[Haskell-cafe] Performance Issue

[Ozgur Akgun]

Richard,

I found this very annoying when I first realised GHC doesn't do any CSE,
given that the result of pure functions only depend on their parameters.

Even though CSE usually sounds good, when you ask, they go and find obscure
examples in which it causes great trouble :)

I think, there should at least be a compiler flag or something similar to
enforce CSE on some structures. But currently, as others pointed out, you
need to bind and do your sub expression elimination yourself.

Best,

On 18 May 2010 17:30, Richard Warburton <richard.warburton at gmail.com> wrote:

> A colleague of mine pointed out that ghc wasn't performing as he
> expected when optimising some code.  I wonder if anyone could offer
> any insight as to why its not noting this common subexpression:
>
> main = print $ newton 4 24
>
> newton a 0 = a
> newton a n =  ((newton a (n-1))^2 + a)/(2*(newton a (n-1)))
>
> Compiled with 'ghc -O3 --make perf.hs', results in:
>
> real    0m5.544s
> user    0m5.492s
> sys     0m0.008s
>
> However if we factor out the repeated call to the newton method:
>
> main = print $ newton2 4 24
>
> newton2 a 0 = a
> newton2 a n =  (x^2 + a)/(2*x)
>        where
>          x = newton2 a (n-1)
>
> real    0m0.004s
> user    0m0.004s
> sys     0m0.004s
>
> It looks to me like Referential transparency should make this a sound
> optimisation to apply, but ghc isn't doing it even on -O3.
>
> regards,
>
>  Richard
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
>



-- 
Ozgur Akgun
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20100522/ac10809c/attachment.html