[Vm-dev] Compact classes not in the compact array

[Eliot Miranda]

On Fri, Apr 6, 2012 at 9:45 AM, Igor Stasenko <siguctua at gmail.com> wrote:

>
> On 6 April 2012 18:37, Hans-Martin Mosner <hmm at heeg.de> wrote:
> >
> > Am 06.04.2012 16:41, schrieb Guillermo Polito:
> >
> >
> >
> >
> >
> > Hi!
> >
> > I was wandering what are the implications of a class being compact or
> not.
> > And also what happens if a class has it's format thinking it's compact
> but
> > it's not in the compact classes array?
> >
> > I'm looking at the implications of bootstrapping a smalltalk inside the
> > same environment, and having two CompiledMethod classes, two Array
> classes,
> > etc, brings me to this issue :).
> >
> > Thanks!
> > Guille
> >
> > Compact classes must be in the compact classes array so their instances
> know their correct class.
> > If you want to bootstrap an image inside another one, do it without
> compact classes.
> > You can always add them later when needed (which is somewhat
> questionable, because their space-saving advantage has a corresponding
> speed disadvantage).
> >
>
> yes, basically you can "convert/copy" any compact class to non-compact
> one. And construct a valid class object with methods etc.
> Everything will be fine unless you want to also create and use
> instances of such classes. Then you are in trouble.
> So, as long as you can guarantee that you don't run any code with
> those classes which involves instantiation,

there will be no problems.
>

Um, no.  Only for certain classes can you freely convert between compact
and uncompact.  The various VMs assume some classes are always compact
(because its faster to check their class).

In the Interpreter BlockContext and MethodContext must be at indices 13 and
14.

In the Stack and Cog VMs the following classes must be at the corresponding
indices
Array at 3
LargeNegativeInteger at 4
LargePositiveInteger at 5
Float at 6
MethodContext at 14

Further, the Stack and Cog VM determines the index of class ByteString at
startup and uses it in debug printing, stack dump generation etc.

If one needs to freely alter the compactClassesArray with a Stack or Cog VM
one needs to generate a special VM where the various compact class indices
(ClassArrayCompactIndex, et al, see ObjectMemory
class>initializeCompactClassIndices) are zero.  This should work, but
hasn't been tested.



> > Cheers,
> > Hans-Martin
>
>
>
> --
> Best regards,
> Igor Stasenko.
>



-- 
best,
Eliot
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