1 /* 2 * Copyright (C) 2015 The Android Open Source Project 3 * 4 * Licensed under the Apache License, Version 2.0 (the "License"); 5 * you may not use this file except in compliance with the License. 6 * You may obtain a copy of the License at 7 * 8 * http://www.apache.org/licenses/LICENSE-2.0 9 * 10 * Unless required by applicable law or agreed to in writing, software 11 * distributed under the License is distributed on an "AS IS" BASIS, 12 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 13 * See the License for the specific language governing permissions and 14 * limitations under the License. 15 */ 16 17 #include "load_store_elimination.h" 18 19 #include "base/array_ref.h" 20 #include "base/scoped_arena_allocator.h" 21 #include "base/scoped_arena_containers.h" 22 #include "escape.h" 23 #include "load_store_analysis.h" 24 #include "side_effects_analysis.h" 25 26 #include <iostream> 27 28 /** 29 * The general algorithm of load-store elimination (LSE). 30 * Load-store analysis in the previous pass collects a list of heap locations 31 * and does alias analysis of those heap locations. 32 * LSE keeps track of a list of heap values corresponding to the heap 33 * locations. It visits basic blocks in reverse post order and for 34 * each basic block, visits instructions sequentially, and processes 35 * instructions as follows: 36 * - If the instruction is a load, and the heap location for that load has a 37 * valid heap value, the load can be eliminated. In order to maintain the 38 * validity of all heap locations during the optimization phase, the real 39 * elimination is delayed till the end of LSE. 40 * - If the instruction is a store, it updates the heap value for the heap 41 * location of the store with the store instruction. The real heap value 42 * can be fetched from the store instruction. Heap values are invalidated 43 * for heap locations that may alias with the store instruction's heap 44 * location. The store instruction can be eliminated unless the value stored 45 * is later needed e.g. by a load from the same/aliased heap location or 46 * the heap location persists at method return/deoptimization. 47 * The store instruction is also needed if it's not used to track the heap 48 * value anymore, e.g. when it fails to merge with the heap values from other 49 * predecessors. 50 * - A store that stores the same value as the heap value is eliminated. 51 * - The list of heap values are merged at basic block entry from the basic 52 * block's predecessors. The algorithm is single-pass, so loop side-effects is 53 * used as best effort to decide if a heap location is stored inside the loop. 54 * - A special type of objects called singletons are instantiated in the method 55 * and have a single name, i.e. no aliases. Singletons have exclusive heap 56 * locations since they have no aliases. Singletons are helpful in narrowing 57 * down the life span of a heap location such that they do not always 58 * need to participate in merging heap values. Allocation of a singleton 59 * can be eliminated if that singleton is not used and does not persist 60 * at method return/deoptimization. 61 * - For newly instantiated instances, their heap values are initialized to 62 * language defined default values. 63 * - Some instructions such as invokes are treated as loading and invalidating 64 * all the heap values, depending on the instruction's side effects. 65 * - Finalizable objects are considered as persisting at method 66 * return/deoptimization. 67 * - Currently this LSE algorithm doesn't handle SIMD graph, e.g. with VecLoad 68 * and VecStore instructions. 69 * - Currently this LSE algorithm doesn't handle graph with try-catch, due to 70 * the special block merging structure. 71 */ 72 73 namespace art { 74 75 // An unknown heap value. Loads with such a value in the heap location cannot be eliminated. 76 // A heap location can be set to kUnknownHeapValue when: 77 // - initially set a value. 78 // - killed due to aliasing, merging, invocation, or loop side effects. 79 static HInstruction* const kUnknownHeapValue = 80 reinterpret_cast<HInstruction*>(static_cast<uintptr_t>(-1)); 81 82 // Default heap value after an allocation. 83 // A heap location can be set to that value right after an allocation. 84 static HInstruction* const kDefaultHeapValue = 85 reinterpret_cast<HInstruction*>(static_cast<uintptr_t>(-2)); 86 87 // Use HGraphDelegateVisitor for which all VisitInvokeXXX() delegate to VisitInvoke(). 