On Sat, Mar 27, 2010 at 12:43 AM, Rafael Almeida <almeidaraf@gmail.com> wrote:

On Fri, Mar 26, 2010 at 6:49 PM, Jason Dagit <dagit@codersbase.com> wrote:
>
>
> On Fri, Mar 26, 2010 at 2:33 PM, Bryan O'Sullivan <bos@serpentine.com>
> wrote:
>>
>> On Fri, Mar 26, 2010 at 10:46 AM, Rafael Cunha de Almeida
>> <almeidaraf@gmail.com> wrote:
>>>
>>> During a talk with a friend I came up with two programs, one written in
>>> C and another in haskell.
>>
>> Your Haskell code builds a huge thunked accumulator value, so of course
>> it's slow (put bang patterns on all arguments). Also, you should use rem
>> instead of mod. Make those tiny changes and you'll get a 5x speedup, to half
>> the performance of the C code.
>
> Interesting. I had to add -fvia-C to get within half the performance of C.
> Just bang patterns and rem and I'm 1/5th of C. I'm on a x86_64 machine. I
> wonder if that plays in.
>
> Jason
>

Using bang patterns didn't help almost anything here. Using rem
instead of mod made the time go from 45s to 40s. Now, using -fvia-C
really helped (when I used rem but not using mod). It went down to
10s.

It's worth pointing out that there's a bit of bang-pattern mysticism going on in this conversation (which has not been uncommon of late!). A non-buggy strictness analyzer should expose the strictness of these functions without difficulty. If bang patterns make any difference at all with a -O flag, either there's a strictness analysis bug, or some very interesting effects from shifting the order of forcing of strict variables.

Putting in bang patterns is a good idea to plug the obvious space leak when run without optimization, but isn't going to make a difference for optimizing compilation of obviously-strict functions.

-Jan-Willem Maessen