临时变量什么时候释放
自动释放池的原理
自动释放池能否嵌套使用
使用xcode创建一个project在创建的时候有会生成一个main.m文件，其中@autoreleasePool的注解
可以通过clang将main.m转义成C++文件，找到源码切入
xcrun -sdk iphonesimulator clang -arch x86_64 -rewrite-objc main.m
打开生成的main.cpp文件
int main(int argc, char * argv[]) {
NSString * appDelegateClassName;
//自动释放池的作用域
/* @autoreleasepool */ { __AtAutoreleasePool __autoreleasepool;
appDelegateClassName = NSStringFromClass(((Class (*)(id, SEL))(void *)objc_msgSend)((id)objc_getClass("AppDelegate"), sel_registerName("class")));
}
return UIApplicationMain(argc, argv, __null, appDelegateClassName);
}
struct __AtAutoreleasePool {
__AtAutoreleasePool() {atautoreleasepoolobj = objc_autoreleasePoolPush();}
~__AtAutoreleasePool() {objc_autoreleasePoolPop(atautoreleasepoolobj);}
void * atautoreleasepoolobj;
};
通过转义后的代码autorelease是一个结构体，结构体变量atautoreleasepoolobj是通过objc_autoreleasePoolPush创建而来，他的作用域在一个{}内。
另外有可以在在@autorelease处打断点，进入汇编调试界面，可以看到注释调用了objc_autoreleasePoolPush方法。
上面我们通过代码转义和汇编两个方式找到进入@autorelease源码的切入点
Autorelease的官方说明
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.
1.指针->栈的结构
- 释放对象+POOL_BOUNDARY（边界或者叫哨兵）
- 页的结构 + 双向链表
- 线程相关
自动释放池使用页，当前正在添加待释放对象的page称为hotpage，已经添加满了的page称为 coldpage。
push
通过objc4-781源码
objc_autoreleasePoolPush->AutoreleasePoolPage::push()
其中AutoreleasePoolPage是继承自 AutoreleasePoolPageData的类
struct AutoreleasePoolPageData
{
magic_t const magic; //用来校验AutoreleasePoolPage的结构是否完整 ，16字节（静态变量不占用结构体内存，它的大小是 uint32_5 m[4] = 4*4 的大小）
__unsafe_unretained id *next; //指向最新添加的autoreleased对象的下一个位置，初始化时指向begin() ，8字节
pthread_t const thread; //指向当前线程 //8字节
AutoreleasePoolPage * const parent; //指向父节点，第一个节点的parent值为nil ，8字节
AutoreleasePoolPage *child; //最后一个节点的child值为nil，8字节
uint32_t const depth; //深度，从0开始，4字节
uint32_t hiwat; //代表high water mark 最大入栈数量标记，4字节
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)
{
}
};
AutoreleasePoolPage对象初始化
AutoreleasePoolPage(AutoreleasePoolPage *newParent) :
AutoreleasePoolPageData(begin(),
objc_thread_self(),
newParent,
newParent ? 1+newParent->depth : 0,
newParent ? newParent->hiwat : 0)
{
if (objc::PageCountWarning != -1) {
checkTooMuchAutorelease();
}
if (parent) {
parent->check();
ASSERT(!parent->child);
parent->unprotect();
parent->child = this;
parent->protect();
}
protect();
}
其中begin()是当前对象的起始地址+ 56个字节，56个字节是AutoreleasePoolPageData中的变量已经占用了这么多，那么一个存放对象的地址应该是(id *) ((uint8_t *)this+sizeof(*this));。
通过源码调试，需要切换到MRC环境
#import <Foundation/Foundation.h>
//自动释放池的打印方法
extern void _objc_autoreleasePoolPrint(void);
int main(int argc, const char * argv[]) {
@autoreleasepool {
// insert code here...
