compiler/optimizing/nodes.cc - LeafOS-Project/android_art - Gitiles

 /*
  * Copyright (C) 2014 The Android Open Source Project
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
  * You may obtain a copy of the License at
  *
  *      http://www.apache.org/licenses/LICENSE-2.0
  *
  * Unless required by applicable law or agreed to in writing, software
  * distributed under the License is distributed on an "AS IS" BASIS,
  * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */

 #include "nodes.h"
 #include "ssa_builder.h"
 #include "utils/growable_array.h"

 namespace art {

 void HGraph::AddBlock(HBasicBlock* block) {
   block->SetBlockId(blocks_.Size());
   blocks_.Add(block);
 }

 void HGraph::FindBackEdges(ArenaBitVector* visited) {
   ArenaBitVector visiting(arena_, blocks_.Size(), false);
   VisitBlockForBackEdges(entry_block_, visited, &visiting);
 }

 void HGraph::RemoveDeadBlocks(const ArenaBitVector& visited) const {
   for (size_t i = 0; i < blocks_.Size(); ++i) {
     if (!visited.IsBitSet(i)) {
       HBasicBlock* block = blocks_.Get(i);
       for (size_t j = 0; j < block->GetSuccessors().Size(); ++j) {
         block->GetSuccessors().Get(j)->RemovePredecessor(block);
       }
       for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
         block->RemovePhi(it.Current()->AsPhi());
       }
       for (HInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) {
         block->RemoveInstruction(it.Current());
       }
     }
   }
 }

 void HGraph::VisitBlockForBackEdges(HBasicBlock* block,
                                     ArenaBitVector* visited,
                                     ArenaBitVector* visiting) {
   int id = block->GetBlockId();
   if (visited->IsBitSet(id)) return;

   visited->SetBit(id);
   visiting->SetBit(id);
   for (size_t i = 0; i < block->GetSuccessors().Size(); i++) {
     HBasicBlock* successor = block->GetSuccessors().Get(i);
     if (visiting->IsBitSet(successor->GetBlockId())) {
       successor->AddBackEdge(block);
     } else {
       VisitBlockForBackEdges(successor, visited, visiting);
     }
   }
   visiting->ClearBit(id);
 }

 void HGraph::BuildDominatorTree() {
   ArenaBitVector visited(arena_, blocks_.Size(), false);

   // (1) Find the back edges in the graph doing a DFS traversal.
   FindBackEdges(&visited);

   // (2) Remove blocks not visited during the initial DFS.
   //     Step (3) requires dead blocks to be removed from the
   //     predecessors list of live blocks.
   RemoveDeadBlocks(visited);

   // (3) Simplify the CFG now, so that we don't need to recompute
   //     dominators and the reverse post order.
   SimplifyCFG();

   // (4) Compute the immediate dominator of each block. We visit
   //     the successors of a block only when all its forward branches
   //     have been processed.
   GrowableArray<size_t> visits(arena_, blocks_.Size());
   visits.SetSize(blocks_.Size());
   reverse_post_order_.Add(entry_block_);
   for (size_t i = 0; i < entry_block_->GetSuccessors().Size(); i++) {
     VisitBlockForDominatorTree(entry_block_->GetSuccessors().Get(i), entry_block_, &visits);
   }
 }

 HBasicBlock* HGraph::FindCommonDominator(HBasicBlock* first, HBasicBlock* second) const {
   ArenaBitVector visited(arena_, blocks_.Size(), false);
   // Walk the dominator tree of the first block and mark the visited blocks.
   while (first != nullptr) {
     visited.SetBit(first->GetBlockId());
     first = first->GetDominator();
   }
   // Walk the dominator tree of the second block until a marked block is found.
   while (second != nullptr) {
     if (visited.IsBitSet(second->GetBlockId())) {
       return second;
     }
     second = second->GetDominator();
   }
   LOG(ERROR) << "Could not find common dominator";
   return nullptr;
 }

 void HGraph::VisitBlockForDominatorTree(HBasicBlock* block,
                                         HBasicBlock* predecessor,
                                         GrowableArray<size_t>* visits) {
   if (block->GetDominator() == nullptr) {
     block->SetDominator(predecessor);
   } else {
     block->SetDominator(FindCommonDominator(block->GetDominator(), predecessor));
   }

