iOS-内存管理分析（下）

前言

iOS-内存管理分析（上）以及iOS-内存管理分析（中）分析了内存的分区，布局，散列表，弱引用计数表等相关知识，这篇文章继续分析内存管理。

1 自动释放池初探

在我们的demo工程的main文件代码如下

#import <Foundation/Foundation.h>

int main(int argc, const char * argv[]) {
 
    @autoreleasepool {
        NSLog(@"Hello, World!");
    }
    return 0;
}

我们通过clang命令，看下底层，我们在main.cpp找到如下代码

int main(int argc, const char * argv[]) {

    /*  */ { __AtAutoreleasePool __autoreleasepool; 
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_r7_rnm6hs1x2jg8bqy7sjwsskx00000gn_T_main_1cecfd_mi_0);
    }
    return 0;
}

@autoreleasepool被注释了，替换的是*__AtAutoreleasePool __autoreleasepool; *这个。
__AtAutoreleasePool是一个结构体，如下

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

这里有一个构造和析构函数，构造函数创建一个objc_autoreleasePoolPush对象，释构函数调用objc_autoreleasePoolPop释入这个objc_autoreleasePoolPush对象。
当然我们还可以通过汇编分析流程，这里不再叙述。

2 自动释放池结构

objc_autoreleasePoolPush与objc_autoreleasePoolPop的底层到底是什么，做了什么操作，我们接下来分析下。
我们在objc(818)的源码中搜索下objc_autoreleasePoolPush这个函数，找到如下代码

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

调用了AutoreleasePoolPage它的push()函数，我们进去看下，如下

class AutoreleasePoolPage : private AutoreleasePoolPageData

AutoreleasePoolPage继承于AutoreleasePoolPageData。

 Autorelease pool implementation

   A thread's autorelease pool is a stack of pointers.
   Each pointer is either an object to release, or POOL_BOUNDARY which is 
     an autorelease pool boundary.
   A pool token is a pointer to the POOL_BOUNDARY for that pool. When 
     the pool is popped, every object hotter than the sentinel is released.
   The stack is divided into a doubly-linked list of pages. Pages are added
     and deleted as necessary. 
   Thread-local storage points to the hot page, where newly autoreleased 
     objects are stored.

自动释放池与线程有关联的。POOL_BOUNDARY就是哨兵对象边（自动释放池边界），不能无限制出栈，如果没有这个边界，不断的出栈，会出现访问野指针。
这里说了自动释放池是一个双向链表。
我们再来看下AutoreleasePoolPageData

struct AutoreleasePoolPageData
{
#if SUPPORT_AUTORELEASEPOOL_DEDUP_PTRS
    struct AutoreleasePoolEntry {
        uintptr_t ptr: 48;
        uintptr_t count: 16;

        static const uintptr_t maxCount = 65535; // 2^16 - 1
    };
    static_assert((AutoreleasePoolEntry){ .ptr = MACH_VM_MAX_ADDRESS }.ptr == MACH_VM_MAX_ADDRESS, "MACH_VM_MAX_ADDRESS doesn't fit into AutoreleasePoolEntry::ptr!");
#endif

    magic_t const magic; // 16
    __unsafe_unretained id *next; // 8
    pthread_t const thread; // 8
    AutoreleasePoolPage * const parent; // 8
    AutoreleasePoolPage *child; // 8
    uint32_t const depth; // 4
    uint32_t hiwat;  // 4
AutoreleasePoolPageData(__unsafe_unretained id* _next, pthread_t _thread, AutoreleasePoolPage* _parent, uint32_t _depth, uint32_t _hiwat)
        : magic(), next(_next), thread(_thread),
          parent(_parent), child(nil),
          depth(_depth), hiwat(_hiwat)
    {
    }
};

magic⽤来校验AutoreleasePoolPage的结构是否完整；
next指向最新添加的autoreleased对象的下⼀个位置，初始化时指向begin()；
thread指向当前线程；
parent指向⽗结点，第⼀个结点的parent值为nil；
child指向⼦结点，最后⼀个结点的child值为nil；
depth代表深度，从0开始，往后递增1；
hiwat代表highwatermark最⼤⼊栈数量标记

