js引擎v8源码解析之对象第三篇（基于v8 0.1.5）

// An abstract superclass, a marker class really, for simple structure classes.
// It doesn't carry much functionality but allows struct classes to me
// identified in the type system.
class Struct: public HeapObject {
 public:
  inline void InitializeBody(int object_size);
  static inline Struct* cast(Object* that);
};

void Struct::InitializeBody(int object_size) {
  for (int offset = kSize; offset < object_size; offset += kPointerSize) {
    WRITE_FIELD(this, offset, Heap::undefined_value());
  }
}

class TemplateInfo: public Struct {
 public:
  DECL_ACCESSORS(tag, Object)
  DECL_ACCESSORS(property_list, Object)

  static const int kTagOffset          = HeapObject::kSize;
  static const int kPropertyListOffset = kTagOffset + kPointerSize;
  static const int kHeaderSize         = kPropertyListOffset + kPointerSize;
 protected:
  friend class AGCCVersionRequiresThisClassToHaveAFriendSoHereItIs;
  DISALLOW_IMPLICIT_CONSTRUCTORS(TemplateInfo);
};

class FunctionTemplateInfo: public TemplateInfo {
 public:
  DECL_ACCESSORS(serial_number, Object)
  DECL_ACCESSORS(call_code, Object)
  DECL_ACCESSORS(property_accessors, Object)
  DECL_ACCESSORS(prototype_template, Object)
  DECL_ACCESSORS(parent_template, Object)
  DECL_ACCESSORS(named_property_handler, Object)
  DECL_ACCESSORS(indexed_property_handler, Object)
  DECL_ACCESSORS(instance_template, Object)
  DECL_ACCESSORS(class_name, Object)
  DECL_ACCESSORS(signature, Object)
  DECL_ACCESSORS(lookup_callback, Object)
  DECL_ACCESSORS(instance_call_handler, Object)
  DECL_ACCESSORS(access_check_info, Object)
  DECL_ACCESSORS(flag, Smi)

  // Following properties use flag bits.
  DECL_BOOLEAN_ACCESSORS(hidden_prototype)
  DECL_BOOLEAN_ACCESSORS(undetectable)
  // If the bit is set, object instances created by this function
  // requires access check.
  DECL_BOOLEAN_ACCESSORS(needs_access_check)

  static inline FunctionTemplateInfo* cast(Object* obj);

#ifdef DEBUG
  void FunctionTemplateInfoPrint();
  void FunctionTemplateInfoVerify();
#endif

  static const int kSerialNumberOffset = TemplateInfo::kHeaderSize;
  static const int kCallCodeOffset = kSerialNumberOffset + kPointerSize;
  static const int kPropertyAccessorsOffset = kCallCodeOffset + kPointerSize;
  static const int kPrototypeTemplateOffset =
      kPropertyAccessorsOffset + kPointerSize;
  static const int kParentTemplateOffset =
      kPrototypeTemplateOffset + kPointerSize;
  static const int kNamedPropertyHandlerOffset =
      kParentTemplateOffset + kPointerSize;
  static const int kIndexedPropertyHandlerOffset =
      kNamedPropertyHandlerOffset + kPointerSize;
  static const int kInstanceTemplateOffset =
      kIndexedPropertyHandlerOffset + kPointerSize;
  static const int kClassNameOffset = kInstanceTemplateOffset + kPointerSize;
  static const int kSignatureOffset = kClassNameOffset + kPointerSize;
  static const int kLookupCallbackOffset = kSignatureOffset + kPointerSize;
  static const int kInstanceCallHandlerOffset =
      kLookupCallbackOffset + kPointerSize;
  static const int kAccessCheckInfoOffset =
      kInstanceCallHandlerOffset + kPointerSize;
  static const int kFlagOffset = kAccessCheckInfoOffset + kPointerSize;
  static const int kSize = kFlagOffset + kPointerSize;

 private:
  DISALLOW_IMPLICIT_CONSTRUCTORS(FunctionTemplateInfo);

