[MS] Don't expect vftables to be provided for extern template instantiations

[rnk]

Summary

MSVC doesn't provide them. PR28223

I left behind the machinery in case we want to resurrect available_externally
vftable emission to support devirtualization.

[majnemer]

LGTM

cfe/trunk/lib/AST/VTableBuilder.cpp

public:
   VFTableBuilder(MicrosoftVTableContext &VTables,
                  const CXXRecordDecl *MostDerivedClass, const VPtrInfo *Which)
       : VTables(VTables),
         Context(MostDerivedClass->getASTContext()),
         MostDerivedClass(MostDerivedClass),
         MostDerivedClassLayout(Context.getASTRecordLayout(MostDerivedClass)),
         WhichVFPtr(*Which),
         Overriders(MostDerivedClass, CharUnits(), MostDerivedClass) {
-    // Only include the RTTI component if we know that we will provide a
-    // definition of the vftable.  We always provide the definition of
-    // dllimported classes.
+    // Provide the RTTI component if RTTIData is enabled. If the vftable would
+    // be available externally, we should not provide the RTTI componenent. It
+    // is currently impossible to get available externally vftables with either
+    // dllimport or extern template instantiations, but eventually we may add a
+    // flag to support additional devirtualization that needs this.
     if (Context.getLangOpts().RTTIData)
-      if (MostDerivedClass->hasAttr<DLLImportAttr>() ||
-          MostDerivedClass->getTemplateSpecializationKind() !=
-              TSK_ExplicitInstantiationDeclaration)
-        HasRTTIComponent = true;
 
     LayoutVFTable();
 
     if (Context.getLangOpts().DumpVTableLayouts)
       dumpLayout(llvm::outs());
   }
 
   uint64_t getNumThunks() const { return Thunks.size(); }

cfe/trunk/lib/CodeGen/CGVTables.cpp

CodeGenModule::getVTableLinkage(const CXXRecordDecl *RD) {
 
   switch (RD->getTemplateSpecializationKind()) {
     case TSK_Undeclared:
     case TSK_ExplicitSpecialization:
     case TSK_ImplicitInstantiation:
       return DiscardableODRLinkage;
 
     case TSK_ExplicitInstantiationDeclaration:
+      // Explicit instantiations in MSVC do not provide vtables, so we must emit
+      // our own.
+      if (getTarget().getCXXABI().isMicrosoft())
+        return DiscardableODRLinkage;
       return shouldEmitAvailableExternallyVTable(*this, RD)
                  ? llvm::GlobalVariable::AvailableExternallyLinkage
                  : llvm::GlobalVariable::ExternalLinkage;
 
     case TSK_ExplicitInstantiationDefinition:
       return NonDiscardableODRLinkage;
   }
 

 ///
 /// The only semantic restriction here is that the object file should
 /// not contain a vtable definition when that vtable is defined
 /// strongly elsewhere.  Otherwise, we'd just like to avoid emitting
 /// vtables when unnecessary.
 bool CodeGenVTables::isVTableExternal(const CXXRecordDecl *RD) {
   assert(RD->isDynamicClass() && "Non-dynamic classes have no VTable.");
 
-  // We always synthesize vtables on the import side regardless of whether or
-  // not it is an explicit instantiation declaration.
-  if (CGM.getTarget().getCXXABI().isMicrosoft() && RD->hasAttr<DLLImportAttr>())
+  // We always synthesize vtables if they are needed in the MS ABI. MSVC doesn't
+  // emit them even if there is an explicit template instantiation.
+  if (CGM.getTarget().getCXXABI().isMicrosoft())
     return false;
 
   // If we have an explicit instantiation declaration (and not a
   // definition), the vtable is defined elsewhere.
   TemplateSpecializationKind TSK = RD->getTemplateSpecializationKind();
   if (TSK == TSK_ExplicitInstantiationDeclaration)
     return true;
 

cfe/trunk/test/CodeGenCXX/microsoft-abi-vbtables.cpp

 namespace Test30 {
 struct A {};
 template <class> struct B : virtual A {
   B() {}
 };
 
 extern template class B<int>;
 template B<int>::B();
-// CHECK-DAG: @"\01??_8?$B@H@Test30@@7B@" = external unnamed_addr constant [2 x i32]{{$}}
+// CHECK-DAG: @"\01??_8?$B@H@Test30@@7B@" = linkonce_odr unnamed_addr constant [2 x i32] [i32 0, i32 4], comdat
 }

cfe/trunk/test/CodeGenCXX/microsoft-abi-vftables.cpp

 
 // RTTI-DAG: [[VTABLE_V:@.*]] = private unnamed_addr constant [2 x i8*] [i8* bitcast ({{.*}} @"\01??_R4V@@6B@" to i8*), i8* bitcast ({{.*}} @"\01??_GV@@UAEPAXI@Z" to i8*)], comdat($"\01??_7V@@6B@")
 // RTTI-DAG: @"\01??_7V@@6B@" = dllexport unnamed_addr alias i8*, getelementptr inbounds ([2 x i8*], [2 x i8*]* [[VTABLE_V]], i32 0, i32 1)
 
 // NO-RTTI-DAG: @"\01??_7V@@6B@" = weak_odr dllexport unnamed_addr constant [1 x i8*] [i8* bitcast ({{.*}} @"\01??_GV@@UAEPAXI@Z" to i8*)]
 
 namespace {
 struct W {
-  virtual ~W();
+  virtual ~W() {}
 } w;
 }
 // RTTI-DAG: [[VTABLE_W:@.*]] = private unnamed_addr constant [2 x i8*] [i8* bitcast ({{.*}} @"\01??_R4W@?A@@6B@" to i8*), i8* bitcast ({{.*}} @"\01??_GW@?A@@UAEPAXI@Z" to i8*)]
 // RTTI-DAG: @"\01??_7W@?A@@6B@" = internal unnamed_addr alias i8*, getelementptr inbounds ([2 x i8*], [2 x i8*]* [[VTABLE_W]], i32 0, i32 1)
 
 // NO-RTTI-DAG: @"\01??_7W@?A@@6B@" = internal unnamed_addr constant [1 x i8*] [i8* bitcast ({{.*}} @"\01??_GW@?A@@UAEPAXI@Z" to i8*)]
 
 struct X {};
 template <class> struct Y : virtual X {
   Y() {}
   virtual ~Y();
 };
 
 extern template class Y<int>;
 template Y<int>::Y();
-// RTTI-DAG: @"\01??_7?$Y@H@@6B@" = external unnamed_addr constant [1 x i8*]
-// NO-RTTI-DAG: @"\01??_7?$Y@H@@6B@" = external unnamed_addr constant [1 x i8*]
+// RTTI-DAG: [[VTABLE_Y:@.*]] = private unnamed_addr constant [2 x i8*] [i8* bitcast (%rtti.CompleteObjectLocator* @"\01??_R4?$Y@H@@6B@" to i8*), i8* bitcast (i8* (%struct.Y*, i32)* @"\01??_G?$Y@H@@UAEPAXI@Z" to i8*)], comdat($"\01??_7?$Y@H@@6B@")
+// RTTI-DAG: @"\01??_7?$Y@H@@6B@" = unnamed_addr alias i8*, getelementptr inbounds ([2 x i8*], [2 x i8*]* [[VTABLE_Y]], i32 0, i32 1)
+
+// NO-RTTI-DAG: @"\01??_7?$Y@H@@6B@" = linkonce_odr unnamed_addr constant [1 x i8*] [i8* bitcast (i8* (%struct.Y*, i32)* @"\01??_G?$Y@H@@UAEPAXI@Z" to i8*)], comdat