8 files changed, 51 insertions, 17 deletions
diff --git a/clang/lib/AST/MicrosoftMangle.cpp b/clang/lib/AST/MicrosoftMangle.cpp
index 0634319cbd1..930cf00d9e3 100644
--- a/clang/lib/AST/MicrosoftMangle.cpp
+++ b/clang/lib/AST/MicrosoftMangle.cpp
@@ -2599,7 +2599,10 @@ void MicrosoftMangleContextImpl::mangleCXXVFTable(
   // NOTE: <cvr-qualifiers> here is always 'B' (const). <storage-class>
   // is always '6' for vftables.
   MicrosoftCXXNameMangler Mangler(*this, Out);
-  Mangler.getStream() << "\01??_7";
+  if (Derived->hasAttr<DLLImportAttr>())
+    Mangler.getStream() << "\01??_S";
+  else
+    Mangler.getStream() << "\01??_7";
   Mangler.mangleName(Derived);
   Mangler.getStream() << "6B"; // '6' for vftable, 'B' for const.
   for (const CXXRecordDecl *RD : BasePath)
diff --git a/clang/lib/AST/VTableBuilder.cpp b/clang/lib/AST/VTableBuilder.cpp
index bae018652f9..e43acc4de78 100644
--- a/clang/lib/AST/VTableBuilder.cpp
+++ b/clang/lib/AST/VTableBuilder.cpp
@@ -2416,7 +2416,7 @@ private:
   MethodVFTableLocationsTy MethodVFTableLocations;
 
   /// \brief Does this class have an RTTI component?
-  bool HasRTTIComponent;
+  bool HasRTTIComponent = false;
 
   /// MethodInfo - Contains information about a method in a vtable.
   /// (Used for computing 'this' pointer adjustment thunks.
@@ -2546,11 +2546,13 @@ public:
         WhichVFPtr(*Which),
         Overriders(MostDerivedClass, CharUnits(), MostDerivedClass) {
     // Only include the RTTI component if we know that we will provide a
-    // definition of the vftable.
-    HasRTTIComponent = Context.getLangOpts().RTTIData &&
-                       !MostDerivedClass->hasAttr<DLLImportAttr>() &&
-                       MostDerivedClass->getTemplateSpecializationKind() !=
-                           TSK_ExplicitInstantiationDeclaration;
+    // definition of the vftable.  We always provide the definition of
+    // dllimported classes.
+    if (Context.getLangOpts().RTTIData)
+      if (MostDerivedClass->hasAttr<DLLImportAttr>() ||
+          MostDerivedClass->getTemplateSpecializationKind() !=
+              TSK_ExplicitInstantiationDeclaration)
+        HasRTTIComponent = true;
 
     LayoutVFTable();
 
diff --git a/clang/lib/CodeGen/CGVTables.cpp b/clang/lib/CodeGen/CGVTables.cpp
index 620e9223b2e..e96a8120207 100644
--- a/clang/lib/CodeGen/CGVTables.cpp
+++ b/clang/lib/CodeGen/CGVTables.cpp
@@ -838,6 +838,11 @@ CodeGenVTables::GenerateClassData(const CXXRecordDecl *RD) {
 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>())
+    return false;
+
   // If we have an explicit instantiation declaration (and not a
   // definition), the vtable is defined elsewhere.
   TemplateSpecializationKind TSK = RD->getTemplateSpecializationKind();
diff --git a/clang/lib/CodeGen/MicrosoftCXXABI.cpp b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
index 2870a91dd5f..2567aa92f25 100644
--- a/clang/lib/CodeGen/MicrosoftCXXABI.cpp
+++ b/clang/lib/CodeGen/MicrosoftCXXABI.cpp
@@ -1673,7 +1673,16 @@ llvm::GlobalVariable *MicrosoftCXXABI::getAddrOfVTable(const CXXRecordDecl *RD,
   SmallString<256> VFTableName;
   mangleVFTableName(getMangleContext(), RD, VFPtr, VFTableName);
 
