src/reduce.c

   1 /* Grammar reduction for Bison.
   2    Copyright 1988, 1989, 2000, 2001  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 /* Reduce the grammar: Find and eliminate unreachable terminals,
  23    nonterminals, and productions.  David S. Bakin.  */
  24
  25 /* Don't eliminate unreachable terminals: They may be used by the
  26    user's parser.  */
  27
  28 #include "system.h"
  29 #include "getargs.h"
  30 #include "files.h"
  31 #include "gram.h"
  32 #include "complain.h"
  33 #include "reduce.h"
  34 #include "reader.h"
  35 #include "getargs.h"
  36
  37 typedef unsigned *BSet;
  38 typedef short *rule;
  39
  40
  41 /* Set of all nonterminals which are not useless.  */
  42 static BSet N;
  43
  44 /* Set of all rules which have no useless nonterminals in their RHS.  */
  45 static BSet P;
  46
  47 /* Set of all accessible symbols.  */
  48 static BSet V;
  49
  50 /* Set of symbols used to define rule precedence (so they are
  51    `useless', but no warning should be issued).  */
  52 static BSet V1;
  53
  54 static int nuseful_productions;
  55 static int nuseless_productions;
  56 static int nuseful_nonterminals;
  57 int nuseless_nonterminals;
  58 \f
  59 static bool
  60 bits_equal (BSet L, BSet R, int n)
  61 {
  62   int i;
  63
  64   for (i = n - 1; i >= 0; i--)
  65     if (L[i] != R[i])
  66       return FALSE;
  67   return TRUE;
  68 }
  69
  70
  71 static int
  72 nbits (unsigned i)
  73 {
  74   int count = 0;
  75
  76   while (i != 0)
  77     {
  78       i ^= (i & ((unsigned) (-(int) i)));
  79       ++count;
  80     }
  81   return count;
  82 }
  83
  84
  85 static int
  86 bits_size (BSet S, int n)
  87 {
  88   int i, count = 0;
  89
  90   for (i = n - 1; i >= 0; i--)
  91     count += nbits (S[i]);
  92   return count;
  93 }
  94 \f
  95 /*-------------------------------------------------------------------.
  96 | Another way to do this would be with a set for each production and |
  97 | then do subset tests against N0, but even for the C grammar the    |
  98 | whole reducing process takes only 2 seconds on my 8Mhz AT.         |
  99 `-------------------------------------------------------------------*/
 100
 101 static bool
 102 useful_production (int i, BSet N0)
 103 {
 104   rule r;
 105   short n;
 106
 107   /* A production is useful if all of the nonterminals in its appear
 108      in the set of useful nonterminals.  */
 109
 110   for (r = &ritem[rule_table[i].rhs]; *r > 0; r++)
 111     if (ISVAR (n = *r))
 112       if (!BITISSET (N0, n - ntokens))
 113         return FALSE;
 114   return TRUE;
 115 }
 116
 117
 118 /*---------------------------------------------------------.
 119 | Remember that rules are 1-origin, symbols are 0-origin.  |
 120 `---------------------------------------------------------*/
 121
 122 static void
 123 useless_nonterminals (void)
 124 {
 125   BSet Np, Ns;
 126   int i;
 127
 128   /* N is set as built.  Np is set being built this iteration. P is
 129      set of all productions which have a RHS all in N.  */
 130
 131   Np = XCALLOC (unsigned, WORDSIZE (nvars));
 132
 133   /* The set being computed is a set of nonterminals which can derive
 134      the empty string or strings consisting of all terminals. At each
 135      iteration a nonterminal is added to the set if there is a
 136      production with that nonterminal as its LHS for which all the
 137      nonterminals in its RHS are already in the set.  Iterate until
 138      the set being computed remains unchanged.  Any nonterminals not
 139      in the set at that point are useless in that they will never be
 140      used in deriving a sentence of the language.
