浅析AutoreleasePool源码

最近在拜读Draveness大佬的一篇文章自动释放池的前世今生 —- 深入解析 autoreleasepool，看到文中给读者留了一个问题：

我到现在也不是很清楚为什么要根据当前页的不同状态 kill 掉不同 child 的页面。

关于AutoreleasePool是什么，强力推荐阅读原文，写的很好。这里就不说了，直接讨论问题。

首先是整个pop方法的实现：

static inline void pop(void *token) 
{
    AutoreleasePoolPage *page;
    id *stop;

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

    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 (PrintPoolHiwat) printHiwat();

    page->releaseUntil(stop);

    // memory: delete empty children
    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 fully
        if (page->lessThanHalfFull()) {
            page->child->kill();
        }
        else if (page->child->child) {
            page->child->child->kill();
        }
    }
}

我们先看看释放的函数releaseUntil，它在释放的时候其实会一直顺着parent往前释放，直到参数stop，也就是说可能一次性释放好几个page。

// 代码有所删减
void releaseUntil(id *stop)
{
    while (this->next != stop) {
        AutoreleasePoolPage *page = hotPage();
        
        while (page->empty()) {
            page = page->parent;
            setHotPage(page);
        }
        
        id obj = *--page->next;
        memset((void*)page->next, SCRIBBLE, sizeof(*page->next));
        
        if (obj != POOL_BOUNDARY) {
            objc_release(obj);
        }
    }
    
    setHotPage(this);
}

然后我们来看看这段有疑问的代码

// memory: delete empty children
if (DebugPoolAllocation  &&  page->empty()) {  // 分支1
    // special case: delete everything during page-per-pool debugging
    AutoreleasePoolPage *parent = page->parent;
    page->kill();
    setHotPage(parent);
} else if (DebugMissingPools  &&  page->empty()  &&  !page->parent) { // 分支2
    // special case: delete everything for pop(top) 
    // when debugging missing autorelease pools
    page->kill();
    setHotPage(nil);
} 
else if (page->child) { // 分支3
    // hysteresis: keep one empty child if page is more than half fully
    if (page->lessThanHalfFull()) {
        page->child->kill();
    }
    else if (page->child->child) {
        page->child->child->kill();
    }
}

这块代码的作用是删除空的子节点，释放内存。pop之后三种情况：

1. 当前page为空，直接kill掉当前page，然后把parent设置为hotpage；
2. 当前page为空，而且没有parent，kill掉当前page，hotpage置为空；
3. 当前page不为空，但是有child，如果当前page的空间占用不到一半，释放child，如果当前page的空间占用超过一半，且child还有child，直接释放这个孙子辈的page。（对于第三步注释中的解释是：keep one empty child if page is more than half fully）

我们再看看kill的实现，可以发现他是会顺着child一直往后释放，保证释放节点的child page都被释放了。

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);
}

到这里就可以得出结论了：

1. pop之后，所有child page肯定都为空了，且当前page一定是hotPage
2. 系统为了节约内存，判断，如果当前page空间使用少于一半，就释放掉所有的child page，如果当前page空间使用大于一半，就从孙子page开始释放，预留一个child page。