《Chrome V8源码》26.Bytecode Handler,字节码的核心

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1 摘要

本篇文章是Builtin专题的第二篇,讲解Bytecode Handler的初始化过程以及相关数据结构。Bytecode handler是采用CAS方式编写的Builtin,它实现了Bytecode的功能,每一条Bytecode对应一条Bytecode handler。本文内容组织方法:Bytecode handler介绍(章节2);Bytecode Handler初始化(章节3)。

 

2 Bytecode Handler介绍

Bytecode handler的地址保存在Dispatch_table数组中,Dispatch_table保存在Isolate中。每一条Bytecode执行完毕后通过Dispatch_table找到下一条Bytecode并完成跳转。Bytecode handler的源码在interpreter-generator.cc文件中,其中Star和Mov的源码如下:

1.  // Store accumulator to register <dst>.
2.  IGNITION_HANDLER(Star, InterpreterAssembler) {
3.    TNode<Object> accumulator = GetAccumulator();
4.    StoreRegisterAtOperandIndex(accumulator, 0);
5.    Dispatch();
6.  }
7.  // Mov <src> <dst>
8.  //
9.  // Stores the value of register <src> to register <dst>.
10.  IGNITION_HANDLER(Mov, InterpreterAssembler) {
11.    TNode<Object> src_value = LoadRegisterAtOperandIndex(0);
12.    StoreRegisterAtOperandIndex(src_value, 1);
13.    Dispatch();
14.  }

上述代码中,Dispatch()利用Isolate读取Dispatch_table并跳转到下一条指令。

 

3 Bytecode Handler初始化

Bytecode handler是用CodeStubAssembler编写的Builtin。第一个Isolate创建时,Bytecode handler完成初始化并存储在dispatch_table中,后续创建新的Isoalte时无需再做初始化,只需要拷贝即可。
Bytecode Handler初始化的入口源码如下:

1.  void SetupIsolateDelegate::SetupBuiltinsInternal(Isolate* isolate) {
2.    Builtins* builtins = isolate->builtins();
3.    int index = 0;
4.    Code code;
5.    //...........省略............................
6.  #define BUILD_BCH(Name, OperandScale, Bytecode)                           \
7.    code = GenerateBytecodeHandler(isolate, index, OperandScale, Bytecode); \
8.    AddBuiltin(builtins, index++, code);
9.   BUILTIN_LIST(BUILD_CPP, BUILD_TFJ, BUILD_TFC, BUILD_TFS, BUILD_TFH, BUILD_BCH,
10.                 BUILD_ASM);
11.    //.........省略............................
12.  }

BUILD_BCHBUILTIN_LISTGenerateBytecodeHandler()共同完成所有Bytecode handler的创建,GenerateBytecodeHandler()源码如下:

Code GenerateBytecodeHandler(Isolate* isolate, int builtin_index,
                             interpreter::OperandScale operand_scale,
                             interpreter::Bytecode bytecode) {
  DCHECK(interpreter::Bytecodes::BytecodeHasHandler(bytecode, operand_scale));
  Handle<Code> code = interpreter::GenerateBytecodeHandler(
      isolate, Builtins::name(builtin_index), bytecode, operand_scale,
      builtin_index, BuiltinAssemblerOptions(isolate, builtin_index));
  return *code;
}

上述代码中Builtins::name(builtin_index)获取Bytecode的名字,源码如下:

const char* Builtins::name(int index) {
  DCHECK(IsBuiltinId(index));
  return builtin_metadata[index].name;
}
//................分隔线.......................
#define DECL_CPP(Name, ...) \
  {#Name, Builtins::CPP, {FUNCTION_ADDR(Builtin_##Name)}},
#define DECL_TFJ(Name, Count, ...) {#Name, Builtins::TFJ, {Count, 0}},
#define DECL_TFC(Name, ...) {#Name, Builtins::TFC, {}},
#define DECL_TFS(Name, ...) {#Name, Builtins::TFS, {}},
#define DECL_TFH(Name, ...) {#Name, Builtins::TFH, {}},
#define DECL_BCH(Name, OperandScale, Bytecode) \
  {#Name, Builtins::BCH, {Bytecode, OperandScale}},
#define DECL_ASM(Name, ...) {#Name, Builtins::ASM, {}},
const BuiltinMetadata builtin_metadata[] = {BUILTIN_LIST(
    DECL_CPP, DECL_TFJ, DECL_TFC, DECL_TFS, DECL_TFH, DECL_BCH, DECL_ASM)};

