mirror of
https://git.savannah.gnu.org/git/bison.git
synced 2026-03-09 20:33:03 +00:00
21a7fa8063
* src/gram.c (grammar_dump): Print the effective number first instead of last. And fix it (remove the incorrect "+1"). Use t/f for Booleans. * src/reduce.c: When asked, always print the reduced grammar, even if there was nothing useless. * tests/sets.at (Reduced Grammar): Check that.
436 lines
14 KiB
C
436 lines
14 KiB
C
/* Grammar reduction for Bison.
|
||
|
||
Copyright (C) 1988-1989, 2000-2003, 2005-2015, 2018-2019 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/>. */
|
||
|
||
|
||
/* Reduce the grammar: Find and eliminate unreachable terminals,
|
||
nonterminals, and productions. David S. Bakin. */
|
||
|
||
/* Don't eliminate unreachable terminals: They may be used by the
|
||
user's parser. */
|
||
|
||
#include <config.h>
|
||
#include "system.h"
|
||
|
||
#include <bitset.h>
|
||
|
||
#include "complain.h"
|
||
#include "files.h"
|
||
#include "getargs.h"
|
||
#include "gram.h"
|
||
#include "print-xml.h"
|
||
#include "reader.h"
|
||
#include "reduce.h"
|
||
#include "symtab.h"
|
||
|
||
/* Set of nonterminals whose language is not empty. */
|
||
static bitset N;
|
||
|
||
/* Set of rules that have no useless nonterminals in their RHS. */
|
||
static bitset P;
|
||
|
||
/* Set of accessible symbols. */
|
||
static bitset V;
|
||
|
||
/* Set of symbols used to define rule precedence (so they are
|
||
'useless', but no warning should be issued). */
|
||
static bitset V1;
|
||
|
||
unsigned nuseless_productions;
|
||
unsigned nuseless_nonterminals;
|
||
|
||
#define bitset_swap(Lhs, Rhs) \
|
||
do { \
|
||
bitset lhs__ = Lhs; \
|
||
Lhs = Rhs; \
|
||
Rhs = lhs__; \
|
||
} while (0)
|
||
|
||
/*-------------------------------------------------------------------.
|
||
| Another way to do this would be with a set for each production and |
|
||
| then do subset tests against N0, but even for the C grammar the |
|
||
| whole reducing process takes only 2 seconds on my 8Mhz AT. |
|
||
`-------------------------------------------------------------------*/
|
||
|
||
static bool
|
||
useful_production (rule_number r, bitset N0)
|
||
{
|
||
/* A production is useful if all of the nonterminals in its appear
|
||
in the set of useful nonterminals. */
|
||
|
||
for (item_number *rhsp = rules[r].rhs; 0 <= *rhsp; ++rhsp)
|
||
if (ISVAR (*rhsp) && !bitset_test (N0, *rhsp - ntokens))
|
||
return false;
|
||
return true;
|
||
}
|
||
|
||
|
||
/*-----------------------------------------------------------------.
|
||
| Compute N, the set of nonterminals whose language is not empty. |
|
||
| |
|
||
| Remember that rules are 1-origin, symbols are 0-origin. |
|
||
`-----------------------------------------------------------------*/
|
||
|
||
static void
|
||
useless_nonterminals (void)
|
||
{
|
||
/* N is set as built. Np is set being built this iteration. P is
|
||
set of all productions which have a RHS all in N. */
|
||
|
||
bitset Np = bitset_create (nvars, BITSET_FIXED);
|
||
|
||
/* The set being computed is a set of nonterminals which can derive
|
||
the empty string or strings consisting of all terminals. At each
|
||
iteration a nonterminal is added to the set if there is a
|
||
production with that nonterminal as its LHS for which all the
|
||
nonterminals in its RHS are already in the set. Iterate until
|
||
the set being computed remains unchanged. Any nonterminals not
|
||
in the set at that point are useless in that they will never be
|
||
used in deriving a sentence of the language.
