iOS底层探索之自动释放池原理解析

#import <Foundation/Foundation.h>
int main(int argc, const char * argv[]) {
 
    @autoreleasepool {
        NSLog(@"Hello, World!");
    }
    return 0;
}

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

    /* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool; 
        NSLog((NSString *)&__NSConstantStringImpl__var_folders_jl_d06jlfkj2ws74_5g45kms07m0000gn_T_main_da0d58_mi_0);
    }
    return 0;
}

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

// 导入 _objc_autoreleasePoolPrint 函数,用于打印自动释放池的结构
extern void _objc_autoreleasePoolPrint(void);
int main(int argc, const char * argv[]) {
    @autoreleasepool {
        NSObject *objc = [[[NSObject alloc] init] autorelease];
        _objc_autoreleasePoolPrint();
    }
    return 0;
}

##############
AUTORELEASE POOLS for thread 0x1000ebe00
2 releases pending.
[0x10700b000]  ................  PAGE  (hot) (cold)
[0x10700b038]  ################  POOL 0x10700b038
[0x10700b040]       0x100705f60  NSObject
##############

void *
objc_autoreleasePoolPush(void)
{
    // 调用 AutoreleasePoolPage 命名空间下的 push 函数
    return AutoreleasePoolPage::push();
}

/***********************************************************************
   Autorelease pool implementation

   A thread's autorelease pool is a stack of pointers. 
   线程的自动释放池是一个指针堆栈
   
   Each pointer is either an object to release, or POOL_BOUNDARY which is an autorelease pool boundary.
   每个指针要么是一个要释放的对象，要么是POOL_BOUNDARY自动释放池边界
     
   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.
   池令牌是指向该池的POOL_BOUNDARY的指针。当池被弹出，每个比哨兵热的对象都被释放
     
   The stack is divided into a doubly-linked list of pages. Pages are added and deleted as necessary. 
   堆栈被分成一个双链接的页面列表。根据需要添加和删除页面

   Thread-local storage points to the hot page, where newly autoreleased objects are stored. 
   线程本地存储指向热页，其中存储新自动释放的对象
**********************************************************************/

class AutoreleasePoolPage : private AutoreleasePoolPageData
{
	friend struct thread_data_t;

public:
	static size_t const SIZE =
#if PROTECT_AUTORELEASEPOOL
		PAGE_MAX_SIZE;  // must be multiple of vm page size
#else
		PAGE_MIN_SIZE;  // size and alignment, power of 2
#endif
    
private:
	static pthread_key_t const key = AUTORELEASE_POOL_KEY;
	static uint8_t const SCRIBBLE = 0xA3;  // 0xA3A3A3A3 after releasing
	static size_t const COUNT = SIZE / sizeof(id);
    static size_t const MAX_FAULTS = 2;
    
    ...
}

class AutoreleasePoolPage;
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;
	__unsafe_unretained id *next;
	pthread_t const thread;
	AutoreleasePoolPage * const parent;
	AutoreleasePoolPage *child;
	uint32_t const depth;
	uint32_t hiwat;

	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)
	{
	}
};

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;
}

int main(int argc, const char * argv[]) {
    @autoreleasepool {
  
        for (int i = 0; i < 505; i++) {
            NSObject *objc = [[[NSObject alloc] init] autorelease];
        }

        _objc_autoreleasePoolPrint();
    }
    return 0;
}

-------------------------
objc[1804]: ##############
objc[1804]: AUTORELEASE POOLS for thread 0x1000ebe00
objc[1804]: 506 releases pending.
objc[1804]: [0x10200c000]  ................  PAGE (full)  (cold)
objc[1804]: [0x10200c038]  ################  POOL 0x10200c038
objc[1804]: [0x10200c040]       0x100638420  NSObject
objc[1804]: [0x10200c048]       0x100637a40  NSObject
objc[1804]: [0x10200c050]       0x100636970  NSObject
...
objc[1804]: [0x100809000]  ................  PAGE  (hot) 
objc[1804]: [0x100809038]       0x10063a0b0  NSObject
objc[1804]: ##############

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

static inline void
pop(void *token)
{
    AutoreleasePoolPage *page;
    id *stop;
    //判断当前对象是否为空占位符
    if (token == (void*)EMPTY_POOL_PLACEHOLDER) {
        // Popping the top-level placeholder pool.
        //获取热页面
        page = hotPage();
        if (!page) {
            // Pool was never used. Clear the placeholder.
            //不存在热页面，将标记设置为nil
            return setHotPage(nil);
        }
        // Pool was used. Pop its contents normally.
        // Pool pages remain allocated for re-use as usual.
        //存在热页面，通过双向链表循环向上找到最冷页面
        page = coldPage();
        //将token设置为起始位置
        token = page->begin();
    } else {
        //获取token所在的页
        page = pageForPointer(token);
    }

