src/reduce.c

   1 /* Grammar reduction for Bison.
   2    Copyright (C) 1988, 1989 Free Software Foundation, Inc.
   3
   4 This file is part of Bison, the GNU Compiler Compiler.
   5
   6 Bison is free software; you can redistribute it and/or modify
   7 it under the terms of the GNU General Public License as published by
   8 the Free Software Foundation; either version 2, or (at your option)
   9 any later version.
  10
  11 Bison is distributed in the hope that it will be useful,
  12 but WITHOUT ANY WARRANTY; without even the implied warranty of
  13 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  14 GNU General Public License for more details.
  15
  16 You should have received a copy of the GNU General Public License
  17 along with Bison; see the file COPYING.  If not, write to
  18 the Free Software Foundation, Inc., 59 Temple Place - Suite 330,
  19 Boston, MA 02111-1307, USA.  */
  20
  21
  22 /*
  23  * Reduce the grammar:  Find and eliminate unreachable terminals,
  24  * nonterminals, and productions.  David S. Bakin.
  25  */
  26
  27 /*
  28  * Don't eliminate unreachable terminals:  They may be used by the user's
  29  * parser.
  30  */
  31
  32 #include <stdio.h>
  33 #include "system.h"
  34 #include "files.h"
  35 #include "gram.h"
  36 #include "machine.h"
  37 #include "alloc.h"
  38 #include "complain.h"
  39
  40
  41 extern char   **tags;           /* reader.c */
  42 extern int      verboseflag;    /* getargs.c */
  43 static int      statisticsflag; /* XXXXXXX */
  44 extern int      fixed_outfiles;
  45
  46 #ifndef TRUE
  47 #define TRUE    (1)
  48 #define FALSE   (0)
  49 #endif
  50 typedef int bool;
  51 typedef unsigned *BSet;
  52 typedef short  *rule;
  53
  54
  55 /*
  56  * N is set of all nonterminals which are not useless.  P is set of all rules
  57  * which have no useless nonterminals in their RHS.  V is the set of all
  58  * accessible symbols.
  59  */
  60
  61 static BSet     N, P, V, V1;
  62
  63 static int      nuseful_productions, nuseless_productions,
  64                 nuseful_nonterminals, nuseless_nonterminals;
  65
  66
  67 bool bits_equal PARAMS((BSet, BSet, int));
  68 int nbits PARAMS((unsigned));
  69 int bits_size PARAMS((BSet, int));
  70 void reduce_grammar PARAMS((void));
  71 static void useless_nonterminals PARAMS((void));
  72 static void inaccessable_symbols PARAMS((void));
  73 static void reduce_grammar_tables PARAMS((void));
  74 static void print_results PARAMS((void));
  75 static void print_notices PARAMS((void));
  76 void dump_grammar PARAMS((void));
  77
  78 \f
  79
  80 bool
  81 bits_equal (BSet L, BSet R, int n)
  82 {
  83   int i;
  84
  85   for (i = n - 1; i >= 0; i--)
  86     if (L[i] != R[i])
  87       return FALSE;
  88   return TRUE;
  89 }
  90
  91
  92 int
  93 nbits (unsigned i)
  94 {
  95   int count = 0;
  96
  97   while (i != 0) {
  98     i ^= (i & ((unsigned) (- (int) i)));
  99     ++count;
 100   }
 101   return count;
 102 }
 103
 104
 105 int
 106 bits_size (BSet S, int n)
 107 {
 108   int i, count = 0;
 109
 110   for (i = n - 1; i >= 0; i--)
 111     count += nbits(S[i]);
 112   return count;
 113 }
 114 \f
 115 void
 116 reduce_grammar (void)
 117 {
 118   bool reduced;
 119
 120   /* Allocate the global sets used to compute the reduced grammar */
 121
 122   N = NEW2(WORDSIZE(nvars), unsigned);
