runtime/vm/compiler/write_barrier_elimination.cc - sdk.git - Git at Google

 // Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.

 #include "vm/compiler/backend/flow_graph.h"
 #include "vm/compiler/compiler_pass.h"
 #include "vm/compiler/write_barrier_elimination.h"

 namespace dart {

 #if defined(DEBUG)
 DEFINE_FLAG(bool,
             trace_write_barrier_elimination,
             false,
             "Trace WriteBarrierElimination pass.");
 #endif

 class DefinitionIndexPairTrait {
  public:
   typedef Definition* Key;
   typedef intptr_t Value;
   struct Pair {
     Definition* definition = nullptr;
     intptr_t index = -1;
     Pair() {}
     Pair(Definition* definition, intptr_t index)
         : definition(definition), index(index) {}
   };

   static Key KeyOf(Pair kv) { return kv.definition; }
   static Value ValueOf(Pair kv) { return kv.index; }
   static inline uword Hash(Key key) {
     return Utils::WordHash(reinterpret_cast<intptr_t>(key));
   }
   static inline bool IsKeyEqual(Pair kv, Key key) {
     return kv.definition == key;
   }
 };

 typedef DirectChainedHashMap<DefinitionIndexPairTrait> DefinitionIndexMap;

 // Inter-block write-barrier elimination.
 //
 // This optimization removes write barriers from some store instructions under
 // certain assumptions which the runtime is responsible to sustain.
 //
 // We can skip a write barrier on a StoreInstanceField to a container object X
 // if we know that either:
 //   - X is in new-space, or
 //   - X is in old-space, and:
 //     - X is in the store buffer, and
 //     - X is in the deferred marking stack (if concurrent marking is enabled)
 //
 // The result of an Allocation instruction (Instruction::IsAllocation()) will
 // satisfy one of these requirements immediately after the instruction
 // if WillAllocateNewOrRemembered() is true.
 //
 // Without runtime support, we would have to assume that any instruction which
 // can trigger a new-space scavenge (Instruction::CanTriggerGC()) might promote
 // a new-space temporary into old-space, and we could not skip a store barrier
 // on a write into it afterward.
 //
 // However, many instructions can trigger GC in unlikely cases, like
 // CheckStackOverflow and Box. To avoid interrupting write barrier elimination
 // across these instructions, the runtime ensures that any live temporaries
 // (except arrays) promoted during a scavenge caused by a non-Dart-call
 // instruction (see Instruction::CanCallDart()) will be added to the store
 // buffer. Additionally, if concurrent marking was initiated, the runtime
 // ensures that all live temporaries are also in the deferred marking stack.
 //
 // See also Thread::RememberLiveTemporaries() and
 // Thread::DeferredMarkLiveTemporaries().
 class WriteBarrierElimination : public ValueObject {
  public:
   WriteBarrierElimination(Zone* zone, FlowGraph* flow_graph);

   void Analyze();
   void SaveResults();

  private:
   void IndexDefinitions(Zone* zone);

   bool AnalyzeBlock(BlockEntryInstr* entry);
   void MergePredecessors(BlockEntryInstr* entry);

   void UpdateVectorForBlock(BlockEntryInstr* entry, bool finalize);

   static intptr_t Index(BlockEntryInstr* entry) {
     return entry->postorder_number();
   }

   intptr_t Index(Definition* def) {
     ASSERT(IsUsable(def));
     return definition_indices_.LookupValue(def);
   }

   bool IsUsable(Definition* def) {
     return def->IsPhi() || (def->IsAllocation() &&
                             def->AsAllocation()->WillAllocateNewOrRemembered());
   }

 #if defined(DEBUG)
   static bool SlotEligibleForWBE(const Slot& slot);
 #endif

   FlowGraph* const flow_graph_;
   const GrowableArray<BlockEntryInstr*>* const block_order_;

   // Number of usable definitions in the graph.
   intptr_t definition_count_ = 0;

   // Maps each usable definition to its index in the bitvectors.
   DefinitionIndexMap definition_indices_;

   // Bitvector with all non-Array-allocation instructions set. Used to
   // un-mark Array allocations as usable.
   BitVector* array_allocations_mask_;

   // Bitvectors for each block of which allocations are new or remembered
   // at the start (after Phis).
   GrowableArray<BitVector*> usable_allocs_in_;