for (int i = 0; i<10; i++) {
NSObject *objc = [[NSObject alloc] autorelease];
}
_objc_autoreleasePoolPrint();
}
return 0;
}
输出
image.png
在释放池中添加505个对象
image.png
自动释放池创建了新的一页，在
0x10180d038位开始的是NSObject对象，不是哨兵对象。在x86架构下，第一页结构是
page的原始结构+504个释放对象 +哨兵对象，大小是（504 + 1）*8 +56 = 4096 = 4kb从第二页开始没有哨兵对象，一共可以存放505个释放对象
AutoreleasePoolPage::push()源码
static inline void *push()
{
id *dest;
//执行slowpath概率很小
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;
}
autoreleaseNewPage()创建一个新的page
static __attribute__((noinline))
id *autoreleaseNewPage(id obj)
{
//1.获取聚焦page
AutoreleasePoolPage *page = hotPage();
//2.存在hotpage就添加
if (page) return autoreleaseFullPage(obj, page);
//3.没有hotpage就表明需要创建一个page
else return autoreleaseNoPage(obj);
}
autoreleaseFast源码
static inline id *autoreleaseFast(id obj)
{
AutoreleasePoolPage *page = hotPage();
//1.有hotpage，切没有满，直接添加
if (page && !page->full()) {
return page->add(obj);
} else if (page) {
//2.如果页满了
return autoreleaseFullPage(obj, page);
} else {
//3.没有获取到page
return autoreleaseNoPage(obj);
}
}
push的流程图如下
image.png
但是由于slowpath的关系，执行autoreleaseNewPage概率很小。
autoreleaseFullPage 页满了的处理情况
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);
//如果hotpage满了，获取下一个非full的页给当前page，跳出循环后，设置当前page为hotpage，并将随访对象添加
do {
if (page->child) page = page->child;
else page = new AutoreleasePoolPage(page);
} while (page->full());
setHotPage(page);
return page->add(obj);
}
- 1.如果hotpage满了，判断是否有child，
- 1.1没有child就创建一个page，复制给当前page，并退出当前会还
- 1.2有就将child复制给当前page，继续while循环判断是否满了
- 2.page没有满，跳出循环，设置当前page为hotpage，
- 3.将释放对象
add到page中
autoreleaseNoPage没有页的情况
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
//No page 可能是没有被东西被压入栈中，或者只有一个占位的哨兵
ASSERT(!hotPage());
bool pushExtraBoundary = false;
if (haveEmptyPoolPlaceholder()) { //如果是初始化需要传入哨兵
// 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.
//初始化返回一个空的占位池
return setEmptyPoolPlaceholder();
}
// We are pushing an object or a non-placeholder'd pool.
// Install the first page. 初始化第一页
AutoreleasePoolPage *page = new AutoreleasePoolPage(nil);
setHotPage(page);
// Push a boundary on behalf of the previously-placeholder'd pool. 传入哨兵
if (pushExtraBoundary) {
page->add(POOL_BOUNDARY);
}
// Push the requested object or pool.
//将释放对象加入page中
return page->add(obj);
}
- 1.判断是否是空占位符，如果是，则压栈哨兵标识符置为YES
- 2.如果对象不是哨兵对象，且没有Pool，则报错
- 3.如果对象是哨兵对象，且没有申请自动释放池内存，则设置一个空占位符存储在tls中，其目的是为了节省内存
- 4.初始化第一页，并设置为hotpage
- 5.压栈哨兵的标识符为YES，则压栈哨兵对象
- 6.压栈对象
add方法
id *add(id obj)
{
ASSERT(!full());
unprotect();
id *ret = next; // faster than `return next-1` because of aliasing
//将obj压栈到next指针位置，然后next进行++，即下一个对象存储的位置
*next++ = obj;
protect();
return ret;
}
autolease:pop
当程序运行出自动释放池的作用域范围会__AtAutoreleasePool调用c++的析构函数，将pop出栈方法写入析构函数，开始释放。
void
objc_autoreleasePoolPop(void *ctxt)
{
AutoreleasePoolPage::pop(ctxt);
}
pop->popPage->releaseUntil
static inline void
pop(void *token)
{
AutoreleasePoolPage *page;
id *stop;
if (token == (void*)EMPTY_POOL_PLACEHOLDER) {
// Popping the top-level placeholder pool.
page = hotPage();
if (!page) {
// Pool was never used. Clear the 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);
}
return popPage<false>(token, page, stop);
}
popPage之前的代码都是在进行容错处理，page = pageForPointer(token);是根据token获取page。
template<bool allowDebug>
static void
popPage(void *token, AutoreleasePoolPage *page, id *stop)
{
//此时传入的allowDebug为false
if (allowDebug && PrintPoolHiwat) printHiwat();
//出栈当前page的对象
page->releaseUntil(stop);
// memory: delete empty children
//删除空子项
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， 如果page超过一半，保存一个空的子集
if (page->lessThanHalfFull()) {
page->child->kill();
}
else if (page->child->child) {
page->child->child->kill();
}
}
}
page->releaseUntil(stop);
void releaseUntil(id *stop)
{
// Not recursive: we don't want to blow out the stack
// if a thread accumulates a stupendous amount of garbage
// 判断下一个对象是否等于stop，如果不等于，则进入while循环
while (this->next != stop) {
// Restart from hotPage() every time, in case -release
// autoreleased more objects
//获取当前操作页面，即hot页面
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);
}
page->unprotect();
//next进行--操作，即出栈
id obj = *--page->next;
//将page的索引位设置为SCRIBBLE，表示已经被释放
memset((void*)page->next, SCRIBBLE, sizeof(*page->next));
page->protect();
//判断是否哨兵
if (obj != POOL_BOUNDARY) {
//释放对象
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
}
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;
//1.获取最后位置的page
while (page->child) page = page->child;
AutoreleasePoolPage *deathptr;
do {
deathptr = page;
//将父parent赋值给当前page
page = page->parent;
if (page) {
page->unprotect();
//产出最后个page
page->child = nil;
page->protect();
}
delete deathptr;
} while (deathptr != this); //最后一个page不等于当前page
}
