[MC] Support case insensitive macros

[jcai19]

Summary

GNU as allows us to specify a macro and then expand with case
insensitive calls. Add this support to match GNU.

[MaskRay]

I am not sure we should copy this legacy cruft from GNU as: http://lists.llvm.org/pipermail/llvm-dev/2020-February/138955.html

See http://lists.llvm.org/pipermail/llvm-dev/2020-February/138937.html for some links.

[jcai19]

This was motived by gas-compatibility reported at https://github.com/ClangBuiltLinux/linux/issues/924. However, after having offline discussion with @MaskRay and reading the shared links, I agree having such support may introduce more complexity into assembly programs. Closing the revision as I will not further purse this change.

llvm/include/llvm/MC/MCContext.h

public:
     void reportWarning(SMLoc L, const Twine &Msg);
     // Unrecoverable error has occurred. Display the best diagnostic we can
     // and bail via exit(1). For now, most MC backend errors are unrecoverable.
     // FIXME: We should really do something about that.
     LLVM_ATTRIBUTE_NORETURN void reportFatalError(SMLoc L,
                                                   const Twine &Msg);
 
     const MCAsmMacro *lookupMacro(StringRef Name) {
-      StringMap<MCAsmMacro>::iterator I = MacroMap.find(Name);
+      StringMap<MCAsmMacro>::iterator I = MacroMap.find(Name.lower());
       return (I == MacroMap.end()) ? nullptr : &I->getValue();
     }
 
     void defineMacro(StringRef Name, MCAsmMacro Macro) {
-      MacroMap.insert(std::make_pair(Name, std::move(Macro)));
+      MacroMap.insert(std::make_pair(Name.lower(), std::move(Macro)));
     }
 
-    void undefineMacro(StringRef Name) { MacroMap.erase(Name); }
+    void undefineMacro(StringRef Name) { MacroMap.erase(Name.lower()); }
   };
 
 } // end namespace llvm
 
 // operator new and delete aren't allowed inside namespaces.
 // The throw specifications are mandated by the standard.
 /// Placement new for using the MCContext's allocator.
 ///

llvm/test/MC/X86/macro_case.s

+# RUN: llvm-mc -filetype=obj -triple=i386 %s -o /dev/null
+
+.macro  mySum from=0, to=5
+        .long   \from
+        .if     \to-\from
+        mySum     "(\from+1)",\to
+        .endif
+        .endm
+
+Mysum 0,5