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   int ruleno;
  50   short *s1;
  51   short *s2;
  52   shorts *p;
  53
  54   short *squeue = XCALLOC (short, 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         if (ritem[rules[ruleno].rhs] >= 0)
  75           {
  76             /* This rule has a non empty RHS. */
  77             short *r;
  78             int any_tokens = 0;
  79             for (r = &ritem[rules[ruleno].rhs]; *r >= 0; ++r)
  80               if (ISTOKEN (*r))
  81                 any_tokens = 1;
  82
  83             /* This rule has only nonterminals: schedule it for the second
  84                pass.  */
  85             if (!any_tokens)
  86               for (r = &ritem[rules[ruleno].rhs]; *r >= 0; ++r)
  87                 {
  88                   rcount[ruleno]++;
  89                   p->next = rsets[*r];
  90                   p->value = ruleno;
  91                   rsets[*r] = p;
  92                   p++;
  93                 }
  94           }
  95         else
  96           {
  97             /* This rule has an empty RHS. */
  98             assert (ritem[rules[ruleno].rhs] == -ruleno);
  99             if (rules[ruleno].useful && !nullable[rules[ruleno].lhs])
 100               {
 101                 nullable[rules[ruleno].lhs] = 1;
 102                 *s2++ = rules[ruleno].lhs;
 103               }
 104           }
 105       }
 106
 107   while (s1 < s2)
 108     for (p = rsets[*s1++]; p; p = p->next)
 109       {
 110         ruleno = p->value;
 111         if (--rcount[ruleno] == 0)
 112           if (rules[ruleno].useful && !nullable[rules[ruleno].lhs])
 113             {
 114               nullable[rules[ruleno].lhs] = 1;
 115               *s2++ = rules[ruleno].lhs;
 116             }
 117       }
 118
 119   XFREE (squeue);
 120   XFREE (rcount);
 121   XFREE (rsets + ntokens);
 122   XFREE (relts);
 123
 124   if (trace_flag)
 125     nullable_print (stderr);
 126 }
 127
 128
 129 void
 130 free_nullable (void)
 131 {
 132   XFREE (nullable + ntokens);
 133 }