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