OLD | NEW |
(Empty) | |
| 1 /* Grammar reduction for Bison. |
| 2 |
| 3 Copyright (C) 1988, 1989, 2000, 2001, 2002, 2003, 2005, 2006, |
| 4 2007, 2008 Free Software Foundation, Inc. |
| 5 |
| 6 This file is part of Bison, the GNU Compiler Compiler. |
| 7 |
| 8 This program is free software: you can redistribute it and/or modify |
| 9 it under the terms of the GNU General Public License as published by |
| 10 the Free Software Foundation, either version 3 of the License, or |
| 11 (at your option) any later version. |
| 12 |
| 13 This program is distributed in the hope that it will be useful, |
| 14 but WITHOUT ANY WARRANTY; without even the implied warranty of |
| 15 MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the |
| 16 GNU General Public License for more details. |
| 17 |
| 18 You should have received a copy of the GNU General Public License |
| 19 along with this program. If not, see <http://www.gnu.org/licenses/>. */ |
| 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 <config.h> |
| 29 #include "system.h" |
| 30 |
| 31 #include <bitset.h> |
| 32 #include <quotearg.h> |
| 33 |
| 34 #include "complain.h" |
| 35 #include "files.h" |
| 36 #include "getargs.h" |
| 37 #include "gram.h" |
| 38 #include "print-xml.h" |
| 39 #include "reader.h" |
| 40 #include "reduce.h" |
| 41 #include "symtab.h" |
| 42 |
| 43 /* Set of all nonterminals which are not useless. */ |
| 44 static bitset N; |
| 45 |
| 46 /* Set of all rules which have no useless nonterminals in their RHS. */ |
| 47 static bitset P; |
| 48 |
| 49 /* Set of all accessible symbols. */ |
| 50 static bitset V; |
| 51 |
| 52 /* Set of symbols used to define rule precedence (so they are |
| 53 `useless', but no warning should be issued). */ |
| 54 static bitset V1; |
| 55 |
| 56 static rule_number nuseful_productions; |
| 57 rule_number nuseless_productions; |
| 58 static int nuseful_nonterminals; |
| 59 symbol_number nuseless_nonterminals; |
| 60 |
| 61 /*-------------------------------------------------------------------. |
| 62 | Another way to do this would be with a set for each production and | |
| 63 | then do subset tests against N0, but even for the C grammar the | |
| 64 | whole reducing process takes only 2 seconds on my 8Mhz AT. | |
| 65 `-------------------------------------------------------------------*/ |
| 66 |
| 67 static bool |
| 68 useful_production (rule_number r, bitset N0) |
| 69 { |
| 70 item_number *rhsp; |
| 71 |
| 72 /* A production is useful if all of the nonterminals in its appear |
| 73 in the set of useful nonterminals. */ |
| 74 |
| 75 for (rhsp = rules[r].rhs; *rhsp >= 0; ++rhsp) |
| 76 if (ISVAR (*rhsp) && !bitset_test (N0, *rhsp - ntokens)) |
| 77 return false; |
| 78 return true; |
| 79 } |
| 80 |
| 81 |
| 82 /*---------------------------------------------------------. |
| 83 | Remember that rules are 1-origin, symbols are 0-origin. | |
| 84 `---------------------------------------------------------*/ |
| 85 |
| 86 static void |
| 87 useless_nonterminals (void) |
| 88 { |
| 89 bitset Np, Ns; |
| 90 rule_number r; |
| 91 |
| 92 /* N is set as built. Np is set being built this iteration. P is |
