[ARM] Improve error messages for .arch_extension directive

[olista01]

Summary

• More informative message when extension name is not an identifier token.
• Stop parsing directive if extension is unknown (avoid duplicate error messages).
• Report unsupported extensions with a source location, rather than report_fatal_error.

[rengolin]

Much better, thanks! LGTM with the additional test.

test/MC/ARM/directive-arch_extension-unsupported.s

2

not testing empty one?

lib/Target/ARM/AsmParser/ARMAsmParser.cpp

 };
 
 /// parseDirectiveArchExtension
 ///   ::= .arch_extension [no]feature
 bool ARMAsmParser::parseDirectiveArchExtension(SMLoc L) {
   MCAsmParser &Parser = getParser();
 
   if (getLexer().isNot(AsmToken::Identifier)) {
-    Error(getLexer().getLoc(), "unexpected token");
+    Error(getLexer().getLoc(), "expected architecture extension name");
     Parser.eatToEndOfStatement();
     return false;
   }
 
   StringRef Name = Parser.getTok().getString();
   SMLoc ExtLoc = Parser.getTok().getLoc();
   Lex();
 
   bool EnableFeature = true;
   if (Name.startswith_lower("no")) {
     EnableFeature = false;
     Name = Name.substr(2);
   }
   unsigned FeatureKind = ARM::parseArchExt(Name);
-  if (FeatureKind == ARM::AEK_INVALID)
+  if (FeatureKind == ARM::AEK_INVALID) {
     Error(ExtLoc, "unknown architectural extension: " + Name);
+    return false;
+  }
 
   for (const auto &Extension : Extensions) {
     if (Extension.Kind != FeatureKind)
       continue;
 
-    if (Extension.Features.none())
-      report_fatal_error("unsupported architectural extension: " + Name);
+    if (Extension.Features.none()) {
+      Error(ExtLoc, "unsupported architectural extension: " + Name);
+      return false;
+    }
 
     if ((getAvailableFeatures() & Extension.ArchCheck) != Extension.ArchCheck) {
       Error(ExtLoc, "architectural extension '" + Name + "' is not "
             "allowed for the current base architecture");
       return false;
     }
 
     MCSubtargetInfo &STI = copySTI();

test/MC/ARM/directive-arch_extension-unsupported.s

+@ RUN: not llvm-mc -triple armv7--none-eabi -filetype asm -o /dev/null 2>&1 %s | FileCheck %s
+
+  .arch_extension os
+CHECK: error: unsupported architectural extension: os
+
+  .arch_extension iwmmxt
+CHECK: error: unsupported architectural extension: iwmmxt
+
+  .arch_extension iwmmxt2
+CHECK: error: unsupported architectural extension: iwmmxt2
+
+  .arch_extension maverick
+CHECK: error: unsupported architectural extension: maverick
+
+  .arch_extension xscale
+CHECK: error: unsupported architectural extension: xscale
+
+  .arch_extension invalid_extension_name
+CHECK: error: unknown architectural extension: invalid_extension_name
+
+  .arch_extension 42
+CHECK: error: expected architecture extension name