[Vm-dev] Problem understanding #cannotInterpret

[Mariano Martinez Peck]

Hi. There is something I don't understand about #cannotInterpret and maybe
you can help me. Since I am not sure if this is more image side or vm, I did
a cross post, sorry for that.
Context: I am implementing proxies for classes. Since I want that a proxy
loads back the original object when receiving the message #class, I have
removed from the compiler the shortcut bytecode for class, with this:

(ParseNode classVarNamed:  'StdSelectors') removeKey: #class ifAbsent: [].

Now, my testcase is something like this:

    | inst proxy |
    "This is the simplest test that does the less assert as possible."
    inst := ClassWith1Var new.
    self assert: inst foo = 'foo'.
    proxy := ClassProxyInstVar new.
    proxy become: ClassWith1Var.
    "here I can send any message, it is just to load back the original
class"
    inst basicIdentityHash.
    self assert: (inst class == ClassWith1Var).
    self assert: inst foo = 'foo'.


ProtoObject subclass: #ClassProxyInstVar
    instanceVariableNames: 'superclass methodDict format actualClass'
    classVariableNames: ''
    poolDictionaries: ''
    category: 'Proxies'


ClassProxyInstVar >> initialize
    super initialize.
    superclass := ProxySuperclass.
    methodDict := nil.
    actualClass := self


So...with this i can use an instance of ClassProxyInstVar like a class. When
an instance of the class receives a message, the methodDict will be in nil,
and then the VM will send the #cannotInterpret. and there I want to put back
the original class. Then I do:

ProxySuperclass >> cannotInterpret: aMessage
    Transcript cr;
        show: 'cannotInterpret:';
        space;
        show: aMessage selector..
    self class restore.
    ^aMessage sendTo: self

Now, this is working, but there is something I DO NOT UNSERSTAND. When I try
to do the "inst basicIdentityHash" the VM will send the #cannotInterpret to
ProxySuperclass because the methodDict was nil in ClassProxyInstVar. Ok,
perfect. What I don't understand is how the method #cannotInterptet can be
run perfeclty in this case, since I am doing a "self class restore" there.
Remember I removed the shortcut bytecode, so #class is sent like any normal
message. So, my question is, when sending "self class", why the VM doesn't
call #cannotInterpter again like it did it when I did "   inst
basicIdentityHash."  ?  in other words...why the VM isn't entering in a loop
?   There is something I am not seeing.

Thanks for any help in advance,

Mariano
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://lists.squeakfoundation.org/pipermail/vm-dev/attachments/20110116/a8e6344f/attachment-0001.htm