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 #if defined(_MSC_VER)
26 // Disables "structure was padded due to __declspec(align())" warnings that are
27 // generated by any class using AlignOf<T> with a manually specified alignment.
28 // Although the warning is disabled in the LLVM project we need this pragma
29 // as AlignOf.h is a published support header that's available for use
30 // out-of-tree, and we would like that to compile cleanly at /W4.
31 #pragma warning(suppress : 4324)
32 #endif
33   T t;
34 private:
AlignmentCalcImplAlignmentCalcImpl35   AlignmentCalcImpl() {} // Never instantiate.
36 };
37 
38 /// AlignOf - A templated class that contains an enum value representing
39 ///  the alignment of the template argument.  For example,
40 ///  AlignOf<int>::Alignment represents the alignment of type "int".  The
41 ///  alignment calculated is the minimum alignment, and not necessarily
42 ///  the "desired" alignment returned by GCC's __alignof__ (for example).  Note
43 ///  that because the alignment is an enum value, it can be used as a
44 ///  compile-time constant (e.g., for template instantiation).
45 template <typename T>
46 struct AlignOf {
47 #ifndef _MSC_VER
48   // Avoid warnings from GCC like:
49   //   comparison between 'enum llvm::AlignOf<X>::<anonymous>' and 'enum
50   //   llvm::AlignOf<Y>::<anonymous>' [-Wenum-compare]
51   // by using constexpr instead of enum.
52   // (except on MSVC, since it doesn't support constexpr yet).
53   static constexpr unsigned Alignment =
54       static_cast<unsigned int>(sizeof(AlignmentCalcImpl<T>) - sizeof(T));
55 #else
56   enum { Alignment =
57          static_cast<unsigned int>(sizeof(AlignmentCalcImpl<T>) - sizeof(T)) };
58 #endif
59   enum { Alignment_GreaterEqual_2Bytes = Alignment >= 2 ? 1 : 0 };
60   enum { Alignment_GreaterEqual_4Bytes = Alignment >= 4 ? 1 : 0 };
61   enum { Alignment_GreaterEqual_8Bytes = Alignment >= 8 ? 1 : 0 };
62   enum { Alignment_GreaterEqual_16Bytes = Alignment >= 16 ? 1 : 0 };
63 
64   enum { Alignment_LessEqual_2Bytes = Alignment <= 2 ? 1 : 0 };
65   enum { Alignment_LessEqual_4Bytes = Alignment <= 4 ? 1 : 0 };
66   enum { Alignment_LessEqual_8Bytes = Alignment <= 8 ? 1 : 0 };
67   enum { Alignment_LessEqual_16Bytes = Alignment <= 16 ? 1 : 0 };
68 };
69 
70 #ifndef _MSC_VER
71 template <typename T> constexpr unsigned AlignOf<T>::Alignment;
72 #endif
73 
74 /// alignOf - A templated function that returns the minimum alignment of
75 ///  of a type.  This provides no extra functionality beyond the AlignOf
76 ///  class besides some cosmetic cleanliness.  Example usage:
77 ///  alignOf<int>() returns the alignment of an int.
78 template <typename T>
alignOf()79 inline unsigned alignOf() { return AlignOf<T>::Alignment; }
80 
81 /// \struct AlignedCharArray
82 /// \brief Helper for building an aligned character array type.
83 ///
84 /// This template is used to explicitly build up a collection of aligned
85 /// character array types. We have to build these up using a macro and explicit
86 /// specialization to cope with old versions of MSVC and GCC where only an
87 /// integer literal can be used to specify an alignment constraint. Once built
88 /// up here, we can then begin to indirect between these using normal C++
89 /// template parameters.
90 
91 // MSVC requires special handling here.
92 #ifndef _MSC_VER
93 
94 #if __has_feature(cxx_alignas)
95 template<std::size_t Alignment, std::size_t Size>
96 struct AlignedCharArray {
97   alignas(Alignment) char buffer[Size];
98 };
99 
100 #elif defined(__GNUC__) || defined(__IBM_ATTRIBUTES)
101 /// \brief Create a type with an aligned char buffer.
