[Haskell-cafe] Can reallyUnsafePtrEquality give false positives?

[Zemyla]

In that case, you probably want to use StableNames [1], 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> wrote:
> Thanks for the feedback, all.
>
> For some more context, I have a concurrency testing library[1], 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.
>
> [1] http://hackage.haskell.org/package/dejafu
>
> --
> Michael Walker (http://www.barrucadu.co.uk)
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