iOS How ARC insert codes?

Question

Im reading some deep implementations of ARC, by read the assemble code generated by xcode, we can see the compiler inserts code like, objc_release, objc_storeStrong, things like that.

if we have a code like:

-(void)foo
{
   NSDate * _date = [[NSDate alloc] init]; 
}

the assemble code is like:

movq    %rdi, -32(%rbp)         ## 8-byte Spill
movq    %rsi, %rdi
movq    -32(%rbp), %rsi         ## 8-byte Reload
callq   _objc_msgSend   (this is call of 'alloc')
movq    L_OBJC_SELECTOR_REFERENCES_2(%rip), %rsi
movq    %rax, %rdi
callq   _objc_msgSend   (this is call of 'init')
movq    %rax, -24(%rbp)
.loc    1 30 0

and at the end of { }, compiler inserts _objc_storeStrong

movq    %rdx, %rdi
callq   _objc_storeStrong  (in this _objc_storeStrong, _date is released)
addq    $64, %rsp
popq    %rbp
retq

compile does _objc_storeStrong(_date,nil); cos no one strong holds the Date variable.

That makes sense

then I write like this : date becomes a strong member variable

@interface MyView : UIView
@property(nonatomic,strong) NSDate * date;
@end

-(void)foo
{
   self.date = [[NSDate alloc] init]; 
}

the assemble code is :

movq    %rax, -48(%rbp)         ## 8-byte Spill
callq   _objc_msgSend  (call alloc)
movq    L_OBJC_SELECTOR_REFERENCES_2(%rip), %rsi
movq    %rax, %rdi
callq   _objc_msgSend  (call init)
movq    L_OBJC_SELECTOR_REFERENCES_4(%rip), %rsi
movq    -48(%rbp), %rdi         ## 8-byte Reload
movq    %rax, %rdx
movq    %rax, -56(%rbp)         ## 8-byte Spill
callq   _objc_msgSend  (call setDate:)
movq    -56(%rbp), %rax         ## 8-byte Reload
movq    %rax, %rdi
callq   _objc_release  ( a magic release,~!!!)

basically, setDate will retain the date, and the last line release it for a balance.(not its not, see below test)

Then I change the code to

@interface MyView : UIView
@property(nonatomic,strong) NSDate * date;
@end

-(void)foo
{
   _date = [[NSDate alloc] init]; 
}

assemble code becomes

callq   _objc_msgSend (call alloc)
movq    L_OBJC_SELECTOR_REFERENCES_2(%rip), %rsi
movq    %rax, %rdi
callq   _objc_msgSend (call init)
movq    -8(%rbp), %rsi
movq    _OBJC_IVAR_$_MyView._date(%rip), %rdi (set date directly)
movq    (%rsi,%rdi), %rcx
movq    %rax, (%rsi,%rdi)
movq    %rcx, %rdi
callq   _objc_release ( a magic release? here? why, we didn't retain it? )

Compiler inserts an _objc_release either, why? we didn't add any retain code here,

this really confused me for a day, by testing it, the _date is not released actually， you could use it then.

I added a symbolic break point on objc_release to trace the retain count, if we use self.date = [[NSDate alloc] init], objc_release breaks 2 times, by checking the retain count (CFGetRetainCount), retainCount is 2 and then 1, which is correct.

if using _date = [[NSDate alloc] init], objc_release breaks only once, the retainCount is 1.

will someone help me to understand this ?

one more question, the symbolic point, I use register read on lldb to read the first register content, and print the retain count, is it correct? generally r0 r1 r2 contains parameters for the method.

Kazuki Sakamoto Kazuki Sakamoto · Accepted Answer · 2014-12-16T22:16:04

The compiler should release the old value that was stored in the __strong variable under ARC environment.

_date = [[NSDate alloc] init];

In this case, the value of the _date variable should be released. Thus, the compiler emits codes as the following.

NSDate *newDate = [[NSDate alloc] init];
NSDate *oldDate = _date;
_date = newDate;
[oldDate release];

callq _objc_release was emitted for releasing the oldDate value.

iOS How ARC insert codes?

1 Answers