[Vm-dev] Special objects array - sometimes everybody cannot be special

[David T. Lewis]

On Tue, May 25, 2010 at 08:19:43AM +0300, Igor Stasenko wrote:
> 
> On 25 May 2010 06:56, David T. Lewis <lewis at mail.msen.com> wrote:
> >
> > We have some contention for slots in the special objects array. The Alien
> > package lays claim to slots 50 - 54, and Igor's improved finalization
> > would like to use slot 50. The Alien claim predates the Finalizer claim
> > (albeit only through an *Extentions in Alien-VMMaker-Support), and there
> > may already be people running VMs that rely on these special object
> > assignments. On the other hand, finalization is part of the core system,
> > so it would have been nice to put ClassWeakFinalizer at slot 50, and the
> > optional Alien assignments at 51 - 55.
> >
> > Would anyone object to the following allocation of slots in the special
> > objects array? This would leave Alien mostly unaffected, but would require
> > Igor's finalization improvements to put WeakFinalizer at position 55 in
> > the special objects array, and it would require that the Alien override
> > be pulled into the VMMaker package such that all VMs would have the Alien
> > assignments in the special objects array (albeit unused in many cases).
> > And recreateSpecialObjectsArray would need updates to match of course.
> >
> > ?? ?? ?? ??SelectorAttemptToAssign := 50.
> > ?? ?? ?? ??"PrimErrTableIndex := 51. in Interpreter class>>initializePrimitiveErrorCodes"
> > ?? ?? ?? ??ClassAlien := 52.
> > ?? ?? ?? ??InvokeCallbackSelector := 53.
> > ?? ?? ?? ??ClassUnsafeAlien := 54.
> > ?? ?? ?? ??ClassWeakFinalizer := 55
> >
> > I seem to recall some discussion of a more flexible special objects
> > registry, although I can't find it right now. Pointers welcome.
> >
> 
> Ouch.. David, it seems like i used the different version of
> #recreateSpecialObjectsArray
> than yours.
> 
> The tail of spl objects in Alien-Core package, found at  SqS/Alien
> looks like following:
> 
> 	newArray at: 50 put: #run:with:in:.
> 	newArray at: 51 put: nil.
> 	newArray at: 52 put: nil.
> 	newArray at: 53 put: (self at: #Alien ifAbsent: []).
> 	newArray at: 54 put: #invokeCallback:stack:registers:jmpbuf:.
> 	newArray at: 55 put: (self at: #UnsafeAlien ifAbsent: []).

Note that ObjectMemory class>>initializeSpecialObjectIndices uses zero-based
indexing, versus one-based indexing in SmalltalkImage>>recreateSpecialObjectsArray,
so I think the difference is not a problem.

> So, i thought its not a problem to reserve the 51 slot for weak
> finalization, because it unused anyways.

Well, the slot for PrimErrTableIndex does appear to be unused at the moment,
but I suspect that either Eliot or John reserved it for a reason. If it is
actually unused, then this would be a good place to put ClassWeakFinalizer
(i.e. ClassWeakFinalizer would go in 51 initializeSpecialObjectIndices, which
would be 52 in the special objects array, right before ClassAlien). 

> Also, maybe we could fill an unused/obsolete slot instead of allocating new one?
>  There are some:
> 
> 	newArray at: 23 put: nil.
> 	newArray at: 41 put: nil.
>

I think that these slots are probably still used in older images. But it
would be good idea if we can safely recycle them.

Dave