Add support for putting constructors and destructos in explicit comdats.

There are situations when clang knows that the C1 and C2 constructors
or the D1 and D2 destructors are identical. We already optimize some
of these cases, but cannot optimize it when the GlobalValue is
weak_odr.

The problem with weak_odr is that an old TU seeing the same code will
have a C1 and a C2 comdat with the corresponding symbols. We cannot
suddenly start putting the C2 symbol in the C1 comdat as we cannot
guarantee that the linker will not pick a .o with only C1 in it.

The solution implemented by GCC is to expand the ABI to have a comdat
whose name uses a C5/D5 suffix and always has both symbols. That is
what this patch implements.

llvm-svn: 217874

clang/include/clang/AST/Mangle.h

@@ -156,6 +156,11 @@ public:
   virtual void mangleItaniumThreadLocalWrapper(const VarDecl *D,
                                                raw_ostream &) = 0;
 
+  virtual void mangleCXXCtorComdat(const CXXConstructorDecl *D,
+                                   raw_ostream &) = 0;
+  virtual void mangleCXXDtorComdat(const CXXDestructorDecl *D,
+                                   raw_ostream &) = 0;
+
   static bool classof(const MangleContext *C) {
     return C->getKind() == MK_Itanium;
   }

clang/include/clang/Basic/ABI.h

@@ -24,14 +24,15 @@ namespace clang {
 enum CXXCtorType {
     Ctor_Complete,          ///< Complete object ctor
     Ctor_Base,              ///< Base object ctor
-    Ctor_CompleteAllocating ///< Complete object allocating ctor
+    Ctor_Comdat             ///< The COMDAT used for ctors
 };
 
 /// \brief C++ destructor types.
 enum CXXDtorType {
     Dtor_Deleting, ///< Deleting dtor
     Dtor_Complete, ///< Complete object dtor
-    Dtor_Base      ///< Base object dtor
+    Dtor_Base,     ///< Base object dtor
+    Dtor_Comdat    ///< The COMDAT used for dtors
 };
 
 /// \brief A return adjustment.

clang/lib/AST/ItaniumMangle.cpp

@@ -150,6 +150,8 @@ public:
   void mangleCXXDtor(const CXXDestructorDecl *D, CXXDtorType Type,
                      raw_ostream &) override;
 
+  void mangleCXXCtorComdat(const CXXConstructorDecl *D, raw_ostream &) override;
+  void mangleCXXDtorComdat(const CXXDestructorDecl *D, raw_ostream &) override;
   void mangleStaticGuardVariable(const VarDecl *D, raw_ostream &) override;
   void mangleDynamicInitializer(const VarDecl *D, raw_ostream &Out) override;
   void mangleDynamicAtExitDestructor(const VarDecl *D,
@@ -3249,8 +3251,8 @@ void CXXNameMangler::mangleFunctionParam(const ParmVarDecl *parm) {
 void CXXNameMangler::mangleCXXCtorType(CXXCtorType T) {
   // <ctor-dtor-name> ::= C1  # complete object constructor
   //                  ::= C2  # base object constructor
-  //                  ::= C3  # complete object allocating constructor
   //
+  // In addition, C5 is a comdat name with C1 and C2 in it.
   switch (T) {
   case Ctor_Complete:
     Out << "C1";
@@ -3258,8 +3260,8 @@ void CXXNameMangler::mangleCXXCtorType(CXXCtorType T) {
   case Ctor_Base:
     Out << "C2";
     break;
-  case Ctor_CompleteAllocating:
+  case Ctor_Comdat:
-    Out << "C3";
+    Out << "C5";
     break;
   }
 }
@@ -3269,6 +3271,7 @@ void CXXNameMangler::mangleCXXDtorType(CXXDtorType T) {
   //                  ::= D1  # complete object destructor
   //                  ::= D2  # base object destructor
   //
+  // In addition, D5 is a comdat name with D1, D2 and, if virtual, D0 in it.
   switch (T) {
   case Dtor_Deleting:
     Out << "D0";
@@ -3279,6 +3282,9 @@ void CXXNameMangler::mangleCXXDtorType(CXXDtorType T) {
   case Dtor_Base:
     Out << "D2";
     break;
+  case Dtor_Comdat:
+    Out << "D5";
+    break;
   }
 }
 
