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Sun, 3 Dec 2017 09:46:57 +0000
On 30 Nov 2017, at 08:52, Richard Frith-Macdonald <address@hidden> wrote:
>> On 29 Nov 2017, at 12:43, David Chisnall <address@hidden> wrote:
>> Hello the list,
>> In trying to reduce lock contention for deallocation, I took a bit from the
>> reference count to indicate that weak references exist to an object (and
>> therefore avoid any serialisation on deallocation). Unfortunately, when
>> Andreas tried testing it, he discovered that this broke
>> NSDecrementExtraRefCountWasZero (and, in theory, NSIncrementExtraRefCount,
>> though not in a way that’s likely to be observable).
>> On OS X, this function is unavailable in ARC mode and comes with a warning
>> that if any code uses ARC then it may give surprising results.
>> Looking in GNUstep, it appears that this is called in only a small number of
>> - NSObject is using it, but probably should just be calling objc_retain() /
>> objc_release() / objc_autorelease() (the runtime will call these directly
>> instead of retain / release anyway). NSProxy, similarly, should simply be
>> calling the relevant functions.
>> - A few classes are calling it to avoid deallocation of objects that are in
>> a table for reuse.It’s also not clear whether this is still a win: we’ll get
>> more cache contention from refcount manipulations if the objects are small,
>> such as NSIndexPath (though it is required for correctness in a few cases,
>> such as NSConnection). In these cases, if the runtime supports ARC then we
>> can simply use a strong-to-weak NSHashTable and get rid of most of this
> That would be good ... at some point it would be great if the core libraries
> could make use of ARC.
This should be fairly easy now, but only if we’re willing to drop support for
GCC - it’s very difficult to maintain code that can compile as both ARC and
>> I’d like to start making these changes, and expose some new libobjc2
>> functions that can provide the functionality of
>> NSDecrementExtraRefCountWasZero (NSIncrementExtraRefCount can be a trivial
>> wrapper around objc_retain(). Currently, the only way I can see of
>> implementing NSDecrementExtraRefCountWasZero on top of the ARC APIs that I
>> can see is to take a weak reference to the object, release it, and then see
>> if the reference is 0.
> Presumably the downside of that is fairly severe inefficiency, but since thew
> functions are rarely used that's probably not a big issue; if it turns out a
> class using the reference count functions has a major performance impace when
> used with ARC, I guess that class could be rerwritten.
Thinking a bit more, it’s actually not a good solution, because that will
deallocate the object, so I will need to add a new runtime hook. This will
probably involve some juggling with weak functions.
>> It’s also worth noting that both NSDecrementExtraRefCountWasZero and
>> NSIncrementExtraRefCount are currently broken (and will cause memory
>> corruption) if called on blocks (or classes, constant strings, and so on).
> I don't think that's a practical issue since it's always been clear that
> those functions are provided for classes to manage their own reference
> counting. I wouldn't worry about trying to make them work for any/all args.
I’ve been going through the uses in -base and converting them to use weak
references. I’ve come across what looks like a bug in NSSocketPort.m. We
create an NSMapTable with NSObjectMapKeyCallBacks and
NSNonOwnedPointerMapValueCallBacks. We then use portNum (a uint16_t) as the
key. I don’t understand how this doesn’t crash at run time (that said, I don’t
know if I’ve ever used anything that uses NSSocketPort).
- Re: NSDecrementExtraRefCountWasZero,
David Chisnall <=