1 //===--- CGCXX.cpp - Emit LLVM Code for declarations ----------------------===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // This contains code dealing with C++ code generation. 10 // 11 //===----------------------------------------------------------------------===// 12 13 // We might split this into multiple files if it gets too unwieldy 14 15 #include "CGCXXABI.h" 16 #include "CodeGenFunction.h" 17 #include "CodeGenModule.h" 18 #include "clang/AST/ASTContext.h" 19 #include "clang/AST/Attr.h" 20 #include "clang/AST/Decl.h" 21 #include "clang/AST/DeclCXX.h" 22 #include "clang/AST/DeclObjC.h" 23 #include "clang/AST/Mangle.h" 24 #include "clang/AST/RecordLayout.h" 25 #include "clang/AST/StmtCXX.h" 26 #include "clang/Basic/CodeGenOptions.h" 27 #include "llvm/ADT/StringExtras.h" 28 using namespace clang; 29 using namespace CodeGen; 30 31 32 /// Try to emit a base destructor as an alias to its primary 33 /// base-class destructor. 34 bool CodeGenModule::TryEmitBaseDestructorAsAlias(const CXXDestructorDecl *D) { 35 if (!getCodeGenOpts().CXXCtorDtorAliases) 36 return true; 37 38 // Producing an alias to a base class ctor/dtor can degrade debug quality 39 // as the debugger cannot tell them apart. 40 if (getCodeGenOpts().OptimizationLevel == 0) 41 return true; 42 43 // If sanitizing memory to check for use-after-dtor, do not emit as 44 // an alias, unless this class owns no members. 45 if (getCodeGenOpts().SanitizeMemoryUseAfterDtor && 46 !D->getParent()->field_empty()) 47 return true; 48 49 // If the destructor doesn't have a trivial body, we have to emit it 50 // separately. 51 if (!D->hasTrivialBody()) 52 return true; 53 54 const CXXRecordDecl *Class = D->getParent(); 55 56 // We are going to instrument this destructor, so give up even if it is 57 // currently empty. 58 if (Class->mayInsertExtraPadding()) 59 return true; 60 61 // If we need to manipulate a VTT parameter, give up. 62 if (Class->getNumVBases()) { 63 // Extra Credit: passing extra parameters is perfectly safe 64 // in many calling conventions, so only bail out if the ctor's 65 // calling convention is nonstandard. 66 return true; 67 } 68 69 // If any field has a non-trivial destructor, we have to emit the 70 // destructor separately. 71 for (const auto *I : Class->fields()) 72 if (I->getType().isDestructedType()) 73 return true; 74 75 // Try to find a unique base class with a non-trivial destructor. 76 const CXXRecordDecl *UniqueBase = nullptr; 77 for (const auto &I : Class->bases()) { 78 79 // We're in the base destructor, so skip virtual bases. 80 if (I.isVirtual()) continue; 81 82 // Skip base classes with trivial destructors. 83 const auto *Base = 84 cast<CXXRecordDecl>(I.getType()->castAs<RecordType>()->getDecl()); 85 if (Base->hasTrivialDestructor()) continue; 86 87 // If we've already found a base class with a non-trivial 88 // destructor, give up. 89 if (UniqueBase) return true; 90 UniqueBase = Base; 91 } 92 93 // If we didn't find any bases with a non-trivial destructor, then 94 // the base destructor is actually effectively trivial, which can 95 // happen if it was needlessly user-defined or if there are virtual 96 // bases with non-trivial destructors. 97 if (!UniqueBase) 98 return true; 99 100 // If the base is at a non-zero offset, give up. 101 const ASTRecordLayout &ClassLayout = Context.getASTRecordLayout(Class); 102 if (!ClassLayout.getBaseClassOffset(UniqueBase).isZero()) 103 return true; 104 105 // Give up if the calling conventions don't match. We could update the call, 106 // but it is probably not worth it. 107 const CXXDestructorDecl *BaseD = UniqueBase->getDestructor(); 108 if (BaseD->getType()->castAs<FunctionType>()->getCallConv() != 109 D->getType()->castAs<FunctionType>()->getCallConv()) 110 return true; 111 112 GlobalDecl AliasDecl(D, Dtor_Base); 113 GlobalDecl TargetDecl(BaseD, Dtor_Base); 114 115 // The alias will use the linkage of the referent. If we can't 116 // support aliases with that linkage, fail. 117 llvm::GlobalValue::LinkageTypes Linkage = getFunctionLinkage(AliasDecl); 118 119 // We can't use an alias if the linkage is not valid for one. 120 if (!llvm::GlobalAlias::isValidLinkage(Linkage)) 121 return true; 122 123 llvm::GlobalValue::LinkageTypes TargetLinkage = 124 getFunctionLinkage(TargetDecl); 125 126 // Check if we have it already. 