G'day all.
On Fri, Apr 19, 2002 at 09:55:46AM +0100, Piers Cawley wrote:
> It's fine for partial continuations certainly, but less fine if you
> want to implement full continations which require you to save the
> state of the entire stack. I was hoping I'd find a way to do this
> without having to wait for Parrot to get its own continuations.
I suspect that using relative offsets for jump instructions might have
been a bad idea in retrospect. Sure it's faster, but now we have
prederef, which could turn absolute offsets into relative offsets at
run time.
> If
>
> set I0, LABEL
>
> sets I0 to an address which I can C<jump> to from anywhere then I have
> what I need.
One possible solution is to introduce a new op:
inline op codeoffset(inconst INT, out INT)
The idea is to turn a relative offset into an absolute address which
can then be passed to "goto ADDRESS()".
Another possibility is:
inline op adjustoffset(in INT, inconst INT, inconst INT, out INT) {
$4 = $1 + $2 - $3;
goto NEXT();
}
The idea is that this:
adjustoffset I0, LABEL1, LABEL2, I1
will adjust an offset I0 (which is relative to LABEL1) to make it
relative to LABEL2. That way, you can canonicalise all your offsets
and just re-adjust them before jumping.
(Note: Yes, this could be done with two existing ops. The benefit of
making it another op is that a JIT compiler can easily compile it away
if it chooses to use only absolute offsets.)
> What's SECD by the way?
http://www.wikipedia.com/wiki/SECD+machine
If you have a well-stocked university library handy:
@book{Henderson80,
author = {Peter Henderson},
title = {{Functional Programming -- Application and Implementation}},
publisher = {Prentice/Hall International},
year = 1980,
series = {Series in Computer Science},
topics = {FP - General,FP - Implementation}
}
Cheers,
Andrew Bromage
