 AlignerImpl(); // Never defined or instantiated.
178 };
179
180 template <typename T1,
181 typename T2 = char, typename T3 = char, typename T4 = char,
182 typename T5 = char, typename T6 = char, typename T7 = char>
183 union SizerImpl {
184 char arr1[sizeof(T1)], arr2[sizeof(T2)], arr3[sizeof(T3)], arr4[sizeof(T4)],
185 arr5[sizeof(T5)], arr6[sizeof(T6)], arr7[sizeof(T7)];
186 };
187 } // end namespace detail
188
189 /// \brief This union template exposes a suitably aligned and sized character
190 /// array member which can hold elements of any of up to four types.
191 ///
192 /// These types may be arrays, structs, or any other types. The goal is to
193 /// expose a char array buffer member which can be used as suitable storage for
194 /// a placement new of any of these types. Support for more than seven types can
195 /// be added at the cost of more boiler plate.
196 template <typename T1,
197 typename T2 = char, typename T3 = char, typename T4 = char,
198 typename T5 = char, typename T6 = char, typename T7 = char>
199 struct AlignedCharArrayUnion : llvm::AlignedCharArray<
200 AlignOf<detail::AlignerImpl<T1, T2, T3, T4, T5, T6, T7> >::Alignment,
201 sizeof(detail::SizerImpl<T1, T2, T3, T4, T5, T6, T7>)> {
202 };
203 } // end namespace llvm
204 #endif
205