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Re: run-once code

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Melvin Smith
January 14, 2004 09:50
Re: run-once code
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At 09:31 AM 1/14/2004 -0800, David Storrs wrote:
>On Wed, Jan 14, 2004 at 10:59:52AM -0500, Melvin Smith wrote:
> > I think Perl6 will allow a hint like so:
> >
> > my int $max_reached;
> >
> > The important thing is that $max_reached is used simply as a conditional,
> > and you don't pass it to a routine or otherwise use it in a way to 
> cause it
> > to be promoted to a PMC
>What uses would cause it to be promoted to a PMC?

Any use that doesn't allow a native type. Currently: passing it to a 
routine or storing it into a hash/list might trigger this.

It depends on how well the optimizer handles hints. Since the optimizer could
decide it only needs to create a temporary PMC for the uses in question,
but leave the base variable as a native. If the optimizer is too aggressive it
might decide that it is less expensive to just declare it as a PMC, so that is
where we have to decide how serious we take hints. :)

If the hint we are talking about becomes a directive, then the compiler just
does what it is told. Any expression needing a coercion will get a temporary
created, but all simple uses of $max_reached (increment, evaluation, 
tests) will still work with a native int register, and this is probably 
what you
want in this case.

On the flip side, for a standard declaration: my $max_reached; (no hint)
it should be possible with data-dependency analysis to decide $max_reached
should be a native, without the hint. We shall see.


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