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iOS 内存原理探究

定时器内存泄漏

NSTimer、CADisplayLink 的基础 API [NSTimer scheduledTimersWithTimeInterval:1 repeat:YES block:nil] 和当前的 VC 都会互相持有，造成环，会存在内存泄漏问题。

定时器内存泄漏原因，解决方案以及高精度定时器，具体可以看这篇 NSTimer 中的内存泄露。

iOS 内存布局

栈、堆、BSS、数据段、代码段

栈（stack）：又称作堆栈，用来存储程序的局部变量（但不包括static声明的变量，static修饰的数据存放于数据段中）。除此之外，在函数被调用时，栈用来传递参数和返回值。栈内存地址越来越少

func a {
    变量 1 地址最大
    变量 2 地址第二大
    // ...
    变量n  地址最小
}

堆（heap）：用于存储程序运行中被动态分配的内存段，它的大小并不固定，可动态的扩张和缩减。操作函数(malloc／free)。分配的内存空间地址越来越大。

BSS段（bss segment）：通常用来存储程序中未被初始化的全局变量和静态变量的一块内存区域。BSS是英文Block Started by Symbol的简称。BSS段输入静态内存分配

数据段（data segment）：通常用来存储程序中已被初始化的全局变量和静态变量和字符串的一块内存区域。数据段包含3部分：

字符串常量。比如 NSString *str = @"杭城小刘";
已初始化数据：已经初始化的全局变量、静态变量等
未初始化数据：未初始化的全局变量、静态变量等

代码段（code segment）：编译之后的代码。通常是指用来存储程序可执行代码的一块内存区域。这部分区域的大小在程序运行前就已经确定，并且内存区域通常属于只读，某些架构也允许代码段为可写，即允许修改程序。

上 Demo 验证

int a = 10;
static int b;
int main () {
    NSString *name = @"杭城小刘";
    int age = 27;
    int height = 177;
    NSObject *obj = [[NSObject alloc] init];
    NSLog(@"\na: %p\nb: %p\n name: %p\nage: %p\n height: %p\nobj:%p", &a, &b, &name, &age, &height, obj);
}
a: 0x107b09b80
b: 0x107b09c48
name: 0x7ff7b83fdbc0
age: 0x7ff7b83fdbbc
height: 0x7ff7b83fdbb8
obj:0x6000012780e0

我们按照内存地址由低到高排个序(如下)，发现和我们总结的规律一致。

// 字符串常量
name:   0x7ff7b83fdbc0
// 已初始化的全局变量、静态变量
a:      0x107b09b80
// 未初始化的全局变量、静态变量
b:      0x107b09c48
// 堆
obj:    0x6000012780e0
// 栈
height: 0x7ff7b83fdbb8
age:    0x7ff7b83fdbbc

NSObject *obj = [[NSObject alloc] init];
NSLog(@"%p %p %@", obj, &obj, obj);

分别打印 obj指针指向的堆上的内存地址、obj 指针在栈上的地址、obj 内容

Tagged Pointer

先来一个 Demo 开启本部分内容（画外音：代码很短，但让我产生了一个大大的问号）

- (bool)isTaggedPointer:(const void *)ptr
{
    return ((uintptr_t)ptr & (1UL<<63)) == (1UL<<63);
}

NSNumber *number = [NSNumber numberWithInt:10]; // 0xb0000000000000a2 b:12  1100
NSLog(@"%p %d %@", number, [self isTaggedPointer:(__bridge const void *)number], number.class);

NSString *name1 = [NSString stringWithFormat:@"ss"]; // 0xa000000000073732 a:11 1011
NSLog(@"%p %d %@", name1, [self isTaggedPointer:(__bridge const void *)name1], name1.class);

前提说明：

创建一个 NSNumer 类型的变量 number，NSString 类型的 name1，代码打印地址、类型。产生一个问题：为什么 NSNumber 是 TaggedPointer，但是 class 却显示 __NSCFNumber ？

static inline bool _objc_isTaggedPointer(const void * _Nullable ptr) 
{
    return ((uintptr_t)ptr & _OBJC_TAG_MASK) == _OBJC_TAG_MASK;
}

通过 objc4 源码研究写了个判断对象是否是 Tagged Pointer 类型的方法。通过系统源码参考写了判断方法 isTaggedPointer。调用方法得到 number 对象是 Tagged Pointer 类型
根据 iOS 平台特性，根据内存地址高位分析确实是 TaggedPointer 类型
同样的 NSString 指针指向的字符串内容比较少，占用内存没必要开创新的内存时，name1 就是 NSTaggedPointerString，打印出 class 也是 NSTaggedPointerString。调用 isTaggedPointer 得到也是 Tageed Pointer 类型

带着问题开始吧

什么是 Tagged Pointer

iOS 从 64bit 开始引入了Tagged Pointer 技术，用于优化 NSNumber、NSDate、NSString等小对象的存储。

在此之前，创建对象需要动态分配内存、维护引用计数等，对象指针存储的是堆中对象的地址值创建一个对象的流程。先在堆上申请一块内存，然后再在栈上增加一个指针类型，指针指向堆上这块内存。假如是 NSNumber *value = [NSNumber numberWithInt:2] value 是指针长度为8字节，堆上内存16字节。加起来24字节就存一个int 2。

此外还需要维护引用技术，沿用一个真正对象那一套，太大材小用了。

Tagged Pointer 格式，对象指针里面存储的数据变成了：Tag + Data，将数据直接存储在了指针中。当指针不够存储数据时，才会使用动态分配内存的方式来存储数据

objc_msgSend 能识别 Tagged Pointer，比如 NSNumber 的 intValue 方法，直接从指针提取数据，节省了调用开销。

经典问题

Demo1

- (void)test {
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    for (NSInteger i = 0; i<1000; i++) {
        dispatch_async(queue, ^{
            self.name = [NSString stringWithFormat:@"和好多好多好多好多事看看上课上课上课"];
        });
    }
}

运行该代码会 Crash，报错信息如下

说明：一开始的报错信息只说坏内存访问，但是并没有显示具体的方法调用堆。想知道具体 Crash 原因还是需要看看堆栈比较方便。输入 bt 查看最后是由于 objc_release 方法造成 crash。

小窍门：利用 LLDB 模式下输入 bt，可以查看堆栈。也就是 backtrace 的缩写。

不仔细想可能发现不了问题，看到 objc_release 就会想到是在多线程情况下 NSString 的 setter 方法内，ARC 代码经过编译器最后会按照 MRC 去运行。所以 Setter 类似下面代码。

-(void)setName:(NSString *)name 
{
    if (_name!=name) {
        [name retain];
        [_name release];
        _name = name;
    }
}

Demo

- (void)test {
    dispatch_queue_t queue = dispatch_get_global_queue(0, 0);
    for (NSInteger i = 0; i<1000; i++) {
        dispatch_async(queue, ^{
            self.name = [NSString stringWithFormat:@"ss"];
            if (i == 100) {
                NSLog(@"%p %@", self.name, self.name.class);
            }
        });
    }
} 
// 0xa000000000073732 NSTaggedPointerString

同样的代码字符串变短居然不 crash 了？因为命中 Tagged Pointer 逻辑了，查看类型是 NSTaggedPointerString

本问题本质是

ARC 代码在编译后真正运行阶段是走 MRC 的，strong、copy 内部都会 release 旧的，copy/retain 新的
多线程情况下访问 setter 需要加锁
字符串在 NSTaggedPointerString 情况下不存在像 OC 对象的 setter 方法内的 release、copy 操作

如何判断一个指针是否为Tagged Pointer

查看 objc4 源码

#if TARGET_OS_OSX && __x86_64__
    // 64-bit Mac - tag bit is LSB
#   define OBJC_MSB_TAGGED_POINTERS 0
#else
    // Everything else - tag bit is MSB
#   define OBJC_MSB_TAGGED_POINTERS 1
#endif

