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