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, 675 Mass Ave, Cambridge, MA 02139, USA.  */
  19
  20
  21 /*
  22  * Reduce the grammar:  Find and eliminate unreachable terminals,
  23  * nonterminals, and productions.  David S. Bakin.
  24  */
  25
  26 /*
  27  * Don't eliminate unreachable terminals:  They may be used by the user's
  28  * parser.
  29  */
  30
  31 #include <stdio.h>
  32 #include "system.h"
  33 #include "files.h"
  34 #include "gram.h"
  35 #include "machine.h"
  36 #include "new.h"
  37
  38
  39 extern char   **tags;           /* reader.c */
  40 extern int      verboseflag;    /* getargs.c */
  41 static int      statisticsflag; /* XXXXXXX */
  42
  43 #ifndef TRUE
  44 #define TRUE    (1)
  45 #define FALSE   (0)
  46 #endif
  47 typedef int bool;
  48 typedef unsigned *BSet;
  49 typedef short  *rule;
  50
  51
  52 /*
  53  * N is set of all nonterminals which are not useless.  P is set of all rules
  54  * which have no useless nonterminals in their RHS.  V is the set of all
  55  * accessible symbols.
  56  */
  57
  58 static BSet     N, P, V, V1;
  59
  60 static int      nuseful_productions, nuseless_productions,
  61                 nuseful_nonterminals, nuseless_nonterminals;
  62
  63
  64 static void useless_nonterminals();
  65 static void inaccessable_symbols();
  66 static void reduce_grammar_tables();
  67 static void print_results();
  68 static void print_notices();
  69 void dump_grammar();
  70
  71 extern void fatals ();
  72 \f
  73
  74 bool
  75 bits_equal (L, R, n)
  76 BSet L;
  77 BSet R;
  78 int n;
  79 {
  80   int i;
  81
  82   for (i = n - 1; i >= 0; i--)
  83     if (L[i] != R[i])
  84       return FALSE;
  85   return TRUE;
  86 }
  87
  88
  89 int
  90 nbits (i)
  91 unsigned i;
  92 {
  93   int count = 0;
  94
  95   while (i != 0) {
  96     i ^= (i & -i);
  97     ++count;
  98   }
  99   return count;
 100 }
 101
 102
 103 int
 104 bits_size (S, n)
 105 BSet S;
 106 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 ()
 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     fatals("Start symbol %s does not derive any sentence.",
 142            tags[start_symbol]);
 143
 144   reduce_grammar_tables();
 145   /* if (verboseflag) {
 146      fprintf(foutput, "REDUCED GRAMMAR\n\n");
 147      dump_grammar();
 148      }
 149      */
 150
 151   /**/ statisticsflag = FALSE; /* someday getopts should handle this */
 152   if (statisticsflag == TRUE)
 153     fprintf(stderr,
 154             "reduced %s defines %d terminal%s, %d nonterminal%s\
 155 , and %d production%s.\n", infile,
 156             ntokens, (ntokens == 1 ? "" : "s"),
 157             nvars,   (nvars   == 1 ? "" : "s"),
 158             nrules,  (nrules  == 1 ? "" : "s"));
 159
 160  done_reducing:
 161
 162   /* Free the global sets used to compute the reduced grammar */
 163
 164   FREE(N);
 165   FREE(V);
 166   FREE(P);
 167
 168 }
 169 \f
 170 /*
 171  * Another way to do this would be with a set for each production and then do
 172  * subset tests against N, but even for the C grammar the whole reducing
 173  * process takes only 2 seconds on my 8Mhz AT.
 174  */
 175
 176 static bool
 177 useful_production (i, N)
 178 int  i;
 179 BSet N;
 180 {
 181   rule  r;
 182   short n;
 183
 184   /*
 185    * A production is useful if all of the nonterminals in its RHS
 186    * appear in the set of useful nonterminals.
 187    */
 188
 189   for (r = &ritem[rrhs[i]]; *r > 0; r++)
 190     if (ISVAR(n = *r))
 191       if (!BITISSET(N, n - ntokens))
 192         return FALSE;
 193   return TRUE;
 194 }
 195
 196
 197 /* Remember that rules are 1-origin, symbols are 0-origin. */
 198
 199 static void
 200 useless_nonterminals ()
 201 {
 202   BSet Np, Ns;
 203   int  i, n;
 204
 205   /*
 206    * N is set as built.  Np is set being built this iteration. P is set
 207    * of all productions which have a RHS all in N.
