js引擎v8源码解析之map对象上篇（基于v8 0.1.5）

theanarkh

发布于 2019-11-23 22:24:34

1.5K0

发布于 2019-11-23 22:24:34

这一篇首先介绍Map类。下面先看类定义

// All heap objects have a Map that describes their structure.
//  A Map contains information about:
//  - Size information about the object
//  - How to iterate over an object (for garbage collection)
class Map: public HeapObject {
 public:
  // instance size.
  inline int instance_size();
  inline void set_instance_size(int value);

  // instance type.
  inline InstanceType instance_type();
  inline void set_instance_type(InstanceType value);

  // tells how many unused property fields are available in the instance.
  // (only used for JSObject in fast mode).
  inline int unused_property_fields();
  inline void set_unused_property_fields(int value);

  // bit field.
  inline byte bit_field();
  inline void set_bit_field(byte value);

  // Tells whether this object has a special lookup behavior.
  void set_special_lookup() {
    set_bit_field(bit_field() | (1 << kHasSpecialLookup));
  }

  bool has_special_lookup() {
    return ((1 << kHasSpecialLookup) & bit_field()) != 0;
  }

  // Tells whether the object in the prototype property will be used
  // for instances created from this function.  If the prototype
  // property is set to a value that is not a JSObject, the prototype
  // property will not be used to create instances of the function.
  // See ECMA-262, 13.2.2.
  inline void set_non_instance_prototype(bool value);
  inline bool has_non_instance_prototype();

  // Tells whether the instance with this map should be ignored by the
  // __proto__ accessor.
  inline void set_is_hidden_prototype() {
    set_bit_field(bit_field() | (1 << kIsHiddenPrototype));
  }

  inline bool is_hidden_prototype() {
    return ((1 << kIsHiddenPrototype) & bit_field()) != 0;
  }

  // Tells whether the instance has a named interceptor.
  inline void set_has_named_interceptor() {
    set_bit_field(bit_field() | (1 << kHasNamedInterceptor));
  }

  inline bool has_named_interceptor() {
    return ((1 << kHasNamedInterceptor) & bit_field()) != 0;
  }

  // Tells whether the instance has a named interceptor.
  inline void set_has_indexed_interceptor() {
    set_bit_field(bit_field() | (1 << kHasIndexedInterceptor));
  }

  inline bool has_indexed_interceptor() {
    return ((1 << kHasIndexedInterceptor) & bit_field()) != 0;
  }

  // Tells whether the instance is undetectable.
  // An undetectable object is a special class of JSObject: 'typeof' operator
  // returns ?developer/article/1543293/undefined, ToBoolean returns false. Otherwise it behaves like
  // a normal JS object.  It is useful for implementing undetectable
  // document.all in Firefox & Safari.
  // See https://bugzilla.mozilla.org/show_bug.cgi?id=248549.
  inline void set_is_undetectable() {
    set_bit_field(bit_field() | (1 << kIsUndetectable));
  }

  inline bool is_undetectable() {
    return ((1 << kIsUndetectable) & bit_field()) != 0;
  }

  // Tells whether the instance has a call-as-function handler.
  inline void set_has_instance_call_handler() {
    set_bit_field(bit_field() | (1 << kHasInstanceCallHandler));
  }

  inline bool has_instance_call_handler() {
    return ((1 << kHasInstanceCallHandler) & bit_field()) != 0;
  }

  // Tells whether the instance needs security checks when accessing its
  // properties.
  inline void set_needs_access_check() {
    set_bit_field(bit_field() | (1 << kNeedsAccessCheck));
  }

  inline bool needs_access_check() {
    return ((1 << kNeedsAccessCheck) & bit_field()) != 0;
  }

  // [prototype]: implicit prototype object.
  /*
        #define DECL_ACCESSORS(name, type)  \
      inline type* name();                 \
      inline void set_##name(type* value)
      宏展开后变成，定义了读写某个属性的函数
      Object * prototype();
      void * set_prototype(Object * value);

      属性的定义如下（宏展开后也是读写某个属性）：
        #define ACCESSORS(holder, name, type, offset)                                   \
          type* holder::name() { return type::cast(READ_FIELD(this, offset)); } \
          void holder::set_##name(type* value) {                                \
            WRITE_FIELD(this, offset, value);                                   \
            WRITE_BARRIER(this, offset);                                        \
          }

        // 定义各个类的读写某属性的函数，第三第四个参数是类型和偏移
        ACCESSORS(Map, instance_descriptors, DescriptorArray,
                  kInstanceDescriptorsOffset)
        ACCESSORS(Map, code_cache, FixedArray, kCodeCacheOffset)
        ACCESSORS(Map, constructor, Object, kConstructorOffset
  */
  DECL_ACCESSORS(prototype, Object)

  // [constructor]: points back to the function responsible for this map.
  DECL_ACCESSORS(constructor, Object)

  // [instance descriptors]: describes the object.
  DECL_ACCESSORS(instance_descriptors, DescriptorArray)

  // [stub cache]: contains stubs compiled for this map.
  DECL_ACCESSORS(code_cache, FixedArray)

  // Returns a copy of the map.
  Object* Copy();

  // Returns the property index for name (only valid for FAST MODE).
  int PropertyIndexFor(String* name);

  // Returns the next free property index (only valid for FAST MODE).
  int NextFreePropertyIndex();

  // Returns the number of properties described in instance_descriptors.
  int NumberOfDescribedProperties();

  // Casting.
  static inline Map* cast(Object* obj);

  // Locate an accessor in the instance descriptor.
  AccessorDescriptor* FindAccessor(String* name);

  // Make sure the instance descriptor has no map transitions
  Object* EnsureNoMapTransitions();

  // Code cache operations.

