Ben Murdoch | 4a90d5f | 2016-03-22 12:00:34 +0000 | [diff] [blame] | 1 | // Copyright 2013 the V8 project authors. All rights reserved. |
| 2 | // Use of this source code is governed by a BSD-style license that can be |
| 3 | // found in the LICENSE file. |
| 4 | |
| 5 | #include "src/crankshaft/hydrogen-load-elimination.h" |
| 6 | |
| 7 | #include "src/crankshaft/hydrogen-alias-analysis.h" |
| 8 | #include "src/crankshaft/hydrogen-flow-engine.h" |
| 9 | #include "src/crankshaft/hydrogen-instructions.h" |
| 10 | |
| 11 | namespace v8 { |
| 12 | namespace internal { |
| 13 | |
| 14 | #define GLOBAL true |
| 15 | #define TRACE(x) if (FLAG_trace_load_elimination) PrintF x |
| 16 | |
| 17 | static const int kMaxTrackedFields = 16; |
| 18 | static const int kMaxTrackedObjects = 5; |
| 19 | |
| 20 | // An element in the field approximation list. |
| 21 | class HFieldApproximation : public ZoneObject { |
| 22 | public: // Just a data blob. |
| 23 | HValue* object_; |
| 24 | HValue* last_value_; |
| 25 | HFieldApproximation* next_; |
| 26 | |
| 27 | // Recursively copy the entire linked list of field approximations. |
| 28 | HFieldApproximation* Copy(Zone* zone) { |
| 29 | HFieldApproximation* copy = new(zone) HFieldApproximation(); |
| 30 | copy->object_ = this->object_; |
| 31 | copy->last_value_ = this->last_value_; |
| 32 | copy->next_ = this->next_ == NULL ? NULL : this->next_->Copy(zone); |
| 33 | return copy; |
| 34 | } |
| 35 | }; |
| 36 | |
| 37 | |
| 38 | // The main datastructure used during load/store elimination. Each in-object |
| 39 | // field is tracked separately. For each field, store a list of known field |
| 40 | // values for known objects. |
| 41 | class HLoadEliminationTable : public ZoneObject { |
| 42 | public: |
| 43 | HLoadEliminationTable(Zone* zone, HAliasAnalyzer* aliasing) |
| 44 | : zone_(zone), fields_(kMaxTrackedFields, zone), aliasing_(aliasing) { } |
| 45 | |
| 46 | // The main processing of instructions. |
| 47 | HLoadEliminationTable* Process(HInstruction* instr, Zone* zone) { |
| 48 | switch (instr->opcode()) { |
| 49 | case HValue::kLoadNamedField: { |
| 50 | HLoadNamedField* l = HLoadNamedField::cast(instr); |
| 51 | TRACE((" process L%d field %d (o%d)\n", |
| 52 | instr->id(), |
| 53 | FieldOf(l->access()), |
| 54 | l->object()->ActualValue()->id())); |
| 55 | HValue* result = load(l); |
| 56 | if (result != instr && l->CanBeReplacedWith(result)) { |
| 57 | // The load can be replaced with a previous load or a value. |
| 58 | TRACE((" replace L%d -> v%d\n", instr->id(), result->id())); |
| 59 | instr->DeleteAndReplaceWith(result); |
| 60 | } |
| 61 | break; |
| 62 | } |
| 63 | case HValue::kStoreNamedField: { |
| 64 | HStoreNamedField* s = HStoreNamedField::cast(instr); |
| 65 | TRACE((" process S%d field %d (o%d) = v%d\n", |
| 66 | instr->id(), |
| 67 | FieldOf(s->access()), |
| 68 | s->object()->ActualValue()->id(), |
| 69 | s->value()->id())); |
| 70 | HValue* result = store(s); |
| 71 | if (result == NULL) { |
| 72 | // The store is redundant. Remove it. |
| 73 | TRACE((" remove S%d\n", instr->id())); |
| 74 | instr->DeleteAndReplaceWith(NULL); |
| 75 | } |
| 76 | break; |
| 77 | } |
| 78 | case HValue::kTransitionElementsKind: { |
| 79 | HTransitionElementsKind* t = HTransitionElementsKind::cast(instr); |
| 80 | HValue* object = t->object()->ActualValue(); |