|
||
|
||
This iteration doesn't use any special traversal over the
|
||
productions. A set is kept of all productions for which all the
|
||
nonterminals in the RHS are in useful. Only productions not in
|
||
this set are scanned on each iteration. At the end, this set is
|
||
saved to be used when finding useful productions: only
|
||
productions in this set will appear in the final grammar. */
|
||
|
||
while (1)
|
||
{
|
||
bitset_copy (Np, N);
|
||
for (rule_number r = 0; r < nrules; ++r)
|
||
if (!bitset_test (P, r)
|
||
&& useful_production (r, N))
|
||
{
|
||
bitset_set (Np, rules[r].lhs->number - ntokens);
|
||
bitset_set (P, r);
|
||
}
|
||
if (bitset_equal_p (N, Np))
|
||
break;
|
||
bitset_swap (N, Np);
|
||
}
|
||
bitset_free (N);
|
||
N = Np;
|
||
}
|
||
|
||
|
||
static void
|
||
inaccessable_symbols (void)
|
||
{
|
||
/* Find out which productions are reachable and which symbols are
|
||
used. Starting with an empty set of productions and a set of
|
||
symbols which only has the start symbol in it, iterate over all
|
||
productions until the set of productions remains unchanged for an
|
||
iteration. For each production which has a LHS in the set of
|
||
reachable symbols, add the production to the set of reachable
|
||
productions, and add all of the nonterminals in the RHS of the
|
||
production to the set of reachable symbols.
|
||
|
||
Consider only the (partially) reduced grammar which has only
|
||
nonterminals in N and productions in P.
|
||
|
||
The result is the set P of productions in the reduced grammar,
|
||
and the set V of symbols in the reduced grammar.
|
||
|
||
Although this algorithm also computes the set of terminals which
|
||
are reachable, no terminal will be deleted from the grammar. Some
|
||
terminals might not be in the grammar but might be generated by
|
||
semantic routines, and so the user might want them available with
|
||
specified numbers. (Is this true?) However, the nonreachable
|
||
terminals are printed (if running in verbose mode) so that the
|
||
user can know. */
|
||
|
||
bitset Vp = bitset_create (nsyms, BITSET_FIXED);
|
||
bitset Pp = bitset_create (nrules, BITSET_FIXED);
|
||
|
||
/* If the start symbol isn't useful, then nothing will be useful. */
|
||
if (bitset_test (N, accept->content->number - ntokens))
|
||
{
|
||
bitset_set (V, accept->content->number);
|
||
|
||
while (1)
|
||
{
|
||
bitset_copy (Vp, V);
|
||
for (rule_number r = 0; r < nrules; ++r)
|
||
if (!bitset_test (Pp, r)
|
||
&& bitset_test (P, r)
|
||
&& bitset_test (V, rules[r].lhs->number))
|
||
{
|
||
for (item_number *rhsp = rules[r].rhs; 0 <= *rhsp; ++rhsp)
|
||
if (ISTOKEN (*rhsp) || bitset_test (N, *rhsp - ntokens))
|
||
bitset_set (Vp, *rhsp);
|
||
bitset_set (Pp, r);
|
||
}
|
||
if (bitset_equal_p (V, Vp))
|
||
break;
|
||
bitset_swap (V, Vp);
|
||
}
|
||
}
|
||
|
||
bitset_free (V);
|
||
V = Vp;
|
||
|
||
/* These tokens (numbered 0, 1, and 2) are internal to Bison.
|
||
Consider them useful. */
|
||
bitset_set (V, endtoken->content->number); /* end-of-input token */
|
||
bitset_set (V, errtoken->content->number); /* error token */
|
||
bitset_set (V, undeftoken->content->number); /* some undefined token */
|
||
|
||
bitset_free (P);
|
||
P = Pp;
|
||
|
||
unsigned nuseful_productions = bitset_count (P);
|
||
nuseless_productions = nrules - nuseful_productions;
|
||
|
||
unsigned nuseful_nonterminals = 0;
|
||
for (symbol_number i = ntokens; i < nsyms; ++i)
|
||
nuseful_nonterminals += bitset_test (V, i);
|
||
nuseless_nonterminals = nvars - nuseful_nonterminals;
|
||
|
||
/* A token that was used in %prec should not be warned about. */
|
||
for (rule_number r = 0; r < nrules; ++r)
|
||
if (rules[r].precsym != 0)
|
||
bitset_set (V1, rules[r].precsym->number);
|
||
}
|
||
|
||
|
||
/*-------------------------------------------------------------------.