我们改下main.m源码，压栈一个对象，

#import <Foundation/Foundation.h>
extern void _objc_autoreleasePoolPrint(void);

int main(int argc, const char * argv[]) {
    @autoreleasepool {
       
        NSObject *objc = [[[NSObject alloc] init] autorelease];
        _objc_autoreleasePoolPrint();

    }
    return 0;
}

这里一定要把arc关掉。
运行项目，如下

objc[27889]: AUTORELEASE POOLS for thread 0x1000ebe00
objc[27889]: 2 releases pending.
objc[27889]: [0x10200c000]  ................  PAGE  (hot) (cold)
objc[27889]: [0x10200c038]  ################  POOL 0x10200c038
objc[27889]: [0x10200c040]       0x10060dfc0  NSObject
objc[27889]: ##############
Program ended with exit code: 0

这里可以看到NSObject对象是压入栈了。
这里有两个对象的原因，是因为哨兵对象也是一个对象。

3 自动释放池的创建和压栈

自动释放池的创建和压栈是如何进行的，我们分析它的源码看下。
我们看下AutoreleasePoolPage::push();的源码，如下

 static inline void *push() 
    {
        id *dest;
        if (slowpath(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;
    }

我们再来看下autoreleaseFast这个函数，源码如下

 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);
        }
    }

这里判断页面是否存在并且没有满，直接add进去。
如果满了调用autoreleaseFullPage处理，如果没有这个页面，调用autoreleaseNoPage创建。
我们再来看下autoreleaseNoPage，如下

 static __attribute__((noinline))
    id *autoreleaseNoPage(id obj)
    {
        // "No page" could mean no pool has been pushed
        // or an empty placeholder pool has been pushed and has no contents yet
        ASSERT(!hotPage());

        bool pushExtraBoundary = false;
        if (haveEmptyPoolPlaceholder()) {
            // We are pushing a second pool over the empty placeholder pool
            // or pushing the first object into the empty placeholder pool.
            // Before doing that, push a pool boundary on behalf of the pool 
            // that is currently represented by the empty placeholder.
            pushExtraBoundary = true;
        }
        else if (obj != POOL_BOUNDARY  &&  DebugMissingPools) {
            // We are pushing an object with no pool in place, 
            // and no-pool debugging was requested by environment.
            _objc_inform("MISSING POOLS: (%p) Object %p of class %s "
                         "autoreleased with no pool in place - "
                         "just leaking - break on "
                         "objc_autoreleaseNoPool() to debug", 
                         objc_thread_self(), (void*)obj, object_getClassName(obj));
            objc_autoreleaseNoPool(obj);
            return nil;
        }
        else if (obj == POOL_BOUNDARY  &&  !DebugPoolAllocation) {
            // We are pushing a pool with no pool in place,
            // and alloc-per-pool debugging was not requested.
            // Install and return the empty pool placeholder.
            return setEmptyPoolPlaceholder();
        }

        // We are pushing an object or a non-placeholder'd pool.

        // Install the first page.
        AutoreleasePoolPage *page = new AutoreleasePoolPage(nil);
        setHotPage(page);
        
        // Push a boundary on behalf of the previously-placeholder'd pool.
        if (pushExtraBoundary) {
            page->add(POOL_BOUNDARY);
        }
        
        // Push the requested object or pool.
        return page->add(obj);
    }

在这里

 AutoreleasePoolPage *page = new AutoreleasePoolPage(nil);
 setHotPage(page);

创建新页面，并设置成hot页面，之后再add对象
我们再看下AutoreleasePoolPage

AutoreleasePoolPage(AutoreleasePoolPage *newParent) :
        AutoreleasePoolPageData(begin(),
                                objc_thread_self(),
                                newParent,
                                newParent ? 1+newParent->depth : 0,
                                newParent ? newParent->hiwat : 0)
    {
        if (objc::PageCountWarning != -1) {
            checkTooMuchAutorelease();
        }

        if (parent) {
            parent->check();
            ASSERT(!parent->child);
            parent->unprotect();
            parent->child = this;
            parent->protect();
        }
        protect();
    }

