Index: cfe/trunk/lib/Driver/Tools.cpp
===================================================================
--- cfe/trunk/lib/Driver/Tools.cpp
+++ cfe/trunk/lib/Driver/Tools.cpp
@@ -743,6 +743,13 @@
   if (const Arg *A = Args.getLastArg(options::OPT_mhwdiv_EQ))
     getARMHWDivFeatures(D, A, Args, Features);
 
+  // -march is handled in getARMCPUForMarch by translating it into a CPU name,
+  // but it needs to return an empty string on invalid arguments. We therefore
+  // check and give an error here if the -march is invalid.
+  if (const Arg *A = Args.getLastArg(options::OPT_march_EQ))
+    if (!Triple.getARMCPUForArch(A->getValue()))
+      D.Diag(diag::err_drv_clang_unsupported) << A->getAsString(Args);
+
   // Setting -msoft-float effectively disables NEON because of the GCC
   // implementation, although the same isn't true of VFP or VFP3.
   if (FloatABI == "soft") {
@@ -5619,7 +5626,13 @@
     }
   }
 
-  return Triple.getARMCPUForArch(MArch);
+  // We need to return an empty string here on invalid MArch values as the
+  // various places that call this function can't cope with a null result.
+  const char *result = Triple.getARMCPUForArch(MArch);
+  if (result)
+    return result;
+  else
+    return "";
 }
 
 /// getARMTargetCPU - Get the (LLVM) name of the ARM cpu we are targeting.
Index: cfe/trunk/test/Driver/arm-cortex-cpus.c
===================================================================
--- cfe/trunk/test/Driver/arm-cortex-cpus.c
+++ cfe/trunk/test/Driver/arm-cortex-cpus.c
@@ -166,11 +166,11 @@
 // RUN: %clang -target arm -march=armebv8a -mbig-endian -mthumb -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-BE-V8A-THUMB %s
 // CHECK-BE-V8A-THUMB: "-cc1"{{.*}} "-triple" "thumbebv8-{{.*}}" "-target-cpu" "cortex-a53"
 
-// ================== Check default CPU on bogus architecture
+// ================== Check that a bogus architecture gives an error
 // RUN: %clang -target arm -march=armbogusv6 -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-BOGUS %s
-// CHECK-BOGUS: "-cc1"{{.*}} "-triple" "armv4t-{{.*}} "-target-cpu" "arm7tdmi"
+// CHECK-BOGUS: error: the clang compiler does not support '-march=armbogusv6' 
 // RUN: %clang -target arm---eabihf -march=armbogusv7 -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-BOGUS-HF %s
-// CHECK-BOGUS-HF: "-cc1"{{.*}} "-triple" "armv6k-{{.*}} "-target-cpu" "arm1176jzf-s"
+// CHECK-BOGUS-HF: error: the clang compiler does not support '-march=armbogusv7' 
 
 // ================== Check default Architecture on each ARM11 CPU
 // RUN: %clang -target arm-linux-gnueabi -mcpu=arm1136j-s -### -c %s 2>&1 | FileCheck -check-prefix=CHECK-CPUV6 %s