#if OBJC_MSB_TAGGED_POINTERS
#   define _OBJC_TAG_MASK (1UL<<63)
#   define _OBJC_TAG_INDEX_SHIFT 60
#   define _OBJC_TAG_SLOT_SHIFT 60
#   define _OBJC_TAG_PAYLOAD_LSHIFT 4
#   define _OBJC_TAG_PAYLOAD_RSHIFT 4
#   define _OBJC_TAG_EXT_MASK (0xfUL<<60)
#   define _OBJC_TAG_EXT_INDEX_SHIFT 52
#   define _OBJC_TAG_EXT_SLOT_SHIFT 52
#   define _OBJC_TAG_EXT_PAYLOAD_LSHIFT 12
#   define _OBJC_TAG_EXT_PAYLOAD_RSHIFT 12
#else
#   define _OBJC_TAG_MASK 1UL
#   define _OBJC_TAG_INDEX_SHIFT 1
#   define _OBJC_TAG_SLOT_SHIFT 0
#   define _OBJC_TAG_PAYLOAD_LSHIFT 0
#   define _OBJC_TAG_PAYLOAD_RSHIFT 4
#   define _OBJC_TAG_EXT_MASK 0xfUL
#   define _O  BJC_TAG_EXT_INDEX_SHIFT 4
#   define _OBJC_TAG_EXT_SLOT_SHIFT 4
#   define _OBJC_TAG_EXT_PAYLOAD_LSHIFT 0
#   define _OBJC_TAG_EXT_PAYLOAD_RSHIFT 12
#endif

static inline bool _objc_isTaggedPointer(const void * _Nullable ptr) 
{
    return ((uintptr_t)ptr & _OBJC_TAG_MASK) == _OBJC_TAG_MASK;
}

可以看到源码通过 _objc_isTaggedPointer 方法判断是否是 Tagged Pointer 类型。传入对象地址，内部通过 _OBJC_TAG_MASK 按位与运算。

其中 _OBJC_TAG_MASK 是一个宏，宏定义外部有个 if 判读，判断是 Mac OS 并且是 x86_64 架构则为0，否则为1。也就是 Mac OS 并且是 x86_64 架构情况下则与 1UL 按位与，否则与 1UL<<63 按位与。

iOS平台｜ Mac 非 x86 平台：最高有效位是1（第64bit）1UL<<63
Mac 且 x86平台：最低有效位是11UL

比如 iOS 平台下

0xb0000000000000a2 b:12  1100

  1100
& 1000
-------
  1000

OC 对象内存管理

iOS 中使用引用计数来管理 OC 对象的内存。一个新创建的 OC 对象引用计数默认是1，当引用计数减为 0，OC 对象就会销毁，释放其占用的内存空间

调用 retain/copy 会让 OC 对象的引用计数 +1，调用 release 会让 OC 对象的引用计数 -1。

内存管理的经验总结

当调用 alloc、new、copy、mutableCopy 方法返回了一个对象，在不需要这个对象时，要调用 release 或者 autorelease 来释放它
想拥有某个对象，就让它的引用计数 +1；不想再拥有某个对象，就让它的引用计数 -1
可以通过以下私有函数来查看自动释放池的情况extern void _objc_autoreleasePoolPrint(void);

僵尸对象：重复释放内存造成的。一个典型场景是多次 setter。setter 内部实现不合理，比如下面 setter。

Person *p = [[Person aloc] init]; // 1
Cat *cat = [[Cat alloc] init]; // 1  
[p setCat:cat];  // 2
[cat release]; // 1
[p setCat:cat]; // 0
[p setCat:cat]; // badAccess
- (void)setCat:(Cat *)cat
{
    [_cat release];
    _cat = [cat retain];
}

改进

- (void)setCat:(Cat *)cat
{    
    if (_cat != cat) {
        [_cat release];
        _cat = [cat retain];
    }
}

早期在 MRC 时代，在 .h 文件中 @property 只会属性的 getter、setter 声明，@synthesize 会自动生成成员变量和属性的 setter、getter 的实现。随着编译器进步，现在 @property 会做完全部的事情。

早期 VC 中使用属性

@property (nonatomic, strong) NSMutableDictionary *dict;

NSMutableDictionary *dict = [[NSMutableDictionary alloc] init];
self.dict = dict;
[dict release];

通过 Foundation 框架中类方法创建出来的对象，会自动调用 autorelease 方法。

简写为 self.dict = [NSMutableDictionary dictionary];

上述可以查看 GUNStep 源码 NSDictionary.m

#define    AUTORELEASE(object)    [(id)(object) autorelease]
+ (id) dictionary {
  return AUTORELEASE([[self allocWithZone: NSDefaultMallocZone()] init]);
}

QA：ARC 做了什么

ARC 其实是 LLVM + Runtime 共同作用的结果。LLVM 编译器自动插入 retain、release 内存管理代码。Runtime 运行时帮我们处理类似 __weak 程序运行过程中弱引用清除掉。

copy/mutableCopy

OC 有2个拷贝方法

copy 不可变拷贝，产生新不可变对象
mutableCopy 可变拷贝，产生新可变对象

上个 Demo1

NSArray *array1 = [[NSArray alloc] initWithObjects:@"1", @"2", nil];
NSLog(@"array1 --- %zd", array1.retainCount);
NSArray *array2 = [array1 copy];
NSLog(@"array1 --- %zd", array1.retainCount);
NSLog(@"array2 --- %zd", array2.retainCount);
NSMutableArray *array3 = [array1 mutableCopy];
NSLog(@"array1 --- %zd", array1.retainCount);
NSLog(@"array2 --- %zd", array2.retainCount);
NSLog(@"array3 --- %zd" array3.retainCount);

[array3 release];
NSLog(@"array3 --- %zd", array3.retainCount);
[array2 release];
NSLog(@"array2 --- %zd", array2.retainCount);
NSLog(@"array1 --- %zd", array1.retainCount);
[array1 release];
NSLog(@"array1 --- %zd", array1.retainCount);
2022-04-12 20:50:43.639296+0800 Main[4408:60897] array1 --- 1
2022-04-12 20:50:43.639715+0800 Main[4408:60897] array1 --- 2
2022-04-12 20:50:43.639772+0800 Main[4408:60897] array2 --- 2
2022-04-12 20:50:43.639846+0800 Main[4408:60897] array1 --- 2
2022-04-12 20:50:43.639899+0800 Main[4408:60897] array2 --- 2
2022-04-12 20:50:43.639957+0800 Main[4408:60897] array3 --- 1
2022-04-12 20:50:43.640013+0800 Main[4408:60897] array3 --- 0
2022-04-12 20:50:43.640059+0800 Main[4408:60897] array2 --- 1
2022-04-12 20:50:43.640105+0800 Main[4408:60897] array1 --- 1
2022-04-12 20:50:43.640159+0800 Main[4408:60897] array1 --- 0

疑问1: 为什么在 array2 创建之后 array2、array1 的引用技术都是2.

因为 array1 指针指向堆上一块内存（NSArray 类型），创建好后 array1 引用计数为1。在创建 array2 的时候发现是对 array1 的浅拷贝，系统为了内存的节省优化，array2 的指针也指向堆上的这一块内存，copy 本身会对 array1 引用技术 +1，变为2。所以这时候 array2 指针指向的内存，引用计数也是2.

基于此，我们稍微修改下，看看 Demo2

NSArray *array1 = [[NSArray alloc] initWithObjects:@"1", @"2", nil];
NSLog(@"array1 --- %zd", array1.retainCount);
NSArray *array2 = [array1 mutableCopy];
NSLog(@"array1 --- %zd", array1.retainCount);
NSLog(@"array2 --- %zd", array2.retainCount);

2022-04-12 20:55:36.539060+0800 Main[4576:65031] array1 --- 1
2022-04-12 20:55:36.539514+0800 Main[4576:65031] array1 --- 1
2022-04-12 20:55:36.539631+0800 Main[4576:65031] array2 --- 1

因为 array1 指针指向堆上一块内存（NSArray 类型），创建好后 array1 引用计数为1。在创建 array2 的时候发现是对 array1 的深拷贝，要产生不可变对象，所以堆上申请内存空间，array2 指针指向这块内存，引用技术为1。

此外 mutableCopy 是 Foundation 针对集合类提供的。如果自定义对象需要支持 copy 方法，需遵循对应的NSCopyint 协议，实现协议方法 -(id)copyWithZone:(NSZone *)zone

总结：

	NSString	NSMutableString	NSArray	NSMutableArray	NSDictionary	NSMutableDictionary
copy	NSString 浅拷贝	NSString 深拷贝	NSArray 浅拷贝	NSArray 深拷贝	NSDictionary 浅拷贝	NSDictionary 深拷贝
mutableCopy	NSMutableString 深拷贝	NSMutableString 深拷贝	NSMutableArray 深拷贝	NSMutableArray 深拷贝	NSMutableDictionary 深拷贝	NSMutableDictionary 深拷贝

