ART世界探险(18) InlineMethod

好，我们还是先复习一下上上节学到的图：

在开始InlineMethod之前，我们再继续补充一点BasicBlock的知识。

BasicBlock中针对MIR的相关操作

AppendMIR

AppendMIR的作用是将MIR增加到一个BasicBlock的结尾。

/* Insert an MIR instruction to the end of a basic block. */
void BasicBlock::AppendMIR(MIR* mir) {// Insert it after the last MIR.InsertMIRListAfter(last_mir_insn, mir, mir);
}

InsertMIRListAfter

一个标准的链表，实现MIR的列表队尾增加元素。

void BasicBlock::InsertMIRListAfter(MIR* insert_after, MIR* first_list_mir, MIR* last_list_mir) {// If no MIR, we are done.if (first_list_mir == nullptr || last_list_mir == nullptr) {return;}// If insert_after is null, assume BB is empty.if (insert_after == nullptr) {first_mir_insn = first_list_mir;last_mir_insn = last_list_mir;last_list_mir->next = nullptr;} else {MIR* after_list = insert_after->next;insert_after->next = first_list_mir;last_list_mir->next = after_list;if (after_list == nullptr) {last_mir_insn = last_list_mir;}}// Set this BB to be the basic block of the MIRs.MIR* last = last_list_mir->next;for (MIR* mir = first_list_mir; mir != last; mir = mir->next) {mir->bb = id;}
}

编译的第一个大步骤是MIRGraph::InlineMethod。
我们上一节准备了指令集和BasicBlock等储备知识，下面我们正式开始分析这第一个大步骤。

MIRGraph::InlineMethod

InlineMethod的作用是将一个Dex方法插入到MIRGraph中的当前插入点中。

void MIRGraph::InlineMethod(const DexFile::CodeItem* code_item, uint32_t access_flags,InvokeType invoke_type ATTRIBUTE_UNUSED, uint16_t class_def_idx,uint32_t method_idx, jobject class_loader, const DexFile& dex_file) {current_code_item_ = code_item;

第一步先把传进来的code_item赋给当前MIRGraph对象的current_mode_item_项目。
它的定义为：

const DexFile::CodeItem* current_code_item_;

第二步将current_method_和current_offset_对压入到method_stack_栈中。
method_stack_是一个MIRLocation类型的ArenaVector。

ArenaVector<MIRLocation> method_stack_;        // Include stack

MIRLocation是在MIRGraph类中定义的整数对。

typedef std::pair<int, int> MIRLocation;       // Insert point, (m_unit_ index, offset)

总之，上面和下面这几句的目的是定位到插入下一处的位置

  method_stack_.push_back(std::make_pair(current_method_, current_offset_));current_method_ = m_units_.size();current_offset_ = 0;

m_units_是DexCompilationUnit的容器，其结构如下：

ArenaVector<DexCompilationUnit*> m_units_;     // List of methods included in this graph

下面就开始往m_units_中push一个新建的DexCompilationUnit。

  m_units_.push_back(new (arena_) DexCompilationUnit(cu_, class_loader, Runtime::Current()->GetClassLinker(), dex_file,current_code_item_, class_def_idx, method_idx, access_flags,cu_->compiler_driver->GetVerifiedMethod(&dex_file, method_idx)));

然后计算代码的首地址和结束地址：

  const uint16_t* code_ptr = current_code_item_->insns_;const uint16_t* code_end =current_code_item_->insns_ + current_code_item_->insns_size_in_code_units_;

下面为新的BasicBlock预留空间。
block_list_是一个BasicBlock的ArenaVector容器：

  ArenaVector<BasicBlock*> block_list_;

先reserve block_list_的空间。然后再定义一个ScopedArenaVector。

  block_list_.reserve(block_list_.size() + current_code_item_->insns_size_in_code_units_);// FindBlock lookup cache.ScopedArenaAllocator allocator(&cu_->arena_stack);ScopedArenaVector<uint16_t> dex_pc_to_block_map(allocator.Adapter());dex_pc_to_block_map.resize(current_code_item_->insns_size_in_code_units_ +1 /* Fall-through on last insn; dead or punt to interpreter. */);
...