这里设置双向链表的数据。
我们看下的begin如下

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

sizeof(*this)这时56的大小，来源于autorealeaPoolDat成员变量

    magic_t const magic;//16
    __unsafe_unretained id *next;//8
    pthread_t const thread; //8
    AutoreleasePoolPage * const parent; //8
    AutoreleasePoolPage *child; //8
    uint32_t const depth;//4
    uint32_t hiwat;//4

magic_t是结构体，有一个uint32_4 m[4]占用16字节。
objc_thread_self(),获取当前线程。

自动释放池的成员变量也是占用空间的。

POOL 0x10080c038上方的这个是我们的哨兵对象。

4 自动释放池的满页临界和压栈出栈

当我们的页面满的时候，是如何处理的，结构又有什么变化呢，我们继续分析下源码才能得到。

我们知道内存是分页的加载的，为什么这么做呢？
假如不分页的话会出现以下情况

不断的压栈出栈，会对这张表操作非常频繁，也就是说对内存操作频繁
压栈出栈的时候，会对整张表进行加锁解锁，非常消耗性能
所有的对象存在这张表，非常复杂，管理复杂
如果某个局部发生问题，整张表都会影响
而分页的方式就不会有这些问题
内存分页的好处
内存可以不连续
性能大大的提高

我们一张表可以存放多少内存，我们探索一下。
我们在改下我们的main.m文件

#import <Foundation/Foundation.h>
extern void _objc_autoreleasePoolPrint(void);

int main(int argc, const char * argv[]) {
    @autoreleasepool {
       
        for (int i = 0; i<4; i++) {
            NSObject *objc = [[[NSObject alloc] init] autorelease];
        }
        _objc_autoreleasePoolPrint();

    }
    return 0;
}

我们运行项目，看下效果，如下所示

objc[29930]: AUTORELEASE POOLS for thread 0x1000ebe00
objc[29930]: 5 releases pending.
objc[29930]: [0x101009000]  ................  PAGE  (hot) (cold)
objc[29930]: [0x101009038]  ################  POOL 0x101009038
objc[29930]: [0x101009040]       0x101a4b020  NSObject
objc[29930]: [0x101009048]       0x101a3f2d0  NSObject
objc[29930]: [0x101009050]       0x101a3ebf0  NSObject
objc[29930]: [0x101009058]       0x101a3dac0  NSObject
objc[29930]: ##############
KCObjcBuild was compiled with optimization - stepping may behave oddly; variables may not be available.

