cmd/gc: do not overflow parser stack on a long chain of else if.

Fixes #2615.

R=dave, minux.ma, iant, daniel.morsing, rsc
CC=golang-dev
https://golang.org/cl/6847078

src/cmd/gc/go.y

@@ -57,7 +57,7 @@ static void fixlbrace(int);
 %type	<node>	compound_stmt dotname embed expr complitexpr
 %type	<node>	expr_or_type
 %type	<node>	fndcl hidden_fndcl fnliteral
-%type	<node>	for_body for_header for_stmt if_header if_stmt else non_dcl_stmt
+%type	<node>	for_body for_header for_stmt if_header if_stmt non_dcl_stmt
 %type	<node>	interfacedcl keyval labelname name
 %type	<node>	name_or_type non_expr_type
 %type	<node>	new_name dcl_name oexpr typedclname
@@ -70,7 +70,7 @@ static void fixlbrace(int);
 
 %type	<list>	xdcl fnbody fnres loop_body dcl_name_list
 %type	<list>	new_name_list expr_list keyval_list braced_keyval_list expr_or_type_list xdcl_list
-%type	<list>	oexpr_list caseblock_list stmt_list oarg_type_list_ocomma arg_type_list
+%type	<list>	oexpr_list caseblock_list elseif elseif_list else stmt_list oarg_type_list_ocomma arg_type_list
 %type	<list>	interfacedcl_list vardcl vardcl_list structdcl structdcl_list
 %type	<list>	common_dcl constdcl constdcl1 constdcl_list typedcl_list
 
@@ -661,25 +661,56 @@ if_stmt:
 	{
 		$3->nbody = $5;
 	}
-	else
+	elseif_list else
 	{
-		popdcl();
+		Node *n;
+		NodeList *nn;
+
 		$$ = $3;
-		if($7 != N)
-			$$->nelse = list1($7);
+		n = $3;
+		popdcl();
+		for(nn = concat($7, $8); nn; nn = nn->next) {
+			if(nn->n->op == OIF)
+				popdcl();
+			n->nelse = list1(nn->n);
+			n = nn->n;
+		}
 	}
 
-else:
+elseif:
+	LELSE LIF 
 	{
-		$$ = N;
+		markdcl();
 	}
-|	LELSE if_stmt
+	if_header loop_body
 	{
-		$$ = $2;
+		if($4->ntest == N)
+			yyerror("missing condition in if statement");
+		$4->nbody = $5;
+		$$ = list1($4);
+	}
+
+elseif_list:
+	{
+		$$ = nil;
+	}
+|	elseif_list elseif
+	{
+		$$ = concat($1, $2);
+	}
+
+else:
+	{
+		$$ = nil;
 	}
 |	LELSE compound_stmt
 	{
-		$$ = $2;
+		NodeList *node;
+		
+		node = mal(sizeof *node);
+		node->n = $2;
+		node->end = node;
+		$$ = node;
 	}
 
 switch_stmt:

src/cmd/gc/y.tab.h

-/* A Bison parser, made by GNU Bison 2.3.  */
+/* A Bison parser, made by GNU Bison 2.6.5.  */
 
-/* Skeleton interface for Bison's Yacc-like parsers in C
+/* Bison interface for Yacc-like parsers in C
    
-   Copyright (C) 1984, 1989, 1990, 2000, 2001, 2002, 2003, 2004, 2005, 2006
-   Free Software Foundation, Inc.
+      Copyright (C) 1984, 1989-1990, 2000-2012 Free Software Foundation, Inc.
    
-   This program is free software; you can redistribute it and/or modify
+   This program is free software: you can redistribute it and/or modify
    it under the terms of the GNU General Public License as published by
-   the Free Software Foundation; either version 2, or (at your option)
-   any later version.
+   the Free Software Foundation, either version 3 of the License, or
+   (at your option) any later version.
    
    This program is distributed in the hope that it will be useful,
    but WITHOUT ANY WARRANTY; without even the implied warranty of
@@ -16,9 +15,7 @@
    GNU General Public License for more details.
    
    You should have received a copy of the GNU General Public License
-   along with this program; if not, write to the Free Software
-   Foundation, Inc., 51 Franklin Street, Fifth Floor,
-   Boston, MA 02110-1301, USA.  */
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
 
 /* As a special exception, you may create a larger work that contains
    part or all of the Bison parser skeleton and distribute that work
@@ -33,6 +30,16 @@
    This special exception was added by the Free Software Foundation in
    version 2.2 of Bison.  */
 
+#ifndef YY_YY_Y_TAB_H_INCLUDED
+# define YY_YY_Y_TAB_H_INCLUDED
+/* Enabling traces.  */
+#ifndef YYDEBUG
+# define YYDEBUG 0
+#endif
+#if YYDEBUG
+extern int yydebug;
+#endif
+
 /* Tokens.  */
 #ifndef YYTOKENTYPE
 # define YYTOKENTYPE
@@ -143,25 +150,42 @@
 
 
 
-
 #if ! defined YYSTYPE && ! defined YYSTYPE_IS_DECLARED
 typedef union YYSTYPE
-#line 28 "go.y"
 {
+/* Line 2042 of yacc.c  */
+#line 28 "go.y"
+
 	Node*		node;
 	NodeList*		list;
 	Type*		type;
 	Sym*		sym;
 	struct	Val	val;
 	int		i;
-}
-/* Line 1529 of yacc.c.  */
-#line 160 "y.tab.h"
-	YYSTYPE;
+
+
+/* Line 2042 of yacc.c  */
+#line 169 "y.tab.h"
+} YYSTYPE;
+# define YYSTYPE_IS_TRIVIAL 1
 # define yystype YYSTYPE /* obsolescent; will be withdrawn */
 # define YYSTYPE_IS_DECLARED 1
-# define YYSTYPE_IS_TRIVIAL 1
 #endif
 
 extern YYSTYPE yylval;
 
+#ifdef YYPARSE_PARAM
+#if defined __STDC__ || defined __cplusplus
+int yyparse (void *YYPARSE_PARAM);
+#else
+int yyparse ();
+#endif
+#else /* ! YYPARSE_PARAM */
+#if defined __STDC__ || defined __cplusplus
+int yyparse (void);
+#else
+int yyparse ();
+#endif
+#endif /* ! YYPARSE_PARAM */
+
+#endif /* !YY_YY_Y_TAB_H_INCLUDED  */

src/cmd/gc/yerr.h

@@ -71,6 +71,6 @@ static struct {
 	112, LNAME,
 	"nested func not allowed",
 
-	615, ';',
+	639, ';',
 	"else must be followed by if or statement block"
 };