上述代码中builtin_metadata数组保存每一条Bytecode的名字、类型和地址指针。如图1中所示,1075号Builtin的名字是DebugBreak0Handler,类型是BCH(Bytecode handler),函数地址是data。

Code GenerateBytecodeHandler()中调用interpreter::GenerateBytecodeHandler(),源码如下:

1.  Handle<Code> GenerateBytecodeHandler(Isolate* isolate, const char* debug_name,
2.                                       Bytecode bytecode,
3.                                       OperandScale operand_scale,
4.                                       int builtin_index,
5.                                       const AssemblerOptions& options) {
6.    Zone zone(isolate->allocator(), ZONE_NAME);
7.    compiler::CodeAssemblerState state(
8.        isolate, &zone, InterpreterDispatchDescriptor{}, Code::BYTECODE_HANDLER,
9.        debug_name,
10.        FLAG_untrusted_code_mitigations
11.            ? PoisoningMitigationLevel::kPoisonCriticalOnly
12.            : PoisoningMitigationLevel::kDontPoison,
13.        builtin_index);
14.    switch (bytecode) {
15.  #define CALL_GENERATOR(Name, ...)                     \
16.    case Bytecode::k##Name:                             \
17.      Name##Assembler::Generate(&state, operand_scale); \
18.      break;
19.      BYTECODE_LIST(CALL_GENERATOR);
20.  #undef CALL_GENERATOR
21.    }
22.    Handle<Code> code = compiler::CodeAssembler::GenerateCode(&state, options);
23.    return code;
24.  }

上述代码中,Bytecode bytecode是枚举变量,源码如下:

// Enumeration of interpreter bytecodes.
enum class Bytecode : uint8_t {
#define DECLARE_BYTECODE(Name, ...) k##Name,
  BYTECODE_LIST(DECLARE_BYTECODE)
#undef DECLARE_BYTECODE
#define COUNT_BYTECODE(x, ...) +1
  // The COUNT_BYTECODE macro will turn this into kLast = -1 +1 +1... which will
  // evaluate to the same value as the last real bytecode.
  kLast = -1 BYTECODE_LIST(COUNT_BYTECODE)
#undef COUNT_BYTECODE
};

GenerateBytecodeHandler()的第14行代码:根据bytecode的类型执行对应的case以完成Bytecode handler的初始化。BYTECODE_LIST源码如下:

// The list of bytecodes which are interpreted by the interpreter.
// Format is V(<bytecode>, <accumulator_use>, <operands>).
#define BYTECODE_LIST(V)                                                       \
  /* Extended width operands */                                                \
  V(Wide, AccumulatorUse::kNone)                                               \
  V(ExtraWide, AccumulatorUse::kNone)                                          \
                                                                               \
  /* Debug Breakpoints - one for each possible size of unscaled bytecodes */   \
  /* and one for each operand widening prefix bytecode                    */   \
  V(DebugBreakWide, AccumulatorUse::kReadWrite)                                \
  V(DebugBreakExtraWide, AccumulatorUse::kReadWrite)                           \
  V(DebugBreak0, AccumulatorUse::kReadWrite)                                   \
  V(DebugBreak1, AccumulatorUse::kReadWrite, OperandType::kReg)                \
  V(DebugBreak2, AccumulatorUse::kReadWrite, OperandType::kReg,                \
    OperandType::kReg)                                                         \
  V(DebugBreak3, AccumulatorUse::kReadWrite, OperandType::kReg,                \
    OperandType::kReg, OperandType::kReg)                                      \
    //省略...................................
//.................................分隔线.......................................
switch (bytecode) {
    case Bytecode::kWide:                             \
      WideAssembler::Generate(&state, operand_scale); \
      break;
    case Bytecode::kLdaSmi:                             \
      LdaSmiAssembler::Generate(&state, operand_scale); \
      break;
//省略...................................
}