88 class LSEVisitor : public HGraphDelegateVisitor { 89 public: 90 LSEVisitor(HGraph* graph, 91 const HeapLocationCollector& heap_locations_collector, 92 const SideEffectsAnalysis& side_effects, 93 OptimizingCompilerStats* stats) 94 : HGraphDelegateVisitor(graph, stats), 95 heap_location_collector_(heap_locations_collector), 96 side_effects_(side_effects), 97 allocator_(graph->GetArenaStack()), 98 heap_values_for_(graph->GetBlocks().size(), 99 ScopedArenaVector<HInstruction*>(heap_locations_collector. 100 GetNumberOfHeapLocations(), 101 kUnknownHeapValue, 102 allocator_.Adapter(kArenaAllocLSE)), 103 allocator_.Adapter(kArenaAllocLSE)), 104 removed_loads_(allocator_.Adapter(kArenaAllocLSE)), 105 substitute_instructions_for_loads_(allocator_.Adapter(kArenaAllocLSE)), 106 possibly_removed_stores_(allocator_.Adapter(kArenaAllocLSE)), 107 singleton_new_instances_(allocator_.Adapter(kArenaAllocLSE)) { 108 } 109 110 void VisitBasicBlock(HBasicBlock* block) override { 111 // Populate the heap_values array for this block. 112 // TODO: try to reuse the heap_values array from one predecessor if possible. 113 if (block->IsLoopHeader()) { 114 HandleLoopSideEffects(block); 115 } else { 116 MergePredecessorValues(block); 117 } 118 HGraphVisitor::VisitBasicBlock(block); 119 } 120 121 HTypeConversion* AddTypeConversionIfNecessary(HInstruction* instruction, 122 HInstruction* value, 123 DataType::Type expected_type) { 124 HTypeConversion* type_conversion = nullptr; 125 // Should never add type conversion into boolean value. 126 if (expected_type != DataType::Type::kBool && 127 !DataType::IsTypeConversionImplicit(value->GetType(), expected_type)) { 128 type_conversion = new (GetGraph()->GetAllocator()) HTypeConversion( 129 expected_type, value, instruction->GetDexPc()); 130 instruction->GetBlock()->InsertInstructionBefore(type_conversion, instruction); 131 } 132 return type_conversion; 133 } 134 135 // Find an instruction's substitute if it's a removed load. 136 // Return the same instruction if it should not be removed. 137 HInstruction* FindSubstitute(HInstruction* instruction) { 138 if (!IsLoad(instruction)) { 139 return instruction; 140 } 141 size_t size = removed_loads_.size(); 142 for (size_t i = 0; i < size; i++) { 143 if (removed_loads_[i] == instruction) { 144 HInstruction* substitute = substitute_instructions_for_loads_[i]; 145 // The substitute list is a flat hierarchy. 146 DCHECK_EQ(FindSubstitute(substitute), substitute); 147 return substitute; 148 } 149 } 150 return instruction; 151 } 152 153 void AddRemovedLoad(HInstruction* load, HInstruction* heap_value) { 154 DCHECK(IsLoad(load)); 155 DCHECK_EQ(FindSubstitute(heap_value), heap_value) << 156 "Unexpected heap_value that has a substitute " << heap_value->DebugName(); 157 removed_loads_.push_back(load); 158 substitute_instructions_for_loads_.push_back(heap_value); 159 } 160 161 // Scan the list of removed loads to see if we can reuse `type_conversion`, if 162 // the other removed load has the same substitute and type and is dominated 163 // by `type_conversion`. 164 void TryToReuseTypeConversion(HInstruction* type_conversion, size_t index) { 165 size_t size = removed_loads_.size(); 166 HInstruction* load = removed_loads_[index]; 167 HInstruction* substitute = substitute_instructions_for_loads_[index]; 168 for (size_t j = index + 1; j < size; j++) { 169 HInstruction* load2 = removed_loads_[j]; 170 HInstruction* substitute2 = substitute_instructions_for_loads_[j]; 171 if (load2 == nullptr) { 172 DCHECK(substitute2->IsTypeConversion()); 173 continue; 174 } 175 DCHECK(load2->IsInstanceFieldGet() || 176 load2->IsStaticFieldGet() || 177 load2->IsArrayGet()); 178 DCHECK(substitute2 != nullptr); 179 if (substitute2 == substitute && 180 load2->GetType() == load->GetType() && 181 type_conversion->GetBlock()->Dominates(load2->GetBlock()) && 182 // Don't share across irreducible loop headers. 183 // TODO: can be more fine-grained than this by testing each dominator. 184 (load2->GetBlock() == type_conversion->GetBlock() || 185 !GetGraph()->HasIrreducibleLoops())) { 186 // The removed_loads_ are added in reverse post order. 187 DCHECK(type_conversion->StrictlyDominates(load2)); 188 load2->ReplaceWith(type_conversion); 189 load2->GetBlock()->RemoveInstruction(load2); 190 removed_loads_[j] = nullptr; 191 substitute_instructions_for_loads_[j] = type_conversion; 192 } 193 } 194 } 195 196 // Remove recorded instructions that should be eliminated. 197 void RemoveInstructions() { 198 size_t size = removed_loads_.size(); 199 DCHECK_EQ(size, substitute_instructions_for_loads_.size()); 200 for (size_t i = 0; i < size; i++) { 201 HInstruction* load = removed_loads_[i]; 202 if (load == nullptr) { 203 // The load has been handled in the scan for type conversion below. 