|
||
| Put the useless productions at the end of RULES, and adjust NRULES |
|
||
| accordingly. |
|
||
`-------------------------------------------------------------------*/
|
||
|
||
static void
|
||
reduce_grammar_tables (void)
|
||
{
|
||
/* Report and flag useless productions. */
|
||
{
|
||
for (rule_number r = 0; r < nrules; ++r)
|
||
rules[r].useful = bitset_test (P, r);
|
||
grammar_rules_useless_report (_("rule useless in grammar"));
|
||
}
|
||
|
||
/* Map the nonterminals to their new index: useful first, useless
|
||
afterwards. Kept for later report. */
|
||
{
|
||
int useful = 0;
|
||
int useless = nrules - nuseless_productions;
|
||
rule *rules_sorted = xnmalloc (nrules, sizeof *rules_sorted);
|
||
for (rule_number r = 0; r < nrules; ++r)
|
||
rules_sorted[rules[r].useful ? useful++ : useless++] = rules[r];
|
||
free (rules);
|
||
rules = rules_sorted;
|
||
|
||
/* Renumber the rules markers in RITEMS. */
|
||
for (rule_number r = 0; r < nrules; ++r)
|
||
{
|
||
item_number *rhsp = rules[r].rhs;
|
||
for (/* Nothing. */; 0 <= *rhsp; ++rhsp)
|
||
continue;
|
||
*rhsp = rule_number_as_item_number (r);
|
||
rules[r].number = r;
|
||
}
|
||
nrules -= nuseless_productions;
|
||
}
|
||
|
||
/* Adjust NRITEMS. */
|
||
for (rule_number r = nrules; r < nrules + nuseless_productions; ++r)
|
||
nritems -= rule_rhs_length (&rules[r]) + 1;
|
||
}
|
||
|
||
|
||
/*------------------------------.
|
||
| Remove useless nonterminals. |
|
||
`------------------------------*/
|
||
|
||
static void
|
||
nonterminals_reduce (void)
|
||
{
|
||
/* Map the nonterminals to their new index: useful first, useless
|
||
afterwards. Kept for later report. */
|
||
|
||
symbol_number *nontermmap = xnmalloc (nvars, sizeof *nontermmap);
|
||
{
|
||
symbol_number n = ntokens;
|
||
for (symbol_number i = ntokens; i < nsyms; ++i)
|
||
if (bitset_test (V, i))
|
||
nontermmap[i - ntokens] = n++;
|
||
for (symbol_number i = ntokens; i < nsyms; ++i)
|
||
if (!bitset_test (V, i))
|
||
{
|
||
nontermmap[i - ntokens] = n++;
|
||
if (symbols[i]->content->status != used)
|
||
complain (&symbols[i]->location, Wother,
|
||
_("nonterminal useless in grammar: %s"),
|
||
symbols[i]->tag);
|
||
}
|
||
}
|
||
|
||
|
||
/* Shuffle elements of tables indexed by symbol number. */
|
||
{
|
||
symbol **symbols_sorted = xnmalloc (nvars, sizeof *symbols_sorted);
|
||
for (symbol_number i = ntokens; i < nsyms; ++i)
|
||
symbols[i]->content->number = nontermmap[i - ntokens];
|
||
for (symbol_number i = ntokens; i < nsyms; ++i)
|
||
symbols_sorted[nontermmap[i - ntokens] - ntokens] = symbols[i];
|
||
for (symbol_number i = ntokens; i < nsyms; ++i)
|
||
symbols[i] = symbols_sorted[i - ntokens];
|
||
free (symbols_sorted);
|
||
}
|
||
|
||
{
|
||
for (rule_number r = 0; r < nrules; ++r)
|
||
for (item_number *rhsp = rules[r].rhs; 0 <= *rhsp; ++rhsp)
|
||
if (ISVAR (*rhsp))
|
||
*rhsp = symbol_number_as_item_number (nontermmap[*rhsp
|
||
- ntokens]);
|
||
accept->content->number = nontermmap[accept->content->number - ntokens];
|
||
}
|
||
|
||
nsyms -= nuseless_nonterminals;
|
||
nvars -= nuseless_nonterminals;
|
||
|
||
free (nontermmap);
|
||
}
|
||
|
||
|
||
/*------------------------------------------------------------------.
