自动释放池
iOS应用的主线程在每次runloop开始的时候创建自动释放池，在runloop结束的时候释放自动释放池。如果在一个runloop内，应用程序创建了大量临时对象，自动释放池可以减少内存峰值。
for (int i = 0; i < 1000000; i++) {
NSString *string = [NSString stringWithFormat:@"hello world!"];
}如果执行上面的代码，使用XCode可以看到有明显的短暂内存上涨。
for (int i = 0; i < 1000000; i++) {
@autoreleasepool {
NSString *string = [NSString stringWithFormat:@"hello world!"];
}
}增加自动释放池以后，程序的内存占用没有明显的上涨，这个时候自动释放池释放了临时创建的变量内存。
自动释放池push和pop时机
autoreleasePool的push/pop和runloop有关。
在runloop进入kCFRunLoopEntry状态时，调用objc_autoreleasePoolPush方法
runloop进入kCFRunLoopBeforeWaiting状态时，调用objc_autoreleasePoolPop方法和objc_autoreleasePoolPush方法
runloop进入kCFRunLoopBeforeExit状态时，调用objc_autoreleasePoolPop方法。
自动释放池数据结构
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 = OBJC_VM_MAX_ADDRESS }.ptr == OBJC_VM_MAX_ADDRESS, "OBJC_VM_MAX_ADDRESS doesn't fit into AutoreleasePoolEntry::ptr!");
#endif
magic_t const magic; //校验page是否完整
__unsafe_unretained id *next; //指向下一个存放数据的地址
objc_thread_t const thread; //当前线程
AutoreleasePoolPage * const parent; //父结点
AutoreleasePoolPage *child; //子结点
uint32_t const depth; //page深度
uint32_t hiwat; //最大入栈数量
AutoreleasePoolPageData(__unsafe_unretained id* _next, objc_thread_t _thread, AutoreleasePoolPage* _parent, uint32_t _depth, uint32_t _hiwat)
: magic(), next(_next), thread(_thread),
parent(_parent), child(nil),
depth(_depth), hiwat(_hiwat)
{
}
};AutoreleasePoolPageData是AutoreleasePoolPage的基类。
自动释放池是双向链表的数据结构，parent指针指向父结点，child指向子结点，每页的数据大小是4096字节大小，每个页面是双向链表的一个结点。next代表的是当前页面可插入数据的位置。
自动释放池实现原理
push过程
void *
objc_autoreleasePoolPush(void)
{
return AutoreleasePoolPage::push();
}当代码中向自动释放池加入对象的时候，Objective-C内部会调用这个方法。
static inline void *push()
{
ReturnAutoreleaseInfo info = getReturnAutoreleaseInfo();
moveTLSAutoreleaseToPool(info);
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 == (id *)EMPTY_POOL_PLACEHOLDER || *dest == POOL_BOUNDARY);
return dest;
}这个方法主要是调用了autoreleaseFast方法。
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);
}
}autoreleaseFast方法用于获取当前的hotPage，如果hotPage没有满直接插入数据，如果已经满了，重新分配一个page的空间然后插入，如果没有获取到page说明当前的自动释放池还没有初始化，需要初始化自动释放池。
id *add(id obj)
{
ASSERT(!full());
unprotect();
id *ret;
ret = next; // faster than `return next-1` because of aliasing
*next++ = obj;
done:
protect();
return ret;
}add方法精简后，可以看到add方法实际是把obj放入到next指针的位置，并把next++，函数的返回值是obj的存放地址。
static __attribute__((noinline))
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);
do {
if (page->child) page = page->child;
else page = new AutoreleasePoolPage(page);
} while (page->full());
setHotPage(page);
return page->add(obj);
}autoreleaseFullPage函数是找到一个没有满的页面，调用add函数插入obj对象
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();
}
// 初始化首页
AutoreleasePoolPage *page = new AutoreleasePoolPage(nil);
setHotPage(page);
// 增加boundry标识
if (pushExtraBoundary) {
page->add(POOL_BOUNDARY);
}
// 添加obj
return page->add(obj);
}如果当前没有自动释放池页面的时候，需要初始化AutoreleasePoolPage，然后调用add方法添加obj。这里setHotPage的作用是标识当前页面。
pop过程
void objc_autoreleasePoolPop(void *ctxt)
{
AutoreleasePoolPage::pop(ctxt);
}自动释放池在pop时会调用这个方法。
static inline void
pop(void *token)
{
// We may have an object in the ReturnAutorelease TLS when the pool is
// otherwise empty. Release that first before checking for an empty pool
// so we don't return prematurely. Loop in case the release placed a new
// object in the TLS.
while (releaseReturnAutoreleaseTLS())
;
AutoreleasePoolPage *page;
id *stop;
if (token == (void*)EMPTY_POOL_PLACEHOLDER) {
// 获取hotPage
page = hotPage();
if (!page) {
// 自动释放池没有使用过，清空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 {
return badPop(token);
}
}
if (slowpath(PrintPoolHiwat || DebugPoolAllocation || DebugMissingPools)) {
return popPageDebug(token, page, stop);
}
//清理自动释放池
return popPage<false>(token, page, stop);
}在pop时会判断是否为EMPTY_POOL_PLACEHOLDER，POOL_BOUNDARY等标记。这个函数最后调用的popPage函数。
template<bool allowDebug>
static void
popPage(void *token, AutoreleasePoolPage *page, id *stop)
{
if (allowDebug && PrintPoolHiwat) printHiwat();
//释放当前页面
page->releaseUntil(stop);
// memory: delete empty children
if (allowDebug && DebugPoolAllocation && page->empty()) {
//debug使用
// 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) {
// 如果页面实际占用空间小于一半，将child结点销毁
if (page->lessThanHalfFull()) {
page->child->kill();
}
如果页面实际占用空间大于一半，将child结点保留
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
do {
while (this->next != stop) {
AutoreleasePoolPage *page = hotPage();
while (page->empty()) {
page = page->parent;
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
id obj = *--page->next;
#endif
memset((void*)page->next, SCRIBBLE, sizeof(*page->next));
page->protect();
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
objc_release(obj);
#endif
}
}
// Stale return autorelease info is conceptually autoreleased. If
// there is any, release the object in the info. If stale info is
// present, we have to loop in case it autoreleased more objects
// when it was released.
} while (releaseReturnAutoreleaseTLS());
setHotPage(this);
#if DEBUG
// 检查子结点是否清空
for (AutoreleasePoolPage *page = child; page; page = page->child) {
ASSERT(page->empty());
}
#endif
}releaseUntil函数会清空一个自动释放池，在释放对象的时候实际调用的是objc_release函数。
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