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

 /*
  * Copyright (C) 2016 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 "block_builder.h"

 #include "base/logging.h"  // FOR VLOG.
 #include "bytecode_utils.h"
 #include "code_item_accessors-inl.h"
 #include "dex_file_exception_helpers.h"
 #include "quicken_info.h"

 namespace art {

 HBasicBlockBuilder::HBasicBlockBuilder(HGraph* graph,
                                        const DexFile* const dex_file,
                                        const CodeItemDebugInfoAccessor& accessor,
                                        ScopedArenaAllocator* local_allocator)
     : allocator_(graph->GetAllocator()),
       graph_(graph),
       dex_file_(dex_file),
       code_item_accessor_(accessor),
       local_allocator_(local_allocator),
       branch_targets_(code_item_accessor_.HasCodeItem()
                           ? code_item_accessor_.InsnsSizeInCodeUnits()
                           : /* fake dex_pc=0 for intrinsic graph */ 1u,
                       nullptr,
                       local_allocator->Adapter(kArenaAllocGraphBuilder)),
       throwing_blocks_(kDefaultNumberOfThrowingBlocks,
                        local_allocator->Adapter(kArenaAllocGraphBuilder)),
       number_of_branches_(0u),
       quicken_index_for_dex_pc_(std::less<uint32_t>(),
                                 local_allocator->Adapter(kArenaAllocGraphBuilder)) {}

 HBasicBlock* HBasicBlockBuilder::MaybeCreateBlockAt(uint32_t dex_pc) {
   return MaybeCreateBlockAt(dex_pc, dex_pc);
 }

 HBasicBlock* HBasicBlockBuilder::MaybeCreateBlockAt(uint32_t semantic_dex_pc,
                                                     uint32_t store_dex_pc) {
   HBasicBlock* block = branch_targets_[store_dex_pc];
   if (block == nullptr) {
     block = new (allocator_) HBasicBlock(graph_, semantic_dex_pc);
     branch_targets_[store_dex_pc] = block;
   }
   DCHECK_EQ(block->GetDexPc(), semantic_dex_pc);
   return block;
 }

 bool HBasicBlockBuilder::CreateBranchTargets() {
   // Create the first block for the dex instructions, single successor of the entry block.
   MaybeCreateBlockAt(0u);

   if (code_item_accessor_.TriesSize() != 0) {
     // Create branch targets at the start/end of the TryItem range. These are
     // places where the program might fall through into/out of the a block and
     // where TryBoundary instructions will be inserted later. Other edges which
     // enter/exit the try blocks are a result of branches/switches.
     for (const DexFile::TryItem& try_item : code_item_accessor_.TryItems()) {
       uint32_t dex_pc_start = try_item.start_addr_;
       uint32_t dex_pc_end = dex_pc_start + try_item.insn_count_;
       MaybeCreateBlockAt(dex_pc_start);
       if (dex_pc_end < code_item_accessor_.InsnsSizeInCodeUnits()) {
         // TODO: Do not create block if the last instruction cannot fall through.
         MaybeCreateBlockAt(dex_pc_end);
       } else if (dex_pc_end == code_item_accessor_.InsnsSizeInCodeUnits()) {
         // The TryItem spans until the very end of the CodeItem and therefore
         // cannot have any code afterwards.
       } else {
         // The TryItem spans beyond the end of the CodeItem. This is invalid code.
         VLOG(compiler) << "Not compiled: TryItem spans beyond the end of the CodeItem";
         return false;
       }
     }

     // Create branch targets for exception handlers.
     const uint8_t* handlers_ptr = code_item_accessor_.GetCatchHandlerData();
     uint32_t handlers_size = DecodeUnsignedLeb128(&handlers_ptr);
     for (uint32_t idx = 0; idx < handlers_size; ++idx) {
       CatchHandlerIterator iterator(handlers_ptr);
       for (; iterator.HasNext(); iterator.Next()) {
         MaybeCreateBlockAt(iterator.GetHandlerAddress());
       }
       handlers_ptr = iterator.EndDataPointer();
     }
   }

   // Iterate over all instructions and find branching instructions. Create blocks for
   // the locations these instructions branch to.
   for (const DexInstructionPcPair& pair : code_item_accessor_) {
     const uint32_t dex_pc = pair.DexPc();
     const Instruction& instruction = pair.Inst();

     if (instruction.IsBranch()) {
       number_of_branches_++;
       MaybeCreateBlockAt(dex_pc + instruction.GetTargetOffset());
     } else if (instruction.IsSwitch()) {
       DexSwitchTable table(instruction, dex_pc);
       for (DexSwitchTableIterator s_it(table); !s_it.Done(); s_it.Advance()) {
         MaybeCreateBlockAt(dex_pc + s_it.CurrentTargetOffset());