| 81 | KillFieldInternal(object, FieldOf(JSArray::kElementsOffset), NULL); |
| 82 | KillFieldInternal(object, FieldOf(JSObject::kMapOffset), NULL); |
| 83 | break; |
| 84 | } |
| 85 | default: { |
| 86 | if (instr->CheckChangesFlag(kInobjectFields)) { |
| 87 | TRACE((" kill-all i%d\n", instr->id())); |
| 88 | Kill(); |
| 89 | break; |
| 90 | } |
| 91 | if (instr->CheckChangesFlag(kMaps)) { |
| 92 | TRACE((" kill-maps i%d\n", instr->id())); |
| 93 | KillOffset(JSObject::kMapOffset); |
| 94 | } |
| 95 | if (instr->CheckChangesFlag(kElementsKind)) { |
| 96 | TRACE((" kill-elements-kind i%d\n", instr->id())); |
| 97 | KillOffset(JSObject::kMapOffset); |
| 98 | KillOffset(JSObject::kElementsOffset); |
| 99 | } |
| 100 | if (instr->CheckChangesFlag(kElementsPointer)) { |
| 101 | TRACE((" kill-elements i%d\n", instr->id())); |
| 102 | KillOffset(JSObject::kElementsOffset); |
| 103 | } |
| 104 | if (instr->CheckChangesFlag(kOsrEntries)) { |
| 105 | TRACE((" kill-osr i%d\n", instr->id())); |
| 106 | Kill(); |
| 107 | } |
| 108 | } |
| 109 | // Improvements possible: |
| 110 | // - learn from HCheckMaps for field 0 |
| 111 | // - remove unobservable stores (write-after-write) |
| 112 | // - track cells |
| 113 | // - track globals |
| 114 | // - track roots |
| 115 | } |
| 116 | return this; |
| 117 | } |
| 118 | |
| 119 | // Support for global analysis with HFlowEngine: Merge given state with |
| 120 | // the other incoming state. |
| 121 | static HLoadEliminationTable* Merge(HLoadEliminationTable* succ_state, |
| 122 | HBasicBlock* succ_block, |
| 123 | HLoadEliminationTable* pred_state, |
| 124 | HBasicBlock* pred_block, |
| 125 | Zone* zone) { |
| 126 | DCHECK(pred_state != NULL); |
| 127 | if (succ_state == NULL) { |
| 128 | return pred_state->Copy(succ_block, pred_block, zone); |
| 129 | } else { |
| 130 | return succ_state->Merge(succ_block, pred_state, pred_block, zone); |
| 131 | } |
| 132 | } |
| 133 | |
| 134 | // Support for global analysis with HFlowEngine: Given state merged with all |
| 135 | // the other incoming states, prepare it for use. |
| 136 | static HLoadEliminationTable* Finish(HLoadEliminationTable* state, |
| 137 | HBasicBlock* block, |
| 138 | Zone* zone) { |
| 139 | DCHECK(state != NULL); |
| 140 | return state; |
| 141 | } |
| 142 | |
| 143 | private: |
| 144 | // Copy state to successor block. |
| 145 | HLoadEliminationTable* Copy(HBasicBlock* succ, HBasicBlock* from_block, |
| 146 | Zone* zone) { |
| 147 | HLoadEliminationTable* copy = |
| 148 | new(zone) HLoadEliminationTable(zone, aliasing_); |
| 149 | copy->EnsureFields(fields_.length()); |
| 150 | for (int i = 0; i < fields_.length(); i++) { |
| 151 | copy->fields_[i] = fields_[i] == NULL ? NULL : fields_[i]->Copy(zone); |
| 152 | } |
| 153 | if (FLAG_trace_load_elimination) { |
| 154 | TRACE((" copy-to B%d\n", succ->block_id())); |
| 155 | copy->Print(); |
| 156 | } |
| 157 | return copy; |
| 158 | } |
| 159 | |
| 160 | // Merge this state with the other incoming state. |
| 161 | HLoadEliminationTable* Merge(HBasicBlock* succ, HLoadEliminationTable* that, |
| 162 | HBasicBlock* that_block, Zone* zone) { |
| 163 | if (that->fields_.length() < fields_.length()) { |
| 164 | // Drop fields not in the other table. |
| 165 | fields_.Rewind(that->fields_.length()); |
| 166 | } |