 123   P = NEW2(WORDSIZE(nrules + 1), unsigned);
 124   V = NEW2(WORDSIZE(nsyms), unsigned);
 125   V1 = NEW2(WORDSIZE(nsyms), unsigned);
 126
 127   useless_nonterminals();
 128   inaccessable_symbols();
 129
 130   reduced = (bool) (nuseless_nonterminals + nuseless_productions > 0);
 131
 132   if (verboseflag)
 133     print_results();
 134
 135   if (reduced == FALSE)
 136     goto done_reducing;
 137
 138   print_notices();
 139
 140   if (!BITISSET(N, start_symbol - ntokens))
 141     fatal (_("Start symbol %s does not derive any sentence"),
 142            tags[start_symbol]);
 143
 144   reduce_grammar_tables();
 145   /*
 146   if (verboseflag)
 147     {
 148       fprintf(foutput, "REDUCED GRAMMAR\n\n");
 149       dump_grammar();
 150     }
 151   */
 152   /**/ statisticsflag = FALSE; /* someday getopts should handle this */
 153   if (statisticsflag == TRUE)
 154     fprintf(stderr,
 155             _("reduced %s defines %d terminal%s, %d nonterminal%s\
 156 , and %d production%s.\n"), infile,
 157             ntokens, (ntokens == 1 ? "" : "s"),
 158             nvars,   (nvars   == 1 ? "" : "s"),
 159             nrules,  (nrules  == 1 ? "" : "s"));
 160
 161  done_reducing:
 162
 163   /* Free the global sets used to compute the reduced grammar */
 164
 165   FREE(N);
 166   FREE(V);
 167   FREE(P);
 168
 169 }
 170 \f
 171 /*
 172  * Another way to do this would be with a set for each production and then do
 173  * subset tests against N0, but even for the C grammar the whole reducing
 174  * process takes only 2 seconds on my 8Mhz AT.
 175  */
 176
 177 static bool
 178 useful_production (int i, BSet N0)
 179 {
 180   rule  r;
 181   short n;
 182
 183   /*
 184    * A production is useful if all of the nonterminals in its RHS
 185    * appear in the set of useful nonterminals.
 186    */
 187
 188   for (r = &ritem[rrhs[i]]; *r > 0; r++)
 189     if (ISVAR(n = *r))
 190       if (!BITISSET(N0, n - ntokens))
 191         return FALSE;
 192   return TRUE;
 193 }
 194
 195
 196 /* Remember that rules are 1-origin, symbols are 0-origin. */
 197
 198 static void
 199 useless_nonterminals (void)
 200 {
 201   BSet Np, Ns;
 202   int  i, n;
 203
 204   /*
 205    * N is set as built.  Np is set being built this iteration. P is set
 206    * of all productions which have a RHS all in N.
 207    */
 208
 209   Np = NEW2(WORDSIZE(nvars), unsigned);
 210
 211   /*
 212    * The set being computed is a set of nonterminals which can derive
 213    * the empty string or strings consisting of all terminals. At each
 214    * iteration a nonterminal is added to the set if there is a
 215    * production with that nonterminal as its LHS for which all the
 216    * nonterminals in its RHS are already in the set.  Iterate until the
 217    * set being computed remains unchanged.  Any nonterminals not in the
 218    * set at that point are useless in that they will never be used in
 219    * deriving a sentence of the language.
 220    *
 221    * This iteration doesn't use any special traversal over the
 222    * productions.  A set is kept of all productions for which all the
 223    * nonterminals in the RHS are in useful.  Only productions not in
 224    * this set are scanned on each iteration.  At the end, this set is
 225    * saved to be used when finding useful productions: only productions
 226    * in this set will appear in the final grammar.