         // Create N-1 blocks where we will insert comparisons of the input value
         // against the Switch's case keys.
         if (table.ShouldBuildDecisionTree() && !s_it.IsLast()) {
           // Store the block under dex_pc of the current key at the switch data
           // instruction for uniqueness but give it the dex_pc of the SWITCH
           // instruction which it semantically belongs to.
           MaybeCreateBlockAt(dex_pc, s_it.GetDexPcForCurrentIndex());
         }
       }
     } else if (instruction.Opcode() == Instruction::MOVE_EXCEPTION) {
       // End the basic block after MOVE_EXCEPTION. This simplifies the later
       // stage of TryBoundary-block insertion.
     } else {
       continue;
     }

     if (instruction.CanFlowThrough()) {
       DexInstructionIterator next(std::next(DexInstructionIterator(pair)));
       if (next == code_item_accessor_.end()) {
         // In the normal case we should never hit this but someone can artificially forge a dex
         // file to fall-through out the method code. In this case we bail out compilation.
         VLOG(compiler) << "Not compiled: Fall-through beyond the CodeItem";
         return false;
       }
       MaybeCreateBlockAt(next.DexPc());
     }
   }

   return true;
 }

 void HBasicBlockBuilder::ConnectBasicBlocks() {
   HBasicBlock* block = graph_->GetEntryBlock();
   graph_->AddBlock(block);

   size_t quicken_index = 0;
   bool is_throwing_block = false;
   // Calculate the qucikening index here instead of CreateBranchTargets since it's easier to
   // calculate in dex_pc order.
   for (const DexInstructionPcPair& pair : code_item_accessor_) {
     const uint32_t dex_pc = pair.DexPc();
     const Instruction& instruction = pair.Inst();

     // Check if this dex_pc address starts a new basic block.
     HBasicBlock* next_block = GetBlockAt(dex_pc);
     if (next_block != nullptr) {
       // We only need quicken index entries for basic block boundaries.
       quicken_index_for_dex_pc_.Put(dex_pc, quicken_index);
       if (block != nullptr) {
         // Last instruction did not end its basic block but a new one starts here.
         // It must have been a block falling through into the next one.
         block->AddSuccessor(next_block);
       }
       block = next_block;
       is_throwing_block = false;
       graph_->AddBlock(block);
     }
     // Make sure to increment this before the continues.
     if (QuickenInfoTable::NeedsIndexForInstruction(&instruction)) {
       ++quicken_index;
     }

     if (block == nullptr) {
       // Ignore dead code.
       continue;
     }

     if (!is_throwing_block && IsThrowingDexInstruction(instruction)) {
       DCHECK(!ContainsElement(throwing_blocks_, block));
       is_throwing_block = true;
       throwing_blocks_.push_back(block);
     }

     if (instruction.IsBranch()) {
       uint32_t target_dex_pc = dex_pc + instruction.GetTargetOffset();
       block->AddSuccessor(GetBlockAt(target_dex_pc));
     } else if (instruction.IsReturn() || (instruction.Opcode() == Instruction::THROW)) {
       block->AddSuccessor(graph_->GetExitBlock());
     } else if (instruction.IsSwitch()) {
       DexSwitchTable table(instruction, dex_pc);
       for (DexSwitchTableIterator s_it(table); !s_it.Done(); s_it.Advance()) {
         uint32_t target_dex_pc = dex_pc + s_it.CurrentTargetOffset();
         block->AddSuccessor(GetBlockAt(target_dex_pc));

         if (table.ShouldBuildDecisionTree() && !s_it.IsLast()) {
           uint32_t next_case_dex_pc = s_it.GetDexPcForCurrentIndex();
           HBasicBlock* next_case_block = GetBlockAt(next_case_dex_pc);
           block->AddSuccessor(next_case_block);
           block = next_case_block;
           graph_->AddBlock(block);
         }
       }
     } else {
       // Remaining code only applies to instructions which end their basic block.
       continue;
     }

     // Go to the next instruction in case we read dex PC below.
     if (instruction.CanFlowThrough()) {
       block->AddSuccessor(GetBlockAt(std::next(DexInstructionIterator(pair)).DexPc()));
     }

     // The basic block ends here. Do not add any more instructions.
     block = nullptr;
   }

   graph_->AddBlock(graph_->GetExitBlock());
 }

 // Returns the TryItem stored for `block` or nullptr if there is no info for it.
 static const DexFile::TryItem* GetTryItem(
     HBasicBlock* block,
     const ScopedArenaSafeMap<uint32_t, const DexFile::TryItem*>& try_block_info) {
   auto iterator = try_block_info.find(block->GetBlockId());
   return (iterator == try_block_info.end()) ? nullptr : iterator->second;
 }

