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 rules.  */
  34 typedef struct rule_list_s
  35 {
  36   struct rule_list_s *next;
  37   rule_t *value;
  38 } rule_list_t;
  39
  40 bool *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 nullable_compute (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   nullable = XCALLOC (bool, nvars) - ntokens;
  70
  71   s1 = s2 = squeue;
  72   p = relts;
  73
  74   for (ruleno = 0; ruleno < nrules; ++ruleno)
  75     if (rules[ruleno].useful)
  76       {
  77         rule_t *rule = &rules[ruleno];
  78         if (rule->rhs[0] >= 0)
  79           {
  80             /* This rule has a non empty RHS. */
  81             item_number_t *r = NULL;
  82             int any_tokens = 0;
  83             for (r = rule->rhs; *r >= 0; ++r)
  84               if (ISTOKEN (*r))
  85                 any_tokens = 1;
  86
  87             /* This rule has only nonterminals: schedule it for the second
  88                pass.  */
  89             if (!any_tokens)
  90               for (r = rule->rhs; *r >= 0; ++r)
  91                 {
  92                   rcount[ruleno]++;
  93                   p->next = rsets[*r];
  94                   p->value = rule;
  95                   rsets[*r] = p;
  96                   p++;
  97                 }
  98           }
  99         else
 100           {
 101             /* This rule has an empty RHS. */
 102             assert (item_number_as_rule_number (rule->rhs[0]) == ruleno);
 103             if (rule->useful && !nullable[rule->lhs->number])
 104               {
 105                 nullable[rule->lhs->number] = 1;
 106                 *s2++ = rule->lhs->number;
 107               }
 108           }
 109       }
 110
 111   while (s1 < s2)
 112     for (p = rsets[*s1++]; p; p = p->next)
 113       {
 114         rule_t *rule = p->value;
 115         if (--rcount[rule->number] == 0)
 116           if (rule->useful && !nullable[rule->lhs->number])
 117             {
 118               nullable[rule->lhs->number] = 1;
 119               *s2++ = rule->lhs->number;
 120             }
 121       }
 122
 123   XFREE (squeue);
 124   XFREE (rcount);
 125   XFREE (rsets + ntokens);
 126   XFREE (relts);
 127
 128   if (trace_flag & trace_sets)
 129     nullable_print (stderr);
 130 }
 131
 132
 133 void
 134 nullable_free (void)
 135 {
 136   XFREE (nullable + ntokens);
 137 }