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136 lines
3.9 KiB
C
136 lines
3.9 KiB
C
/* Calculate which nonterminals can expand into the null string for Bison.
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Copyright (C) 1984, 1989, 2000-2006, 2009-2015, 2018-2021 Free
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Software Foundation, Inc.
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This file is part of Bison, the GNU Compiler Compiler.
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This program is free software: you can redistribute it and/or modify
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it under the terms of the GNU General Public License as published by
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the Free Software Foundation, either version 3 of the License, or
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(at your option) any later version.
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This program is distributed in the hope that it will be useful,
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but WITHOUT ANY WARRANTY; without even the implied warranty of
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
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GNU General Public License for more details.
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You should have received a copy of the GNU General Public License
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along with this program. If not, see <https://www.gnu.org/licenses/>. */
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/* Set up NULLABLE, a vector saying which nonterminals can expand into
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the null string. NULLABLE[I - NTOKENS] is nonzero if symbol I can
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do so. */
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#include <config.h>
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#include "system.h"
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#include "getargs.h"
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#include "gram.h"
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#include "nullable.h"
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#include "reduce.h"
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#include "symtab.h"
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/* Linked list of rules. */
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typedef struct rule_list
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{
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struct rule_list *next;
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const rule *value;
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} rule_list;
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bool *nullable = NULL;
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static void
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nullable_print (FILE *out)
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{
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fputs ("NULLABLE\n", out);
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for (int i = ntokens; i < nsyms; i++)
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fprintf (out, " %s: %s\n", symbols[i]->tag,
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nullable[i - ntokens] ? "yes" : "no");
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fputs ("\n\n", out);
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}
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void
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nullable_compute (void)
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{
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nullable = xcalloc (nnterms, sizeof *nullable);
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size_t *rcount = xcalloc (nrules, sizeof *rcount);
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/* RITEM contains all the rules, including useless productions.
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Hence we must allocate room for useless nonterminals too. */
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rule_list **rsets = xcalloc (nnterms, sizeof *rsets);
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/* This is said to be more elements than we actually use.
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Supposedly NRITEMS - NRULES is enough. But why take the risk? */
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rule_list *relts = xnmalloc (nritems + nnterms + 1, sizeof *relts);
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symbol_number *squeue = xnmalloc (nnterms, sizeof *squeue);
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symbol_number *s2 = squeue;
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{
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rule_list *p = relts;
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for (rule_number ruleno = 0; ruleno < nrules; ++ruleno)
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if (rules[ruleno].useful)
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{
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const rule *r = &rules[ruleno];
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if (r->rhs[0] >= 0)
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{
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/* This rule has a non empty RHS. */
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bool any_tokens = false;
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for (item_number *rp = r->rhs; *rp >= 0; ++rp)
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if (ISTOKEN (*rp))
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any_tokens = true;
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/* This rule has only nonterminals: schedule it for the second
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pass. */
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if (!any_tokens)
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for (item_number *rp = r->rhs; *rp >= 0; ++rp)
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{
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rcount[ruleno]++;
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p->next = rsets[*rp - ntokens];
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p->value = r;
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rsets[*rp - ntokens] = p;
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p++;
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}
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}
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else
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{
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/* This rule has an empty RHS. */
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if (r->useful
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&& ! nullable[r->lhs->number - ntokens])
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{
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nullable[r->lhs->number - ntokens] = true;
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*s2++ = r->lhs->number;
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}
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}
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}
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}
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symbol_number *s1 = squeue;
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while (s1 < s2)
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for (rule_list *p = rsets[*s1++ - ntokens]; p; p = p->next)
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{
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const rule *r = p->value;
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if (--rcount[r->number] == 0)
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if (r->useful && ! nullable[r->lhs->number - ntokens])
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{
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nullable[r->lhs->number - ntokens] = true;
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*s2++ = r->lhs->number;
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}
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}
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free (squeue);
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free (rcount);
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free (rsets);
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free (relts);
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if (trace_flag & trace_sets)
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nullable_print (stderr);
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}
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void
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nullable_free (void)
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{
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free (nullable);
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}
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