   // Bitvectors for each block of which allocations are new or remembered
   // at the end of the block.
   GrowableArray<BitVector*> usable_allocs_out_;

   // Remaining blocks to process.
   GrowableArray<BlockEntryInstr*> worklist_;

   // Temporary used in many functions to avoid repeated zone allocation.
   BitVector* vector_;

   // Bitvector of blocks which have been processed, to ensure each block
   // is processed at least once.
   BitVector* processed_blocks_;

 #if defined(DEBUG)
   bool tracing_ = false;
 #else
   static constexpr bool tracing_ = false;
 #endif
 };

 WriteBarrierElimination::WriteBarrierElimination(Zone* zone,
                                                  FlowGraph* flow_graph)
     : flow_graph_(flow_graph), block_order_(&flow_graph->postorder()) {
 #if defined(DEBUG)
   if (flow_graph->should_print() && FLAG_trace_write_barrier_elimination) {
     tracing_ = true;
   }
 #endif

   IndexDefinitions(zone);

   for (intptr_t i = 0; i < block_order_->length(); ++i) {
     usable_allocs_in_.Add(new (zone) BitVector(zone, definition_count_));
     usable_allocs_in_[i]->CopyFrom(vector_);

     usable_allocs_out_.Add(new (zone) BitVector(zone, definition_count_));
     usable_allocs_out_[i]->CopyFrom(vector_);
   }

   processed_blocks_ = new (zone) BitVector(zone, block_order_->length());
 }

 void WriteBarrierElimination::Analyze() {
   for (intptr_t i = 0; i < block_order_->length(); ++i) {
     worklist_.Add(block_order_->At(i));
   }

   while (!worklist_.is_empty()) {
     auto* const entry = worklist_.RemoveLast();
     if (AnalyzeBlock(entry)) {
       for (intptr_t i = 0; i < entry->last_instruction()->SuccessorCount();
            ++i) {
         if (tracing_) {
           THR_Print("Enqueueing block %" Pd "\n", entry->block_id());
         }
         worklist_.Add(entry->last_instruction()->SuccessorAt(i));
       }
     }
   }
 }

 void WriteBarrierElimination::SaveResults() {
   for (intptr_t i = 0; i < block_order_->length(); ++i) {
     vector_->CopyFrom(usable_allocs_in_[i]);
     UpdateVectorForBlock(block_order_->At(i), /*finalize=*/true);
   }
 }

 void WriteBarrierElimination::IndexDefinitions(Zone* zone) {
   BitmapBuilder array_allocations;

   GrowableArray<Definition*> create_array_worklist;

   for (intptr_t i = 0; i < block_order_->length(); ++i) {
     BlockEntryInstr* const block = block_order_->At(i);
     if (auto join_block = block->AsJoinEntry()) {
       for (PhiIterator it(join_block); !it.Done(); it.Advance()) {
         array_allocations.Set(definition_count_, false);
         definition_indices_.Insert({it.Current(), definition_count_++});
 #if defined(DEBUG)
         if (tracing_) {
           THR_Print("Definition (%" Pd ") has index %" Pd ".\n",
                     it.Current()->ssa_temp_index(), definition_count_ - 1);
         }
 #endif
       }
     }
     for (ForwardInstructionIterator it(block); !it.Done(); it.Advance()) {
       if (Definition* current = it.Current()->AsDefinition()) {
         if (IsUsable(current)) {
           const bool is_create_array = current->IsCreateArray();
           array_allocations.Set(definition_count_, is_create_array);
           definition_indices_.Insert({current, definition_count_++});
           if (is_create_array) {
             create_array_worklist.Add(current);
           }
 #if defined(DEBUG)
           if (tracing_) {
             THR_Print("Definition (%" Pd ") has index %" Pd ".\n",
                       current->ssa_temp_index(), definition_count_ - 1);
           }
 #endif
         }
       }
     }
   }

   while (!create_array_worklist.is_empty()) {
     auto instr = create_array_worklist.RemoveLast();
     for (Value::Iterator it(instr->input_use_list()); !it.Done();
          it.Advance()) {
       if (auto phi_use = it.Current()->instruction()->AsPhi()) {
         const intptr_t index = Index(phi_use);
         if (!array_allocations.Get(index)) {
           array_allocations.Set(index, /*can_be_create_array=*/true);
           create_array_worklist.Add(phi_use);
         }
       }
     }
   }

