cmd/7a: labels are local to functions

Replay of https://codereview.appspot.com/159670043/

Fixes golang#44

Author: 4ad

src/cmd/7a/a.h

@@ -107,6 +107,7 @@ void	newio(void);
 void	newfile(char*, int);
 Sym*	slookup(char*);
 Sym*	lookup(void);
+Sym*	labellookup(Sym*);
 void	settext(LSym*);
 void	syminit(Sym*);
 int32	yylex(void);

src/cmd/7a/a.y

@@ -41,15 +41,11 @@ prog:
 |	prog line
 
 line:
-	LLAB ':'
-	{
-		if($1->value != pc)
-			yyerror("redeclaration of %s", $1->name);
-		$1->value = pc;
-	}
-	line
 |	LNAME ':'
 	{
+		$1 = labellookup($1);
+		if($1->type == LLAB && $1->value != pc)
+			yyerror("redeclaration of %s", $1->labelname);
 		$1->type = LLAB;
 		$1->value = pc;
 	}
@@ -435,15 +431,10 @@ rel:
 	}
 |	LNAME offset
 	{
+		$1 = labellookup($1);
 		$$ = nullgen;
-		if(pass == 2)
-			yyerror("undefined label: %s", $1->name);
-		$$.type = D_BRANCH;
-		$$.offset = $2;
-	}
-|	LLAB offset
-	{
-		$$ = nullgen;
+		if(pass == 2 && $1->type != LLAB)
+			yyerror("undefined label: %s", $1->labelname);
 		$$.type = D_BRANCH;
 		$$.offset = $1->value + $2;
 	}

src/cmd/7a/y.tab.c

@@ -186,15 +186,15 @@ extern int yydebug;
 typedef union YYSTYPE YYSTYPE;
 union YYSTYPE
 {
-#line 9 "a.y" /* yacc.c:1915  */
+#line 9 "a.y" /* yacc.c:1909  */
 
 	Sym	*sym;
 	vlong	lval;
 	double	dval;
 	char	sval[NSNAME];
 	Addr	addr;
 
-#line 198 "y.tab.h" /* yacc.c:1915  */
+#line 198 "y.tab.h" /* yacc.c:1909  */
 };
 # define YYSTYPE_IS_TRIVIAL 1
 # define YYSTYPE_IS_DECLARED 1