Re: {maint} yacc.c: initialize yylval in pure-parser mode

[Paul Eggert]

I looked into the matter further, and it turns out that
GCC has two classes of warnings in this area, -Wuninitialized
and -Wmaybe-uninitialized.  It's the latter that are causing
trouble, and initializing yylval suppresses them; but it
also suppresses the trouble-free warnings, which is undesirable.

So, how about the following idea instead?  It follows
your suggestion for random compilers, but for new-enough GCC it
simply disables the bogus warning directly.  This should cause
recent GCC to issue a warning for my most-recent test grammar
(which is what we prefer) while not issuing warnings for the
cases where warnings are bothering people.

>From 41eb9c0b8568ab9cc79395fa27dce6b2646379a3 Mon Sep 17 00:00:00 2001
From: Paul Eggert <address@hidden>
Date: Fri, 5 Oct 2012 10:41:51 -0700
Subject: [PATCH] yacc: suppress unwanted GCC -Wmaybe-uninitalized diagnostic

* data/yacc.c (YY_IGNORE_MAYBE_UNINITIALIZED_BEGIN)
(YY_IGNORE_MAYBE_UNINITIALIZED_END, YYLVAL_INITIALIZE):
New macros.  Use them to suppress an unwanted GCC diagnostic.
---
 data/yacc.c | 28 ++++++++++++++++++++++++++++
 1 file changed, 28 insertions(+)

diff --git a/data/yacc.c b/data/yacc.c
index 44f96dd..e5fc28c 100644
--- a/data/yacc.c
+++ b/data/yacc.c
@@ -172,6 +172,28 @@ m4_define([b4_declare_scanner_communication_variables], [[
 /* The lookahead symbol.  */
 int yychar;
 
+]b4_pure_if([[
+#if defined __GNUC__ && (4 < __GNUC__ + (6 <= __GNUC_MINOR__))
+/* Suppress an incorrect diagnostic about yylval being uninitialized.  */
+# define YY_IGNORE_MAYBE_UNINITIALIZED_BEGIN \
+    _Pragma ("GCC diagnostic push") \
+    _Pragma ("GCC diagnostic ignored \"-Wmaybe-uninitialized\"")
+# define YY_IGNORE_MAYBE_UNINITIALIZED_END \
+    _Pragma ("GCC diagnostic pop")
+#else
+/* Default value used for initialization, for pacifying older GCCs
+   or non-GCC compilers.  */
+static YYSTYPE yyval_default;
+# define YYLVAL_INITIALIZE() (yylval = yyval_default)
+#endif]])[
+#ifndef YYLVAL_INITIALIZE
+# define YYLVAL_INITIALIZE()
+#endif
+#ifndef YY_IGNORE_MAYBE_UNINITIALIZED_BEGIN
+# define YY_IGNORE_MAYBE_UNINITIALIZED_BEGIN
+# define YY_IGNORE_MAYBE_UNINITIALIZED_END
+#endif
+
 /* The semantic value of the lookahead symbol.  */
 YYSTYPE yylval;]b4_locations_if([[
 
@@ -1456,6 +1478,8 @@ b4_function_define([[yyparse]], [[int]], b4_parse_param)[
   yyvsp = yyvs;]b4_locations_if([[
   yylsp = yyls;
 
+  YYLVAL_INITIALIZE ();
+
 #if defined ]b4_api_PREFIX[LTYPE_IS_TRIVIAL && ]b4_api_PREFIX[LTYPE_IS_TRIVIAL
   /* Initialize the default location before parsing starts.  */
   yylloc.first_line   = yylloc.last_line   = ]b4_location_initial_line[;
@@ -1641,7 +1665,9 @@ yyread_pushed_token:]])[
   YY_LAC_DISCARD ("shift");]])[
 
   yystate = yyn;
+  YY_IGNORE_MAYBE_UNINITIALIZED_BEGIN
   *++yyvsp = yylval;
+  YY_IGNORE_MAYBE_UNINITIALIZED_END
 ]b4_locations_if([  *++yylsp = yylloc;])[
   goto yynewstate;
 
@@ -1861,7 +1887,9 @@ yyerrlab1:
      current lookahead token, the shift below will for sure.  */
   YY_LAC_DISCARD ("error recovery");]])[
 
+  YY_IGNORE_MAYBE_UNINITIALIZED_BEGIN
   *++yyvsp = yylval;
+  YY_IGNORE_MAYBE_UNINITIALIZED_END
 ]b4_locations_if([[
   yyerror_range[2] = yylloc;
   /* Using YYLLOC is tempting, but would change the location of
-- 
1.7.11.4