127 StringRef MangledName = getMangledName(AliasDecl); 128 llvm::GlobalValue *Entry = GetGlobalValue(MangledName); 129 if (Entry && !Entry->isDeclaration()) 130 return false; 131 if (Replacements.count(MangledName)) 132 return false; 133 134 // Derive the type for the alias. 135 llvm::Type *AliasValueType = getTypes().GetFunctionType(AliasDecl); 136 llvm::PointerType *AliasType = AliasValueType->getPointerTo(); 137 138 // Find the referent. Some aliases might require a bitcast, in 139 // which case the caller is responsible for ensuring the soundness 140 // of these semantics. 141 auto *Ref = cast<llvm::GlobalValue>(GetAddrOfGlobal(TargetDecl)); 142 llvm::Constant *Aliasee = Ref; 143 if (Ref->getType() != AliasType) 144 Aliasee = llvm::ConstantExpr::getBitCast(Ref, AliasType); 145 146 // Instead of creating as alias to a linkonce_odr, replace all of the uses 147 // of the aliasee. 148 if (llvm::GlobalValue::isDiscardableIfUnused(Linkage) && 149 !(TargetLinkage == llvm::GlobalValue::AvailableExternallyLinkage && 150 TargetDecl.getDecl()->hasAttr<AlwaysInlineAttr>())) { 151 // FIXME: An extern template instantiation will create functions with 152 // linkage "AvailableExternally". In libc++, some classes also define 153 // members with attribute "AlwaysInline" and expect no reference to 154 // be generated. It is desirable to reenable this optimisation after 155 // corresponding LLVM changes. 156 addReplacement(MangledName, Aliasee); 157 return false; 158 } 159 160 // If we have a weak, non-discardable alias (weak, weak_odr), like an extern 161 // template instantiation or a dllexported class, avoid forming it on COFF. 162 // A COFF weak external alias cannot satisfy a normal undefined symbol 163 // reference from another TU. The other TU must also mark the referenced 164 // symbol as weak, which we cannot rely on. 165 if (llvm::GlobalValue::isWeakForLinker(Linkage) && 166 getTriple().isOSBinFormatCOFF()) { 167 return true; 168 } 169 170 // If we don't have a definition for the destructor yet or the definition is 171 // avaialable_externally, don't emit an alias. We can't emit aliases to 172 // declarations; that's just not how aliases work. 173 if (Ref->isDeclarationForLinker()) 174 return true; 175 176 // Don't create an alias to a linker weak symbol. This avoids producing 177 // different COMDATs in different TUs. Another option would be to 178 // output the alias both for weak_odr and linkonce_odr, but that 179 // requires explicit comdat support in the IL. 180 if (llvm::GlobalValue::isWeakForLinker(TargetLinkage)) 181 return true; 182 183 // Create the alias with no name. 184 auto *Alias = llvm::GlobalAlias::create(AliasValueType, 0, Linkage, "", 185 Aliasee, &getModule()); 186 187 // Destructors are always unnamed_addr. 188 Alias->setUnnamedAddr(llvm::GlobalValue::UnnamedAddr::Global); 189 190 // Switch any previous uses to the alias. 191 if (Entry) { 192 assert(Entry->getType() == AliasType && 193 "declaration exists with different type"); 194 Alias->takeName(Entry); 195 Entry->replaceAllUsesWith(Alias); 196 Entry->eraseFromParent(); 197 } else { 198 Alias->setName(MangledName); 199 } 200 201 // Finally, set up the alias with its proper name and attributes. 