204 DCHECK(substitute_instructions_for_loads_[i]->IsTypeConversion()); 205 continue; 206 } 207 DCHECK(load->IsInstanceFieldGet() || 208 load->IsStaticFieldGet() || 209 load->IsArrayGet()); 210 HInstruction* substitute = substitute_instructions_for_loads_[i]; 211 DCHECK(substitute != nullptr); 212 // We proactively retrieve the substitute for a removed load, so 213 // a load that has a substitute should not be observed as a heap 214 // location value. 215 DCHECK_EQ(FindSubstitute(substitute), substitute); 216 217 // The load expects to load the heap value as type load->GetType(). 218 // However the tracked heap value may not be of that type. An explicit 219 // type conversion may be needed. 220 // There are actually three types involved here: 221 // (1) tracked heap value's type (type A) 222 // (2) heap location (field or element)'s type (type B) 223 // (3) load's type (type C) 224 // We guarantee that type A stored as type B and then fetched out as 225 // type C is the same as casting from type A to type C directly, since 226 // type B and type C will have the same size which is guarenteed in 227 // HInstanceFieldGet/HStaticFieldGet/HArrayGet's SetType(). 228 // So we only need one type conversion from type A to type C. 229 HTypeConversion* type_conversion = AddTypeConversionIfNecessary( 230 load, substitute, load->GetType()); 231 if (type_conversion != nullptr) { 232 TryToReuseTypeConversion(type_conversion, i); 233 load->ReplaceWith(type_conversion); 234 substitute_instructions_for_loads_[i] = type_conversion; 235 } else { 236 load->ReplaceWith(substitute); 237 } 238 load->GetBlock()->RemoveInstruction(load); 239 } 240 241 // At this point, stores in possibly_removed_stores_ can be safely removed. 242 for (HInstruction* store : possibly_removed_stores_) { 243 DCHECK(store->IsInstanceFieldSet() || store->IsStaticFieldSet() || store->IsArraySet()); 244 store->GetBlock()->RemoveInstruction(store); 245 } 246 247 // Eliminate singleton-classified instructions: 248 // * - Constructor fences (they never escape this thread). 249 // * - Allocations (if they are unused). 250 for (HInstruction* new_instance : singleton_new_instances_) { 251 size_t removed = HConstructorFence::RemoveConstructorFences(new_instance); 252 MaybeRecordStat(stats_, 253 MethodCompilationStat::kConstructorFenceRemovedLSE, 254 removed); 255 256 if (!new_instance->HasNonEnvironmentUses()) { 257 new_instance->RemoveEnvironmentUsers(); 258 new_instance->GetBlock()->RemoveInstruction(new_instance); 259 } 260 } 261 } 262 263 private: 264 static bool IsLoad(HInstruction* instruction) { 265 if (instruction == kUnknownHeapValue || instruction == kDefaultHeapValue) { 266 return false; 267 } 268 // Unresolved load is not treated as a load. 269 return instruction->IsInstanceFieldGet() || 270 instruction->IsStaticFieldGet() || 271 instruction->IsArrayGet(); 272 } 273 274 static bool IsStore(HInstruction* instruction) { 275 if (instruction == kUnknownHeapValue || instruction == kDefaultHeapValue) { 276 return false; 277 } 278 // Unresolved store is not treated as a store. 279 return instruction->IsInstanceFieldSet() || 280 instruction->IsArraySet() || 281 instruction->IsStaticFieldSet(); 282 } 283 284 // Returns the real heap value by finding its substitute or by "peeling" 285 // a store instruction. 286 HInstruction* GetRealHeapValue(HInstruction* heap_value) { 287 if (IsLoad(heap_value)) { 288 return FindSubstitute(heap_value); 289 } 290 if (!IsStore(heap_value)) { 291 return heap_value; 292 } 293 294 // We keep track of store instructions as the heap values which might be 295 // eliminated if the stores are later found not necessary. The real stored 296 // value needs to be fetched from the store instruction. 297 if (heap_value->IsInstanceFieldSet()) { 298 heap_value = heap_value->AsInstanceFieldSet()->GetValue(); 299 } else if (heap_value->IsStaticFieldSet()) { 300 heap_value = heap_value->AsStaticFieldSet()->GetValue(); 301 } else { 302 DCHECK(heap_value->IsArraySet()); 303 heap_value = heap_value->AsArraySet()->GetValue(); 304 } 305 // heap_value may already be a removed load. 306 return FindSubstitute(heap_value); 307 } 308 309 // If heap_value is a store, need to keep the store. 310 // This is necessary if a heap value is killed or replaced by another value, 311 // so that the store is no longer used to track heap value. 