Implement %define lr.default_rules.

Its value describes the states that are permitted to contain
default rules: "all", "consistent", or "accepting".
* src/reader.c (reader): Default lr.default_rules to "all".
Check for a valid lr.default_rules value.
* src/lalr.c (state_lookahead_tokens_count): If lr.default_rules
is "accepting", then only mark the accepting state as
consistent.
(initialize_LA): Tell state_lookahead_tokens_count whether
lr.default_rules is "accepting".
* src/tables.c (action_row): If lr.default_rules is not "all",
then disable default rules in inconsistent states.
* src/print.c (print_reductions): Use this opportunity to
perform some assertions about whether lr.default_rules was
obeyed correctly.
* tests/local.at (AT_TEST_TABLES_AND_PARSE): New macro that
helps with checking the parser tables for a grammar.
* tests/input.at (%define lr.default_rules invalid values): New
test group.
* tests/reduce.at (AT_TEST_LR_DEFAULT_RULES): New macro using
AT_TEST_TABLES_AND_PARSE.
(`no %define lr.default_rules'): New test group generated by
AT_TEST_LR_DEFAULT_RULES.
(`%define lr.default_rules "all"'): Likewise.
(`%define lr.default_rules "consistent"'): Likewise.
(`%define lr.default_rules "accepting"'): Likewise.

src/lalr.c

@@ -1,7 +1,7 @@
 /* Compute lookahead criteria for Bison.
 
    Copyright (C) 1984, 1986, 1989, 2000, 2001, 2002, 2003, 2004, 2005,
-   2006, 2007 Free Software Foundation, Inc.
+   2006, 2007, 2008, 2009 Free Software Foundation, Inc.
 
    This file is part of Bison, the GNU Compiler Compiler.
 
@@ -36,6 +36,7 @@
 #include "getargs.h"
 #include "gram.h"
 #include "lalr.h"
+#include "muscle_tab.h"
 #include "nullable.h"
 #include "reader.h"
 #include "relation.h"
@@ -336,22 +337,30 @@ compute_lookahead_tokens (void)
 `----------------------------------------------------*/
 
 static int
-state_lookahead_tokens_count (state *s)
+state_lookahead_tokens_count (state *s, bool default_rule_only_for_accept)
 {
   int n_lookahead_tokens = 0;
   reductions *rp = s->reductions;
   transitions *sp = s->transitions;
 
-  /* We need a lookahead either to distinguish different
-     reductions (i.e., there are two or more), or to distinguish a
-     reduction from a shift.  Otherwise, it is straightforward,
-     and the state is `consistent'.  There is no need to check that
-     transition 0 hasn't been disabled before checking if it is a
-     shift since transitions are only disabled during conflict
-     resolution, and that hasn't happened yet.  */
+  /* Transitions are only disabled during conflict resolution, and that
+     hasn't happened yet, so there should be no need to check that
+     transition 0 hasn't been disabled before checking if it is a shift.
+     However, this check was performed at one time, so we leave it as an
+     aver.  */
   aver (sp->num == 0 || !TRANSITION_IS_DISABLED (sp, 0));
+
+  /* We need a lookahead either to distinguish different reductions
+     (i.e., there are two or more), or to distinguish a reduction from a
+     shift.  Otherwise, it is straightforward, and the state is
+     `consistent'.  However, for states that have any rules, treat only
+     the accepting state as consistent (since there is never a lookahead
+     token that makes sense there, and so no lookahead token should be
+     read) if the user has otherwise disabled default rules.  */
   if (rp->num > 1
-      || (rp->num == 1 && sp->num && TRANSITION_IS_SHIFT (sp, 0)))
+      || (rp->num == 1 && sp->num && TRANSITION_IS_SHIFT (sp, 0))
+      || (rp->num == 1 && rp->rules[0]->number != 0
+          && default_rule_only_for_accept))
     n_lookahead_tokens += rp->num;
   else
     s->consistent = 1;
@@ -369,11 +378,18 @@ initialize_LA (void)
 {
   state_number i;
   bitsetv pLA;
+  bool default_rule_only_for_accept;
+  {
+    char *default_rules = muscle_percent_define_get ("lr.default_rules");
+    default_rule_only_for_accept = 0 == strcmp (default_rules, "accepting");
+    free (default_rules);
+  }
 
   /* Compute the total number of reductions requiring a lookahead.  */
   nLA = 0;
   for (i = 0; i < nstates; i++)
-    nLA += state_lookahead_tokens_count (states[i]);
+    nLA +=
+      state_lookahead_tokens_count (states[i], default_rule_only_for_accept);
   /* Avoid having to special case 0.  */
   if (!nLA)
     nLA = 1;
@@ -385,7 +401,8 @@ initialize_LA (void)
      require lookahead tokens.  */
   for (i = 0; i < nstates; i++)
     {
-      int count = state_lookahead_tokens_count (states[i]);
+      int count =
+        state_lookahead_tokens_count (states[i], default_rule_only_for_accept);
       if (count)
 	{
 	  states[i]->reductions->lookahead_tokens = pLA;

src/print.c

@@ -1,7 +1,7 @@
 /* Print information on generated parser, for bison,
 
-   Copyright (C) 1984, 1986, 1989, 2000, 2001, 2002, 2003, 2004, 2005, 2007
-   Free Software Foundation, Inc.
+   Copyright (C) 1984, 1986, 1989, 2000, 2001, 2002, 2003, 2004, 2005,
+   2007, 2009 Free Software Foundation, Inc.
 
