diff --git a/llvm/lib/Support/Mutex.cpp b/llvm/lib/Support/Mutex.cpp
index b1d5e7c..be501e8 100644
--- a/llvm/lib/Support/Mutex.cpp
+++ b/llvm/lib/Support/Mutex.cpp
@@ -119,9 +119,9 @@
 
 #elif defined(LLVM_ON_UNIX)
 #include "Unix/Mutex.inc"
-#elif defined( LLVM_ON_WIN32)
+#elif defined( _WIN32)
 #include "Windows/Mutex.inc"
 #else
-#warning Neither LLVM_ON_UNIX nor LLVM_ON_WIN32 was set in Support/Mutex.cpp
+#warning Neither LLVM_ON_UNIX nor _WIN32 was set in Support/Mutex.cpp
 #endif
 #endif