Re: [Haskell-cafe] GHC optimisations

Hugh Perkins wrote:

On 8/22/07, Twan van Laarhoven wrote:

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But Double is already quite badly behaved:

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let x = 1e20 Prelude> 1 + (x - x) 1.0 Prelude> (1 + x) - x 0.0

Ewwww. Whilst that's understandable and unavoidable, that kindof rings alarm bells for folds of Doubles in an automatic threading situation. Ie, if we split our fold between 4 cores, we might get a different answer than if we split it between 8 cores?

Yep, Double's (+) isn't associative. I was trying to come up with ANY common mathematical property that Double fulfills and this was what I came up with: --> for all x::Float. not (x > x + 1) I'm not even sure it's true in the presence of very large numbers, optimizations and extra precision being added in the CPU. Note that it's not (x <= x + 1) because of NaN. Is Double's (+) commutative? I don't know for sure. Its (==) isn't reflexive (is it transitive? probably, at least if there aren't too many optimizations, but floating-point transitive equality isn't very useful). --> Okay... so (+), (*), (==) are probably symmetric too. Any others? Isaac