src/nullable.c

   1 /* Calculate which nonterminals can expand into the null string for Bison.
   2
   3    Copyright (C) 1984, 1989, 2000-2006, 2009-2011 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
  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
  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
 139 nullable_free (void)
 140 {
 141   free (nullable);
 142 }