595 lines
14 KiB
C
595 lines
14 KiB
C
/* Grammar reduction for Bison.
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Copyright (C) 1988, 1989 Free Software Foundation, Inc.
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This file is part of Bison, the GNU Compiler Compiler.
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Bison 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 2, or (at your option)
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any later version.
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Bison 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 Bison; see the file COPYING. If not, write to
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the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
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Boston, MA 02111-1307, USA. */
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/*
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* Reduce the grammar: Find and eliminate unreachable terminals,
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* nonterminals, and productions. David S. Bakin.
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*/
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/*
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* Don't eliminate unreachable terminals: They may be used by the user's
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* parser.
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*/
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#include <stdio.h>
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#include "system.h"
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#include "files.h"
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#include "gram.h"
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#include "machine.h"
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#include "alloc.h"
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extern char **tags; /* reader.c */
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extern int verboseflag; /* getargs.c */
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static int statisticsflag; /* XXXXXXX */
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extern int fixed_outfiles;
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#ifndef TRUE
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#define TRUE (1)
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#define FALSE (0)
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#endif
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typedef int bool;
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typedef unsigned *BSet;
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typedef short *rule;
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/*
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* N is set of all nonterminals which are not useless. P is set of all rules
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* which have no useless nonterminals in their RHS. V is the set of all
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* accessible symbols.
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*/
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static BSet N, P, V, V1;
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static int nuseful_productions, nuseless_productions,
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nuseful_nonterminals, nuseless_nonterminals;
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bool bits_equal PARAMS((BSet, BSet, int));
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int nbits PARAMS((unsigned));
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int bits_size PARAMS((BSet, int));
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void reduce_grammar PARAMS((void));
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static void useless_nonterminals PARAMS((void));
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static void inaccessable_symbols PARAMS((void));
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static void reduce_grammar_tables PARAMS((void));
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static void print_results PARAMS((void));
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static void print_notices PARAMS((void));
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void dump_grammar PARAMS((void));
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extern void fatals PARAMS((char *, char *));
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bool
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bits_equal (BSet L, BSet R, int n)
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{
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int i;
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for (i = n - 1; i >= 0; i--)
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if (L[i] != R[i])
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return FALSE;
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return TRUE;
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}
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int
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nbits (unsigned i)
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{
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int count = 0;
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while (i != 0) {
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i ^= (i & ((unsigned) (- (int) i)));
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++count;
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}
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return count;
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}
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int
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bits_size (BSet S, int n)
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{
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int i, count = 0;
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for (i = n - 1; i >= 0; i--)
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count += nbits(S[i]);
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return count;
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}
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void
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reduce_grammar (void)
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{
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bool reduced;
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/* Allocate the global sets used to compute the reduced grammar */
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N = NEW2(WORDSIZE(nvars), unsigned);
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P = NEW2(WORDSIZE(nrules + 1), unsigned);
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V = NEW2(WORDSIZE(nsyms), unsigned);
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V1 = NEW2(WORDSIZE(nsyms), unsigned);
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useless_nonterminals();
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inaccessable_symbols();
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reduced = (bool) (nuseless_nonterminals + nuseless_productions > 0);
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if (verboseflag)
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print_results();
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if (reduced == FALSE)
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goto done_reducing;
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print_notices();
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if (!BITISSET(N, start_symbol - ntokens))
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fatals(_("Start symbol %s does not derive any sentence"),
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tags[start_symbol]);
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reduce_grammar_tables();
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/* if (verboseflag) {
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fprintf(foutput, "REDUCED GRAMMAR\n\n");
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dump_grammar();
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}
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*/
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/**/ statisticsflag = FALSE; /* someday getopts should handle this */
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if (statisticsflag == TRUE)
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fprintf(stderr,
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_("reduced %s defines %d terminal%s, %d nonterminal%s\
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, and %d production%s.\n"), infile,
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ntokens, (ntokens == 1 ? "" : "s"),
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nvars, (nvars == 1 ? "" : "s"),
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nrules, (nrules == 1 ? "" : "s"));
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done_reducing:
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/* Free the global sets used to compute the reduced grammar */
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FREE(N);
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FREE(V);
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FREE(P);
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}
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/*
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* Another way to do this would be with a set for each production and then do
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* subset tests against N0, but even for the C grammar the whole reducing
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* process takes only 2 seconds on my 8Mhz AT.
