这一篇首先介绍Map类。下面先看类定义
代码语言：javascript复制// 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 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 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。
代码语言：javascript复制// 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。