    //赋值给stop
    stop = (id *)token;
    //当前位置不是哨兵对象
    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
            
            //最冷页面的起始可能不是POOL_BOUNDARY:
            //1. 弹出顶级池，保留冷页面
            //2. 对象在没有池的情况下被自动释放
        } else {
            // Error. For bincompat purposes this is not
            // fatal in executables built with old SDKs.
            //出现异常情况
            return badPop(token);
        }
    }

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

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

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

    //当前页中对象出栈，到stop位置停止
    page->releaseUntil(stop);

    // memory: delete empty children
    if (allowDebug && DebugPoolAllocation  &&  page->empty()) {
        // special case: delete everything during page-per-pool debugging
        //特殊情况:在逐页池调试期间删除所有内容
        
        //获取父页面
        AutoreleasePoolPage *parent = page->parent;
        //销毁当前页面
        page->kill();
        //将父页面设置为热页面
        setHotPage(parent);
    } else if (allowDebug && DebugMissingPools  &&  page->empty()  &&  !page->parent) {
        // special case: delete everything for pop(top)
        // when debugging missing autorelease pools
        //特殊情况:删除所有的pop
        
        //销毁当前页面
        page->kill();
        //将热页面标记设置为nil
        setHotPage(nil);
    } else if (page->child) {
        // hysteresis: keep one empty child if page is more than half full
        //如果页面超过一半，则保留一个空子页面
        if (page->lessThanHalfFull()) {
            page->child->kill();
        }
        else if (page->child->child) {
            page->child->child->kill();
        }
    }
}

void releaseUntil(id *stop) 
{
    // Not recursive: we don't want to blow out the stack 
    // if a thread accumulates a stupendous amount of garbage
    
    //向下遍历，到stop停止
    while (this->next != stop) {
        // Restart from hotPage() every time, in case -release 
        // autoreleased more objects
        
        //获取热页面
        AutoreleasePoolPage *page = hotPage();

        // fixme I think this `while` can be `if`, but I can't prove it
        //如果当前页面中没有对象
        while (page->empty()) {
            //获取父页面
            page = page->parent;
            //标记为热页面
            setHotPage(page);
        }

        page->unprotect();
#if SUPPORT_AUTORELEASEPOOL_DEDUP_PTRS
        AutoreleasePoolEntry* entry = (AutoreleasePoolEntry*) --page->next;

        // create an obj with the zeroed out top byte and release that
        id obj = (id)entry->ptr;
        int count = (int)entry->count;  // grab these before memset
#else
        //内存平移，获取对象
        id obj = *--page->next;
#endif
        memset((void*)page->next, SCRIBBLE, sizeof(*page->next));
        page->protect();

        //当前对象不是哨兵对象
        if (obj != POOL_BOUNDARY) {
#if SUPPORT_AUTORELEASEPOOL_DEDUP_PTRS
            // release count+1 times since it is count of the additional
            // autoreleases beyond the first one
            for (int i = 0; i < count + 1; i++) {
                objc_release(obj);
            }
#else
            //将其释放
            objc_release(obj);
#endif
        }
    }

    //将当前页面标记为热页面
    setHotPage(this);

#if DEBUG
    // we expect any children to be completely empty
    for (AutoreleasePoolPage *page = child; page; page = page->child) {
        ASSERT(page->empty());
    }
#endif
}

 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;
    //循环找到最后一个子页面
    while (page->child) page = page->child;

    AutoreleasePoolPage *deathptr;
    do {
        deathptr = page;
        //找到父页面
        page = page->parent;
        if (page) {
            //将子页面设置为nil
            page->unprotect();
            page->child = nil;
            page->protect();
        }
        //销毁子页面
        delete deathptr;
        
        //遍历销毁到this为止
    } while (deathptr != this);
}

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

        @autoreleasepool {
            NSObject *objc = [[[NSObject alloc] init] autorelease];
        }
        
        _objc_autoreleasePoolPrint();
    }
    return 0;
}

-------------------------
objc[2511]: ##############
objc[2511]: AUTORELEASE POOLS for thread 0x1000ebe00
objc[2511]: 4 releases pending.
objc[2511]: [0x10680d000]  ................  PAGE  (hot) (cold)
objc[2511]: [0x10680d038]  ################  POOL 0x10680d038
objc[2511]: [0x10680d040]       0x101370c40  NSObject
objc[2511]: [0x10680d048]  ################  POOL 0x10680d048
objc[2511]: [0x10680d050]       0x101365fb0  NSObject
objc[2511]: ##############

for (int i = 0; i<100000000; i++) {
    @autoreleasepool {
        NSLog(@"%d",i);
        __autoreleasing LZPerson *p =[LZPerson new];
    }
}