iOS 自動釋放池autoreleasepool(二)

時間 2019-11-08

原文原文鏈接

繼上一篇文章介紹了autoreleasepool的AutoreleasepoolPage，這篇文章咱們就來主要講講AutoreleasepoolPage::Push跟AutoreleasepoolPage::Pop 還有一個autoreleasebash

AutoreleasepoolPage::Push

static inline void *push() 
    {
        id *dest;
        if (DebugPoolAllocation) {
            // Each autorelease pool starts on a new pool page.
            // 每一個autoreleasepool從一個新的page開始
            dest = autoreleaseNewPage(POOL_BOUNDARY);
        } else {
            // 已經有線程的頁
            dest = autoreleaseFast(POOL_BOUNDARY);
        }
        assert(dest == EMPTY_POOL_PLACEHOLDER || *dest == POOL_BOUNDARY);
        return dest;
    }
複製代碼

這裏分開兩條路，咱們先看第一個less

autoreleaseNewPage

先說一下大概流程ui

1.獲取一個活躍的頁 2.判斷這頁是否是滿了，滿的就新建一個page 3.若是沒獲取到page，也是新建一個頁 4.把要push的obj，push到對應的頁裏this

一、
   id *autoreleaseNewPage(id obj)
    {
        //獲取一個活躍的頁
        AutoreleasePoolPage *page = hotPage();
        //若是有就返回，而且判斷是不是滿的，滿的就會新建一個page
        if (page) return autoreleaseFullPage(obj, page);
        //若是沒獲取到page
        else return autoreleaseNoPage(obj);
    }
---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
二、
    id *autoreleaseFullPage(id obj, AutoreleasePoolPage *page)
    {
        // The hot page is full. 
        // Step to the next non-full page, adding a new page if necessary.
        // Then add the object to that page.
        assert(page == hotPage());
        assert(page->full()  ||  DebugPoolAllocation);

        // 若是滿了就循環，這一頁有child的話 就指向child，再繼續判斷是否滿
        // 直到沒有child 就建立一個新的頁
        do {
            if (page->child) page = page->child;
            else page = new AutoreleasePoolPage(page);
        } while (page->full());
        // 設置成活躍的頁
        setHotPage(page);
        // 將Obj 添加到這頁裏
        return page->add(obj);
    }
---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
三、
    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());

        // 翻譯：pushExtraBoundary 推入一個額外邊界
        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.
            //當push一個新頁或者第一頁的時候，在push以前須要先push一個poolboundary(邊界)
            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", 
                         pthread_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);
    }
複製代碼

到此，第一條路就走完了。spa

autoreleaseFast

一、
    static inline id *autoreleaseFast(id obj)
    {
        //一樣獲取一個活頁
        AutoreleasePoolPage *page = hotPage();
        if (page && !page->full()) {//若是page沒滿的話
            return page->add(obj);
        } else if (page) {//若是page滿了
            return autoreleaseFullPage(obj, page);
        } else {//沒有獲取到活頁
            return autoreleaseNoPage(obj);
        }
    }
---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
二、
    id *autoreleaseFullPage(id obj, AutoreleasePoolPage *page)
    {
        // The hot page is full. 
        // Step to the next non-full page, adding a new page if necessary.
        // Then add the object to that page.
        assert(page == hotPage());
        assert(page->full()  ||  DebugPoolAllocation);

        // 若是滿了就循環，這一頁有child的話 就指向child，再繼續判斷是否滿
        // 直到沒有child 就建立一個新的頁
        do {
            if (page->child) page = page->child;
            else page = new AutoreleasePoolPage(page);
        } while (page->full());
        // 設置成活躍的頁
        setHotPage(page);
        // 將Obj 添加到這頁裏
        return page->add(obj);
    }
---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
三、 
    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());

        // 翻譯：pushExtraBoundary 推入一個額外邊界
        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.
            //當push一個新頁或者第一頁的時候，在push以前須要先push一個poolboundary(邊界)
            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", 
                         pthread_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);
    }
複製代碼

其實這兩條路是差很少的，在底層中都走了這兩個的方法
autoreleaseFullPage
autoreleaseNoPage
感受徹底能夠合起來啊- -，不知道蘋果這麼處理是爲了什麼。
#####page->add()
push的最終操做也就是這個add，看看這裏都幹了什麼線程

id *add(id obj)
    {
        assert(!full());
        unprotect(); //取消保護
        id *ret = next;  // faster than `return next-1` because of aliasing
        *next++ = obj;
        protect(); // 保護
        return ret;
    }
複製代碼