202 SetCommonAttributes(AliasDecl, Alias); 203 204 return false; 205 } 206 207 llvm::Function *CodeGenModule::codegenCXXStructor(GlobalDecl GD) { 208 const CGFunctionInfo &FnInfo = getTypes().arrangeCXXStructorDeclaration(GD); 209 auto *Fn = cast<llvm::Function>( 210 getAddrOfCXXStructor(GD, &FnInfo, /*FnType=*/nullptr, 211 /*DontDefer=*/true, ForDefinition)); 212 213 setFunctionLinkage(GD, Fn); 214 215 CodeGenFunction(*this).GenerateCode(GD, Fn, FnInfo); 216 setNonAliasAttributes(GD, Fn); 217 SetLLVMFunctionAttributesForDefinition(cast<CXXMethodDecl>(GD.getDecl()), Fn); 218 return Fn; 219 } 220 221 llvm::FunctionCallee CodeGenModule::getAddrAndTypeOfCXXStructor( 222 GlobalDecl GD, const CGFunctionInfo *FnInfo, llvm::FunctionType *FnType, 223 bool DontDefer, ForDefinition_t IsForDefinition) { 224 auto *MD = cast<CXXMethodDecl>(GD.getDecl()); 225 226 if (isa<CXXDestructorDecl>(MD)) { 227 // Always alias equivalent complete destructors to base destructors in the 228 // MS ABI. 229 if (getTarget().getCXXABI().isMicrosoft() && 230 GD.getDtorType() == Dtor_Complete && 231 MD->getParent()->getNumVBases() == 0) 232 GD = GD.getWithDtorType(Dtor_Base); 233 } 234 235 if (!FnType) { 236 if (!FnInfo) 237 FnInfo = &getTypes().arrangeCXXStructorDeclaration(GD); 238 FnType = getTypes().GetFunctionType(*FnInfo); 239 } 240 241 llvm::Constant *Ptr = GetOrCreateLLVMFunction( 242 getMangledName(GD), FnType, GD, /*ForVTable=*/false, DontDefer, 243 /*IsThunk=*/false, /*ExtraAttrs=*/llvm::AttributeList(), IsForDefinition); 244 return {FnType, Ptr}; 245 } 246 247 static CGCallee BuildAppleKextVirtualCall(CodeGenFunction &CGF, 248 GlobalDecl GD, 249 llvm::Type *Ty, 250 const CXXRecordDecl *RD) { 251 assert(!CGF.CGM.getTarget().getCXXABI().isMicrosoft() && 252 "No kext in Microsoft ABI"); 253 CodeGenModule &CGM = CGF.CGM; 254 llvm::Value *VTable = CGM.getCXXABI().getAddrOfVTable(RD, CharUnits()); 255 Ty = Ty->getPointerTo(); 256 VTable = CGF.Builder.CreateBitCast(VTable, Ty->getPointerTo()); 257 assert(VTable && "BuildVirtualCall = kext vtbl pointer is null"); 258 uint64_t VTableIndex = CGM.getItaniumVTableContext().getMethodVTableIndex(GD); 259 const VTableLayout &VTLayout = CGM.getItaniumVTableContext().getVTableLayout(RD); 260 VTableLayout::AddressPointLocation AddressPoint = 261 VTLayout.getAddressPoint(BaseSubobject(RD, CharUnits::Zero())); 262 VTableIndex += VTLayout.getVTableOffset(AddressPoint.VTableIndex) + 263 AddressPoint.AddressPointIndex; 264 llvm::Value *VFuncPtr = 265 CGF.Builder.CreateConstInBoundsGEP1_64(Ty, VTable, VTableIndex, "vfnkxt"); 266 llvm::Value *VFunc = CGF.Builder.CreateAlignedLoad( 267 Ty, VFuncPtr, llvm::Align(CGF.PointerAlignInBytes)); 268 CGCallee Callee(GD, VFunc); 269 return Callee; 270 } 271 272 /// BuildAppleKextVirtualCall - This routine is to support gcc's kext ABI making 273 /// indirect call to virtual functions. It makes the call through indexing 274 /// into the vtable. 275 CGCallee 276 CodeGenFunction::BuildAppleKextVirtualCall(const CXXMethodDecl *MD, 277 NestedNameSpecifier *Qual, 278 llvm::Type *Ty) { 279 assert((Qual->getKind() == NestedNameSpecifier::TypeSpec) && 280 "BuildAppleKextVirtualCall - bad Qual kind"); 281 282 const Type *QTy = Qual->getAsType(); 283 QualType T = QualType(QTy, 0); 284 const RecordType *RT = T->getAs<RecordType>(); 285 assert(RT && "BuildAppleKextVirtualCall - Qual type must be record"); 286 const auto *RD = cast<CXXRecordDecl>(RT->getDecl()); 287 288 if (const auto *DD = dyn_cast<CXXDestructorDecl>(MD)) 289 return BuildAppleKextVirtualDestructorCall(DD, Dtor_Complete, RD); 290 291 return ::BuildAppleKextVirtualCall(*this, MD, Ty, RD); 292 } 293 294 /// BuildVirtualCall - This routine makes indirect vtable call for 295 /// call to virtual destructors. It returns 0 if it could not do it. 296 CGCallee 297 CodeGenFunction::BuildAppleKextVirtualDestructorCall( 298 const CXXDestructorDecl *DD, 299 CXXDtorType Type, 300 const CXXRecordDecl *RD) { 301 assert(DD->isVirtual() && Type != Dtor_Base); 302 // Compute the function type we're calling. 303 const CGFunctionInfo &FInfo = CGM.getTypes().arrangeCXXStructorDeclaration( 304 GlobalDecl(DD, Dtor_Complete)); 305 llvm::Type *Ty = CGM.getTypes().GetFunctionType(FInfo); 306 return ::BuildAppleKextVirtualCall(*this, GlobalDecl(DD, Type), Ty, RD); 307 } 308