 // Iterates over the exception handlers of `try_item`, finds the corresponding
 // catch blocks and makes them successors of `try_boundary`. The order of
 // successors matches the order in which runtime exception delivery searches
 // for a handler.
 static void LinkToCatchBlocks(HTryBoundary* try_boundary,
                               const CodeItemDataAccessor& accessor,
                               const DexFile::TryItem* try_item,
                               const ScopedArenaSafeMap<uint32_t, HBasicBlock*>& catch_blocks) {
   for (CatchHandlerIterator it(accessor.GetCatchHandlerData(try_item->handler_off_));
       it.HasNext();
       it.Next()) {
     try_boundary->AddExceptionHandler(catch_blocks.Get(it.GetHandlerAddress()));
   }
 }

 bool HBasicBlockBuilder::MightHaveLiveNormalPredecessors(HBasicBlock* catch_block) {
   if (kIsDebugBuild) {
     DCHECK_NE(catch_block->GetDexPc(), kNoDexPc) << "Should not be called on synthetic blocks";
     DCHECK(!graph_->GetEntryBlock()->GetSuccessors().empty())
         << "Basic blocks must have been created and connected";
     for (HBasicBlock* predecessor : catch_block->GetPredecessors()) {
       DCHECK(!predecessor->IsSingleTryBoundary())
           << "TryBoundary blocks must not have not been created yet";
     }
   }

   const Instruction& first = code_item_accessor_.InstructionAt(catch_block->GetDexPc());
   if (first.Opcode() == Instruction::MOVE_EXCEPTION) {
     // Verifier guarantees that if a catch block begins with MOVE_EXCEPTION then
     // it has no live normal predecessors.
     return false;
   } else if (catch_block->GetPredecessors().empty()) {
     // Normal control-flow edges have already been created. Since block's list of
     // predecessors is empty, it cannot have any live or dead normal predecessors.
     return false;
   }

   // The catch block has normal predecessors but we do not know which are live
   // and which will be removed during the initial DCE. Return `true` to signal
   // that it may have live normal predecessors.
   return true;
 }

 void HBasicBlockBuilder::InsertTryBoundaryBlocks() {
   if (code_item_accessor_.TriesSize() == 0) {
     return;
   }

   // Keep a map of all try blocks and their respective TryItems. We do not use
   // the block's pointer but rather its id to ensure deterministic iteration.
   ScopedArenaSafeMap<uint32_t, const DexFile::TryItem*> try_block_info(
       std::less<uint32_t>(), local_allocator_->Adapter(kArenaAllocGraphBuilder));

   // Obtain TryItem information for blocks with throwing instructions, and split
   // blocks which are both try & catch to simplify the graph.
   for (HBasicBlock* block : graph_->GetBlocks()) {
     if (block->GetDexPc() == kNoDexPc) {
       continue;
     }

     // Do not bother creating exceptional edges for try blocks which have no
     // throwing instructions. In that case we simply assume that the block is
     // not covered by a TryItem. This prevents us from creating a throw-catch
     // loop for synchronized blocks.
     if (ContainsElement(throwing_blocks_, block)) {
       // Try to find a TryItem covering the block.
       const DexFile::TryItem* try_item = code_item_accessor_.FindTryItem(block->GetDexPc());
       if (try_item != nullptr) {
         // Block throwing and in a TryItem. Store the try block information.
         try_block_info.Put(block->GetBlockId(), try_item);
       }
     }
   }

   // Map from a handler dex_pc to the corresponding catch block.
   ScopedArenaSafeMap<uint32_t, HBasicBlock*> catch_blocks(
       std::less<uint32_t>(), local_allocator_->Adapter(kArenaAllocGraphBuilder));

   // Iterate over catch blocks, create artifical landing pads if necessary to
   // simplify the CFG, and set metadata.
   const uint8_t* handlers_ptr = code_item_accessor_.GetCatchHandlerData();
   uint32_t handlers_size = DecodeUnsignedLeb128(&handlers_ptr);
   for (uint32_t idx = 0; idx < handlers_size; ++idx) {
     CatchHandlerIterator iterator(handlers_ptr);
     for (; iterator.HasNext(); iterator.Next()) {
       uint32_t address = iterator.GetHandlerAddress();
       if (catch_blocks.find(address) != catch_blocks.end()) {
         // Catch block already processed.
         continue;
       }

