我有一个方法 createATestObject。顾名思义，它创建一个对象并返回它。代码很简单，在ARC下。

- (TestObj *)createATestObj
{
    return [[TestObj alloc] init] ;
}

我汇编文件并得到下面的汇编代码。

Lfunc_begin4:
    .cfi_startproc
@ BB#0:
    push    {r7, lr}
    mov r7, sp
    sub sp, #8
    @DEBUG_VALUE: -[ViewController createATestObj]:self <- undef
    @DEBUG_VALUE: -[ViewController createATestObj]:_cmd <- undef
    str r0, [sp, #4]
    str r1, [sp]
    movw    r0, :lower16:(L_OBJC_CLASSLIST_REFERENCES_$_-(LPC4_0+4))
    movt    r0, :upper16:(L_OBJC_CLASSLIST_REFERENCES_$_-(LPC4_0+4))
LPC4_0:
    add r0, pc
    ldr r0, [r0]
    movw    r1, :lower16:(L_OBJC_SELECTOR_REFERENCES_10-(LPC4_1+4))
    movt    r1, :upper16:(L_OBJC_SELECTOR_REFERENCES_10-(LPC4_1+4))
LPC4_1:
    add r1, pc
    ldr r1, [r1]
    blx _objc_msgSend
    movw    r1, :lower16:(L_OBJC_SELECTOR_REFERENCES_-(LPC4_2+4))
    movt    r1, :upper16:(L_OBJC_SELECTOR_REFERENCES_-(LPC4_2+4))
LPC4_2:
    add r1, pc
    ldr r1, [r1]
    blx _objc_msgSend
    add sp, #8
    pop.w   {r7, lr}
    b.w _objc_autoreleaseReturnValue
Ltmp13:
Lfunc_end4:
    .cfi_endproc

我注意到函数 _objc_autoreleaseReturnValue 并从 here 获得了关于它的引用.但我不知道这是什么意思。任何人都可以详细说明吗？非常感谢。

id objc_autoreleaseReturnValue(id value);

Precondition: value is null or a pointer to a valid object.

If value is null, this call has no effect. Otherwise, it makes a best effort to hand off ownership of a retain count on the object to a call to objc_retainAutoreleasedReturnValue for the same object in an enclosing call frame. If this is not possible, the object is autoreleased as above.

Always returns value.

最佳答案

首先感谢@Darren，感谢您的链接。

我引用了 link 中的一些段落.

ARC has a trick that keeps returned objects out of autorelease pools if both the caller and callee are ARC.

But how does that work? One of the features of ARC is that old compiled-before-ARC code (MRC code) can call ARC code and vice-versa. But if ARC code doesn’t put a returned object in an autorelease pool that MRC code is expecting, then the object would just leak.

So ARC-ified clang code emits this function call when returning an object: objc_autoreleaseReturnValue(id).

If you look at objc_autoreleaseReturnValue's implementation, it calls callerAcceptsFastAutorelease(). Even if you don’t read x86_64 or ARM assembly, the code’s comment is straight-forward:

/*
  Fast handling of returned autoreleased values.
  The caller and callee cooperate to keep the returned object 
  out of the autorelease pool.

  Caller:
    ret = callee();
    objc_retainAutoreleasedReturnValue(ret);
    // use ret here

  Callee:
    // compute ret
    [ret retain];
    return objc_autoreleaseReturnValue(ret);

  objc_autoreleaseReturnValue() examines the caller's instructions following
  the return. If the caller's instructions immediately call
  objc_autoreleaseReturnValue, then the callee omits the -autorelease and saves
  the result in thread-local storage. If the caller does not look like it
  cooperates, then the callee calls -autorelease as usual.

  objc_autoreleaseReturnValue checks if the returned value is the same as the
  one in thread-local storage. If it is, the value is used directly. If not,
  the value is assumed to be truly autoreleased and is retained again.  In
  either case, the caller now has a retained reference to the value.

