Paolo Molaro wrote:
> If anyone has any
> evidence that coding a stack-based virtual machine or a register one
> provides for better instructions scheduling in the dispatch code,
> please step forward.
I think we're going to have some evidence in a few weeks. I'm not
sure which side the evidence is going to support though... ;)
Eric Raymond posted on python-dev that he's doubtful, but has
a "wait and see" approach. Seems sensible. I think even Dan would
say he's just hopeful, not commited.
> I believe that a stack-based machine will have roughly the same
> performance when interpreted as a register-based machine, but
> it easily allows to take a step further and JIT compile the bytecode
> to machine code.
I don't really see much difference. You don't have to map the VM
registers to hardware registers to pick up a lot of speed. If the
JIT wanted to, it could have an optional peep-hole optimization
pass. That actually sounds easier to do with a register-based VM
than a stack-based one. (Of course the run-time environment is a
big wild-card here. We need a fast interface between the dispatcher
and the run-time. That's going to want registers too.)
> With the difference that the registers are malloc()ed while the eval
> stack in a stack machine is in the actual cpu stack.
Is there really a difference in memory access between the heap
and the stack? I've alway thought a page is a page and it doesn't
matter where in memory the page is. I'm not a hardware guy though...
Allocating register pools on the heap (making sure that malloc()
is used sensibly), might be faster if you want your VM to handle
continuations and co-routines. Check out stackless Python for
a good example. I'm not sure if Appel was the first, but he has
written quite a bit about the advantages of allocating activation
records on the heap. (He also points out that a garbage collector
can make heap allocation as fast as stack allocation.)
- Ken