@@ -3689,6 +3695,18 @@ void ItaniumMangleContextImpl::mangleCXXDtor(const CXXDestructorDecl *D,
   Mangler.mangle(D);
 }
 
+void ItaniumMangleContextImpl::mangleCXXCtorComdat(const CXXConstructorDecl *D,
+                                                   raw_ostream &Out) {
+  CXXNameMangler Mangler(*this, Out, D, Ctor_Comdat);
+  Mangler.mangle(D);
+}
+
+void ItaniumMangleContextImpl::mangleCXXDtorComdat(const CXXDestructorDecl *D,
+                                                   raw_ostream &Out) {
+  CXXNameMangler Mangler(*this, Out, D, Dtor_Comdat);
+  Mangler.mangle(D);
+}
+
 void ItaniumMangleContextImpl::mangleThunk(const CXXMethodDecl *MD,
                                            const ThunkInfo &Thunk,
                                            raw_ostream &Out) {

clang/lib/AST/MicrosoftMangle.cpp

@@ -854,6 +854,8 @@ void MicrosoftCXXNameMangler::mangleCXXDtorType(CXXDtorType T) {
   // <operator-name> ::= ?_E # vector deleting destructor
   // FIXME: Add a vector deleting dtor type.  It goes in the vtable, so we need
   // it.
+  case Dtor_Comdat:
+    llvm_unreachable("not expecting a COMDAT");
   }
   llvm_unreachable("Unsupported dtor type?");
 }

clang/lib/CodeGen/CGClass.cpp

@@ -1292,6 +1292,9 @@ void CodeGenFunction::EmitDestructorBody(FunctionArgList &Args) {
   // we'd introduce *two* handler blocks.  In the Microsoft ABI, we 
   // always delegate because we might not have a definition in this TU.
   switch (DtorType) {
+  case Dtor_Comdat:
+    llvm_unreachable("not expecting a COMDAT");
+
   case Dtor_Deleting: llvm_unreachable("already handled deleting case");
 
   case Dtor_Complete:

clang/lib/CodeGen/CodeGenModule.cpp

@@ -198,6 +198,10 @@ void CodeGenModule::createCUDARuntime() {
   CUDARuntime = CreateNVCUDARuntime(*this);
 }
 
+void CodeGenModule::addReplacement(StringRef Name, llvm::Constant *C) {
+  Replacements[Name] = C;
+}
+
 void CodeGenModule::applyReplacements() {
   for (ReplacementsTy::iterator I = Replacements.begin(),
                                 E = Replacements.end();

clang/lib/CodeGen/CodeGenModule.h

@@ -1058,9 +1058,17 @@ public:
   void setFunctionDefinitionAttributes(const FunctionDecl *D,
                                        llvm::Function *F);
 
-private:
   llvm::GlobalValue *GetGlobalValue(StringRef Ref);
 
+  /// Set attributes which are common to any form of a global definition (alias,
+  /// Objective-C method, function, global variable).
+  ///
+  /// NOTE: This should only be called for definitions.
+  void SetCommonAttributes(const Decl *D, llvm::GlobalValue *GV);
+
+  void addReplacement(StringRef Name, llvm::Constant *C);
+private:
+
   llvm::Constant *
   GetOrCreateLLVMFunction(StringRef MangledName, llvm::Type *Ty, GlobalDecl D,
                           bool ForVTable, bool DontDefer = false,
@@ -1070,12 +1078,6 @@ private:
                                         llvm::PointerType *PTy,
                                         const VarDecl *D);
 
