[Vm-dev] Re: Multiple Bytecode Sets

[Eliot Miranda]

On Tue, Oct 16, 2012 at 1:48 PM, Guido Chari <charig at gmail.com> wrote:

>
> Claus' VM? I don't know it and i'm interested on the topic, any precise
> pointer?
>

Not sure how much there is on the VM, but you could try

http://www.esug.org/wiki/pier/Conferences/2011/Schedule-And-Talks/A-Guided-Tour-through-Smalltalk


>
> Cheers,
> Guido.
>
> 2012/10/16 Stefan Marr <smalltalk at stefan-marr.de>
>
>>
>> Hi Eliot:
>>
>> On 16 Oct 2012, at 22:32, Eliot Miranda wrote:
>>
>> > StackInterpreter methods
>> > fetchNextBytecode
>> >       "This method fetches the next instruction (bytecode)."
>> >       currentBytecode := self fetchByte + bytecodeSetSelector
>>
>> Thanks, that's what I was looking for.
>>
>> > I couldn't see much of an impact when I measured it, but I don't have
>> my measurements handy.  Modern processors cache the read so well that even
>> in the interpreter impact is minimal (it was certainly less than 10%,
>> probably less than 5%, in a pure interpreter), and of course in the JIT it
>> matters not at all.  But the impact was enough that I ended up
>> conditionalizing the code so it reads
>>
>> Hm, ok. Sounds better than the 5-30% hit I have for having a check inside
>> the bytecodes to have different semantics.
>>
>> > I don't suppose its something people do a lot.  Claus' VM is the only
>> one I was aware of.  I think its useful to support multiple languages and
>> to support migrating the bytecode set using incremental development, but in
>> a production VM if one didn't need the support I think one would end up
>> disabling it.
>>
>> The other example I found is the LuaJIT2 interpreter.
>> It is used to enable and disable debugging hooks, as far as I understand.
>> But since it is written in assembler, it is just patching the dispatch
>> table
>> and let's the bytecode jump to different code when the debugging hooks
>> are enabled.
>> But I didn't see any performance numbers for that. Would guess it would
>> take some interaction with the OS to deactivate protection on executable
>> memory pages to change the dispatch table.
>>
>> Thanks
>> Stefan
>>
>> --
>> Stefan Marr
>> Software Languages Lab
>> Vrije Universiteit Brussel
>> Pleinlaan 2 / B-1050 Brussels / Belgium
>> http://soft.vub.ac.be/~smarr
>> Phone: +32 2 629 2974
>> Fax:   +32 2 629 3525
>>
>>
>
>


-- 
best,
Eliot
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