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*/
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static bool
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useful_production (int i, BSet N0)
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{
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rule r;
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short n;
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/*
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* A production is useful if all of the nonterminals in its RHS
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* appear in the set of useful nonterminals.
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*/
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for (r = &ritem[rrhs[i]]; *r > 0; r++)
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if (ISVAR(n = *r))
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if (!BITISSET(N0, n - ntokens))
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return FALSE;
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return TRUE;
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}
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/* Remember that rules are 1-origin, symbols are 0-origin. */
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static void
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useless_nonterminals (void)
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{
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BSet Np, Ns;
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int i, n;
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/*
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* N is set as built. Np is set being built this iteration. P is set
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* of all productions which have a RHS all in N.
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*/
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Np = NEW2(WORDSIZE(nvars), unsigned);
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/*
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* The set being computed is a set of nonterminals which can derive
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* the empty string or strings consisting of all terminals. At each
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* iteration a nonterminal is added to the set if there is a
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* production with that nonterminal as its LHS for which all the
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* nonterminals in its RHS are already in the set. Iterate until the
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* set being computed remains unchanged. Any nonterminals not in the
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* set at that point are useless in that they will never be used in
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* deriving a sentence of the language.
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*
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* This iteration doesn't use any special traversal over the
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* productions. A set is kept of all productions for which all the
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* nonterminals in the RHS are in useful. Only productions not in
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* this set are scanned on each iteration. At the end, this set is
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* saved to be used when finding useful productions: only productions
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* in this set will appear in the final grammar.
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*/
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n = 0;
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while (1)
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{
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for (i = WORDSIZE(nvars) - 1; i >= 0; i--)
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Np[i] = N[i];
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for (i = 1; i <= nrules; i++)
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{
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if (!BITISSET(P, i))
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{
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if (useful_production(i, N))
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{
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SETBIT(Np, rlhs[i] - ntokens);
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SETBIT(P, i);
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}
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}
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}
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if (bits_equal(N, Np, WORDSIZE(nvars)))
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break;
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Ns = Np;
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Np = N;
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N = Ns;
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}
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FREE(N);
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N = Np;
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}
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static void
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inaccessable_symbols (void)
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{
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BSet Vp, Vs, Pp;
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int i, n;
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short t;
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rule r;
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/*
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* Find out which productions are reachable and which symbols are
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* used. Starting with an empty set of productions and a set of
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* symbols which only has the start symbol in it, iterate over all
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* productions until the set of productions remains unchanged for an
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* iteration. For each production which has a LHS in the set of
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* reachable symbols, add the production to the set of reachable
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* productions, and add all of the nonterminals in the RHS of the
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* production to the set of reachable symbols.
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*
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* Consider only the (partially) reduced grammar which has only
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* nonterminals in N and productions in P.
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*
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* The result is the set P of productions in the reduced grammar, and
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* the set V of symbols in the reduced grammar.
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*
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* Although this algorithm also computes the set of terminals which are
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* reachable, no terminal will be deleted from the grammar. Some
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* terminals might not be in the grammar but might be generated by
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* semantic routines, and so the user might want them available with
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* specified numbers. (Is this true?) However, the nonreachable
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* terminals are printed (if running in verbose mode) so that the user
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* can know.
