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

[Igor Stasenko]

On 6 April 2012 19:26, Eliot Miranda <eliot.miranda at gmail.com> wrote:
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> On Fri, Apr 6, 2012 at 9:45 AM, Igor Stasenko <siguctua at gmail.com> wrote:
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>> On 6 April 2012 18:37, Hans-Martin Mosner <hmm at heeg.de> wrote:
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>> > Am 06.04.2012 16:41, schrieb Guillermo Polito:
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>> > 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.
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> 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).
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> In the Interpreter BlockContext and MethodContext must be at indices 13 and 14.
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What i meant is that you can replicate both of those i.e. create a
uncompact BlockContext' and MethodContext'
and can compile methods for them.. but should avoid instantiation.


> 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
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> Further, the Stack and Cog VM determines the index of class ByteString at startup and uses it in debug printing, stack dump generation etc.
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> 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.
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>> > Cheers,
>> > Hans-Martin
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>>
>>
>> --
>> Best regards,
>> Igor Stasenko.
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> --
> best,
> Eliot
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>



-- 
Best regards,
Igor Stasenko.