www.pudn.com > bison.zip > reduce.c
/* Grammar reduction for Bison. Copyright (C) 1988, 1989 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, 675 Mass Ave, Cambridge, MA 02139, 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#include "system.h" #include "files.h" #include "gram.h" #include "machine.h" #include "new.h" extern char **tags; /* reader.c */ extern int verboseflag; /* getargs.c */ static int statisticsflag; /* XXXXXXX */ #ifndef TRUE #define TRUE (1) #define FALSE (0) #endif typedef int bool; typedef unsigned *BSet; typedef short *rule; /* * N is set of all nonterminals which are not useless. P is set of all rules * which have no useless nonterminals in their RHS. V is the set of all * accessible symbols. */ static BSet N, P, V, V1; static int nuseful_productions, nuseless_productions, nuseful_nonterminals, nuseless_nonterminals; static void useless_nonterminals(); static void inaccessable_symbols(); static void reduce_grammar_tables(); static void print_results(); static void print_notices(); void dump_grammar(); extern void fatals (); bool bits_equal (L, R, n) BSet L; BSet R; int n; { int i; for (i = n - 1; i >= 0; i--) if (L[i] != R[i]) return FALSE; return TRUE; } int nbits (i) unsigned i; { int count = 0; while (i != 0) { i ^= (i & -i); ++count; } return count; } int bits_size (S, n) BSet S; int n; { int i, count = 0; for (i = n - 1; i >= 0; i--) count += nbits(S[i]); return count; } void reduce_grammar () { bool reduced; /* Allocate the global sets used to compute the reduced grammar */ N = NEW2(WORDSIZE(nvars), unsigned); P = NEW2(WORDSIZE(nrules + 1), unsigned); V = NEW2(WORDSIZE(nsyms), unsigned); V1 = NEW2(WORDSIZE(nsyms), unsigned); useless_nonterminals(); inaccessable_symbols(); reduced = (bool) (nuseless_nonterminals + nuseless_productions > 0); if (verboseflag) print_results(); if (reduced == FALSE) goto done_reducing; print_notices(); if (!BITISSET(N, start_symbol - ntokens)) fatals("Start symbol %s does not derive any sentence.", tags[start_symbol]); reduce_grammar_tables(); /* if (verboseflag) { fprintf(foutput, "REDUCED GRAMMAR\n\n"); dump_grammar(); } */ /**/ statisticsflag = FALSE; /* someday getopts should handle this */ if (statisticsflag == TRUE) fprintf(stderr, "reduced %s defines %d terminal%s, %d nonterminal%s\ , and %d production%s.\n", infile, ntokens, (ntokens == 1 ? "" : "s"), nvars, (nvars == 1 ? "" : "s"), nrules, (nrules == 1 ? "" : "s")); done_reducing: /* Free the global sets used to compute the reduced grammar */ FREE(N); FREE(V); FREE(P); } /* * Another way to do this would be with a set for each production and then do * subset tests against N, but even for the C grammar the whole reducing * process takes only 2 seconds on my 8Mhz AT. */ static bool useful_production (i, N) int i; BSet N; { rule r; short n; /* * A production is useful if all of the nonterminals in its RHS * appear in the set of useful nonterminals. */ for (r = &ritem[rrhs[i]]; *r > 0; r++) if (ISVAR(n = *r)) if (!BITISSET(N, n - ntokens)) return FALSE; return TRUE; } /* Remember that rules are 1-origin, symbols are 0-origin. */ static void useless_nonterminals () { BSet Np, Ns; int i, n; /* * 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 = NEW2(WORDSIZE(nvars), unsigned); /* * 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. */ n = 0; 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, rlhs[i] - ntokens); SETBIT(P, i); } } } if (bits_equal(N, Np, WORDSIZE(nvars))) break; Ns = Np; Np = N; N = Ns; } FREE(N); N = Np; } static void inaccessable_symbols () { BSet Vp, Vs, Pp; int i, n; 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 = NEW2(WORDSIZE(nsyms), unsigned); Pp = NEW2(WORDSIZE(nrules + 1), unsigned); /* If the start symbol isn't useful, then nothing will be