Index: lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp
===================================================================
--- lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp
+++ lib/Target/ARM/MCTargetDesc/ARMMCTargetDesc.cpp
@@ -153,6 +153,13 @@
       ARMArchFeature += ",+nacl-trap";
   }
 
+  if (TT.isOSWindows()) {
+    if (ARMArchFeature.empty())
+      ARMArchFeature = "+noarm";
+    else
+      ARMArchFeature += ",+noarm";
+  }
+
   return ARMArchFeature;
 }
 
Index: test/MC/ARM/arm-thumb-cpus.s
===================================================================
--- test/MC/ARM/arm-thumb-cpus.s
+++ test/MC/ARM/arm-thumb-cpus.s
@@ -19,6 +19,9 @@
 @ RUN: not llvm-mc -show-encoding -triple=armv6sm-eabi < %s 2>&1 \
 @ RUN:  | FileCheck %s --check-prefix=CHECK-THUMB-ONLY
 
+@ RUN: not llvm-mc -show-encoding -triple=thumbv7-win32-gnu < %s 2>&1 \
+@ RUN:  | FileCheck %s --check-prefix=CHECK-THUMB-ONLY
+
         @ Make sure correct diagnostics are given for CPUs without support for
         @ one or other of the execution states.
         .thumb