  Tagged pointer objects do participate in the fast autorelease scheme, 
  because it saves message sends. They are not entered in the autorelease 
  pool in the slow case.
*/

我的东西

我们来看objc_autoreleaseReturnValue的源码。

id 
objc_autoreleaseReturnValue(id obj)
{
#if SUPPORT_RETURN_AUTORELEASE
    assert(tls_get_direct(AUTORELEASE_POOL_RECLAIM_KEY) == NULL);

    if (callerAcceptsFastAutorelease(__builtin_return_address(0))) {
        tls_set_direct(AUTORELEASE_POOL_RECLAIM_KEY, obj);
        return obj;
    }
#endif

    return objc_autorelease(obj);
}

__builtin_return_address(0) returns the return address of current function ，然后我们看callerAcceptsFastAutorelease对于arm版本的实现:

static bool callerAcceptsFastAutorelease(const void *ra)
{
    // if the low bit is set, we're returning to thumb mode
    if ((uintptr_t)ra & 1) {
        // 3f 46          mov r7, r7
        // we mask off the low bit via subtraction
        if (*(uint16_t *)((uint8_t *)ra - 1) == 0x463f) {
            return true;
        }
    } else {
        // 07 70 a0 e1    mov r7, r7
        if (*(uint32_t *)ra == 0xe1a07007) {
            return true;
        }
    }
    return false;
}

在该方法中，它会查找指令mov r7, r7，它是objc_retainAutoreleaseReturnValue 的标记，如果找到，则该方法返回true ，因此被调用者将省略自动释放。

你可以看到调用者的汇编代码，在我的例子中是testFun1。

- (void)testFun1:(ViewController *)vc
{
    [vc createATestObj] ;
}

下面是汇编代码，你可以找到“mov r7, r7 @marker for objc_retainAutoreleaseReturnValue”这一行

    .cfi_startproc
@ BB#0:
    push    {r7, lr}
    mov r7, sp
    sub sp, #16
    add r3, sp, #4
    movw    r9, #0
    movt    r9, #0
    str r0, [sp, #12]
    str r1, [sp, #8]
    str.w   r9, [sp, #4]
    mov r0, r3
    mov r1, r2
    bl  _objc_storeStrong
    movw    r0, :lower16:(L_objc_msgSend$non_lazy_ptr-(LPC5_0+4))
    movt    r0, :upper16:(L_objc_msgSend$non_lazy_ptr-(LPC5_0+4))
LPC5_0:
    add r0, pc
    ldr r0, [r0]
    movw    r1, :lower16:(L_OBJC_SELECTOR_REFERENCES_12-(LPC5_1+4))
    movt    r1, :upper16:(L_OBJC_SELECTOR_REFERENCES_12-(LPC5_1+4))
LPC5_1:
    add r1, pc
Ltmp14:
    ldr r2, [sp, #4]
    ldr r1, [r1]
    str r0, [sp]                @ 4-byte Spill
    mov r0, r2
    ldr r2, [sp]                @ 4-byte Reload
    blx r2
    @ InlineAsm Start
    mov r7, r7      @ marker for objc_retainAutoreleaseReturnValue
    .code   16
    @ InlineAsm End
    bl  _objc_retainAutoreleasedReturnValue
    bl  _objc_release
    movs    r1, #0
    movt    r1, #0
    add r0, sp, #4
    bl  _objc_storeStrong
    add sp, #16
    pop {r7, pc}
Ltmp15:
Lfunc_end5:
    .cfi_endproc

更新:

我认为apple's implementation的评论有些错误.

正确的评论应该是这样的:

/*
  Fast handling of returned autoreleased values.
  The caller and callee cooperate to keep the returned object 
  out of the autorelease pool.

  Caller:
    ret = callee();
    objc_retainAutoreleasedReturnValue(ret);
    // use ret here

  Callee:
    // compute ret
    [ret retain];
    return objc_autoreleaseReturnValue(ret);

  objc_autoreleaseReturnValue() examines the caller's instructions following
  the return. If the caller's instructions immediately call
  objc_retainAutoreleasedReturnValue, then the callee omits the -autorelease and saves
  the result in thread-local storage. If the caller does not look like it
  cooperates, then the callee calls -autorelease as usual.

  objc_retainAutoreleasedReturnValue checks if the returned value is the same as the
  one in thread-local storage. If it is, the value is used directly. If not,
  the value is assumed to be truly autoreleased and is retained again.  In
  either case, the caller now has a retained reference to the value.

  Tagged pointer objects do participate in the fast autorelease scheme, 
  because it saves message sends. They are not entered in the autorelease 
  pool in the slow case.
*/

如果您有其他想法，请发表评论。谢谢!

关于ios - objc_autoreleaseReturnValue 是什么意思？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/29249171/