下面开始处理第一个方法，为其创建BasicBlock对象：null_block对象，entry_block_对象和exit_block_对象。
CreateNewBB的逻辑在前面我们已经讲过了。

  // If this is the first method, set up default entry and exit blocks.if (current_method_ == 0) {DCHECK(entry_block_ == nullptr);DCHECK(exit_block_ == nullptr);DCHECK_EQ(GetNumBlocks(), 0U);// Use id 0 to represent a null block.BasicBlock* null_block = CreateNewBB(kNullBlock);DCHECK_EQ(null_block->id, NullBasicBlockId);null_block->hidden = true;entry_block_ = CreateNewBB(kEntryBlock);exit_block_ = CreateNewBB(kExitBlock);} else {UNIMPLEMENTED(FATAL) << "Nested inlining not implemented.";/** Will need to manage storage for ins & outs, push prevous state and update* insert point.*/}

null块，入口块和出口块都是默认的。下面再创建代码块：

  /* Current block to record parsed instructions */BasicBlock* cur_block = CreateNewBB(kDalvikByteCode);DCHECK_EQ(current_offset_, 0U);cur_block->start_offset = current_offset_;// TODO: for inlining support, insert at the insert point rather than entry block.entry_block_->fall_through = cur_block->id;cur_block->predecessors.push_back(entry_block_->id);

下面开始处理try块所管辖的区间。

  /* Identify code range in try blocks and set up the empty catch blocks */ProcessTryCatchBlocks(&dex_pc_to_block_map);

我们看一下ProcessTryCatchBlock的处理逻辑。
主要思路是：
* 遍历，寻找块中的每一个try语句
* 针对每一个try，计算catch需要处理的区间，然后加入到CatchHandler中。


/* Identify code range in try blocks and set up the empty catch blocks */
void MIRGraph::ProcessTryCatchBlocks(ScopedArenaVector<uint16_t>* dex_pc_to_block_map) {int tries_size = current_code_item_->tries_size_;DexOffset offset;if (tries_size == 0) {return;}for (int i = 0; i < tries_size; i++) {const DexFile::TryItem* pTry =DexFile::GetTryItems(*current_code_item_, i);DexOffset start_offset = pTry->start_addr_;DexOffset end_offset = start_offset + pTry->insn_count_;for (offset = start_offset; offset < end_offset; offset++) {try_block_addr_->SetBit(offset);}}// Iterate over each of the handlers to enqueue the empty Catch blocks.const uint8_t* handlers_ptr = DexFile::GetCatchHandlerData(*current_code_item_, 0);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();FindBlock(address, true /*create*/, /* immed_pred_block_p */ nullptr, dex_pc_to_block_map);}handlers_ptr = iterator.EndDataPointer();}
}

下面开始处理每一条指令，将其转化成MIR。

  uint64_t merged_df_flags = 0u;/* Parse all instructions and put them into containing basic blocks */while (code_ptr < code_end) {MIR *insn = NewMIR();insn->offset = current_offset_;insn->m_unit_index = current_method_;int width = ParseInsn(code_ptr, &insn->dalvikInsn);Instruction::Code opcode = insn->dalvikInsn.opcode;if (opcode_count_ != nullptr) {opcode_count_[static_cast<int>(opcode)]++;}int flags = insn->dalvikInsn.FlagsOf();int verify_flags = Instruction::VerifyFlagsOf(insn->dalvikInsn.opcode);

前面都是跟上节讲到的Dalvik指令集密切相关，相关信息可以参考上节。
下面开始处理一些特殊的标志。

    uint64_t df_flags = GetDataFlowAttributes(insn);merged_df_flags |= df_flags;if (df_flags & DF_HAS_DEFS) {def_count_ += (df_flags & DF_A_WIDE) ? 2 : 1;}if (df_flags & DF_LVN) {cur_block->use_lvn = true;  // Run local value numbering on this basic block.}