 208    */
 209
 210   Np = NEW2(WORDSIZE(nvars), unsigned);
 211
 212   /*
 213    * The set being computed is a set of nonterminals which can derive
 214    * the empty string or strings consisting of all terminals. At each
 215    * iteration a nonterminal is added to the set if there is a
 216    * production with that nonterminal as its LHS for which all the
 217    * nonterminals in its RHS are already in the set.  Iterate until the
 218    * set being computed remains unchanged.  Any nonterminals not in the
 219    * set at that point are useless in that they will never be used in
 220    * deriving a sentence of the language.
 221    *
 222    * This iteration doesn't use any special traversal over the
 223    * productions.  A set is kept of all productions for which all the
 224    * nonterminals in the RHS are in useful.  Only productions not in
 225    * this set are scanned on each iteration.  At the end, this set is
 226    * saved to be used when finding useful productions: only productions
 227    * in this set will appear in the final grammar.
 228    */
 229
 230   n = 0;
 231   while (1)
 232     {
 233       for (i = WORDSIZE(nvars) - 1; i >= 0; i--)
 234         Np[i] = N[i];
 235       for (i = 1; i <= nrules; i++)
 236         {
 237           if (!BITISSET(P, i))
 238             {
 239               if (useful_production(i, N))
 240                 {
 241                   SETBIT(Np, rlhs[i] - ntokens);
 242                   SETBIT(P, i);
 243                 }
 244             }
 245         }
 246       if (bits_equal(N, Np, WORDSIZE(nvars)))
 247         break;
 248       Ns = Np;
 249       Np = N;
 250       N = Ns;
 251     }
 252   FREE(N);
 253   N = Np;
 254 }
 255 \f
 256 static void
 257 inaccessable_symbols ()
 258 {
 259   BSet  Vp, Vs, Pp;
 260   int   i, n;
 261   short t;
 262   rule  r;
 263
 264   /*
 265    * Find out which productions are reachable and which symbols are
 266    * used.  Starting with an empty set of productions and a set of
 267    * symbols which only has the start symbol in it, iterate over all
 268    * productions until the set of productions remains unchanged for an
 269    * iteration.  For each production which has a LHS in the set of
 270    * reachable symbols, add the production to the set of reachable
 271    * productions, and add all of the nonterminals in the RHS of the
 272    * production to the set of reachable symbols.
 273    *
 274    * Consider only the (partially) reduced grammar which has only
 275    * nonterminals in N and productions in P.
 276    *
 277    * The result is the set P of productions in the reduced grammar, and
 278    * the set V of symbols in the reduced grammar.
 279    *
 280    * Although this algorithm also computes the set of terminals which are
 281    * reachable, no terminal will be deleted from the grammar. Some
 282    * terminals might not be in the grammar but might be generated by
 283    * semantic routines, and so the user might want them available with
 284    * specified numbers.  (Is this true?)  However, the nonreachable
 285    * terminals are printed (if running in verbose mode) so that the user
 286    * can know.
 287    */
 288
 289   Vp = NEW2(WORDSIZE(nsyms), unsigned);
 290   Pp = NEW2(WORDSIZE(nrules + 1), unsigned);
 291
 292   /* If the start symbol isn't useful, then nothing will be useful. */
 293   if (!BITISSET(N, start_symbol - ntokens))
 294     goto end_iteration;
 295
 296   SETBIT(V, start_symbol);
 297
 298   n = 0;
 299   while (1)
 300     {
 301       for (i = WORDSIZE(nsyms) - 1; i >= 0; i--)
 302         Vp[i] = V[i];
 303       for (i = 1; i <= nrules; i++)
 304         {
 305           if (!BITISSET(Pp, i) && BITISSET(P, i) &&
 306               BITISSET(V, rlhs[i]))
 307             {
 308               for (r = &ritem[rrhs[i]]; *r >= 0; r++)
 309                 {
 310                   if (ISTOKEN(t = *r)
 311                       || BITISSET(N, t - ntokens))
 312                     {
 313                       SETBIT(Vp, t);
 314                     }
 315                 }
 316               SETBIT(Pp, i);
 317             }
 318         }
 319       if (bits_equal(V, Vp, WORDSIZE(nsyms)))