引用计数

union isa_t {
    Class cls;
    uintptr_t bits;
    struct {
        uintptr_t nonpointer        : 1;
        uintptr_t has_assoc         : 1;
        uintptr_t has_cxx_dtor      : 1;
        uintptr_t shiftcls          : 33; // MACH_VM_MAX_ADDRESS 0x1000000000
        uintptr_t magic             : 6;
        uintptr_t weakly_referenced : 1;
        uintptr_t deallocating      : 1;
        uintptr_t has_sidetable_rc  : 1;
        uintptr_t extra_rc          : 19;
    };
}

iOS 从 64 位开始开始，对 isa 进行了优化，信息存放于 union 结构中

extra_rc 存储引用计数信息-1，可以看到是 19位。存储引用计数器 -1
has_sidetable_rc 引用计数是否过大无法存储在 isa。当过大无法存储与 isa 中时，has_sidetable_rc 这位会变为1，引用计数存储在 SideTable 的类的属性中

也就是说，iOS 从64位开始，引用计数存放于 isa 结构体的一个 union 中，字段为 extra_rc，值为对象引用计数值 -1。当引用计数过大无法存放的时候 union 中 has_sidetable_rc 为 1，则引用计数存放于 SideTable 结构体中。

SideTable 结构如下

struct SideTable {
    spinlock_t slock;
    RefcountMap refcnts;
    weak_table_t weak_table;
};

其中 refcnts 是一个存放着对象引用计数的散列表

查看 objc4 关于引用计数的实现

uintptr_t _objc_rootRetainCount(id obj) {
    assert(obj);
    return obj->rootRetainCount();
}

inline uintptr_t objc_object::rootRetainCount() {
    if (isTaggedPointer()) return (uintptr_t)this;
    sidetable_lock();
    isa_t bits = LoadExclusive(&isa.bits);
    ClearExclusive(&isa.bits);
    if (bits.nonpointer) { // 优化过的 isa
        uintptr_t rc = 1 + bits.extra_rc; 
        if (bits.has_sidetable_rc) { // 引用计数不是存储在 isa 中，而是 SideTable 
            rc += sidetable_getExtraRC_nolock();
        }
        sidetable_unlock();
        return rc;
    }
    sidetable_unlock();
    return sidetable_retainCount();
}

size_t  objc_object::sidetable_getExtraRC_nolock() {
    assert(isa.nonpointer);
    SideTable& table = SideTables()[this];
    RefcountMap::iterator it = table.refcnts.find(this); // key 拿值
    if (it == table.refcnts.end()) return 0;
    else return it->second >> SIDE_TABLE_RC_SHIFT;
}

__unsafe_unretained 不安全如何体现？上 Demo

__weak Person *p2;
__unsafe_unretained Person *p3;
{
    Person *p = [[Person alloc] init];
    p2 = p;
}
NSLog(@"%@", p2);
2022-04-12 21:39:30.308917+0800 Main[5307:98296] -[Person dealloc]
2022-04-12 21:39:30.309413+0800 Main[5307:98296] (null)

可以看到出了代码块，之后 p2 虽然指向 p，但是 p 没有强指针指向，所以回收了，此时打印 p2，是 null。

__unsafe_unretained Person *p3;
{
    Person *p = [[Person alloc] init];
    p3 = p;
}
NSLog(@"%@", p3);
2022-04-12 21:40:47.558581+0800 Main[5342:99598] -[Person dealloc]
2022-04-12 21:40:47.559330+0800 Main[5342:99598] <Person: 0x101206130>

当用 __unsafe_unretained 修饰后，虽然释放了，但是内存还没回收，这时候去使用很容易出错。

dealloc 是如何工作的？

在 MRC 时代，写完代码都需要显示在 dealloc 方法中做一些内存回收之类的工作。对象析构时将内部对象先 release 掉，非 OC 对象(比如定时器、c 对象、CF 对象等) 也需要回收内存，最后调用 [super dealloc] 继续将父类对象做析构。

- (void)dealloc {
    CFRelease(XX);
    self.timer = nil;
    [super dealloc];
}

但在 ARC 时代，dealloc 中一般只需要写一些非 OC 对象的内存释放工作，比如 CFRelease()

带来2个问题：

类中的实例变量在哪释放？
当前类中没有显示调用 [super dealloc] ，父类的析构如何触发？

LLVM 文档对 dealloc 的描述

LLVM ARC 文档对 dealloc 描述如下

A class may provide a method definition for an instance method named dealloc. This method will be called after the final release of the object but before it is deallocated or any of its instance variables are destroyed. The superclass’s implementation of dealloc will be called automatically when the method returns.

The instance variables for an ARC-compiled class will be destroyed at some point after control enters the dealloc method for the root class of the class. The ordering of the destruction of instance variables is unspecified, both within a single class and between subclasses and superclasses.

根据描述可以看到 dealloc 方法在最后一次 release 方法调用后触发，但实例变量（ivars）还未释放，父类的 dealloc 方法将会在子类 dealloc 方法返回后自动调用。

ARC 模式下，对象的实例变量会在根类 [NSObject dealloc] 中释放，但是释放的顺序是不一定的。

也就是说会自动调用 [super dealloc]，那到底如何实现的，探究下。

查看 objc4 源码

- (void)dealloc {
    _objc_rootDealloc(self);
}

void _objc_rootDealloc(id obj) {
    assert(obj);
    obj->rootDealloc();
}

inline void objc_object::rootDealloc() {
    if (isTaggedPointer()) return;  // fixme necessary?
    // fastpath 判断当前对象是否满足条件。
    if (fastpath(isa.nonpointer  &&      // nonpointer
                 !isa.weakly_referenced  &&   // 是否有弱引用
                 !isa.has_assoc  &&   // 关联对象
                 !isa.has_cxx_dtor  &&   // c++ 析构函数
                 !isa.has_sidetable_rc)) // 是否有 SideTable
    {
        assert(!sidetable_present());
        free(this);
    }  else {
        object_dispose((id)this);
    }
}

id object_dispose(id obj){
    if (!obj) return nil;
    objc_destructInstance(obj);    
    free(obj);
    return nil;
}

void *objc_destructInstance(id obj) {
    if (obj) {
        // Read all of the flags at once for performance.
        bool cxx = obj->hasCxxDtor();
        bool assoc = obj->hasAssociatedObjects();

        // This order is important.
        if (cxx) object_cxxDestruct(obj); // 清除成员变量
        if (assoc) _object_remove_assocations(obj);
        obj->clearDeallocating(); // 将指向当前对象的弱指针置为 nil
    }
    return obj;
}

inline void objc_object::clearDeallocating() {
    if (slowpath(!isa.nonpointer)) {
        // Slow path for raw pointer isa.
        sidetable_clearDeallocating();
    }
    else if (slowpath(isa.weakly_referenced  ||  isa.has_sidetable_rc)) {
        // Slow path for non-pointer isa with weak refs and/or side table data.
        clearDeallocating_slow();
    }
    assert(!sidetable_present());
}
void objc_object::sidetable_clearDeallocating(){
    SideTable& table = SideTables()[this];
    // clear any weak table items
    // clear extra retain count and deallocating bit
    // (fixme warn or abort if extra retain count == 0 ?)
    table.lock();
    RefcountMap::iterator it = table.refcnts.find(this);
    if (it != table.refcnts.end()) {
        if (it->second & SIDE_TABLE_WEAKLY_REFERENCED) {
            weak_clear_no_lock(&table.weak_table, (id)this);
        }
        table.refcnts.erase(it);
    }
    table.unlock();
}

void 
weak_clear_no_lock(weak_table_t *weak_table, id referent_id) 
{
    objc_object *referent = (objc_object *)referent_id;

    weak_entry_t *entry = weak_entry_for_referent(weak_table, referent);
    if (entry == nil) {
        /// XXX shouldn't happen, but does with mismatched CF/objc
        //printf("XXX no entry for clear deallocating %p\n", referent);
        return;
    }

    // zero out references
    weak_referrer_t *referrers;
    size_t count;

    if (entry->out_of_line()) {
        referrers = entry->referrers;
        count = TABLE_SIZE(entry);
    } 
    else {
        referrers = entry->inline_referrers;
        count = WEAK_INLINE_COUNT;
    }

    for (size_t i = 0; i < count; ++i) {
        objc_object **referrer = referrers[i];
        if (referrer) {
            if (*referrer == referent) {
                *referrer = nil;
            }
            else if (*referrer) {
                _objc_inform("__weak variable at %p holds %p instead of %p. "
                             "This is probably incorrect use of "
                             "objc_storeWeak() and objc_loadWeak(). "
                             "Break on objc_weak_error to debug.\n", 
                             referrer, (void*)*referrer, (void*)referent);
                objc_weak_error();
            }
        }
    }

    weak_entry_remove(weak_table, entry);
}

可以清楚看到在 objc_destructInstance 方法中调用了3个核心方法

object_cxxDestruct(obj)：清除成员变量
object_remove_assocations(obj)：去除该对象相关的关联属性(Category 添加的)
obj->clearDeallocating()：清空引用技术表和弱引用表，将 weak 引用设置为 nil

