Index: cfe/trunk/lib/CodeGen/CGDecl.cpp
===================================================================
--- cfe/trunk/lib/CodeGen/CGDecl.cpp
+++ cfe/trunk/lib/CodeGen/CGDecl.cpp
@@ -210,8 +210,7 @@
   GV->setAlignment(getContext().getDeclAlign(&D).getQuantity());
   setGlobalVisibility(GV, &D);
 
-  if (supportsCOMDAT() && GV->isWeakForLinker())
-    GV->setComdat(TheModule.getOrInsertComdat(GV->getName()));
+  maybeSetTrivialComdat(*GV);
 
   if (D.getTLSKind())
     setTLSMode(GV, D);
Index: cfe/trunk/lib/CodeGen/CGVTT.cpp
===================================================================
--- cfe/trunk/lib/CodeGen/CGVTT.cpp
+++ cfe/trunk/lib/CodeGen/CGVTT.cpp
@@ -94,8 +94,7 @@
   // Set the correct linkage.
   VTT->setLinkage(Linkage);
 
-  if (CGM.supportsCOMDAT() && VTT->isWeakForLinker())
-    VTT->setComdat(CGM.getModule().getOrInsertComdat(VTT->getName()));
+  CGM.maybeSetTrivialComdat(*VTT);
 
   // Set the right visibility.
   CGM.setGlobalVisibility(VTT, RD);
@@ -177,4 +176,3 @@
   
   return I->second;
 }
-
Index: cfe/trunk/lib/CodeGen/CGVTables.cpp
===================================================================
--- cfe/trunk/lib/CodeGen/CGVTables.cpp
+++ cfe/trunk/lib/CodeGen/CGVTables.cpp
@@ -378,9 +378,6 @@
   // Set the right linkage.
   CGM.setFunctionLinkage(GD, Fn);
 
-  if (CGM.supportsCOMDAT() && Fn->isWeakForLinker())
-    Fn->setComdat(CGM.getModule().getOrInsertComdat(Fn->getName()));
-
   // Set the right visibility.
   const CXXMethodDecl *MD = cast<CXXMethodDecl>(GD.getDecl());
   setThunkVisibility(CGM, MD, Thunk, Fn);
@@ -461,6 +458,7 @@
     CGM.getCXXABI().setThunkLinkage(ThunkFn, ForVTable, GD,
                                     !Thunk.Return.isEmpty());
   }
+  CGM.maybeSetTrivialComdat(*ThunkFn);
 }
 
 void CodeGenVTables::maybeEmitThunkForVTable(GlobalDecl GD,
Index: cfe/trunk/lib/CodeGen/CodeGenModule.h
===================================================================
--- cfe/trunk/lib/CodeGen/CodeGenModule.h
+++ cfe/trunk/lib/CodeGen/CodeGenModule.h
@@ -608,6 +608,7 @@
   const llvm::Triple &getTriple() const;
   bool supportsCOMDAT() const;
   void maybeSetTrivialComdat(const Decl &D, llvm::GlobalObject &GO);
+  void maybeSetTrivialComdat(llvm::GlobalObject &GO);
 
   CGCXXABI &getCXXABI() const { return *ABI; }
   llvm::LLVMContext &getLLVMContext() { return VMContext; }
Index: cfe/trunk/lib/CodeGen/CodeGenModule.cpp
===================================================================
--- cfe/trunk/lib/CodeGen/CodeGenModule.cpp
+++ cfe/trunk/lib/CodeGen/CodeGenModule.cpp
@@ -1298,8 +1298,7 @@
   auto *GV = new llvm::GlobalVariable(
       getModule(), Init->getType(),
       /*isConstant=*/true, llvm::GlobalValue::LinkOnceODRLinkage, Init, Name);
-  if (supportsCOMDAT())
-    GV->setComdat(TheModule.getOrInsertComdat(GV->getName()));
+  maybeSetTrivialComdat(*GV);
   return GV;
 }
 
@@ -1850,9 +1849,7 @@
     OldGV->eraseFromParent();
   }
 
-  if (supportsCOMDAT() && GV->isWeakForLinker() &&
-      !GV->hasAvailableExternallyLinkage())
-    GV->setComdat(TheModule.getOrInsertComdat(GV->getName()));
+  maybeSetTrivialComdat(*GV);
 
   return GV;
 }
@@ -1985,6 +1982,14 @@
   GO.setComdat(TheModule.getOrInsertComdat(GO.getName()));
 }
 