| 93 set of all productions which have a RHS all in N. */ |
| 94 |
| 95 Np = bitset_create (nvars, BITSET_FIXED); |
| 96 |
| 97 |
| 98 /* The set being computed is a set of nonterminals which can derive |
| 99 the empty string or strings consisting of all terminals. At each |
| 100 iteration a nonterminal is added to the set if there is a |
| 101 production with that nonterminal as its LHS for which all the |
| 102 nonterminals in its RHS are already in the set. Iterate until |
| 103 the set being computed remains unchanged. Any nonterminals not |
| 104 in the set at that point are useless in that they will never be |
| 105 used in deriving a sentence of the language. |
| 106 |
| 107 This iteration doesn't use any special traversal over the |
| 108 productions. A set is kept of all productions for which all the |
| 109 nonterminals in the RHS are in useful. Only productions not in |
| 110 this set are scanned on each iteration. At the end, this set is |
| 111 saved to be used when finding useful productions: only |
| 112 productions in this set will appear in the final grammar. */ |
| 113 |
| 114 while (1) |
| 115 { |
| 116 bitset_copy (Np, N); |
| 117 for (r = 0; r < nrules; r++) |
| 118 if (!bitset_test (P, r) |
| 119 && useful_production (r, N)) |
| 120 { |
| 121 bitset_set (Np, rules[r].lhs->number - ntokens); |
| 122 bitset_set (P, r); |
| 123 } |
| 124 if (bitset_equal_p (N, Np)) |
| 125 break; |
| 126 Ns = Np; |
| 127 Np = N; |
| 128 N = Ns; |
| 129 } |
| 130 bitset_free (N); |
| 131 N = Np; |
| 132 } |
| 133 |
| 134 |
| 135 static void |
| 136 inaccessable_symbols (void) |
| 137 { |
| 138 bitset Vp, Vs, Pp; |
| 139 |
| 140 /* Find out which productions are reachable and which symbols are |
| 141 used. Starting with an empty set of productions and a set of |
| 142 symbols which only has the start symbol in it, iterate over all |
| 143 productions until the set of productions remains unchanged for an |
| 144 iteration. For each production which has a LHS in the set of |
| 145 reachable symbols, add the production to the set of reachable |
| 146 productions, and add all of the nonterminals in the RHS of the |
| 147 production to the set of reachable symbols. |
| 148 |
| 149 Consider only the (partially) reduced grammar which has only |
| 150 nonterminals in N and productions in P. |
| 151 |
| 152 The result is the set P of productions in the reduced grammar, |
| 153 and the set V of symbols in the reduced grammar. |
| 154 |
| 155 Although this algorithm also computes the set of terminals which |
| 156 are reachable, no terminal will be deleted from the grammar. Some |
| 157 terminals might not be in the grammar but might be generated by |
| 158 semantic routines, and so the user might want them available with |
| 159 specified numbers. (Is this true?) However, the nonreachable |
| 160 terminals are printed (if running in verbose mode) so that the |
| 161 user can know. */ |
| 162 |
| 163 Vp = bitset_create (nsyms, BITSET_FIXED); |
| 164 Pp = bitset_create (nrules, BITSET_FIXED); |
| 165 |
| 166 /* If the start symbol isn't useful, then nothing will be useful. */ |
