Instead of mapping the LHS of unused rules to -1, keep the LHS

valid, but flag the rules as invalid.
* src/gram.h (rule_t): `useful' is a new member.
* src/print.c (print_grammar): Adjust.
* src/derives.c (set_derives): Likewise.
* src/reader.c (packgram, reduce_output): Likewise.
* src/reduce.c (reduce_grammar_tables): Likewise.
* tests/reduce.at (Underivable Rules, Useless Rules): New.

src/derives.c

@@ -63,7 +63,6 @@ void
 set_derives (void)
 {
   int i;
-  int lhs;
   shorts *p;
   short *q;
   shorts **dset;
@@ -74,16 +73,14 @@ set_derives (void)
 
   p = delts;
   for (i = nrules; i > 0; i--)
-    {
-      lhs = rule_table[i].lhs;
-      if (lhs >= 0)
-	{
-	  p->next = dset[lhs];
-	  p->value = i;
-	  dset[lhs] = p;
-	  p++;
-	}
-    }
+    if (rule_table[i].useful)
+      {
+	int lhs = rule_table[i].lhs;
+	p->next = dset[lhs];
+	p->value = i;
+	dset[lhs] = p;
+	p++;
+      }
 
   derives = XCALLOC (short *, nvars) - ntokens;
   q = XCALLOC (short, nvars + nrules);

src/gram.h

@@ -59,6 +59,8 @@
 
    RULE_TABLE[R].line -- the line where R was defined.
 
+   RULE_TABLE[R].useful -- TRUE iff the rule is used.
+
    The right hand side is stored as symbol numbers in a portion of
    RITEM.
 
@@ -119,6 +121,7 @@ typedef struct rule_s
   short precsym;
   short assoc;
   short line;
+  bool useful;
 } rule_t;
 
 extern struct rule_s *rule_table;

src/print.c

@@ -216,7 +216,7 @@ print_grammar (FILE *out)
   fprintf (out, "  %s\n", _("Number, Line, Rule"));
   for (i = 1; i <= nrules; i++)
     /* Don't print rules disabled in reduce_grammar_tables.  */
-    if (rule_table[i].lhs >= 0)
+    if (rule_table[i].useful)
       {
 	fprintf (out, _("  %3d %3d %s ->"),
 		 i, rule_table[i].line, tags[rule_table[i].lhs]);

src/reader.c

@@ -1877,6 +1877,7 @@ packgram (void)
       rule_table[ruleno].lhs = p->sym->value;
       rule_table[ruleno].rhs = itemno;
       rule_table[ruleno].line = p->line;
+      rule_table[ruleno].useful = TRUE;
 
       p = p->next;
       while (p && p->sym)

src/reduce.c

@@ -310,20 +310,12 @@ reduce_grammar_tables (void)
 
     }
 #endif /* 0 */
-  /* Disable useless productions,
-     since they may contain useless nonterms
-     that would get mapped below to -1 and confuse everyone.  */
+  /* Disable useless productions. */
   if (nuseless_productions > 0)
     {
       int pn;
-
       for (pn = 1; pn <= nrules; pn++)
-	{
-	  if (!BITISSET (P, pn))
-	    {
-	      rule_table[pn].lhs = -1;
-	    }
-	}
+	rule_table[pn].useful = BITISSET (P, pn);
     }
 }
 
@@ -338,8 +330,8 @@ nonterminals_reduce (void)
   int i, n;
   rule r;
 
-  /* Create a map of nonterminal number to new nonterminal number. -1
-     in the map means it was useless and is being eliminated.  */
+  /* Map the nonterminals to their new index: useful first, useless
+     afterwards.  Kept for later report.  */
 
   short *nontermmap = XCALLOC (short, nvars) - ntokens;
   n = ntokens;
@@ -379,9 +371,7 @@ nonterminals_reduce (void)
 
   for (i = 1; i <= nrules; i++)
     {
-      /* Ignore the rules disabled above.  */
-      if (rule_table[i].lhs >= 0)
-	rule_table[i].lhs = nontermmap[rule_table[i].lhs];
+      rule_table[i].lhs = nontermmap[rule_table[i].lhs];
       if (ISVAR (rule_table[i].precsym))
 	/* Can this happen?  */
 	rule_table[i].precsym = nontermmap[rule_table[i].precsym];
@@ -396,7 +386,7 @@ nonterminals_reduce (void)
   nsyms -= nuseless_nonterminals;
   nvars -= nuseless_nonterminals;
 
-  free (&nontermmap[ntokens]);
+  free (nontermmap + ntokens);
 }
 
 
@@ -436,11 +426,11 @@ reduce_output (FILE *out)
       int i;
       fprintf (out, "%s\n\n", _("Useless rules:"));
       for (i = 1; i <= nrules; i++)
-	if (!BITISSET (P, i))
+	if (!rule_table[i].useful)
 	  {
 	    rule r;
 	    fprintf (out, "#%-4d  ", i);
-	    fprintf (out, "%s :\t", tags[rule_table[i].lhs]);
+	    fprintf (out, "%s:", tags[rule_table[i].lhs]);
 	    for (r = &ritem[rule_table[i].rhs]; *r >= 0; r++)
 	      fprintf (out, " %s", tags[*r]);
 	    fputs (";\n", out);