312 void KeepIfIsStore(HInstruction* heap_value) { 313 if (!IsStore(heap_value)) { 314 return; 315 } 316 auto idx = std::find(possibly_removed_stores_.begin(), 317 possibly_removed_stores_.end(), heap_value); 318 if (idx != possibly_removed_stores_.end()) { 319 // Make sure the store is kept. 320 possibly_removed_stores_.erase(idx); 321 } 322 } 323 324 // If a heap location X may alias with heap location at `loc_index` 325 // and heap_values of that heap location X holds a store, keep that store. 326 // It's needed for a dependent load that's not eliminated since any store 327 // that may put value into the load's heap location needs to be kept. 328 void KeepStoresIfAliasedToLocation(ScopedArenaVector<HInstruction*>& heap_values, 329 size_t loc_index) { 330 for (size_t i = 0; i < heap_values.size(); i++) { 331 if ((i == loc_index) || heap_location_collector_.MayAlias(i, loc_index)) { 332 KeepIfIsStore(heap_values[i]); 333 } 334 } 335 } 336 337 void HandleLoopSideEffects(HBasicBlock* block) { 338 DCHECK(block->IsLoopHeader()); 339 int block_id = block->GetBlockId(); 340 ScopedArenaVector<HInstruction*>& heap_values = heap_values_for_[block_id]; 341 HBasicBlock* pre_header = block->GetLoopInformation()->GetPreHeader(); 342 ScopedArenaVector<HInstruction*>& pre_header_heap_values = 343 heap_values_for_[pre_header->GetBlockId()]; 344 345 // Don't eliminate loads in irreducible loops. 346 // Also keep the stores before the loop. 347 if (block->GetLoopInformation()->IsIrreducible()) { 348 if (kIsDebugBuild) { 349 for (size_t i = 0; i < heap_values.size(); i++) { 350 DCHECK_EQ(heap_values[i], kUnknownHeapValue); 351 } 352 } 353 for (size_t i = 0; i < heap_values.size(); i++) { 354 KeepIfIsStore(pre_header_heap_values[i]); 355 } 356 return; 357 } 358 359 // Inherit the values from pre-header. 360 for (size_t i = 0; i < heap_values.size(); i++) { 361 heap_values[i] = pre_header_heap_values[i]; 362 } 363 364 // We do a single pass in reverse post order. For loops, use the side effects as a hint 365 // to see if the heap values should be killed. 366 if (side_effects_.GetLoopEffects(block).DoesAnyWrite()) { 367 for (size_t i = 0; i < heap_values.size(); i++) { 368 HeapLocation* location = heap_location_collector_.GetHeapLocation(i); 369 ReferenceInfo* ref_info = location->GetReferenceInfo(); 370 if (ref_info->IsSingleton() && !location->IsValueKilledByLoopSideEffects()) { 371 // A singleton's field that's not stored into inside a loop is 372 // invariant throughout the loop. Nothing to do. 373 } else { 374 // heap value is killed by loop side effects. 375 KeepIfIsStore(pre_header_heap_values[i]); 376 heap_values[i] = kUnknownHeapValue; 377 } 378 } 379 } else { 380 // The loop doesn't kill any value. 381 } 382 } 383 384 void MergePredecessorValues(HBasicBlock* block) { 385 ArrayRef<HBasicBlock* const> predecessors(block->GetPredecessors()); 386 if (predecessors.size() == 0) { 387 return; 388 } 389 if (block->IsExitBlock()) { 390 // Exit block doesn't really merge values since the control flow ends in 391 // its predecessors. Each predecessor needs to make sure stores are kept 392 // if necessary. 393 return; 394 } 395 396 ScopedArenaVector<HInstruction*>& heap_values = heap_values_for_[block->GetBlockId()]; 397 for (size_t i = 0; i < heap_values.size(); i++) { 398 HInstruction* merged_value = nullptr; 399 // If we can merge the store itself from the predecessors, we keep 400 // the store as the heap value as long as possible. In case we cannot 401 // merge the store, we try to merge the values of the stores. 402 HInstruction* merged_store_value = nullptr; 403 // Whether merged_value is a result that's merged from all predecessors. 404 bool from_all_predecessors = true; 405 ReferenceInfo* ref_info = heap_location_collector_.GetHeapLocation(i)->GetReferenceInfo(); 406 HInstruction* ref = ref_info->GetReference(); 407 HInstruction* singleton_ref = nullptr; 408 if (ref_info->IsSingleton()) { 409 // We do more analysis based on singleton's liveness when merging 410 // heap values for such cases. 411 singleton_ref = ref; 412 } 413 414 for (HBasicBlock* predecessor : predecessors) { 415 HInstruction* pred_value = heap_values_for_[predecessor->GetBlockId()][i]; 416 if (!IsStore(pred_value)) { 417 pred_value = FindSubstitute(pred_value); 418 } 419 DCHECK(pred_value != nullptr); 420 HInstruction* pred_store_value = GetRealHeapValue(pred_value); 421 if ((singleton_ref != nullptr) && 422 !singleton_ref->GetBlock()->Dominates(predecessor)) { 423 // singleton_ref is not live in this predecessor. No need to merge 424 // since singleton_ref is not live at the beginning of this block. 425 DCHECK_EQ(pred_value, kUnknownHeapValue); 426 from_all_predecessors = false; 427 break; 428 } 429 if (merged_value == nullptr) { 430 // First seen heap value. 