> On Aug 10, 2017, at 11:13 AM, Charles Srstka <cocoa...@charlessoft.com> wrote:
>> On Aug 10, 2017, at 9:44 AM, Alastair Houghton 
>> <alast...@alastairs-place.net> wrote:
>> On 10 Aug 2017, at 15:24, Jeremy Hughes <moon.rab...@virginmedia.com 
>> <mailto:moon.rab...@virginmedia.com>> wrote:
>>>> On 10 Aug 2017, at 15:15, Alastair Houghton <alast...@alastairs-place.net 
>>>> <mailto:alast...@alastairs-place.net>> wrote:
>>>> On 10 Aug 2017, at 15:09, Charles Srstka <cocoa...@charlessoft.com 
>>>> <mailto:cocoa...@charlessoft.com>> wrote:
>>>>> They’re equivalent syntactically, but performance-wise, +array and 
>>>>> friends will cause the object to be put into an autorelease pool. 
>>>>> Therefore, +new is better for performance.
>>>> Not with ARC they don’t.  The ARC logic circumvents the autorelease pool 
>>>> in that case.
>>> Are you sure?
>> Yes, I’m sure.  At the call site, ARC causes the compiler to emit a call to 
>> objc_retainAutoreleasedReturnValue() or 
>> objc_unsafeClaimAutoreleasedReturnValue(), while in the method itself, ARC 
>> will use objc_autoreleaseReturnValue() or 
>> objc_retainAutoreleaseReturnValue().  The latter looks at the code for the 
>> call site and, assuming it matches, it will *not* do the autorelease and 
>> will set a flag that causes objc_retainAutoreleasedReturnValue() to 
>> eliminate the retain.
> The frameworks (and thus, the implementation of +array) are not built using 
> ARC. The -autorelease method is called manually, and the object is put in the 
> autorelease pool. You can see this for yourself by making a small test app 
> that calls [NSMutableArray array] and running it in Instruments, where the 
> autorelease will be clearly visible.

Autoreleases done by -autorelease can still be reclaimed by 

The autorelease-reclaim optimization is not reliable, and there are a number of 
caveats that make testing it tricky.  That is why we generally refer to it as 
an optimization, rather than claiming it as a semantic guarantee.  In your test 
app, you are almost certainly running into a well-known problem where the first 
attempt to reclaim an autorelease fails on x86-64 (because of a detail of 
dynamic linking).


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