|
||
| Output the detailed results of the reductions. For FILE.output. |
|
||
`------------------------------------------------------------------*/
|
||
|
||
void
|
||
reduce_output (FILE *out)
|
||
{
|
||
if (nuseless_nonterminals)
|
||
{
|
||
fprintf (out, "%s\n\n", _("Nonterminals useless in grammar"));
|
||
for (int i = 0; i < nuseless_nonterminals; ++i)
|
||
fprintf (out, " %s\n", symbols[nsyms + i]->tag);
|
||
fputs ("\n\n", out);
|
||
}
|
||
|
||
{
|
||
bool b = false;
|
||
for (int i = 0; i < ntokens; ++i)
|
||
if (reduce_token_unused_in_grammar (i))
|
||
{
|
||
if (!b)
|
||
fprintf (out, "%s\n\n", _("Terminals unused in grammar"));
|
||
b = true;
|
||
fprintf (out, " %s\n", symbols[i]->tag);
|
||
}
|
||
if (b)
|
||
fputs ("\n\n", out);
|
||
}
|
||
|
||
if (nuseless_productions)
|
||
grammar_rules_partial_print (out, _("Rules useless in grammar"),
|
||
rule_useless_in_grammar_p);
|
||
}
|
||
|
||
|
||
/*-------------------------------.
|
||
| Report the results to STDERR. |
|
||
`-------------------------------*/
|
||
|
||
static void
|
||
reduce_print (void)
|
||
{
|
||
if (nuseless_nonterminals)
|
||
complain (NULL, Wother, ngettext ("%d nonterminal useless in grammar",
|
||
"%d nonterminals useless in grammar",
|
||
nuseless_nonterminals),
|
||
nuseless_nonterminals);
|
||
if (nuseless_productions)
|
||
complain (NULL, Wother, ngettext ("%d rule useless in grammar",
|
||
"%d rules useless in grammar",
|
||
nuseless_productions),
|
||
nuseless_productions);
|
||
}
|
||
|
||
void
|
||
reduce_grammar (void)
|
||
{
|
||
/* Allocate the global sets used to compute the reduced grammar */
|
||
|
||
N = bitset_create (nvars, BITSET_FIXED);
|
||
P = bitset_create (nrules, BITSET_FIXED);
|
||
V = bitset_create (nsyms, BITSET_FIXED);
|
||
V1 = bitset_create (nsyms, BITSET_FIXED);
|
||
|
||
useless_nonterminals ();
|
||
inaccessable_symbols ();
|
||
|
||
/* Did we reduce something? */
|
||
if (nuseless_nonterminals || nuseless_productions)
|
||
{
|
||
reduce_print ();
|
||
|
||
if (!bitset_test (N, accept->content->number - ntokens))
|
||
complain (&startsymbol_location, fatal,
|
||
_("start symbol %s does not derive any sentence"),
|
||
startsymbol->tag);
|
||
|
||
/* First reduce the nonterminals, as they renumber themselves in the
|
||
whole grammar. If you change the order, nonterms would be
|
||
renumbered only in the reduced grammar. */
|
||
if (nuseless_nonterminals)
|
||
nonterminals_reduce ();
|
||
if (nuseless_productions)
|
||
reduce_grammar_tables ();
|
||
}
|
||
|
||
if (trace_flag & trace_grammar)
|
||
{
|
||
grammar_dump (stderr, "Reduced Grammar");
|
||
|
||
fprintf (stderr, "reduced %s defines %d terminals, %d nonterminals"
|
||
", and %d productions.\n",
|
||
grammar_file, ntokens, nvars, nrules);
|
||
}
|
||
}
|
||
|
||
bool
|
||
reduce_token_unused_in_grammar (symbol_number i)
|
||
{
|
||
aver (i < ntokens);
|
||
return !bitset_test (V, i) && !bitset_test (V1, i);
|
||
}
|
||
|
||
bool
|
||
reduce_nonterminal_useless_in_grammar (const sym_content *sym)
|
||
{
|
||
symbol_number n = sym->number;
|
||
aver (ntokens <= n && n < nsyms + nuseless_nonterminals);
|
||
return nsyms <= n;
|
||
}
|
||
|
||
/*-----------------------------------------------------------.
|
||
| Free the global sets used to compute the reduced grammar. |
|
||
`-----------------------------------------------------------*/
|
||
|
||
void
|
||
reduce_free (void)
|
||
{
|
||
bitset_free (N);
|
||
bitset_free (V);
|
||
bitset_free (V1);
|
||
bitset_free (P);
|
||
}
|