继续看看 object_cxxDestruct 方法内部细节。

神秘的 cxx_destruct

object_cxxDestruct 方法最终会调用到 object_cxxDestructFromClass

void object_cxxDestruct(id obj) {
    if (_objc_isTaggedPointerOrNil(obj)) return;
    object_cxxDestructFromClass(obj, obj->ISA());
}

static void object_cxxDestructFromClass(id obj, Class cls) {
    void (*dtor)(id);
    // Call cls's dtor first, then superclasses's dtors.
    for ( ; cls; cls = cls->getSuperclass()) {
        if (!cls->hasCxxDtor()) return; 
        dtor = (void(*)(id))
            lookupMethodInClassAndLoadCache(cls, SEL_cxx_destruct);
        // 调用
        if (dtor != (void(*)(id))_objc_msgForward_impcache) {
            if (PrintCxxCtors) {
                _objc_inform("CXX: calling C++ destructors for class %s", 
                             cls->nameForLogging());
            }
            (*dtor)(obj);
        }
    }
}

做的事情就是遍历，不断寻找父类中 SEL_cxx_destruct这个 selector，找到函数实现并调用。

void sel_init(size_t selrefCount){
#if SUPPORT_PREOPT
    if (PrintPreopt) {
        _objc_inform("PREOPTIMIZATION: using dyld selector opt");
    }
#endif
  namedSelectors.init((unsigned)selrefCount);
    // Register selectors used by libobjc
    mutex_locker_t lock(selLock)
    SEL_cxx_construct = sel_registerNameNoLock(".cxx_construct", NO);
    SEL_cxx_destruct = sel_registerNameNoLock(".cxx_destruct", NO);
}

继续翻阅源码发现 SEL_cxx_destruct 其实就是 .cxx_destruct。在《Effective Objective-C 2.0》中说明：

When the compiler saw that an object contained C++ objects, it would generate a method called .cxx_destruct. ARC piggybacks on this method and emits the required cleanup code within it.

也就是说，当编译器看到 C++ 对象的时候，它将会生成 .cxx_destruct 析构方法，但是 ARC 借用这个方法，并在其中插入了代码以实现自动内存释放的功能。

探究啥时候生成 .cxx_destruct 方法

@interface Person : NSObject
@property (nonatomic, strong) NSString *name;
@end
// 
- (void)viewDidLoad {
    [super viewDidLoad];
    {
        NSLog(@"comes");
        Person *p = [[Person alloc] init];
        p.name = @"杭城小刘";
        NSLog(@"gone");
    }
}

在 gone 处加断点，利用 runtime 查看类中的方法信息

发现存在 .cxx_destruct 方法。

我们一开要研究的是 ivars 啥时候释放，所以控制变量，将属性改为成员对象

@interface Person : NSObject
{
    @public
    NSString *name;
}
@end

{
    NSLog(@"comes");
    Person *p = [[Person alloc] init];
    p->name = @"杭城小刘";
    NSLog(@"gone");
}

也有 .cxx_destruct 方法

将成员变量换为基本数据类型

@interface Person : NSObject
{
    @public
    int age;
}
@end

Tips：@property 会自动生成成员变量，另外类后面加 {} 在内部也可以加成员变量，假如成员变量是对象类型，比如 NSString，则叫实例变量。

得出结论：

只有 ARC 模式下才有 .cxx_destruct 方法
类拥有实例变量的时候({} 或者 @property) 才有 .cxx_destruct，父类成员对象的实例变量不会让子类拥有该方法

使用 watchpoint 观察内存释放时机

在 gone 的地方加断点，输入 watchpoint set variable p->_name，则会将 _name 实例变量加入 watchpoint，当变量被修改时会触发断点，可以看出从某个值变为 0x0，也就是 nil。此时边上调用堆栈显示在 objc_storestrong 方法中，被设置为 nil.

深入 .cxx_destruct

简单梳理下，在 ARC 模式下，类拥有实例变量的时候会在 .cxx_destruct 方法内调用 objc_storeStrong 去释放的内存。

我们也知道 .cxx_destruct 是编译器生成的代码。去查询资料 .cxx_destruct site:clang.llvm.org

在 clang 的 doxygen 文档中 CodeGenModule 模块源码发现了相关逻辑。在 5907 行代码

void CodeGenModule::EmitObjCIvarInitializations(ObjCImplementationDecl *D) {
  // We might need a .cxx_destruct even if we don't have any ivar initializers.
  if (needsDestructMethod(D)) {
    IdentifierInfo *II = &getContext().Idents.get(".cxx_destruct");
    Selector cxxSelector = getContext().Selectors.getSelector(0, &II);
    ObjCMethodDecl *DTORMethod = ObjCMethodDecl::Create(
        getContext(), D->getLocation(), D->getLocation(), cxxSelector,
        getContext().VoidTy, nullptr, D,
        /*isInstance=*/true, /*isVariadic=*/false,
        /*isPropertyAccessor=*/true, /*isSynthesizedAccessorStub=*/false,
        /*isImplicitlyDeclared=*/true,
        /*isDefined=*/false, ObjCMethodDecl::Required);
    D->addInstanceMethod(DTORMethod);
    CodeGenFunction(*this).GenerateObjCCtorDtorMethod(D, DTORMethod, false);
    D->setHasDestructors(true);
  }

  // If the implementation doesn't have any ivar initializers, we don't need
  // a .cxx_construct.
  if (D->getNumIvarInitializers() == 0 ||
      AllTrivialInitializers(*this, D))
    return;

  IdentifierInfo *II = &getContext().Idents.get(".cxx_construct");
  Selector cxxSelector = getContext().Selectors.getSelector(0, &II);
  // The constructor returns 'self'.
  ObjCMethodDecl *CTORMethod = ObjCMethodDecl::Create(
      getContext(), D->getLocation(), D->getLocation(), cxxSelector,
      getContext().getObjCIdType(), nullptr, D, /*isInstance=*/true,
      /*isVariadic=*/false,
      /*isPropertyAccessor=*/true, /*isSynthesizedAccessorStub=*/false,
      /*isImplicitlyDeclared=*/true,
      /*isDefined=*/false, ObjCMethodDecl::Required);
  D->addInstanceMethod(CTORMethod);
  CodeGenFunction(*this).GenerateObjCCtorDtorMethod(D, CTORMethod, true);
  D->setHasNonZeroConstructors(true);
}

源码大概做的事情就是：获取 .cxx_destructor 的 selector，创建 Method，然后将新创建的 Method 插入到 class 方法列表中。调用 GenerateObjCCtorDtorMethod 方法，才创建这个方法的实现。查看 GenerateObjCCtorDtorMethod 的实现。在 https://clang.llvm.org/doxygen/CGObjC_8cpp_source.html 的1626行处。

static void emitCXXDestructMethod(CodeGenFunction &CGF,
                                   ObjCImplementationDecl *impl) {
   CodeGenFunction::RunCleanupsScope scope(CGF);

   llvm::Value *self = CGF.LoadObjCSelf();

   const ObjCInterfaceDecl *iface = impl->getClassInterface();
   for (const ObjCIvarDecl *ivar = iface->all_declared_ivar_begin();
        ivar; ivar = ivar->getNextIvar()) {
     QualType type = ivar->getType();

     // Check whether the ivar is a destructible type.
     QualType::DestructionKind dtorKind = type.isDestructedType();
     if (!dtorKind) continue;

     CodeGenFunction::Destroyer *destroyer = nullptr;

     // Use a call to objc_storeStrong to destroy strong ivars, for the
     // general benefit of the tools.
     if (dtorKind == QualType::DK_objc_strong_lifetime) {
       destroyer = destroyARCStrongWithStore;

     // Otherwise use the default for the destruction kind.
     } else {
       destroyer = CGF.getDestroyer(dtorKind);
     }

     CleanupKind cleanupKind = CGF.getCleanupKind(dtorKind);

     CGF.EHStack.pushCleanup<DestroyIvar>(cleanupKind, self, ivar, destroyer,
                                          cleanupKind & EHCleanup);
   }

   assert(scope.requiresCleanups() && "nothing to do in .cxx_destruct?");
 }

可以看到：遍历了当前对象的所有实例变量，调用 objc_storeStrong，从 clang 文档上可以看出

id objc_storeStrong(id *object, id value) {
  value = [value retain];
  id oldValue = *object;
  *object = value;
  [oldValue release];
  return value;
}