   vector_ = new (zone) BitVector(zone, definition_count_);
   vector_->SetAll();
   array_allocations_mask_ = new (zone) BitVector(zone, definition_count_);
   for (intptr_t i = 0; i < definition_count_; ++i) {
     if (!array_allocations.Get(i)) array_allocations_mask_->Add(i);
   }
 }

 void WriteBarrierElimination::MergePredecessors(BlockEntryInstr* entry) {
   vector_->Clear();
   for (intptr_t i = 0; i < entry->PredecessorCount(); ++i) {
     BitVector* predecessor_set =
         usable_allocs_out_[Index(entry->PredecessorAt(i))];
     if (i == 0) {
       vector_->AddAll(predecessor_set);
     } else {
       vector_->Intersect(predecessor_set);
     }
   }

   if (JoinEntryInstr* join = entry->AsJoinEntry()) {
     // A Phi is usable if and only if all its inputs are usable.
     for (PhiIterator it(join); !it.Done(); it.Advance()) {
       PhiInstr* phi = it.Current();
       ASSERT(phi->InputCount() == entry->PredecessorCount());
       bool is_usable = true;
       for (intptr_t i = 0; i < phi->InputCount(); ++i) {
         BitVector* const predecessor_set =
             usable_allocs_out_[Index(entry->PredecessorAt(i))];
         Definition* const origin = phi->InputAt(i)->definition();
         if (!IsUsable(origin) || !predecessor_set->Contains(Index(origin))) {
           is_usable = false;
           break;
         }
       }
       vector_->Set(Index(phi), is_usable);
     }

 #if defined(DEBUG)
     if (tracing_) {
       THR_Print("Merge predecessors for %" Pd ".\n", entry->block_id());
       for (PhiIterator it(join); !it.Done(); it.Advance()) {
         PhiInstr* phi = it.Current();
         THR_Print("%" Pd ": %s\n", phi->ssa_temp_index(),
                   vector_->Contains(Index(phi)) ? "true" : "false");
       }
     }
 #endif
   }
 }

 bool WriteBarrierElimination::AnalyzeBlock(BlockEntryInstr* entry) {
   // Recompute the usable allocs in-set.
   MergePredecessors(entry);

   // If the in-set has not changed, there's no work to do.
   BitVector* const in_set = usable_allocs_in_[Index(entry)];
   ASSERT(vector_->SubsetOf(*in_set));  // convergence
   if (vector_->Equals(*in_set) && processed_blocks_->Contains(Index(entry))) {
     if (tracing_) {
       THR_Print("Bailout of block %" Pd ": inputs didn't change.\n",
                 entry->block_id());
     }
     return false;
   } else if (tracing_) {
     THR_Print("Inputs of block %" Pd " changed: ", entry->block_id());
     in_set->Print();
     THR_Print(" -> ");
     vector_->Print();
     THR_Print("\n");
   }

   usable_allocs_in_[Index(entry)]->CopyFrom(vector_);
   UpdateVectorForBlock(entry, /*finalize=*/false);

   processed_blocks_->Add(Index(entry));

   // Successors only need to be updated if the out-set changes.
   if (vector_->Equals(*usable_allocs_out_[Index(entry)])) {
     if (tracing_) {
       THR_Print("Bailout of block %" Pd ": out-set didn't change.\n",
                 entry->block_id());
     }
     return false;
   }

   BitVector* const out_set = usable_allocs_out_[Index(entry)];
   ASSERT(vector_->SubsetOf(*out_set));  // convergence
   out_set->CopyFrom(vector_);
   if (tracing_) {
     THR_Print("Block %" Pd " changed.\n", entry->block_id());
   }
   return true;
 }

 #if defined(DEBUG)
 bool WriteBarrierElimination::SlotEligibleForWBE(const Slot& slot) {
   // We assume that Dart code only stores into Instances, Contexts, and
   // UnhandledExceptions. This assumption is used in
   // RestoreWriteBarrierInvariantVisitor::VisitPointers.

   switch (slot.kind()) {
     case Slot::Kind::kCapturedVariable:  // Context
       return true;
     case Slot::Kind::kDartField:  // Instance
       return true;