   visits->Increment(block->GetBlockId());
   // Once all the forward edges have been visited, we know the immediate
   // dominator of the block. We can then start visiting its successors.
   if (visits->Get(block->GetBlockId()) ==
       block->GetPredecessors().Size() - block->NumberOfBackEdges()) {
     reverse_post_order_.Add(block);
     for (size_t i = 0; i < block->GetSuccessors().Size(); i++) {
       VisitBlockForDominatorTree(block->GetSuccessors().Get(i), block, visits);
     }
   }
 }

 void HGraph::TransformToSSA() {
   DCHECK(!reverse_post_order_.IsEmpty());
   SsaBuilder ssa_builder(this);
   ssa_builder.BuildSsa();
 }

 void HGraph::SplitCriticalEdge(HBasicBlock* block, HBasicBlock* successor) {
   // Insert a new node between `block` and `successor` to split the
   // critical edge.
   HBasicBlock* new_block = new (arena_) HBasicBlock(this);
   AddBlock(new_block);
   new_block->AddInstruction(new (arena_) HGoto());
   block->ReplaceSuccessor(successor, new_block);
   new_block->AddSuccessor(successor);
   if (successor->IsLoopHeader()) {
     // If we split at a back edge boundary, make the new block the back edge.
     HLoopInformation* info = successor->GetLoopInformation();
     if (info->IsBackEdge(block)) {
       info->RemoveBackEdge(block);
       info->AddBackEdge(new_block);
     }
   }
 }

 void HGraph::SimplifyLoop(HBasicBlock* header) {
   HLoopInformation* info = header->GetLoopInformation();

   // If there are more than one back edge, make them branch to the same block that
   // will become the only back edge. This simplifies finding natural loops in the
   // graph.
   if (info->NumberOfBackEdges() > 1) {
     HBasicBlock* new_back_edge = new (arena_) HBasicBlock(this);
     AddBlock(new_back_edge);
     new_back_edge->AddInstruction(new (arena_) HGoto());
     for (size_t pred = 0, e = info->GetBackEdges().Size(); pred < e; ++pred) {
       HBasicBlock* back_edge = info->GetBackEdges().Get(pred);
       back_edge->ReplaceSuccessor(header, new_back_edge);
     }
     info->ClearBackEdges();
     info->AddBackEdge(new_back_edge);
     new_back_edge->AddSuccessor(header);
   }

   // Make sure the loop has only one pre header. This simplifies SSA building by having
   // to just look at the pre header to know which locals are initialized at entry of the
   // loop.
   size_t number_of_incomings = header->GetPredecessors().Size() - info->NumberOfBackEdges();
   if (number_of_incomings != 1) {
     HBasicBlock* pre_header = new (arena_) HBasicBlock(this);
     AddBlock(pre_header);
     pre_header->AddInstruction(new (arena_) HGoto());

     ArenaBitVector back_edges(arena_, GetBlocks().Size(), false);
     HBasicBlock* back_edge = info->GetBackEdges().Get(0);
     for (size_t pred = 0; pred < header->GetPredecessors().Size(); ++pred) {
       HBasicBlock* predecessor = header->GetPredecessors().Get(pred);
       if (predecessor != back_edge) {
         predecessor->ReplaceSuccessor(header, pre_header);
         pred--;
       }
     }
     pre_header->AddSuccessor(header);
   }
 }

 void HGraph::SimplifyCFG() {
   // Simplify the CFG for future analysis, and code generation:
   // (1): Split critical edges.
   // (2): Simplify loops by having only one back edge, and one preheader.
   for (size_t i = 0; i < blocks_.Size(); ++i) {
     HBasicBlock* block = blocks_.Get(i);
     if (block->GetSuccessors().Size() > 1) {
       for (size_t j = 0; j < block->GetSuccessors().Size(); ++j) {
         HBasicBlock* successor = block->GetSuccessors().Get(j);
         if (successor->GetPredecessors().Size() > 1) {
           SplitCriticalEdge(block, successor);
           --j;
         }
       }
     }
     if (block->IsLoopHeader()) {
       SimplifyLoop(block);
     }
   }
 }