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*/
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Vp = NEW2(WORDSIZE(nsyms), unsigned);
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Pp = NEW2(WORDSIZE(nrules + 1), unsigned);
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/* If the start symbol isn't useful, then nothing will be useful. */
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if (!BITISSET(N, start_symbol - ntokens))
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goto end_iteration;
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SETBIT(V, start_symbol);
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n = 0;
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while (1)
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{
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for (i = WORDSIZE(nsyms) - 1; i >= 0; i--)
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Vp[i] = V[i];
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for (i = 1; i <= nrules; i++)
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{
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if (!BITISSET(Pp, i) && BITISSET(P, i) &&
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BITISSET(V, rlhs[i]))
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{
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for (r = &ritem[rrhs[i]]; *r >= 0; r++)
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{
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if (ISTOKEN(t = *r)
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|| BITISSET(N, t - ntokens))
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{
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SETBIT(Vp, t);
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}
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}
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SETBIT(Pp, i);
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}
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}
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if (bits_equal(V, Vp, WORDSIZE(nsyms)))
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{
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break;
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}
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Vs = Vp;
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Vp = V;
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V = Vs;
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}
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end_iteration:
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FREE(V);
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V = Vp;
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/* Tokens 0, 1, and 2 are internal to Bison. Consider them useful. */
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SETBIT(V, 0); /* end-of-input token */
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SETBIT(V, 1); /* error token */
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SETBIT(V, 2); /* some undefined token */
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FREE(P);
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P = Pp;
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nuseful_productions = bits_size(P, WORDSIZE(nrules + 1));
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nuseless_productions = nrules - nuseful_productions;
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nuseful_nonterminals = 0;
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for (i = ntokens; i < nsyms; i++)
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if (BITISSET(V, i))
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nuseful_nonterminals++;
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nuseless_nonterminals = nvars - nuseful_nonterminals;
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/* A token that was used in %prec should not be warned about. */
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for (i = 1; i < nrules; i++)
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if (rprecsym[i] != 0)
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SETBIT(V1, rprecsym[i]);
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}
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static void
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reduce_grammar_tables (void)
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{
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/* This is turned off because we would need to change the numbers
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in the case statements in the actions file. */
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#if 0
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/* remove useless productions */
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if (nuseless_productions > 0)
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{
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short np, pn, ni, pi;
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np = 0;
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ni = 0;
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for (pn = 1; pn <= nrules; pn++)
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{
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if (BITISSET(P, pn))
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{
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np++;
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if (pn != np)
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{
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rlhs[np] = rlhs[pn];
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rline[np] = rline[pn];
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rprec[np] = rprec[pn];
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rassoc[np] = rassoc[pn];
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rrhs[np] = rrhs[pn];
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if (rrhs[np] != ni)
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{
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pi = rrhs[np];
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rrhs[np] = ni;
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while (ritem[pi] >= 0)
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ritem[ni++] = ritem[pi++];
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ritem[ni++] = -np;
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}
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} else {
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while (ritem[ni++] >= 0);
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}
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}
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}
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ritem[ni] = 0;
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nrules -= nuseless_productions;
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nitems = ni;
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/*
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* Is it worth it to reduce the amount of memory for the
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* grammar? Probably not.
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*/
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}
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#endif /* 0 */
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/* Disable useless productions,
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since they may contain useless nonterms
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that would get mapped below to -1 and confuse everyone. */
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if (nuseless_productions > 0)
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{
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int pn;
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for (pn = 1; pn <= nrules; pn++)
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{
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if (!BITISSET(P, pn))
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{
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rlhs[pn] = -1;
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}
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}
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}
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/* remove useless symbols */
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if (nuseless_nonterminals > 0)
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{
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int i, n;
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/* short j; JF unused */
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short *nontermmap;
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rule r;
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/*
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* create a map of nonterminal number to new nonterminal
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* number. -1 in the map means it was useless and is being
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* eliminated.