 #define FOR_EACH_NATIVE_SLOT(class, underlying_type, field, __, ___)           \
   case Slot::Kind::k##class##_##field:                                         \
     return std::is_base_of<UntaggedInstance, underlying_type>::value ||        \
            std::is_base_of<UntaggedContext, underlying_type>::value ||         \
            std::is_base_of<UntaggedUnhandledException,                         \
                            underlying_type>::value;

       NATIVE_SLOTS_LIST(FOR_EACH_NATIVE_SLOT)
 #undef FOR_EACH_NATIVE_SLOT

     default:
       return false;
   }
 }
 #endif

 void WriteBarrierElimination::UpdateVectorForBlock(BlockEntryInstr* entry,
                                                    bool finalize) {
   for (ForwardInstructionIterator it(entry); !it.Done(); it.Advance()) {
     Instruction* const current = it.Current();

     if (finalize) {
       if (StoreInstanceFieldInstr* instr = current->AsStoreInstanceField()) {
         Definition* const container = instr->instance()->definition();
         if (IsUsable(container) && vector_->Contains(Index(container))) {
           DEBUG_ASSERT(SlotEligibleForWBE(instr->slot()));
           instr->set_emit_store_barrier(kNoStoreBarrier);
         }
       } else if (StoreIndexedInstr* instr = current->AsStoreIndexed()) {
         Definition* const array = instr->array()->definition();
         if (IsUsable(array) && vector_->Contains(Index(array))) {
           instr->set_emit_store_barrier(StoreBarrierType::kNoStoreBarrier);
         }
       }
     }

     if (current->CanCallDart()) {
       vector_->Clear();
     } else if (current->CanTriggerGC()) {
       // Clear array allocations. These are not added to the remembered set
       // by Thread::RememberLiveTemporaries() after a scavenge.
       vector_->Intersect(array_allocations_mask_);
     }

     if (AllocationInstr* const alloc = current->AsAllocation()) {
       if (alloc->WillAllocateNewOrRemembered()) {
         vector_->Add(Index(alloc));
       }
     }
   }
 }

 void EliminateWriteBarriers(FlowGraph* flow_graph) {
   WriteBarrierElimination elimination(Thread::Current()->zone(), flow_graph);
   elimination.Analyze();
   elimination.SaveResults();
 }

 }  // namespace dart
	// Copyright (c) 2020, the Dart project authors. Please see the AUTHORS file
	// for details. All rights reserved. Use of this source code is governed by a
	// BSD-style license that can be found in the LICENSE file.

	#include "vm/compiler/backend/flow_graph.h"
	#include "vm/compiler/compiler_pass.h"
	#include "vm/compiler/write_barrier_elimination.h"

	namespace dart {

	#if defined(DEBUG)
	DEFINE_FLAG(bool,
	trace_write_barrier_elimination,
	false,
	"Trace WriteBarrierElimination pass.");
	#endif

	class DefinitionIndexPairTrait {
	public:
	typedef Definition* Key;
	typedef intptr_t Value;
	struct Pair {
	Definition* definition = nullptr;
	intptr_t index = -1;
	Pair() {}
	Pair(Definition* definition, intptr_t index)
	: definition(definition), index(index) {}
	};

	static Key KeyOf(Pair kv) { return kv.definition; }
	static Value ValueOf(Pair kv) { return kv.index; }
	static inline uword Hash(Key key) {
	return Utils::WordHash(reinterpret_cast<intptr_t>(key));
	}
	static inline bool IsKeyEqual(Pair kv, Key key) {
	return kv.definition == key;
	}
	};

	typedef DirectChainedHashMap<DefinitionIndexPairTrait> DefinitionIndexMap;

	// Inter-block write-barrier elimination.
	//
	// This optimization removes write barriers from some store instructions under
	// certain assumptions which the runtime is responsible to sustain.
	//
	// We can skip a write barrier on a StoreInstanceField to a container object X
	// if we know that either:
	// - X is in new-space, or
	// - X is in old-space, and:
	// - X is in the store buffer, and
	// - X is in the deferred marking stack (if concurrent marking is enabled)
	//
	// The result of an Allocation instruction (Instruction::IsAllocation()) will
	// satisfy one of these requirements immediately after the instruction
	// if WillAllocateNewOrRemembered() is true.
	//
	// Without runtime support, we would have to assume that any instruction which
	// can trigger a new-space scavenge (Instruction::CanTriggerGC()) might promote
	// a new-space temporary into old-space, and we could not skip a store barrier
	// on a write into it afterward.
	//
	// However, many instructions can trigger GC in unlikely cases, like
	// CheckStackOverflow and Box. To avoid interrupting write barrier elimination
	// across these instructions, the runtime ensures that any live temporaries
	// (except arrays) promoted during a scavenge caused by a non-Dart-call
	// instruction (see Instruction::CanCallDart()) will be added to the store
	// buffer. Additionally, if concurrent marking was initiated, the runtime
	// ensures that all live temporaries are also in the deferred marking stack.
	//
	// See also Thread::RememberLiveTemporaries() and
	// Thread::DeferredMarkLiveTemporaries().
	class WriteBarrierElimination : public ValueObject {
	public:
	WriteBarrierElimination(Zone* zone, FlowGraph* flow_graph);