       // Check if we should create an artifical landing pad for the catch block.
       // We create one if the catch block is also a try block because we do not
       // have a strategy for inserting TryBoundaries on exceptional edges.
       // We also create one if the block might have normal predecessors so as to
       // simplify register allocation.
       HBasicBlock* catch_block = GetBlockAt(address);
       bool is_try_block = (try_block_info.find(catch_block->GetBlockId()) != try_block_info.end());
       if (is_try_block || MightHaveLiveNormalPredecessors(catch_block)) {
         HBasicBlock* new_catch_block = new (allocator_) HBasicBlock(graph_, address);
         new_catch_block->AddInstruction(new (allocator_) HGoto(address));
         new_catch_block->AddSuccessor(catch_block);
         graph_->AddBlock(new_catch_block);
         catch_block = new_catch_block;
       }

       catch_blocks.Put(address, catch_block);
       catch_block->SetTryCatchInformation(
         new (allocator_) TryCatchInformation(iterator.GetHandlerTypeIndex(), *dex_file_));
     }
     handlers_ptr = iterator.EndDataPointer();
   }

   // Do a pass over the try blocks and insert entering TryBoundaries where at
   // least one predecessor is not covered by the same TryItem as the try block.
   // We do not split each edge separately, but rather create one boundary block
   // that all predecessors are relinked to. This preserves loop headers (b/23895756).
   for (const auto& entry : try_block_info) {
     uint32_t block_id = entry.first;
     const DexFile::TryItem* try_item = entry.second;
     HBasicBlock* try_block = graph_->GetBlocks()[block_id];
     for (HBasicBlock* predecessor : try_block->GetPredecessors()) {
       if (GetTryItem(predecessor, try_block_info) != try_item) {
         // Found a predecessor not covered by the same TryItem. Insert entering
         // boundary block.
         HTryBoundary* try_entry = new (allocator_) HTryBoundary(
             HTryBoundary::BoundaryKind::kEntry, try_block->GetDexPc());
         try_block->CreateImmediateDominator()->AddInstruction(try_entry);
         LinkToCatchBlocks(try_entry, code_item_accessor_, try_item, catch_blocks);
         break;
       }
     }
   }

   // Do a second pass over the try blocks and insert exit TryBoundaries where
   // the successor is not in the same TryItem.
   for (const auto& entry : try_block_info) {
     uint32_t block_id = entry.first;
     const DexFile::TryItem* try_item = entry.second;
     HBasicBlock* try_block = graph_->GetBlocks()[block_id];
     // NOTE: Do not use iterators because SplitEdge would invalidate them.
     for (size_t i = 0, e = try_block->GetSuccessors().size(); i < e; ++i) {
       HBasicBlock* successor = try_block->GetSuccessors()[i];

       // If the successor is a try block, all of its predecessors must be
       // covered by the same TryItem. Otherwise the previous pass would have
       // created a non-throwing boundary block.
       if (GetTryItem(successor, try_block_info) != nullptr) {
         DCHECK_EQ(try_item, GetTryItem(successor, try_block_info));
         continue;
       }

       // Insert TryBoundary and link to catch blocks.
       HTryBoundary* try_exit =
           new (allocator_) HTryBoundary(HTryBoundary::BoundaryKind::kExit, successor->GetDexPc());
       graph_->SplitEdge(try_block, successor)->AddInstruction(try_exit);
       LinkToCatchBlocks(try_exit, code_item_accessor_, try_item, catch_blocks);
     }
   }
 }

 bool HBasicBlockBuilder::Build() {
   DCHECK(code_item_accessor_.HasCodeItem());
   DCHECK(graph_->GetBlocks().empty());

   graph_->SetEntryBlock(new (allocator_) HBasicBlock(graph_, kNoDexPc));
   graph_->SetExitBlock(new (allocator_) HBasicBlock(graph_, kNoDexPc));

   // TODO(dbrazdil): Do CreateBranchTargets and ConnectBasicBlocks in one pass.
   if (!CreateBranchTargets()) {
     return false;
   }

   ConnectBasicBlocks();
   InsertTryBoundaryBlocks();

   return true;
 }

 void HBasicBlockBuilder::BuildIntrinsic() {
   DCHECK(!code_item_accessor_.HasCodeItem());
   DCHECK(graph_->GetBlocks().empty());

   // Create blocks.
   HBasicBlock* entry_block = new (allocator_) HBasicBlock(graph_, kNoDexPc);
   HBasicBlock* exit_block = new (allocator_) HBasicBlock(graph_, kNoDexPc);
   HBasicBlock* body = MaybeCreateBlockAt(/* semantic_dex_pc */ kNoDexPc, /* store_dex_pc */ 0u);

   // Add blocks to the graph.
   graph_->AddBlock(entry_block);
   graph_->AddBlock(body);
   graph_->AddBlock(exit_block);
   graph_->SetEntryBlock(entry_block);
   graph_->SetExitBlock(exit_block);

   // Connect blocks.
   entry_block->AddSuccessor(body);
   body->AddSuccessor(exit_block);
 }

 size_t HBasicBlockBuilder::GetQuickenIndex(uint32_t dex_pc) const {
   return quicken_index_for_dex_pc_.Get(dex_pc);
 }