通过展开kWideKLdaSmi可以看到各自的生成函数,图2给出了此时的调用堆栈。

下面讲解LdaSmi的初始化,LdaSmi的初始化函数由宏IGNITION_HANDLERIGNITION_HANDLER(LdaSmi, InterpreterAssembler)共同组成,展开后的源码如下:

1.    class LdaSmiAssembler : public InterpreterAssembler { 
2.     public:                                                        
3.      explicit LdaSmiAssembler(compiler::CodeAssemblerState* state,    
4.                               Bytecode bytecode, OperandScale scale)  
5.          : InterpreterAssembler(state, bytecode, scale) {}                 
6.      static void Generate(compiler::CodeAssemblerState* state,        
7.                           OperandScale scale);                        
8.     private:                                                           
9.      void GenerateImpl();                                              
10.      DISALLOW_COPY_AND_ASSIGN(LdaSmiAssembler);                        
11.    };                                                                  
12.    void LdaSmiAssembler::Generate(compiler::CodeAssemblerState* state, 
13.                                   OperandScale scale) {                
14.      LdaSmiAssembler assembler(state, Bytecode::kLdaSmi, scale);       
15.      state->SetInitialDebugInformation("LdaSmi", __FILE__, __LINE__);  
16.      assembler.GenerateImpl();                                        
17.    }                                                                 
18.    void LdaSmiAssembler::GenerateImpl(){
19.    TNode<Smi> smi_int = BytecodeOperandImmSmi(0);
20.    SetAccumulator(smi_int);
21.    Dispatch();
22.  }

上述代码满足case Bytecode::kLdaSmi条件,执行第12行代码Generate();在Generate()中调用GenerateImpl()(第18行代码);第19行代码生成小整数smi_int;第20行代码把smi_int存入累加寄存器。第1行代码InterpreterAssembler的父类InterpreterAssembler实现了很多Bytecode handler的基础功能,源码如下:

TNode<Object> InterpreterAssembler::GetAccumulator() {
  DCHECK(Bytecodes::ReadsAccumulator(bytecode_));
  accumulator_use_ = accumulator_use_ | AccumulatorUse::kRead;
  return TaggedPoisonOnSpeculation(GetAccumulatorUnchecked());
}
//分隔线.................................
void InterpreterAssembler::SetAccumulator(SloppyTNode<Object> value) {
  DCHECK(Bytecodes::WritesAccumulator(bytecode_));
  accumulator_use_ = accumulator_use_ | AccumulatorUse::kWrite;
  accumulator_ = value;
}
//分隔线.................................
TNode<ExternalReference> InterpreterAssembler::DispatchTablePointer() {
  if (Bytecodes::MakesCallAlongCriticalPath(bytecode_) && made_call_ &&
      (dispatch_table_.value() ==
       Parameter(InterpreterDispatchDescriptor::kDispatchTable))) {
    dispatch_table_ = ExternalConstant(
        ExternalReference::interpreter_dispatch_table_address(isolate()));
  }
  return dispatch_table_.value();
}

上述代码中GetAccumulator()获取累加器的值;SetAccumulator设置累加器的值;DispatchTablePointer()获取dispatchtable的地址;这些函数的源码在interpreter-assembler.cc文件中。
技术总结
(1) 调试Bytecode handler的方法是在Code GenerateBytecodeHandler()中设置断点;
(2) Bytecode在宏BYTECODELIST中的位置是Bytecode的编号index,index是Bytecode handler在dispatch_table中的下标;
(3) InterpreterAssembler中定义了Bytecode handler的底层功能,CodeAssembler中定义了更底层的原子操作。

好了,今天到这里,下次见。

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