 227    */
 228
 229   n = 0;
 230   while (1)
 231     {
 232       for (i = WORDSIZE(nvars) - 1; i >= 0; i--)
 233         Np[i] = N[i];
 234       for (i = 1; i <= nrules; i++)
 235         {
 236           if (!BITISSET(P, i))
 237             {
 238               if (useful_production(i, N))
 239                 {
 240                   SETBIT(Np, rlhs[i] - ntokens);
 241                   SETBIT(P, i);
 242                 }
 243             }
 244         }
 245       if (bits_equal(N, Np, WORDSIZE(nvars)))
 246         break;
 247       Ns = Np;
 248       Np = N;
 249       N = Ns;
 250     }
 251   FREE(N);
 252   N = Np;
 253 }
 254 \f
 255 static void
 256 inaccessable_symbols (void)
 257 {
 258   BSet  Vp, Vs, Pp;
 259   int   i, n;
 260   short t;
 261   rule  r;
 262
 263   /*
 264    * Find out which productions are reachable and which symbols are
 265    * used.  Starting with an empty set of productions and a set of
 266    * symbols which only has the start symbol in it, iterate over all
 267    * productions until the set of productions remains unchanged for an
 268    * iteration.  For each production which has a LHS in the set of
 269    * reachable symbols, add the production to the set of reachable
 270    * productions, and add all of the nonterminals in the RHS of the
 271    * production to the set of reachable symbols.
 272    *
 273    * Consider only the (partially) reduced grammar which has only
 274    * nonterminals in N and productions in P.
 275    *
 276    * The result is the set P of productions in the reduced grammar, and
 277    * the set V of symbols in the reduced grammar.
 278    *
 279    * Although this algorithm also computes the set of terminals which are
 280    * reachable, no terminal will be deleted from the grammar. Some
 281    * terminals might not be in the grammar but might be generated by
 282    * semantic routines, and so the user might want them available with
 283    * specified numbers.  (Is this true?)  However, the nonreachable
 284    * terminals are printed (if running in verbose mode) so that the user
 285    * can know.
 286    */
 287
 288   Vp = NEW2(WORDSIZE(nsyms), unsigned);
 289   Pp = NEW2(WORDSIZE(nrules + 1), unsigned);
 290
 291   /* If the start symbol isn't useful, then nothing will be useful. */
 292   if (!BITISSET(N, start_symbol - ntokens))
 293     goto end_iteration;
 294
 295   SETBIT(V, start_symbol);
 296
 297   n = 0;
 298   while (1)
 299     {
 300       for (i = WORDSIZE(nsyms) - 1; i >= 0; i--)
 301         Vp[i] = V[i];
 302       for (i = 1; i <= nrules; i++)
 303         {
 304           if (!BITISSET(Pp, i) && BITISSET(P, i) &&
 305               BITISSET(V, rlhs[i]))
 306             {
 307               for (r = &ritem[rrhs[i]]; *r >= 0; r++)
 308                 {
 309                   if (ISTOKEN(t = *r)
 310                       || BITISSET(N, t - ntokens))
 311                     {
 312                       SETBIT(Vp, t);
 313                     }
 314                 }
 315               SETBIT(Pp, i);
 316             }
 317         }
 318       if (bits_equal(V, Vp, WORDSIZE(nsyms)))
 319         {
 320           break;
 321         }
 322       Vs = Vp;
 323       Vp = V;
 324       V = Vs;
 325     }
 326  end_iteration:
 327
 328   FREE(V);
 329   V = Vp;
 330
 331   /* Tokens 0, 1, and 2 are internal to Bison.  Consider them useful. */
 332   SETBIT(V, 0);                 /* end-of-input token */
 333   SETBIT(V, 1);                 /* error token */
 334   SETBIT(V, 2);                 /* some undefined token */
 335
 336   FREE(P);
 337   P = Pp;
 338
 339   nuseful_productions = bits_size(P, WORDSIZE(nrules + 1));
 340   nuseless_productions = nrules - nuseful_productions;
 341
 342   nuseful_nonterminals = 0;
 343   for (i = ntokens; i < nsyms; i++)
 344     if (BITISSET(V, i))
 345       nuseful_nonterminals++;
 346   nuseless_nonterminals = nvars - nuseful_nonterminals;
 347
 348   /* A token that was used in %prec should not be warned about.  */
 349   for (i = 1; i < nrules; i++)
 350     if (rprecsym[i] != 0)
 351       SETBIT(V1, rprecsym[i]);
 352 }
 353 \f
 354 static void
 355 reduce_grammar_tables (void)
 356 {
 357 /* This is turned off because we would need to change the numbers
 358    in the case statements in the actions file.  */
 359 #if 0
 360   /* remove useless productions */
 361   if (nuseless_productions > 0)
 362     {
 363       short np, pn, ni, pi;
 364
 365       np = 0;
 366       ni = 0;
 367       for (pn = 1; pn <= nrules; pn++)
 368         {
 369           if (BITISSET(P, pn))
 370             {
 371               np++;
 372               if (pn != np)
 373                 {
 374                   rlhs[np] = rlhs[pn];
 375                   rline[np] = rline[pn];
 376                   rprec[np] = rprec[pn];
 377                   rassoc[np] = rassoc[pn];
 378                   rrhs[np] = rrhs[pn];
 379                   if (rrhs[np] != ni)
 380                     {
 381                       pi = rrhs[np];
 382                       rrhs[np] = ni;
 383                       while (ritem[pi] >= 0)
 384                         ritem[ni++] = ritem[pi++];
 385                       ritem[ni++] = -np;
 386                     }
 387                 } else {
 388                   while (ritem[ni++] >= 0);
 389                 }
 390             }
 391         }
 392       ritem[ni] = 0;
 393       nrules -= nuseless_productions;
 394       nitems = ni;
 395
 396       /*
 397        * Is it worth it to reduce the amount of memory for the
 398        * grammar? Probably not.