我们可以看到没有产生分页的效果，我们再把循环改下505，再看下效果

objc[29984]: AUTORELEASE POOLS for thread 0x1000ebe00
objc[29984]: 506 releases pending.
objc[29984]: [0x10480d000]  ................  PAGE (full)  (cold)
objc[29984]: [0x10480d038]  ################  POOL 0x10480d038
objc[29984]: [0x10480d040]       0x10132a9f0  NSObject
objc[29984]: [0x10480d048]       0x101328f20  NSObject
objc[29984]: [0x10480d050]       0x101328b70  NSObject
objc[29984]: [0x10480d058]       0x101328bd0  NSObject
objc[29984]: [0x10480d060]       0x101327b10  NSObject
objc[29984]: [0x10480d068]       0x101327b50  NSObject
objc[29984]: [0x10480d070]       0x101327100  NSObject
objc[29984]: [0x10480d078]       0x101325fd0  NSObject
objc[29984]: [0x10480d080]       0x101326010  NSObject
objc[29984]: [0x10480d088]       0x101326040  NSObject
objc[29984]: [0x10480d090]       0x101325ae0  NSObject
objc[29984]: [0x10480d098]       0x101324fc0  NSObject
objc[29984]: [0x10480d0a0]       0x1013249a0  NSObject
objc[29984]: [0x10480d0a8]       0x101324500  NSObject
objc[29984]: [0x10480d0b0]       0x101323ff0  NSObject
objc[29984]: [0x10480d0b8]       0x101323290  NSObject
objc[29984]: [0x10480d0c0]       0x101322d90  NSObject
objc[29984]: [0x10480d0c8]       0x101322630  NSObject
objc[29984]: [0x10480d0d0]       0x101322660  NSObject
objc[29984]: [0x10480d0d8]       0x10130f190  NSObject
objc[29984]: [0x10480d0e0]       0x10130f1c0  NSObject
objc[29984]: [0x10480d0e8]       0x10130e510  NSObject
objc[29984]: [0x10480d0f0]       0x10130e7d0  NSObject
objc[29984]: [0x10480d0f8]       0x10130d7b0  NSObject
objc[29984]: [0x10480d100]       0x10130d080  NSObject
objc[29984]: [0x10480d108]       0x10130d140  NSObject
objc[29984]: [0x10480d110]       0x10130be70  NSObject
objc[29984]: [0x10480d118]       0x10130ade0  NSObject
objc[29984]: [0x10480d120]       0x10130a1b0  NSObject
objc[29984]: [0x10480d128]       0x101305530  NSObject
objc[29984]: [0x10480d130]       0x101308ca0  NSObject
objc[29984]: [0x10480d138]       0x101308e60  NSObject
objc[29984]: [0x10480d140]       0x101304900  NSObject
objc[29984]: [0x10480d148]       0x1013387a0  NSObject
objc[29984]: [0x10480d150]       0x1013387b0  NSObject
objc[29984]: [0x10480d158]       0x1013387c0  NSObject
objc[29984]: [0x10480d160]       0x1013387d0  NSObject
objc[29984]: [0x10480d168]       0x1013387e0  NSObject
objc[29984]: [0x10480d170]       0x1013387f0  NSObject
objc[29984]: [0x10480d178]       0x101338800  NSObject
objc[29984]: [0x10480d180]       0x101338810  NSObject
objc[29984]: [0x10480d188]       0x101338820  NSObject
objc[29984]: [0x10480d190]       0x101338830  NSObject
objc[29984]: [0x10480d198]       0x101338840  NSObject
objc[29984]: [0x10480d1a0]       0x101338850  NSObject
objc[29984]: [0x10480d1a8]       0x101338860  NSObject
objc[29984]: [0x10480d1b0]       0x101338870  NSObject
objc[29984]: [0x10480d1b8]       0x101338880  NSObject
objc[29984]: [0x10480d1c0]       0x101338890  NSObject
objc[29984]: [0x10480d1c8]       0x101338bb0  NSObject
objc[29984]: [0x10480d1d0]       0x101338bc0  NSObject
objc[29984]: [0x10480d1d8]       0x101338bd0  NSObject
objc[29984]: [0x10480d1e0]       0x101338be0  NSObject
objc[29984]: [0x10480d1e8]       0x101338bf0  NSObject
objc[29984]: [0x10480d1f0]       0x101338c00  NSObject
objc[29984]: [0x10480d1f8]       0x101338c10  NSObject
objc[29984]: [0x10480d200]       0x101338c20  NSObject
objc[29984]: [0x10480d208]       0x101338c30  NSObject
objc[29984]: [0x10480d210]       0x101338c40  NSObject
objc[29984]: [0x10480d218]       0x101338c50  NSObject
objc[29984]: [0x10480d220]       0x101338c60  NSObject
objc[29984]: [0x10480d228]       0x101338c70  NSObject
objc[29984]: [0x10480d230]       0x101338c80  NSObject
objc[29984]: [0x10480d238]       0x101338c90  NSObject
objc[29984]: [0x10480d240]       0x101338ca0  NSObject
objc[29984]: [0x10480d248]       0x101338cb0  NSObject