-  llvm::GlobalValue::LinkageTypes VFTableLinkage = CGM.getVTableLinkage(RD);
+  // Classes marked __declspec(dllimport) need vftables generated on the
+  // import-side in order to support features like constexpr.  No other
+  // translation unit relies on the emission of the local vftable, translation
+  // units are expected to generate them as needed.
+  //
+  // Because of this unique behavior, we maintain this logic here instead of
+  // getVTableLinkage.
+  llvm::GlobalValue::LinkageTypes VFTableLinkage =
+      RD->hasAttr<DLLImportAttr>() ? llvm::GlobalValue::LinkOnceODRLinkage
+                                   : CGM.getVTableLinkage(RD);
   bool VFTableComesFromAnotherTU =
       llvm::GlobalValue::isAvailableExternallyLinkage(VFTableLinkage) ||
       llvm::GlobalValue::isExternalLinkage(VFTableLinkage);
@@ -1746,9 +1755,7 @@ llvm::GlobalVariable *MicrosoftCXXABI::getAddrOfVTable(const CXXRecordDecl *RD,
   if (C)
     VTable->setComdat(C);
 
-  if (RD->hasAttr<DLLImportAttr>())
-    VFTable->setDLLStorageClass(llvm::GlobalValue::DLLImportStorageClass);
-  else if (RD->hasAttr<DLLExportAttr>())
+  if (RD->hasAttr<DLLExportAttr>())
     VFTable->setDLLStorageClass(llvm::GlobalValue::DLLExportStorageClass);
 
   VFTablesMap[ID] = VFTable;
diff --git a/clang/test/CodeGenCXX/PR26569.cpp b/clang/test/CodeGenCXX/PR26569.cpp
new file mode 100644
index 00000000000..666a6466aa4
--- /dev/null
+++ b/clang/test/CodeGenCXX/PR26569.cpp
@@ -0,0 +1,17 @@
+// RUN: %clang_cc1 -triple i686-pc-win32 -fms-extensions -emit-llvm -O1 -disable-llvm-optzns %s -o - | FileCheck %s
+
+class A {
+  virtual void m_fn1();
+};
+template <typename>
+class B : A {};
+
+extern template class __declspec(dllimport) B<int>;
+class __declspec(dllexport) C : B<int> {};
+
+// CHECK: @[[VTABLE_C:.*]] = private unnamed_addr constant [2 x i8*] [i8* bitcast (%rtti.CompleteObjectLocator* @"\01??_R4C@@6B@" to i8*), i8* bitcast (void (%class.A*)* @"\01?m_fn1@A@@EAEXXZ" to i8*)]
+// CHECK: @[[VTABLE_B:.*]] = private unnamed_addr constant [2 x i8*] [i8* bitcast (%rtti.CompleteObjectLocator* @"\01??_R4?$B@H@@6B@" to i8*), i8* bitcast (void (%class.A*)* @"\01?m_fn1@A@@EAEXXZ" to i8*)], comdat($"\01??_S?$B@H@@6B@")
+// CHECK: @[[VTABLE_A:.*]] = private unnamed_addr constant [2 x i8*] [i8* bitcast (%rtti.CompleteObjectLocator* @"\01??_R4A@@6B@" to i8*), i8* bitcast (void (%class.A*)* @"\01?m_fn1@A@@EAEXXZ" to i8*)], comdat($"\01??_7A@@6B@")
+// CHECK: @"\01??_7C@@6B@" = dllexport unnamed_addr alias i8*, getelementptr inbounds ([2 x i8*], [2 x i8*]* @[[VTABLE_C]], i32 0, i32 1)
+// CHECK: @"\01??_S?$B@H@@6B@" = unnamed_addr alias i8*, getelementptr inbounds ([2 x i8*], [2 x i8*]* @[[VTABLE_B]], i32 0, i32 1)
+// CHECK: @"\01??_7A@@6B@" = unnamed_addr alias i8*, getelementptr inbounds ([2 x i8*], [2 x i8*]* @[[VTABLE_A]], i32 0, i32 1)
diff --git a/clang/test/CodeGenCXX/dllimport-rtti.cpp b/clang/test/CodeGenCXX/dllimport-rtti.cpp
index 071ce278a5b..4baee50fcff 100644
--- a/clang/test/CodeGenCXX/dllimport-rtti.cpp
+++ b/clang/test/CodeGenCXX/dllimport-rtti.cpp
@@ -4,7 +4,8 @@
 struct __declspec(dllimport) S {
   virtual void f() {}
 } s;
-// MSVC-DAG: @"\01??_7S@@6B@" = available_externally dllimport
+// MSVC: [[VF_S:.*]] = private unnamed_addr constant [2 x i8*]
+// MSVC-DAG: @"\01??_SS@@6B@" = unnamed_addr alias i8*, getelementptr inbounds ([2 x i8*], [2 x i8*]* [[VF_S]], i32 0, i32 1)
 // MSVC-DAG: @"\01??_R0?AUS@@@8" = linkonce_odr
 // MSVC-DAG: @"\01??_R1A@?0A@EA@S@@8" = linkonce_odr
 // MSVC-DAG: @"\01??_R2S@@8" = linkonce_odr
diff --git a/clang/test/CodeGenCXX/dllimport.cpp b/clang/test/CodeGenCXX/dllimport.cpp
index b9c850b8b87..e336bfb6924 100644
--- a/clang/test/CodeGenCXX/dllimport.cpp
+++ b/clang/test/CodeGenCXX/dllimport.cpp
@@ -614,7 +614,7 @@ USEMEMFUNC(V, foo)
 struct __declspec(dllimport) W { virtual void foo() {} };
 USECLASS(W)
 // vftable:
-// MO1-DAG: @"\01??_7W@@6B@" = available_externally dllimport unnamed_addr constant [1 x i8*] [i8* bitcast (void (%struct.W*)* @"\01?foo@W@@UAEXXZ" to i8*)]
+// MO1-DAG: @"\01??_SW@@6B@" = linkonce_odr unnamed_addr constant [1 x i8*] [i8* bitcast (void (%struct.W*)* @"\01?foo@W@@UAEXXZ" to i8*)]
 // GO1-DAG: @_ZTV1W = available_externally dllimport unnamed_addr constant [3 x i8*] [i8* null, i8* null, i8* bitcast (void (%struct.W*)* @_ZN1W3fooEv to i8*)]
 
