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+size2) 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+size4) 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 + size2]);
    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;
}