-  /// Set attributes which are common to any form of a global definition (alias,
-  /// Objective-C method, function, global variable).
-  ///
-  /// NOTE: This should only be called for definitions.
-  void SetCommonAttributes(const Decl *D, llvm::GlobalValue *GV);
-
   void setNonAliasAttributes(const Decl *D, llvm::GlobalObject *GO);
 
   /// Set function attributes for a function declaration.

clang/lib/CodeGen/CodeGenTypes.h

@@ -80,8 +80,8 @@ inline StructorType getFromCtorType(CXXCtorType T) {
     return StructorType::Complete;
   case Ctor_Base:
     return StructorType::Base;
-  case Ctor_CompleteAllocating:
+  case Ctor_Comdat:
-    llvm_unreachable("invalid enum");
+    llvm_unreachable("not expecting a COMDAT");
   }
   llvm_unreachable("not a CXXCtorType");
 }
@@ -106,6 +106,8 @@ inline StructorType getFromDtorType(CXXDtorType T) {
     return StructorType::Complete;
   case Dtor_Base:
     return StructorType::Base;
+  case Dtor_Comdat:
+    llvm_unreachable("not expecting a COMDAT");
   }
   llvm_unreachable("not a CXXDtorType");
 }

clang/lib/CodeGen/ItaniumCXXABI.cpp

@@ -2999,35 +2999,102 @@ ItaniumCXXABI::RTTIUniquenessKind ItaniumCXXABI::classifyRTTIUniqueness(
   return RUK_NonUniqueVisible;
 }
 
-static void emitCXXConstructor(CodeGenModule &CGM,
+// Find out how to codegen the complete destructor and constructor
-                               const CXXConstructorDecl *ctor,
+namespace {
-                               StructorType ctorType) {
+enum class StructorCodegen { Emit, RAUW, Alias, COMDAT };
-  if (!ctor->getParent()->getNumVBases() &&
+}
-      (ctorType == StructorType::Complete || ctorType == StructorType::Base)) {
+static StructorCodegen getCodegenToUse(CodeGenModule &CGM,
-    // The complete constructor is equivalent to the base constructor
+                                       const CXXMethodDecl *MD) {
-    // for classes with no virtual bases.  Try to emit it as an alias.
+  if (!CGM.getCodeGenOpts().CXXCtorDtorAliases)
-    bool ProducedAlias = !CGM.TryEmitDefinitionAsAlias(
+    return StructorCodegen::Emit;
-        GlobalDecl(ctor, Ctor_Complete), GlobalDecl(ctor, Ctor_Base), true);
-    if (ctorType == StructorType::Complete && ProducedAlias)
-      return;
-  }
 
-  CGM.codegenCXXStructor(ctor, ctorType);
+  // The complete and base structors are not equivalent if there are any virtual
+  // bases, so emit separate functions.
+  if (MD->getParent()->getNumVBases())
+    return StructorCodegen::Emit;
+
+  GlobalDecl AliasDecl;
+  if (const auto *DD = dyn_cast<CXXDestructorDecl>(MD)) {
+    AliasDecl = GlobalDecl(DD, Dtor_Complete);
+  } else {
+    const auto *CD = cast<CXXConstructorDecl>(MD);
+    AliasDecl = GlobalDecl(CD, Ctor_Complete);
+  }
+  llvm::GlobalValue::LinkageTypes Linkage = CGM.getFunctionLinkage(AliasDecl);
+
+  if (llvm::GlobalValue::isDiscardableIfUnused(Linkage))
+    return StructorCodegen::RAUW;
+
+  // FIXME: Should we allow available_externally aliases?
+  if (!llvm::GlobalAlias::isValidLinkage(Linkage))
+    return StructorCodegen::RAUW;
+
+  if (llvm::GlobalValue::isWeakForLinker(Linkage))
+    return StructorCodegen::COMDAT;
+
+  return StructorCodegen::Alias;
 }
 