  // Bit position in the flag, from least significant bit position.
  static const int kHiddenPrototypeBit   = 0;
  static const int kUndetectableBit      = 1;
  static const int kNeedsAccessCheckBit  = 2;
};

ACCESSORS(FunctionTemplateInfo, serial_number, Object, kSerialNumberOffset)
ACCESSORS(FunctionTemplateInfo, call_code, Object, kCallCodeOffset)
ACCESSORS(FunctionTemplateInfo, property_accessors, Object,
          kPropertyAccessorsOffset)
ACCESSORS(FunctionTemplateInfo, prototype_template, Object,
          kPrototypeTemplateOffset)
ACCESSORS(FunctionTemplateInfo, parent_template, Object, kParentTemplateOffset)
ACCESSORS(FunctionTemplateInfo, named_property_handler, Object,
          kNamedPropertyHandlerOffset)
ACCESSORS(FunctionTemplateInfo, indexed_property_handler, Object,
          kIndexedPropertyHandlerOffset)
ACCESSORS(FunctionTemplateInfo, instance_template, Object,
          kInstanceTemplateOffset)
ACCESSORS(FunctionTemplateInfo, class_name, Object, kClassNameOffset)
ACCESSORS(FunctionTemplateInfo, signature, Object, kSignatureOffset)
ACCESSORS(FunctionTemplateInfo, lookup_callback, Object, kLookupCallbackOffset)
ACCESSORS(FunctionTemplateInfo, instance_call_handler, Object,
          kInstanceCallHandlerOffset)
ACCESSORS(FunctionTemplateInfo, access_check_info, Object,
          kAccessCheckInfoOffset)
ACCESSORS(FunctionTemplateInfo, flag, Smi, kFlagOffset)

BOOL_ACCESSORS(FunctionTemplateInfo, flag, hidden_prototype,
               kHiddenPrototypeBit)
BOOL_ACCESSORS(FunctionTemplateInfo, flag, undetectable, kUndetectableBit)
BOOL_ACCESSORS(FunctionTemplateInfo, flag, needs_access_check,
               kNeedsAccessCheckBit)

class ObjectTemplateInfo: public TemplateInfo {
 public:
  DECL_ACCESSORS(constructor, Object)
  DECL_ACCESSORS(internal_field_count, Object)

  static inline ObjectTemplateInfo* cast(Object* obj);

#ifdef DEBUG
  void ObjectTemplateInfoPrint();
  void ObjectTemplateInfoVerify();
#endif

  static const int kConstructorOffset = TemplateInfo::kHeaderSize;
  static const int kInternalFieldCountOffset =
      kConstructorOffset + kPointerSize;
  static const int kSize = kInternalFieldCountOffset + kHeaderSize;
};

ACCESSORS(ObjectTemplateInfo, constructor, Object, kConstructorOffset)
ACCESSORS(ObjectTemplateInfo, internal_field_count, Object,
          kInternalFieldCountOffset)

// BooleanBit is a helper class for setting and getting a bit in an
// integer or Smi.
// 比特位读写类
class BooleanBit : public AllStatic {
 public:
  static inline bool get(Smi* smi, int bit_position) {
    return get(smi->value(), bit_position);
  }

  static inline bool get(int value, int bit_position) {
    return (value & (1 << bit_position)) != 0;
  }

  static inline Smi* set(Smi* smi, int bit_position, bool v) {
    return Smi::FromInt(set(smi->value(), bit_position, v));
  }

  static inline int set(int value, int bit_position, bool v) {
    if (v) {
      // 设置
      value |= (1 << bit_position);
    } else {
      // 清除
      value &= ~(1 << bit_position);
    }
    return value;
  }
};