useful. */ if (!BITISSET(N, start_symbol - ntokens)) goto end_iteration; SETBIT(V, start_symbol); n = 0; 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, rlhs[i])) { for (r = &ritem[rrhs[i]]; *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; } end_iteration: FREE(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 */ FREE(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 (rprecsym[i] != 0) SETBIT(V1, rprecsym[i]); } static void reduce_grammar_tables () { /* This is turned off because we would need to change the numbers in the case statements in the actions file. */ #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) { rlhs[np] = rlhs[pn]; rline[np] = rline[pn]; rprec[np] = rprec[pn]; rassoc[np] = rassoc[pn]; rrhs[np] = rrhs[pn]; if (rrhs[np] != ni) { pi = rrhs[np]; rrhs[np] = 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. */ } #endif /* 0 */ /* Disable useless productions, since they may contain useless nonterms that would get mapped below to -1 and confuse everyone. */ if (nuseless_productions > 0) { int pn; for (pn = 1; pn <= nrules; pn++) { if (!BITISSET(P, pn)) { rlhs[pn] = -1; } } } /* remove useless symbols */ if (nuseless_nonterminals > 0) { int i, n; /* short j; JF unused */ short *nontermmap; rule r; /* * create a map of nonterminal number to new nonterminal * number. -1 in the map means it was useless and is being * eliminated. */ nontermmap = NEW2(nvars, short) - ntokens; for (i = ntokens; i < nsyms; i++) nontermmap[i] = -1; n = ntokens; for (i = ntokens; i < nsyms; i++) if (BITISSET(V, i)) nontermmap[i] = n++; /* Shuffle elements of tables indexed by symbol number. */ for (i = ntokens; i < nsyms; i++) { n = nontermmap[i]; if (n >= 0) { sassoc[n] = sassoc[i]; sprec[n] = sprec[i]; tags[n] = tags[i]; } else { free(tags[i]); } } /* Replace all symbol numbers in valid data structures. */ for (i = 1; i <= nrules; i++) { /* Ignore the rules disabled above. */ if (rlhs[i] >= 0) rlhs[i] = nontermmap[rlhs[i]]; if (ISVAR (rprecsym[i])) /* Can this happen? */ rprecsym[i] = nontermmap[rprecsym[i]]; } 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]); } } static void print_results () { int i; /* short j; JF unused */ rule r; bool b; if (nuseless_nonterminals > 0) { fprintf(foutput, "Useless nonterminals:\n\n"); for (i = ntokens; i < nsyms; i++) if (!BITISSET(V, i)) fprintf(foutput, " %s\n", tags[i]); } b = FALSE; for (i = 0; i < ntokens; i++) { if (!BITISSET(V, i) && !BITISSET(V1, i)) { if (!b) { fprintf(foutput, "\n\nTerminals which are not used:\n\n"); b = TRUE; } fprintf(foutput, " %s\n", tags[i]); } } if (nuseless_productions > 0) { fprintf(foutput, "\n\nUseless rules:\n\n"); for (i = 1; i <= nrules; i++) { if (!BITISSET(P, i)) { fprintf(foutput, "#%-4d ", i); fprintf(foutput, "%s :\t", tags[rlhs[i]]); for (r = &ritem[rrhs[i]]; *r >= 0; r++) { fprintf(foutput, " %s", tags[*r]); } fprintf(foutput, ";\n"); } } } if (nuseless_nonterminals > 0 || nuseless_productions > 0 || b) fprintf(foutput, "\n\n"); } void dump_grammar () { int i; rule r; fprintf(foutput, "ntokens = %d, nvars = %d, nsyms = %d, nrules = %d, nitems = %d\n\n", ntokens, nvars, nsyms, nrules, nitems); fprintf(foutput, "Variables\n---------\n\n"); fprintf(foutput, "Value Sprec Sassoc Tag\n"); for (i = ntokens; i < nsyms; i++) fprintf(foutput, "%5d %5d %5d %s\n", i, sprec[i], sassoc[i], tags[i]); fprintf(foutput, "\n\n"); fprintf(foutput, "Rules\n-----\n\n"); for (i = 1; i <= nrules; i++) { fprintf(foutput, "%-5d(%5d%5d)%5d : (@%-5d)", i, rprec[i], rassoc[i], rlhs[i], rrhs[i]); for (r = &ritem[rrhs[i]]; *r > 0; r++) fprintf(foutput, "%5d", *r); fprintf(foutput, " [%d]\n", -(*r)); } 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 () { extern int fixed_outfiles; 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); }