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Annotation of GNUtools/bison/reduce.c, revision 1.1

1.1     ! root        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: 
        !            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: 
        !           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: 
        !           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: 
        !           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);                        /* illegal 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: 
        !           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: 
        !           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: 
        !           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: }

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