在 .cxx_destruct 方法内部会对所有的实例变量调用 objc_storeStrong(&ivar, null) ，实例变量就会 release 。

自动调用 [super dealloc] 的原理

同理，CodeGen 也会做自动调用 [super dealloc] 的事情。https://clang.llvm.org/doxygen/CGObjC_8cpp_source.html，第751行 StartObjCMethod 方法。

  751 void CodeGenFunction::StartObjCMethod(const ObjCMethodDecl *OMD,
  752                                       const ObjCContainerDecl *CD) {
  // ...  
  789   // In ARC, certain methods get an extra cleanup.
  790   if (CGM.getLangOpts().ObjCAutoRefCount &&
  791       OMD->isInstanceMethod() &&
  792       OMD->getSelector().isUnarySelector()) {
  793     const IdentifierInfo *ident =
  794       OMD->getSelector().getIdentifierInfoForSlot(0);
  795     if (ident->isStr("dealloc"))
  796       EHStack.pushCleanup<FinishARCDealloc>(getARCCleanupKind());
  797   }
  798 }

可以看到在调用到 dealloc 方法时，插入了代码，实现如下

struct FinishARCDealloc : EHScopeStack::Cleanup {
   void Emit(CodeGenFunction &CGF, Flags flags) override {
     const ObjCMethodDecl *method = cast<ObjCMethodDecl>(CGF.CurCodeDecl);

     const ObjCImplDecl *impl = cast<ObjCImplDecl>(method->getDeclContext());
     const ObjCInterfaceDecl *iface = impl->getClassInterface();
     if (!iface->getSuperClass()) return;

     bool isCategory = isa<ObjCCategoryImplDecl>(impl);

     // Call [super dealloc] if we have a superclass.
     llvm::Value *self = CGF.LoadObjCSelf();

     CallArgList args;
     CGF.CGM.getObjCRuntime().GenerateMessageSendSuper(CGF, ReturnValueSlot(),
                                                       CGF.getContext().VoidTy,
                                                       method->getSelector(),
                                                       iface,
                                                       isCategory,
                                                       self,
                                                       /*is class msg*/ false,
                                                       args,
                                                       method);
   }
};

代码大概就是向父类转发 dealloc 的调用实现，内部自动调用 [super dealloc] 方法。

总结下：

ARC 模式下，实例变量由编译器插入 .cxx_destruct 方法自动释放
ARC 模式下 [super dealloc] 由 llvm 编译器自动插入(CodeGen)

自动释放池底层原理探索

上 Demo

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        Person *p = [[[Person alloc] init] autorelease];
    }
    return 0;
}

clang 转为 c++ xcrun -sdk iphonesimulator clang -rewrite-objc main.m

int main(int argc, const char * argv[]) {
    /* @autoreleasepool */ {  __AtAutoreleasePool __autoreleasepool; 
        Person *p = ((Person *(*)(id, SEL))(void *)objc_msgSend)((id)((Person *(*)(id, SEL))(void *)objc_msgSend)((id)((Person *(*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("Person"), sel_registerName("alloc")), sel_registerName("init")), sel_registerName("autorelease"));
    }
    return 0;
}

下面的代码其实就是 objc_msgSend，有效代码是 __AtAutoreleasePool __autoreleasepool;

继续查找

struct __AtAutoreleasePool {
    __AtAutoreleasePool() {
        atautoreleasepoolobj = objc_autoreleasePoolPush();
    }
    ~__AtAutoreleasePool() {
        objc_autoreleasePoolPop(atautoreleasepoolobj);
    }
    void * atautoreleasepoolobj;
};

OC 对象本质就是结构体

__AtAutoreleasePool 结构体中 __AtAutoreleasePool 是构造方法，在创建结构体的时候调用
~__AtAutoreleasePool 是析构函数，在结构体销毁的时候调用

main 内的代码作用域，离开代表销毁。所以上面代码等价于

atautoreleasepoolobj = objc_autoreleasePoolPush();
Person *p = [[[Person alloc] init] autorelease];
objc_autoreleasePoolPop(atautoreleasepoolobj);

利用关键函数继续查看 objc4 源码

void *objc_autoreleasePoolPush(void) {
    return AutoreleasePoolPage::push();
}
void objc_autoreleasePoolPop(void *ctxt) {
    AutoreleasePoolPage::pop(ctxt);
}

自动释放池的主要实现依靠2个对象：__AtAutoreleasePool、AutoreleasePoolPage

objc_autoreleasePoolPush、objc_autoreleasePoolPop 底层都是调用了 AutoreleasePoolPage 对象来管理的。

查看源码

class AutoreleasePoolPage {
    magic_t const magic;
    id *next;
    pthread_t const thread;
    AutoreleasePoolPage * const parent;
    AutoreleasePoolPage *child;
    uint32_t const depth;
    uint32_t hiwat;
}

每个 AutoreleasePoolPage 对象占用 4096 字节内存，除了用来存放它内部的成员变量，剩下的空间用来存放 autorelease 对象的地址
所有的 AutoreleasePoolPage 对象通过双向链表的形式连接在一起。child 指向下一个对象，parent 指向上一个对象

id * begin() {
    return (id *) ((uint8_t *)this+sizeof(*this));
}

id * end() {
    return (id *) ((uint8_t *)this+SIZE);
}

其中 begin 方法返回 autoreleasePoolPage 对象中开始存储 autorelease 对象的开始地址

end 方法返回 autoreleasePoolPage 对象中结束存储 autorelease 对象的开始地址

调用 AutoreleasePoolPage::push 方法会将一个 POOL_BOUNDARY 入栈，并且返回其存放的内存地址

调用 AutoreleasePoolPage::pop 方法时传入一个 POOL_BOUNDARY 的内存地址，系统会从最后一个入栈的对象开始发送 release消息，直到遇到这个 POOL_BOUNDARY

id *next 指向了下一个能存放 autorelease 对象地址的区域

static inline void *push() {
    id *dest;
    if (DebugPoolAllocation) {
        // Each autorelease pool starts on a new pool page.
        dest = autoreleaseNewPage(POOL_BOUNDARY);
    } else {
        dest = autoreleaseFast(POOL_BOUNDARY);
    }
    assert(dest == EMPTY_POOL_PLACEHOLDER || *dest == POOL_BOUNDARY);
    return dest;
}

来个骚一些的例子

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        Person *p1 = [[[Person alloc] init] autorelease];
        Person *p2 = [[[Person alloc] init] autorelease];
        @autoreleasepool {
            Person *p3 = [[[Person alloc] init] autorelease];
            @autoreleasepool {
                Person *p4 = [[[Person alloc] init] autorelease];
            }
        }
    }
    return 0;
}

main 方法内部3个 autoreleasepool 底层怎么样工作的？

3个@auto releasepool，系统遇到第一个的时候底层就是初始化一个结构体 __AtAutoreleasePool，结构体构造方法内部调用 AutoreleasePoolPage::push 方法，系统给 AutoreleasePoolPage 真正保存 autorelease 对象的地方存储进一个 POOL_BOUNDARY 对象，然后储存 P1、P2 对象地址，遇到第二个则继续初始化结构体，调用 push 方法，存储一个 POOL_BOUNDARY 对象，继续保存 P3，遇到第三个则继续初始化结构体，调用 push 方法，存储一个 POOL_BOUNDARY 对象，继续保存 P4。

当结束第三个大括号的时候，第三个结构体对象，调用析构函数，内部调用 AutoreleasePoolPage::pop 方法，会从最后一个入栈的对象开始发送 release 消息，直到遇到 POOL_BOUNDARY 对象。

紧接着第二个大括号结束，第二个结构体对象析构函数执行，内部调用 AutoreleasePoolPage::pop 方法，会从最后一个入栈的对象开始发送 release 消息，直到遇到 POOL_BOUNDARY 对象。

第一个同理。

小窍门，对于上述原理的分析可以用源码中看到的 AutoreleasePoolPage 对象的 printAll 方法。

static void printAll() {        
    _objc_inform("##############");
    _objc_inform("AUTORELEASE POOLS for thread %p", pthread_self());

