diff --git a/lib/Target/R600/SIISelLowering.cpp b/lib/Target/R600/SIISelLowering.cpp
index e6c1614eb27..b3e3997a305 100644
--- a/lib/Target/R600/SIISelLowering.cpp
+++ b/lib/Target/R600/SIISelLowering.cpp
@@ -12,6 +12,12 @@
 //
 //===----------------------------------------------------------------------===//
 
+#ifdef _MSC_VER
+// Provide M_PI.
+#define _USE_MATH_DEFINES
+#include <cmath>
+#endif
+
 #include "SIISelLowering.h"
 #include "AMDGPU.h"
 #include "AMDGPUIntrinsicInfo.h"