Index: clang/lib/CodeGen/ItaniumCXXABI.cpp
===================================================================
--- clang/lib/CodeGen/ItaniumCXXABI.cpp
+++ clang/lib/CodeGen/ItaniumCXXABI.cpp
@@ -2690,7 +2690,9 @@
 
     if (Init) {
       Init->setVisibility(Var->getVisibility());
-      Init->setDSOLocal(Var->isDSOLocal());
+      // Don't mark an extern_weak function DSO local on windows.
+      if (!CGM.getTriple().isOSWindows() || !Init->hasExternalWeakLinkage())
+        Init->setDSOLocal(Var->isDSOLocal());
     }
 
     llvm::LLVMContext &Context = CGM.getModule().getContext();
Index: clang/test/CodeGenCXX/mingw-thread-local.cpp
===================================================================
--- /dev/null
+++ clang/test/CodeGenCXX/mingw-thread-local.cpp
@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -std=c++11 -emit-llvm %s -o - -triple x86_64-w64-mingw32 | FileCheck %s
+
+extern thread_local int var;
+
+int get() {
+  return var;
+}
+
+// CHECK: declare extern_weak void @_ZTH3var()