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 iOS 64-bit ABI is follows ARM's published 64-bit ABI more 67 /// closely, but we don't guarantee to follow it perfectly. 68 /// 69 /// It is documented here: 70 /// http://infocenter.arm.com 71 /// /help/topic/com.arm.doc.ihi0059a/IHI0059A_cppabi64.pdf 72 iOS64, 73 74 /// The generic AArch64 ABI is also a modified version of the Itanium ABI, 75 /// but it has fewer divergences than the 32-bit ARM ABI. 76 /// 77 /// The relevant changes from the generic ABI in this case are: 78 /// - representation of member function pointers adjusted as in ARM. 79 /// - guard variables are smaller. 80 GenericAArch64, 81 82 /// The Microsoft ABI is the ABI used by Microsoft Visual Studio (and 83 /// compatible compilers). 84 /// 85 /// FIXME: should this be split into Win32 and Win64 variants? 86 /// 87 /// Only scattered and incomplete official documentation exists. 88 Microsoft 89 }; 90 91 private: 92 // Right now, this class is passed around as a cheap value type. 93 // If you add more members, especially non-POD members, please 94 // audit the users to pass it by reference instead. 95 Kind TheKind; 96 97 public: 98 /// A bogus initialization of the platform ABI. TargetCXXABI()99 TargetCXXABI() : TheKind(GenericItanium) {} 100 TargetCXXABI(Kind kind)101 TargetCXXABI(Kind kind) : TheKind(kind) {} 102 set(Kind kind)103 void set(Kind kind) { 104 TheKind = kind; 105 } 106 getKind()107 Kind getKind() const { return TheKind; } 108 109 /// \brief Does this ABI generally fall into the Itanium family of ABIs? isItaniumFamily()110 bool isItaniumFamily() const { 111 switch (getKind()) { 112 case GenericAArch64: 113 case GenericItanium: 114 case GenericARM: 115 case iOS: 116 case iOS64: 117 return true; 118 119 case Microsoft: 120 return false; 121 } 122 llvm_unreachable("bad ABI kind"); 123 } 124 125 /// \brief Is this ABI an MSVC-compatible ABI? isMicrosoft()126 bool isMicrosoft() const { 127 switch (getKind()) { 128 case GenericAArch64: 129 case GenericItanium: 130 case GenericARM: 131 case iOS: 132 case iOS64: 133 return false; 134 135 case Microsoft: 136 return true; 137 } 138 llvm_unreachable("bad ABI kind"); 139 } 140 141 /// \brief Is the default C++ member function calling convention 142 /// the same as the default calling convention? isMemberFunctionCCDefault()143 bool isMemberFunctionCCDefault() const { 144 // Right now, this is always false for Microsoft. 145 return !isMicrosoft(); 146 } 147 148 /// Are arguments to a call destroyed left to right in the callee? 149 /// This is a fundamental language change, since it implies that objects 150 /// passed by value do *not* live to the end of the full expression. 151 /// Temporaries passed to a function taking a const reference live to the end 152 /// of the full expression as usual. Both the caller and the callee must 153 /// have access to the destructor, while only the caller needs the 154 /// destructor if this is false. areArgsDestroyedLeftToRightInCallee()155 bool areArgsDestroyedLeftToRightInCallee() const { 156 return isMicrosoft(); 157 } 158 159 /// \brief Does this ABI have different entrypoints for complete-object 160 /// and base-subobject constructors? hasConstructorVariants()161 bool hasConstructorVariants() const { 162 return isItaniumFamily(); 163 } 164 165 /// \brief Does this ABI allow virtual bases to be primary base classes? hasPrimaryVBases()166 bool hasPrimaryVBases() const { 167 return isItaniumFamily(); 168 } 169 170 /// \brief Does this ABI use key functions? If so, class data such as the 171 /// vtable is emitted with strong linkage by the TU containing the key 172 /// function. hasKeyFunctions()173 bool hasKeyFunctions() const { 174 return isItaniumFamily(); 175 } 176 177 /// \brief Can an out-of-line inline function serve as a key function? 178 /// 179 /// This flag is only useful in ABIs where type data (for example, 180 /// v-tables and type_info objects) are emitted only after processing 181 /// the definition of a special "key" virtual function. (This is safe 182 /// because the ODR requires that every virtual function be defined 183 /// somewhere in a program.) This usually permits such data to be 184 /// emitted in only a single object file, as opposed to redundantly 185 /// in every object file that requires it. 186 /// 187 /// One simple and common definition of "key function" is the first 188 /// virtual function in the class definition which is not defined there. 