[Vm-dev] Re: Multiple Bytecode Sets
charig at gmail.com
Tue Oct 16 20:48:18 UTC 2012
Claus' VM? I don't know it and i'm interested on the topic, any precise
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
> and let's the bytecode jump to different code when the debugging hooks are
> 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.
> Stefan Marr
> Software Languages Lab
> Vrije Universiteit Brussel
> Pleinlaan 2 / B-1050 Brussels / Belgium
> Phone: +32 2 629 2974
> Fax: +32 2 629 3525
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