-static void emitCXXDestructor(CodeGenModule &CGM, const CXXDestructorDecl *dtor,
+static void emitConstructorDestructorAlias(CodeGenModule &CGM,
-                              StructorType dtorType) {
+                                           GlobalDecl AliasDecl,
-  // The complete destructor is equivalent to the base destructor for
+                                           GlobalDecl TargetDecl) {
-  // classes with no virtual bases, so try to emit it as an alias.
+  llvm::GlobalValue::LinkageTypes Linkage = CGM.getFunctionLinkage(AliasDecl);
-  if (!dtor->getParent()->getNumVBases() &&
+
-      (dtorType == StructorType::Complete || dtorType == StructorType::Base)) {
+  StringRef MangledName = CGM.getMangledName(AliasDecl);
-    bool ProducedAlias = !CGM.TryEmitDefinitionAsAlias(
+  llvm::GlobalValue *Entry = CGM.GetGlobalValue(MangledName);
-        GlobalDecl(dtor, Dtor_Complete), GlobalDecl(dtor, Dtor_Base), true);
+  if (Entry && !Entry->isDeclaration())
-    if (ProducedAlias) {
+    return;
-      if (dtorType == StructorType::Complete)
+
-        return;
+  auto *Aliasee = cast<llvm::GlobalValue>(CGM.GetAddrOfGlobal(TargetDecl));
-      if (dtor->isVirtual())
+  llvm::PointerType *AliasType = Aliasee->getType();
-        CGM.getVTables().EmitThunks(GlobalDecl(dtor, Dtor_Complete));
+
+  // Create the alias with no name.
+  auto *Alias = llvm::GlobalAlias::create(
+      AliasType->getElementType(), 0, Linkage, "", Aliasee, &CGM.getModule());
+
+  // Switch any previous uses to the alias.
+  if (Entry) {
+    assert(Entry->getType() == AliasType &&
+           "declaration exists with different type");
+    Alias->takeName(Entry);
+    Entry->replaceAllUsesWith(Alias);
+    Entry->eraseFromParent();
+  } else {
+    Alias->setName(MangledName);
+  }
+
+  // Finally, set up the alias with its proper name and attributes.
+  CGM.SetCommonAttributes(cast<NamedDecl>(AliasDecl.getDecl()), Alias);
+}
+
+void ItaniumCXXABI::emitCXXStructor(const CXXMethodDecl *MD,
+                                    StructorType Type) {
+  auto *CD = dyn_cast<CXXConstructorDecl>(MD);
+  const CXXDestructorDecl *DD = CD ? nullptr : cast<CXXDestructorDecl>(MD);
+
+  StructorCodegen CGType = getCodegenToUse(CGM, MD);
+
+  if (Type == StructorType::Complete) {
+    GlobalDecl CompleteDecl;
+    GlobalDecl BaseDecl;
+    if (CD) {
+      CompleteDecl = GlobalDecl(CD, Ctor_Complete);
+      BaseDecl = GlobalDecl(CD, Ctor_Base);
+    } else {
+      CompleteDecl = GlobalDecl(DD, Dtor_Complete);
+      BaseDecl = GlobalDecl(DD, Dtor_Base);
+    }
+
+    if (CGType == StructorCodegen::Alias || CGType == StructorCodegen::COMDAT) {
+      emitConstructorDestructorAlias(CGM, CompleteDecl, BaseDecl);
+      return;
+    }
+
+    if (CGType == StructorCodegen::RAUW) {
+      StringRef MangledName = CGM.getMangledName(CompleteDecl);
+      auto *Aliasee = cast<llvm::GlobalValue>(CGM.GetAddrOfGlobal(BaseDecl));
+      CGM.addReplacement(MangledName, Aliasee);
+      return;
     }
   }
 
