[Vm-dev] Interpreter versus StackInterpreter hierarchy

[Clément Bera]

On Thu, May 19, 2016 at 3:42 PM, Ben Coman <btc at openinworld.com> wrote:

>
> The hierarchy goes
>   VMClass
>      InterpreterPrimitives
>         StackIntepreter
>           StackIntepreterPrimitives
>              CoInterpreter
>                CoInterpreterPrimitives
>
> from which a tick / tock (XXInterpreter / XXInterpreterPrimitives)
> pattern is apparent,
> and I wonder why Interpreter is missing above InterpreterPrimitives
> sitting off to the side.  I guess it is something to do with
> InterpreterPrimitives composing objectMemory as an instance variable
> rather than inheriting from ObjectMemory as Interpeter does?
>

I think this is exactly that.


> The reason I ask is that I'm trying a new approach to the OwnedLock
> primitives
> where if waitAcquire primitive sleeps, when the process wakes up it
> retries the primitive.
> So effectively the process sleeps at the top of the waitAquire rather
> than the bottom,
> and the process truly cannot proceed past that point until it gains
> the lock. I have this working if StackInterpreterPrimitives holds the
> primitive, but if I move the primitive to InterpreterPrimitives,
> instructionPointer is unknown.
>
>
In general, you implement in InterpreterPrimitives the primitives that
would work with the Interpreter, and in StackInterpreterPrimitives the ones
specific to the StackInterpreter. If you have a bug in
InterpreterPrimitives, it's likely to be specific to the StackInterpreter.

In the future the Cog branch may merge with the trunk, hence Interpreter
will be back in the hierarchy.


>
> P.S. For the curious, here is the proof of concept I tried.  A single
> call to the primitive counts up to four by backing up the
> instructionPointer such that the primitive is executed again, until
> the exit condition of four is reached.
>
> (Note the use of Semaphore and ExcessSignalsIndex is not significant,
> just an expedient template I was familiar with.)
>
> # VM SIDE...
>
> StackInterpreterPrimitives >> primitiveRetryExperiment
>     | excessSignals stackTop |
>     stackTop := self stackTop.
>     excessSignals := objectMemory fetchInteger: ExcessSignalsIndex
> ofObject: stackTop.
>     excessSignals := excessSignals + 1.
>     objectMemory storeInteger: ExcessSignalsIndex
>         ofObject: stackTop
>         withValue: excessSignals.
>     [ excessSignals > 3 ] ifFalse: [ instructionPointer :=
> instructionPointer - 1 ].
>
> StackInterpreter class >> initializePrimitiveTable
>     (234 primitiveRetryExperiment)
>
>
... Why instructionPointer - 1 ? It works if the send is encoded in a
single byte as by chance is the case in your example, else your
interpretation get misaligned, which is a complete nonsense (unless you're
working with a Smalltalk-78 VM ;-) ). Going backward in the instructions is
not that trivial, you need a scanner to find the previous pc. See
#skipBackBeforeJump for example, which goes backward 1 instruction in the
image side. You need to implement something similar in the interpreter if
you want to do that... but you don't.

Although going back one instruction is very funny, it won't work with the
JIT, unless you do something completely evil, crazy and hackish. Most
likely you want in fact to implement your primitive with a while loop
instead, so you don't need that ip modification hack. Why didn't you do a
while loop in the first place ? Primitive calls are not interrupt points
anyway, it would be exactly the same behavior, wouldn't it ?


>
> # IMAGE SIDE...
>
> Semaphore subclass: #PrimExp
>     instanceVariableNames: ''
>     classVariableNames: ''
>     package: '0PrimitiveRetryExperiment'
>
> PrimExp >> initialize
>     excessSignals := 1.
>
> PrimExp >> primRetryExperiment
>     <primitive: 234>
>
> PrimExp >> excessSignals
>     ^ excessSignals
>
> # TEST CASE...
>
> PrimExp new primRetryExperiment excessSignals
>     --> 4
>
> I've only done this with the Stack VM so far.  I'll report further
> when I try it with Cog.
>
>
It's nice to see people trying to hack the VM :-). I'll try to answer your
other questions.


> cheers -ben
>
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