下面先处理空指令。
空指令虽然只有一个字节，而且也没有操作要执行。但是处理起来也是有不少工程上的细节。
1. 首先要判断是否是因为对齐，而占用的字节数大于1.
2. 如果只占一个字节，则AppendMIR这条空指令
3. 否则可能存在不可达指令，对此要做一些针对性的处理

    // Check for inline data block signatures.if (opcode == Instruction::NOP) {// A simple NOP will have a width of 1 at this point, embedded data NOP > 1.if ((width == 1) && ((current_offset_ & 0x1) == 0x1) && ((code_end - code_ptr) > 1)) {// Could be an aligning nop.  If an embedded data NOP follows, treat pair as single unit.uint16_t following_raw_instruction = code_ptr[1];if ((following_raw_instruction == Instruction::kSparseSwitchSignature) ||(following_raw_instruction == Instruction::kPackedSwitchSignature) ||(following_raw_instruction == Instruction::kArrayDataSignature)) {width += Instruction::At(code_ptr + 1)->SizeInCodeUnits();}}if (width == 1) {// It is a simple nop - treat normally.cur_block->AppendMIR(insn);} else {DCHECK(cur_block->fall_through == NullBasicBlockId);DCHECK(cur_block->taken == NullBasicBlockId);// Unreachable instruction, mark for no continuation and end basic block.flags &= ~Instruction::kContinue;FindBlock(current_offset_ + width, /* create */ true,/* immed_pred_block_p */ nullptr, &dex_pc_to_block_map);}

如果不是空指令的话，直接AppendMIR。

    } else {cur_block->AppendMIR(insn);}

下面开始处理跳转相关的指令：

    // Associate the starting dex_pc for this opcode with its containing basic block.dex_pc_to_block_map[insn->offset] = cur_block->id;code_ptr += width;if (flags & Instruction::kBranch) {cur_block = ProcessCanBranch(cur_block, insn, current_offset_,width, flags, code_ptr, code_end, &dex_pc_to_block_map);

处理返回相关的操作：

    } else if (flags & Instruction::kReturn) {cur_block->terminated_by_return = true;cur_block->fall_through = exit_block_->id;exit_block_->predecessors.push_back(cur_block->id);/** Terminate the current block if there are instructions* afterwards.*/if (code_ptr < code_end) {/** Create a fallthrough block for real instructions* (incl. NOP).*/FindBlock(current_offset_ + width, /* create */ true,/* immed_pred_block_p */ nullptr, &dex_pc_to_block_map);}

处理抛出异常指令：

    } else if (flags & Instruction::kThrow) {cur_block = ProcessCanThrow(cur_block, insn, current_offset_, width, flags, try_block_addr_,code_ptr, code_end, &dex_pc_to_block_map);

处理分支指令：

    } else if (flags & Instruction::kSwitch) {cur_block = ProcessCanSwitch(cur_block, insn, current_offset_, width,flags, &dex_pc_to_block_map);}
...

寻找下一个BasicBlock. 找到之后，就把它们关联起来。
周而复始，我们就将它们画成了一张图。

    current_offset_ += width;BasicBlock* next_block = FindBlock(current_offset_, /* create */ false,/* immed_pred_block_p */ nullptr,&dex_pc_to_block_map);if (next_block) {/** The next instruction could be the target of a previously parsed* forward branch so a block is already created. If the current* instruction is not an unconditional branch, connect them through* the fall-through link.*/DCHECK(cur_block->fall_through == NullBasicBlockId ||GetBasicBlock(cur_block->fall_through) == next_block ||GetBasicBlock(cur_block->fall_through) == exit_block_);if ((cur_block->fall_through == NullBasicBlockId) && (flags & Instruction::kContinue)) {cur_block->fall_through = next_block->id;next_block->predecessors.push_back(cur_block->id);}cur_block = next_block;}}merged_df_flags_ = merged_df_flags;
...