| 167 | for (int i = 0; i < fields_.length(); i++) { |
| 168 | // Merge the field approximations for like fields. |
| 169 | HFieldApproximation* approx = fields_[i]; |
| 170 | HFieldApproximation* prev = NULL; |
| 171 | while (approx != NULL) { |
| 172 | // TODO(titzer): Merging is O(N * M); sort? |
| 173 | HFieldApproximation* other = that->Find(approx->object_, i); |
| 174 | if (other == NULL || !Equal(approx->last_value_, other->last_value_)) { |
| 175 | // Kill an entry that doesn't agree with the other value. |
| 176 | if (prev != NULL) { |
| 177 | prev->next_ = approx->next_; |
| 178 | } else { |
| 179 | fields_[i] = approx->next_; |
| 180 | } |
| 181 | approx = approx->next_; |
| 182 | continue; |
| 183 | } |
| 184 | prev = approx; |
| 185 | approx = approx->next_; |
| 186 | } |
| 187 | } |
| 188 | if (FLAG_trace_load_elimination) { |
| 189 | TRACE((" merge-to B%d\n", succ->block_id())); |
| 190 | Print(); |
| 191 | } |
| 192 | return this; |
| 193 | } |
| 194 | |
| 195 | friend class HLoadEliminationEffects; // Calls Kill() and others. |
| 196 | friend class HLoadEliminationPhase; |
| 197 | |
| 198 | private: |
| 199 | // Process a load instruction, updating internal table state. If a previous |
| 200 | // load or store for this object and field exists, return the new value with |
| 201 | // which the load should be replaced. Otherwise, return {instr}. |
| 202 | HValue* load(HLoadNamedField* instr) { |
| 203 | // There must be no loads from non observable in-object properties. |
| 204 | DCHECK(!instr->access().IsInobject() || |
| 205 | instr->access().existing_inobject_property()); |
| 206 | |
| 207 | int field = FieldOf(instr->access()); |
| 208 | if (field < 0) return instr; |
| 209 | |
| 210 | HValue* object = instr->object()->ActualValue(); |
| 211 | HFieldApproximation* approx = FindOrCreate(object, field); |
| 212 | |
| 213 | if (approx->last_value_ == NULL) { |
| 214 | // Load is not redundant. Fill out a new entry. |
| 215 | approx->last_value_ = instr; |
| 216 | return instr; |
| 217 | } else if (approx->last_value_->block()->EqualToOrDominates( |
| 218 | instr->block())) { |
| 219 | // Eliminate the load. Reuse previously stored value or load instruction. |
| 220 | return approx->last_value_; |
| 221 | } else { |
| 222 | return instr; |
| 223 | } |
| 224 | } |
| 225 | |
| 226 | // Process a store instruction, updating internal table state. If a previous |
| 227 | // store to the same object and field makes this store redundant (e.g. because |
| 228 | // the stored values are the same), return NULL indicating that this store |
| 229 | // instruction is redundant. Otherwise, return {instr}. |
| 230 | HValue* store(HStoreNamedField* instr) { |
| 231 | if (instr->access().IsInobject() && |
| 232 | !instr->access().existing_inobject_property()) { |
| 233 | TRACE((" skipping non existing property initialization store\n")); |
| 234 | return instr; |
| 235 | } |
| 236 | |
| 237 | int field = FieldOf(instr->access()); |
| 238 | if (field < 0) return KillIfMisaligned(instr); |
| 239 | |
| 240 | HValue* object = instr->object()->ActualValue(); |
| 241 | HValue* value = instr->value(); |
| 242 | |
| 243 | if (instr->has_transition()) { |
| 244 | // A transition introduces a new field and alters the map of the object. |
| 245 | // Since the field in the object is new, it cannot alias existing entries. |