  // Clears the code cache.
  inline void ClearCodeCache();

  // Update code cache.
  Object* UpdateCodeCache(String* name, Code* code);

  // Returns the found code or ?developer/article/1543293/undefined if absent.
  Object* FindInCodeCache(String* name, Code::Flags flags);

  // Tells whether code is in the code cache.
  bool IncludedInCodeCache(Code* code);

  // Dispatched behavior.
  void MapIterateBody(ObjectVisitor* v);
#ifdef DEBUG
  void MapPrint();
  void MapVerify();
#endif

  // Layout description.
  static const int kInstanceAttributesOffset = HeapObject::kSize;
  static const int kPrototypeOffset = kInstanceAttributesOffset + kIntSize;
  static const int kConstructorOffset = kPrototypeOffset + kPointerSize;
  static const int kInstanceDescriptorsOffset =
      kConstructorOffset + kPointerSize;
  static const int kCodeCacheOffset = kInstanceDescriptorsOffset + kPointerSize;
  static const int kSize = kCodeCacheOffset + kIntSize;

  // Byte offsets within kInstanceAttributesOffset attributes.
  static const int kInstanceSizeOffset = kInstanceAttributesOffset + 0;
  static const int kInstanceTypeOffset = kInstanceAttributesOffset + 1;
  static const int kUnusedPropertyFieldsOffset = kInstanceAttributesOffset + 2;
  static const int kBitFieldOffset = kInstanceAttributesOffset + 3;

  // kBitFieldOffset对应的一个字节，下面分别是该一个字节各比特位的标记
  static const int kHasSpecialLookup = 0;
  static const int kHasNonInstancePrototype = 1;
  static const int kIsHiddenPrototype = 2;
  static const int kHasNamedInterceptor = 3;
  static const int kHasIndexedInterceptor = 4;
  static const int kIsUndetectable = 5;
  static const int kHasInstanceCallHandler = 6;
  static const int kNeedsAccessCheck = 7;
 private:
  DISALLOW_IMPLICIT_CONSTRUCTORS(Map);
};

下面的map的属性内存布局。

我们逐个函数分析他的实现。首先看objects-inl.h中的实现。

// 获取对象某个属性的地址，p是对象的首地址，offset是偏移，kHeapObjectTag是对象的标记，算地址的时候需要减掉
#define FIELD_ADDR(p, offset) \
  (reinterpret_cast<byte*>(p) + offset - kHeapObjectTag)

// 读写一个字节的内容
#define READ_BYTE_FIELD(p, offset) \
  (*reinterpret_cast<byte*>(FIELD_ADDR(p, offset)))

#define WRITE_BYTE_FIELD(p, offset, value) \
  (*reinterpret_cast<byte*>(FIELD_ADDR(p, offset)) = value)


void Map::set_instance_size(int value) {
  ASSERT(0 <= value && value < 256);
  WRITE_BYTE_FIELD(this, kInstanceSizeOffset, static_cast<byte>(value));
}


InstanceType Map::instance_type() {
  return static_cast<InstanceType>(READ_BYTE_FIELD(this, kInstanceTypeOffset));
}


void Map::set_instance_type(InstanceType value) {
  ASSERT(0 <= value && value < 256);
  WRITE_BYTE_FIELD(this, kInstanceTypeOffset, value);
}


int Map::unused_property_fields() {
  return READ_BYTE_FIELD(this, kUnusedPropertyFieldsOffset);
}


void Map::set_unused_property_fields(int value) {
  WRITE_BYTE_FIELD(this, kUnusedPropertyFieldsOffset, Min(value, 255));
}

// 读写一个字节的内容，每个比特都记录着一个标记
byte Map::bit_field() {
  return READ_BYTE_FIELD(this, kBitFieldOffset);
}


void Map::set_bit_field(byte value) {
  WRITE_BYTE_FIELD(this, kBitFieldOffset, value);
}


void Map::set_non_instance_prototype(bool value) {
  if (value) {
    // 设置该位
    set_bit_field(bit_field() | (1 << kHasNonInstancePrototype));
  } else {
    // 清除该位
    set_bit_field(bit_field() & ~(1 << kHasNonInstancePrototype));
  }
}

// 是否设置了某位
bool Map::has_non_instance_prototype() {
  return ((1 << kHasNonInstancePrototype) & bit_field()) != 0;
}

void Map::ClearCodeCache() {
  // No write barrier is needed since empty_fixed_array is not in new space.
  // Please note this function is used during marking:
  //  - MarkCompactCollector::MarkUnmarkedObject
  ASSERT(!Heap::InNewSpace(Heap::empty_fixed_array()));
  WRITE_FIELD(this, kCodeCacheOffset, Heap::empty_fixed_array());
}

从上面的代码中我们知道，只是对某些属性或标记进行读写。根据对象的内存布局对号入座就行，至于每个属性和标记的意义，后续再慢慢探讨。map还有很多函数，但是会涉及很多其他的类，等后面分析完了，再继续分析。

本文参与?腾讯云自媒体分享计划，分享自微信公众号。

原始发表：2019-11-17，如有侵权请联系?cloudcommunity@tencent.com 删除

map