| 167 if (bitset_test (N, accept->number - ntokens)) |
| 168 { |
| 169 bitset_set (V, accept->number); |
| 170 |
| 171 while (1) |
| 172 { |
| 173 rule_number r; |
| 174 bitset_copy (Vp, V); |
| 175 for (r = 0; r < nrules; r++) |
| 176 { |
| 177 if (!bitset_test (Pp, r) |
| 178 && bitset_test (P, r) |
| 179 && bitset_test (V, rules[r].lhs->number)) |
| 180 { |
| 181 item_number *rhsp; |
| 182 for (rhsp = rules[r].rhs; *rhsp >= 0; rhsp++) |
| 183 if (ISTOKEN (*rhsp) || bitset_test (N, *rhsp - ntokens)) |
| 184 bitset_set (Vp, *rhsp); |
| 185 bitset_set (Pp, r); |
| 186 } |
| 187 } |
| 188 if (bitset_equal_p (V, Vp)) |
| 189 break; |
| 190 Vs = Vp; |
| 191 Vp = V; |
| 192 V = Vs; |
| 193 } |
| 194 } |
| 195 |
| 196 bitset_free (V); |
| 197 V = Vp; |
| 198 |
| 199 /* Tokens 0, 1, and 2 are internal to Bison. Consider them useful. */ |
| 200 bitset_set (V, endtoken->number); /* end-of-input token */ |
| 201 bitset_set (V, errtoken->number); /* error token */ |
| 202 bitset_set (V, undeftoken->number); /* some undefined token */ |
| 203 |
| 204 bitset_free (P); |
| 205 P = Pp; |
| 206 |
| 207 nuseful_productions = bitset_count (P); |
| 208 nuseless_productions = nrules - nuseful_productions; |
| 209 |
| 210 nuseful_nonterminals = 0; |
| 211 { |
| 212 symbol_number i; |
| 213 for (i = ntokens; i < nsyms; i++) |
| 214 if (bitset_test (V, i)) |
| 215 nuseful_nonterminals++; |
| 216 } |
| 217 nuseless_nonterminals = nvars - nuseful_nonterminals; |
| 218 |
| 219 /* A token that was used in %prec should not be warned about. */ |
| 220 { |
| 221 rule_number r; |
| 222 for (r = 0; r < nrules; ++r) |
| 223 if (rules[r].precsym != 0) |
| 224 bitset_set (V1, rules[r].precsym->number); |
| 225 } |
| 226 } |
| 227 |
| 228 |
| 229 /*-------------------------------------------------------------------. |
| 230 | Put the useless productions at the end of RULES, and adjust NRULES | |
| 231 | accordingly. | |
| 232 `-------------------------------------------------------------------*/ |
| 233 |
| 234 static void |
| 235 reduce_grammar_tables (void) |
| 236 { |
| 237 /* Report and flag useless productions. */ |
| 238 { |
| 239 rule_number r; |
| 240 for (r = 0; r < nrules; r++) |
| 241 rules[r].useful = bitset_test (P, r); |
| 242 grammar_rules_useless_report (_("rule useless in grammar")); |
| 243 } |
| 244 |
| 245 /* Map the nonterminals to their new index: useful first, useless |
| 246 afterwards. Kept for later report. */ |
| 247 { |
| 248 int useful = 0; |
| 249 int useless = nrules - nuseless_productions; |
| 250 rule *rules_sorted = xnmalloc (nrules, sizeof *rules_sorted); |
| 251 rule_number r; |
| 252 for (r = 0; r < nrules; ++r) |
| 253 rules_sorted[rules[r].useful ? useful++ : useless++] = rules[r]; |
| 254 free (rules); |
| 255 rules = rules_sorted; |
| 256 |
| 257 /* Renumber the rules markers in RITEMS. */ |
| 258 for (r = 0; r < nrules; ++r) |
| 259 { |
| 260 item_number *rhsp = rules[r].rhs; |
| 261 for (/* Nothing. */; *rhsp >= 0; ++rhsp) |
| 262 /* Nothing. */; |
| 263 *rhsp = rule_number_as_item_number (r); |
| 264 rules[r].number = r; |
| 265 } |
| 266 nrules -= nuseless_productions; |
| 267 } |
| 268 |
| 269 /* Adjust NRITEMS. */ |