Ben Murdoch | 4a90d5f | 2016-03-22 12:00:34 +0000 | [diff] [blame] | 246 | KillFieldInternal(object, FieldOf(JSObject::kMapOffset), NULL); |
| 247 | } else { |
| 248 | // Kill non-equivalent may-alias entries. |
| 249 | KillFieldInternal(object, field, value); |
| 250 | } |
| 251 | HFieldApproximation* approx = FindOrCreate(object, field); |
| 252 | |
| 253 | if (Equal(approx->last_value_, value)) { |
| 254 | // The store is redundant because the field already has this value. |
| 255 | return NULL; |
| 256 | } else { |
| 257 | // The store is not redundant. Update the entry. |
| 258 | approx->last_value_ = value; |
| 259 | return instr; |
| 260 | } |
| 261 | } |
| 262 | |
| 263 | // Kill everything in this table. |
| 264 | void Kill() { |
| 265 | fields_.Rewind(0); |
| 266 | } |
| 267 | |
| 268 | // Kill all entries matching the given offset. |
| 269 | void KillOffset(int offset) { |
| 270 | int field = FieldOf(offset); |
| 271 | if (field >= 0 && field < fields_.length()) { |
| 272 | fields_[field] = NULL; |
| 273 | } |
| 274 | } |
| 275 | |
| 276 | // Kill all entries aliasing the given store. |
| 277 | void KillStore(HStoreNamedField* s) { |
| 278 | int field = FieldOf(s->access()); |
| 279 | if (field >= 0) { |
| 280 | KillFieldInternal(s->object()->ActualValue(), field, s->value()); |
| 281 | } else { |
| 282 | KillIfMisaligned(s); |
| 283 | } |
| 284 | } |
| 285 | |
| 286 | // Kill multiple entries in the case of a misaligned store. |
| 287 | HValue* KillIfMisaligned(HStoreNamedField* instr) { |
| 288 | HObjectAccess access = instr->access(); |
| 289 | if (access.IsInobject()) { |
| 290 | int offset = access.offset(); |
| 291 | if ((offset % kPointerSize) != 0) { |
| 292 | // Kill the field containing the first word of the access. |
| 293 | HValue* object = instr->object()->ActualValue(); |
| 294 | int field = offset / kPointerSize; |
| 295 | KillFieldInternal(object, field, NULL); |
| 296 | |
| 297 | // Kill the next field in case of overlap. |
| 298 | int size = access.representation().size(); |
| 299 | int next_field = (offset + size - 1) / kPointerSize; |
| 300 | if (next_field != field) KillFieldInternal(object, next_field, NULL); |
| 301 | } |
| 302 | } |
| 303 | return instr; |
| 304 | } |
| 305 | |
| 306 | // Find an entry for the given object and field pair. |
| 307 | HFieldApproximation* Find(HValue* object, int field) { |
| 308 | // Search for a field approximation for this object. |
| 309 | HFieldApproximation* approx = fields_[field]; |
| 310 | while (approx != NULL) { |
| 311 | if (aliasing_->MustAlias(object, approx->object_)) return approx; |
| 312 | approx = approx->next_; |
| 313 | } |
| 314 | return NULL; |
| 315 | } |
| 316 | |
| 317 | // Find or create an entry for the given object and field pair. |
| 318 | HFieldApproximation* FindOrCreate(HValue* object, int field) { |
| 319 | EnsureFields(field + 1); |
| 320 | |
| 321 | // Search for a field approximation for this object. |
| 322 | HFieldApproximation* approx = fields_[field]; |
| 323 | int count = 0; |
| 324 | while (approx != NULL) { |
| 325 | if (aliasing_->MustAlias(object, approx->object_)) return approx; |
| 326 | count++; |
| 327 | approx = approx->next_; |
| 328 | } |
| 329 | |
| 330 | if (count >= kMaxTrackedObjects) { |
| 331 | // Pull the last entry off the end and repurpose it for this object. |