最后再检查一下是不是有落空的代码跳出去了。

  // Check if there's been a fall-through out of the method code.BasicBlockId out_bb_id = dex_pc_to_block_map[current_code_item_->insns_size_in_code_units_];if (UNLIKELY(out_bb_id != NullBasicBlockId)) {// Eagerly calculate DFS order to determine if the block is dead.DCHECK(!DfsOrdersUpToDate());ComputeDFSOrders();BasicBlock* out_bb = GetBasicBlock(out_bb_id);DCHECK(out_bb != nullptr);if (out_bb->block_type != kDead) {LOG(WARNING) << "Live fall-through out of method in " << PrettyMethod(method_idx, dex_file);SetPuntToInterpreter(true);}}
}

以上，便完成了一次MIRGraph的生成过程。后面我们会举例子，详细分析生成代码时这个流程是如何走的。
但是，我们还有一些细节还没有讲，我们先过一下它们。

ProcessCanBranch

ProcessCanBranch方法，会处理下面这些跟跳转相关的指令：
* 无条件跳转指令
* GOTO
* GOTO_16
* GOTO_32
* 条件跳转指令
* IF_EQ: 等于
* IF_NE: 不等于
* IF_LT: 小于
* IF_GE: 大于或等于
* IF_GT: 大于
* IF_LE: 小于或等于

另外，还有两参数的指令：IF_XXZ。
上节看指令格式的时候我们可以看到，IF_EQ是三参数的：。而对应的IF_EQZ是两个参数的：IF_EQZ vAA, +BBBB

首先是根据指令晒参数：

/* Process instructions with the kBranch flag */
BasicBlock* MIRGraph::ProcessCanBranch(BasicBlock* cur_block, MIR* insn, DexOffset cur_offset,int width, int flags, const uint16_t* code_ptr,const uint16_t* code_end,ScopedArenaVector<uint16_t>* dex_pc_to_block_map) {DexOffset target = cur_offset;switch (insn->dalvikInsn.opcode) {case Instruction::GOTO:case Instruction::GOTO_16:case Instruction::GOTO_32:target += insn->dalvikInsn.vA;break;case Instruction::IF_EQ:case Instruction::IF_NE:case Instruction::IF_LT:case Instruction::IF_GE:case Instruction::IF_GT:case Instruction::IF_LE:cur_block->conditional_branch = true;target += insn->dalvikInsn.vC;break;case Instruction::IF_EQZ:case Instruction::IF_NEZ:case Instruction::IF_LTZ:case Instruction::IF_GEZ:case Instruction::IF_GTZ:case Instruction::IF_LEZ:cur_block->conditional_branch = true;target += insn->dalvikInsn.vB;break;default:LOG(FATAL) << "Unexpected opcode(" << insn->dalvikInsn.opcode << ") with kBranch set";}

后面根据参数情况查找要跳转的代码块：

  CountBranch(target);BasicBlock* taken_block = FindBlock(target, /* create */ true,/* immed_pred_block_p */ &cur_block,dex_pc_to_block_map);DCHECK(taken_block != nullptr);cur_block->taken = taken_block->id;taken_block->predecessors.push_back(cur_block->id);

下面处理continue退出块的情况：

  /* Always terminate the current block for conditional branches */if (flags & Instruction::kContinue) {BasicBlock* fallthrough_block = FindBlock(cur_offset +  width,/* create */true,/* immed_pred_block_p */&cur_block,dex_pc_to_block_map);DCHECK(fallthrough_block != nullptr);cur_block->fall_through = fallthrough_block->id;fallthrough_block->predecessors.push_back(cur_block->id);} else if (code_ptr < code_end) {FindBlock(cur_offset + width, /* create */ true, /* immed_pred_block_p */ nullptr, dex_pc_to_block_map);}return cur_block;
}

