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Message from discussion NULL Px proposal (was Re: [BUG] IMCC looking in P3[0] for 1st arg)
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Melvin Smith  
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 More options Oct 26 2003, 1:48 pm
Newsgroups: perl.perl6.internals
From: mrjoltc...@mindspring.com (Melvin Smith)
Date: Sun, 26 Oct 2003 13:39:24 -0500
Local: Sun, Oct 26 2003 1:39 pm
Subject: NULL Px proposal (was Re: [BUG] IMCC looking in P3[0] for 1st arg)
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At 07:21 PM 10/26/2003 +0100, Leopold Toetsch wrote:

>Steve Fink <st...@fink.com> 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.

The downside is fast initialization of register blocks. memsetting with
NULL (0)
will not be possible, but I'd have to actually go check and see if that
is really all that common.

-Melvin


 
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