 399        */
 400
 401     }
 402 #endif /* 0 */
 403   /* Disable useless productions,
 404      since they may contain useless nonterms
 405      that would get mapped below to -1 and confuse everyone.  */
 406   if (nuseless_productions > 0)
 407     {
 408       int pn;
 409
 410       for (pn = 1; pn <= nrules; pn++)
 411         {
 412           if (!BITISSET(P, pn))
 413             {
 414               rlhs[pn] = -1;
 415             }
 416         }
 417     }
 418
 419   /* remove useless symbols */
 420   if (nuseless_nonterminals > 0)
 421     {
 422
 423       int    i, n;
 424 /*      short  j; JF unused */
 425       short *nontermmap;
 426       rule   r;
 427
 428       /*
 429        * create a map of nonterminal number to new nonterminal
 430        * number. -1 in the map means it was useless and is being
 431        * eliminated.
 432        */
 433
 434       nontermmap = NEW2(nvars, short) - ntokens;
 435       for (i = ntokens; i < nsyms; i++)
 436         nontermmap[i] = -1;
 437
 438       n = ntokens;
 439       for (i = ntokens; i < nsyms; i++)
 440         if (BITISSET(V, i))
 441           nontermmap[i] = n++;
 442
 443       /* Shuffle elements of tables indexed by symbol number.  */
 444
 445       for (i = ntokens; i < nsyms; i++)
 446         {
 447           n = nontermmap[i];
 448           if (n >= 0)
 449             {
 450               sassoc[n] = sassoc[i];
 451               sprec[n] = sprec[i];
 452               tags[n] = tags[i];
 453             } else {
 454               free(tags[i]);
 455             }
 456         }
 457
 458       /* Replace all symbol numbers in valid data structures.  */
 459
 460       for (i = 1; i <= nrules; i++)
 461         {
 462           /* Ignore the rules disabled above.  */
 463           if (rlhs[i] >= 0)
 464             rlhs[i] = nontermmap[rlhs[i]];
 465           if (ISVAR (rprecsym[i]))
 466             /* Can this happen?  */
 467             rprecsym[i] = nontermmap[rprecsym[i]];
 468         }
 469
 470       for (r = ritem; *r; r++)
 471         if (ISVAR(*r))
 472           *r = nontermmap[*r];
 473
 474       start_symbol = nontermmap[start_symbol];
 475
 476       nsyms -= nuseless_nonterminals;
 477       nvars -= nuseless_nonterminals;
 478
 479       free(&nontermmap[ntokens]);
 480     }
 481 }
 482 \f
 483 static void
 484 print_results (void)
 485 {
 486   int   i;
 487 /*  short j; JF unused */
 488   rule  r;
 489   bool  b;
 490
 491   if (nuseless_nonterminals > 0)
 492     {
 493       fprintf(foutput, _("Useless nonterminals:\n\n"));
 494       for (i = ntokens; i < nsyms; i++)
 495         if (!BITISSET(V, i))
 496           fprintf(foutput, "   %s\n", tags[i]);
 497     }
 498   b = FALSE;
 499   for (i = 0; i < ntokens; i++)
 500     {
 501       if (!BITISSET(V, i) && !BITISSET(V1, i))
 502         {
 503           if (!b)
 504             {
 505               fprintf(foutput, _("\n\nTerminals which are not used:\n\n"));