} }

class AccessorInfo: public Struct {
 public:
  DECL_ACCESSORS(getter, Object)
  DECL_ACCESSORS(setter, Object)
  DECL_ACCESSORS(data, Object)
  DECL_ACCESSORS(name, Object)
  DECL_ACCESSORS(flag, Smi)

  inline bool all_can_read();
  inline void set_all_can_read(bool value);

  inline bool all_can_write();
  inline void set_all_can_write(bool value);

  inline PropertyAttributes property_attributes();
  inline void set_property_attributes(PropertyAttributes attributes);

  static inline AccessorInfo* cast(Object* obj);

  static const int kGetterOffset = HeapObject::kSize;
  static const int kSetterOffset = kGetterOffset + kPointerSize;
  static const int kDataOffset = kSetterOffset + kPointerSize;
  static const int kNameOffset = kDataOffset + kPointerSize;
  static const int kFlagOffset = kNameOffset + kPointerSize;
  static const int kSize = kFlagOffset + kPointerSize;

 private:
  DISALLOW_IMPLICIT_CONSTRUCTORS(AccessorInfo);

  // Bit positions in flag.
  static const int kAllCanReadBit  = 0;
  static const int kAllCanWriteBit = 1;
  class AttributesField: public BitField<PropertyAttributes, 2, 3> {};
};

ACCESSORS(AccessorInfo, getter, Object, kGetterOffset)
ACCESSORS(AccessorInfo, setter, Object, kSetterOffset)
ACCESSORS(AccessorInfo, data, Object, kDataOffset)
ACCESSORS(AccessorInfo, name, Object, kNameOffset)
ACCESSORS(AccessorInfo, flag, Smi, kFlagOffset)

// 低两位是可读写标记
class AttributesField: public BitField<PropertyAttributes, 2, 3> {};

// BitField is a help template for encoding and decode bitfield with unsigned
// content.
template<class T, int shift, int size>
class BitField {
 public:
  // Tells whether the provided value fits into the bit field.
  // 判断低几位之外的其他位是否都等于0
  static bool is_valid(T value) {
    return (static_cast<uint32_t>(value) & ~((1U << (size)) - 1)) == 0;
  }

  // Returns a uint32_t mask of bit field.
  // 从第shift+1到shift+size位为1，其他位为0。这里是3到5位
  static uint32_t mask() {
    return (1U << (size + shift)) - (1U << shift);
  }

  // Returns a uint32_t with the bit field value encoded.
  // 设置某位的值，这里是跳过低2位，因为低两位是可读写标记
  static uint32_t encode(T value) {
    ASSERT(is_valid(value));
    return static_cast<uint32_t>(value) << shift;
  }

  // Extracts the bit field from the value.
  // 取某位的值，需要屏蔽其他不相关的位的值，这里值的范围是小于等于7
  static T decode(uint32_t value) {
    return static_cast<T>((value >> shift) & ((1U << (size)) - 1));
  }
};

// 判断某个值的比特是否已设置
bool AccessorInfo::all_can_read() {
  return BooleanBit::get(flag(), kAllCanReadBit);
}

// 设置某个值的某个比特
void AccessorInfo::set_all_can_read(bool value) {
  set_flag(BooleanBit::set(flag(), kAllCanReadBit, value));
}


bool AccessorInfo::all_can_write() {
  return BooleanBit::get(flag(), kAllCanWriteBit);
}


void AccessorInfo::set_all_can_write(bool value) {
  set_flag(BooleanBit::set(flag(), kAllCanWriteBit, value));
}

// 获flag里取某位的值
PropertyAttributes AccessorInfo::property_attributes() {
  return AttributesField::decode(static_cast<uint32_t>(flag()->value()));
}

/*
  // Ecma-262 3rd 8.6.1
  enum PropertyAttributes {
    NONE              = v8::None,
    READ_ONLY         = v8::ReadOnly,
    DONT_ENUM         = v8::DontEnum,
    DONT_DELETE       = v8::DontDelete,
    INTERCEPTED       = 1 << 3,
    ABSENT            = 16  // Used in runtime to indicate a property is absent.
  };

 设置flag某位的值，见AttributesField
 */
void AccessorInfo::set_property_attributes(PropertyAttributes attributes) {
  ASSERT(AttributesField::is_valid(attributes));
  // 清除相关的位
  int rest_value = flag()->value() & ~AttributesField::mask();
  // 重新设置某位
  set_flag(Smi::FromInt(rest_value | AttributesField::encode(attributes)));
}