Index: lib/CodeGen/CodeGenModule.cpp
===================================================================
--- lib/CodeGen/CodeGenModule.cpp
+++ lib/CodeGen/CodeGenModule.cpp
@@ -763,6 +763,13 @@
         !GV->isThreadLocal())
       return false;
   }
+
+  // On COFF, don't mark 'extern_weak' symbols as DSO local. If these symbols
+  // remain unresolved in the link, they can be resolved to zero, which is
+  // outside the current DSO.
+  if (TT.isOSBinFormatCOFF() && GV->hasExternalWeakLinkage())
+    return false;
+
   // Every other GV is local on COFF.
   // Make an exception for windows OS in the triple: Some firmware builds use
   // *-win32-macho triples. This (accidentally?) produced windows relocations
Index: test/CodeGen/dso-local-executable.c
===================================================================
--- test/CodeGen/dso-local-executable.c
+++ test/CodeGen/dso-local-executable.c
@@ -1,6 +1,6 @@
 // RUN: %clang_cc1 -triple x86_64-pc-win32 -emit-llvm %s -o - | FileCheck -allow-deprecated-dag-overlap --check-prefix=COFF %s
 // COFF-DAG: @bar = external dso_local global i32
-// COFF-DAG: @weak_bar = extern_weak dso_local global i32
+// COFF-DAG: @weak_bar = extern_weak global i32
 // COFF-DAG: declare dso_local void @foo()
 // COFF-DAG: @baz = dso_local global i32 42
 // COFF-DAG: define dso_local i32* @zed()