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 21namespace llvm { 22 23template <typename T> 24struct AlignmentCalcImpl { 25 char x; 26 T t; 27private: 28 AlignmentCalcImpl() {} // Never instantiate. 29}; 30 31/// AlignOf - A templated class that contains an enum value representing 32/// the alignment of the template argument. For example, 33/// AlignOf<int>::Alignment represents the alignment of type "int". The 34/// alignment calculated is the minimum alignment, and not necessarily 35/// the "desired" alignment returned by GCC's __alignof__ (for example). Note 36/// that because the alignment is an enum value, it can be used as a 37/// compile-time constant (e.g., for template instantiation). 38template <typename T> 39struct AlignOf { 40 enum { Alignment = 41 static_cast<unsigned int>(sizeof(AlignmentCalcImpl<T>) - sizeof(T)) }; 42 43 enum { Alignment_GreaterEqual_2Bytes = Alignment >= 2 ? 1 : 0 }; 44 enum { Alignment_GreaterEqual_4Bytes = Alignment >= 4 ? 1 : 0 }; 45 enum { Alignment_GreaterEqual_8Bytes = Alignment >= 8 ? 1 : 0 }; 46 enum { Alignment_GreaterEqual_16Bytes = Alignment >= 16 ? 1 : 0 }; 47 48 enum { Alignment_LessEqual_2Bytes = Alignment <= 2 ? 1 : 0 }; 49 enum { Alignment_LessEqual_4Bytes = Alignment <= 4 ? 1 : 0 }; 50 enum { Alignment_LessEqual_8Bytes = Alignment <= 8 ? 1 : 0 }; 51 enum { Alignment_LessEqual_16Bytes = Alignment <= 16 ? 1 : 0 }; 52 53}; 54 55/// alignOf - A templated function that returns the minimum alignment of 56/// of a type. This provides no extra functionality beyond the AlignOf 57/// class besides some cosmetic cleanliness. Example usage: 58/// alignOf<int>() returns the alignment of an int. 59template <typename T> 60inline unsigned alignOf() { return AlignOf<T>::Alignment; } 61 62 63/// \brief Helper for building an aligned character array type. 64/// 65/// This template is used to explicitly build up a collection of aligned 66/// character types. We have to build these up using a macro and explicit 67/// specialization to cope with old versions of MSVC and GCC where only an 68/// integer literal can be used to specify an alignment constraint. Once built 69/// up here, we can then begin to indirect between these using normal C++ 70/// template parameters. 71template <size_t Alignment> struct AlignedCharArrayImpl; 72 73// MSVC requires special handling here. 74#ifndef _MSC_VER 75 76#if __has_feature(cxx_alignas) 77#define LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(x) \ 78 template <> struct AlignedCharArrayImpl<x> { \ 79 char alignas(x) aligned; \ 80 } 81#elif defined(__GNUC__) 82#define LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(x) \ 83 template <> struct AlignedCharArrayImpl<x> { \ 84 char aligned __attribute__((aligned(x))); \ 85 } 86#else 87# error No supported align as directive. 88#endif 89 90LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(1); 91LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(2); 92LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(4); 93LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(8); 94LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(16); 95LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(32); 96LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(64); 97LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(128); 98LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(512); 99LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(1024); 100LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(2048); 101LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(4096); 102LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(8192); 103 104#undef LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT 105 106#else // _MSC_VER 107 108// We provide special variations of this template for the most common 109// alignments because __declspec(align(...)) doesn't actually work when it is 110// a member of a by-value function argument in MSVC, even if the alignment 111// request is something reasonably like 8-byte or 16-byte. 112template <> struct AlignedCharArrayImpl<1> { char aligned; }; 113template <> struct AlignedCharArrayImpl<2> { short aligned; }; 114template <> struct AlignedCharArrayImpl<4> { int aligned; }; 115template <> struct AlignedCharArrayImpl<8> { double aligned; }; 116 117#define LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(x) \ 118 template <> struct AlignedCharArrayImpl<x> { \ 119 __declspec(align(x)) char aligned; \ 120 } 121LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(16); 122LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(32); 123LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(64); 124LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(128); 125LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(512); 126LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(1024); 127LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(2048); 128LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(4096); 129LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT(8192); 130// Any larger and MSVC complains. 131#undef LLVM_ALIGNEDCHARARRAY_TEMPLATE_ALIGNMENT 132 133#endif // _MSC_VER 134 135/// \brief This union template exposes a suitably aligned and sized character 136/// array member which can hold elements of any of up to four types. 137/// 138/// These types may be arrays, structs, or any other types. The goal is to 139/// produce a union type containing a character array which, when used, forms 140/// storage suitable to placement new any of these types over. Support for more 141/// than four types can be added at the cost of more boiler plate. 142template <typename T1, 143 typename T2 = char, typename T3 = char, typename T4 = char> 144union AlignedCharArrayUnion { 145private: 146 class AlignerImpl { 147 T1 t1; T2 t2; T3 t3; T4 t4; 148 149 AlignerImpl(); // Never defined or instantiated. 150 }; 151 union SizerImpl { 152 char arr1[sizeof(T1)], arr2[sizeof(T2)], arr3[sizeof(T3)], arr4[sizeof(T4)]; 153 }; 154 155public: 156 /// \brief The character array buffer for use by clients. 157 /// 158 /// No other member of this union should be referenced. The exist purely to 159 /// constrain the layout of this character array. 160 char buffer[sizeof(SizerImpl)]; 161 162private: 163 // Tests seem to indicate that both Clang and GCC will properly register the 164 // alignment of a struct containing an aligned member, and this alignment 165 // should carry over to the character array in the union. 166 llvm::AlignedCharArrayImpl<AlignOf<AlignerImpl>::Alignment> nonce_member; 167}; 168 169} // end namespace llvm 170#endif 171