 }  // namespace art
	/*
	* Copyright (C) 2016 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 "block_builder.h"

	#include "base/logging.h" // FOR VLOG.
	#include "bytecode_utils.h"
	#include "code_item_accessors-inl.h"
	#include "dex_file_exception_helpers.h"
	#include "quicken_info.h"

	namespace art {

	HBasicBlockBuilder::HBasicBlockBuilder(HGraph* graph,
	const DexFile* const dex_file,
	const CodeItemDebugInfoAccessor& accessor,
	ScopedArenaAllocator* local_allocator)
	: allocator_(graph->GetAllocator()),
	graph_(graph),
	dex_file_(dex_file),
	code_item_accessor_(accessor),
	local_allocator_(local_allocator),
	branch_targets_(code_item_accessor_.HasCodeItem()
	? code_item_accessor_.InsnsSizeInCodeUnits()
	: /* fake dex_pc=0 for intrinsic graph */ 1u,
	nullptr,
	local_allocator->Adapter(kArenaAllocGraphBuilder)),
	throwing_blocks_(kDefaultNumberOfThrowingBlocks,
	local_allocator->Adapter(kArenaAllocGraphBuilder)),
	number_of_branches_(0u),
	quicken_index_for_dex_pc_(std::less<uint32_t>(),
	local_allocator->Adapter(kArenaAllocGraphBuilder)) {}

	HBasicBlock* HBasicBlockBuilder::MaybeCreateBlockAt(uint32_t dex_pc) {
	return MaybeCreateBlockAt(dex_pc, dex_pc);
	}

	HBasicBlock* HBasicBlockBuilder::MaybeCreateBlockAt(uint32_t semantic_dex_pc,
	uint32_t store_dex_pc) {
	HBasicBlock* block = branch_targets_[store_dex_pc];
	if (block == nullptr) {
	block = new (allocator_) HBasicBlock(graph_, semantic_dex_pc);
	branch_targets_[store_dex_pc] = block;
	}
	DCHECK_EQ(block->GetDexPc(), semantic_dex_pc);
	return block;
	}

	bool HBasicBlockBuilder::CreateBranchTargets() {
	// Create the first block for the dex instructions, single successor of the entry block.
	MaybeCreateBlockAt(0u);

	if (code_item_accessor_.TriesSize() != 0) {
	// Create branch targets at the start/end of the TryItem range. These are
	// places where the program might fall through into/out of the a block and
	// where TryBoundary instructions will be inserted later. Other edges which
	// enter/exit the try blocks are a result of branches/switches.
	for (const DexFile::TryItem& try_item : code_item_accessor_.TryItems()) {
	uint32_t dex_pc_start = try_item.start_addr_;
	uint32_t dex_pc_end = dex_pc_start + try_item.insn_count_;
	MaybeCreateBlockAt(dex_pc_start);
	if (dex_pc_end < code_item_accessor_.InsnsSizeInCodeUnits()) {
	// TODO: Do not create block if the last instruction cannot fall through.
	MaybeCreateBlockAt(dex_pc_end);
	} else if (dex_pc_end == code_item_accessor_.InsnsSizeInCodeUnits()) {
	// The TryItem spans until the very end of the CodeItem and therefore
	// cannot have any code afterwards.
	} else {
	// The TryItem spans beyond the end of the CodeItem. This is invalid code.
	VLOG(compiler) << "Not compiled: TryItem spans beyond the end of the CodeItem";
	return false;
	}
	}

	// Create branch targets for exception handlers.
	const uint8_t* handlers_ptr = code_item_accessor_.GetCatchHandlerData();
	uint32_t handlers_size = DecodeUnsignedLeb128(&handlers_ptr);
	for (uint32_t idx = 0; idx < handlers_size; ++idx) {
	CatchHandlerIterator iterator(handlers_ptr);
	for (; iterator.HasNext(); iterator.Next()) {
	MaybeCreateBlockAt(iterator.GetHandlerAddress());
	}
	handlers_ptr = iterator.EndDataPointer();
	}
	}

	// Iterate over all instructions and find branching instructions. Create blocks for
	// the locations these instructions branch to.
	for (const DexInstructionPcPair& pair : code_item_accessor_) {
	const uint32_t dex_pc = pair.DexPc();
	const Instruction& instruction = pair.Inst();

	if (instruction.IsBranch()) {
	number_of_branches_++;
	MaybeCreateBlockAt(dex_pc + instruction.GetTargetOffset());
	} else if (instruction.IsSwitch()) {
	DexSwitchTable table(instruction, dex_pc);
	for (DexSwitchTableIterator s_it(table); !s_it.Done(); s_it.Advance()) {
	MaybeCreateBlockAt(dex_pc + s_it.CurrentTargetOffset());

