[Driver] Error if ARM mode was selected explicitly for M-profile CPUs.

[fhahn]

Summary

M-class profiles do not support ARM execution mode, so providing
-marm/-mno-thumb does not make sense in combination with -mcpu/-march
options that support the M-profile.

This is a follow-up patch to D35569 and it seemed pretty clear that we
should emit an error in the driver in this case.

We probably also should warn/error if the provided -mcpu/-march options
do not match, e.g. -mcpu=cortex-m0 -march=armv8-a is invalid, as
cortex-m0 does not support armv8-a. But that should be a separate patch
I think.

[rengolin]

This makes sense to me. LGTM. Thanks!

include/clang/Basic/DiagnosticDriverKinds.td

def err_drv_invalid_libcxx_deployment : Error<
   "invalid deployment target for -stdlib=libc++ (requires %0 or later)">;
 def err_drv_invalid_argument_to_fdebug_prefix_map : Error<
   "invalid argument '%0' to -fdebug-prefix-map">;
 def err_drv_malformed_sanitizer_blacklist : Error<
   "malformed sanitizer blacklist: '%0'">;
 
 def err_target_unsupported_arch
   : Error<"the target architecture '%0' is not supported by the target '%1'">;
+def err_cpu_unsupported_isa
+  : Error<"CPU '%0' does not support '%1' execution mode">;
+def err_arch_unsupported_isa
+  : Error<"Architecture '%0' does not support '%1' execution mode">;
 
 def err_drv_I_dash_not_supported : Error<
   "'%0' not supported, please use -iquote instead">;
 def err_drv_unknown_argument : Error<"unknown argument: '%0'">;
 def warn_drv_unknown_argument_clang_cl : Warning<
   "unknown argument ignored in clang-cl: '%0'">,
   InGroup<UnknownArgument>;
 

lib/Driver/ToolChain.cpp

case llvm::Triple::thumbeb: {
     if (getTriple().isOSWindows())
       ThumbDefault = true;
     std::string ArchName;
     if (IsBigEndian)
       ArchName = "armeb";
     else
       ArchName = "arm";
 
+    // Check if ARM ISA was explicitly selected (using -mno-thumb or -marm) for
+    // M-Class CPUs/architecture variants, which is not supported.
+    bool ARMModeRequested = !Args.hasFlag(options::OPT_mthumb,
+                                          options::OPT_mno_thumb, ThumbDefault);
+    if (IsMProfile && ARMModeRequested) {
+      if (!MCPU.empty())
+        getDriver().Diag(diag::err_cpu_unsupported_isa) << CPU << "ARM";
+       else
+        getDriver().Diag(diag::err_arch_unsupported_isa)
+          << tools::arm::getARMArch(MArch, getTriple()) << "ARM";
+    }
+
     // Assembly files should start in ARM mode, unless arch is M-profile.
     // Windows is always thumb.
     if ((InputType != types::TY_PP_Asm && Args.hasFlag(options::OPT_mthumb,
          options::OPT_mno_thumb, ThumbDefault)) || IsMProfile ||
          getTriple().isOSWindows()) {
       if (IsBigEndian)
         ArchName = "thumbeb";
       else

test/Driver/arm-thumb-only-cores.c

+// RUN: not %clang -target arm-unknown-linux -marm -mcpu=cortex-m0 %s -o /dev/null 2>&1 \
+// RUN:   | FileCheck --check-prefix M0 %s
+// M0: error: CPU 'cortex-m0' does not support 'ARM' execution mode
+
+// RUN: not %clang -target arm-unknown-linux -marm -march=armv7m %s -o /dev/null 2>&1 \
+// RUN:   | FileCheck --check-prefix ARMV7M %s
+// RUN: not %clang -target armv7m-unknown-linux -mno-thumb %s -o /dev/null 2>&1 \
+// RUN:   | FileCheck --check-prefix ARMV7M %s
+// ARMV7M: error: Architecture 'armv7m' does not support 'ARM' execution mode
+//
+// RUN: %clang -S -emit-llvm -target arm-unknown-linux -mcpu=cortex-m0 %s -o /dev/null 2>&1
+// M-Profile CPUs default to Thumb mode even if arm triples are provided.