[Haskell-cafe] Can reallyUnsafePtrEquality give false positives?

[Erik Hesselink]

According to Edwark Kmett it can give false positives as well, or at least
could in 2010:
https://mail.haskell.org/pipermail/haskell-cafe/2010-June/079532.html

Erik

On 22 November 2017 at 20:06, 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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