 506               b = TRUE;
 507             }
 508           fprintf(foutput, "   %s\n", tags[i]);
 509         }
 510     }
 511
 512   if (nuseless_productions > 0)
 513     {
 514       fprintf(foutput, _("\n\nUseless rules:\n\n"));
 515       for (i = 1; i <= nrules; i++)
 516         {
 517           if (!BITISSET(P, i))
 518             {
 519               fprintf(foutput, "#%-4d  ", i);
 520               fprintf(foutput, "%s :\t", tags[rlhs[i]]);
 521               for (r = &ritem[rrhs[i]]; *r >= 0; r++)
 522                 {
 523                   fprintf(foutput, " %s", tags[*r]);
 524                 }
 525               fprintf(foutput, ";\n");
 526             }
 527         }
 528     }
 529   if (nuseless_nonterminals > 0 || nuseless_productions > 0 || b)
 530     fprintf(foutput, "\n\n");
 531 }
 532 \f
 533 void
 534 dump_grammar (void)
 535 {
 536   int i;
 537   rule r;
 538
 539   fprintf(foutput,
 540           "ntokens = %d, nvars = %d, nsyms = %d, nrules = %d, nitems = %d\n\n",
 541           ntokens, nvars, nsyms, nrules, nitems);
 542   fprintf(foutput, _("Variables\n---------\n\n"));
 543   fprintf(foutput, _("Value  Sprec    Sassoc    Tag\n"));
 544   for (i = ntokens; i < nsyms; i++)
 545     fprintf(foutput, "%5d  %5d  %5d  %s\n",
 546             i, sprec[i], sassoc[i], tags[i]);
 547   fprintf(foutput, "\n\n");
 548   fprintf(foutput, _("Rules\n-----\n\n"));
 549   for (i = 1; i <= nrules; i++)
 550     {
 551       fprintf(foutput, "%-5d(%5d%5d)%5d : (@%-5d)",
 552               i, rprec[i], rassoc[i], rlhs[i], rrhs[i]);
 553       for (r = &ritem[rrhs[i]]; *r > 0; r++)
 554         fprintf(foutput, "%5d", *r);
 555       fprintf(foutput, " [%d]\n", -(*r));
 556     }
 557   fprintf(foutput, "\n\n");
 558   fprintf(foutput, _("Rules interpreted\n-----------------\n\n"));
 559   for (i = 1; i <= nrules; i++)
 560     {
 561       fprintf(foutput, "%-5d  %s :", i, tags[rlhs[i]]);
 562       for (r = &ritem[rrhs[i]]; *r > 0; r++)
 563         fprintf(foutput, " %s", tags[*r]);
 564       fprintf(foutput, "\n");
 565     }
 566   fprintf(foutput, "\n\n");
 567 }
 568
 569
 570 static void
 571 print_notices (void)
 572 {
 573   if (fixed_outfiles && nuseless_productions)
 574     fprintf(stderr, _("%d rules never reduced\n"), nuseless_productions);
 575
 576   fprintf(stderr, _("%s contains "), infile);
 577
 578   if (nuseless_nonterminals > 0)
 579     {
 580       fprintf(stderr, _("%d useless nonterminal%s"),
 581               nuseless_nonterminals,
 582               (nuseless_nonterminals == 1 ? "" : "s"));
 583     }
 584   if (nuseless_nonterminals > 0 && nuseless_productions > 0)
 585     fprintf(stderr, _(" and "));
 586
 587   if (nuseless_productions > 0)
 588     {
 589       fprintf(stderr, _("%d useless rule%s"),
 590               nuseless_productions,
 591               (nuseless_productions == 1 ? "" : "s"));
 592     }
 593   fprintf(stderr, "\n");
 594   fflush(stderr);
 595 }