	void Analyze();
	void SaveResults();

	private:
	void IndexDefinitions(Zone* zone);

	bool AnalyzeBlock(BlockEntryInstr* entry);
	void MergePredecessors(BlockEntryInstr* entry);

	void UpdateVectorForBlock(BlockEntryInstr* entry, bool finalize);

	static intptr_t Index(BlockEntryInstr* entry) {
	return entry->postorder_number();
	}

	intptr_t Index(Definition* def) {
	ASSERT(IsUsable(def));
	return definition_indices_.LookupValue(def);
	}

	bool IsUsable(Definition* def) {
	return def->IsPhi() \|\| (def->IsAllocation() &&
	def->AsAllocation()->WillAllocateNewOrRemembered());
	}

	#if defined(DEBUG)
	static bool SlotEligibleForWBE(const Slot& slot);
	#endif

	FlowGraph* const flow_graph_;
	const GrowableArray<BlockEntryInstr> const block_order_;

	// Number of usable definitions in the graph.
	intptr_t definition_count_ = 0;

	// Maps each usable definition to its index in the bitvectors.
	DefinitionIndexMap definition_indices_;

	// Bitvector with all non-Array-allocation instructions set. Used to
	// un-mark Array allocations as usable.
	BitVector* array_allocations_mask_;

	// Bitvectors for each block of which allocations are new or remembered
	// at the start (after Phis).
	GrowableArray<BitVector*> usable_allocs_in_;

	// Bitvectors for each block of which allocations are new or remembered
	// at the end of the block.
	GrowableArray<BitVector*> usable_allocs_out_;

	// Remaining blocks to process.
	GrowableArray<BlockEntryInstr*> worklist_;

	// Temporary used in many functions to avoid repeated zone allocation.
	BitVector* vector_;

	// Bitvector of blocks which have been processed, to ensure each block
	// is processed at least once.
	BitVector* processed_blocks_;

	#if defined(DEBUG)
	bool tracing_ = false;
	#else
	static constexpr bool tracing_ = false;
	#endif
	};

	WriteBarrierElimination::WriteBarrierElimination(Zone* zone,
	FlowGraph* flow_graph)
	: flow_graph_(flow_graph), block_order_(&flow_graph->postorder()) {
	#if defined(DEBUG)
	if (flow_graph->should_print() && FLAG_trace_write_barrier_elimination) {
	tracing_ = true;
	}
	#endif

	IndexDefinitions(zone);

	for (intptr_t i = 0; i < block_order_->length(); ++i) {
	usable_allocs_in_.Add(new (zone) BitVector(zone, definition_count_));
	usable_allocs_in_[i]->CopyFrom(vector_);

	usable_allocs_out_.Add(new (zone) BitVector(zone, definition_count_));
	usable_allocs_out_[i]->CopyFrom(vector_);
	}

	processed_blocks_ = new (zone) BitVector(zone, block_order_->length());
	}

	void WriteBarrierElimination::Analyze() {
	for (intptr_t i = 0; i < block_order_->length(); ++i) {
	worklist_.Add(block_order_->At(i));
	}

	while (!worklist_.is_empty()) {
	auto* const entry = worklist_.RemoveLast();
	if (AnalyzeBlock(entry)) {
	for (intptr_t i = 0; i < entry->last_instruction()->SuccessorCount();
	++i) {
	if (tracing_) {
	THR_Print("Enqueueing block %" Pd "\n", entry->block_id());
	}
	worklist_.Add(entry->last_instruction()->SuccessorAt(i));
	}
	}
	}
	}

	void WriteBarrierElimination::SaveResults() {
	for (intptr_t i = 0; i < block_order_->length(); ++i) {
	vector_->CopyFrom(usable_allocs_in_[i]);
	UpdateVectorForBlock(block_order_->At(i), /finalize=/true);
	}
	}

	void WriteBarrierElimination::IndexDefinitions(Zone* zone) {
	BitmapBuilder array_allocations;

