mirror of
https://git.savannah.gnu.org/git/bison.git
synced 2026-08-11 11:45:13 +00:00
Akim Demaille
5844cd0cfa
removal of useless rules via CPP but via `if (0)', so that the compiler still check the code is valid. For instance, it should have noticed `rline' no longer exists: use the `line' member of rule_t. * src/gram.c (dummy, rline): Remove, unused.
581 lines
15 KiB
C
581 lines
15 KiB
C
/* Grammar reduction for Bison.
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Copyright 1988, 1989, 2000, 2001 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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/* Reduce the grammar: Find and eliminate unreachable terminals,
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nonterminals, and productions. David S. Bakin. */
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/* Don't eliminate unreachable terminals: They may be used by the
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user's parser. */
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#include "system.h"
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#include "getargs.h"
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#include "files.h"
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#include "gram.h"
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#include "complain.h"
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#include "reduce.h"
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#include "reader.h"
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#include "getargs.h"
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typedef unsigned *BSet;
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typedef short *rule;
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/* Set of all nonterminals which are not useless. */
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static BSet N;
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/* Set of all rules which have no useless nonterminals in their RHS. */
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static BSet P;
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/* Set of all accessible symbols. */
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static BSet V;
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/* Set of symbols used to define rule precedence (so they are
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`useless', but no warning should be issued). */
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static BSet V1;
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static int nuseful_productions;
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static int nuseless_productions;
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static int nuseful_nonterminals;
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int nuseless_nonterminals;
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static 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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static 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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{
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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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static 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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/*-------------------------------------------------------------------.
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| Another way to do this would be with a set for each production and |
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| then do subset tests against N0, but even for the C grammar the |
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| whole reducing 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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/* A production is useful if all of the nonterminals in its appear
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in the set of useful nonterminals. */
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for (r = &ritem[rule_table[i].rhs]; *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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/*---------------------------------------------------------.
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| Remember that rules are 1-origin, symbols are 0-origin. |
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`---------------------------------------------------------*/
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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;
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/* N is set as built. Np is set being built this iteration. P is
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set of all productions which have a RHS all in N. */
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Np = XCALLOC (unsigned, WORDSIZE (nvars));
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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
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the set being computed remains unchanged. Any nonterminals not
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in the set at that point are useless in that they will never be
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used in deriving a sentence of the language.
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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
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productions in this set will appear in the final grammar. */
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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, rule_table[i].lhs - 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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XFREE (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;
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short t;
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rule r;
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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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Consider only the (partially) reduced grammar which has only
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nonterminals in N and productions in P.
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The result is the set P of productions in the reduced grammar,
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and the set V of symbols in the reduced grammar.
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Although this algorithm also computes the set of terminals which
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are 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
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user can know. */
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Vp = XCALLOC (unsigned, WORDSIZE (nsyms));
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Pp = XCALLOC (unsigned, WORDSIZE (nrules + 1));
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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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{
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SETBIT (V, start_symbol);
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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)
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&& BITISSET (P, i)
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&& BITISSET (V, rule_table[i].lhs))
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{
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for (r = &ritem[rule_table[i].rhs]; *r >= 0; r++)
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if (ISTOKEN (t = *r) || BITISSET (N, t - ntokens))
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SETBIT (Vp, t);
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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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break;
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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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}
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XFREE (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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XFREE (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 (rule_table[i].precsym != 0)
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SETBIT (V1, rule_table[i].precsym);
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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 in
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the case statements in the actions file.
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We don't disable it via CPP so that it is still checked with the
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rest of the code, to avoid its becoming completely obsolete.
