src/nullable.c

   1 /* Part of the bison parser generator,
   2    Copyright (C) 1984, 1989, 2000, 2001, 2002 Free Software Foundation, Inc.
   3
   4    This file is part of Bison, the GNU Compiler Compiler.
   5
   6    Bison is free software; you can redistribute it and/or modify
   7    it under the terms of the GNU General Public License as published by
   8    the Free Software Foundation; either version 2, or (at your option)
   9    any later version.
  10
  11    Bison is distributed in the hope that it will be useful,
  12    but WITHOUT ANY WARRANTY; without even the implied warranty of
  13    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14    GNU General Public License for more details.
  15
  16    You should have received a copy of the GNU General Public License
  17    along with Bison; see the file COPYING.  If not, write to
  18    the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  19    Boston, MA 02111-1307, USA.  */
  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 "system.h"
  27 #include "getargs.h"
  28 #include "symtab.h"
  29 #include "gram.h"
  30 #include "reduce.h"
  31 #include "nullable.h"
  32
  33 /* Linked list of rule numbers.  */
  34 typedef struct rule_list_s
  35 {
  36   struct rule_list_s *next;
  37   rule_number_t value;
  38 } rule_list_t;
  39
  40 char *nullable = NULL;
  41
  42 static void
  43 nullable_print (FILE *out)
  44 {
  45   int i;
  46   fputs ("NULLABLE\n", out);
  47   for (i = ntokens; i < nsyms; i++)
  48     fprintf (out, "\t%s: %s\n", symbols[i]->tag, nullable[i] ? "yes" : "no");
  49   fputs ("\n\n", out);
  50 }
  51
  52 void
  53 set_nullable (void)
  54 {
  55   rule_number_t ruleno;
  56   symbol_number_t *s1;
  57   symbol_number_t *s2;
  58   rule_list_t *p;
  59
  60   symbol_number_t *squeue = XCALLOC (symbol_number_t, nvars);
  61   short *rcount = XCALLOC (short, nrules);
  62   /* RITEM contains all the rules, including useless productions.
  63      Hence we must allocate room for useless nonterminals too.  */
  64   rule_list_t **rsets = XCALLOC (rule_list_t *, nvars) - ntokens;
  65   /* This is said to be more elements than we actually use.
  66      Supposedly NRITEMS - NRULES is enough.  But why take the risk?  */
  67   rule_list_t *relts = XCALLOC (rule_list_t, nritems + nvars + 1);
  68
  69   if (trace_flag)
  70     fprintf (stderr, "Entering set_nullable\n");
  71
  72   nullable = XCALLOC (char, nvars) - ntokens;
  73
  74   s1 = s2 = squeue;
  75   p = relts;
  76
  77   for (ruleno = 0; ruleno < nrules; ++ruleno)
  78     if (rules[ruleno].useful)
  79       {
  80         rule_t *rule = &rules[ruleno];
  81         if (rule->rhs[0] >= 0)
  82           {
  83             /* This rule has a non empty RHS. */
  84             item_number_t *r = NULL;
  85             int any_tokens = 0;
  86             for (r = rule->rhs; *r >= 0; ++r)
  87               if (ISTOKEN (*r))
  88                 any_tokens = 1;
  89
  90             /* This rule has only nonterminals: schedule it for the second
  91                pass.  */
  92             if (!any_tokens)
  93               for (r = rule->rhs; *r >= 0; ++r)
  94                 {
  95                   rcount[ruleno]++;
  96                   p->next = rsets[*r];
  97                   p->value = ruleno;
  98                   rsets[*r] = p;
  99                   p++;
 100                 }
 101           }
 102         else
 103           {
 104             /* This rule has an empty RHS. */
 105             assert (item_number_as_rule_number (rule->rhs[0]) == ruleno);
 106             if (rule->useful && !nullable[rule->lhs->number])
 107               {
 108                 nullable[rule->lhs->number] = 1;
 109                 *s2++ = rule->lhs->number;
 110               }
 111           }
 112       }
 113
 114   while (s1 < s2)
 115     for (p = rsets[*s1++]; p; p = p->next)
 116       {
 117         ruleno = p->value;
 118         if (--rcount[ruleno] == 0)
 119           if (rules[ruleno].useful && !nullable[rules[ruleno].lhs->number])
 120             {
 121               nullable[rules[ruleno].lhs->number] = 1;
 122               *s2++ = rules[ruleno].lhs->number;
 123             }
 124       }
 125
 126   XFREE (squeue);
 127   XFREE (rcount);
 128   XFREE (rsets + ntokens);
 129   XFREE (relts);
 130
 131   if (trace_flag)
 132     nullable_print (stderr);
 133 }
 134
 135
 136 void
 137 free_nullable (void)
 138 {
 139   XFREE (nullable + ntokens);
 140 }