深入理解 autorelease

先上一段代码，分别实现了包含 autorelease pool 不包含 autorelease pool 的函数

void autoreleasepool() {
    @autoreleasepool {
        NSObject *a = [[NSObject alloc] init];
    }
}

void noautoreleasepool() {
    NSObject *a = [[NSObject alloc] init];
}

编译后再用hopper disassembler 反编译，然后看 autorelasepool函数的汇编代码。

 _autoreleasepool:
0000000100006b80         stp        x29, x30, [sp, #0xfffffff0]!                ; XREF=_main+24
0000000100006b84         mov        x29, sp
0000000100006b88         sub        sp, sp, #0x10
0000000100006b8c         bl         imp___stubs__objc_autoreleasePoolPush
0000000100006b90         adrp       x8, #0x100008000                            ; imp___got_dyld_stub_binder
0000000100006b94         add        x8, x8, #0xc30                              ; @selector(alloc)
0000000100006b98         adrp       x9, #0x100008000                            ; imp___got_dyld_stub_binder
0000000100006b9c         add        x9, x9, #0xc40                              ; objc_cls_ref_NSObject
0000000100006ba0         ldr        x9, [x9]                                    ; objc_cls_ref_NSObject
0000000100006ba4         ldr        x1, [x8]                                    ; @selector(alloc)
0000000100006ba8         str        x0, [sp]
0000000100006bac         mov        x0, x9
0000000100006bb0         bl         imp___stubs__objc_msgSend
0000000100006bb4         adrp       x8, #0x100008000                            ; imp___got_dyld_stub_binder
0000000100006bb8         add        x8, x8, #0xc38                              ; @selector(init)
0000000100006bbc         ldr        x1, [x8]                                    ; @selector(init)
0000000100006bc0         bl         imp___stubs__objc_msgSend
0000000100006bc4         movz       x8, #0x0
0000000100006bc8         add        x9, sp, #0x8
0000000100006bcc         str        x0, [sp, #0x8]
0000000100006bd0         mov        x0, x9
0000000100006bd4         mov        x1, x8
0000000100006bd8         bl         imp___stubs__objc_storeStrong
0000000100006bdc         ldr        x0, [sp]
0000000100006be0         bl         imp___stubs__objc_autoreleasePoolPop
0000000100006be4         mov        sp, x29
0000000100006be8         ldp        x29, x30, [sp], #0x10
0000000100006bec         ret        
                        ; endp

再看 noautoreleasepool 函数的反汇编代码：

   _noautoreleasepool:
0000000100006bf0         stp        x29, x30, [sp, #0xfffffff0]!                ; XREF=_main+28
0000000100006bf4         mov        x29, sp
0000000100006bf8         sub        sp, sp, #0x10
0000000100006bfc         adrp       x8, #0x100008000                            ; imp___got_dyld_stub_binder
0000000100006c00         add        x8, x8, #0xc30                              ; @selector(alloc)
0000000100006c04         adrp       x9, #0x100008000                            ; imp___got_dyld_stub_binder
0000000100006c08         add        x9, x9, #0xc40                              ; objc_cls_ref_NSObject
0000000100006c0c         ldr        x9, [x9]                                    ; objc_cls_ref_NSObject
0000000100006c10         ldr        x1, [x8]                                    ; @selector(alloc)
0000000100006c14         mov        x0, x9
0000000100006c18         bl         imp___stubs__objc_msgSend
0000000100006c1c         adrp       x8, #0x100008000                            ; imp___got_dyld_stub_binder
0000000100006c20         add        x8, x8, #0xc38                              ; @selector(init)
0000000100006c24         ldr        x1, [x8]                                    ; @selector(init)
0000000100006c28         bl         imp___stubs__objc_msgSend
0000000100006c2c         movz       x8, #0x0
0000000100006c30         add        x9, sp, #0x8
0000000100006c34         str        x0, [sp, #0x8]
0000000100006c38         mov        x0, x9
0000000100006c3c         mov        x1, x8
0000000100006c40         bl         imp___stubs__objc_storeStrong
0000000100006c44         mov        sp, x29
0000000100006c48         ldp        x29, x30, [sp], #0x10
0000000100006c4c         ret        
                        ; endp

两者的区别是 autorelease 版本多了如下内容

0000000100006b8c         bl         imp___stubs__objc_autoreleasePoolPush  ; 调用 objc_autoreleasePoolPush方法
0000000100006ba8         str        x0, [sp]                               ; 把objc_autoreleasePoolPush的返回值入栈

......

0000000100006bdc         ldr        x0, [sp]                               ; 把之前入栈的返回值出栈
0000000100006be0         bl         imp___stubs__objc_autoreleasePoolPop   ; 调用objc_autoreleasePoolPop方法

发现了和预想不一样的地方，按照常理，在 autoreleasepool 里面的对象初始化的时候，似乎要调用-[NSObject autorelease] 来让他自动释放，然而，并没有。

看看 objc 的源码 objc_autoreleasePoolPush 和 objc_autoreleasePoolPop 的实现。

objc_autoreleasePoolPush

void *
objc_autoreleasePoolPush(void)
{
    if (UseGC) return nil;
    return AutoreleasePoolPage::push();
}

static inline void *push() 
{
    id *dest;
    if (DebugPoolAllocation) {
        // Each autorelease pool starts on a new pool page.
        dest = autoreleaseNewPage(POOL_SENTINEL);
    } else {
        dest = autoreleaseFast(POOL_SENTINEL);
    }
    assert(*dest == POOL_SENTINEL);
    return dest;
}

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);
    }
}

static __attribute__((noinline))
id *autoreleaseNoPage(id obj)
{
    // No pool in place.
    assert(!hotPage());

    if (obj != POOL_SENTINEL  &&  DebugMissingPools) {
        // We are pushing an object with no pool in place, 
        // and no-pool debugging was requested by environment.
        _objc_inform("MISSING POOLS: Object %p of class %s "
                     "autoreleased with no pool in place - "
                     "just leaking - break on "
                     "objc_autoreleaseNoPool() to debug", 
                     (void*)obj, object_getClassName(obj));
        objc_autoreleaseNoPool(obj);
        return nil;
    }

    // Install the first page.
    AutoreleasePoolPage *page = new AutoreleasePoolPage(nil);
    setHotPage(page);

    // Push an autorelease pool boundary if it wasn't already requested.
    if (obj != POOL_SENTINEL) {
        page->add(POOL_SENTINEL);
    }

    // Push the requested object.
    return page->add(obj);
}

push 的主要作用是选一个/建一个autoreleasePoolPage ，设定为hotpage备用，流程图如下

objc_autoreleasePoolPop