In my test it works ~20% faster than s2 and ~20% slower than s1. Did you use -O2 flag?
2012/6/25 Lorenzo Bolla <[email protected]> > I wonder why this performs really badly, though (I would expect it to be > the same as s2): > > s3 :: Int -> Int > s3 n = sum [gcd x y | x <- [ 0 .. n-1 ], y <- [ 0 .. n-1 ]] > > From the links posted by Dmitry, it might be that the code generated is > made of 2 recursive calls: in fact, what I observe is a "stack space > overflow" error on runtime... > > L. > > > > > On Mon, Jun 25, 2012 at 10:09 AM, Dmitry Olshansky > <[email protected]>wrote: > >> s1 ~ sum $ map (sum . flip map [0..n] . gcd) [0..n] >> s2 ~ sum $ concatMap (flip map [0..n] . gcd) [0..n] >> >> There are some posts from Joachim Breitner investigated fusion for >> concatMap: >> >> http://www.haskell.org/pipermail/haskell-cafe/2011-December/thread.html#97227 >> >> >> >> 2012/6/25 Johannes Waldmann <[email protected]> >> >>> Dear all, >>> >>> while doing some benchmarking (*) >>> I noticed that function s1 is considerably faster than s2 >>> (but I wanted s2 because it looks more natural) >>> (for n = 10000, s1 takes 20 s, s2 takes 13 s; compiled by ghc-7.4.2 -O2) >>> >>> s1 :: Int -> Int >>> s1 n = sum $ do >>> x <- [ 0 .. n-1 ] >>> return $ sum $ do >>> y <- [ 0 .. n-1 ] >>> return $ gcd x y >>> >>> s2 :: Int -> Int >>> s2 n = sum $ do >>> x <- [ 0 .. n-1 ] >>> y <- [ 0 .. n-1 ] >>> return $ gcd x y >>> >>> I was expecting that in both programs, >>> all lists will be fused away (are they?) >>> so the code generator essentially can produce straightforward >>> assembly code (no allocations, no closures, etc.) >>> >>> >>> For reference, I also wrote the equivalent imperative program >>> (two nested loops, one accumulator for the sum) >>> (with the straightforward recursive gcd) >>> and runtimes are (for same input as above) >>> >>> C/gcc: 7.3 s , Java: 7.7 s, C#/Mono: 8.7 s >>> >>> >>> So, they sort of agree with each other, but disagree with ghc. >>> Where does the factor 2 come from? Lists? Laziness? >>> Does ghc turn the tail recursion (in gcd) into a loop? (gcc does). >>> (I am looking at -ddump-asm but can't quite see through it.) >>> >>> >>> (*) benchmarking to show that today's compilers are clever enough >>> such that the choice of paradigm/language does not really matter >>> for this kind of low-level programming. >>> >>> >>> >>> >>> >>> >>> _______________________________________________ >>> Haskell-Cafe mailing list >>> [email protected] >>> http://www.haskell.org/mailman/listinfo/haskell-cafe >>> >> >> >> _______________________________________________ >> Haskell-Cafe mailing list >> [email protected] >> http://www.haskell.org/mailman/listinfo/haskell-cafe >> >> >
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