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FIXME: I think the comment above demonstrates this code must be
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turned off for *semantic* parser, not in the general case. Try
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to understand this better --akim. */
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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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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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rule_table[np].lhs = rule_table[pn].lhs;
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rule_table[np].line = rule_table[pn].line;
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rule_table[np].prec = rule_table[pn].prec;
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rule_table[np].assoc = rule_table[pn].assoc;
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rule_table[np].rhs = rule_table[pn].rhs;
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if (rule_table[np].rhs != ni)
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{
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pi = rule_table[np].rhs;
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rule_table[np].rhs = 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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}
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else
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{
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while (ritem[ni++] >= 0);
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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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/* 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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/* Disable useless productions. */
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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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rule_table[pn].useful = BITISSET (P, pn);
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}
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}
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/*------------------------------.
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| Remove useless nonterminals. |
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`------------------------------*/
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static void
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nonterminals_reduce (void)
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{
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int i, n;
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rule r;
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/* Map the nonterminals to their new index: useful first, useless
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afterwards. Kept for later report. */
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short *nontermmap = XCALLOC (short, nvars) - ntokens;
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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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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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{
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short *sassoc_sorted = XMALLOC (short, nvars) - ntokens;
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short *sprec_sorted = XMALLOC (short, nvars) - ntokens;
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char **tags_sorted = XMALLOC (char *, nvars) - ntokens;
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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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sassoc_sorted[n] = sassoc[i];
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sprec_sorted[n] = sprec[i];
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tags_sorted[n] = tags[i];
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}
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for (i = ntokens; i < nsyms; i++)
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{
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sassoc[i] = sassoc_sorted[i];
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sprec[i] = sprec_sorted[i];
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tags[i] = tags_sorted[i];
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}
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free (sassoc_sorted + ntokens);
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free (sprec_sorted + ntokens);
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free (tags_sorted + ntokens);
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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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rule_table[i].lhs = nontermmap[rule_table[i].lhs];
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if (ISVAR (rule_table[i].precsym))
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/* Can this happen? */
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rule_table[i].precsym = nontermmap[rule_table[i].precsym];
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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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| Output the detailed results of the reductions. For FILE.output. |
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`------------------------------------------------------------------*/
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void
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reduce_output (FILE *out)
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{
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if (nuseless_nonterminals > 0)
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{
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int i;
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fprintf (out, "%s\n\n", _("Useless nonterminals:"));
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for (i = 0; i < nuseless_nonterminals; ++i)
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fprintf (out, " %s\n", tags[nsyms + i]);
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fputs ("\n\n", out);
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}
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{
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bool b = FALSE;
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int i;
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for (i = 0; i < ntokens; i++)
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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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fprintf (out, "%s\n\n", _("Terminals which are not used:"));
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b = TRUE;
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fprintf (out, " %s\n", tags[i]);
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}
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if (b)
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fputs ("\n\n", out);
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}
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if (nuseless_productions > 0)
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{
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int i;
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fprintf (out, "%s\n\n", _("Useless rules:"));
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for (i = 1; i <= nrules; i++)
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if (!rule_table[i].useful)
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{
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rule r;
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fprintf (out, "#%-4d ", i);
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fprintf (out, "%s:", tags[rule_table[i].lhs]);
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for (r = &ritem[rule_table[i].rhs]; *r >= 0; r++)
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fprintf (out, " %s", tags[*r]);
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fputs (";\n", out);
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}
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fputs ("\n\n", out);
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}
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}
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static void
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dump_grammar (FILE *out)
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{
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int i;
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rule r;
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fprintf (out, "REDUCED GRAMMAR\n\n");
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fprintf (out,
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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 (out, "Variables\n---------\n\n");
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fprintf (out, "Value Sprec Sassoc Tag\n");
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for (i = ntokens; i < nsyms; i++)
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fprintf (out, "%5d %5d %5d %s\n", i, sprec[i], sassoc[i], tags[i]);
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fprintf (out, "\n\n");
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fprintf (out, "Rules\n-----\n\n");
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fprintf (out, "Num (Prec, Assoc, Useful, Ritem Range) Lhs -> Rhs (Ritem range) [Num]\n");
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for (i = 1; i <= nrules; i++)
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{
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int rhs_count = 0;
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/* Find the last RHS index in ritems. */
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for (r = &ritem[rule_table[i].rhs]; *r > 0; ++r)
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++rhs_count;
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fprintf (out, "%3d (%2d, %2d, %2d, %2d-%2d) %2d ->",
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i,
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rule_table[i].prec, rule_table[i].assoc, rule_table[i].useful,
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rule_table[i].rhs, rule_table[i].rhs + rhs_count - 1,
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rule_table[i].lhs);
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/* Dumped the RHS. */
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for (r = &ritem[rule_table[i].rhs]; *r > 0; r++)
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fprintf (out, "%3d", *r);
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fprintf (out, " [%d]\n", -(*r));
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}
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fprintf (out, "\n\n");
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fprintf (out, "Rules interpreted\n-----------------\n\n");
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for (i = 1; i <= nrules; i++)
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{
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fprintf (out, "%-5d %s :", i, tags[rule_table[i].lhs]);
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for (r = &ritem[rule_table[i].rhs]; *r > 0; r++)
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fprintf (out, " %s", tags[*r]);
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fputc ('\n', out);
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}
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fprintf (out, "\n\n");
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}
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/*-------------------------------.