    This file is part of Bison, the GNU Compiler Compiler.
 
@@ -31,6 +31,7 @@
 #include "getargs.h"
 #include "gram.h"
 #include "lalr.h"
+#include "muscle_tab.h"
 #include "print.h"
 #include "reader.h"
 #include "reduce.h"
@@ -244,6 +245,7 @@ print_reductions (FILE *out, state *s)
   rule *default_rule = NULL;
   size_t width = 0;
   int i, j;
+  bool non_default_action = false;
 
   if (reds->num == 0)
     return;
@@ -296,6 +298,8 @@ print_reductions (FILE *out, state *s)
       {
 	bool defaulted = false;
 	bool count = bitset_test (no_reduce_set, i);
+        if (count)
+          non_default_action = true;
 
 	for (j = 0; j < reds->num; ++j)
 	  if (bitset_test (reds->lookahead_tokens[j], i))
@@ -303,15 +307,19 @@ print_reductions (FILE *out, state *s)
 	      if (! count)
 		{
 		  if (reds->rules[j] != default_rule)
-		    print_reduction (out, width,
-				     symbols[i]->tag,
-				     reds->rules[j], true);
+                    {
+                      non_default_action = true;
+                      print_reduction (out, width,
+                                       symbols[i]->tag,
+                                       reds->rules[j], true);
+                    }
 		  else
 		    defaulted = true;
 		  count = true;
 		}
 	      else
 		{
+                  non_default_action = true;
 		  if (defaulted)
 		    print_reduction (out, width,
 				     symbols[i]->tag,
@@ -325,8 +333,15 @@ print_reductions (FILE *out, state *s)
       }
 
   if (default_rule)
-    print_reduction (out, width,
-		     _("$default"), default_rule, true);
+    {
+      char *default_rules = muscle_percent_define_get ("lr.default_rules");
+      print_reduction (out, width, _("$default"), default_rule, true);
+      aver (0 == strcmp (default_rules, "all")
+            || (0 == strcmp (default_rules, "consistent")
+                && !non_default_action)
+            || (reds->num == 1 && reds->rules[0]->number == 0));
+      free (default_rules);
+    }
 }
 
 

src/reader.c

@@ -1,7 +1,7 @@
 /* Input parser for Bison
 
    Copyright (C) 1984, 1986, 1989, 1992, 1998, 2000, 2001, 2002, 2003,
-   2005, 2006, 2007 Free Software Foundation, Inc.
+   2005, 2006, 2007, 2009 Free Software Foundation, Inc.
 
    This file is part of Bison, the GNU Compiler Compiler.
 
@@ -555,6 +555,17 @@ reader (void)
   gram_scanner_initialize ();
   gram_parse ();
 
+  muscle_percent_define_default ("lr.default_rules", "all");
+
+  /* Check front-end %define variable values.  */
+  {
+    static char const * const values[] = {
+      "lr.default_rules", "all", "consistent", "accepting", NULL,
+      NULL
+    };
+    muscle_percent_define_check_values (values);
+  }
+
   if (! complaint_issued)
     check_and_convert_grammar ();
 

src/tables.c

@@ -1,7 +1,7 @@
 /* Output the generated parsing program for Bison.
 
    Copyright (C) 1984, 1986, 1989, 1992, 2000, 2001, 2002, 2003, 2004,
-   2005, 2006 Free Software Foundation, Inc.
+   2005, 2006, 2009 Free Software Foundation, Inc.
 
    This file is part of Bison, the GNU Compiler Compiler.
 
@@ -30,6 +30,7 @@
 #include "getargs.h"
 #include "gram.h"
 #include "lalr.h"
+#include "muscle_tab.h"
 #include "reader.h"
 #include "symtab.h"
 #include "tables.h"
@@ -303,6 +304,16 @@ action_row (state *s)
       actrow[sym->number] = ACTION_NUMBER_MINIMUM;
     }
 
+  /* Turn off default rules where requested by the user.  See
+     state_lookahead_tokens_count in lalr.c to understand when states are
+     labeled as consistent.  */
+  {
+    char *default_rules = muscle_percent_define_get ("lr.default_rules");
+    if (0 != strcmp (default_rules, "all") && !s->consistent)
+      nodefault = true;
+    free (default_rules);
+  }
+
   /* Now find the most common reduction and make it the default action
      for this state.  */