431 DCHECK(pred_value != nullptr); 432 merged_value = pred_value; 433 } else if (pred_value != merged_value) { 434 // There are conflicting values. 435 merged_value = kUnknownHeapValue; 436 // We may still be able to merge store values. 437 } 438 439 // Conflicting stores may be storing the same value. We do another merge 440 // of real stored values. 441 if (merged_store_value == nullptr) { 442 // First seen store value. 443 DCHECK(pred_store_value != nullptr); 444 merged_store_value = pred_store_value; 445 } else if (pred_store_value != merged_store_value) { 446 // There are conflicting store values. 447 merged_store_value = kUnknownHeapValue; 448 // There must be conflicting stores also. 449 DCHECK_EQ(merged_value, kUnknownHeapValue); 450 // No need to merge anymore. 451 break; 452 } 453 } 454 455 if (merged_value == nullptr) { 456 DCHECK(!from_all_predecessors); 457 DCHECK(singleton_ref != nullptr); 458 } 459 if (from_all_predecessors) { 460 if (ref_info->IsSingletonAndRemovable() && 461 (block->IsSingleReturnOrReturnVoidAllowingPhis() || 462 (block->EndsWithReturn() && (merged_value != kUnknownHeapValue || 463 merged_store_value != kUnknownHeapValue)))) { 464 // Values in the singleton are not needed anymore: 465 // (1) if this block consists of a sole return, or 466 // (2) if this block returns and a usable merged value is obtained 467 // (loads prior to the return will always use that value). 468 } else if (!IsStore(merged_value)) { 469 // We don't track merged value as a store anymore. We have to 470 // hold the stores in predecessors live here. 471 for (HBasicBlock* predecessor : predecessors) { 472 ScopedArenaVector<HInstruction*>& pred_values = 473 heap_values_for_[predecessor->GetBlockId()]; 474 KeepIfIsStore(pred_values[i]); 475 } 476 } 477 } else { 478 DCHECK(singleton_ref != nullptr); 479 // singleton_ref is non-existing at the beginning of the block. There is 480 // no need to keep the stores. 481 } 482 483 if (!from_all_predecessors) { 484 DCHECK(singleton_ref != nullptr); 485 DCHECK((singleton_ref->GetBlock() == block) || 486 !singleton_ref->GetBlock()->Dominates(block)) 487 << "method: " << GetGraph()->GetMethodName(); 488 // singleton_ref is not defined before block or defined only in some of its 489 // predecessors, so block doesn't really have the location at its entry. 490 heap_values[i] = kUnknownHeapValue; 491 } else if (predecessors.size() == 1) { 492 // Inherit heap value from the single predecessor. 493 DCHECK_EQ(heap_values_for_[predecessors[0]->GetBlockId()][i], merged_value); 494 heap_values[i] = merged_value; 495 } else { 496 DCHECK(merged_value == kUnknownHeapValue || 497 merged_value == kDefaultHeapValue || 498 merged_value->GetBlock()->Dominates(block)); 499 if (merged_value != kUnknownHeapValue) { 500 heap_values[i] = merged_value; 501 } else { 502 // Stores in different predecessors may be storing the same value. 503 heap_values[i] = merged_store_value; 504 } 505 } 506 } 507 } 508 509 // `instruction` is being removed. Try to see if the null check on it 510 // can be removed. This can happen if the same value is set in two branches 511 // but not in dominators. Such as: 512 // int[] a = foo(); 513 // if () { 514 // a[0] = 2; 515 // } else { 516 // a[0] = 2; 517 // } 518 // // a[0] can now be replaced with constant 2, and the null check on it can be removed. 519 void TryRemovingNullCheck(HInstruction* instruction) { 520 HInstruction* prev = instruction->GetPrevious(); 521 if ((prev != nullptr) && prev->IsNullCheck() && (prev == instruction->InputAt(0))) { 522 // Previous instruction is a null check for this instruction. Remove the null check. 523 prev->ReplaceWith(prev->InputAt(0)); 524 prev->GetBlock()->RemoveInstruction(prev); 525 } 526 } 527 528 HInstruction* GetDefaultValue(DataType::Type type) { 529 switch (type) { 530 case DataType::Type::kReference: 531 return GetGraph()->GetNullConstant(); 532 case DataType::Type::kBool: 533 case DataType::Type::kUint8: 534 case DataType::Type::kInt8: 535 case DataType::Type::kUint16: 536 case DataType::Type::kInt16: 537 case DataType::Type::kInt32: 538 return GetGraph()->GetIntConstant(0); 539 case DataType::Type::kInt64: 540 return GetGraph()->GetLongConstant(0); 541 case DataType::Type::kFloat32: 542 return GetGraph()->GetFloatConstant(0); 543 case DataType::Type::kFloat64: 544 return GetGraph()->GetDoubleConstant(0); 545 default: 546 UNREACHABLE(); 547 } 548 } 549 550 void VisitGetLocation(HInstruction* instruction, size_t idx) { 551 DCHECK_NE(idx, HeapLocationCollector::kHeapLocationNotFound); 552 ScopedArenaVector<HInstruction*>& heap_values = 553 heap_values_for_[instruction->GetBlock()->GetBlockId()]; 554 HInstruction* heap_value = heap_values[idx]; 555 if (heap_value == kDefaultHeapValue) { 556 HInstruction* constant = GetDefaultValue(instruction->GetType()); 557 AddRemovedLoad(instruction, constant); 558 heap_values[idx] = constant; 559 return; 560 } 561 heap_value = GetRealHeapValue(heap_value); 562 if (heap_value == kUnknownHeapValue) { 563 // Load isn't eliminated. Put the load as the value into the HeapLocation. 