 bool HGraph::FindNaturalLoops() const {
   for (size_t i = 0; i < blocks_.Size(); ++i) {
     HBasicBlock* block = blocks_.Get(i);
     if (block->IsLoopHeader()) {
       HLoopInformation* info = block->GetLoopInformation();
       if (!info->Populate()) {
         // Abort if the loop is non natural. We currently bailout in such cases.
         return false;
       }
     }
   }
   return true;
 }

 void HLoopInformation::PopulateRecursive(HBasicBlock* block) {
   if (blocks_.IsBitSet(block->GetBlockId())) {
     return;
   }

   blocks_.SetBit(block->GetBlockId());
   block->SetInLoop(this);
   for (size_t i = 0, e = block->GetPredecessors().Size(); i < e; ++i) {
     PopulateRecursive(block->GetPredecessors().Get(i));
   }
 }

 bool HLoopInformation::Populate() {
   DCHECK_EQ(GetBackEdges().Size(), 1u);
   HBasicBlock* back_edge = GetBackEdges().Get(0);
   DCHECK(back_edge->GetDominator() != nullptr);
   if (!header_->Dominates(back_edge)) {
     // This loop is not natural. Do not bother going further.
     return false;
   }

   // Populate this loop: starting with the back edge, recursively add predecessors
   // that are not already part of that loop. Set the header as part of the loop
   // to end the recursion.
   // This is a recursive implementation of the algorithm described in
   // "Advanced Compiler Design & Implementation" (Muchnick) p192.
   blocks_.SetBit(header_->GetBlockId());
   PopulateRecursive(back_edge);
   return true;
 }

 HBasicBlock* HLoopInformation::GetPreHeader() const {
   DCHECK_EQ(header_->GetPredecessors().Size(), 2u);
   return header_->GetDominator();
 }

 bool HLoopInformation::Contains(const HBasicBlock& block) const {
   return blocks_.IsBitSet(block.GetBlockId());
 }

 bool HLoopInformation::IsIn(const HLoopInformation& other) const {
   return other.blocks_.IsBitSet(header_->GetBlockId());
 }

 bool HBasicBlock::Dominates(HBasicBlock* other) const {
   // Walk up the dominator tree from `other`, to find out if `this`
   // is an ancestor.
   HBasicBlock* current = other;
   while (current != nullptr) {
     if (current == this) {
       return true;
     }
     current = current->GetDominator();
   }
   return false;
 }

 void HBasicBlock::InsertInstructionBefore(HInstruction* instruction, HInstruction* cursor) {
   DCHECK(cursor->AsPhi() == nullptr);
   DCHECK(instruction->AsPhi() == nullptr);
   DCHECK_EQ(instruction->GetId(), -1);
   DCHECK_NE(cursor->GetId(), -1);
   DCHECK_EQ(cursor->GetBlock(), this);
   DCHECK(!instruction->IsControlFlow());
   instruction->next_ = cursor;
   instruction->previous_ = cursor->previous_;
   cursor->previous_ = instruction;
   if (GetFirstInstruction() == cursor) {
     instructions_.first_instruction_ = instruction;
   } else {
     instruction->previous_->next_ = instruction;
   }
   instruction->SetBlock(this);
   instruction->SetId(GetGraph()->GetNextInstructionId());
 }

 static void Add(HInstructionList* instruction_list,
                 HBasicBlock* block,
                 HInstruction* instruction) {
   DCHECK(instruction->GetBlock() == nullptr);
   DCHECK_EQ(instruction->GetId(), -1);
   instruction->SetBlock(block);
   instruction->SetId(block->GetGraph()->GetNextInstructionId());
   instruction_list->AddInstruction(instruction);
 }

 void HBasicBlock::AddInstruction(HInstruction* instruction) {
   Add(&instructions_, this, instruction);
 }

 void HBasicBlock::AddPhi(HPhi* phi) {
   Add(&phis_, this, phi);
 }

 static void Remove(HInstructionList* instruction_list,
                    HBasicBlock* block,
                    HInstruction* instruction) {
   DCHECK_EQ(block, instruction->GetBlock());
   DCHECK(instruction->GetUses() == nullptr);
   DCHECK(instruction->GetEnvUses() == nullptr);
   instruction->SetBlock(nullptr);
   instruction_list->RemoveInstruction(instruction);

   for (size_t i = 0; i < instruction->InputCount(); i++) {
     instruction->InputAt(i)->RemoveUser(instruction, i);
   }
 }

