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