CGCXX.cpp revision 87d948ecccffea9e9e37d0d053b246e2d6d6c47b 
1//===--- CGCXX.cpp - Emit LLVM Code for declarations ----------------------===//
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// This contains code dealing with C++ code generation.
11//
12//===----------------------------------------------------------------------===//
13
14// We might split this into multiple files if it gets too unwieldy
15
16#include "CodeGenModule.h"
17#include "CGCXXABI.h"
18#include "CodeGenFunction.h"
19#include "clang/AST/ASTContext.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/Frontend/CodeGenOptions.h"
27#include "llvm/ADT/StringExtras.h"
28using namespace clang;
29using namespace CodeGen;
30
31
32/// Try to emit a base destructor as an alias to its primary
33/// base-class destructor.
34bool 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()->getAs<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()->getAs<FunctionType>()->getCallConv() !=
109      D->getType()->getAs<FunctionType>()->getCallConv())
110    return true;
111
112  return TryEmitDefinitionAsAlias(GlobalDecl(D, Dtor_Base),
113                                  GlobalDecl(BaseD, Dtor_Base),
114                                  false);
115}
116
117/// Try to emit a definition as a global alias for another definition.
118/// If \p InEveryTU is true, we know that an equivalent alias can be produced
119/// in every translation unit.
120bool CodeGenModule::TryEmitDefinitionAsAlias(GlobalDecl AliasDecl,
121                                             GlobalDecl TargetDecl,
122                                             bool InEveryTU) {
123  if (!getCodeGenOpts().CXXCtorDtorAliases)
124    return true;
125
126  // The alias will use the linkage of the referent.  If we can't
127  // support aliases with that linkage, fail.
128  llvm::GlobalValue::LinkageTypes Linkage = getFunctionLinkage(AliasDecl);
129
130  // We can't use an alias if the linkage is not valid for one.
131  if (!llvm::GlobalAlias::isValidLinkage(Linkage))
132    return true;
133
134  llvm::GlobalValue::LinkageTypes TargetLinkage =
135      getFunctionLinkage(TargetDecl);
136
137  // Check if we have it already.
138  StringRef MangledName = getMangledName(AliasDecl);
139  llvm::GlobalValue *Entry = GetGlobalValue(MangledName);
140  if (Entry && !Entry->isDeclaration())
141    return false;
142  if (Replacements.count(MangledName))
143    return false;
144
145  // Derive the type for the alias.
146  llvm::Type *AliasValueType = getTypes().GetFunctionType(AliasDecl);
147  llvm::PointerType *AliasType = AliasValueType->getPointerTo();
148
149  // Find the referent.  Some aliases might require a bitcast, in
150  // which case the caller is responsible for ensuring the soundness
151  // of these semantics.
152  auto *Ref = cast<llvm::GlobalValue>(GetAddrOfGlobal(TargetDecl));
153  llvm::Constant *Aliasee = Ref;
154  if (Ref->getType() != AliasType)
155    Aliasee = llvm::ConstantExpr::getBitCast(Ref, AliasType);
156
157  // Instead of creating as alias to a linkonce_odr, replace all of the uses
158  // of the aliasee.
159  if (llvm::GlobalValue::isDiscardableIfUnused(Linkage) &&
160     (TargetLinkage != llvm::GlobalValue::AvailableExternallyLinkage ||
161      !TargetDecl.getDecl()->hasAttr<AlwaysInlineAttr>())) {
162    // FIXME: An extern template instantiation will create functions with
163    // linkage "AvailableExternally". In libc++, some classes also define
164    // members with attribute "AlwaysInline" and expect no reference to
165    // be generated. It is desirable to reenable this optimisation after
166    // corresponding LLVM changes.
167    Replacements[MangledName] = Aliasee;
168    return false;
169  }
170
171  // If we have a weak, non-discardable alias (weak, weak_odr), like an extern
172  // template instantiation or a dllexported class, avoid forming it on COFF.