564 // This acts like GVN but with better aliasing analysis. 565 heap_values[idx] = instruction; 566 KeepStoresIfAliasedToLocation(heap_values, idx); 567 } else { 568 // Load is eliminated. 569 AddRemovedLoad(instruction, heap_value); 570 TryRemovingNullCheck(instruction); 571 } 572 } 573 574 bool Equal(HInstruction* heap_value, HInstruction* value) { 575 DCHECK(!IsStore(value)) << value->DebugName(); 576 if (heap_value == kUnknownHeapValue) { 577 // Don't compare kUnknownHeapValue with other values. 578 return false; 579 } 580 if (heap_value == value) { 581 return true; 582 } 583 if (heap_value == kDefaultHeapValue && GetDefaultValue(value->GetType()) == value) { 584 return true; 585 } 586 HInstruction* real_heap_value = GetRealHeapValue(heap_value); 587 if (real_heap_value != heap_value) { 588 return Equal(real_heap_value, value); 589 } 590 return false; 591 } 592 593 void VisitSetLocation(HInstruction* instruction, size_t idx, HInstruction* value) { 594 DCHECK_NE(idx, HeapLocationCollector::kHeapLocationNotFound); 595 DCHECK(!IsStore(value)) << value->DebugName(); 596 // value may already have a substitute. 597 value = FindSubstitute(value); 598 ScopedArenaVector<HInstruction*>& heap_values = 599 heap_values_for_[instruction->GetBlock()->GetBlockId()]; 600 HInstruction* heap_value = heap_values[idx]; 601 bool possibly_redundant = false; 602 603 if (Equal(heap_value, value)) { 604 // Store into the heap location with the same value. 605 // This store can be eliminated right away. 606 instruction->GetBlock()->RemoveInstruction(instruction); 607 return; 608 } else { 609 HLoopInformation* loop_info = instruction->GetBlock()->GetLoopInformation(); 610 if (loop_info == nullptr) { 611 // Store is not in a loop. We try to precisely track the heap value by 612 // the store. 613 possibly_redundant = true; 614 } else if (!loop_info->IsIrreducible()) { 615 // instruction is a store in the loop so the loop must do write. 616 DCHECK(side_effects_.GetLoopEffects(loop_info->GetHeader()).DoesAnyWrite()); 617 ReferenceInfo* ref_info = heap_location_collector_.GetHeapLocation(idx)->GetReferenceInfo(); 618 if (ref_info->IsSingleton() && !loop_info->IsDefinedOutOfTheLoop(ref_info->GetReference())) { 619 // original_ref is created inside the loop. Value stored to it isn't needed at 620 // the loop header. This is true for outer loops also. 621 possibly_redundant = true; 622 } else { 623 // Keep the store since its value may be needed at the loop header. 624 } 625 } else { 626 // Keep the store inside irreducible loops. 627 } 628 } 629 if (possibly_redundant) { 630 possibly_removed_stores_.push_back(instruction); 631 } 632 633 // Put the store as the heap value. If the value is loaded or needed after 634 // return/deoptimization later, this store isn't really redundant. 635 heap_values[idx] = instruction; 636 637 // This store may kill values in other heap locations due to aliasing. 638 for (size_t i = 0; i < heap_values.size(); i++) { 639 if (i == idx) { 640 continue; 641 } 642 if (Equal(heap_values[i], value)) { 643 // Same value should be kept even if aliasing happens. 644 continue; 645 } 646 if (heap_values[i] == kUnknownHeapValue) { 647 // Value is already unknown, no need for aliasing check. 648 continue; 649 } 650 if (heap_location_collector_.MayAlias(i, idx)) { 651 // Kill heap locations that may alias and as a result if the heap value 652 // is a store, the store needs to be kept. 653 KeepIfIsStore(heap_values[i]); 654 heap_values[i] = kUnknownHeapValue; 655 } 656 } 657 } 658 659 void VisitInstanceFieldGet(HInstanceFieldGet* instruction) override { 660 HInstruction* object = instruction->InputAt(0); 661 const FieldInfo& field = instruction->GetFieldInfo(); 662 VisitGetLocation(instruction, heap_location_collector_.GetFieldHeapLocation(object, &field)); 663 } 664 665 void VisitInstanceFieldSet(HInstanceFieldSet* instruction) override { 666 HInstruction* object = instruction->InputAt(0); 667 const FieldInfo& field = instruction->GetFieldInfo(); 668 HInstruction* value = instruction->InputAt(1); 