[Haskell-cafe] Very fast loops. Now!

[Lennart Augustsson]

GCC 4.x gets a pass on this test. :)
You can do (much) better than that, of course.
But it's what I'd expect without going over board.

	-- Lennart

On Feb 10, 2007, at 16:45 , Donald Bruce Stewart wrote:

> dons:
>> The following C program was described on #haskell
>>
>>     #include <stdio.h>
>>
>>     int main()
>>     {
>>         double x = 1.0/3.0;
>>         double y = 3.0;
>>         int i    = 1;
>>         for (; i<=1000000000; i++) {
>>             x = x*y/3.0;
>>             y = x*9.0;
>>         }
>>         printf("%f\n", x+y);
>>     }
>>
>>
>> Which was translated to the following Haskell:
>>
>>     {-# OPTIONS -fexcess-precision #-}
>>
>>     import Text.Printf
>>
>>     main = go (1/3) 3 1
>>
>>     go :: Double -> Double -> Int -> IO ()
>>     go !x !y !i
>>         | i == 1000000000 = printf "%f\n" (x+y)
>>         | otherwise       = go (x*y/3) (x*9) (i+1)
>>
>>
>> To everyone's surprise, GHC 6.6 beats GCC (3.3.5) here, at least  
>> the two test machines:
>>
>>
>>     $ ghc -O -fexcess-precision -fbang-patterns -optc-O3 -optc- 
>> ffast-math -optc-mfpmath=sse -optc-msse2 A.hs -o a
>>
>>     $ time ./a
>>     3.333333
>>     ./a  0.96s user 0.01s system 99% cpu 0.969 total
>>                                          ^^^^^
>>
>> Versus gcc 3.3.5:
>>
>>     $ gcc -O3 -ffast-math -mfpmath=sse -msse2 -std=c99 t.c -o c_loop
>>     $ time ./c_loop
>>     3.333333
>>     ./c_loop  1.01s user 0.01s system 97% cpu 1.046 total
>>                                               ^^^^^
>>
>> Note that newer gcc's will statically compute that loop. Note also  
>> that
>> -fexcess-precision must currently be provided as a pragma only.
>>
>> I declare GHC Haskell numerics (with -fexcess-precision) not so  
>> shabby!
>>
>
>
> GCC 4.x seems to do a much better job, turning the inner loop into:
>
>     .L2:
>         mulsd   %xmm3, %xmm0
>         mulsd   %xmm1, %xmm0
>         movapd  %xmm0, %xmm1
>         mulsd   %xmm2, %xmm1
>         addl    $1, %eax
>         cmpl    $100000001, %eax
>         jne .L2
>
> -- Don
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