@@ -3035,17 +3102,20 @@ static void emitCXXDestructor(CodeGenModule &CGM, const CXXDestructorDecl *dtor,
   // base class if there is exactly one non-virtual base class with a
   // non-trivial destructor, there are no fields with a non-trivial
   // destructor, and the body of the destructor is trivial.
-  if (dtorType == StructorType::Base && !CGM.TryEmitBaseDestructorAsAlias(dtor))
+  if (DD && Type == StructorType::Base && CGType != StructorCodegen::COMDAT &&
+      !CGM.TryEmitBaseDestructorAsAlias(DD))
     return;
 
-  CGM.codegenCXXStructor(dtor, dtorType);
+  llvm::Function *Fn = CGM.codegenCXXStructor(MD, Type);
-}
 
-void ItaniumCXXABI::emitCXXStructor(const CXXMethodDecl *MD,
+  if (CGType == StructorCodegen::COMDAT) {
-                                    StructorType Type) {
+    SmallString<256> Buffer;
-  if (auto *CD = dyn_cast<CXXConstructorDecl>(MD)) {
+    llvm::raw_svector_ostream Out(Buffer);
-    emitCXXConstructor(CGM, CD, Type);
+    if (DD)
-    return;
+      getMangleContext().mangleCXXDtorComdat(DD, Out);
+    else
+      getMangleContext().mangleCXXCtorComdat(CD, Out);
+    llvm::Comdat *C = CGM.getModule().getOrInsertComdat(Out.str());
+    Fn->setComdat(C);
   }
-  emitCXXDestructor(CGM, cast<CXXDestructorDecl>(MD), Type);
 }

clang/test/CodeGenCXX/ctor-dtor-alias.cpp

@@ -6,18 +6,23 @@
 // RUN: FileCheck --check-prefix=CHECK3 --input-file=%t %s
 // RUN: FileCheck --check-prefix=CHECK4 --input-file=%t %s
 // RUN: FileCheck --check-prefix=CHECK5 --input-file=%t %s
+// RUN: FileCheck --check-prefix=CHECK6 --input-file=%t %s
 
 namespace test1 {
-// test that we don't produce an alias when the destructor is weak_odr. The
+// Test that we produce the apropriate comdats when creating aliases to
-// reason to avoid it that another TU might have no explicit template
+// weak_odr constructors and destructors.
-// instantiation definition or declaration, causing it to to output only
-// one of the destructors as linkonce_odr, producing a different comdat.
 
-// CHECK1: define weak_odr void @_ZN5test16foobarIvEC2Ev
+// CHECK1: @_ZN5test16foobarIvEC1Ev = weak_odr alias void {{.*}} @_ZN5test16foobarIvEC2Ev
-// CHECK1: define weak_odr void @_ZN5test16foobarIvEC1Ev
+// CHECK1: @_ZN5test16foobarIvED1Ev = weak_odr alias void (%"struct.test1::foobar"*)* @_ZN5test16foobarIvED2Ev
+// CHECK1: define weak_odr void @_ZN5test16foobarIvEC2Ev({{.*}} comdat $_ZN5test16foobarIvEC5Ev
+// CHECK1: define weak_odr void @_ZN5test16foobarIvED2Ev({{.*}} comdat $_ZN5test16foobarIvED5Ev
+// CHECK1: define weak_odr void @_ZN5test16foobarIvED0Ev({{.*}} comdat $_ZN5test16foobarIvED5Ev
+// CHECK1-NOT: comdat
 
-template <typename T> struct foobar {
+template <typename T>
+struct foobar {
   foobar() {}
+  virtual ~foobar() {}
 };
 
