diff --git a/clang/lib/Driver/ToolChains/CrossWindows.cpp b/clang/lib/Driver/ToolChains/CrossWindows.cpp
--- a/clang/lib/Driver/ToolChains/CrossWindows.cpp
+++ b/clang/lib/Driver/ToolChains/CrossWindows.cpp
@@ -94,7 +94,8 @@
   CmdArgs.push_back("-m");
   switch (TC.getArch()) {
   default:
-    llvm_unreachable("unsupported architecture");
+    D.Diag(diag::err_target_unknown_triple) << TC.getEffectiveTriple().str();
+    break;
   case llvm::Triple::arm:
   case llvm::Triple::thumb:
     // FIXME: this is incorrect for WinCE
diff --git a/clang/lib/Driver/ToolChains/MinGW.cpp b/clang/lib/Driver/ToolChains/MinGW.cpp
--- a/clang/lib/Driver/ToolChains/MinGW.cpp
+++ b/clang/lib/Driver/ToolChains/MinGW.cpp
@@ -135,7 +135,7 @@
     CmdArgs.push_back("arm64pe");
     break;
   default:
-    llvm_unreachable("Unsupported target architecture.");
+    D.Diag(diag::err_target_unknown_triple) << TC.getEffectiveTriple().str();
   }
 
   Arg *SubsysArg =
diff --git a/clang/test/Driver/unsupported-target-arch.c b/clang/test/Driver/unsupported-target-arch.c
--- a/clang/test/Driver/unsupported-target-arch.c
+++ b/clang/test/Driver/unsupported-target-arch.c
@@ -23,3 +23,11 @@
 // RUN: not %clang --target=noarch-unknown-nacl -o %t.o %s 2> %t.err
 // RUN: FileCheck --input-file=%t.err --check-prefix=CHECK-NOARCH-NACL %s
 // CHECK-NOARCH-NACL:  error: the target architecture 'noarch' is not supported by the target 'Native Client'
+
+// RUN: not %clang --target=noarch-unknown-windows-gnu -o %t.o %s 2> %t.err
+// RUN: FileCheck --input-file=%t.err --check-prefix=CHECK-NOARCH-MINGW %s
+// CHECK-NOARCH-MINGW: error: unknown target triple 'noarch-unknown-windows-gnu'
+
+// RUN: not %clang --target=noarch-unknown-windows-itanium -o %t.o %s 2> %t.err
+// RUN: FileCheck --input-file=%t.err --check-prefix=CHECK-NOARCH-CROSSWINDOWS %s
+// CHECK-NOARCH-CROSSWINDOWS: error: unknown target triple 'noarch-unknown-windows-itanium'