| 270 { |
| 271 rule_number r; |
| 272 int length; |
| 273 for (r = nrules; r < nrules + nuseless_productions; ++r) |
| 274 { |
| 275 length = rule_rhs_length (&rules[r]); |
| 276 nritems -= length + 1; |
| 277 } |
| 278 } |
| 279 } |
| 280 |
| 281 |
| 282 /*------------------------------. |
| 283 | Remove useless nonterminals. | |
| 284 `------------------------------*/ |
| 285 |
| 286 static void |
| 287 nonterminals_reduce (void) |
| 288 { |
| 289 symbol_number i, n; |
| 290 |
| 291 /* Map the nonterminals to their new index: useful first, useless |
| 292 afterwards. Kept for later report. */ |
| 293 |
| 294 symbol_number *nontermmap = xnmalloc (nvars, sizeof *nontermmap); |
| 295 n = ntokens; |
| 296 for (i = ntokens; i < nsyms; i++) |
| 297 if (bitset_test (V, i)) |
| 298 nontermmap[i - ntokens] = n++; |
| 299 for (i = ntokens; i < nsyms; i++) |
| 300 if (!bitset_test (V, i)) |
| 301 { |
| 302 nontermmap[i - ntokens] = n++; |
| 303 warn_at (symbols[i]->location, _("nonterminal useless in grammar: %s"), |
| 304 symbols[i]->tag); |
| 305 } |
| 306 |
| 307 |
| 308 /* Shuffle elements of tables indexed by symbol number. */ |
| 309 { |
| 310 symbol **symbols_sorted = xnmalloc (nvars, sizeof *symbols_sorted); |
| 311 |
| 312 for (i = ntokens; i < nsyms; i++) |
| 313 symbols[i]->number = nontermmap[i - ntokens]; |
| 314 for (i = ntokens; i < nsyms; i++) |
| 315 symbols_sorted[nontermmap[i - ntokens] - ntokens] = symbols[i]; |
| 316 for (i = ntokens; i < nsyms; i++) |
| 317 symbols[i] = symbols_sorted[i - ntokens]; |
| 318 free (symbols_sorted); |
| 319 } |
| 320 |
| 321 { |
| 322 rule_number r; |
| 323 for (r = 0; r < nrules; ++r) |
| 324 { |
| 325 item_number *rhsp; |
| 326 for (rhsp = rules[r].rhs; *rhsp >= 0; ++rhsp) |
| 327 if (ISVAR (*rhsp)) |
| 328 *rhsp = symbol_number_as_item_number (nontermmap[*rhsp |
| 329 - ntokens]); |
| 330 } |
| 331 accept->number = nontermmap[accept->number - ntokens]; |
| 332 } |
| 333 |
| 334 nsyms -= nuseless_nonterminals; |
| 335 nvars -= nuseless_nonterminals; |
| 336 |
| 337 free (nontermmap); |
| 338 } |
| 339 |
| 340 |
| 341 /*------------------------------------------------------------------. |
| 342 | Output the detailed results of the reductions. For FILE.output. | |
| 343 `------------------------------------------------------------------*/ |
| 344 |
| 345 void |
| 346 reduce_output (FILE *out) |
| 347 { |
| 348 if (nuseless_nonterminals > 0) |
| 349 { |
| 350 int i; |
| 351 fprintf (out, "%s\n\n", _("Nonterminals useless in grammar")); |
| 352 for (i = 0; i < nuseless_nonterminals; ++i) |
| 353 fprintf (out, " %s\n", symbols[nsyms + i]->tag); |
| 354 fputs ("\n\n", out); |
| 355 } |
| 356 |
| 357 { |
| 358 bool b = false; |
| 359 int i; |
| 360 for (i = 0; i < ntokens; i++) |
| 361 if (reduce_token_unused_in_grammar (i)) |
| 362 { |
| 363 if (!b) |
| 364 fprintf (out, "%s\n\n", _("Terminals unused in grammar")); |
| 365 b = true; |
| 366 fprintf (out, " %s\n", symbols[i]->tag); |
| 367 } |
| 368 if (b) |
| 369 fputs ("\n\n", out); |
| 370 } |
| 371 |
| 372 if (nuseless_productions > 0) |
| 373 grammar_rules_partial_print (out, _("Rules useless in grammar"), |