 void HBasicBlock::RemoveInstruction(HInstruction* instruction) {
   Remove(&instructions_, this, instruction);
 }

 void HBasicBlock::RemovePhi(HPhi* phi) {
   Remove(&phis_, this, phi);
 }

 void HInstruction::RemoveUser(HInstruction* user, size_t input_index) {
   HUseListNode<HInstruction>* previous = nullptr;
   HUseListNode<HInstruction>* current = uses_;
   while (current != nullptr) {
     if (current->GetUser() == user && current->GetIndex() == input_index) {
       if (previous == NULL) {
         uses_ = current->GetTail();
       } else {
         previous->SetTail(current->GetTail());
       }
     }
     previous = current;
     current = current->GetTail();
   }
 }

 void HInstructionList::AddInstruction(HInstruction* instruction) {
   if (first_instruction_ == nullptr) {
     DCHECK(last_instruction_ == nullptr);
     first_instruction_ = last_instruction_ = instruction;
   } else {
     last_instruction_->next_ = instruction;
     instruction->previous_ = last_instruction_;
     last_instruction_ = instruction;
   }
   for (size_t i = 0; i < instruction->InputCount(); i++) {
     instruction->InputAt(i)->AddUseAt(instruction, i);
   }
 }

 void HInstructionList::RemoveInstruction(HInstruction* instruction) {
   if (instruction->previous_ != nullptr) {
     instruction->previous_->next_ = instruction->next_;
   }
   if (instruction->next_ != nullptr) {
     instruction->next_->previous_ = instruction->previous_;
   }
   if (instruction == first_instruction_) {
     first_instruction_ = instruction->next_;
   }
   if (instruction == last_instruction_) {
     last_instruction_ = instruction->previous_;
   }
 }

 void HInstruction::ReplaceWith(HInstruction* other) {
   DCHECK(other != nullptr);
   for (HUseIterator<HInstruction> it(GetUses()); !it.Done(); it.Advance()) {
     HUseListNode<HInstruction>* current = it.Current();
     HInstruction* user = current->GetUser();
     size_t input_index = current->GetIndex();
     user->SetRawInputAt(input_index, other);
     other->AddUseAt(user, input_index);
   }

   for (HUseIterator<HEnvironment> it(GetEnvUses()); !it.Done(); it.Advance()) {
     HUseListNode<HEnvironment>* current = it.Current();
     HEnvironment* user = current->GetUser();
     size_t input_index = current->GetIndex();
     user->SetRawEnvAt(input_index, other);
     other->AddEnvUseAt(user, input_index);
   }

   uses_ = nullptr;
   env_uses_ = nullptr;
 }

 size_t HInstruction::EnvironmentSize() const {
   return HasEnvironment() ? environment_->Size() : 0;
 }

 void HPhi::AddInput(HInstruction* input) {
   DCHECK(input->GetBlock() != nullptr);
   inputs_.Add(input);
   input->AddUseAt(this, inputs_.Size() - 1);
 }

 #define DEFINE_ACCEPT(name)                                                    \
 void H##name::Accept(HGraphVisitor* visitor) {                                 \
   visitor->Visit##name(this);                                                  \
 }

 FOR_EACH_INSTRUCTION(DEFINE_ACCEPT)

 #undef DEFINE_ACCEPT

 void HGraphVisitor::VisitInsertionOrder() {
   const GrowableArray<HBasicBlock*>& blocks = graph_->GetBlocks();
   for (size_t i = 0 ; i < blocks.Size(); i++) {
     VisitBasicBlock(blocks.Get(i));
   }
 }

 void HGraphVisitor::VisitBasicBlock(HBasicBlock* block) {
   for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
     it.Current()->Accept(this);
   }
   for (HInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) {
     it.Current()->Accept(this);
   }
 }


 bool HCondition::NeedsMaterialization() const {
   if (!HasOnlyOneUse()) {
     return true;
   }
   HUseListNode<HInstruction>* uses = GetUses();
   HInstruction* user = uses->GetUser();
   if (!user->IsIf()) {
     return true;
   }

   // TODO: should we allow intervening instructions with no side-effect between this condition
   // and the If instruction?
   if (GetNext() != user) {
     return true;
   }
   return false;
 }