173  // A COFF weak external alias cannot satisfy a normal undefined symbol
174  // reference from another TU. The other TU must also mark the referenced
175  // symbol as weak, which we cannot rely on.
176  if (llvm::GlobalValue::isWeakForLinker(Linkage) &&
177      getTriple().isOSBinFormatCOFF()) {
178    return true;
179  }
180
181  if (!InEveryTU) {
182    // If we don't have a definition for the destructor yet, don't
183    // emit.  We can't emit aliases to declarations; that's just not
184    // how aliases work.
185    if (Ref->isDeclaration())
186      return true;
187  }
188
189  // Don't create an alias to a linker weak symbol. This avoids producing
190  // different COMDATs in different TUs. Another option would be to
191  // output the alias both for weak_odr and linkonce_odr, but that
192  // requires explicit comdat support in the IL.
193  if (llvm::GlobalValue::isWeakForLinker(TargetLinkage))
194    return true;
195
196  // Create the alias with no name.
197  auto *Alias = llvm::GlobalAlias::create(AliasValueType, 0, Linkage, "",
198                                          Aliasee, &getModule());
199
200  // Switch any previous uses to the alias.
201  if (Entry) {
202    assert(Entry->getType() == AliasType &&
203           "declaration exists with different type");
204    Alias->takeName(Entry);
205    Entry->replaceAllUsesWith(Alias);
206    Entry->eraseFromParent();
207  } else {
208    Alias->setName(MangledName);
209  }
210
211  // Finally, set up the alias with its proper name and attributes.
212  setAliasAttributes(cast<NamedDecl>(AliasDecl.getDecl()), Alias);
213
214  return false;
215}
216
217llvm::Function *CodeGenModule::codegenCXXStructor(const CXXMethodDecl *MD,
218                                                  StructorType Type) {
219  const CGFunctionInfo &FnInfo =
220      getTypes().arrangeCXXStructorDeclaration(MD, Type);
221  auto *Fn = cast<llvm::Function>(
222      getAddrOfCXXStructor(MD, Type, &FnInfo, /*FnType=*/nullptr,
223                           /*DontDefer=*/true, /*IsForDefinition=*/true));
224
225  GlobalDecl GD;
226  if (const auto *DD = dyn_cast<CXXDestructorDecl>(MD)) {
227    GD = GlobalDecl(DD, toCXXDtorType(Type));
228  } else {
229    const auto *CD = cast<CXXConstructorDecl>(MD);
230    GD = GlobalDecl(CD, toCXXCtorType(Type));
231  }
232
233  setFunctionLinkage(GD, Fn);
234  setFunctionDLLStorageClass(GD, Fn);
235
236  CodeGenFunction(*this).GenerateCode(GD, Fn, FnInfo);
237  setFunctionDefinitionAttributes(MD, Fn);
238  SetLLVMFunctionAttributesForDefinition(MD, Fn);
239  return Fn;
240}
241
242llvm::Constant *CodeGenModule::getAddrOfCXXStructor(
243    const CXXMethodDecl *MD, StructorType Type, const CGFunctionInfo *FnInfo,
244    llvm::FunctionType *FnType, bool DontDefer, bool IsForDefinition) {
245  GlobalDecl GD;
246  if (auto *CD = dyn_cast<CXXConstructorDecl>(MD)) {
247    GD = GlobalDecl(CD, toCXXCtorType(Type));
248  } else {
249    GD = GlobalDecl(cast<CXXDestructorDecl>(MD), toCXXDtorType(Type));
250  }
251
252  if (!FnType) {
253    if (!FnInfo)
254      FnInfo = &getTypes().arrangeCXXStructorDeclaration(MD, Type);
255    FnType = getTypes().GetFunctionType(*FnInfo);
256  }
257
258  return GetOrCreateLLVMFunction(
259      getMangledName(GD), FnType, GD, /*ForVTable=*/false, DontDefer,
