[Vm-dev] Context>>copyTo: doesn't update closure home

[Clément Bera]

The way I understand it the closure is shared between the original and the
copied stack. If you change the closure outerContext to match the copied
context, then non local returns / debugging won't work any more on the
original stack.

As you mentioned, updating all the closures referencing the context as
their outer context is non trivial. I believe you can do it lazily (at
least that's my conclusion from the sista experience where I need to do
that if the outer context is deoptimized). The only cases where the
outerContext is used are non local returns and debugging. If a non local
return fails, it calls the #cannotReturn: call-back. You can change the
code there to do the right think if the outer context is not correct (like
setting the correct outer context and retrying the non local return). You
can also change the BlockClosure code to check at each outerContext
in-image read if the outerContext is valid or not. I think that's much
better than updating ahead of time all the closure instances.

Alternatively, if you feel like it, you could try something like 'context
becomeForward: copyOfContext'.

On Fri, Jun 3, 2016 at 11:02 AM, Max Leske <maxleske at gmail.com> wrote:

>
> I see at least one problem: at the point where we make a copy of a
> context, the closures that reference it are either in callees or not on the
> stack at all. Therefore we would need to take note of all the closures we
> meet on the way and update the outer contexts only if we find them on the
> stack. That makes copying a whole lot harder.
>
> Max
>
> > Hi,
> >
> > Is anybody aware of a reason for why Context>>copyTo: should not update
> the home of closures (which is currently the case)? This behaviour becomes
> a problem when I want to simulate #stepThrough: on a copied stack, since
> the home of a closure will never be found on the stack.
> >
> > My proposal is to implement
> >
> > Context>>postCopy
> >       closureOrNil ifNil: [ ^ self ].
> >
> >       closureOrNil outerContext: self
> >
> > and
> >
> > BlockClosure>>outerContext: aContext
> >       outerContext := aContext
> >
> >
> > Thoughts?
> >
> > Cheers,
> > Max
>
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