 }  // namespace art
	/*
	* Copyright (C) 2014 The Android Open Source Project
	*
	* Licensed under the Apache License, Version 2.0 (the "License");
	* you may not use this file except in compliance with the License.
	* You may obtain a copy of the License at
	*
	* http://www.apache.org/licenses/LICENSE-2.0
	*
	* Unless required by applicable law or agreed to in writing, software
	* distributed under the License is distributed on an "AS IS" BASIS,
	* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
	* See the License for the specific language governing permissions and
	* limitations under the License.
	*/

	#include "nodes.h"
	#include "ssa_builder.h"
	#include "utils/growable_array.h"

	namespace art {

	void HGraph::AddBlock(HBasicBlock* block) {
	block->SetBlockId(blocks_.Size());
	blocks_.Add(block);
	}

	void HGraph::FindBackEdges(ArenaBitVector* visited) {
	ArenaBitVector visiting(arena_, blocks_.Size(), false);
	VisitBlockForBackEdges(entry_block_, visited, &visiting);
	}

	void HGraph::RemoveDeadBlocks(const ArenaBitVector& visited) const {
	for (size_t i = 0; i < blocks_.Size(); ++i) {
	if (!visited.IsBitSet(i)) {
	HBasicBlock* block = blocks_.Get(i);
	for (size_t j = 0; j < block->GetSuccessors().Size(); ++j) {
	block->GetSuccessors().Get(j)->RemovePredecessor(block);
	}
	for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
	block->RemovePhi(it.Current()->AsPhi());
	}
	for (HInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) {
	block->RemoveInstruction(it.Current());
	}
	}
	}
	}

	void HGraph::VisitBlockForBackEdges(HBasicBlock* block,
	ArenaBitVector* visited,
	ArenaBitVector* visiting) {
	int id = block->GetBlockId();
	if (visited->IsBitSet(id)) return;

	visited->SetBit(id);
	visiting->SetBit(id);
	for (size_t i = 0; i < block->GetSuccessors().Size(); i++) {
	HBasicBlock* successor = block->GetSuccessors().Get(i);
	if (visiting->IsBitSet(successor->GetBlockId())) {
	successor->AddBackEdge(block);
	} else {
	VisitBlockForBackEdges(successor, visited, visiting);
	}
	}
	visiting->ClearBit(id);
	}

	void HGraph::BuildDominatorTree() {
	ArenaBitVector visited(arena_, blocks_.Size(), false);

	// (1) Find the back edges in the graph doing a DFS traversal.
	FindBackEdges(&visited);

	// (2) Remove blocks not visited during the initial DFS.
	// Step (3) requires dead blocks to be removed from the
	// predecessors list of live blocks.
	RemoveDeadBlocks(visited);

	// (3) Simplify the CFG now, so that we don't need to recompute
	// dominators and the reverse post order.
	SimplifyCFG();

	// (4) Compute the immediate dominator of each block. We visit
	// the successors of a block only when all its forward branches
	// have been processed.
	GrowableArray<size_t> visits(arena_, blocks_.Size());
	visits.SetSize(blocks_.Size());
	reverse_post_order_.Add(entry_block_);
	for (size_t i = 0; i < entry_block_->GetSuccessors().Size(); i++) {
	VisitBlockForDominatorTree(entry_block_->GetSuccessors().Get(i), entry_block_, &visits);
	}
	}

	HBasicBlock* HGraph::FindCommonDominator(HBasicBlock* first, HBasicBlock* second) const {
	ArenaBitVector visited(arena_, blocks_.Size(), false);
	// Walk the dominator tree of the first block and mark the visited blocks.
	while (first != nullptr) {
	visited.SetBit(first->GetBlockId());
	first = first->GetDominator();
	}
	// Walk the dominator tree of the second block until a marked block is found.
	while (second != nullptr) {
	if (visited.IsBitSet(second->GetBlockId())) {
	return second;
	}
	second = second->GetDominator();
	}
	LOG(ERROR) << "Could not find common dominator";
	return nullptr;
	}

	void HGraph::VisitBlockForDominatorTree(HBasicBlock* block,
	HBasicBlock* predecessor,
	GrowableArray<size_t>* visits) {
	if (block->GetDominator() == nullptr) {
	block->SetDominator(predecessor);
	} else {
	block->SetDominator(FindCommonDominator(block->GetDominator(), predecessor));
	}