objc[29984]: [0x10480d250]       0x101338cc0  NSObject
objc[29984]: [0x10480d258]       0x101338cd0  NSObject
objc[29984]: [0x10480d260]       0x101338ce0  NSObject
objc[29984]: [0x10480d268]       0x101338cf0  NSObject
objc[29984]: [0x10480d270]       0x101338d00  NSObject
objc[29984]: [0x10480d278]       0x101338d10  NSObject
objc[29984]: [0x10480d280]       0x101338d20  NSObject
objc[29984]: [0x10480d288]       0x101338d30  NSObject
objc[29984]: [0x10480d290]       0x101338d40  NSObject
objc[29984]: [0x10480d298]       0x101338d50  NSObject
objc[29984]: [0x10480d2a0]       0x101338d60  NSObject
objc[29984]: [0x10480d2a8]       0x101338d70  NSObject
objc[29984]: [0x10480d2b0]       0x101338d80  NSObject
objc[29984]: [0x10480d2b8]       0x101338d90  NSObject
objc[29984]: [0x10480d2c0]       0x101338da0  NSObject
objc[29984]: [0x10480d2c8]       0x101338db0  NSObject
objc[29984]: [0x10480d2d0]       0x101338dc0  NSObject
objc[29984]: [0x10480d2d8]       0x101338dd0  NSObject
objc[29984]: [0x10480d2e0]       0x101338de0  NSObject
objc[29984]: [0x10480d2e8]       0x101338df0  NSObject
objc[29984]: [0x10480d2f0]       0x101338e00  NSObject
objc[29984]: [0x10480d2f8]       0x101338e10  NSObject
objc[29984]: [0x10480d300]       0x101338e20  NSObject
objc[29984]: [0x10480d308]       0x101338e30  NSObject
objc[29984]: [0x10480d310]       0x101338e40  NSObject
objc[29984]: [0x10480d318]       0x101338e50  NSObject
objc[29984]: [0x10480d320]       0x101338e60  NSObject
objc[29984]: [0x10480d328]       0x101338e70  NSObject
objc[29984]: [0x10480d330]       0x101338e80  NSObject
objc[29984]: [0x10480d338]       0x101338e90  NSObject
objc[29984]: [0x10480d340]       0x101338ea0  NSObject
objc[29984]: [0x10480d348]       0x101338eb0  NSObject
objc[29984]: [0x10480d350]       0x101338ec0  NSObject
objc[29984]: [0x10480d358]       0x101338ed0  NSObject
objc[29984]: [0x10480d360]       0x101338ee0  NSObject
objc[29984]: [0x10480d368]       0x101338ef0  NSObject
objc[29984]: [0x10480d370]       0x101338f00  NSObject
objc[29984]: [0x10480d378]       0x101338f10  NSObject
objc[29984]: [0x10480d380]       0x101338f20  NSObject
objc[29984]: [0x10480d388]       0x101338f30  NSObject
objc[29984]: [0x10480d390]       0x101338f40  NSObject
objc[29984]: [0x10480d398]       0x101338f50  NSObject
objc[29984]: [0x10480d3a0]       0x101338f60  NSObject
objc[29984]: [0x10480d3a8]       0x101338f70  NSObject
objc[29984]: [0x10480d3b0]       0x101338f80  NSObject
objc[29984]: [0x10480d3b8]       0x101338f90  NSObject
objc[29984]: [0x10480d3c0]       0x101338fa0  NSObject
objc[29984]: [0x10480d3c8]       0x101338fb0  NSObject
objc[29984]: [0x10480d3d0]       0x101338fc0  NSObject
objc[29984]: [0x10480d3d8]       0x101338fd0  NSObject
objc[29984]: [0x10480d3e0]       0x101338fe0  NSObject
objc[29984]: [0x10480d3e8]       0x101338ff0  NSObject
objc[29984]: [0x10480d3f0]       0x101339000  NSObject
objc[29984]: [0x10480d3f8]       0x101339010  NSObject
objc[29984]: [0x10480d400]       0x101339020  NSObject
objc[29984]: [0x10480d408]       0x101339030  NSObject
objc[29984]: [0x10480d410]       0x101339040  NSObject
objc[29984]: [0x10480d418]       0x101339050  NSObject
objc[29984]: [0x10480d420]       0x101339060  NSObject
objc[29984]: [0x10480d428]       0x101339070  NSObject
objc[29984]: [0x10480d430]       0x101339080  NSObject
objc[29984]: [0x10480d438]       0x101339090  NSObject
objc[29984]: [0x10480d440]       0x1013390a0  NSObject
objc[29984]: [0x10480d448]       0x1013390b0  NSObject
objc[29984]: [0x10480d450]       0x1013390c0  NSObject
objc[29984]: [0x10480d458]       0x1013390d0  NSObject
objc[29984]: [0x10480d460]       0x1013390e0  NSObject
objc[29984]: [0x10480d468]       0x1013390f0  NSObject
objc[29984]: [0x10480d470]       0x101339100  NSObject
objc[29984]: [0x10480d478]       0x101339110  NSObject
objc[29984]: [0x10480d480]       0x101339120  NSObject
