1 //===--- TargetCXXABI.h - C++ ABI Target Configuration ----------*- 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 /// \file 11 /// \brief Defines the TargetCXXABI class, which abstracts details of the 12 /// C++ ABI that we're targeting. 13 /// 14 //===----------------------------------------------------------------------===// 15 16 #ifndef LLVM_CLANG_TARGETCXXABI_H 17 #define LLVM_CLANG_TARGETCXXABI_H 18 19 #include "llvm/ADT/Triple.h" 20 #include "llvm/Support/ErrorHandling.h" 21 22 namespace clang { 23 24 /// \brief The basic abstraction for the target C++ ABI. 25 class TargetCXXABI { 26 public: 27 /// \brief The basic C++ ABI kind. 28 enum Kind { 29 /// The generic Itanium ABI is the standard ABI of most open-source 30 /// and Unix-like platforms. It is the primary ABI targeted by 31 /// many compilers, including Clang and GCC. 32 /// 33 /// It is documented here: 34 /// http://www.codesourcery.com/public/cxx-abi/ 35 GenericItanium, 36 37 /// The generic ARM ABI is a modified version of the Itanium ABI 38 /// proposed by ARM for use on ARM-based platforms. 39 /// 40 /// These changes include: 41 /// - the representation of member function pointers is adjusted 42 /// to not conflict with the 'thumb' bit of ARM function pointers; 43 /// - constructors and destructors return 'this'; 44 /// - guard variables are smaller; 45 /// - inline functions are never key functions; 46 /// - array cookies have a slightly different layout; 47 /// - additional convenience functions are specified; 48 /// - and more! 49 /// 50 /// It is documented here: 51 /// http://infocenter.arm.com 52 /// /help/topic/com.arm.doc.ihi0041c/IHI0041C_cppabi.pdf 53 GenericARM, 54 55 /// The iOS ABI is a partial implementation of the ARM ABI. 56 /// Several of the features of the ARM ABI were not fully implemented 57 /// in the compilers that iOS was launched with. 58 /// 59 /// Essentially, the iOS ABI includes the ARM changes to: 60 /// - member function pointers, 61 /// - guard variables, 62 /// - array cookies, and 63 /// - constructor/destructor signatures. 64 iOS, 65 66 /// The generic AArch64 ABI is also a modified version of the Itanium ABI, 67 /// but it has fewer divergences than the 32-bit ARM ABI. 68 /// 69 /// The relevant changes from the generic ABI in this case are: 70 /// - representation of member function pointers adjusted as in ARM. 71 /// - guard variables are smaller. 72 GenericAArch64, 73 74 /// The Microsoft ABI is the ABI used by Microsoft Visual Studio (and 75 /// compatible compilers). 76 /// 77 /// FIXME: should this be split into Win32 and Win64 variants? 78 /// 79 /// Only scattered and incomplete official documentation exists. 80 Microsoft 81 }; 82 83 private: 84 // Right now, this class is passed around as a cheap value type. 85 // If you add more members, especially non-POD members, please 86 // audit the users to pass it by reference instead. 87 Kind TheKind; 88 89 public: 90 /// A bogus initialization of the platform ABI. 91 TargetCXXABI() : TheKind(GenericItanium) {} 92 93 TargetCXXABI(Kind kind) : TheKind(kind) {} 94 95 void set(Kind kind) { 96 TheKind = kind; 97 } 98 99 Kind getKind() const { return TheKind; } 100 101 /// \brief Does this ABI generally fall into the Itanium family of ABIs? 102 bool isItaniumFamily() const { 103 switch (getKind()) { 104 case GenericAArch64: 105 case GenericItanium: 106 case GenericARM: 107 case iOS: 108 return true; 109 110 case Microsoft: 111 return false; 112 } 113 llvm_unreachable("bad ABI kind"); 114 } 115 116 /// \brief Is this ABI an MSVC-compatible ABI? 117 bool isMicrosoft() const { 118 switch (getKind()) { 119 case GenericAArch64: 120 case GenericItanium: 121 case GenericARM: 122 case iOS: 123 return false; 124 125 case Microsoft: 126 return true; 127 } 128 llvm_unreachable("bad ABI kind"); 129 } 130 131 /// \brief Is the default C++ member function calling convention 132 /// the same as the default calling convention? 133 bool isMemberFunctionCCDefault() const { 134 // Right now, this is always false for Microsoft. 135 return !isMicrosoft(); 136 } 137 138 /// Are temporary objects passed by value to a call destroyed by the callee? 139 /// This is a fundamental language change, since it implies that objects 140 /// passed by value do *not* live to the end of the full expression. 141 /// Temporaries passed to a function taking a const reference live to the end 142 /// of the full expression as usual. Both the caller and the callee must 143 /// have access to the destructor, while only the caller needs the 144 /// destructor if this is false. 145 bool isArgumentDestroyedByCallee() const { 146 return isMicrosoft(); 147 } 148 149 /// \brief Does this ABI have different entrypoints for complete-object 150 /// and base-subobject constructors? 151 bool hasConstructorVariants() const { 152 return isItaniumFamily(); 153 } 154 155 /// \brief Does this ABI allow virtual bases to be primary base classes? 