	GrowableArray<Definition*> create_array_worklist;

	for (intptr_t i = 0; i < block_order_->length(); ++i) {
	BlockEntryInstr* const block = block_order_->At(i);
	if (auto join_block = block->AsJoinEntry()) {
	for (PhiIterator it(join_block); !it.Done(); it.Advance()) {
	array_allocations.Set(definition_count_, false);
	definition_indices_.Insert({it.Current(), definition_count_++});
	#if defined(DEBUG)
	if (tracing_) {
	THR_Print("Definition (%" Pd ") has index %" Pd ".\n",
	it.Current()->ssa_temp_index(), definition_count_ - 1);
	}
	#endif
	}
	}
	for (ForwardInstructionIterator it(block); !it.Done(); it.Advance()) {
	if (Definition* current = it.Current()->AsDefinition()) {
	if (IsUsable(current)) {
	const bool is_create_array = current->IsCreateArray();
	array_allocations.Set(definition_count_, is_create_array);
	definition_indices_.Insert({current, definition_count_++});
	if (is_create_array) {
	create_array_worklist.Add(current);
	}
	#if defined(DEBUG)
	if (tracing_) {
	THR_Print("Definition (%" Pd ") has index %" Pd ".\n",
	current->ssa_temp_index(), definition_count_ - 1);
	}
	#endif
	}
	}
	}
	}

	while (!create_array_worklist.is_empty()) {
	auto instr = create_array_worklist.RemoveLast();
	for (Value::Iterator it(instr->input_use_list()); !it.Done();
	it.Advance()) {
	if (auto phi_use = it.Current()->instruction()->AsPhi()) {
	const intptr_t index = Index(phi_use);
	if (!array_allocations.Get(index)) {
	array_allocations.Set(index, /can_be_create_array=/true);
	create_array_worklist.Add(phi_use);
	}
	}
	}
	}

	vector_ = new (zone) BitVector(zone, definition_count_);
	vector_->SetAll();
	array_allocations_mask_ = new (zone) BitVector(zone, definition_count_);
	for (intptr_t i = 0; i < definition_count_; ++i) {
	if (!array_allocations.Get(i)) array_allocations_mask_->Add(i);
	}
	}

	void WriteBarrierElimination::MergePredecessors(BlockEntryInstr* entry) {
	vector_->Clear();
	for (intptr_t i = 0; i < entry->PredecessorCount(); ++i) {
	BitVector* predecessor_set =
	usable_allocs_out_[Index(entry->PredecessorAt(i))];
	if (i == 0) {
	vector_->AddAll(predecessor_set);
	} else {
	vector_->Intersect(predecessor_set);
	}
	}

	if (JoinEntryInstr* join = entry->AsJoinEntry()) {
	// A Phi is usable if and only if all its inputs are usable.
	for (PhiIterator it(join); !it.Done(); it.Advance()) {
	PhiInstr* phi = it.Current();
	ASSERT(phi->InputCount() == entry->PredecessorCount());
	bool is_usable = true;
	for (intptr_t i = 0; i < phi->InputCount(); ++i) {
	BitVector* const predecessor_set =
	usable_allocs_out_[Index(entry->PredecessorAt(i))];
	Definition* const origin = phi->InputAt(i)->definition();
	if (!IsUsable(origin) \|\| !predecessor_set->Contains(Index(origin))) {
	is_usable = false;
	break;
	}
	}
	vector_->Set(Index(phi), is_usable);
	}

	#if defined(DEBUG)
	if (tracing_) {
	THR_Print("Merge predecessors for %" Pd ".\n", entry->block_id());
	for (PhiIterator it(join); !it.Done(); it.Advance()) {
	PhiInstr* phi = it.Current();
	THR_Print("%" Pd ": %s\n", phi->ssa_temp_index(),
	vector_->Contains(Index(phi)) ? "true" : "false");
	}
	}
	#endif
	}
	}

	bool WriteBarrierElimination::AnalyzeBlock(BlockEntryInstr* entry) {
	// Recompute the usable allocs in-set.
	MergePredecessors(entry);