	// Create N-1 blocks where we will insert comparisons of the input value
	// against the Switch's case keys.
	if (table.ShouldBuildDecisionTree() && !s_it.IsLast()) {
	// Store the block under dex_pc of the current key at the switch data
	// instruction for uniqueness but give it the dex_pc of the SWITCH
	// instruction which it semantically belongs to.
	MaybeCreateBlockAt(dex_pc, s_it.GetDexPcForCurrentIndex());
	}
	}
	} else if (instruction.Opcode() == Instruction::MOVE_EXCEPTION) {
	// End the basic block after MOVE_EXCEPTION. This simplifies the later
	// stage of TryBoundary-block insertion.
	} else {
	continue;
	}

	if (instruction.CanFlowThrough()) {
	DexInstructionIterator next(std::next(DexInstructionIterator(pair)));
	if (next == code_item_accessor_.end()) {
	// In the normal case we should never hit this but someone can artificially forge a dex
	// file to fall-through out the method code. In this case we bail out compilation.
	VLOG(compiler) << "Not compiled: Fall-through beyond the CodeItem";
	return false;
	}
	MaybeCreateBlockAt(next.DexPc());
	}
	}

	return true;
	}

	void HBasicBlockBuilder::ConnectBasicBlocks() {
	HBasicBlock* block = graph_->GetEntryBlock();
	graph_->AddBlock(block);

	size_t quicken_index = 0;
	bool is_throwing_block = false;
	// Calculate the qucikening index here instead of CreateBranchTargets since it's easier to
	// calculate in dex_pc order.
	for (const DexInstructionPcPair& pair : code_item_accessor_) {
	const uint32_t dex_pc = pair.DexPc();
	const Instruction& instruction = pair.Inst();

	// Check if this dex_pc address starts a new basic block.
	HBasicBlock* next_block = GetBlockAt(dex_pc);
	if (next_block != nullptr) {
	// We only need quicken index entries for basic block boundaries.
	quicken_index_for_dex_pc_.Put(dex_pc, quicken_index);
	if (block != nullptr) {
	// Last instruction did not end its basic block but a new one starts here.
	// It must have been a block falling through into the next one.
	block->AddSuccessor(next_block);
	}
	block = next_block;
	is_throwing_block = false;
	graph_->AddBlock(block);
	}
	// Make sure to increment this before the continues.
	if (QuickenInfoTable::NeedsIndexForInstruction(&instruction)) {
	++quicken_index;
	}

	if (block == nullptr) {
	// Ignore dead code.
	continue;
	}

	if (!is_throwing_block && IsThrowingDexInstruction(instruction)) {
	DCHECK(!ContainsElement(throwing_blocks_, block));
	is_throwing_block = true;
	throwing_blocks_.push_back(block);
	}

	if (instruction.IsBranch()) {
	uint32_t target_dex_pc = dex_pc + instruction.GetTargetOffset();
	block->AddSuccessor(GetBlockAt(target_dex_pc));
	} else if (instruction.IsReturn() \|\| (instruction.Opcode() == Instruction::THROW)) {
	block->AddSuccessor(graph_->GetExitBlock());
	} else if (instruction.IsSwitch()) {
	DexSwitchTable table(instruction, dex_pc);
	for (DexSwitchTableIterator s_it(table); !s_it.Done(); s_it.Advance()) {
	uint32_t target_dex_pc = dex_pc + s_it.CurrentTargetOffset();
	block->AddSuccessor(GetBlockAt(target_dex_pc));

	if (table.ShouldBuildDecisionTree() && !s_it.IsLast()) {
	uint32_t next_case_dex_pc = s_it.GetDexPcForCurrentIndex();
	HBasicBlock* next_case_block = GetBlockAt(next_case_dex_pc);
	block->AddSuccessor(next_case_block);
	block = next_case_block;
	graph_->AddBlock(block);
	}
	}
	} else {
	// Remaining code only applies to instructions which end their basic block.
	continue;
	}

	// Go to the next instruction in case we read dex PC below.
	if (instruction.CanFlowThrough()) {
	block->AddSuccessor(GetBlockAt(std::next(DexInstructionIterator(pair)).DexPc()));
	}

	// The basic block ends here. Do not add any more instructions.
	block = nullptr;
	}

	graph_->AddBlock(graph_->GetExitBlock());
	}

	// Returns the TryItem stored for `block` or nullptr if there is no info for it.
	static const DexFile::TryItem* GetTryItem(
	HBasicBlock* block,
	const ScopedArenaSafeMap<uint32_t, const DexFile::TryItem*>& try_block_info) {
	auto iterator = try_block_info.find(block->GetBlockId());
	return (iterator == try_block_info.end()) ? nullptr : iterator->second;
	}

