[Vm-dev] Interpreter versus StackInterpreter hierarchy

Ben Coman btc at openinworld.com
Fri May 20 17:51:29 UTC 2016

On Fri, May 20, 2016 at 9:25 PM, Clément Bera <bera.clement at gmail.com> wrote:
> 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 ?

A random experiment before moving on to -2, -3 and -4.   Obviously I
don't know enough yet to have properly judged its impact.  Just poking
it with a stick to see what pops out, and following that old adage
that if you need something to be true, "assume it" until you learn
otherwise.  And whadayaknow..., it worked for the StackInterpreter.
And I've since learnt its not so easy for the JIT.

> 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.

I thought *maybe* that since the layout of the receiver is "known" (to
be an OwnedLock) the previous pc could be assumed at a known distance
up the stack - but maybe that is bad practice and trouble if the
layout changes for anyone subclassing OwnedLock.

> 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.

I'll take a look just for interest, but point taken.

> Although going back one instruction is very funny, it won't work with the JIT, unless you do something completely evil, crazy and hackish.

That was the advice I was looking for.  I don't want to be hackish ;)
 At least not in the final result.

> 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 ?

A loop in the Image would work, but I'm attempting to keep it in the
VM.  IIUC a loop won't work in the VM because the primitive sleeps and
changes context if the lock is held by someone else.

> 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.

So I found the approach doesn't work with Cog.
cheers -ben

> It's nice to see people trying to hack the VM :-). I'll try to answer your other questions.

More information about the Vm-dev mailing list