 320         {
 321           break;
 322         }
 323       Vs = Vp;
 324       Vp = V;
 325       V = Vs;
 326     }
 327  end_iteration:
 328
 329   FREE(V);
 330   V = Vp;
 331
 332   /* Tokens 0, 1, and 2 are internal to Bison.  Consider them useful. */
 333   SETBIT(V, 0);                 /* end-of-input token */
 334   SETBIT(V, 1);                 /* error token */
 335   SETBIT(V, 2);                 /* some undefined token */
 336
 337   FREE(P);
 338   P = Pp;
 339
 340   nuseful_productions = bits_size(P, WORDSIZE(nrules + 1));
 341   nuseless_productions = nrules - nuseful_productions;
 342
 343   nuseful_nonterminals = 0;
 344   for (i = ntokens; i < nsyms; i++)
 345     if (BITISSET(V, i))
 346       nuseful_nonterminals++;
 347   nuseless_nonterminals = nvars - nuseful_nonterminals;
 348
 349   /* A token that was used in %prec should not be warned about.  */
 350   for (i = 1; i < nrules; i++)
 351     if (rprecsym[i] != 0)
 352       SETBIT(V1, rprecsym[i]);
 353 }
 354 \f
 355 static void
 356 reduce_grammar_tables ()
 357 {
 358 /* This is turned off because we would need to change the numbers
 359    in the case statements in the actions file.  */
 360 #if 0
 361   /* remove useless productions */
 362   if (nuseless_productions > 0)
 363     {
 364       short np, pn, ni, pi;
 365
 366       np = 0;
 367       ni = 0;
 368       for (pn = 1; pn <= nrules; pn++)
 369         {
 370           if (BITISSET(P, pn))
 371             {
 372               np++;
 373               if (pn != np)
 374                 {
 375                   rlhs[np] = rlhs[pn];
 376                   rline[np] = rline[pn];
 377                   rprec[np] = rprec[pn];
 378                   rassoc[np] = rassoc[pn];
 379                   rrhs[np] = rrhs[pn];
 380                   if (rrhs[np] != ni)
 381                     {
 382                       pi = rrhs[np];
 383                       rrhs[np] = ni;
 384                       while (ritem[pi] >= 0)
 385                         ritem[ni++] = ritem[pi++];
 386                       ritem[ni++] = -np;
 387                     }
 388                 } else {
 389                   while (ritem[ni++] >= 0);
 390                 }
 391             }
 392         }
 393       ritem[ni] = 0;
 394       nrules -= nuseless_productions;
 395       nitems = ni;
 396
 397       /*
 398        * Is it worth it to reduce the amount of memory for the
 399        * grammar? Probably not.
 400        */
 401
 402     }
 403 #endif /* 0 */
 404   /* Disable useless productions,
 405      since they may contain useless nonterms
 406      that would get mapped below to -1 and confuse everyone.  */
 407   if (nuseless_productions > 0)
 408     {
 409       int pn;
 410
 411       for (pn = 1; pn <= nrules; pn++)
 412         {
 413           if (!BITISSET(P, pn))
 414             {
 415               rlhs[pn] = -1;
 416             }
 417         }
 418     }
 419
 420   /* remove useless symbols */
 421   if (nuseless_nonterminals > 0)
 422     {
 423
 424       int    i, n;
 425 /*      short  j; JF unused */
 426       short *nontermmap;
 427       rule   r;
 428
 429       /*
 430        * create a map of nonterminal number to new nonterminal
 431        * number. -1 in the map means it was useless and is being
 432        * eliminated.
 433        */
 434
 435       nontermmap = NEW2(nvars, short) - ntokens;
 436       for (i = ntokens; i < nsyms; i++)
 437         nontermmap[i] = -1;
 438
 439       n = ntokens;
 440       for (i = ntokens; i < nsyms; i++)
 441         if (BITISSET(V, i))
 442           nontermmap[i] = n++;
 443
 444       /* Shuffle elements of tables indexed by symbol number.  */
 445
 446       for (i = ntokens; i < nsyms; i++)
 447         {
 448           n = nontermmap[i];
 449           if (n >= 0)
 450             {
 451               sassoc[n] = sassoc[i];
 452               sprec[n] = sprec[i];
 453               tags[n] = tags[i];
 454             } else {
 455               free(tags[i]);
 456             }
 457         }
 458
 459       /* Replace all symbol numbers in valid data structures.  */
 460
 461       for (i = 1; i <= nrules; i++)
 462         {