102 template<std::size_t Alignment, std::size_t Size>
103 struct AlignedCharArray;
104 
105 #define LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(x) \
106   template<std::size_t Size> \
107   struct AlignedCharArray<x, Size> { \
108     __attribute__((aligned(x))) char buffer[Size]; \
109   };
110 
111 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(1)
112 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(2)
113 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(4)
114 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(8)
115 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(16)
116 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(32)
117 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(64)
118 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(128)
119 
120 #undef LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT
121 
122 #else
123 # error No supported align as directive.
124 #endif
125 
126 #else // _MSC_VER
127 
128 /// \brief Create a type with an aligned char buffer.
129 template<std::size_t Alignment, std::size_t Size>
130 struct AlignedCharArray;
131 
132 // We provide special variations of this template for the most common
133 // alignments because __declspec(align(...)) doesn't actually work when it is
134 // a member of a by-value function argument in MSVC, even if the alignment
135 // request is something reasonably like 8-byte or 16-byte. Note that we can't
136 // even include the declspec with the union that forces the alignment because
137 // MSVC warns on the existence of the declspec despite the union member forcing
138 // proper alignment.
139 
140 template<std::size_t Size>
141 struct AlignedCharArray<1, Size> {
142   union {
143     char aligned;
144     char buffer[Size];
145   };
146 };
147 
148 template<std::size_t Size>
149 struct AlignedCharArray<2, Size> {
150   union {
151     short aligned;
152     char buffer[Size];
153   };
154 };
155 
156 template<std::size_t Size>
157 struct AlignedCharArray<4, Size> {
158   union {
159     int aligned;
160     char buffer[Size];
161   };
162 };
163 
164 template<std::size_t Size>
165 struct AlignedCharArray<8, Size> {
166   union {
167     double aligned;
168     char buffer[Size];
169   };
170 };
171 
172 
173 // The rest of these are provided with a __declspec(align(...)) and we simply
174 // can't pass them by-value as function arguments on MSVC.
175 
176 #define LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(x) \
177   template<std::size_t Size> \
178   struct AlignedCharArray<x, Size> { \
179     __declspec(align(x)) char buffer[Size]; \
180   };
181 
182 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(16)
183 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(32)
184 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(64)
185 LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(128)
186 
187 #undef LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT
188 
189 #endif // _MSC_VER
190 
191 namespace detail {
192 template <typename T1,
193           typename T2 = char, typename T3 = char, typename T4 = char,
194           typename T5 = char, typename T6 = char, typename T7 = char,
195           typename T8 = char, typename T9 = char, typename T10 = char>
196 class AlignerImpl {
197   T1 t1; T2 t2; T3 t3; T4 t4; T5 t5; T6 t6; T7 t7; T8 t8; T9 t9; T10 t10;
198 
199   AlignerImpl(); // Never defined or instantiated.
200 };
201 
202 template <typename T1,
203           typename T2 = char, typename T3 = char, typename T4 = char,
204           typename T5 = char, typename T6 = char, typename T7 = char,
205           typename T8 = char, typename T9 = char, typename T10 = char>
206 union SizerImpl {
207   char arr1[sizeof(T1)], arr2[sizeof(T2)], arr3[sizeof(T3)], arr4[sizeof(T4)],
208        arr5[sizeof(T5)], arr6[sizeof(T6)], arr7[sizeof(T7)], arr8[sizeof(T8)],
209        arr9[sizeof(T9)], arr10[sizeof(T10)];
210 };
211 } // end namespace detail
212 
213 /// \brief This union template exposes a suitably aligned and sized character
214 /// array member which can hold elements of any of up to ten types.
215 ///
216 /// These types may be arrays, structs, or any other types. The goal is to
217 /// expose a char array buffer member which can be used as suitable storage for
218 /// a placement new of any of these types. Support for more than ten types can
219 /// be added at the cost of more boilerplate.
220 template <typename T1,
221           typename T2 = char, typename T3 = char, typename T4 = char,
222           typename T5 = char, typename T6 = char, typename T7 = char,
223           typename T8 = char, typename T9 = char, typename T10 = char>
224 struct AlignedCharArrayUnion : llvm::AlignedCharArray<
225     AlignOf<detail::AlignerImpl<T1, T2, T3, T4, T5,
226                                 T6, T7, T8, T9, T10> >::Alignment,
227     sizeof(detail::SizerImpl<T1, T2, T3, T4, T5,
228                              T6, T7, T8, T9, T10>)> {
229 };
230 } // end namespace llvm
231 #endif
232