OC底层探索28-autoreleasepool自动释放池OC底层探索28-autoreleasepool自动释放池

int main(int argc, const char * argv[]) {
    @autoreleasepool {        
    }
    return 0;
}

int main(int argc, const char * argv[]) {
{ __AtAutoreleasePool __autoreleasepool; }
    return 0;
}

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

class AutoreleasePoolPage : private AutoreleasePoolPageData
{
    friend struct thread_data_t;

    //一页大小4098
#define PAGE_MIN_SIZE           (1 << PAGE_MIN_SHIFT) 
    static size_t const SIZE = PAGE_MAX_SIZE

    // 构造函数
    AutoreleasePoolPage(AutoreleasePoolPage *newParent) :
        AutoreleasePoolPageData(begin(),
                                objc_thread_self(),
                                newParent,
                                newParent ? 1+newParent->depth : 0,
                                newParent ? newParent->hiwat : 0)
    {
        ...
    }
    // 析构函数
    ~AutoreleasePoolPage() 
    {...}
}

/***********************************************************************
   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.
**********************************************************************/

struct AutoreleasePoolPageData
{
    magic_t const magic;
    __unsafe_unretained id *next;
    pthread_t const thread;
    AutoreleasePoolPage * const parent;
    AutoreleasePoolPage *child;
    uint32_t const depth;
    uint32_t hiwat;

//对照：`AutoreleasePoolPageData(begin(), objc_thread_self(),nil newParent, 0, 0)`
    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 id *autoreleaseFast(id obj)
{
    AutoreleasePoolPage *page = hotPage();
    // 1. 如果不满，则直接插入
    if (page && !page->full()) {
        return page->add(obj);
    } else if (page) {  // 2. 如果满了,创建新页并将当前页传入建立关系
        return autoreleaseFullPage(obj, page);
    } else {
        return autoreleaseNoPage(obj);
    }
}

id *add(id obj)
{
    id *ret = next;  // faster than `return next-1` because of aliasing
    // 对象赋值，并移动next
    *next++ = obj;
    return ret;
}

template<bool allowDebug>
    static void
    popPage(void *token, AutoreleasePoolPage *page, id *stop)
{
// 一口气释放页内所有对象
    page->releaseUntil(stop);

    // 释放页
    // 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();
        }
    }
}

void kill() 
{
    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);
}

void releaseUntil(id *stop) 
{
    while (this->next != stop) {
        // 获取当前页，当前页有可能是空的
        AutoreleasePoolPage *page = hotPage();
        //找到非空页
        while (page->empty()) {
            page = page->parent;
            // 拿到当前页的父类，并设为当前页
            setHotPage(page);
        }
        page->unprotect();
        // 找到前一个要释放的对象
        id obj = *--page->next;
        memset((void*)page->next, SCRIBBLE, sizeof(*page->next));
        page->protect();
        // 如果是哨兵则释放完成
        if (obj != POOL_BOUNDARY) {
        //// 如果不是哨兵对象则继续释放
#if SUPPORT_AUTORELEASEPOOL_DEDUP_PTRS
            for (int i = 0; i < count + 1; i++) {
                objc_release(obj);
            }
#else
            objc_release(obj);
#endif
        }
    }
    setHotPage(this);
}

_objc_autoreleasePollPrint()