	visits->Increment(block->GetBlockId());
	// Once all the forward edges have been visited, we know the immediate
	// dominator of the block. We can then start visiting its successors.
	if (visits->Get(block->GetBlockId()) ==
	block->GetPredecessors().Size() - block->NumberOfBackEdges()) {
	reverse_post_order_.Add(block);
	for (size_t i = 0; i < block->GetSuccessors().Size(); i++) {
	VisitBlockForDominatorTree(block->GetSuccessors().Get(i), block, visits);
	}
	}
	}

	void HGraph::TransformToSSA() {
	DCHECK(!reverse_post_order_.IsEmpty());
	SsaBuilder ssa_builder(this);
	ssa_builder.BuildSsa();
	}

	void HGraph::SplitCriticalEdge(HBasicBlock* block, HBasicBlock* successor) {
	// Insert a new node between `block` and `successor` to split the
	// critical edge.
	HBasicBlock* new_block = new (arena_) HBasicBlock(this);
	AddBlock(new_block);
	new_block->AddInstruction(new (arena_) HGoto());
	block->ReplaceSuccessor(successor, new_block);
	new_block->AddSuccessor(successor);
	if (successor->IsLoopHeader()) {
	// If we split at a back edge boundary, make the new block the back edge.
	HLoopInformation* info = successor->GetLoopInformation();
	if (info->IsBackEdge(block)) {
	info->RemoveBackEdge(block);
	info->AddBackEdge(new_block);
	}
	}
	}

	void HGraph::SimplifyLoop(HBasicBlock* header) {
	HLoopInformation* info = header->GetLoopInformation();

	// If there are more than one back edge, make them branch to the same block that
	// will become the only back edge. This simplifies finding natural loops in the
	// graph.
	if (info->NumberOfBackEdges() > 1) {
	HBasicBlock* new_back_edge = new (arena_) HBasicBlock(this);
	AddBlock(new_back_edge);
	new_back_edge->AddInstruction(new (arena_) HGoto());
	for (size_t pred = 0, e = info->GetBackEdges().Size(); pred < e; ++pred) {
	HBasicBlock* back_edge = info->GetBackEdges().Get(pred);
	back_edge->ReplaceSuccessor(header, new_back_edge);
	}
	info->ClearBackEdges();
	info->AddBackEdge(new_back_edge);
	new_back_edge->AddSuccessor(header);
	}

	// Make sure the loop has only one pre header. This simplifies SSA building by having
	// to just look at the pre header to know which locals are initialized at entry of the
	// loop.
	size_t number_of_incomings = header->GetPredecessors().Size() - info->NumberOfBackEdges();
	if (number_of_incomings != 1) {
	HBasicBlock* pre_header = new (arena_) HBasicBlock(this);
	AddBlock(pre_header);
	pre_header->AddInstruction(new (arena_) HGoto());

	ArenaBitVector back_edges(arena_, GetBlocks().Size(), false);
	HBasicBlock* back_edge = info->GetBackEdges().Get(0);
	for (size_t pred = 0; pred < header->GetPredecessors().Size(); ++pred) {
	HBasicBlock* predecessor = header->GetPredecessors().Get(pred);
	if (predecessor != back_edge) {
	predecessor->ReplaceSuccessor(header, pre_header);
	pred--;
	}
	}
	pre_header->AddSuccessor(header);
	}
	}

	void HGraph::SimplifyCFG() {
	// Simplify the CFG for future analysis, and code generation:
	// (1): Split critical edges.
	// (2): Simplify loops by having only one back edge, and one preheader.
	for (size_t i = 0; i < blocks_.Size(); ++i) {
	HBasicBlock* block = blocks_.Get(i);
	if (block->GetSuccessors().Size() > 1) {
	for (size_t j = 0; j < block->GetSuccessors().Size(); ++j) {
	HBasicBlock* successor = block->GetSuccessors().Get(j);
	if (successor->GetPredecessors().Size() > 1) {
	SplitCriticalEdge(block, successor);
	--j;
	}
	}
	}
	if (block->IsLoopHeader()) {
	SimplifyLoop(block);
	}
	}
	}

	bool HGraph::FindNaturalLoops() const {
	for (size_t i = 0; i < blocks_.Size(); ++i) {
	HBasicBlock* block = blocks_.Get(i);
	if (block->IsLoopHeader()) {
	HLoopInformation* info = block->GetLoopInformation();
	if (!info->Populate()) {
	// Abort if the loop is non natural. We currently bailout in such cases.
	return false;
	}
	}
	}
	return true;
	}