objc[29984]: [0x10480d488]       0x101339130  NSObject
objc[29984]: [0x10480d490]       0x101339140  NSObject
objc[29984]: [0x10480d498]       0x101339150  NSObject
objc[29984]: [0x10480d4a0]       0x101339160  NSObject
objc[29984]: [0x10480d4a8]       0x101339170  NSObject
objc[29984]: [0x10480d4b0]       0x101339180  NSObject
objc[29984]: [0x10480d4b8]       0x101339190  NSObject
objc[29984]: [0x10480d4c0]       0x1013391a0  NSObject
objc[29984]: [0x10480d4c8]       0x1013391b0  NSObject
objc[29984]: [0x10480d4d0]       0x1013391c0  NSObject
objc[29984]: [0x10480d4d8]       0x1013391d0  NSObject
objc[29984]: [0x10480d4e0]       0x1013391e0  NSObject
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objc[29984]: [0x10480de48]       0x1013350e0  NSObject
objc[29984]: [0x10480de50]       0x1013350f0  NSObject
objc[29984]: [0x10480de58]       0x101335100  NSObject
objc[29984]: [0x10480de60]       0x101335110  NSObject
objc[29984]: [0x10480de68]       0x101335120  NSObject
objc[29984]: [0x10480de70]       0x101335130  NSObject
objc[29984]: [0x10480de78]       0x101335140  NSObject
objc[29984]: [0x10480de80]       0x101335150  NSObject
objc[29984]: [0x10480de88]       0x101335160  NSObject
objc[29984]: [0x10480de90]       0x101335170  NSObject
objc[29984]: [0x10480de98]       0x101335180  NSObject
objc[29984]: [0x10480dea0]       0x101335190  NSObject
objc[29984]: [0x10480dea8]       0x1013351a0  NSObject
objc[29984]: [0x10480deb0]       0x1013351b0  NSObject
objc[29984]: [0x10480deb8]       0x1013351c0  NSObject
objc[29984]: [0x10480dec0]       0x1013351d0  NSObject
objc[29984]: [0x10480dec8]       0x1013351e0  NSObject
objc[29984]: [0x10480ded0]       0x1013351f0  NSObject
objc[29984]: [0x10480ded8]       0x101335200  NSObject
objc[29984]: [0x10480dee0]       0x101335210  NSObject
objc[29984]: [0x10480dee8]       0x101335220  NSObject
objc[29984]: [0x10480def0]       0x101335230  NSObject
objc[29984]: [0x10480def8]       0x101335240  NSObject
objc[29984]: [0x10480df00]       0x101335250  NSObject
objc[29984]: [0x10480df08]       0x101335260  NSObject
objc[29984]: [0x10480df10]       0x101335270  NSObject
objc[29984]: [0x10480df18]       0x101335280  NSObject
objc[29984]: [0x10480df20]       0x101335290  NSObject
objc[29984]: [0x10480df28]       0x1013352a0  NSObject
objc[29984]: [0x10480df30]       0x1013352b0  NSObject
objc[29984]: [0x10480df38]       0x1013352c0  NSObject
objc[29984]: [0x10480df40]       0x1013352d0  NSObject
objc[29984]: [0x10480df48]       0x1013352e0  NSObject
objc[29984]: [0x10480df50]       0x1013352f0  NSObject
objc[29984]: [0x10480df58]       0x101335300  NSObject
objc[29984]: [0x10480df60]       0x101335310  NSObject
objc[29984]: [0x10480df68]       0x101335320  NSObject
objc[29984]: [0x10480df70]       0x101335330  NSObject
objc[29984]: [0x10480df78]       0x101335340  NSObject
objc[29984]: [0x10480df80]       0x101335350  NSObject
objc[29984]: [0x10480df88]       0x101335360  NSObject
objc[29984]: [0x10480df90]       0x101335370  NSObject
objc[29984]: [0x10480df98]       0x101335380  NSObject
objc[29984]: [0x10480dfa0]       0x101335390  NSObject
objc[29984]: [0x10480dfa8]       0x1013353a0  NSObject
objc[29984]: [0x10480dfb0]       0x1013353b0  NSObject
objc[29984]: [0x10480dfb8]       0x1013353c0  NSObject
objc[29984]: [0x10480dfc0]       0x1013353d0  NSObject
objc[29984]: [0x10480dfc8]       0x1013353e0  NSObject
objc[29984]: [0x10480dfd0]       0x1013353f0  NSObject
objc[29984]: [0x10480dfd8]       0x101335400  NSObject
objc[29984]: [0x10480dfe0]       0x101335410  NSObject
objc[29984]: [0x10480dfe8]       0x101335420  NSObject
objc[29984]: [0x10480dff0]       0x101335430  NSObject
objc[29984]: [0x10480dff8]       0x101335440  NSObject
objc[29984]: [0x104809000]  ................  PAGE  (hot) 
objc[29984]: [0x104809038]       0x101335450  NSObject

