MCSymbol: Remove isUnset

The isUnset state lacks significance and should be treated as equivalent
to an undefined symbol.

Equated and common symbols seem to have subtle semantic distinctions in
GAS, justifying the use of distinct `SymContents*` values.

TODO: Ensure common symbols have a fragment, so that `isDefined()`
returns true, rejecting `.comm c, 4, 4; .set c, 4`

Author: MaskRay

llvm/include/llvm/MC/MCSymbol.h

@@ -41,11 +41,9 @@ class raw_ostream;
 /// it is a reference to an external entity, it has a null section.
 class MCSymbol {
 protected:
-  /// A symbol can contain an Offset, or Value, or be Common, but never more
-  /// than one of these.
+  // A symbol can be regular, equated to an expression, or a common symbol.
   enum Contents : uint8_t {
     SymContentsUnset,
-    SymContentsOffset,
     SymContentsVariable,
     SymContentsCommon,
     SymContentsTargetCommon, // Index stores the section index
@@ -294,20 +292,15 @@ class MCSymbol {
     Index = Value;
   }
 
-  bool isUnset() const { return SymbolContents == SymContentsUnset; }
-
   uint64_t getOffset() const {
-    assert((SymbolContents == SymContentsUnset ||
-            SymbolContents == SymContentsOffset) &&
+    assert(SymbolContents == SymContentsUnset &&
            "Cannot get offset for a common/variable symbol");
     return Offset;
   }
   void setOffset(uint64_t Value) {
-    assert((SymbolContents == SymContentsUnset ||
-            SymbolContents == SymContentsOffset) &&
+    assert(SymbolContents == SymContentsUnset &&
            "Cannot set offset for a common/variable symbol");
     Offset = Value;
-    SymbolContents = SymContentsOffset;
   }
 
   /// Return the size of a 'common' symbol.

llvm/lib/MC/MCParser/AsmParser.cpp

@@ -6273,7 +6273,8 @@ bool parseAssignmentExpression(StringRef Name, bool allow_redef,
   // used as a symbol, or it is an absolute symbol).
   Sym = Parser.getContext().lookupSymbol(Name);
   if (Sym) {
-    if (!Sym->isUnset() && (!allow_redef || !Sym->isRedefinable()))
+    if ((Sym->isVariable() || Sym->isDefined()) &&
+        (!allow_redef || !Sym->isRedefinable()))
       return Parser.Error(EqualLoc, "redefinition of '" + Name + "'");
     // If the symbol is redefinable, clone it and update the symbol table
     // to the new symbol. Existing references to the original symbol remain