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