156 bool hasPrimaryVBases() const { 157 return isItaniumFamily(); 158 } 159 160 /// \brief Does this ABI use key functions? If so, class data such as the 161 /// vtable is emitted with strong linkage by the TU containing the key 162 /// function. 163 bool hasKeyFunctions() const { 164 return isItaniumFamily(); 165 } 166 167 /// \brief Can an out-of-line inline function serve as a key function? 168 /// 169 /// This flag is only useful in ABIs where type data (for example, 170 /// v-tables and type_info objects) are emitted only after processing 171 /// the definition of a special "key" virtual function. (This is safe 172 /// because the ODR requires that every virtual function be defined 173 /// somewhere in a program.) This usually permits such data to be 174 /// emitted in only a single object file, as opposed to redundantly 175 /// in every object file that requires it. 176 /// 177 /// One simple and common definition of "key function" is the first 178 /// virtual function in the class definition which is not defined there. 179 /// This rule works very well when that function has a non-inline 180 /// definition in some non-header file. Unfortunately, when that 181 /// function is defined inline, this rule requires the type data 182 /// to be emitted weakly, as if there were no key function. 183 /// 184 /// The ARM ABI observes that the ODR provides an additional guarantee: 185 /// a virtual function is always ODR-used, so if it is defined inline, 186 /// that definition must appear in every translation unit that defines 187 /// the class. Therefore, there is no reason to allow such functions 188 /// to serve as key functions. 189 /// 190 /// Because this changes the rules for emitting type data, 191 /// it can cause type data to be emitted with both weak and strong 192 /// linkage, which is not allowed on all platforms. Therefore, 193 /// exploiting this observation requires an ABI break and cannot be 194 /// done on a generic Itanium platform. 195 bool canKeyFunctionBeInline() const { 196 switch (getKind()) { 197 case GenericARM: 198 return false; 199 200 case GenericAArch64: 201 case GenericItanium: 202 case iOS: // old iOS compilers did not follow this rule 203 case Microsoft: 204 return true; 205 } 206 llvm_unreachable("bad ABI kind"); 207 } 208 209 /// When is record layout allowed to allocate objects in the tail 210 /// padding of a base class? 211 /// 212 /// This decision cannot be changed without breaking platform ABI 213 /// compatibility, and yet it is tied to language guarantees which 214 /// the committee has so far seen fit to strengthen no less than 215 /// three separate times: 216 /// - originally, there were no restrictions at all; 217 /// - C++98 declared that objects could not be allocated in the 218 /// tail padding of a POD type; 219 /// - C++03 extended the definition of POD to include classes 220 /// containing member pointers; and 221 /// - C++11 greatly broadened the definition of POD to include 222 /// all trivial standard-layout classes. 223 /// Each of these changes technically took several existing 224 /// platforms and made them permanently non-conformant. 225 enum TailPaddingUseRules { 226 /// The tail-padding of a base class is always theoretically 227 /// available, even if it's POD. This is not strictly conforming 228 /// in any language mode. 229 AlwaysUseTailPadding, 230 231 /// Only allocate objects in the tail padding of a base class if 232 /// the base class is not POD according to the rules of C++ TR1. 233 /// This is non strictly conforming in C++11 mode. 234 UseTailPaddingUnlessPOD03, 235 236 /// Only allocate objects in the tail padding of a base class if 237 /// the base class is not POD according to the rules of C++11. 238 UseTailPaddingUnlessPOD11 239 }; 240 TailPaddingUseRules getTailPaddingUseRules() const { 241 switch (getKind()) { 242 // To preserve binary compatibility, the generic Itanium ABI has 243 // permanently locked the definition of POD to the rules of C++ TR1, 244 // and that trickles down to all the derived ABIs. 245 case GenericItanium: 246 case GenericAArch64: 247 case GenericARM: 248 case iOS: 249 return UseTailPaddingUnlessPOD03; 250 251 // MSVC always allocates fields in the tail-padding of a base class 252 // subobject, even if they're POD. 253 case Microsoft: 254 return AlwaysUseTailPadding; 255 } 256 llvm_unreachable("bad ABI kind"); 257 } 258 259 /// Try to parse an ABI name, returning false on error. 260 bool tryParse(llvm::StringRef name); 261 262 friend bool operator==(const TargetCXXABI &left, const TargetCXXABI &right) { 263 return left.getKind() == right.getKind(); 264 } 265 266 friend bool operator!=(const TargetCXXABI &left, const TargetCXXABI &right) { 267 return !(left == right); 268 } 269 }; 270 271 } // end namespace clang 272 273 #endif 274