	// Iterates over the exception handlers of `try_item`, finds the corresponding
	// catch blocks and makes them successors of `try_boundary`. The order of
	// successors matches the order in which runtime exception delivery searches
	// for a handler.
	static void LinkToCatchBlocks(HTryBoundary* try_boundary,
	const CodeItemDataAccessor& accessor,
	const DexFile::TryItem* try_item,
	const ScopedArenaSafeMap<uint32_t, HBasicBlock*>& catch_blocks) {
	for (CatchHandlerIterator it(accessor.GetCatchHandlerData(try_item->handler_off_));
	it.HasNext();
	it.Next()) {
	try_boundary->AddExceptionHandler(catch_blocks.Get(it.GetHandlerAddress()));
	}
	}

	bool HBasicBlockBuilder::MightHaveLiveNormalPredecessors(HBasicBlock* catch_block) {
	if (kIsDebugBuild) {
	DCHECK_NE(catch_block->GetDexPc(), kNoDexPc) << "Should not be called on synthetic blocks";
	DCHECK(!graph_->GetEntryBlock()->GetSuccessors().empty())
	<< "Basic blocks must have been created and connected";
	for (HBasicBlock* predecessor : catch_block->GetPredecessors()) {
	DCHECK(!predecessor->IsSingleTryBoundary())
	<< "TryBoundary blocks must not have not been created yet";
	}
	}

	const Instruction& first = code_item_accessor_.InstructionAt(catch_block->GetDexPc());
	if (first.Opcode() == Instruction::MOVE_EXCEPTION) {
	// Verifier guarantees that if a catch block begins with MOVE_EXCEPTION then
	// it has no live normal predecessors.
	return false;
	} else if (catch_block->GetPredecessors().empty()) {
	// Normal control-flow edges have already been created. Since block's list of
	// predecessors is empty, it cannot have any live or dead normal predecessors.
	return false;
	}

	// The catch block has normal predecessors but we do not know which are live
	// and which will be removed during the initial DCE. Return `true` to signal
	// that it may have live normal predecessors.
	return true;
	}

	void HBasicBlockBuilder::InsertTryBoundaryBlocks() {
	if (code_item_accessor_.TriesSize() == 0) {
	return;
	}

	// Keep a map of all try blocks and their respective TryItems. We do not use
	// the block's pointer but rather its id to ensure deterministic iteration.
	ScopedArenaSafeMap<uint32_t, const DexFile::TryItem*> try_block_info(
	std::less<uint32_t>(), local_allocator_->Adapter(kArenaAllocGraphBuilder));

	// Obtain TryItem information for blocks with throwing instructions, and split
	// blocks which are both try & catch to simplify the graph.
	for (HBasicBlock* block : graph_->GetBlocks()) {
	if (block->GetDexPc() == kNoDexPc) {
	continue;
	}

	// Do not bother creating exceptional edges for try blocks which have no
	// throwing instructions. In that case we simply assume that the block is
	// not covered by a TryItem. This prevents us from creating a throw-catch
	// loop for synchronized blocks.
	if (ContainsElement(throwing_blocks_, block)) {
	// Try to find a TryItem covering the block.
	const DexFile::TryItem* try_item = code_item_accessor_.FindTryItem(block->GetDexPc());
	if (try_item != nullptr) {
	// Block throwing and in a TryItem. Store the try block information.
	try_block_info.Put(block->GetBlockId(), try_item);
	}
	}
	}

	// Map from a handler dex_pc to the corresponding catch block.
	ScopedArenaSafeMap<uint32_t, HBasicBlock*> catch_blocks(
	std::less<uint32_t>(), local_allocator_->Adapter(kArenaAllocGraphBuilder));

	// Iterate over catch blocks, create artifical landing pads if necessary to
	// simplify the CFG, and set metadata.
	const uint8_t* handlers_ptr = code_item_accessor_.GetCatchHandlerData();
	uint32_t handlers_size = DecodeUnsignedLeb128(&handlers_ptr);
	for (uint32_t idx = 0; idx < handlers_size; ++idx) {
	CatchHandlerIterator iterator(handlers_ptr);
	for (; iterator.HasNext(); iterator.Next()) {
	uint32_t address = iterator.GetHandlerAddress();
	if (catch_blocks.find(address) != catch_blocks.end()) {
	// Catch block already processed.
	continue;
	}

