src/nullable.c

/* Calculate which nonterminals can expand into the null string for Bison.

   Copyright (C) 1984, 1989, 2000, 2001, 2002, 2003, 2004, 2005, 2006
   Free Software Foundation, Inc.

   This file is part of Bison, the GNU Compiler Compiler.

   Bison is free software; you can redistribute it and/or modify
   it under the terms of the GNU General Public License as published by
   the Free Software Foundation; either version 2, or (at your option)
   any later version.

   Bison is distributed in the hope that it will be useful,
   but WITHOUT ANY WARRANTY; without even the implied warranty of
   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
   GNU General Public License for more details.

   You should have received a copy of the GNU General Public License
   along with Bison; see the file COPYING.  If not, write to
   the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
   Boston, MA 02110-1301, USA.  */


/* Set up NULLABLE, a vector saying which nonterminals can expand into
   the null string.  NULLABLE[I - NTOKENS] is nonzero if symbol I can
   do so.  */

#include <config.h>
#include "system.h"

#include "getargs.h"
#include "gram.h"
#include "nullable.h"
#include "reduce.h"
#include "symtab.h"

/* Linked list of rules.  */
typedef struct rule_list
{
  struct rule_list *next;
  rule *value;
} rule_list;

bool *nullable = NULL;

static void
nullable_print (FILE *out)
{
  int i;
  fputs ("NULLABLE\n", out);
  for (i = ntokens; i < nsyms; i++)
    fprintf (out, "\t%s: %s\n", symbols[i]->tag,
	     nullable[i - ntokens] ? "yes" : "no");
  fputs ("\n\n", out);
}

void
nullable_compute (void)
{
  rule_number ruleno;
  symbol_number *s1;
  symbol_number *s2;
  rule_list *p;

  symbol_number *squeue = xnmalloc (nvars, sizeof *squeue);
  size_t *rcount = xcalloc (nrules, sizeof *rcount);
  /* RITEM contains all the rules, including useless productions.
     Hence we must allocate room for useless nonterminals too.  */
  rule_list **rsets = xcalloc (nvars, sizeof *rsets);
  /* This is said to be more elements than we actually use.
     Supposedly NRITEMS - NRULES is enough.  But why take the risk?  */
  rule_list *relts = xnmalloc (nritems + nvars + 1, sizeof *relts);

  nullable = xcalloc (nvars, sizeof *nullable);

  s1 = s2 = squeue;
  p = relts;

  for (ruleno = 0; ruleno < nrules; ++ruleno)
    if (rules[ruleno].useful)
      {
	rule *rules_ruleno = &rules[ruleno];
	if (rules_ruleno->rhs[0] >= 0)
	  {
	    /* This rule has a non empty RHS. */
	    item_number *rp = NULL;
	    bool any_tokens = false;
	    for (rp = rules_ruleno->rhs; *rp >= 0; ++rp)
	      if (ISTOKEN (*rp))
		any_tokens = true;

	    /* This rule has only nonterminals: schedule it for the second
	       pass.  */
	    if (!any_tokens)
	      for (rp = rules_ruleno->rhs; *rp >= 0; ++rp)
		{
		  rcount[ruleno]++;
		  p->next = rsets[*rp - ntokens];
		  p->value = rules_ruleno;
		  rsets[*rp - ntokens] = p;
		  p++;
		}
	  }
	else
	  {
	    /* This rule has an empty RHS. */
	    assert (item_number_as_rule_number (rules_ruleno->rhs[0])
		    == ruleno);
	    if (rules_ruleno->useful
		&& ! nullable[rules_ruleno->lhs->number - ntokens])
	      {
		nullable[rules_ruleno->lhs->number - ntokens] = true;
		*s2++ = rules_ruleno->lhs->number;
	      }
	  }
      }

  while (s1 < s2)
    for (p = rsets[*s1++ - ntokens]; p; p = p->next)
      {
	rule *r = p->value;
	if (--rcount[r->number] == 0)
	  if (r->useful && ! nullable[r->lhs->number - ntokens])
	    {
	      nullable[r->lhs->number - ntokens] = true;
	      *s2++ = r->lhs->number;
	    }
      }

  free (squeue);
  free (rcount);
  free (rsets);
  free (relts);

  if (trace_flag & trace_sets)
    nullable_print (stderr);
}


void
nullable_free (void)
{
  free (nullable);
}