diff --git a/llvm/lib/Support/Threading.cpp b/llvm/lib/Support/Threading.cpp
index a4e1e44..de2ff6b 100644
--- a/llvm/lib/Support/Threading.cpp
+++ b/llvm/lib/Support/Threading.cpp
@@ -72,6 +72,11 @@
 #include "Windows/WindowsSupport.h"
 #include <process.h>
 
+// Windows will at times define MemoryFence.
+#ifdef MemoryFence
+#undef MemoryFence
+#endif
+
 struct ThreadInfo {
   void (*func)(void*);
   void *param;