669 size_t idx = heap_location_collector_.GetFieldHeapLocation(object, &field); 670 VisitSetLocation(instruction, idx, value); 671 } 672 673 void VisitStaticFieldGet(HStaticFieldGet* instruction) override { 674 HInstruction* cls = instruction->InputAt(0); 675 const FieldInfo& field = instruction->GetFieldInfo(); 676 VisitGetLocation(instruction, heap_location_collector_.GetFieldHeapLocation(cls, &field)); 677 } 678 679 void VisitStaticFieldSet(HStaticFieldSet* instruction) override { 680 HInstruction* cls = instruction->InputAt(0); 681 const FieldInfo& field = instruction->GetFieldInfo(); 682 size_t idx = heap_location_collector_.GetFieldHeapLocation(cls, &field); 683 VisitSetLocation(instruction, idx, instruction->InputAt(1)); 684 } 685 686 void VisitArrayGet(HArrayGet* instruction) override { 687 VisitGetLocation(instruction, heap_location_collector_.GetArrayHeapLocation(instruction)); 688 } 689 690 void VisitArraySet(HArraySet* instruction) override { 691 size_t idx = heap_location_collector_.GetArrayHeapLocation(instruction); 692 VisitSetLocation(instruction, idx, instruction->InputAt(2)); 693 } 694 695 void VisitDeoptimize(HDeoptimize* instruction) override { 696 const ScopedArenaVector<HInstruction*>& heap_values = 697 heap_values_for_[instruction->GetBlock()->GetBlockId()]; 698 for (HInstruction* heap_value : heap_values) { 699 // A store is kept as the heap value for possibly removed stores. 700 // That value stored is generally observeable after deoptimization, except 701 // for singletons that don't escape after deoptimization. 702 if (IsStore(heap_value)) { 703 if (heap_value->IsStaticFieldSet()) { 704 KeepIfIsStore(heap_value); 705 continue; 706 } 707 HInstruction* reference = heap_value->InputAt(0); 708 if (heap_location_collector_.FindReferenceInfoOf(reference)->IsSingleton()) { 709 if (reference->IsNewInstance() && reference->AsNewInstance()->IsFinalizable()) { 710 // Finalizable objects alway escape. 711 KeepIfIsStore(heap_value); 712 continue; 713 } 714 // Check whether the reference for a store is used by an environment local of 715 // HDeoptimize. If not, the singleton is not observed after 716 // deoptimizion. 717 for (const HUseListNode<HEnvironment*>& use : reference->GetEnvUses()) { 718 HEnvironment* user = use.GetUser(); 719 if (user->GetHolder() == instruction) { 720 // The singleton for the store is visible at this deoptimization 721 // point. Need to keep the store so that the heap value is 722 // seen by the interpreter. 723 KeepIfIsStore(heap_value); 724 } 725 } 726 } else { 727 KeepIfIsStore(heap_value); 728 } 729 } 730 } 731 } 732 733 // Keep necessary stores before exiting a method via return/throw. 734 void HandleExit(HBasicBlock* block) { 735 const ScopedArenaVector<HInstruction*>& heap_values = 736 heap_values_for_[block->GetBlockId()]; 737 for (size_t i = 0; i < heap_values.size(); i++) { 738 HInstruction* heap_value = heap_values[i]; 739 ReferenceInfo* ref_info = heap_location_collector_.GetHeapLocation(i)->GetReferenceInfo(); 740 if (!ref_info->IsSingletonAndRemovable()) { 741 KeepIfIsStore(heap_value); 742 } 743 } 744 } 745 746 void VisitReturn(HReturn* instruction) override { 747 HandleExit(instruction->GetBlock()); 748 } 749 750 void VisitReturnVoid(HReturnVoid* return_void) override { 751 HandleExit(return_void->GetBlock()); 752 } 753 754 void VisitThrow(HThrow* throw_instruction) override { 755 HandleExit(throw_instruction->GetBlock()); 756 } 757 758 void HandleInvoke(HInstruction* instruction) { 759 SideEffects side_effects = instruction->GetSideEffects(); 760 ScopedArenaVector<HInstruction*>& heap_values = 761 heap_values_for_[instruction->GetBlock()->GetBlockId()]; 762 for (size_t i = 0; i < heap_values.size(); i++) { 763 ReferenceInfo* ref_info = heap_location_collector_.GetHeapLocation(i)->GetReferenceInfo(); 764 if (ref_info->IsSingleton()) { 765 // Singleton references cannot be seen by the callee. 766 } else { 767 if (side_effects.DoesAnyRead()) { 768 // Invocation may read the heap value. 769 KeepIfIsStore(heap_values[i]); 770 } 771 if (side_effects.DoesAnyWrite()) { 772 // Keep the store since it's not used to track the heap value anymore. 773 KeepIfIsStore(heap_values[i]); 774 heap_values[i] = kUnknownHeapValue; 775 } 776 } 777 } 778 } 779 780 void VisitInvoke(HInvoke* invoke) override { 781 HandleInvoke(invoke); 782 } 783 784 void VisitClinitCheck(HClinitCheck* clinit) override { 785 HandleInvoke(clinit); 786 } 787 788 void VisitUnresolvedInstanceFieldGet(HUnresolvedInstanceFieldGet* instruction) override { 789 // Conservatively treat it as an invocation. 