 struct __declspec(dllimport) KeyFuncClass {
diff --git a/clang/test/CodeGenCXX/microsoft-abi-vftables.cpp b/clang/test/CodeGenCXX/microsoft-abi-vftables.cpp
index f32bff09a8a..d94c9c1a909 100644
--- a/clang/test/CodeGenCXX/microsoft-abi-vftables.cpp
+++ b/clang/test/CodeGenCXX/microsoft-abi-vftables.cpp
@@ -4,8 +4,6 @@
 // RTTI-DAG: $"\01??_7S@@6B@" = comdat largest
 // RTTI-DAG: $"\01??_7V@@6B@" = comdat largest
 
-// RTTI-NOT: @"\01??_R4U@@6B@"
-
 struct S {
   virtual ~S();
 } s;
@@ -19,9 +17,10 @@ struct __declspec(dllimport) U {
   virtual ~U();
 } u;
 
-// RTTI-DAG: @"\01??_7U@@6B@" = available_externally dllimport unnamed_addr constant [1 x i8*] [i8* bitcast ({{.*}} @"\01??_GU@@UAEPAXI@Z" to i8*)]
+// RTTI-DAG: [[VTABLE_U:@.*]] = private unnamed_addr constant [2 x i8*] [i8* bitcast ({{.*}} @"\01??_R4U@@6B@" to i8*), i8* bitcast ({{.*}} @"\01??_GU@@UAEPAXI@Z" to i8*)]
+// RTTI-DAG: @"\01??_SU@@6B@" = unnamed_addr alias i8*, getelementptr inbounds ([2 x i8*], [2 x i8*]* [[VTABLE_U]], i32 0, i32 1)
 
-// NO-RTTI-DAG: @"\01??_7U@@6B@" = available_externally dllimport unnamed_addr constant [1 x i8*] [i8* bitcast ({{.*}} @"\01??_GU@@UAEPAXI@Z" to i8*)]
+// NO-RTTI-DAG: @"\01??_SU@@6B@" = linkonce_odr unnamed_addr constant [1 x i8*] [i8* bitcast ({{.*}} @"\01??_GU@@UAEPAXI@Z" to i8*)]
 
 struct __declspec(dllexport) V {
   virtual ~V();