1/* Calculate which nonterminals can expand into the null string for Bison.
2
3   Copyright (C) 1984, 1989, 2000-2006, 2009-2012 Free Software
4   Foundation, Inc.
5
6   This file is part of Bison, the GNU Compiler Compiler.
7
8   This program is free software: you can redistribute it and/or modify
9   it under the terms of the GNU General Public License as published by
10   the Free Software Foundation, either version 3 of the License, or
11   (at your option) any later version.
12
13   This program is distributed in the hope that it will be useful,
14   but WITHOUT ANY WARRANTY; without even the implied warranty of
15   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
16   GNU General Public License for more details.
17
18   You should have received a copy of the GNU General Public License
19   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
20
21
22/* Set up NULLABLE, a vector saying which nonterminals can expand into
23   the null string.  NULLABLE[I - NTOKENS] is nonzero if symbol I can
24   do so.  */
25
26#include <config.h>
27#include "system.h"
28
29#include "getargs.h"
30#include "gram.h"
31#include "nullable.h"
32#include "reduce.h"
33#include "symtab.h"
34
35/* Linked list of rules.  */
36typedef struct rule_list
37{
38  struct rule_list *next;
39  rule *value;
40} rule_list;
41
42bool *nullable = NULL;
43
44static void
45nullable_print (FILE *out)
46{
47  int i;
48  fputs ("NULLABLE\n", out);
49  for (i = ntokens; i < nsyms; i++)
50    fprintf (out, "\t%s: %s\n", symbols[i]->tag,
51	     nullable[i - ntokens] ? "yes" : "no");
52  fputs ("\n\n", out);
53}
54
55void
56nullable_compute (void)
57{
58  rule_number ruleno;
59  symbol_number *s1;
60  symbol_number *s2;
61  rule_list *p;
62
63  symbol_number *squeue = xnmalloc (nvars, sizeof *squeue);
64  size_t *rcount = xcalloc (nrules, sizeof *rcount);
65  /* RITEM contains all the rules, including useless productions.
66     Hence we must allocate room for useless nonterminals too.  */
67  rule_list **rsets = xcalloc (nvars, sizeof *rsets);
68  /* This is said to be more elements than we actually use.
69     Supposedly NRITEMS - NRULES is enough.  But why take the risk?  */
70  rule_list *relts = xnmalloc (nritems + nvars + 1, sizeof *relts);
71
72  nullable = xcalloc (nvars, sizeof *nullable);
73
74  s1 = s2 = squeue;
75  p = relts;
76
77  for (ruleno = 0; ruleno < nrules; ++ruleno)
78    if (rules[ruleno].useful)
79      {
80	rule *rules_ruleno = &rules[ruleno];
81	if (rules_ruleno->rhs[0] >= 0)
82	  {
83	    /* This rule has a non empty RHS. */
84	    item_number *rp = NULL;
85	    bool any_tokens = false;
86	    for (rp = rules_ruleno->rhs; *rp >= 0; ++rp)
87	      if (ISTOKEN (*rp))
88		any_tokens = true;
89
90	    /* This rule has only nonterminals: schedule it for the second
91	       pass.  */
92	    if (!any_tokens)
93	      for (rp = rules_ruleno->rhs; *rp >= 0; ++rp)
94		{
95		  rcount[ruleno]++;
96		  p->next = rsets[*rp - ntokens];
97		  p->value = rules_ruleno;
98		  rsets[*rp - ntokens] = p;
99		  p++;
100		}
101	  }
102	else
103	  {
104	    /* This rule has an empty RHS. */
105	    aver (item_number_as_rule_number (rules_ruleno->rhs[0])
106		  == ruleno);
107	    if (rules_ruleno->useful
108		&& ! nullable[rules_ruleno->lhs->number - ntokens])
109	      {
110		nullable[rules_ruleno->lhs->number - ntokens] = true;
111		*s2++ = rules_ruleno->lhs->number;
112	      }
113	  }
114      }
115
116  while (s1 < s2)
117    for (p = rsets[*s1++ - ntokens]; p; p = p->next)
118      {
119	rule *r = p->value;
120	if (--rcount[r->number] == 0)
121	  if (r->useful && ! nullable[r->lhs->number - ntokens])
122	    {
123	      nullable[r->lhs->number - ntokens] = true;
124	      *s2++ = r->lhs->number;
125	    }
126      }
127
128  free (squeue);
129  free (rcount);
130  free (rsets);
131  free (relts);
132
133  if (trace_flag & trace_sets)
134    nullable_print (stderr);
135}
136
137
138void
139nullable_free (void)
140{
141  free (nullable);
142}
143