[PATCH] D55869: Convert some ObjC retain/release msgSends to runtime calls

[John McCall via Phabricator via cfe-commits]

rjmccall added a comment.

In D55869#1337733 <https://reviews.llvm.org/D55869#1337733>, @pete wrote:

> In D55869#1337723 <https://reviews.llvm.org/D55869#1337723>, @js wrote:
>
> > In D55869#1337711 <https://reviews.llvm.org/D55869#1337711>, @dexonsmith wrote:
> >
> > > In D55869#1337706 <https://reviews.llvm.org/D55869#1337706>, @js wrote:
> > >
> > > > The ObjFW runtime itself does not contain anything about release, retain or autorelease - these are all part of ObjFW (as in the framework). As ObjFW also supports the Apple runtime, as well as mixing with Foundation code, it so far only provides objc_retain and objc_release when they are missing, to make ARC work. They just call into the corresponding methods on the object and do nothing else.
> > > >
> > > > How will this change work from the Apple runtime? Will objc_retain/objc_release call the retain/release on the object if it implements its own? Should I drop my own retain/release implementation from my root object if I am compiling against the Apple runtime?
> > >
> > >
> > > Yes, the idea is that the specialized runtime functions are fast paths for the common case, but they still fall back if the object has overridden them.
> >
> >
> > I am guessing not just overridden, but also a root class providing one? IOW, the fast path is used when the class does not have the retain or release method at all? In that case, LGTM as is.
>
>
> The Apple runtime is using a bit on each realized class to track the overriding.  The root class defaults to not having a custom RR, ie, it appears that its version of RR is considered equivalent to objc_retain.
>
> Presumably that would apply to other root classes too, although i'm not 100% sure.  I did see some special handling of NSObject in there too so perhaps only its RR is equivalent to objc_retain/objc_release.


`objc_retain` and `objc_release` are always defined as being equivalent to doing a normal message send of the corresponding messages.  Apple's implementations detect when a class simply inherits `NSObject`'s implementations: if so, they just directly use the `NSObject` RR implementation, and if no, they do a normal message send.  That fast path is only triggered if the class's root class is `NSObject` and it doesn't override any of the RR methods.


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https://reviews.llvm.org/D55869