Index: include/clang/Basic/DiagnosticCommonKinds.td
===================================================================
--- include/clang/Basic/DiagnosticCommonKinds.td
+++ include/clang/Basic/DiagnosticCommonKinds.td
@@ -123,6 +123,8 @@
 def err_target_unknown_fpmath : Error<"unknown FP unit '%0'">;
 def err_target_unsupported_fpmath : Error<
     "the '%0' unit is not supported with this instruction set">;
+def err_target_unsupported_unaligned : Error<
+  "the %0 sub-architecture does not support unaligned accesses">;
 
 // Source manager
 def err_cannot_open_file : Error<"cannot open file '%0': %1">, DefaultFatal;
Index: lib/Driver/Tools.cpp
===================================================================
--- lib/Driver/Tools.cpp
+++ lib/Driver/Tools.cpp
@@ -806,8 +806,12 @@
       CmdArgs.push_back("-backend-option");
       if (A->getOption().matches(options::OPT_mno_unaligned_access))
         CmdArgs.push_back("-arm-strict-align");
-      else
+      else {
+        if (getToolChain().getTriple().getSubArch() ==
+            llvm::Triple::SubArchType::ARMSubArch_v6m)
+          D.Diag(diag::err_target_unsupported_unaligned) << "v6m";
         CmdArgs.push_back("-arm-no-strict-align");
+      }
     }
   }