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.  */
36 typedef struct rule_list
37 {
38   struct rule_list *next;
39   rule *value;
40 } rule_list;
41 
42 bool *nullable = NULL;
43 
44 static void
nullable_print(FILE * out)45 nullable_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 
55 void
nullable_compute(void)56 nullable_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 
138 void
nullable_free(void)139 nullable_free (void)
140 {
141   free (nullable);
142 }
143