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Re: NSDecrementExtraRefCountWasZero


From: Richard Frith-Macdonald
Subject: Re: NSDecrementExtraRefCountWasZero
Date: Thu, 30 Nov 2017 08:52:42 +0000


> 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 
> places:
> 
> - 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 logic.

That would be good ... at some point it would be great if the core libraries 
could make use of ARC.

> 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.

> 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.




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