ProcessCanSwitch

处理switch语句：


/* Process instructions with the kSwitch flag */
BasicBlock* MIRGraph::ProcessCanSwitch(BasicBlock* cur_block, MIR* insn, DexOffset cur_offset,int width, int flags,ScopedArenaVector<uint16_t>* dex_pc_to_block_map) {UNUSED(flags);const uint16_t* switch_data =reinterpret_cast<const uint16_t*>(GetCurrentInsns() + cur_offset +static_cast<int32_t>(insn->dalvikInsn.vB));int size;const int* keyTable;const int* target_table;int i;int first_key;

switch的case以压缩的格式存储的话：

  /** Packed switch data format:*  ushort ident = 0x0100   magic value*  ushort size             number of entries in the table*  int first_key           first (and lowest) switch case value*  int targets[size]       branch targets, relative to switch opcode** Total size is (4+size*2) 16-bit code units.*/if (insn->dalvikInsn.opcode == Instruction::PACKED_SWITCH) {DCHECK_EQ(static_cast<int>(switch_data[0]),static_cast<int>(Instruction::kPackedSwitchSignature));size = switch_data[1];first_key = switch_data[2] | (switch_data[3] << 16);target_table = reinterpret_cast<const int*>(&switch_data[4]);keyTable = nullptr;        // Make the compiler happy.

以非压缩的稀疏方式存储的情况：

  /** Sparse switch data format:*  ushort ident = 0x0200   magic value*  ushort size             number of entries in the table; > 0*  int keys[size]          keys, sorted low-to-high; 32-bit aligned*  int targets[size]       branch targets, relative to switch opcode** Total size is (2+size*4) 16-bit code units.*/} else {DCHECK_EQ(static_cast<int>(switch_data[0]),static_cast<int>(Instruction::kSparseSwitchSignature));size = switch_data[1];keyTable = reinterpret_cast<const int*>(&switch_data[2]);target_table = reinterpret_cast<const int*>(&switch_data[2 + size*2]);first_key = 0;   // To make the compiler happy.}
...

下面去查找对应的代码块，并把它们组织起来。

  cur_block->successor_block_list_type =(insn->dalvikInsn.opcode == Instruction::PACKED_SWITCH) ?  kPackedSwitch : kSparseSwitch;cur_block->successor_blocks.reserve(size);for (i = 0; i < size; i++) {BasicBlock* case_block = FindBlock(cur_offset + target_table[i],  /* create */ true,/* immed_pred_block_p */ &cur_block,dex_pc_to_block_map);DCHECK(case_block != nullptr);SuccessorBlockInfo* successor_block_info =static_cast<SuccessorBlockInfo*>(arena_->Alloc(sizeof(SuccessorBlockInfo),kArenaAllocSuccessor));successor_block_info->block = case_block->id;successor_block_info->key =(insn->dalvikInsn.opcode == Instruction::PACKED_SWITCH) ?first_key + i : keyTable[i];cur_block->successor_blocks.push_back(successor_block_info);case_block->predecessors.push_back(cur_block->id);}

下面处理落空的情况，就是default的情况了。

  /* Fall-through case */BasicBlock* fallthrough_block = FindBlock(cur_offset +  width, /* create */ true,/* immed_pred_block_p */ nullptr,dex_pc_to_block_map);DCHECK(fallthrough_block != nullptr);cur_block->fall_through = fallthrough_block->id;fallthrough_block->predecessors.push_back(cur_block->id);return cur_block;
}

ProcessCanThrow - 处理异常的情况

/* Process instructions with the kThrow flag */
BasicBlock* MIRGraph::ProcessCanThrow(BasicBlock* cur_block, MIR* insn, DexOffset cur_offset,int width, int flags, ArenaBitVector* try_block_addr,const uint16_t* code_ptr, const uint16_t* code_end,ScopedArenaVector<uint16_t>* dex_pc_to_block_map) {UNUSED(flags);bool in_try_block = try_block_addr->IsBitSet(cur_offset);bool is_throw = (insn->dalvikInsn.opcode == Instruction::THROW);