| 332 | approx = ReuseLastApproximation(field); |
| 333 | } else { |
| 334 | // Allocate a new entry. |
| 335 | approx = new(zone_) HFieldApproximation(); |
| 336 | } |
| 337 | |
| 338 | // Insert the entry at the head of the list. |
| 339 | approx->object_ = object; |
| 340 | approx->last_value_ = NULL; |
| 341 | approx->next_ = fields_[field]; |
| 342 | fields_[field] = approx; |
| 343 | |
| 344 | return approx; |
| 345 | } |
| 346 | |
| 347 | // Kill all entries for a given field that _may_ alias the given object |
| 348 | // and do _not_ have the given value. |
| 349 | void KillFieldInternal(HValue* object, int field, HValue* value) { |
| 350 | if (field >= fields_.length()) return; // Nothing to do. |
| 351 | |
| 352 | HFieldApproximation* approx = fields_[field]; |
| 353 | HFieldApproximation* prev = NULL; |
| 354 | while (approx != NULL) { |
| 355 | if (aliasing_->MayAlias(object, approx->object_)) { |
| 356 | if (!Equal(approx->last_value_, value)) { |
| 357 | // Kill an aliasing entry that doesn't agree on the value. |
| 358 | if (prev != NULL) { |
| 359 | prev->next_ = approx->next_; |
| 360 | } else { |
| 361 | fields_[field] = approx->next_; |
| 362 | } |
| 363 | approx = approx->next_; |
| 364 | continue; |
| 365 | } |
| 366 | } |
| 367 | prev = approx; |
| 368 | approx = approx->next_; |
| 369 | } |
| 370 | } |
| 371 | |
| 372 | bool Equal(HValue* a, HValue* b) { |
| 373 | if (a == b) return true; |
| 374 | if (a != NULL && b != NULL && a->CheckFlag(HValue::kUseGVN)) { |
| 375 | return a->Equals(b); |
| 376 | } |
| 377 | return false; |
| 378 | } |
| 379 | |
| 380 | // Remove the last approximation for a field so that it can be reused. |
| 381 | // We reuse the last entry because it was the first inserted and is thus |
| 382 | // farthest away from the current instruction. |
| 383 | HFieldApproximation* ReuseLastApproximation(int field) { |
| 384 | HFieldApproximation* approx = fields_[field]; |
| 385 | DCHECK(approx != NULL); |
| 386 | |
| 387 | HFieldApproximation* prev = NULL; |
| 388 | while (approx->next_ != NULL) { |
| 389 | prev = approx; |
| 390 | approx = approx->next_; |
| 391 | } |
| 392 | if (prev != NULL) prev->next_ = NULL; |
| 393 | return approx; |
| 394 | } |
| 395 | |
| 396 | // Compute the field index for the given object access; -1 if not tracked. |
| 397 | int FieldOf(HObjectAccess access) { |
| 398 | return access.IsInobject() ? FieldOf(access.offset()) : -1; |
| 399 | } |
| 400 | |
| 401 | // Compute the field index for the given in-object offset; -1 if not tracked. |
| 402 | int FieldOf(int offset) { |
| 403 | if (offset >= kMaxTrackedFields * kPointerSize) return -1; |
Ben Murdoch | 4a90d5f | 2016-03-22 12:00:34 +0000 | [diff] [blame] | 404 | if ((offset % kPointerSize) != 0) return -1; // Ignore misaligned accesses. |
| 405 | return offset / kPointerSize; |
| 406 | } |
| 407 | |
| 408 | // Ensure internal storage for the given number of fields. |
| 409 | void EnsureFields(int num_fields) { |
| 410 | if (fields_.length() < num_fields) { |
| 411 | fields_.AddBlock(NULL, num_fields - fields_.length(), zone_); |
| 412 | } |
| 413 | } |
| 414 | |
| 415 | // Print this table to stdout. |
| 416 | void Print() { |
| 417 | for (int i = 0; i < fields_.length(); i++) { |
| 418 | PrintF(" field %d: ", i); |