| 374 rule_useless_in_grammar_p); |
| 375 } |
| 376 |
| 377 |
| 378 /*-------------------------------. |
| 379 | Report the results to STDERR. | |
| 380 `-------------------------------*/ |
| 381 |
| 382 static void |
| 383 reduce_print (void) |
| 384 { |
| 385 if (nuseless_nonterminals > 0) |
| 386 { |
| 387 fprintf (stderr, "%s: %s: ", grammar_file, _("warning")); |
| 388 fprintf (stderr, ngettext ("%d nonterminal useless in grammar", |
| 389 "%d nonterminals useless in grammar", |
| 390 nuseless_nonterminals), |
| 391 nuseless_nonterminals); |
| 392 fprintf (stderr, "\n"); |
| 393 } |
| 394 if (nuseless_productions > 0) |
| 395 { |
| 396 fprintf (stderr, "%s: %s: ", grammar_file, _("warning")); |
| 397 fprintf (stderr, ngettext ("%d rule useless in grammar", |
| 398 "%d rules useless in grammar", |
| 399 nuseless_productions), |
| 400 nuseless_productions); |
| 401 fprintf (stderr, "\n"); |
| 402 } |
| 403 } |
| 404 |
| 405 void |
| 406 reduce_grammar (void) |
| 407 { |
| 408 bool reduced; |
| 409 |
| 410 /* Allocate the global sets used to compute the reduced grammar */ |
| 411 |
| 412 N = bitset_create (nvars, BITSET_FIXED); |
| 413 P = bitset_create (nrules, BITSET_FIXED); |
| 414 V = bitset_create (nsyms, BITSET_FIXED); |
| 415 V1 = bitset_create (nsyms, BITSET_FIXED); |
| 416 |
| 417 useless_nonterminals (); |
| 418 inaccessable_symbols (); |
| 419 |
| 420 reduced = (nuseless_nonterminals + nuseless_productions > 0); |
| 421 if (!reduced) |
| 422 return; |
| 423 |
| 424 reduce_print (); |
| 425 |
| 426 if (!bitset_test (N, accept->number - ntokens)) |
| 427 fatal_at (startsymbol_location, |
| 428 _("start symbol %s does not derive any sentence"), |
| 429 startsymbol->tag); |
| 430 |
| 431 /* First reduce the nonterminals, as they renumber themselves in the |
| 432 whole grammar. If you change the order, nonterms would be |
| 433 renumbered only in the reduced grammar. */ |
| 434 if (nuseless_nonterminals > 0) |
| 435 nonterminals_reduce (); |
| 436 if (nuseless_productions > 0) |
| 437 reduce_grammar_tables (); |
| 438 |
| 439 if (trace_flag & trace_grammar) |
| 440 { |
| 441 grammar_dump (stderr, "Reduced Grammar"); |
| 442 |
| 443 fprintf (stderr, "reduced %s defines %d terminals, %d nonterminals\ |
| 444 , and %d productions.\n", |
| 445 grammar_file, ntokens, nvars, nrules); |
| 446 } |
| 447 } |
| 448 |
| 449 bool |
| 450 reduce_token_unused_in_grammar (symbol_number i) |
| 451 { |
| 452 aver (i < ntokens); |
| 453 return !bitset_test (V, i) && !bitset_test (V1, i); |
| 454 } |
| 455 |
| 456 bool |
| 457 reduce_nonterminal_useless_in_grammar (symbol_number i) |
| 458 { |
| 459 aver (ntokens <= i && i < nsyms + nuseless_nonterminals); |
| 460 return nsyms <= i; |
| 461 } |
| 462 |
| 463 /*-----------------------------------------------------------. |
| 464 | Free the global sets used to compute the reduced grammar. | |
| 465 `-----------------------------------------------------------*/ |
| 466 |
| 467 void |
| 468 reduce_free (void) |
| 469 { |
| 470 bitset_free (N); |
| 471 bitset_free (V); |
| 472 bitset_free (V1); |
| 473 bitset_free (P); |
| 474 } |
OLD | NEW |