	void HLoopInformation::PopulateRecursive(HBasicBlock* block) {
	if (blocks_.IsBitSet(block->GetBlockId())) {
	return;
	}

	blocks_.SetBit(block->GetBlockId());
	block->SetInLoop(this);
	for (size_t i = 0, e = block->GetPredecessors().Size(); i < e; ++i) {
	PopulateRecursive(block->GetPredecessors().Get(i));
	}
	}

	bool HLoopInformation::Populate() {
	DCHECK_EQ(GetBackEdges().Size(), 1u);
	HBasicBlock* back_edge = GetBackEdges().Get(0);
	DCHECK(back_edge->GetDominator() != nullptr);
	if (!header_->Dominates(back_edge)) {
	// This loop is not natural. Do not bother going further.
	return false;
	}

	// Populate this loop: starting with the back edge, recursively add predecessors
	// that are not already part of that loop. Set the header as part of the loop
	// to end the recursion.
	// This is a recursive implementation of the algorithm described in
	// "Advanced Compiler Design & Implementation" (Muchnick) p192.
	blocks_.SetBit(header_->GetBlockId());
	PopulateRecursive(back_edge);
	return true;
	}

	HBasicBlock* HLoopInformation::GetPreHeader() const {
	DCHECK_EQ(header_->GetPredecessors().Size(), 2u);
	return header_->GetDominator();
	}

	bool HLoopInformation::Contains(const HBasicBlock& block) const {
	return blocks_.IsBitSet(block.GetBlockId());
	}

	bool HLoopInformation::IsIn(const HLoopInformation& other) const {
	return other.blocks_.IsBitSet(header_->GetBlockId());
	}

	bool HBasicBlock::Dominates(HBasicBlock* other) const {
	// Walk up the dominator tree from `other`, to find out if `this`
	// is an ancestor.
	HBasicBlock* current = other;
	while (current != nullptr) {
	if (current == this) {
	return true;
	}
	current = current->GetDominator();
	}
	return false;
	}

	void HBasicBlock::InsertInstructionBefore(HInstruction* instruction, HInstruction* cursor) {
	DCHECK(cursor->AsPhi() == nullptr);
	DCHECK(instruction->AsPhi() == nullptr);
	DCHECK_EQ(instruction->GetId(), -1);
	DCHECK_NE(cursor->GetId(), -1);
	DCHECK_EQ(cursor->GetBlock(), this);
	DCHECK(!instruction->IsControlFlow());
	instruction->next_ = cursor;
	instruction->previous_ = cursor->previous_;
	cursor->previous_ = instruction;
	if (GetFirstInstruction() == cursor) {
	instructions_.first_instruction_ = instruction;
	} else {
	instruction->previous_->next_ = instruction;
	}
	instruction->SetBlock(this);
	instruction->SetId(GetGraph()->GetNextInstructionId());
	}

	static void Add(HInstructionList* instruction_list,
	HBasicBlock* block,
	HInstruction* instruction) {
	DCHECK(instruction->GetBlock() == nullptr);
	DCHECK_EQ(instruction->GetId(), -1);
	instruction->SetBlock(block);
	instruction->SetId(block->GetGraph()->GetNextInstructionId());
	instruction_list->AddInstruction(instruction);
	}

	void HBasicBlock::AddInstruction(HInstruction* instruction) {
	Add(&instructions_, this, instruction);
	}

	void HBasicBlock::AddPhi(HPhi* phi) {
	Add(&phis_, this, phi);
	}

	static void Remove(HInstructionList* instruction_list,
	HBasicBlock* block,
	HInstruction* instruction) {
	DCHECK_EQ(block, instruction->GetBlock());
	DCHECK(instruction->GetUses() == nullptr);
	DCHECK(instruction->GetEnvUses() == nullptr);
	instruction->SetBlock(nullptr);
	instruction_list->RemoveInstruction(instruction);

	for (size_t i = 0; i < instruction->InputCount(); i++) {
	instruction->InputAt(i)->RemoveUser(instruction, i);
	}
	}

	void HBasicBlock::RemoveInstruction(HInstruction* instruction) {
	Remove(&instructions_, this, instruction);
	}

