Index: lib/CodeGen/ItaniumCXXABI.cpp
===================================================================
--- lib/CodeGen/ItaniumCXXABI.cpp
+++ lib/CodeGen/ItaniumCXXABI.cpp
@@ -284,6 +284,14 @@
     // linkage together with vtables when needed.
     if (ForVTable && !Thunk->hasLocalLinkage())
       Thunk->setLinkage(llvm::GlobalValue::AvailableExternallyLinkage);
+
+    // Propagate dllexport storage, to enable the linker to generate import
+    // thunks as necessary (e.g. when a parent class has a key function and a
+    // child class doesn't, and the construction vtable for the parent in the
+    // child needs to reference the parent's thunks).
+    const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
+    if (MD->hasAttr<DLLExportAttr>())
+      Thunk->setDLLStorageClass(llvm::GlobalValue::DLLExportStorageClass);
   }
 
   llvm::Value *performThisAdjustment(CodeGenFunction &CGF, Address This,
Index: test/CodeGenCXX/dllexport-vtable-thunks.cpp
===================================================================
--- /dev/null
+++ test/CodeGenCXX/dllexport-vtable-thunks.cpp
@@ -0,0 +1,23 @@
+// RUN: %clang_cc1 -triple x86_64-windows-gnu -fdeclspec -emit-llvm -o - %s | FileCheck %s
+// RUN: %clang_cc1 -triple x86_64-windows-itanium -fdeclspec -emit-llvm -o - %s | FileCheck %s
+
+struct __declspec(dllexport) A {
+  virtual void m();
+};
+struct __declspec(dllexport) B {
+  virtual void m();
+};
+struct __declspec(dllexport) C : A, B {
+  virtual void m();
+};
+void C::m() {}
+// CHECK: define dllexport void @_ZThn8_N1C1mEv
+
+struct Base {
+  virtual void m();
+};
+struct __declspec(dllexport) Derived : virtual Base {
+  virtual void m();
+};
+void Derived::m() {}
+// CHECK: define dllexport void @_ZTv0_n24_N7Derived1mEv