189 /// This rule works very well when that function has a non-inline 190 /// definition in some non-header file. Unfortunately, when that 191 /// function is defined inline, this rule requires the type data 192 /// to be emitted weakly, as if there were no key function. 193 /// 194 /// The ARM ABI observes that the ODR provides an additional guarantee: 195 /// a virtual function is always ODR-used, so if it is defined inline, 196 /// that definition must appear in every translation unit that defines 197 /// the class. Therefore, there is no reason to allow such functions 198 /// to serve as key functions. 199 /// 200 /// Because this changes the rules for emitting type data, 201 /// it can cause type data to be emitted with both weak and strong 202 /// linkage, which is not allowed on all platforms. Therefore, 203 /// exploiting this observation requires an ABI break and cannot be 204 /// done on a generic Itanium platform. canKeyFunctionBeInline()205 bool canKeyFunctionBeInline() const { 206 switch (getKind()) { 207 case GenericARM: 208 case iOS64: 209 return false; 210 211 case GenericAArch64: 212 case GenericItanium: 213 case iOS: // old iOS compilers did not follow this rule 214 case Microsoft: 215 return true; 216 } 217 llvm_unreachable("bad ABI kind"); 218 } 219 220 /// When is record layout allowed to allocate objects in the tail 221 /// padding of a base class? 222 /// 223 /// This decision cannot be changed without breaking platform ABI 224 /// compatibility, and yet it is tied to language guarantees which 225 /// the committee has so far seen fit to strengthen no less than 226 /// three separate times: 227 /// - originally, there were no restrictions at all; 228 /// - C++98 declared that objects could not be allocated in the 229 /// tail padding of a POD type; 230 /// - C++03 extended the definition of POD to include classes 231 /// containing member pointers; and 232 /// - C++11 greatly broadened the definition of POD to include 233 /// all trivial standard-layout classes. 234 /// Each of these changes technically took several existing 235 /// platforms and made them permanently non-conformant. 236 enum TailPaddingUseRules { 237 /// The tail-padding of a base class is always theoretically 238 /// available, even if it's POD. This is not strictly conforming 239 /// in any language mode. 240 AlwaysUseTailPadding, 241 242 /// Only allocate objects in the tail padding of a base class if 243 /// the base class is not POD according to the rules of C++ TR1. 244 /// This is non-strictly conforming in C++11 mode. 245 UseTailPaddingUnlessPOD03, 246 247 /// Only allocate objects in the tail padding of a base class if 248 /// the base class is not POD according to the rules of C++11. 249 UseTailPaddingUnlessPOD11 250 }; getTailPaddingUseRules()251 TailPaddingUseRules getTailPaddingUseRules() const { 252 switch (getKind()) { 253 // To preserve binary compatibility, the generic Itanium ABI has 254 // permanently locked the definition of POD to the rules of C++ TR1, 255 // and that trickles down to all the derived ABIs. 256 case GenericItanium: 257 case GenericAArch64: 258 case GenericARM: 259 case iOS: 260 return UseTailPaddingUnlessPOD03; 261 262 // iOS on ARM64 uses the C++11 POD rules. It does not honor the 263 // Itanium exception about classes with over-large bitfields. 264 case iOS64: 265 return UseTailPaddingUnlessPOD11; 266 267 // MSVC always allocates fields in the tail-padding of a base class 268 // subobject, even if they're POD. 269 case Microsoft: 270 return AlwaysUseTailPadding; 271 } 272 llvm_unreachable("bad ABI kind"); 273 } 274 275 /// Try to parse an ABI name, returning false on error. 276 bool tryParse(llvm::StringRef name); 277 278 friend bool operator==(const TargetCXXABI &left, const TargetCXXABI &right) { 279 return left.getKind() == right.getKind(); 280 } 281 282 friend bool operator!=(const TargetCXXABI &left, const TargetCXXABI &right) { 283 return !(left == right); 284 } 285 }; 286 287 } // end namespace clang 288 289 #endif 290