    AutoreleasePoolPage *page;
    ptrdiff_t objects = 0;
    for (page = coldPage(); page; page = page->child) {
        objects += page->next - page->begin();
    }
    _objc_inform("%llu releases pending.", (unsigned long long)objects);

    if (haveEmptyPoolPlaceholder()) {
        _objc_inform("[%p]  ................  PAGE (placeholder)", 
                        EMPTY_POOL_PLACEHOLDER);
        _objc_inform("[%p]  ################  POOL (placeholder)", 
                        EMPTY_POOL_PLACEHOLDER);
    }
    else {
        for (page = coldPage(); page; page = page->child) {
            page->print();
        }
    }
    _objc_inform("##############");
}


void _objc_autoreleasePoolPrint(void) {
    AutoreleasePoolPage::printAll();
}

查了下 printAll 函数的使用方，就只有 _objc_autoreleasePoolPrint 函数。且可以看到在 objc4 objc-internal.h 头文件中有将该函数 export 出去，也就是可以在外部链接该符号。

OBJC_EXPORT void _objc_autoreleasePoolPrint(void) OBJC_AVAILABLE(10.7, 5.0, 9.0, 1.0, 2.0);

所以我们在测试 Demo 中将 _objc_autoreleasePoolPrint 函数声明下。在打印下

extern void _objc_autoreleasePoolPrint(void);
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        Person *p1 = [[[Person alloc] init] autorelease];
        Person *p2 = [[[Person alloc] init] autorelease];
        @autoreleasepool {
            Person *p3 = [[[Person alloc] init] autorelease];
            @autoreleasepool {
                Person *p4 = [[[Person alloc] init] autorelease];
                _objc_autoreleasePoolPrint();
            }
        }
    }
    return 0;
}
objc[23132]: ##############
objc[23132]: AUTORELEASE POOLS for thread 0x100094600
objc[23132]: 7 releases pending.
objc[23132]: [0x10080a000]  ................  PAGE  (hot) (cold)
objc[23132]: [0x10080a038]  ################  POOL 0x10080a038
objc[23132]: [0x10080a040]       0x10075f060  Person
objc[23132]: [0x10080a048]       0x10075f0c0  Person
objc[23132]: [0x10080a050]  ################  POOL 0x10080a050
objc[23132]: [0x10080a058]       0x10075f0e0  Person
objc[23132]: [0x10080a060]  ################  POOL 0x10080a060
objc[23132]: [0x10080a068]       0x10075f100  Person
objc[23132]: ##############

可以看到打印结果和上面的分析是一致的（和上面的图片对比看看）

再来个 Demo，验证下 AutoreleasePoolPage 一页满情况

extern void _objc_autoreleasePoolPrint(void);
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        Person *p1 = [[[Person alloc] init] autorelease];
        Person *p2 = [[[Person alloc] init] autorelease];
        @autoreleasepool {
            for (NSInteger index = 0; index<600; index++) {
                Person *p3 = [[[Person alloc] init] autorelease];
            }
            @autoreleasepool {
                Person *p4 = [[[Person alloc] init] autorelease];
                _objc_autoreleasePoolPrint();
            }
        }
    }
    return 0;
}