 463           /* Ignore the rules disabled above.  */
 464           if (rlhs[i] >= 0)
 465             rlhs[i] = nontermmap[rlhs[i]];
 466           if (ISVAR (rprecsym[i]))
 467             /* Can this happen?  */
 468             rprecsym[i] = nontermmap[rprecsym[i]];
 469         }
 470
 471       for (r = ritem; *r; r++)
 472         if (ISVAR(*r))
 473           *r = nontermmap[*r];
 474
 475       start_symbol = nontermmap[start_symbol];
 476
 477       nsyms -= nuseless_nonterminals;
 478       nvars -= nuseless_nonterminals;
 479
 480       free(&nontermmap[ntokens]);
 481     }
 482 }
 483 \f
 484 static void
 485 print_results ()
 486 {
 487   int   i;
 488 /*  short j; JF unused */
 489   rule  r;
 490   bool  b;
 491
 492   if (nuseless_nonterminals > 0)
 493     {
 494       fprintf(foutput, "Useless nonterminals:\n\n");
 495       for (i = ntokens; i < nsyms; i++)
 496         if (!BITISSET(V, i))
 497           fprintf(foutput, "   %s\n", tags[i]);
 498     }
 499   b = FALSE;
 500   for (i = 0; i < ntokens; i++)
 501     {
 502       if (!BITISSET(V, i) && !BITISSET(V1, i))
 503         {
 504           if (!b)
 505             {
 506               fprintf(foutput, "\n\nTerminals which are not used:\n\n");
 507               b = TRUE;
 508             }
 509           fprintf(foutput, "   %s\n", tags[i]);
 510         }
 511     }
 512
 513   if (nuseless_productions > 0)
 514     {
 515       fprintf(foutput, "\n\nUseless rules:\n\n");
 516       for (i = 1; i <= nrules; i++)
 517         {
 518           if (!BITISSET(P, i))
 519             {
 520               fprintf(foutput, "#%-4d  ", i);
 521               fprintf(foutput, "%s :\t", tags[rlhs[i]]);
 522               for (r = &ritem[rrhs[i]]; *r >= 0; r++)
 523                 {
 524                   fprintf(foutput, " %s", tags[*r]);
 525                 }
 526               fprintf(foutput, ";\n");
 527             }
 528         }
 529     }
 530   if (nuseless_nonterminals > 0 || nuseless_productions > 0 || b)
 531     fprintf(foutput, "\n\n");
 532 }
 533 \f
 534 void
 535 dump_grammar ()
 536 {
 537   int i;
 538   rule r;
 539
 540   fprintf(foutput,
 541           "ntokens = %d, nvars = %d, nsyms = %d, nrules = %d, nitems = %d\n\n",
 542           ntokens, nvars, nsyms, nrules, nitems);
 543   fprintf(foutput, "Variables\n---------\n\n");
 544   fprintf(foutput, "Value  Sprec    Sassoc    Tag\n");
 545   for (i = ntokens; i < nsyms; i++)
 546     fprintf(foutput, "%5d  %5d  %5d  %s\n",
 547             i, sprec[i], sassoc[i], tags[i]);
 548   fprintf(foutput, "\n\n");
 549   fprintf(foutput, "Rules\n-----\n\n");
 550   for (i = 1; i <= nrules; i++)
 551     {
 552       fprintf(foutput, "%-5d(%5d%5d)%5d : (@%-5d)",
 553               i, rprec[i], rassoc[i], rlhs[i], rrhs[i]);
 554       for (r = &ritem[rrhs[i]]; *r > 0; r++)
 555         fprintf(foutput, "%5d", *r);
 556       fprintf(foutput, " [%d]\n", -(*r));
 557     }
 558   fprintf(foutput, "\n\n");
 559   fprintf(foutput, "Rules interpreted\n-----------------\n\n");
 560   for (i = 1; i <= nrules; i++)
 561     {
 562       fprintf(foutput, "%-5d  %s :", i, tags[rlhs[i]]);
 563       for (r = &ritem[rrhs[i]]; *r > 0; r++)
 564         fprintf(foutput, " %s", tags[*r]);
 565       fprintf(foutput, "\n");
 566     }
 567   fprintf(foutput, "\n\n");
 568 }
 569
 570
 571 static void
 572 print_notices ()
 573 {
 574   extern int fixed_outfiles;
 575
 576   if (fixed_outfiles && nuseless_productions)
 577     fprintf(stderr, "%d rules never reduced\n", nuseless_productions);
 578
 579   fprintf(stderr, "%s contains ", infile);
 580
 581   if (nuseless_nonterminals > 0)
 582     {
 583       fprintf(stderr, "%d useless nonterminal%s",
 584               nuseless_nonterminals,
 585               (nuseless_nonterminals == 1 ? "" : "s"));
 586     }
 587   if (nuseless_nonterminals > 0 && nuseless_productions > 0)
 588     fprintf(stderr, " and ");
 589
 590   if (nuseless_productions > 0)
 591     {
 592       fprintf(stderr, "%d useless rule%s",
 593               nuseless_productions,
 594               (nuseless_productions == 1 ? "" : "s"));
 595     }
 596   fprintf(stderr, ".\n");
 597   fflush(stderr);
 598 }