260      /*isThunk=*/false, /*ExtraAttrs=*/llvm::AttributeSet(), IsForDefinition);
261}
262
263static llvm::Value *BuildAppleKextVirtualCall(CodeGenFunction &CGF,
264                                              GlobalDecl GD,
265                                              llvm::Type *Ty,
266                                              const CXXRecordDecl *RD) {
267  assert(!CGF.CGM.getTarget().getCXXABI().isMicrosoft() &&
268         "No kext in Microsoft ABI");
269  GD = GD.getCanonicalDecl();
270  CodeGenModule &CGM = CGF.CGM;
271  llvm::Value *VTable = CGM.getCXXABI().getAddrOfVTable(RD, CharUnits());
272  Ty = Ty->getPointerTo()->getPointerTo();
273  VTable = CGF.Builder.CreateBitCast(VTable, Ty);
274  assert(VTable && "BuildVirtualCall = kext vtbl pointer is null");
275  uint64_t VTableIndex = CGM.getItaniumVTableContext().getMethodVTableIndex(GD);
276  uint64_t AddressPoint =
277    CGM.getItaniumVTableContext().getVTableLayout(RD)
278       .getAddressPoint(BaseSubobject(RD, CharUnits::Zero()));
279  VTableIndex += AddressPoint;
280  llvm::Value *VFuncPtr =
281    CGF.Builder.CreateConstInBoundsGEP1_64(VTable, VTableIndex, "vfnkxt");
282  return CGF.Builder.CreateAlignedLoad(VFuncPtr, CGF.PointerAlignInBytes);
283}
284
285/// BuildAppleKextVirtualCall - This routine is to support gcc's kext ABI making
286/// indirect call to virtual functions. It makes the call through indexing
287/// into the vtable.
288llvm::Value *
289CodeGenFunction::BuildAppleKextVirtualCall(const CXXMethodDecl *MD,
290                                  NestedNameSpecifier *Qual,
291                                  llvm::Type *Ty) {
292  assert((Qual->getKind() == NestedNameSpecifier::TypeSpec) &&
293         "BuildAppleKextVirtualCall - bad Qual kind");
294
295  const Type *QTy = Qual->getAsType();
296  QualType T = QualType(QTy, 0);
297  const RecordType *RT = T->getAs<RecordType>();
298  assert(RT && "BuildAppleKextVirtualCall - Qual type must be record");
299  const auto *RD = cast<CXXRecordDecl>(RT->getDecl());
300
301  if (const auto *DD = dyn_cast<CXXDestructorDecl>(MD))
302    return BuildAppleKextVirtualDestructorCall(DD, Dtor_Complete, RD);
303
304  return ::BuildAppleKextVirtualCall(*this, MD, Ty, RD);
305}
306
307/// BuildVirtualCall - This routine makes indirect vtable call for
308/// call to virtual destructors. It returns 0 if it could not do it.
309llvm::Value *
310CodeGenFunction::BuildAppleKextVirtualDestructorCall(
311                                            const CXXDestructorDecl *DD,
312                                            CXXDtorType Type,
313                                            const CXXRecordDecl *RD) {
314  const auto *MD = cast<CXXMethodDecl>(DD);
315  // FIXME. Dtor_Base dtor is always direct!!
316  // It need be somehow inline expanded into the caller.
317  // -O does that. But need to support -O0 as well.
318  if (MD->isVirtual() && Type != Dtor_Base) {
319    // Compute the function type we're calling.
320    const CGFunctionInfo &FInfo = CGM.getTypes().arrangeCXXStructorDeclaration(
321        DD, StructorType::Complete);
322    llvm::Type *Ty = CGM.getTypes().GetFunctionType(FInfo);
323    return ::BuildAppleKextVirtualCall(*this, GlobalDecl(DD, Type), Ty, RD);
324  }
325  return nullptr;
326}
327