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

Zemlya, I think you dropped this. [1] https://hackage.haskell.org/package/base-4.10.0.0/docs/System-Mem-StableName... On Thu, Nov 23, 2017 at 8:03 PM, Zemyla wrote:

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 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) _______________________________________________ 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.

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