 141
 142      This iteration doesn't use any special traversal over the
 143      productions.  A set is kept of all productions for which all the
 144      nonterminals in the RHS are in useful.  Only productions not in
 145      this set are scanned on each iteration.  At the end, this set is
 146      saved to be used when finding useful productions: only
 147      productions in this set will appear in the final grammar.  */
 148
 149   while (1)
 150     {
 151       for (i = WORDSIZE (nvars) - 1; i >= 0; i--)
 152         Np[i] = N[i];
 153       for (i = 1; i <= nrules; i++)
 154         {
 155           if (!BITISSET (P, i))
 156             {
 157               if (useful_production (i, N))
 158                 {
 159                   SETBIT (Np, rule_table[i].lhs - ntokens);
 160                   SETBIT (P, i);
 161                 }
 162             }
 163         }
 164       if (bits_equal (N, Np, WORDSIZE (nvars)))
 165         break;
 166       Ns = Np;
 167       Np = N;
 168       N = Ns;
 169     }
 170   XFREE (N);
 171   N = Np;
 172 }
 173
 174
 175 static void
 176 inaccessable_symbols (void)
 177 {
 178   BSet Vp, Vs, Pp;
 179   int i;
 180   short t;
 181   rule r;
 182
 183   /* Find out which productions are reachable and which symbols are
 184      used.  Starting with an empty set of productions and a set of
 185      symbols which only has the start symbol in it, iterate over all
 186      productions until the set of productions remains unchanged for an
 187      iteration.  For each production which has a LHS in the set of
 188      reachable symbols, add the production to the set of reachable
 189      productions, and add all of the nonterminals in the RHS of the
 190      production to the set of reachable symbols.
 191
 192      Consider only the (partially) reduced grammar which has only
 193      nonterminals in N and productions in P.
 194
 195      The result is the set P of productions in the reduced grammar,
 196      and the set V of symbols in the reduced grammar.
 197
 198      Although this algorithm also computes the set of terminals which
 199      are reachable, no terminal will be deleted from the grammar. Some
 200      terminals might not be in the grammar but might be generated by
 201      semantic routines, and so the user might want them available with
 202      specified numbers.  (Is this true?)  However, the nonreachable
 203      terminals are printed (if running in verbose mode) so that the
 204      user can know.  */
 205
 206   Vp = XCALLOC (unsigned, WORDSIZE (nsyms));
 207   Pp = XCALLOC (unsigned, WORDSIZE (nrules + 1));
 208
 209   /* If the start symbol isn't useful, then nothing will be useful. */
 210   if (BITISSET (N, start_symbol - ntokens))
 211     {
 212       SETBIT (V, start_symbol);
 213
 214       while (1)
 215         {
 216           for (i = WORDSIZE (nsyms) - 1; i >= 0; i--)
 217             Vp[i] = V[i];
 218           for (i = 1; i <= nrules; i++)
 219             {
 220               if (!BITISSET (Pp, i)
 221                   && BITISSET (P, i)
 222                   && BITISSET (V, rule_table[i].lhs))
 223                 {
 224                   for (r = &ritem[rule_table[i].rhs]; *r >= 0; r++)
 225                     if (ISTOKEN (t = *r) || BITISSET (N, t - ntokens))
 226                       SETBIT (Vp, t);
 227                   SETBIT (Pp, i);
 228                 }
 229             }
 230           if (bits_equal (V, Vp, WORDSIZE (nsyms)))
 231             break;
 232           Vs = Vp;
 233           Vp = V;
 234           V = Vs;
 235         }
 236     }
 237
 238   XFREE (V);
 239   V = Vp;
 240
 241   /* Tokens 0, 1, and 2 are internal to Bison.  Consider them useful. */
 242   SETBIT (V, 0);                /* end-of-input token */
 243   SETBIT (V, 1);                /* error token */
 244   SETBIT (V, 2);                /* some undefined token */
 245
 246   XFREE (P);
 247   P = Pp;
 248
 249   nuseful_productions = bits_size (P, WORDSIZE (nrules + 1));
 250   nuseless_productions = nrules - nuseful_productions;
 251
 252   nuseful_nonterminals = 0;
 253   for (i = ntokens; i < nsyms; i++)
 254     if (BITISSET (V, i))
 255       nuseful_nonterminals++;
 256   nuseless_nonterminals = nvars - nuseful_nonterminals;
 257
 258   /* A token that was used in %prec should not be warned about.  */
 259   for (i = 1; i < nrules; i++)
 260     if (rule_table[i].precsym != 0)
 261       SETBIT (V1, rule_table[i].precsym);
 262 }
 263
 264 static void
 265 reduce_grammar_tables (void)
 266 {
 267   /* This is turned off because we would need to change the numbers in
 268      the case statements in the actions file.