| 419 | for (HFieldApproximation* a = fields_[i]; a != NULL; a = a->next_) { |
| 420 | PrintF("[o%d =", a->object_->id()); |
| 421 | if (a->last_value_ != NULL) PrintF(" v%d", a->last_value_->id()); |
| 422 | PrintF("] "); |
| 423 | } |
| 424 | PrintF("\n"); |
| 425 | } |
| 426 | } |
| 427 | |
| 428 | Zone* zone_; |
| 429 | ZoneList<HFieldApproximation*> fields_; |
| 430 | HAliasAnalyzer* aliasing_; |
| 431 | }; |
| 432 | |
| 433 | |
| 434 | // Support for HFlowEngine: collect store effects within loops. |
| 435 | class HLoadEliminationEffects : public ZoneObject { |
| 436 | public: |
| 437 | explicit HLoadEliminationEffects(Zone* zone) |
| 438 | : zone_(zone), stores_(5, zone) { } |
| 439 | |
| 440 | inline bool Disabled() { |
| 441 | return false; // Effects are _not_ disabled. |
| 442 | } |
| 443 | |
| 444 | // Process a possibly side-effecting instruction. |
| 445 | void Process(HInstruction* instr, Zone* zone) { |
| 446 | if (instr->IsStoreNamedField()) { |
| 447 | stores_.Add(HStoreNamedField::cast(instr), zone_); |
| 448 | } else { |
| 449 | flags_.Add(instr->ChangesFlags()); |
| 450 | } |
| 451 | } |
| 452 | |
| 453 | // Apply these effects to the given load elimination table. |
| 454 | void Apply(HLoadEliminationTable* table) { |
| 455 | // Loads must not be hoisted past the OSR entry, therefore we kill |
| 456 | // everything if we see an OSR entry. |
| 457 | if (flags_.Contains(kInobjectFields) || flags_.Contains(kOsrEntries)) { |
| 458 | table->Kill(); |
| 459 | return; |
| 460 | } |
| 461 | if (flags_.Contains(kElementsKind) || flags_.Contains(kMaps)) { |
| 462 | table->KillOffset(JSObject::kMapOffset); |
| 463 | } |
| 464 | if (flags_.Contains(kElementsKind) || flags_.Contains(kElementsPointer)) { |
| 465 | table->KillOffset(JSObject::kElementsOffset); |
| 466 | } |
| 467 | |
| 468 | // Kill non-agreeing fields for each store contained in these effects. |
| 469 | for (int i = 0; i < stores_.length(); i++) { |
| 470 | table->KillStore(stores_[i]); |
| 471 | } |
| 472 | } |
| 473 | |
| 474 | // Union these effects with the other effects. |
| 475 | void Union(HLoadEliminationEffects* that, Zone* zone) { |
| 476 | flags_.Add(that->flags_); |
| 477 | for (int i = 0; i < that->stores_.length(); i++) { |
| 478 | stores_.Add(that->stores_[i], zone); |
| 479 | } |
| 480 | } |
| 481 | |
| 482 | private: |
| 483 | Zone* zone_; |
| 484 | GVNFlagSet flags_; |
| 485 | ZoneList<HStoreNamedField*> stores_; |
| 486 | }; |
| 487 | |
| 488 | |
| 489 | // The main routine of the analysis phase. Use the HFlowEngine for either a |
| 490 | // local or a global analysis. |
| 491 | void HLoadEliminationPhase::Run() { |
| 492 | HFlowEngine<HLoadEliminationTable, HLoadEliminationEffects> |
| 493 | engine(graph(), zone()); |
| 494 | HAliasAnalyzer aliasing; |
| 495 | HLoadEliminationTable* table = |
| 496 | new(zone()) HLoadEliminationTable(zone(), &aliasing); |
| 497 | |
| 498 | if (GLOBAL) { |
| 499 | // Perform a global analysis. |
| 500 | engine.AnalyzeDominatedBlocks(graph()->blocks()->at(0), table); |
| 501 | } else { |
| 502 | // Perform only local analysis. |
| 503 | for (int i = 0; i < graph()->blocks()->length(); i++) { |
| 504 | table->Kill(); |
| 505 | engine.AnalyzeOneBlock(graph()->blocks()->at(i), table); |
| 506 | } |
| 507 | } |
| 508 | } |
| 509 | |
| 510 | } // namespace internal |
| 511 | } // namespace v8 |