	void HBasicBlock::RemovePhi(HPhi* phi) {
	Remove(&phis_, this, phi);
	}

	void HInstruction::RemoveUser(HInstruction* user, size_t input_index) {
	HUseListNode<HInstruction>* previous = nullptr;
	HUseListNode<HInstruction>* current = uses_;
	while (current != nullptr) {
	if (current->GetUser() == user && current->GetIndex() == input_index) {
	if (previous == NULL) {
	uses_ = current->GetTail();
	} else {
	previous->SetTail(current->GetTail());
	}
	}
	previous = current;
	current = current->GetTail();
	}
	}

	void HInstructionList::AddInstruction(HInstruction* instruction) {
	if (first_instruction_ == nullptr) {
	DCHECK(last_instruction_ == nullptr);
	first_instruction_ = last_instruction_ = instruction;
	} else {
	last_instruction_->next_ = instruction;
	instruction->previous_ = last_instruction_;
	last_instruction_ = instruction;
	}
	for (size_t i = 0; i < instruction->InputCount(); i++) {
	instruction->InputAt(i)->AddUseAt(instruction, i);
	}
	}

	void HInstructionList::RemoveInstruction(HInstruction* instruction) {
	if (instruction->previous_ != nullptr) {
	instruction->previous_->next_ = instruction->next_;
	}
	if (instruction->next_ != nullptr) {
	instruction->next_->previous_ = instruction->previous_;
	}
	if (instruction == first_instruction_) {
	first_instruction_ = instruction->next_;
	}
	if (instruction == last_instruction_) {
	last_instruction_ = instruction->previous_;
	}
	}

	void HInstruction::ReplaceWith(HInstruction* other) {
	DCHECK(other != nullptr);
	for (HUseIterator<HInstruction> it(GetUses()); !it.Done(); it.Advance()) {
	HUseListNode<HInstruction>* current = it.Current();
	HInstruction* user = current->GetUser();
	size_t input_index = current->GetIndex();
	user->SetRawInputAt(input_index, other);
	other->AddUseAt(user, input_index);
	}

	for (HUseIterator<HEnvironment> it(GetEnvUses()); !it.Done(); it.Advance()) {
	HUseListNode<HEnvironment>* current = it.Current();
	HEnvironment* user = current->GetUser();
	size_t input_index = current->GetIndex();
	user->SetRawEnvAt(input_index, other);
	other->AddEnvUseAt(user, input_index);
	}

	uses_ = nullptr;
	env_uses_ = nullptr;
	}

	size_t HInstruction::EnvironmentSize() const {
	return HasEnvironment() ? environment_->Size() : 0;
	}

	void HPhi::AddInput(HInstruction* input) {
	DCHECK(input->GetBlock() != nullptr);
	inputs_.Add(input);
	input->AddUseAt(this, inputs_.Size() - 1);
	}

	#define DEFINE_ACCEPT(name) \
	void H##name::Accept(HGraphVisitor* visitor) { \
	visitor->Visit##name(this); \
	}

	FOR_EACH_INSTRUCTION(DEFINE_ACCEPT)

	#undef DEFINE_ACCEPT

	void HGraphVisitor::VisitInsertionOrder() {
	const GrowableArray<HBasicBlock*>& blocks = graph_->GetBlocks();
	for (size_t i = 0 ; i < blocks.Size(); i++) {
	VisitBasicBlock(blocks.Get(i));
	}
	}

	void HGraphVisitor::VisitBasicBlock(HBasicBlock* block) {
	for (HInstructionIterator it(block->GetPhis()); !it.Done(); it.Advance()) {
	it.Current()->Accept(this);
	}
	for (HInstructionIterator it(block->GetInstructions()); !it.Done(); it.Advance()) {
	it.Current()->Accept(this);
	}
	}


	bool HCondition::NeedsMaterialization() const {
	if (!HasOnlyOneUse()) {
	return true;
	}
	HUseListNode<HInstruction>* uses = GetUses();
	HInstruction* user = uses->GetUser();
	if (!user->IsIf()) {
	return true;
	}

	// TODO: should we allow intervening instructions with no side-effect between this condition
	// and the If instruction?
	if (GetNext() != user) {
	return true;
	}
	return false;
	}

	} // namespace art