 269
 270      We don't disable it via CPP so that it is still checked with the
 271      rest of the code, to avoid its becoming completely obsolete.
 272
 273      FIXME: I think the comment above demonstrates this code must be
 274      turned off for *semantic* parser, not in the general case.  Try
 275      to understand this better --akim.  */
 276
 277   if (0)
 278     /* remove useless productions */
 279     if (nuseless_productions > 0)
 280       {
 281         short np, pn, ni, pi;
 282
 283         np = 0;
 284         ni = 0;
 285         for (pn = 1; pn <= nrules; pn++)
 286           if (BITISSET (P, pn))
 287             {
 288               np++;
 289               if (pn != np)
 290                 {
 291                   rule_table[np].lhs   = rule_table[pn].lhs;
 292                   rule_table[np].line  = rule_table[pn].line;
 293                   rule_table[np].prec  = rule_table[pn].prec;
 294                   rule_table[np].assoc = rule_table[pn].assoc;
 295                   rule_table[np].rhs   = rule_table[pn].rhs;
 296                   if (rule_table[np].rhs != ni)
 297                     {
 298                       pi = rule_table[np].rhs;
 299                       rule_table[np].rhs = ni;
 300                       while (ritem[pi] >= 0)
 301                         ritem[ni++] = ritem[pi++];
 302                       ritem[ni++] = -np;
 303                     }
 304                 }
 305               else
 306                 {
 307                   while (ritem[ni++] >= 0);
 308                 }
 309             }
 310
 311         ritem[ni] = 0;
 312         nrules -= nuseless_productions;
 313         nitems = ni;
 314
 315         /* Is it worth it to reduce the amount of memory for the
 316            grammar? Probably not.  */
 317       }
 318
 319   /* Disable useless productions. */
 320   if (nuseless_productions > 0)
 321     {
 322       int pn;
 323       for (pn = 1; pn <= nrules; pn++)
 324         rule_table[pn].useful = BITISSET (P, pn);
 325     }
 326 }
 327
 328
 329 /*------------------------------.
 330 | Remove useless nonterminals.  |
 331 `------------------------------*/
 332
 333 static void
 334 nonterminals_reduce (void)
 335 {
 336   int i, n;
 337   rule r;
 338
 339   /* Map the nonterminals to their new index: useful first, useless
 340      afterwards.  Kept for later report.  */
 341
 342   short *nontermmap = XCALLOC (short, nvars) - ntokens;
 343   n = ntokens;
 344   for (i = ntokens; i < nsyms; i++)
 345     if (BITISSET (V, i))
 346       nontermmap[i] = n++;
 347   for (i = ntokens; i < nsyms; i++)
 348     if (!BITISSET (V, i))
 349       nontermmap[i] = n++;
 350
 351
 352   /* Shuffle elements of tables indexed by symbol number.  */
 353   {
 354     short *sassoc_sorted = XMALLOC (short, nvars) - ntokens;
 355     short *sprec_sorted  = XMALLOC (short, nvars) - ntokens;
 356     char **tags_sorted   = XMALLOC (char *, nvars) - ntokens;
 357
 358     for (i = ntokens; i < nsyms; i++)
 359       {
 360         n = nontermmap[i];
 361         sassoc_sorted[n] = sassoc[i];
 362         sprec_sorted[n]  = sprec[i];
 363         tags_sorted[n]   = tags[i];
 364       }
 365     for (i = ntokens; i < nsyms; i++)
 366       {
 367         sassoc[i] = sassoc_sorted[i];
 368         sprec[i]  = sprec_sorted[i];
 369         tags[i]   = tags_sorted[i];
 370       }
 371     free (sassoc_sorted + ntokens);
 372     free (sprec_sorted + ntokens);
 373     free (tags_sorted + ntokens);
 374   }
 375
 376   /* Replace all symbol numbers in valid data structures.  */
 377
 378   for (i = 1; i <= nrules; i++)
 379     {
 380       rule_table[i].lhs = nontermmap[rule_table[i].lhs];
 381       if (ISVAR (rule_table[i].precsym))
 382         /* Can this happen?  */
 383         rule_table[i].precsym = nontermmap[rule_table[i].precsym];
 384     }
 385
 386   for (r = ritem; *r; r++)
 387     if (ISVAR (*r))
 388       *r = nontermmap[*r];
 389
 390   start_symbol = nontermmap[start_symbol];
 391
 392   nsyms -= nuseless_nonterminals;
 393   nvars -= nuseless_nonterminals;
 394
 395   free (nontermmap + ntokens);
 396 }
 397
 398
 399 /*------------------------------------------------------------------.