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| Report the results to STDERR. |
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`-------------------------------*/
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static void
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reduce_print (void)
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{
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if (yacc_flag && nuseless_productions)
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fprintf (stderr, ngettext ("%d rule never reduced\n",
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"%d rules never reduced\n",
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nuseless_productions),
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nuseless_productions);
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fprintf (stderr, _("%s contains "), infile);
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if (nuseless_nonterminals > 0)
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fprintf (stderr, ngettext ("%d useless nonterminal",
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"%d useless nonterminals",
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nuseless_nonterminals),
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nuseless_nonterminals);
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if (nuseless_nonterminals > 0 && nuseless_productions > 0)
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fprintf (stderr, _(" and "));
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|
|
if (nuseless_productions > 0)
|
|
fprintf (stderr, ngettext ("%d useless rule",
|
|
"%d useless rules",
|
|
nuseless_productions),
|
|
nuseless_productions);
|
|
fprintf (stderr, "\n");
|
|
fflush (stderr);
|
|
}
|
|
|
|
void
|
|
reduce_grammar (void)
|
|
{
|
|
bool reduced;
|
|
|
|
/* Allocate the global sets used to compute the reduced grammar */
|
|
|
|
N = XCALLOC (unsigned, WORDSIZE (nvars));
|
|
P = XCALLOC (unsigned, WORDSIZE (nrules + 1));
|
|
V = XCALLOC (unsigned, WORDSIZE (nsyms));
|
|
V1 = XCALLOC (unsigned, WORDSIZE (nsyms));
|
|
|
|
useless_nonterminals ();
|
|
inaccessable_symbols ();
|
|
|
|
reduced = (bool) (nuseless_nonterminals + nuseless_productions > 0);
|
|
|
|
if (!reduced)
|
|
return;
|
|
|
|
reduce_print ();
|
|
|
|
if (!BITISSET (N, start_symbol - ntokens))
|
|
fatal (_("Start symbol %s does not derive any sentence"),
|
|
tags[start_symbol]);
|
|
|
|
reduce_grammar_tables ();
|
|
if (nuseless_nonterminals > 0)
|
|
nonterminals_reduce ();
|
|
|
|
if (trace_flag)
|
|
{
|
|
dump_grammar (stderr);
|
|
|
|
fprintf (stderr, "reduced %s defines %d terminals, %d nonterminals\
|
|
, and %d productions.\n",
|
|
infile, ntokens, nvars, nrules);
|
|
}
|
|
}
|
|
|
|
|
|
/*-----------------------------------------------------------.
|
|
| Free the global sets used to compute the reduced grammar. |
|
|
`-----------------------------------------------------------*/
|
|
|
|
void
|
|
reduce_free (void)
|
|
{
|
|
XFREE (N);
|
|
XFREE (V);
|
|
XFREE (V1);
|
|
XFREE (P);
|
|
}
|