[Haskell-cafe] Can reallyUnsafePtrEquality give false positives?

[Michael Walker]

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)