	// If the in-set has not changed, there's no work to do.
	BitVector* const in_set = usable_allocs_in_[Index(entry)];
	ASSERT(vector_->SubsetOf(*in_set)); // convergence
	if (vector_->Equals(*in_set) && processed_blocks_->Contains(Index(entry))) {
	if (tracing_) {
	THR_Print("Bailout of block %" Pd ": inputs didn't change.\n",
	entry->block_id());
	}
	return false;
	} else if (tracing_) {
	THR_Print("Inputs of block %" Pd " changed: ", entry->block_id());
	in_set->Print();
	THR_Print(" -> ");
	vector_->Print();
	THR_Print("\n");
	}

	usable_allocs_in_[Index(entry)]->CopyFrom(vector_);
	UpdateVectorForBlock(entry, /finalize=/false);

	processed_blocks_->Add(Index(entry));

	// Successors only need to be updated if the out-set changes.
	if (vector_->Equals(*usable_allocs_out_[Index(entry)])) {
	if (tracing_) {
	THR_Print("Bailout of block %" Pd ": out-set didn't change.\n",
	entry->block_id());
	}
	return false;
	}

	BitVector* const out_set = usable_allocs_out_[Index(entry)];
	ASSERT(vector_->SubsetOf(*out_set)); // convergence
	out_set->CopyFrom(vector_);
	if (tracing_) {
	THR_Print("Block %" Pd " changed.\n", entry->block_id());
	}
	return true;
	}

	#if defined(DEBUG)
	bool WriteBarrierElimination::SlotEligibleForWBE(const Slot& slot) {
	// We assume that Dart code only stores into Instances, Contexts, and
	// UnhandledExceptions. This assumption is used in
	// RestoreWriteBarrierInvariantVisitor::VisitPointers.

	switch (slot.kind()) {
	case Slot::Kind::kCapturedVariable: // Context
	return true;
	case Slot::Kind::kDartField: // Instance
	return true;

	#define FOR_EACH_NATIVE_SLOT(class, underlying_type, field, __, ___) \
	case Slot::Kind::k##class##_##field: \
	return std::is_base_of<UntaggedInstance, underlying_type>::value \|\| \
	std::is_base_of<UntaggedContext, underlying_type>::value \|\| \
	std::is_base_of<UntaggedUnhandledException, \
	underlying_type>::value;

	NATIVE_SLOTS_LIST(FOR_EACH_NATIVE_SLOT)
	#undef FOR_EACH_NATIVE_SLOT

	default:
	return false;
	}
	}
	#endif

	void WriteBarrierElimination::UpdateVectorForBlock(BlockEntryInstr* entry,
	bool finalize) {
	for (ForwardInstructionIterator it(entry); !it.Done(); it.Advance()) {
	Instruction* const current = it.Current();

	if (finalize) {
	if (StoreInstanceFieldInstr* instr = current->AsStoreInstanceField()) {
	Definition* const container = instr->instance()->definition();
	if (IsUsable(container) && vector_->Contains(Index(container))) {
	DEBUG_ASSERT(SlotEligibleForWBE(instr->slot()));
	instr->set_emit_store_barrier(kNoStoreBarrier);
	}
	} else if (StoreIndexedInstr* instr = current->AsStoreIndexed()) {
	Definition* const array = instr->array()->definition();
	if (IsUsable(array) && vector_->Contains(Index(array))) {
	instr->set_emit_store_barrier(StoreBarrierType::kNoStoreBarrier);
	}
	}
	}

	if (current->CanCallDart()) {
	vector_->Clear();
	} else if (current->CanTriggerGC()) {
	// Clear array allocations. These are not added to the remembered set
	// by Thread::RememberLiveTemporaries() after a scavenge.
	vector_->Intersect(array_allocations_mask_);
	}

	if (AllocationInstr* const alloc = current->AsAllocation()) {
	if (alloc->WillAllocateNewOrRemembered()) {
	vector_->Add(Index(alloc));
	}
	}
	}
	}

	void EliminateWriteBarriers(FlowGraph* flow_graph) {
	WriteBarrierElimination elimination(Thread::Current()->zone(), flow_graph);
	elimination.Analyze();
	elimination.SaveResults();
	}

	} // namespace dart