Re: [Haskell-cafe] Can reallyUnsafePtrEquality give false positives?

This is out of curiosity: does the runtime actually provide guarantees about when the GC must kick in? Or is this implementation defined but common knowledge? Thanks, Siddharth On Wed 22 Nov, 2017, 20:20 Brandon Allbery, wrote:

Actually, I'd say that it can, but only if used incorrectly. Which is why it's reallyUnsafe.

The OP is correct in that a gc at the wrong time can lead to spurious positives. The key to this is that, if you force everything to be evaluated to WHNF (so you actually have the pointers) and then gc, you have some determinacy as to when the next gc will happen: force to WHNF first, gc, make sure any subsequent operations between that and your reallyUnsafePtrEquality either don't allocate or fit in the nursery --- and in this case, you can trust the result. So it requires a fair amount of care, but there is a window in which it is safe.

On Wed, Nov 22, 2017 at 2:06 PM, Andrew Martin wrote:

...

It cannot give false positives. If it could, that would make it totally worthless.

Sent from my iPhone

...

On Nov 22, 2017, at 12:22 PM, Michael Walker wrote:

Hello,

Can reallyUnsafePtrEquality give false positives? I can see how it can give false negatives (eg, compiler optimisations increasing or decreasing sharing), but I'm not so sure if it can give false positives.

I don't see how in a garbage collected language two live values could compare reference equal. Unless the implementation is something like:

reallyUnsafePtrEquality a b = getptr a == getptr b

...as that then means that if the GC moves things after `getptr a` is evaluated but before `getptr b` is, then you could get a false positive. But that doesn't seem like a sensible implementation to me, because then reallyUnsafePtrEquality would surely be totally useless.

-- Michael Walker (http://www.barrucadu.co.uk) _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

---

Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- brandon s allbery kf8nh sine nomine associates allbery.b@gmail.com ballbery@sinenomine.net unix, openafs, kerberos, infrastructure, xmonad http://sinenomine.net _______________________________________________ Haskell-Cafe mailing list To (un)subscribe, modify options or view archives go to: http://mail.haskell.org/cgi-bin/mailman/listinfo/haskell-cafe Only members subscribed via the mailman list are allowed to post.

-- Sending this from my phone, please excuse any typos!