[Haskell-cafe] Re: newbie optimization question

[Rodrigo Queiro]

rem is faster because it has slightly different behaviour to mod, and
there happens to be an intel instruction that maps more directly to
rem than to mod, thus making it much faster on intel processors.

Why do you expose perfect and divisors? Maybe if you just expose main,
perfect and divisors will be inlined (although this will only save
10,000 function entries, so will probably have negligible effect).

Rodrigo

On 29/10/2007, Peter Hercek <peter at syncad.com> wrote:
> Derek Elkins wrote:
> >
> > Try with rem instead of mod.
> >
> > (What the heck is with bottom?)
>
> The bottom was there by error and I was lazy to redo
>   the tests so I rather posted exactly what I was
>   doing. I do not know the compiler that good to be
>   absolutely sure it cannot have impact on result
>   ... so I rather did not doctor what I did :-)
>
> Ok, rem did help quite a bit. Any comments why it is so?
>
> Here are summary of results for those interested. I run
>   all the tests once again. Haskell was about 13% slower
>   than C++.
>
> MS cl.exe options: /Ox /G7 /MD
> ghc options: -O2
>
> C++ version:  1.000; 0.984; 0.984
> Haskell version specialized to Int: 1.125; 1.125; 1.109
> Haskell version specialized to Integer: 8.781; 8.813; 8.813
> Haskell version specialized to Int64: 9.781; 9.766; 9.831
>
> The code:
>
> % cat b.hs
> module Main (divisors, perfect, main) where
> import Data.Int
>
> divisors :: Int -> [Int]
> divisors i = [j | j<-[1..i-1], i `rem` j == 0]
>
> perfect :: [Int]
> perfect = [i | i<-[1..10000], i == sum (divisors i)]
>
> main = print perfect
>
> % cat b.cpp
> #include <iostream>
> using namespace std;
>
> int main() {
>    for (int i = 1; i <= 10000; i++) {
>      int sum = 0;
>      for (int j = 1; j < i; j++)
>        if (i % j == 0)
>          sum += j;
>      if (sum == i)
>        cout << i << " ";
>    }
>    return 0;
> }
>
> %
>
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