	// Check if we should create an artifical landing pad for the catch block.
	// We create one if the catch block is also a try block because we do not
	// have a strategy for inserting TryBoundaries on exceptional edges.
	// We also create one if the block might have normal predecessors so as to
	// simplify register allocation.
	HBasicBlock* catch_block = GetBlockAt(address);
	bool is_try_block = (try_block_info.find(catch_block->GetBlockId()) != try_block_info.end());
	if (is_try_block \|\| MightHaveLiveNormalPredecessors(catch_block)) {
	HBasicBlock* new_catch_block = new (allocator_) HBasicBlock(graph_, address);
	new_catch_block->AddInstruction(new (allocator_) HGoto(address));
	new_catch_block->AddSuccessor(catch_block);
	graph_->AddBlock(new_catch_block);
	catch_block = new_catch_block;
	}

	catch_blocks.Put(address, catch_block);
	catch_block->SetTryCatchInformation(
	new (allocator_) TryCatchInformation(iterator.GetHandlerTypeIndex(), *dex_file_));
	}
	handlers_ptr = iterator.EndDataPointer();
	}

	// Do a pass over the try blocks and insert entering TryBoundaries where at
	// least one predecessor is not covered by the same TryItem as the try block.
	// We do not split each edge separately, but rather create one boundary block
	// that all predecessors are relinked to. This preserves loop headers (b/23895756).
	for (const auto& entry : try_block_info) {
	uint32_t block_id = entry.first;
	const DexFile::TryItem* try_item = entry.second;
	HBasicBlock* try_block = graph_->GetBlocks()[block_id];
	for (HBasicBlock* predecessor : try_block->GetPredecessors()) {
	if (GetTryItem(predecessor, try_block_info) != try_item) {
	// Found a predecessor not covered by the same TryItem. Insert entering
	// boundary block.
	HTryBoundary* try_entry = new (allocator_) HTryBoundary(
	HTryBoundary::BoundaryKind::kEntry, try_block->GetDexPc());
	try_block->CreateImmediateDominator()->AddInstruction(try_entry);
	LinkToCatchBlocks(try_entry, code_item_accessor_, try_item, catch_blocks);
	break;
	}
	}
	}

	// Do a second pass over the try blocks and insert exit TryBoundaries where
	// the successor is not in the same TryItem.
	for (const auto& entry : try_block_info) {
	uint32_t block_id = entry.first;
	const DexFile::TryItem* try_item = entry.second;
	HBasicBlock* try_block = graph_->GetBlocks()[block_id];
	// NOTE: Do not use iterators because SplitEdge would invalidate them.
	for (size_t i = 0, e = try_block->GetSuccessors().size(); i < e; ++i) {
	HBasicBlock* successor = try_block->GetSuccessors()[i];

	// If the successor is a try block, all of its predecessors must be
	// covered by the same TryItem. Otherwise the previous pass would have
	// created a non-throwing boundary block.
	if (GetTryItem(successor, try_block_info) != nullptr) {
	DCHECK_EQ(try_item, GetTryItem(successor, try_block_info));
	continue;
	}

	// Insert TryBoundary and link to catch blocks.
	HTryBoundary* try_exit =
	new (allocator_) HTryBoundary(HTryBoundary::BoundaryKind::kExit, successor->GetDexPc());
	graph_->SplitEdge(try_block, successor)->AddInstruction(try_exit);
	LinkToCatchBlocks(try_exit, code_item_accessor_, try_item, catch_blocks);
	}
	}
	}

	bool HBasicBlockBuilder::Build() {
	DCHECK(code_item_accessor_.HasCodeItem());
	DCHECK(graph_->GetBlocks().empty());

	graph_->SetEntryBlock(new (allocator_) HBasicBlock(graph_, kNoDexPc));
	graph_->SetExitBlock(new (allocator_) HBasicBlock(graph_, kNoDexPc));

	// TODO(dbrazdil): Do CreateBranchTargets and ConnectBasicBlocks in one pass.
	if (!CreateBranchTargets()) {
	return false;
	}

	ConnectBasicBlocks();
	InsertTryBoundaryBlocks();

	return true;
	}

	void HBasicBlockBuilder::BuildIntrinsic() {
	DCHECK(!code_item_accessor_.HasCodeItem());
	DCHECK(graph_->GetBlocks().empty());

	// Create blocks.
	HBasicBlock* entry_block = new (allocator_) HBasicBlock(graph_, kNoDexPc);
	HBasicBlock* exit_block = new (allocator_) HBasicBlock(graph_, kNoDexPc);
	HBasicBlock* body = MaybeCreateBlockAt(/* semantic_dex_pc / kNoDexPc, / store_dex_pc */ 0u);

	// Add blocks to the graph.
	graph_->AddBlock(entry_block);
	graph_->AddBlock(body);
	graph_->AddBlock(exit_block);
	graph_->SetEntryBlock(entry_block);
	graph_->SetExitBlock(exit_block);

	// Connect blocks.
	entry_block->AddSuccessor(body);
	body->AddSuccessor(exit_block);
	}

	size_t HBasicBlockBuilder::GetQuickenIndex(uint32_t dex_pc) const {
	return quicken_index_for_dex_pc_.Get(dex_pc);
	}

	} // namespace art