+void CodeGenModule::maybeSetTrivialComdat(llvm::GlobalObject &GO) {
+  if (!supportsCOMDAT())
+    return;
+  if (GO.isWeakForLinker() && !GO.hasAvailableExternallyLinkage()) {
+    GO.setComdat(getModule().getOrInsertComdat(GO.getName()));
+  }
+}
+
 void CodeGenModule::EmitGlobalVarDefinition(const VarDecl *D) {
   llvm::Constant *Init = nullptr;
   QualType ASTTy = D->getType();
@@ -2924,10 +2929,7 @@
       nullptr, llvm::GlobalVariable::NotThreadLocal, AddrSpace);
   GV->setAlignment(Alignment);
   GV->setUnnamedAddr(true);
-  if (GV->isWeakForLinker()) {
-    assert(CGM.supportsCOMDAT() && "Only COFF uses weak string literals");
-    GV->setComdat(M.getOrInsertComdat(GV->getName()));
-  }
+  CGM.maybeSetTrivialComdat(*GV);
 
   return GV;
 }
@@ -3109,8 +3111,7 @@
   setGlobalVisibility(GV, VD);
   GV->setAlignment(
       getContext().getTypeAlignInChars(MaterializedType).getQuantity());
-  if (supportsCOMDAT() && GV->isWeakForLinker())
-    GV->setComdat(TheModule.getOrInsertComdat(GV->getName()));
+  maybeSetTrivialComdat(*GV);
   if (VD->getTLSKind())
     setTLSMode(GV, *VD);
   Slot = GV;
Index: cfe/trunk/lib/CodeGen/ItaniumCXXABI.cpp
===================================================================
--- cfe/trunk/lib/CodeGen/ItaniumCXXABI.cpp
+++ cfe/trunk/lib/CodeGen/ItaniumCXXABI.cpp
@@ -1328,8 +1328,7 @@
   // Set the correct linkage.
   VTable->setLinkage(Linkage);
 
-  if (CGM.supportsCOMDAT() && VTable->isWeakForLinker())
-    VTable->setComdat(CGM.getModule().getOrInsertComdat(VTable->getName()));
+  CGM.maybeSetTrivialComdat(*VTable);
 
   // Set the right visibility.
   CGM.setGlobalVisibility(VTable, RD);
@@ -1796,8 +1795,8 @@
     if (!D.isLocalVarDecl() && C) {
       guard->setComdat(C);
       CGF.CurFn->setComdat(C);
-    } else if (CGM.supportsCOMDAT() && guard->isWeakForLinker()) {
-      guard->setComdat(CGM.getModule().getOrInsertComdat(guard->getName()));
+    } else {
+      CGM.maybeSetTrivialComdat(*guard);
     }
 
     CGM.setStaticLocalDeclGuardAddress(&D, guard);
@@ -2803,8 +2802,7 @@
       new llvm::GlobalVariable(M, Init->getType(),
                                /*Constant=*/true, Linkage, Init, Name);
 
-  if (CGM.supportsCOMDAT() && GV->isWeakForLinker())
-    GV->setComdat(M.getOrInsertComdat(GV->getName()));
+  CGM.maybeSetTrivialComdat(*GV);
 
   // If there's already an old global variable, replace it with the new one.
   if (OldGV) {
@@ -3598,8 +3596,7 @@
     // we don't want it to turn into an exported symbol.
     fn->setLinkage(llvm::Function::LinkOnceODRLinkage);
     fn->setVisibility(llvm::Function::HiddenVisibility);
-    if (CGM.supportsCOMDAT())
-      fn->setComdat(CGM.getModule().getOrInsertComdat(fn->getName()));
+    CGM.maybeSetTrivialComdat(*fn);
 
     // Set up the function.
     llvm::BasicBlock *entry =
Index: cfe/trunk/test/CodeGenCXX/thunks.cpp
===================================================================
--- cfe/trunk/test/CodeGenCXX/thunks.cpp
+++ cfe/trunk/test/CodeGenCXX/thunks.cpp
@@ -1,5 +1,5 @@
 // RUN: %clang_cc1 %s -triple=x86_64-pc-linux-gnu -munwind-tables -emit-llvm -o - | FileCheck %s
-// RUN: %clang_cc1 %s -triple=x86_64-pc-linux-gnu -munwind-tables -emit-llvm -o - -O1 -disable-llvm-optzns | FileCheck %s
+// RUN: %clang_cc1 %s -triple=x86_64-pc-linux-gnu -munwind-tables -emit-llvm -o - -O1 -disable-llvm-optzns | FileCheck %s --check-prefix=CHECK --check-prefix=CHECKOPT
 
 namespace Test1 {
 
@@ -361,6 +361,28 @@
   // CHECK: declare void @_ZThn8_N6Test151C1fEiz
 }
 
+namespace Test16 {
+
+// Check that the thunk for 'B::f' has available_externally linkage
+// and is not in a comdat.
+
+template <class C>
+struct A {
+  virtual void f();
+};
+
+template <class D>
+struct B : virtual A<D> {
+  virtual void f() { }
+};
+
+extern template struct B<int>;
+
+void f(B<int> b) {
+  b.f();
+}
+}
+
 /**** The following has to go at the end of the file ****/
 
 // This is from Test5:
@@ -371,4 +393,7 @@
 // CHECK-LABEL: define linkonce_odr void @_ZN6Test101C3fooEv
 // CHECK-LABEL: define linkonce_odr void @_ZThn8_N6Test101C3fooEv
 
+// CHECKOPT-LABEL: define available_externally void @_ZTv0_n24_N6Test161BIiE1fEv
+// CHECKOPT-NOT: comdat
+
 // CHECK: attributes [[NUW]] = { nounwind uwtable{{.*}} }