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/* Calculate which nonterminals can expand into the null string for Bison.
Copyright (C) 1984, 1989, 2000-2006, 2009-2015, 2018-2020 Free
Software Foundation, Inc.
This file is part of Bison, the GNU Compiler Compiler.
This program 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 3 of the License, or
(at your option) any later version.
This program 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 this program. If not, see <http://www.gnu.org/licenses/>. */
/* 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, " %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. */
aver (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);
}
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