我们看到这里有产生PAGE (hot) ，我们再以下两行打印

objc[29984]: [0x10480d000]  ................  PAGE (full)  (cold)
objc[29984]: [0x104809000]  ................  PAGE  (hot)

当前页满的时候，新建了一个PAGE,并设置hot
这里可能看出504刚好满一页，我们算下，页面的大小是多少？
504 8 + 56=4088+8(哨兵对象)=4096=10244=4k。
通过这里可以看出PAGESIZE在MAC下是4k。
我们再看下AutoreleasePoolPage中的#define PAGE_MIN_SIZE (1 << PAGE_MIN_SHIFT)，而#define PAGE_MIN_SHIFT 12，1左移12位，也就是2的12次方，就是4096。iOS是16k的在大小
在分页的时候哨兵对象并没有创建，因为我们已经有一个了，不需要再创建。
所以第二个对象就可以创建505个对象了。

那我们的pop是如何操作的，我们来看下objc_autoreleasePoolPop函数

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

我们再看一这里的pop函数

 static inline void
    pop(void *token)
    {
        AutoreleasePoolPage *page;
        id *stop;
        if (token == (void*)EMPTY_POOL_PLACEHOLDER) {
            // Popping the top-level placeholder pool.
            page = hotPage();
            if (!page) {
                // Pool was never used. Clear the placeholder.
                return setHotPage(nil);
            }
            // Pool was used. Pop its contents normally.
            // Pool pages remain allocated for re-use as usual.
            page = coldPage();
            token = page->begin();
        } else {
            page = pageForPointer(token);
        }

        stop = (id *)token;
        if (*stop != POOL_BOUNDARY) {
            if (stop == page->begin()  &&  !page->parent) {
                // Start of coldest page may correctly not be POOL_BOUNDARY:
                // 1. top-level pool is popped, leaving the cold page in place
                // 2. an object is autoreleased with no pool
            } else {
                // Error. For bincompat purposes this is not 
                // fatal in executables built with old SDKs.
                return badPop(token);
            }
        }

        if (slowpath(PrintPoolHiwat || DebugPoolAllocation || DebugMissingPools)) {
            return popPageDebug(token, page, stop);
        }

        return popPage<false>(token, page, stop);
    }

这里获取到page = hotPage();这个hot页，如果不是hot。

 if (!page) {
                // Pool was never used. Clear the placeholder.
                return setHotPage(nil);
            }

置空

 page = coldPage();
 token = page->begin();