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*/
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nontermmap = NEW2(nvars, short) - ntokens;
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for (i = ntokens; i < nsyms; i++)
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nontermmap[i] = -1;
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n = ntokens;
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for (i = ntokens; i < nsyms; i++)
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if (BITISSET(V, i))
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nontermmap[i] = n++;
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/* Shuffle elements of tables indexed by symbol number. */
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for (i = ntokens; i < nsyms; i++)
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{
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n = nontermmap[i];
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if (n >= 0)
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{
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sassoc[n] = sassoc[i];
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sprec[n] = sprec[i];
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tags[n] = tags[i];
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} else {
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free(tags[i]);
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}
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}
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/* Replace all symbol numbers in valid data structures. */
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for (i = 1; i <= nrules; i++)
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{
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/* Ignore the rules disabled above. */
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if (rlhs[i] >= 0)
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rlhs[i] = nontermmap[rlhs[i]];
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if (ISVAR (rprecsym[i]))
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/* Can this happen? */
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rprecsym[i] = nontermmap[rprecsym[i]];
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}
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for (r = ritem; *r; r++)
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if (ISVAR(*r))
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*r = nontermmap[*r];
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start_symbol = nontermmap[start_symbol];
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nsyms -= nuseless_nonterminals;
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nvars -= nuseless_nonterminals;
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free(&nontermmap[ntokens]);
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}
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}
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static void
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print_results (void)
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{
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int i;
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/* short j; JF unused */
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rule r;
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bool b;
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if (nuseless_nonterminals > 0)
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{
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fprintf(foutput, _("Useless nonterminals:\n\n"));
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for (i = ntokens; i < nsyms; i++)
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if (!BITISSET(V, i))
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fprintf(foutput, " %s\n", tags[i]);
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}
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b = FALSE;
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for (i = 0; i < ntokens; i++)
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{
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if (!BITISSET(V, i) && !BITISSET(V1, i))
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{
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if (!b)
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{
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fprintf(foutput, _("\n\nTerminals which are not used:\n\n"));
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b = TRUE;
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}
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fprintf(foutput, " %s\n", tags[i]);
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}
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}
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if (nuseless_productions > 0)
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{
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fprintf(foutput, _("\n\nUseless rules:\n\n"));
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for (i = 1; i <= nrules; i++)
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{
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if (!BITISSET(P, i))
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{
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fprintf(foutput, "#%-4d ", i);
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fprintf(foutput, "%s :\t", tags[rlhs[i]]);
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for (r = &ritem[rrhs[i]]; *r >= 0; r++)
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{
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fprintf(foutput, " %s", tags[*r]);
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}
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fprintf(foutput, ";\n");
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}
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}
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}
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if (nuseless_nonterminals > 0 || nuseless_productions > 0 || b)
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fprintf(foutput, "\n\n");
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}
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void
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dump_grammar (void)
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{
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int i;
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rule r;
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fprintf(foutput,
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"ntokens = %d, nvars = %d, nsyms = %d, nrules = %d, nitems = %d\n\n",
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ntokens, nvars, nsyms, nrules, nitems);
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fprintf(foutput, _("Variables\n---------\n\n"));
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fprintf(foutput, _("Value Sprec Sassoc Tag\n"));
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for (i = ntokens; i < nsyms; i++)
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fprintf(foutput, "%5d %5d %5d %s\n",
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i, sprec[i], sassoc[i], tags[i]);
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fprintf(foutput, "\n\n");
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fprintf(foutput, _("Rules\n-----\n\n"));
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for (i = 1; i <= nrules; i++)
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{
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fprintf(foutput, "%-5d(%5d%5d)%5d : (@%-5d)",
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i, rprec[i], rassoc[i], rlhs[i], rrhs[i]);
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for (r = &ritem[rrhs[i]]; *r > 0; r++)
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fprintf(foutput, "%5d", *r);
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fprintf(foutput, " [%d]\n", -(*r));
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}
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fprintf(foutput, "\n\n");
|
||
fprintf(foutput, _("Rules interpreted\n-----------------\n\n"));
|
||
for (i = 1; i <= nrules; i++)
|
||
{
|
||
fprintf(foutput, "%-5d %s :", i, tags[rlhs[i]]);
|
||
for (r = &ritem[rrhs[i]]; *r > 0; r++)
|
||
fprintf(foutput, " %s", tags[*r]);
|
||
fprintf(foutput, "\n");
|
||
}
|
||
fprintf(foutput, "\n\n");
|
||
}
|
||
|
||
|
||
static void
|
||
print_notices (void)
|
||
{
|
||
if (fixed_outfiles && nuseless_productions)
|
||
fprintf(stderr, _("%d rules never reduced\n"), nuseless_productions);
|
||
|
||
fprintf(stderr, _("%s contains "), infile);
|
||
|
||
if (nuseless_nonterminals > 0)
|
||
{
|
||
fprintf(stderr, _("%d useless nonterminal%s"),
|
||
nuseless_nonterminals,
|
||
(nuseless_nonterminals == 1 ? "" : "s"));
|
||
}
|
||
if (nuseless_nonterminals > 0 && nuseless_productions > 0)
|
||
fprintf(stderr, _(" and "));
|
||
|
||
if (nuseless_productions > 0)
|
||
{
|
||
fprintf(stderr, _("%d useless rule%s"),
|
||
nuseless_productions,
|
||
(nuseless_productions == 1 ? "" : "s"));
|
||
}
|
||
fprintf(stderr, "\n");
|
||
fflush(stderr);
|
||
}
|