[Haskell-cafe] Can reallyUnsafePtrEquality give false positives?

[Siddharth Bhat]

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, <allbery.b at gmail.com> 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 <andrew.thaddeus at gmail.com>
> 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 <mike at barrucadu.co.uk>
>> 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)
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>
>
>
> --
> brandon s allbery kf8nh                               sine nomine
> associates
> allbery.b at gmail.com
> ballbery at sinenomine.net
> unix, openafs, kerberos, infrastructure, xmonad
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-- 
Sending this from my phone, please excuse any typos!
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