 400 | Output the detailed results of the reductions.  For FILE.output.  |
 401 `------------------------------------------------------------------*/
 402
 403 void
 404 reduce_output (FILE *out)
 405 {
 406   if (nuseless_nonterminals > 0)
 407     {
 408       int i;
 409       fprintf (out, "%s\n\n", _("Useless nonterminals:"));
 410       for (i = 0; i < nuseless_nonterminals; ++i)
 411         fprintf (out, "   %s\n", tags[nsyms + i]);
 412       fputs ("\n\n", out);
 413     }
 414
 415   {
 416     bool b = FALSE;
 417     int i;
 418     for (i = 0; i < ntokens; i++)
 419       if (!BITISSET (V, i) && !BITISSET (V1, i))
 420         {
 421           if (!b)
 422             fprintf (out, "%s\n\n", _("Terminals which are not used:"));
 423           b = TRUE;
 424           fprintf (out, "   %s\n", tags[i]);
 425         }
 426     if (b)
 427       fputs ("\n\n", out);
 428   }
 429
 430   if (nuseless_productions > 0)
 431     {
 432       int i;
 433       fprintf (out, "%s\n\n", _("Useless rules:"));
 434       for (i = 1; i <= nrules; i++)
 435         if (!rule_table[i].useful)
 436           {
 437             rule r;
 438             fprintf (out, "#%-4d  ", i - 1);
 439             fprintf (out, "%s:", tags[rule_table[i].lhs]);
 440             for (r = &ritem[rule_table[i].rhs]; *r >= 0; r++)
 441               fprintf (out, " %s", tags[*r]);
 442             fputs (";\n", out);
 443           }
 444       fputs ("\n\n", out);
 445     }
 446 }
 447 \f
 448 static void
 449 dump_grammar (FILE *out)
 450 {
 451   int i;
 452   rule r;
 453
 454   fprintf (out, "REDUCED GRAMMAR\n\n");
 455   fprintf (out,
 456            "ntokens = %d, nvars = %d, nsyms = %d, nrules = %d, nitems = %d\n\n",
 457            ntokens, nvars, nsyms, nrules, nitems);
 458   fprintf (out, "Variables\n---------\n\n");
 459   fprintf (out, "Value  Sprec  Sassoc  Tag\n");
 460   for (i = ntokens; i < nsyms; i++)
 461     fprintf (out, "%5d  %5d   %5d  %s\n", i, sprec[i], sassoc[i], tags[i]);
 462   fprintf (out, "\n\n");
 463   fprintf (out, "Rules\n-----\n\n");
 464   fprintf (out, "Num (Prec, Assoc, Useful, Ritem Range) Lhs -> Rhs (Ritem range) [Num]\n");
 465   for (i = 1; i <= nrules; i++)
 466     {
 467       int rhs_count = 0;
 468       /* Find the last RHS index in ritems. */
 469       for (r = &ritem[rule_table[i].rhs]; *r > 0; ++r)
 470         ++rhs_count;
 471       fprintf (out, "%3d (%2d, %2d, %2d, %2d-%2d)   %2d ->",
 472                i,
 473                rule_table[i].prec, rule_table[i].assoc, rule_table[i].useful,
 474                rule_table[i].rhs, rule_table[i].rhs + rhs_count - 1,
 475                rule_table[i].lhs);
 476       /* Dumped the RHS. */
 477       for (r = &ritem[rule_table[i].rhs]; *r > 0; r++)
 478         fprintf (out, "%3d", *r);
 479       fprintf (out, "  [%d]\n", -(*r));
 480     }
 481   fprintf (out, "\n\n");
 482   fprintf (out, "Rules interpreted\n-----------------\n\n");
 483   for (i = 1; i <= nrules; i++)
 484     {
 485       fprintf (out, "%-5d  %s :", i, tags[rule_table[i].lhs]);
 486       for (r = &ritem[rule_table[i].rhs]; *r > 0; r++)
 487         fprintf (out, " %s", tags[*r]);
 488       fputc ('\n', out);
 489     }
 490   fprintf (out, "\n\n");
 491 }
 492
 493
 494
 495 /*-------------------------------.
