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 "reader.h"
  29 #include "types.h"
  30 #include "gram.h"
  31 #include "nullable.h"
  32
  33 char *nullable = NULL;
  34
  35 static void
  36 nullable_print (FILE *out)
  37 {
  38   int i;
  39   fputs ("NULLABLE\n", out);
  40   for (i = ntokens; i < nsyms; i++)
  41     fprintf (out, "\t%s: %s\n", tags[i], nullable[i] ? "yes" : "no");
  42   fputs ("\n\n", out);
  43 }
  44
  45 void
  46 set_nullable (void)
  47 {
  48   short *r;
  49   short *s1;
  50   short *s2;
  51   shorts *p;
  52
  53   short *squeue;
  54   short *rcount;
  55   shorts **rsets;
  56   shorts *relts;
  57
  58   if (trace_flag)
  59     fprintf (stderr, "Entering set_nullable\n");
  60
  61   nullable = XCALLOC (char, nvars) - ntokens;
  62
  63   squeue = XCALLOC (short, nvars);
  64   s1 = s2 = squeue;
  65
  66   rcount = XCALLOC (short, nrules + 1);
  67   rsets = XCALLOC (shorts *, nvars) - ntokens;
  68   /* This is said to be more elements than we actually use.
  69      Supposedly nitems - nrules is enough.
  70      But why take the risk?  */
  71   relts = XCALLOC (shorts, nitems + nvars + 1);
  72   p = relts;
  73
  74   for (r = ritem; *r; ++r)
  75     {
  76       /* Walk RITEM to find (i), if there are any tokens in the
  77          RHS, and (ii), to find RULENO. */
  78       int ruleno;
  79       int any_tokens = 0;
  80       short *r1;
  81       for (r1 = r; *r1 > 0; ++r1)
  82         if (ISTOKEN (*r1))
  83           any_tokens = 1;
  84       ruleno = -*r1;
  85
  86       /* Examine the RHS of the rule.  */
  87       if (!any_tokens)
  88         for (/* Nothing. */; *r > 0; ++r)
  89           {
  90             rcount[ruleno]++;
  91             p->next = rsets[*r];
  92             p->value = ruleno;
  93             rsets[*r] = p;
  94             p++;
  95           }
  96
  97       /* Examine its LHS. */
  98       if (rule_table[ruleno].useful && !nullable[rule_table[ruleno].lhs])
  99         {
 100           nullable[rule_table[ruleno].lhs] = 1;
 101           *s2++ = rule_table[ruleno].lhs;
 102         }
 103     }
 104
 105   while (s1 < s2)
 106     {
 107       p = rsets[*s1++];
 108       while (p)
 109         {
 110           int ruleno = p->value;
 111           p = p->next;
 112           if (--rcount[ruleno] == 0)
 113             {
 114               int symbol = rule_table[ruleno].lhs;
 115               if (symbol >= 0 && !nullable[symbol])
 116                 {
 117                   nullable[symbol] = 1;
 118                   *s2++ = symbol;
 119                 }
 120             }
 121         }
 122     }
 123
 124   XFREE (squeue);
 125   XFREE (rcount);
 126   XFREE (rsets + ntokens);
 127   XFREE (relts);
 128
 129   if (trace_flag)
 130     nullable_print (stderr);
 131 }
 132
 133
 134 void
 135 free_nullable (void)
 136 {
 137   XFREE (nullable + ntokens);
 138 }