[Vm-dev] Re: [squeak-dev] Trying to understand thisContext

[Eliot Miranda]

Hi Matthias,

    cc'ing vm-dev, because this is more of a VM question

On Mon, Jun 8, 2015 at 6:45 AM, matthias.springer <me at matthiasspringer.de>
wrote:

> Hi there,
>
> I am trying to understand what the meaning and implementation of
> "thisContext" is.
>
> On a recent COG VM, "[ |a b| a:=b:=1. thisContext basicSize ] value"
> returns
> 3. Adding more temp vars increases that value.
>

That's correct.  thisContext basicSize effectively answers the value of the
stack pointer of a context.  In Squeak (as opposed to, say, the BLueBook)
the Interpreter was optimized by changing the garbage collector to never
look at fields in a context past the stack pointer.  In the blue book,
thisContext basicSize answers the number of stack slots in the context,
rather than the stack pointer, and the garbage collector might look beyond
the stack pointer for reachable object references, but if pop nils out the
top of stack, there should only ever be nils beyond the stack pointer.

IMO, this difference could have been hidden in the VM and thisCOntext
basicSize could have always answered a constant, but that's the current
definition in Squeak/Pharo.


>
> On a Cocoa VM (http://squeakvm.org/mac/release/Squeak%205.7.4.1.zip), the
> same code always returns 0. Also, "thisContext at: 1" etc. is out of
> bounds.
>

This is not a closure VM right?  So while the code reads the same, the
bytecode and implementation is quite different.  In this VM, which has
blue-book blocks, block arguments are popped into temporary variables in
the home context by the block prolog, so that by the time thisCOntext is
sent basicSize its stack is empty.  So given the compilayion of blocks,
this answer is also correct.

What I suggest is that you write the following and examine the differences
in the bytecode:

{ [ |a b| a:=b:=1. thisContext basicSize ] value. thisContext method
symbolic }


>
> Which one is correct? Any ideas?
>

Both.  They differ because they implement blocks quite differently.  You
can read more on the difference in the implementations in
http://www.mirandabanda.org/cogblog/2008/06/07/closures-part-i/ et al.



>
> The reason why I am asking is because there is code that seems to rely on
> this (e.g. SystemTracer2>>writeContext:) and I am trying to get it running
> on RSqueak.
>

Again, let me instead suggest that you adapt the code in SpurBootstrap in
the Cog package at http://source.squeak.org/VMMaker, instead of using
SystemTracer.  It is a lot more direct, and less hairy than an image
tracing itself.

-- 
best,
Eliot
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