 template struct foobar<void>;
@@ -187,3 +192,38 @@ void
 fn1() {
   new C;
 }
+
+namespace test10 {
+// Test that if a destructor is in a comdat, we don't try to emit is as an
+// alias to a base class destructor.
+struct bar {
+  ~bar();
+};
+bar::~bar() {
+}
+} // closing the namespace causes ~bar to be sent to CodeGen
+namespace test10 {
+template <typename T>
+struct foo : public bar {
+  ~foo();
+};
+template <typename T>
+foo<T>::~foo() {}
+template class foo<int>;
+// CHECK5: define weak_odr void @_ZN6test103fooIiED2Ev({{.*}} comdat $_ZN6test103fooIiED5Ev
+}
+
+namespace test11 {
+// Test that when we don't have to worry about COMDATs we produce an alias
+// from complate to base and from base to base class base.
+struct bar {
+  ~bar();
+};
+bar::~bar() {}
+struct foo : public bar {
+  ~foo();
+};
+foo::~foo() {}
+// CHECK6: @_ZN6test113fooD2Ev = alias {{.*}} @_ZN6test113barD2Ev
+// CHECK6: @_ZN6test113fooD1Ev = alias {{.*}} @_ZN6test113fooD2Ev
+}

clang/test/CodeGenCXX/destructors.cpp

@@ -204,18 +204,14 @@ namespace test3 {
   // CHECK4: call void @_ZN5test312_GLOBAL__N_11DD0Ev(
   // CHECK4: ret void
 
-  // CHECK4-LABEL: define internal void @_ZThn8_N5test312_GLOBAL__N_11CD1Ev(
+  // CHECK4-LABEL: declare void @_ZN5test31BD2Ev(
-  // CHECK4: getelementptr inbounds i8* {{.*}}, i64 -8
+  // CHECK4-LABEL: declare void @_ZN5test31AD2Ev(
-  // CHECK4: call void @_ZN5test312_GLOBAL__N_11CD2Ev(
-  // CHECK4: ret void
 
   // CHECK4-LABEL: define internal void @_ZN5test312_GLOBAL__N_11CD2Ev(%"struct.test3::(anonymous namespace)::C"* %this) unnamed_addr
   // CHECK4: invoke void @_ZN5test31BD2Ev(
   // CHECK4: call void @_ZN5test31AD2Ev(
   // CHECK4: ret void
 
-  // CHECK4: declare void @_ZN5test31BD2Ev(
-  // CHECK4: declare void @_ZN5test31AD2Ev(
 
   // CHECK4-LABEL: define internal void @_ZN5test312_GLOBAL__N_11CD0Ev(%"struct.test3::(anonymous namespace)::C"* %this) unnamed_addr
   // CHECK4: invoke void @_ZN5test312_GLOBAL__N_11CD2Ev(
@@ -226,6 +222,11 @@ namespace test3 {
   // CHECK4: call void @_ZdlPv({{.*}}) [[NUW]]
   // CHECK4: resume { i8*, i32 }
 
+  // CHECK4-LABEL: define internal void @_ZThn8_N5test312_GLOBAL__N_11CD1Ev(
+  // CHECK4: getelementptr inbounds i8* {{.*}}, i64 -8
+  // CHECK4: call void @_ZN5test312_GLOBAL__N_11CD2Ev(
+  // CHECK4: ret void
+
   // CHECK4-LABEL: define internal void @_ZThn8_N5test312_GLOBAL__N_11CD0Ev(
   // CHECK4: getelementptr inbounds i8* {{.*}}, i64 -8
   // CHECK4: call void @_ZN5test312_GLOBAL__N_11CD0Ev(

clang/test/CodeGenCXX/virtual-destructor-calls.cpp

@@ -17,8 +17,8 @@ struct B : A {
 // CHECK: @_ZN1BD1Ev = alias {{.*}} @_ZN1BD2Ev
 
 // (aliases from C)
-// CHECK: @_ZN1CD1Ev = alias {{.*}} @_ZN1CD2Ev
 // CHECK: @_ZN1CD2Ev = alias bitcast {{.*}} @_ZN1BD2Ev
+// CHECK: @_ZN1CD1Ev = alias {{.*}} @_ZN1CD2Ev
 
 // Base dtor: actually calls A's base dtor.
 // CHECK-LABEL: define void @_ZN1BD2Ev(%struct.B* %this) unnamed_addr