objc[23504]: ##############
objc[23504]: AUTORELEASE POOLS for thread 0x100094600
objc[23504]: 606 releases pending.
objc[23504]: [0x10080d000]  ................  PAGE (full)  (cold)
objc[23504]: [0x10080d038]  ################  POOL 0x10080d038
objc[23504]: [0x10080d040]       0x1007092f0  Person
objc[23504]: [0x10080d048]       0x100709350  Person
objc[23504]: [0x10080d050]  ################  POOL 0x10080d050
objc[23504]: [0x10080d058]       0x100753250  Person
objc[23504]: [0x10080d060]       0x100753270  Person
objc[23504]: [0x10080d068]       0x100753290  Person
objc[23504]: [0x10080d070]       0x1007532b0  Person
objc[23504]: [0x10080d078]       0x1007532d0  Person
objc[23504]: [0x10080d080]       0x1007532f0  Person
objc[23504]: [0x10080d088]       0x100753310  Person
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objc[23504]: [0x10080da60]       0x1007509f0  Person
objc[23504]: [0x10080da68]       0x100750a10  Person
objc[23504]: [0x10080da70]       0x100750a30  Person
objc[23504]: [0x10080da78]       0x100750a50  Person
objc[23504]: [0x10080da80]       0x100750a70  Person
objc[23504]: [0x10080da88]       0x100750a90  Person
objc[23504]: [0x10080da90]       0x100750ab0  Person
objc[23504]: [0x10080da98]       0x100750ad0  Person
objc[23504]: [0x10080daa0]       0x100750af0  Person
objc[23504]: [0x10080daa8]       0x100750b10  Person
objc[23504]: [0x10080dab0]       0x100750b30  Person
objc[23504]: [0x10080dab8]       0x100750b50  Person
objc[23504]: [0x10080dac0]       0x100750b70  Person
objc[23504]: [0x10080dac8]       0x100750b90  Person
objc[23504]: [0x10080dad0]       0x100750bb0  Person
objc[23504]: [0x10080dad8]       0x100750bd0  Person
objc[23504]: [0x10080dae0]       0x100750bf0  Person
objc[23504]: [0x10080dae8]       0x100750c10  Person
objc[23504]: [0x10080daf0]       0x100750c30  Person
objc[23504]: [0x10080daf8]       0x100750c50  Person
objc[23504]: [0x10080db00]       0x100750c70  Person
objc[23504]: [0x10080db08]       0x100750c90  Person
objc[23504]: [0x10080db10]       0x100750cb0  Person
objc[23504]: [0x10080db18]       0x100750cd0  Person
objc[23504]: [0x10080db20]       0x100750cf0  Person
objc[23504]: [0x10080db28]       0x100750d10  Person
objc[23504]: [0x10080db30]       0x100750d30  Person
objc[23504]: [0x10080db38]       0x100750d50  Person
objc[23504]: [0x10080db40]       0x100750d70  Person
objc[23504]: [0x10080db48]       0x100750d90  Person
objc[23504]: [0x10080db50]       0x100750db0  Person
objc[23504]: [0x10080db58]       0x100750dd0  Person
objc[23504]: [0x10080db60]       0x100750df0  Person
objc[23504]: [0x10080db68]       0x100750e10  Person
objc[23504]: [0x10080db70]       0x100750e30  Person
objc[23504]: [0x10080db78]       0x100750e50  Person
objc[23504]: [0x10080db80]       0x100750e70  Person
objc[23504]: [0x10080db88]       0x100750e90  Person
objc[23504]: [0x10080db90]       0x100750eb0  Person
objc[23504]: [0x10080db98]       0x100750ed0  Person
objc[23504]: [0x10080dba0]       0x100750ef0  Person
objc[23504]: [0x10080dba8]       0x100750f10  Person
objc[23504]: [0x10080dbb0]       0x100750f30  Person
objc[23504]: [0x10080dbb8]       0x100750f50  Person
objc[23504]: [0x10080dbc0]       0x100750f70  Person
objc[23504]: [0x10080dbc8]       0x100750f90  Person
objc[23504]: [0x10080dbd0]       0x100750fb0  Person
objc[23504]: [0x10080dbd8]       0x100750fd0  Person
objc[23504]: [0x10080dbe0]       0x100750ff0  Person
objc[23504]: [0x10080dbe8]       0x100751010  Person
objc[23504]: [0x10080dbf0]       0x100751030  Person
objc[23504]: [0x10080dbf8]       0x100751050  Person
objc[23504]: [0x10080dc00]       0x100751070  Person
objc[23504]: [0x10080dc08]       0x100751090  Person
objc[23504]: [0x10080dc10]       0x1007510b0  Person
objc[23504]: [0x10080dc18]       0x1007510d0  Person
objc[23504]: [0x10080dc20]       0x1007510f0  Person
objc[23504]: [0x10080dc28]       0x100751110  Person
objc[23504]: [0x10080dc30]       0x100751130  Person
objc[23504]: [0x10080dc38]       0x100751150  Person
objc[23504]: [0x10080dc40]       0x100751170  Person
objc[23504]: [0x10080dc48]       0x100751190  Person
objc[23504]: [0x10080dc50]       0x1007511b0  Person
objc[23504]: [0x10080dc58]       0x1007511d0  Person
objc[23504]: [0x10080dc60]       0x1007511f0  Person
objc[23504]: [0x10080dc68]       0x100751210  Person
objc[23504]: [0x10080dc70]       0x100751230  Person
objc[23504]: [0x10080dc78]       0x100751250  Person
objc[23504]: [0x10080dc80]       0x100751270  Person
objc[23504]: [0x10080dc88]       0x100751290  Person
objc[23504]: [0x10080dc90]       0x1007512b0  Person
objc[23504]: [0x10080dc98]       0x1007512d0  Person
objc[23504]: [0x10080dca0]       0x1007512f0  Person
objc[23504]: [0x10080dca8]       0x100751310  Person
objc[23504]: [0x10080dcb0]       0x100751330  Person
objc[23504]: [0x10080dcb8]       0x100751350  Person
objc[23504]: [0x10080dcc0]       0x100751370  Person
objc[23504]: [0x10080dcc8]       0x100751390  Person
objc[23504]: [0x10080dcd0]       0x1007513b0  Person
objc[23504]: [0x10080dcd8]       0x1007513d0  Person
objc[23504]: [0x10080dce0]       0x1007513f0  Person
objc[23504]: [0x10080dce8]       0x100751410  Person
objc[23504]: [0x10080dcf0]       0x100751430  Person
objc[23504]: [0x10080dcf8]       0x100751450  Person
objc[23504]: [0x10080dd00]       0x100751470  Person
objc[23504]: [0x10080dd08]       0x100751490  Person
objc[23504]: [0x10080dd10]       0x1007514b0  Person
objc[23504]: [0x10080dd18]       0x1007514d0  Person
objc[23504]: [0x10080dd20]       0x1007514f0  Person
objc[23504]: [0x10080dd28]       0x100751510  Person
objc[23504]: [0x10080dd30]       0x100751530  Person
objc[23504]: [0x10080dd38]       0x100751550  Person
objc[23504]: [0x10080dd40]       0x100751570  Person
objc[23504]: [0x10080dd48]       0x100751590  Person
objc[23504]: [0x10080dd50]       0x1007515b0  Person
objc[23504]: [0x10080dd58]       0x1007515d0  Person
objc[23504]: [0x10080dd60]       0x1007515f0  Person
objc[23504]: [0x10080dd68]       0x100751610  Person
objc[23504]: [0x10080dd70]       0x100751630  Person
objc[23504]: [0x10080dd78]       0x100751650  Person
objc[23504]: [0x10080dd80]       0x100751670  Person
objc[23504]: [0x10080dd88]       0x100751690  Person
objc[23504]: [0x10080dd90]       0x1007516b0  Person
objc[23504]: [0x10080dd98]       0x1007516d0  Person
objc[23504]: [0x10080dda0]       0x1007516f0  Person
objc[23504]: [0x10080dda8]       0x100751710  Person
objc[23504]: [0x10080ddb0]       0x100751730  Person
objc[23504]: [0x10080ddb8]       0x100751750  Person
objc[23504]: [0x10080ddc0]       0x100751770  Person
objc[23504]: [0x10080ddc8]       0x100751790  Person
objc[23504]: [0x10080ddd0]       0x1007517b0  Person
objc[23504]: [0x10080ddd8]       0x1007517d0  Person
objc[23504]: [0x10080dde0]       0x1007517f0  Person
objc[23504]: [0x10080dde8]       0x100751810  Person
objc[23504]: [0x10080ddf0]       0x100751830  Person
objc[23504]: [0x10080ddf8]       0x100751850  Person
objc[23504]: [0x10080de00]       0x100751870  Person
objc[23504]: [0x10080de08]       0x100751890  Person
objc[23504]: [0x10080de10]       0x1007518b0  Person
objc[23504]: [0x10080de18]       0x1007518d0  Person
objc[23504]: [0x10080de20]       0x1007518f0  Person
objc[23504]: [0x10080de28]       0x100751910  Person
objc[23504]: [0x10080de30]       0x100751930  Person
objc[23504]: [0x10080de38]       0x100751950  Person
objc[23504]: [0x10080de40]       0x100751970  Person
objc[23504]: [0x10080de48]       0x100751990  Person
objc[23504]: [0x10080de50]       0x1007519b0  Person
objc[23504]: [0x10080de58]       0x1007519d0  Person
objc[23504]: [0x10080de60]       0x1007519f0  Person
objc[23504]: [0x10080de68]       0x100751a10  Person
objc[23504]: [0x10080de70]       0x100751a30  Person
objc[23504]: [0x10080de78]       0x100751a50  Person
objc[23504]: [0x10080de80]       0x100751a70  Person
objc[23504]: [0x10080de88]       0x100751a90  Person
objc[23504]: [0x10080de90]       0x100751ab0  Person
objc[23504]: [0x10080de98]       0x100751ad0  Person
objc[23504]: [0x10080dea0]       0x100751af0  Person
objc[23504]: [0x10080dea8]       0x100751b10  Person
objc[23504]: [0x10080deb0]       0x100751b30  Person
objc[23504]: [0x10080deb8]       0x100751b50  Person
objc[23504]: [0x10080dec0]       0x100751b70  Person
objc[23504]: [0x10080dec8]       0x100751b90  Person
objc[23504]: [0x10080ded0]       0x100751bb0  Person
objc[23504]: [0x10080ded8]       0x100751bd0  Person
objc[23504]: [0x10080dee0]       0x100751bf0  Person
objc[23504]: [0x10080dee8]       0x100751c10  Person
objc[23504]: [0x10080def0]       0x100751c30  Person
objc[23504]: [0x10080def8]       0x100751c50  Person
objc[23504]: [0x10080df00]       0x100751c70  Person
objc[23504]: [0x10080df08]       0x100751c90  Person
objc[23504]: [0x10080df10]       0x100751cb0  Person
objc[23504]: [0x10080df18]       0x100751cd0  Person
objc[23504]: [0x10080df20]       0x100751cf0  Person
objc[23504]: [0x10080df28]       0x100751d10  Person
objc[23504]: [0x10080df30]       0x100751d30  Person
objc[23504]: [0x10080df38]       0x100751d50  Person
objc[23504]: [0x10080df40]       0x100751d70  Person
objc[23504]: [0x10080df48]       0x100751d90  Person
objc[23504]: [0x10080df50]       0x100751db0  Person
objc[23504]: [0x10080df58]       0x100751dd0  Person
objc[23504]: [0x10080df60]       0x100751df0  Person
objc[23504]: [0x10080df68]       0x100751e10  Person
objc[23504]: [0x10080df70]       0x100751e30  Person
objc[23504]: [0x10080df78]       0x100751e50  Person
objc[23504]: [0x10080df80]       0x100751e70  Person
objc[23504]: [0x10080df88]       0x100751e90  Person
objc[23504]: [0x10080df90]       0x100751eb0  Person
objc[23504]: [0x10080df98]       0x100751ed0  Person
objc[23504]: [0x10080dfa0]       0x100751ef0  Person
objc[23504]: [0x10080dfa8]       0x100751f10  Person
objc[23504]: [0x10080dfb0]       0x100751f30  Person
objc[23504]: [0x10080dfb8]       0x100751f50  Person
objc[23504]: [0x10080dfc0]       0x100751f70  Person
objc[23504]: [0x10080dfc8]       0x100751f90  Person
objc[23504]: [0x10080dfd0]       0x100751fb0  Person
objc[23504]: [0x10080dfd8]       0x100751fd0  Person
objc[23504]: [0x10080dfe0]       0x100751ff0  Person
objc[23504]: [0x10080dfe8]       0x100752010  Person
objc[23504]: [0x10080dff0]       0x100752030  Person
objc[23504]: [0x10080dff8]       0x100752050  Person
objc[23504]: [0x100817000]  ................  PAGE  (hot) 
objc[23504]: [0x100817038]       0x100752070  Person
objc[23504]: [0x100817040]       0x100752090  Person
objc[23504]: [0x100817048]       0x1007520b0  Person
objc[23504]: [0x100817050]       0x1007520d0  Person
objc[23504]: [0x100817058]       0x1007520f0  Person
objc[23504]: [0x100817060]       0x100752110  Person
objc[23504]: [0x100817068]       0x100752130  Person
objc[23504]: [0x100817070]       0x100752150  Person
objc[23504]: [0x100817078]       0x100752170  Person
objc[23504]: [0x100817080]       0x100752190  Person
objc[23504]: [0x100817088]       0x1007521b0  Person
objc[23504]: [0x100817090]       0x1007521d0  Person
objc[23504]: [0x100817098]       0x1007521f0  Person
objc[23504]: [0x1008170a0]       0x100752210  Person
objc[23504]: [0x1008170a8]       0x100752230  Person
objc[23504]: [0x1008170b0]       0x100752250  Person
objc[23504]: [0x1008170b8]       0x100752270  Person
objc[23504]: [0x1008170c0]       0x100752290  Person
objc[23504]: [0x1008170c8]       0x1007522b0  Person
objc[23504]: [0x1008170d0]       0x1007522d0  Person
objc[23504]: [0x1008170d8]       0x1007522f0  Person
objc[23504]: [0x1008170e0]       0x100752310  Person
objc[23504]: [0x1008170e8]       0x100752330  Person
objc[23504]: [0x1008170f0]       0x100752350  Person
objc[23504]: [0x1008170f8]       0x100752370  Person
objc[23504]: [0x100817100]       0x100752390  Person
objc[23504]: [0x100817108]       0x1007523b0  Person
objc[23504]: [0x100817110]       0x1007523d0  Person
objc[23504]: [0x100817118]       0x1007523f0  Person
objc[23504]: [0x100817120]       0x100752410  Person
objc[23504]: [0x100817128]       0x100752430  Person
objc[23504]: [0x100817130]       0x100752450  Person
objc[23504]: [0x100817138]       0x100752470  Person
objc[23504]: [0x100817140]       0x100752490  Person
objc[23504]: [0x100817148]       0x1007524b0  Person
objc[23504]: [0x100817150]       0x1007524d0  Person
objc[23504]: [0x100817158]       0x1007524f0  Person
objc[23504]: [0x100817160]       0x100752510  Person
objc[23504]: [0x100817168]       0x100752530  Person
objc[23504]: [0x100817170]       0x100752550  Person
objc[23504]: [0x100817178]       0x1007556d0  Person
objc[23504]: [0x100817180]       0x1007556f0  Person
objc[23504]: [0x100817188]       0x100755710  Person
objc[23504]: [0x100817190]       0x100755730  Person
objc[23504]: [0x100817198]       0x100755750  Person
objc[23504]: [0x1008171a0]       0x100755770  Person
objc[23504]: [0x1008171a8]       0x100755790  Person
objc[23504]: [0x1008171b0]       0x1007557b0  Person
objc[23504]: [0x1008171b8]       0x1007557d0  Person
objc[23504]: [0x1008171c0]       0x1007557f0  Person
objc[23504]: [0x1008171c8]       0x100755810  Person
objc[23504]: [0x1008171d0]       0x100755830  Person
objc[23504]: [0x1008171d8]       0x100755850  Person
objc[23504]: [0x1008171e0]       0x100755870  Person
objc[23504]: [0x1008171e8]       0x100755890  Person
objc[23504]: [0x1008171f0]       0x1007558b0  Person
objc[23504]: [0x1008171f8]       0x1007558d0  Person
objc[23504]: [0x100817200]       0x1007558f0  Person
objc[23504]: [0x100817208]       0x100755910  Person
objc[23504]: [0x100817210]       0x100755930  Person
objc[23504]: [0x100817218]       0x100755950  Person
objc[23504]: [0x100817220]       0x100755970  Person
objc[23504]: [0x100817228]       0x100755990  Person
objc[23504]: [0x100817230]       0x1007559b0  Person
objc[23504]: [0x100817238]       0x1007559d0  Person
objc[23504]: [0x100817240]       0x1007559f0  Person
objc[23504]: [0x100817248]       0x100755a10  Person
objc[23504]: [0x100817250]       0x100755a30  Person
objc[23504]: [0x100817258]       0x100755a50  Person
objc[23504]: [0x100817260]       0x100755a70  Person
objc[23504]: [0x100817268]       0x100755a90  Person
objc[23504]: [0x100817270]       0x100755ab0  Person
objc[23504]: [0x100817278]       0x100755ad0  Person
objc[23504]: [0x100817280]       0x100755af0  Person
objc[23504]: [0x100817288]       0x100755b10  Person
objc[23504]: [0x100817290]       0x100755b30  Person
objc[23504]: [0x100817298]       0x100755b50  Person
objc[23504]: [0x1008172a0]       0x100755b70  Person
objc[23504]: [0x1008172a8]       0x100755b90  Person
objc[23504]: [0x1008172b0]       0x100755bb0  Person
objc[23504]: [0x1008172b8]       0x100755bd0  Person
objc[23504]: [0x1008172c0]       0x100755bf0  Person
objc[23504]: [0x1008172c8]       0x100755c10  Person
objc[23504]: [0x1008172d0]       0x100755c30  Person
objc[23504]: [0x1008172d8]       0x100755c50  Person
objc[23504]: [0x1008172e0]       0x100755c70  Person
objc[23504]: [0x1008172e8]       0x100755c90  Person
objc[23504]: [0x1008172f0]       0x100755cb0  Person
objc[23504]: [0x1008172f8]       0x100755cd0  Person
objc[23504]: [0x100817300]       0x100755cf0  Person
objc[23504]: [0x100817308]       0x100755d10  Person
objc[23504]: [0x100817310]       0x100755d30  Person
objc[23504]: [0x100817318]       0x100755d50  Person
objc[23504]: [0x100817320]       0x100755d70  Person
objc[23504]: [0x100817328]       0x100755d90  Person
objc[23504]: [0x100817330]       0x100755db0  Person
objc[23504]: [0x100817338]       0x100755dd0  Person
objc[23504]: [0x100817340]       0x100755df0  Person
objc[23504]: [0x100817348]       0x100755e10  Person
objc[23504]: [0x100817350]  ################  POOL 0x100817350
objc[23504]: [0x100817358]       0x100755e30  Person
objc[23504]: ##############

