On Sun, 26 Oct 2003, Melvin Smith wrote:
> At 07:21 PM 10/26/2003 +0100, Leopold Toetsch wrote:
> >Steve Fink <[EMAIL PROTECTED]> wrote:
> >
> > > Although this does bring up another issue -- should parrot really be
> > > seg faulting when it gets a uninitialized (null) PMC?
> >
> >The problem is of course that we call pmc->vtable->some_meth_od() on a
> >NULL PMC. We could do the checks always, slowing down each PMC
> >operation. But OTOH a correct HLL compiler wouldn't produce such broken
> >code (hopefully). So that's probably something for the bounds-checking
>
> This is an old discussion. While I agree with Leo (and Dan) on not putting
> checks everywhere, I also think segfaulting on faulty bytecode is
> violating the "Virtual Machine" contract.
>
> I also don't wan't to see the performance hit of checking PMC regs everywhere.
>
> I think a compromise would be to do define a interpreter global PMCNull
> and point (or init) all Px registers to it.
>
> All methods in this class would be implemented to throw an exception or
> dump the bytecode for debugging and we could always trap a "NULL"
> register access.
>
> This way we never have a NULL pointer access with regards to Px regs.
This works. The safe code still needs to check, and null registers are
still a possibility but the null op and the register block-clearing code
can do this when they clear out register blocks. The Null PMC should pitch
a fatal exception for everything it does, so we can catch this stuff.
Probably should be a compile-time option so it can be turned on for
debugging/development runs and off for general usage.
Dan
--------------------------------------"it's like this"-------------------
Dan Sugalski even samurai
[EMAIL PROTECTED] have teddy bears and even
teddy bears get drunk