 496 | Report the results to STDERR.  |
 497 `-------------------------------*/
 498
 499 static void
 500 reduce_print (void)
 501 {
 502   if (yacc_flag && nuseless_productions)
 503     fprintf (stderr, ngettext ("%d rule never reduced\n",
 504                                "%d rules never reduced\n",
 505                                nuseless_productions),
 506              nuseless_productions);
 507
 508   fprintf (stderr, _("%s contains "), infile);
 509
 510   if (nuseless_nonterminals > 0)
 511     fprintf (stderr, ngettext ("%d useless nonterminal",
 512                                "%d useless nonterminals",
 513                                nuseless_nonterminals),
 514              nuseless_nonterminals);
 515
 516   if (nuseless_nonterminals > 0 && nuseless_productions > 0)
 517     fprintf (stderr, _(" and "));
 518
 519   if (nuseless_productions > 0)
 520     fprintf (stderr, ngettext ("%d useless rule",
 521                                "%d useless rules",
 522                                nuseless_productions),
 523              nuseless_productions);
 524   fprintf (stderr, "\n");
 525   fflush (stderr);
 526 }
 527 \f
 528 void
 529 reduce_grammar (void)
 530 {
 531   bool reduced;
 532
 533   /* Allocate the global sets used to compute the reduced grammar */
 534
 535   N = XCALLOC (unsigned, WORDSIZE (nvars));
 536   P = XCALLOC (unsigned, WORDSIZE (nrules + 1));
 537   V = XCALLOC (unsigned, WORDSIZE (nsyms));
 538   V1 = XCALLOC (unsigned, WORDSIZE (nsyms));
 539
 540   useless_nonterminals ();
 541   inaccessable_symbols ();
 542
 543   reduced = (bool) (nuseless_nonterminals + nuseless_productions > 0);
 544
 545   if (!reduced)
 546     return;
 547
 548   reduce_print ();
 549
 550   if (!BITISSET (N, start_symbol - ntokens))
 551     fatal (_("Start symbol %s does not derive any sentence"),
 552            tags[start_symbol]);
 553
 554   reduce_grammar_tables ();
 555   if (nuseless_nonterminals > 0)
 556     nonterminals_reduce ();
 557
 558   if (trace_flag)
 559     {
 560       dump_grammar (stderr);
 561
 562       fprintf (stderr, "reduced %s defines %d terminals, %d nonterminals\
 563 , and %d productions.\n",
 564                infile, ntokens, nvars, nrules);
 565     }
 566 }
 567
 568
 569 /*-----------------------------------------------------------.
 570 | Free the global sets used to compute the reduced grammar.  |
 571 `-----------------------------------------------------------*/
 572
 573 void
 574 reduce_free (void)
 575 {
 576   XFREE (N);
 577   XFREE (V);
 578   XFREE (V1);
 579   XFREE (P);
 580 }