可以看到当600*8=4800字节，所以一页肯定存不下，可以看到

................ PAGE (full) (cold) page 右边有个 cold、hot。cold 代表不是当前页，hot 代表当前页。

继续看看对象调用 autorelease 方法做了什么事情？

- (id)autorelease {
    return ((id)self)->rootAutorelease();
}

inline id objc_object::rootAutorelease() {
    if (isTaggedPointer()) return (id)this;
    if (prepareOptimizedReturn(ReturnAtPlus1)) return (id)this;
    return rootAutorelease2();
}
_attribute__((noinline,used)) id objc_object::rootAutorelease2() {
    assert(!isTaggedPointer());
    return AutoreleasePoolPage::autorelease((id)this);
}

static inline id autorelease(id obj) {
    assert(obj);
    assert(!obj->isTaggedPointer());
    id *dest __unused = autoreleaseFast(obj);
    assert(!dest  ||  dest == EMPTY_POOL_PLACEHOLDER  ||  *dest == obj);
    return obj;
}

static inline id *autoreleaseFast(id obj) {
    AutoreleasePoolPage *page = hotPage();
    if (page && !page->full()) {
        return page->add(obj);
    } else if (page) {
        return autoreleaseFullPage(obj, page);
    } else {
        return autoreleaseNoPage(obj);
    }
}

查看 NSObject autorelease 方法调用链路可以看到最后还是调用 AutoreleasePoolPage 的 add 方法（会判断有没有页、有没有满）

autorelease 对象什么时候调用 release 方法

其实也就是 autorelease 和 RunLoop 的关系。

iOS 在主线程的 Runloop 中注册了2个 Observer

第1个 Observer 监听了 kCFRunLoopEntry 事件，会调用objc_autoreleasePoolPush()
第2个 Observer 监听了 kCFRunLoopBeforeWaiting 事件，会调用objc_autoreleasePoolPop()、objc_autoreleasePoolPush()。还监听了kCFRunLoopBeforeExit事件，会调用 objc_autoreleasePoolPop()

结合 RunLoop 运行图

01 通知 Observer 进入 Loop 会调用 objc_autoreleasePoolPush
做一堆其他事情
07 在将要休眠的时候先调用 objc_autoreleasePoolPop，再调用 objc_autoreleasePoolPush
等待唤醒做一堆其他事情，回到第二步
07 又开始休眠，先调用 objc_autoreleasePoolPop，再调用 objc_autoreleasePoolPush
11 没任务将要休眠，调用 objc_autoreleasePoolPop

可以看到 objc_autoreleasePoolPush、objc_autoreleasePoolPop 成对调用，贯穿 RunLoop

77 KiB Raw Blame History Unescape Escape

iOS 内存原理探究

定时器内存泄漏

iOS 内存布局

Tagged Pointer

什么是 Tagged Pointer

经典问题

如何判断一个指针是否为Tagged Pointer

OC 对象内存管理

copy/mutableCopy

引用计数

dealloc 是如何工作的？

LLVM 文档对 dealloc 的描述

查看 objc4 源码

神秘的 cxx_destruct

探究啥时候生成 .cxx_destruct 方法

深入 .cxx_destruct

自动调用 [super dealloc] 的原理

自动释放池底层原理探索

autorelease 对象什么时候调用 release 方法

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