/* Grammar reduction for Bison. Copyright 1988, 1989, 2000, 2001 Free Software Foundation, Inc. This file is part of Bison, the GNU Compiler Compiler. Bison 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 2, or (at your option) any later version. Bison 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 Bison; see the file COPYING. If not, write to the Free Software Foundation, Inc., 59 Temple Place - Suite 330, Boston, MA 02111-1307, USA. */ /* 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 "system.h" #include "getargs.h" #include "files.h" #include "gram.h" #include "complain.h" #include "reduce.h" #include "reader.h" #include "getargs.h" typedef unsigned *BSet; typedef short *rule; /* Set of all nonterminals which are not useless. */ static BSet N; /* Set of all rules which have no useless nonterminals in their RHS. */ static BSet P; /* Set of all accessible symbols. */ static BSet V; /* Set of symbols used to define rule precedence (so they are `useless', but no warning should be issued). */ static BSet V1; static int nuseful_productions; static int nuseless_productions; static int nuseful_nonterminals; int nuseless_nonterminals; static bool bits_equal (BSet L, BSet R, int n) { int i; for (i = n - 1; i >= 0; i--) if (L[i] != R[i]) return FALSE; return TRUE; } static int nbits (unsigned i) { int count = 0; while (i != 0) { i ^= (i & ((unsigned) (-(int) i))); ++count; } return count; } static int bits_size (BSet S, int n) { int i, count = 0; for (i = n - 1; i >= 0; i--) count += nbits (S[i]); return count; } /*-------------------------------------------------------------------. | 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 (int i, BSet N0) { rule r; short n; /* A production is useful if all of the nonterminals in its appear in the set of useful nonterminals. */ for (r = &ritem[rule_table[i].rhs]; *r > 0; r++) if (ISVAR (n = *r)) if (!BITISSET (N0, n - ntokens)) return FALSE; return TRUE; } /*---------------------------------------------------------. | Remember that rules are 1-origin, symbols are 0-origin. | `---------------------------------------------------------*/ static void useless_nonterminals (void) { BSet Np, Ns; int i; /* 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. */ Np = XCALLOC (unsigned, WORDSIZE (nvars)); /* 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) { for (i = WORDSIZE (nvars) - 1; i >= 0; i--) Np[i] = N[i]; for (i = 1; i <= nrules; i++) { if (!BITISSET (P, i)) { if (useful_production (i, N)) { SETBIT (Np, rule_table[i].lhs - ntokens); SETBIT (P, i); } } } if (bits_equal (N, Np, WORDSIZE (nvars))) break; Ns = Np; Np = N; N = Ns; } XFREE (N); N = Np; } static void inaccessable_symbols (void) { BSet Vp, Vs, Pp; int i; short t; rule r; /* 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. */ Vp = XCALLOC (unsigned, WORDSIZE (nsyms)); Pp = XCALLOC (unsigned, WORDSIZE (nrules + 1)); /* If the start symbol isn't useful, then nothing will be useful. */ if (BITISSET (N, start_symbol - ntokens)) { SETBIT (V, start_symbol); while (1) { for (i = WORDSIZE (nsyms) - 1; i >= 0; i--) Vp[i] = V[i]; for (i = 1; i <= nrules; i++) { if (!BITISSET (Pp, i) && BITISSET (P, i) && BITISSET (V, rule_table[i].lhs)) { for (r = &ritem[rule_table[i].rhs]; *r >= 0; r++) if (ISTOKEN (t = *r) || BITISSET (N, t - ntokens)) SETBIT (Vp, t); SETBIT (Pp, i); } } if (bits_equal (V, Vp, WORDSIZE (nsyms))) break; Vs = Vp; Vp = V; V = Vs; } } XFREE (V); V = Vp; /* Tokens 0, 1, and 2 are internal to Bison. Consider them useful. */ SETBIT (V, 0); /* end-of-input token */ SETBIT (V, 1); /* error token */ SETBIT (V, 2); /* some undefined token */ XFREE (P); P = Pp; nuseful_productions = bits_size (P, WORDSIZE (nrules + 1)); nuseless_productions = nrules - nuseful_productions; nuseful_nonterminals = 0; for (i = ntokens; i < nsyms; i++) if (BITISSET (V, i)) nuseful_nonterminals++; nuseless_nonterminals = nvars - nuseful_nonterminals; /* A token that was used in %prec should not be warned about. */ for (i = 1; i < nrules; i++) if (rule_table[i].precsym != 0) SETBIT (V1, rule_table[i].precsym); } static void reduce_grammar_tables (void) { /* This is turned off because we would need to change the numbers in the case statements in the actions file. We don't disable it via CPP so that it is still checked with the rest of the code, to avoid its becoming completely obsolete. FIXME: I think the comment above demonstrates this code must be turned off for *semantic* parser, not in the general case. Try to understand this better --akim. */ if (0) /* remove useless productions */ if (nuseless_productions > 0) { short np, pn, ni, pi; np = 0; ni = 0; for (pn = 1; pn <= nrules; pn++) if (BITISSET (P, pn)) { np++; if (pn != np) { rule_table[np].lhs = rule_table[pn].lhs; rule_table[np].line = rule_table[pn].line; rule_table[np].prec = rule_table[pn].prec; rule_table[np].assoc = rule_table[pn].assoc; rule_table[np].rhs = rule_table[pn].rhs; if (rule_table[np].rhs != ni) { pi = rule_table[np].rhs; rule_table[np].rhs = ni; while (ritem[pi] >= 0) ritem[ni++] = ritem[pi++]; ritem[ni++] = -np; } } else { while (ritem[ni++] >= 0); } } ritem[ni] = 0; nrules -= nuseless_productions; nitems = ni; /* Is it worth it to reduce the amount of memory for the grammar? Probably not. */ } /* Disable useless productions. */ if (nuseless_productions > 0) { int pn; for (pn = 1; pn <= nrules; pn++) rule_table[pn].useful = BITISSET (P, pn); } } /*------------------------------. | Remove useless nonterminals. | `------------------------------*/ static void nonterminals_reduce (void) { int i, n; rule r; /* Map the nonterminals to their new index: useful first, useless afterwards. Kept for later report. */ short *nontermmap = XCALLOC (short, nvars) - ntokens; n = ntokens; for (i = ntokens; i < nsyms; i++) if (BITISSET (V, i)) nontermmap[i] = n++; for (i = ntokens; i < nsyms; i++) if (!BITISSET (V, i)) nontermmap[i] = n++; /* Shuffle elements of tables indexed by symbol number. */ { short *sassoc_sorted = XMALLOC (short, nvars) - ntokens; short *sprec_sorted = XMALLOC (short, nvars) - ntokens; char **tags_sorted = XMALLOC (char *, nvars) - ntokens; for (i = ntokens; i < nsyms; i++) { n = nontermmap[i]; sassoc_sorted[n] = sassoc[i]; sprec_sorted[n] = sprec[i]; tags_sorted[n] = tags[i]; } for (i = ntokens; i < nsyms; i++) { sassoc[i] = sassoc_sorted[i]; sprec[i] = sprec_sorted[i]; tags[i] = tags_sorted[i]; } free (sassoc_sorted + ntokens); free (sprec_sorted + ntokens); free (tags_sorted + ntokens); } /* Replace all symbol numbers in valid data structures. */ for (i = 1; i <= nrules; i++) { rule_table[i].lhs = nontermmap[rule_table[i].lhs]; if (ISVAR (rule_table[i].precsym)) /* Can this happen? */ rule_table[i].precsym = nontermmap[rule_table[i].precsym]; } for (r = ritem; *r; r++) if (ISVAR (*r)) *r = nontermmap[*r]; start_symbol = nontermmap[start_symbol]; nsyms -= nuseless_nonterminals; nvars -= nuseless_nonterminals; free (nontermmap + ntokens); } /*------------------------------------------------------------------. | Output the detailed results of the reductions. For FILE.output. | `------------------------------------------------------------------*/ void reduce_output (FILE *out) { if (nuseless_nonterminals > 0) { int i; fprintf (out, "%s\n\n", _("Useless nonterminals:")); for (i = 0; i < nuseless_nonterminals; ++i) fprintf (out, " %s\n", tags[nsyms + i]); fputs ("\n\n", out); } { bool b = FALSE; int i; for (i = 0; i < ntokens; i++) if (!BITISSET (V, i) && !BITISSET (V1, i)) { if (!b) fprintf (out, "%s\n\n", _("Terminals which are not used:")); b = TRUE; fprintf (out, " %s\n", tags[i]); } if (b) fputs ("\n\n", out); } if (nuseless_productions > 0) { int i; fprintf (out, "%s\n\n", _("Useless rules:")); for (i = 1; i <= nrules; i++) if (!rule_table[i].useful) { rule r; fprintf (out, "#%-4d ", i); fprintf (out, "%s:", tags[rule_table[i].lhs]); for (r = &ritem[rule_table[i].rhs]; *r >= 0; r++) fprintf (out, " %s", tags[*r]); fputs (";\n", out); } fputs ("\n\n", out); } } static void dump_grammar (FILE *out) { int i; rule r; fprintf (out, "REDUCED GRAMMAR\n\n"); fprintf (out, "ntokens = %d, nvars = %d, nsyms = %d, nrules = %d, nitems = %d\n\n", ntokens, nvars, nsyms, nrules, nitems); fprintf (out, "Variables\n---------\n\n"); fprintf (out, "Value Sprec Sassoc Tag\n"); for (i = ntokens; i < nsyms; i++) fprintf (out, "%5d %5d %5d %s\n", i, sprec[i], sassoc[i], tags[i]); fprintf (out, "\n\n"); fprintf (out, "Rules\n-----\n\n"); fprintf (out, "Num (Prec, Assoc, Useful, Ritem Range) Lhs -> Rhs (Ritem range) [Num]\n"); for (i = 1; i <= nrules; i++) { int rhs_count = 0; /* Find the last RHS index in ritems. */ for (r = &ritem[rule_table[i].rhs]; *r > 0; ++r) ++rhs_count; fprintf (out, "%3d (%2d, %2d, %2d, %2d-%2d) %2d ->", i, rule_table[i].prec, rule_table[i].assoc, rule_table[i].useful, rule_table[i].rhs, rule_table[i].rhs + rhs_count - 1, rule_table[i].lhs); /* Dumped the RHS. */ for (r = &ritem[rule_table[i].rhs]; *r > 0; r++) fprintf (out, "%3d", *r); fprintf (out, " [%d]\n", -(*r)); } fprintf (out, "\n\n"); fprintf (out, "Rules interpreted\n-----------------\n\n"); for (i = 1; i <= nrules; i++) { fprintf (out, "%-5d %s :", i, tags[rule_table[i].lhs]); for (r = &ritem[rule_table[i].rhs]; *r > 0; r++) fprintf (out, " %s", tags[*r]); fputc ('\n', out); } fprintf (out, "\n\n"); } /*-------------------------------. | Report the results to STDERR. | `-------------------------------*/ static void reduce_print (void) { if (yacc_flag && nuseless_productions) fprintf (stderr, ngettext ("%d rule never reduced\n", "%d rules never reduced\n", nuseless_productions), nuseless_productions); fprintf (stderr, _("%s contains "), infile); if (nuseless_nonterminals > 0) fprintf (stderr, ngettext ("%d useless nonterminal", "%d useless nonterminals", nuseless_nonterminals), nuseless_nonterminals); if (nuseless_nonterminals > 0 && nuseless_productions > 0) fprintf (stderr, _(" and ")); 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); }