src/nullable.c

   1 /* Part of the bison parser generator,
   2    Copyright 1984, 1989, 2000, 2001 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 "types.h"
  30 #include "gram.h"
  31 #include "reduce.h"
  32 #include "nullable.h"
  33
  34 char *nullable = NULL;
  35
  36 static void
  37 nullable_print (FILE *out)
  38 {
  39   int i;
  40   fputs ("NULLABLE\n", out);
  41   for (i = ntokens; i < nsyms; i++)
  42     fprintf (out, "\t%s: %s\n", symbols[i]->tag, nullable[i] ? "yes" : "no");
  43   fputs ("\n\n", out);
  44 }
  45
  46 void
  47 set_nullable (void)
  48 {
  49   rule_number_t ruleno;
  50   symbol_number_t *s1;
  51   symbol_number_t *s2;
  52   shorts *p;
  53
  54   symbol_number_t *squeue = XCALLOC (symbol_number_t, nvars);
  55   short *rcount = XCALLOC (short, nrules + 1);
  56   /* RITEM contains all the rules, including useless productions.
  57      Hence we must allocate room for useless nonterminals too.  */
  58   shorts **rsets = XCALLOC (shorts *, nvars) - ntokens;
  59   /* This is said to be more elements than we actually use.
  60      Supposedly NRITEMS - NRULES is enough.  But why take the risk?  */
  61   shorts *relts = XCALLOC (shorts, nritems + nvars + 1);
  62
  63   if (trace_flag)
  64     fprintf (stderr, "Entering set_nullable\n");
  65
  66   nullable = XCALLOC (char, nvars) - ntokens;
  67
  68   s1 = s2 = squeue;
  69   p = relts;
  70
  71   for (ruleno = 1; ruleno < nrules + 1; ++ruleno)
  72     if (rules[ruleno].useful)
  73       {
  74         rule_t *rule = &rules[ruleno];
  75         if (rule->rhs[0] >= 0)
  76           {
  77             /* This rule has a non empty RHS. */
  78             item_number_t *r = NULL;
  79             int any_tokens = 0;
  80             for (r = rule->rhs; *r >= 0; ++r)
  81               if (ISTOKEN (*r))
  82                 any_tokens = 1;
  83
  84             /* This rule has only nonterminals: schedule it for the second
  85                pass.  */
  86             if (!any_tokens)
  87               for (r = rule->rhs; *r >= 0; ++r)
  88                 {
  89                   rcount[ruleno]++;
  90                   p->next = rsets[*r];
  91                   p->value = ruleno;
  92                   rsets[*r] = p;
  93                   p++;
  94                 }
  95           }
  96         else
  97           {
  98             /* This rule has an empty RHS. */
  99             assert (rule_number_of_item_number (rule->rhs[0]) == ruleno);
 100             if (rule->useful && !nullable[rule->lhs->number])
 101               {
 102                 nullable[rule->lhs->number] = 1;
 103                 *s2++ = rule->lhs->number;
 104               }
 105           }
 106       }
 107
 108   while (s1 < s2)
 109     for (p = rsets[*s1++]; p; p = p->next)
 110       {
 111         ruleno = p->value;
 112         if (--rcount[ruleno] == 0)
 113           if (rules[ruleno].useful && !nullable[rules[ruleno].lhs->number])
 114             {
 115               nullable[rules[ruleno].lhs->number] = 1;
 116               *s2++ = rules[ruleno].lhs->number;
 117             }
 118       }
 119
 120   XFREE (squeue);
 121   XFREE (rcount);
 122   XFREE (rsets + ntokens);
 123   XFREE (relts);
 124
 125   if (trace_flag)
 126     nullable_print (stderr);
 127 }
 128
 129
 130 void
 131 free_nullable (void)
 132 {
 133   XFREE (nullable + ntokens);
 134 }