Index: lib/CodeGen/CodeGenModule.cpp
===================================================================
--- lib/CodeGen/CodeGenModule.cpp
+++ lib/CodeGen/CodeGenModule.cpp
@@ -3761,6 +3761,11 @@
       }
     }
   }
+  // COFF doesn't support alignments greater than 32, so these cannot be
+  // in common.
+  if (Context.getTargetInfo().getTriple().isKnownWindowsMSVCEnvironment() &&
+      Context.getTypeAlignIfKnown(D->getType()) > 32)
+    return true;
 
   return false;
 }
Index: test/CodeGen/microsoft-no-common-align.c
===================================================================
--- test/CodeGen/microsoft-no-common-align.c
+++ test/CodeGen/microsoft-no-common-align.c
@@ -0,0 +1,8 @@
+// RUN: %clang_cc1 -triple x86_64-windows-msvc -emit-llvm -o - %s | FileCheck %s
+typedef float TooLargeAlignment __attribute__((__vector_size__(64)));
+typedef float NormalAlignment __attribute__((__vector_size__(4)));
+
+TooLargeAlignment TooBig;
+// CHECK: @TooBig = dso_local global <16 x float>  zeroinitializer, align 64
+NormalAlignment JustRight;
+// CHECK: @JustRight = common dso_local global <1 x float>  zeroinitializer, align 4