790 HandleInvoke(instruction); 791 } 792 793 void VisitUnresolvedInstanceFieldSet(HUnresolvedInstanceFieldSet* instruction) override { 794 // Conservatively treat it as an invocation. 795 HandleInvoke(instruction); 796 } 797 798 void VisitUnresolvedStaticFieldGet(HUnresolvedStaticFieldGet* instruction) override { 799 // Conservatively treat it as an invocation. 800 HandleInvoke(instruction); 801 } 802 803 void VisitUnresolvedStaticFieldSet(HUnresolvedStaticFieldSet* instruction) override { 804 // Conservatively treat it as an invocation. 805 HandleInvoke(instruction); 806 } 807 808 void VisitNewInstance(HNewInstance* new_instance) override { 809 ReferenceInfo* ref_info = heap_location_collector_.FindReferenceInfoOf(new_instance); 810 if (ref_info == nullptr) { 811 // new_instance isn't used for field accesses. No need to process it. 812 return; 813 } 814 if (ref_info->IsSingletonAndRemovable() && !new_instance->NeedsChecks()) { 815 DCHECK(!new_instance->IsFinalizable()); 816 // new_instance can potentially be eliminated. 817 singleton_new_instances_.push_back(new_instance); 818 } 819 ScopedArenaVector<HInstruction*>& heap_values = 820 heap_values_for_[new_instance->GetBlock()->GetBlockId()]; 821 for (size_t i = 0; i < heap_values.size(); i++) { 822 HInstruction* ref = 823 heap_location_collector_.GetHeapLocation(i)->GetReferenceInfo()->GetReference(); 824 size_t offset = heap_location_collector_.GetHeapLocation(i)->GetOffset(); 825 if (ref == new_instance && offset >= mirror::kObjectHeaderSize) { 826 // Instance fields except the header fields are set to default heap values. 827 heap_values[i] = kDefaultHeapValue; 828 } 829 } 830 } 831 832 void VisitNewArray(HNewArray* new_array) override { 833 ReferenceInfo* ref_info = heap_location_collector_.FindReferenceInfoOf(new_array); 834 if (ref_info == nullptr) { 835 // new_array isn't used for array accesses. No need to process it. 836 return; 837 } 838 if (ref_info->IsSingletonAndRemovable()) { 839 if (new_array->GetLength()->IsIntConstant() && 840 new_array->GetLength()->AsIntConstant()->GetValue() >= 0) { 841 // new_array can potentially be eliminated. 842 singleton_new_instances_.push_back(new_array); 843 } else { 844 // new_array may throw NegativeArraySizeException. Keep it. 845 } 846 } 847 ScopedArenaVector<HInstruction*>& heap_values = 848 heap_values_for_[new_array->GetBlock()->GetBlockId()]; 849 for (size_t i = 0; i < heap_values.size(); i++) { 850 HeapLocation* location = heap_location_collector_.GetHeapLocation(i); 851 HInstruction* ref = location->GetReferenceInfo()->GetReference(); 852 if (ref == new_array && location->GetIndex() != nullptr) { 853 // Array elements are set to default heap values. 854 heap_values[i] = kDefaultHeapValue; 855 } 856 } 857 } 858 859 const HeapLocationCollector& heap_location_collector_; 860 const SideEffectsAnalysis& side_effects_; 861 862 // Use local allocator for allocating memory. 863 ScopedArenaAllocator allocator_; 864 865 // One array of heap values for each block. 866 ScopedArenaVector<ScopedArenaVector<HInstruction*>> heap_values_for_; 867 868 // We record the instructions that should be eliminated but may be 869 // used by heap locations. They'll be removed in the end. 870 ScopedArenaVector<HInstruction*> removed_loads_; 871 ScopedArenaVector<HInstruction*> substitute_instructions_for_loads_; 872 873 // Stores in this list may be removed from the list later when it's 874 // found that the store cannot be eliminated. 875 ScopedArenaVector<HInstruction*> possibly_removed_stores_; 876 877 ScopedArenaVector<HInstruction*> singleton_new_instances_; 878 879 DISALLOW_COPY_AND_ASSIGN(LSEVisitor); 880 }; 881 882 bool LoadStoreElimination::Run() { 883 if (graph_->IsDebuggable() || graph_->HasTryCatch()) { 884 // Debugger may set heap values or trigger deoptimization of callers. 885 // Try/catch support not implemented yet. 886 // Skip this optimization. 887 return false; 888 } 889 const HeapLocationCollector& heap_location_collector = lsa_.GetHeapLocationCollector(); 890 if (heap_location_collector.GetNumberOfHeapLocations() == 0) { 891 // No HeapLocation information from LSA, skip this optimization. 892 return false; 893 } 894 895 // TODO: analyze VecLoad/VecStore better. 896 if (graph_->HasSIMD()) { 897 return false; 898 } 899 900 LSEVisitor lse_visitor(graph_, heap_location_collector, side_effects_, stats_); 901 for (HBasicBlock* block : graph_->GetReversePostOrder()) { 902 lse_visitor.VisitBasicBlock(block); 903 } 904 lse_visitor.RemoveInstructions(); 905 906 return true; 907 } 908 909 } // namespace art 910