這裏作了什麼呢翻譯

這裏面有unprotect() protect() 這兩個能夠理解爲加鎖相似NSLock

建立一個臨時變量ret 來存儲next

next ++ 內存地址移動。相似 0x0001 -> 0x0002

將obj存儲到新移動出來的內存地址上

那麼push就先到這裏，接下來講說autoreleasedebug

autorelease

在瞭解過push以後，咱們能夠很容易理解這個autorelease。看下面的代碼跳轉， autorelease最終走到了一個上文出現過的方法autoreleaseFast()，那麼其實autorelease作的操做也跟push同樣code

- (id)autorelease {
    return ((id)self)->rootAutorelease();
}
---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
objc_object::rootAutorelease()
{
    if (isTaggedPointer()) return (id)this;
    if (prepareOptimizedReturn(ReturnAtPlus1)) return (id)this;

    return rootAutorelease2();
}
---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
objc_object::rootAutorelease2()
{
    assert(!isTaggedPointer());
    return AutoreleasePoolPage::autorelease((id)this);
}
---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
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;
}
複製代碼

AutoreleasepoolPage::Pop

先簡單說一下都作了什麼orm

判斷當前pop對象是不是空池佔位符，是的話就推出去，按道理通常不多進去

經過棧頂的地址找到對應的page

記錄最高水位標記

開始pop，objc_release(obj);

kill空白頁

static inline void pop(void *token) 
    {
        AutoreleasePoolPage *page;
        id *stop;
        //只有在push的時候 push了一個邊界符， 纔有可能出現EMPTY_POOL_PLACEHOLDER
        if (token == (void*)EMPTY_POOL_PLACEHOLDER) {
            // Popping the top-level placeholder pool.
            // 推出最頂上的佔位池
            if (hotPage()) {
                // Pool was used. Pop its contents normally.
                // Pool pages remain allocated for re-use as usual.
                // 遞歸 coldPage調用begin
                pop(coldPage()->begin());
            } else {
                // Pool was never used. Clear the placeholder.
                setHotPage(nil);
            }
            return;
        }

        // 經過棧頂的地址找到對應的page
        page = pageForPointer(token);
        //token賦值給stop
        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);
            }
        }

        // 記錄最高水位標記
        if (PrintPoolHiwat) printHiwat();
        // 從棧頂開始操做出棧，並向棧中的對象發送release消息，直到遇到第一個邊界符
        page->releaseUntil(stop);

        // memory: delete empty children
        // 刪除空的child頁
        if (DebugPoolAllocation  &&  page->empty()) {
            // special case: delete everything during page-per-pool debugging
            AutoreleasePoolPage *parent = page->parent;
            page->kill();
            setHotPage(parent);
        } else if (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();
            }
        }
    }

---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
    void releaseUntil(id *stop) 
    {
        // Not recursive: we don't want to blow out the stack // if a thread accumulates a stupendous amount of garbage while (this->next != stop) { // Restart from hotPage() every time, in case -release // autoreleased more objects AutoreleasePoolPage *page = hotPage(); // fixme I think this `while` can be `if`, but I can't prove it
            while (page->empty()) {
                page = page->parent;
                setHotPage(page);
            }

            //相似push時候的操做
            page->unprotect();
            id obj = *--page->next;
            memset((void*)page->next, SCRIBBLE, sizeof(*page->next));
            page->protect();

            if (obj != POOL_BOUNDARY) {
                //若是當前對象不是邊界符，就釋放對象
                objc_release(obj);
            }
        }

        setHotPage(this);

#if DEBUG
        // we expect any children to be completely empty
        for (AutoreleasePoolPage *page = child; page; page = page->child) {
            assert(page->empty());
        }
#endif
    }
---------------------------------------- 華麗的分割線 ---------------------------------------------------------------------
    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; //若是page有child節點 就不斷循環直到再也不有child while (page->child) page = page->child; //建立一個deathptr臨時變量來存儲page AutoreleasePoolPage *deathptr; //若是page不等於this(能夠理解成self) //就不斷的循環把child節點一層一層的置爲nil do { deathptr = page; page = page->parent; if (page) { page->unprotect(); page->child = nil; page->protect(); } delete deathptr; } while (deathptr != this); } 複製代碼

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