Re: [Haskell-cafe] Performance Issue

Actually, in this case it would be safe to do CSS. Because a) the function is strict in both arguments so GHC creates a worker which only uses unboxed types b) this cannot cause any space leaks (it contains no pointers) The generated Core does look pretty weird, though: $wnewton = \ (ww_s115 :: Double#) (ww1_s119 :: Int#) -> case ww1_s119 of ds_Xr8 { __DEFAULT -> case ^_r11D (case $wnewton ww_s115 (-# ds_Xr8 1) of ww2_s11d { __DEFAULT -> D# ww2_s11d -- box the result of $wnewton }) lvl_r11B of _ { D# x_avk -> -- unbox it again case $wnewton ww_s115 (-# ds_Xr8 1) of ww2_s11d { __DEFAULT -> /## (+## x_avk ww_s115) (*## 2.0 ww2_s11d) } }; 0 -> ww_s115 } On 22 May 2010 13:30, Ozgur Akgun wrote:

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 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@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

-- Ozgur Akgun

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