[ASM] Fix define symbol bug

Assembler's symbol table is set by Assembler::encode. If sytmbol
definition happens before any encode, symbol table of Assembler is
nullptr and unexpected undefined symbol error arise.

driver/asm_directive.cpp

@@ -186,8 +186,9 @@ Error AsmDirective::defineSymbol(
             return setError(symbol, DUPLICATE_LABEL);
     }
 
+    auto parserCtx = _assembler.parserContext(&context.symbols);
     ErrorAt error;
-    context.value = _assembler.parseExpr(scan, error);
+    context.value = _assembler.parser().eval(scan, error, parserCtx);
     if (error.hasError()) {
         context.value.clear();
         return setError(scan, error);

src/asm_base.cpp

@@ -118,8 +118,12 @@ Value Assembler::parseInteger(StrScanner &expr, ErrorAt &error, char delim) cons
     return value;
 }
 
+ParserContext Assembler::parserContext(const SymbolTable *symtab, char delim) const {
+    return ParserContext{_currentLocation, symtab, delim, _inputRadix};
+}
+
 Value Assembler::parseExpr(StrScanner &expr, ErrorAt &error, char delim) const {
-    ParserContext context{_currentLocation, _symtab, delim, _inputRadix};
+    auto context = parserContext(_symtab, delim);
     return _parser.eval(expr, error, context);
 }
 

src/asm_base.h

@@ -38,6 +38,7 @@ struct Assembler {
     virtual void reset();
 
     const ValueParser &parser() const { return _parser; }
+    ParserContext parserContext(const SymbolTable *symtab, char delim = 0) const;
     bool endOfLine(StrScanner &scan) const { return _parser.endOfLine(scan); }
     /** Parse |expr| text as an integer expression and get value. */
     Value parseInteger(StrScanner &expr, ErrorAt &error, char delim = 0) const;