首先是处理try块：

  /* In try block */if (in_try_block) {CatchHandlerIterator iterator(*current_code_item_, cur_offset);if (cur_block->successor_block_list_type != kNotUsed) {LOG(INFO) << PrettyMethod(cu_->method_idx, *cu_->dex_file);LOG(FATAL) << "Successor block list already in use: "<< static_cast<int>(cur_block->successor_block_list_type);}for (; iterator.HasNext(); iterator.Next()) {BasicBlock* catch_block = FindBlock(iterator.GetHandlerAddress(), false /* create */,nullptr /* immed_pred_block_p */,dex_pc_to_block_map);if (insn->dalvikInsn.opcode == Instruction::MONITOR_EXIT &&IsBadMonitorExitCatch(insn->offset, catch_block->start_offset)) {// Don't allow monitor-exit to catch its own exception, http://b/15745363 .continue;}if (cur_block->successor_block_list_type == kNotUsed) {cur_block->successor_block_list_type = kCatch;}catch_block->catch_entry = true;if (kIsDebugBuild) {catches_.insert(catch_block->start_offset);}SuccessorBlockInfo* successor_block_info = reinterpret_cast<SuccessorBlockInfo*>(arena_->Alloc(sizeof(SuccessorBlockInfo), kArenaAllocSuccessor));successor_block_info->block = catch_block->id;successor_block_info->key = iterator.GetHandlerTypeIndex();cur_block->successor_blocks.push_back(successor_block_info);catch_block->predecessors.push_back(cur_block->id);}in_try_block = (cur_block->successor_block_list_type != kNotUsed);}bool build_all_edges =(cu_->disable_opt & (1 << kSuppressExceptionEdges)) || is_throw || in_try_block;if (!in_try_block && build_all_edges) {BasicBlock* eh_block = CreateNewBB(kExceptionHandling);cur_block->taken = eh_block->id;eh_block->start_offset = cur_offset;eh_block->predecessors.push_back(cur_block->id);}

如果有异常要抛出，就需要构建一个catch块去处理：

  if (is_throw) {cur_block->explicit_throw = true;if (code_ptr < code_end) {// Force creation of new block following THROW via side-effect.FindBlock(cur_offset + width, /* create */ true, /* immed_pred_block_p */ nullptr, dex_pc_to_block_map);}if (!in_try_block) {// Don't split a THROW that can't rethrow - we're done.return cur_block;}}if (!build_all_edges) {/** Even though there is an exception edge here, control cannot return to this* method.  Thus, for the purposes of dataflow analysis and optimization, we can* ignore the edge.  Doing this reduces compile time, and increases the scope* of the basic-block level optimization pass.*/return cur_block;}

下面是对catch的处理。注释里有详细的说明，我们后面再讨论细节。
这个阶段重要的是大家对于整个流程有个概念，可以不必过于关注细节。

  /** Split the potentially-throwing instruction into two parts.* The first half will be a pseudo-op that captures the exception* edges and terminates the basic block.  It always falls through.* Then, create a new basic block that begins with the throwing instruction* (minus exceptions).  Note: this new basic block must NOT be entered into* the block_map.  If the potentially-throwing instruction is the target of a* future branch, we need to find the check psuedo half.  The new* basic block containing the work portion of the instruction should* only be entered via fallthrough from the block containing the* pseudo exception edge MIR.  Note also that this new block is* not automatically terminated after the work portion, and may* contain following instructions.** Note also that the dex_pc_to_block_map entry for the potentially* throwing instruction will refer to the original basic block.*/BasicBlock* new_block = CreateNewBB(kDalvikByteCode);new_block->start_offset = insn->offset;cur_block->fall_through = new_block->id;new_block->predecessors.push_back(cur_block->id);MIR* new_insn = NewMIR();*new_insn = *insn;insn->dalvikInsn.opcode = static_cast<Instruction::Code>(kMirOpCheck);// Associate the two halves.insn->meta.throw_insn = new_insn;new_block->AppendMIR(new_insn);return new_block;
}