[Haskell-cafe] Can reallyUnsafePtrEquality give false positives?
Theodore Lief Gannon
tanuki at gmail.com
Sat Nov 25 00:51:58 UTC 2017
Zemlya, I think you dropped this.
On Thu, Nov 23, 2017 at 8:03 PM, Zemyla <zemyla at gmail.com> wrote:
> In that case, you probably want to use StableNames , which are
> unique and persistent even across GCs, and give you something you can
> use for a hash table.
> On Thu, Nov 23, 2017 at 6:48 AM, Michael Walker <mike at barrucadu.co.uk>
> > Thanks for the feedback, all.
> > For some more context, I have a concurrency testing library, which
> > uses an algorithm called DPOR to reduce the space of possible
> > schedules it needs to explore. There's an algorithm called MCR which
> > has the potential to reduce this space significantly further. One of
> > the differences in MCR is taking into account reference equality of
> > values written to shared mutable variables.
> > I can't just use Eq because (a) putMVar/etc doesn't have an Eq
> > constraint in the type, and (b) that would change strictness. So I
> > looked to reallyUnsafePtrEquality# as a possible solution. In this
> > setting, false negatives are fine. If the algorithm thinks two equal
> > values are distinct, then it doesn't run as fast as it could
> > potentially do, but should still outperform DPOR due to its other
> > improvements. But false positives are not fine, and lead to
> > unsoundness.
> > An alternative is just using "\_ _ -> False", but if
> > reallyUnsafePtrEquality# will sometimes identify two equal things
> > without false positives, then it's effectively a free performance
> > boost in the cases where it happens to work.
> >> - Extending the previous scenario, if purity is violated in the
> >> evaluation of the initially shared thunk, then x and y may become
> >> distinct values later on.
> > Fortunately, I don't need to worry about impure actions leading to a
> > shared thunk evaluating to different things in different places, as
> > any nondeterminism beyond scheduler nondeterminism and relaxed memory
> > is already explicitly out of scope.
> >  http://hackage.haskell.org/package/dejafu
> > --
> > Michael Walker (http://www.barrucadu.co.uk)
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