Danger Will Robinson! Missing API!

[Charles Oliver Nutter]

On Thu, May 26, 2011 at 12:55 AM, Ola Bini <ola.bini at gmail.com> wrote:
> Actually, it was Remi who came up with this gem when we discussed adding
> an isInvalid() method on SwitchPoint.

I don't say it's not clever. It's just not something anyone is
typically going to figure out on their own. There will be constant
questions "if I can invalidate a SwitchPoint, why can't I ask if it
has been invalidated?"

> The thing is, at least where I've used SwitchPoint, having a way of
> asking is not as useful as being able to use the pattern to change work
> later. So, for example, in Seph I allow "if" to be overridden. If that
> doesn't happen I have a fastpath for ifstatements, guarded by a
> switchpoint. When someone overrides "if" I want to stop checking for
> "if", so I just add the method to check if "if" is overridden to the
> switchpoint itself, with a fallback of not doing anything.
>
> I suspect that those kind of patterns are going to be much more common.

That's certainly true, but in the case I'm considering I want to use
it as both an inline guard and as a trigger to alter future
compilation. Specifically, I want to use it to test whether Fixnum or
Float have been modified, so I can stop emitting direct paths for
their calls. The hope is that I can avoid bounding through a few field
loads to check a JRuby-instance-local flag indicating whether they've
been overridden, and simply have a switchpoint around the fast path
math logic. I want SwitchPoint.isValid so that once it has been
invalidated I only have a single place to check to know whether I
should even bother trying to insert the fast path in the future.

Yes, it could be another volatile boolean I set around the same time
as invalidating the SwitchPoint, but I'm duplicating SwitchPoint's
logic then.

I can use the trick for now, but this seems like an obvious missing
feature. SwitchPoint has state: is or is not valid. You can modify
that state directly via invalidateAll. You should also be able to
query it without doing a MH dance.

- Charlie

- Charlie