不断的移动这个页面，移除popPage<false>(token, page, stop);这里的调用

 template<bool allowDebug>
    static void
    popPage(void *token, AutoreleasePoolPage *page, id *stop)
    {
        if (allowDebug && PrintPoolHiwat) printHiwat();

        page->releaseUntil(stop);

        // memory: delete empty children
        if (allowDebug && DebugPoolAllocation  &&  page->empty()) {
            // special case: delete everything during page-per-pool debugging
            AutoreleasePoolPage *parent = page->parent;
            page->kill();
            setHotPage(parent);
        } else if (allowDebug && DebugMissingPools  &&  page->empty()  &&  !page->parent) {
            // special case: delete everything for pop(top)
            // when debugging missing autorelease pools
            page->kill();
            setHotPage(nil);
        } else if (page->child) {
            // hysteresis: keep one empty child if page is more than half full
            if (page->lessThanHalfFull()) {
                page->child->kill();
            }
            else if (page->child->child) {
                page->child->child->kill();
            }
        }
    }

这里先

AutoreleasePoolPage *parent = page->parent;

拿到parent页面，接着

page->kill();

杀掉当前页，然后

setHotPage(parent);

把父页设置成hot。
而kill就是不断的内存平移

 void kill() 
    {
        // Not recursive: we don't want to blow out the stack 
        // if a thread accumulates a stupendous amount of garbage
        AutoreleasePoolPage *page = this;
        while (page->child) page = page->child;

        AutoreleasePoolPage *deathptr;
        do {
            deathptr = page;
            page = page->parent;
            if (page) {
                page->unprotect();
                page->child = nil;
                page->protect();
            }
            delete deathptr;
        } while (deathptr != this);
    }

先入后出，不断的递归。
通过哨兵对象是否等于哨兵边界来判断标记完成了。

5 自动释放池的拓展

临时变量什么时候释放
自动释放池原理
自动释放池能否嵌套使用
临时变量什么时候释放就是在作用域的范围
自动释放池原理就是上述所讲
自动释放池可以嵌套使用
我们看下代码

#import <Foundation/Foundation.h>
extern void _objc_autoreleasePoolPrint(void);

int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSObject *objc = [[[NSObject alloc] init] autorelease];
        
        @autoreleasepool {
            NSObject *objc2 = [[[NSObject alloc] init] autorelease];
            NSLog(@"%@", objc2);
            _objc_autoreleasePoolPrint();
        }
        
        _objc_autoreleasePoolPrint();

    }
    return 0;
}

我们运行下项目，看下效果，如下

objc[30334]: ##############
objc[30334]: AUTORELEASE POOLS for thread 0x1000ebe00
objc[30334]: 4 releases pending.
objc[30334]: [0x100811000]  ................  PAGE  (hot) (cold)
objc[30334]: [0x100811038]  ################  POOL 0x100811038
objc[30334]: [0x100811040]       0x10060d9b0  NSObject
objc[30334]: [0x100811048]  ################  POOL 0x100811048
objc[30334]: [0x100811050]       0x10060cdd0  NSObject
objc[30334]: ##############
objc[30334]: ##############
objc[30334]: AUTORELEASE POOLS for thread 0x1000ebe00
objc[30334]: 2 releases pending.
objc[30334]: [0x100811000]  ................  PAGE  (hot) (cold)
objc[30334]: [0x100811038]  ################  POOL 0x100811038
objc[30334]: [0x100811040]       0x10060d9b0  NSObject
objc[30334]: ##############
KCObjcBuild was compiled with optimization - stepping may behave oddly; variables may not be available.

从这可以看出objc2是加入到objc2的自动释放池中，这里也说明是可以嵌套使用，相互不影响。
但是如果不autorelease是不加入到自动释放池中的（在MAC中）
但是以alloc、new、copy、mutablecopy创建的是不会加入到自动释放池中的

总结

这篇文章主要介绍了自动释放池的结构、创建、压栈等相关知识，到这里内存管理的相关知识，我们已经基本介绍完了，但这只是个人的一份见解，内存相关的知识还有很多很多，本人也只是对其有那么一点点的研究，如有遗漏，还望海涵。

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