[Haskell-cafe] Haskell, C and Matrix Multiplication

[Henning Thielemann]

Ketil Malde wrote:
> Blake Rain <blake.rain at gmail.com> writes:
> 
>> Here is the one in C:
>>
>> void multiplyM4 (float m1[4][4], float m2[4][4], float m3[4][4]) {
>> m1[0][0] = m2[0][0] * m3[0][0] + m2[0][1] * m3[1][0] + m2[0][2] * m3[2][0] + m2[0][3] * m3[3][0];
>> m1[0][1] = m2[0][0] * m3[0][1] + m2[0][1] * m3[1][1] + m2[0][2] * m3[2][1] + m2[0][3] * m3[3][1];
>     (etc)
>     :
> 
>> Some points about the C version:
>     :
>>   6. It's pretty fast (being written in C):
>>
>>      Performing  100,000,000 matrix  multiples  takes on  average
>>      6.117s (with gcc -O2).
>     :
> 
>> Haskell version:
>>
>> [foldl (+) 0 $ zipWith (*) x y | x <- m1, y <- transpose m2]
>     :
>>   5. It's very fast (being written in Haskell):
>>
>>      Performing  100,000,000  matrix  multiples takes  on  average
>>      1.435s  (with ghc  -O2).
> 
> This is surprising: a C program doing an explicit list of arithmetic
> operation is beaten by a factor of 4 or so by a polymorphic, lazy,
> list-consuming Haskell program?  Are you sure? What am I missing? 

Constant folding? The numbers certainly depend on the kind of the test 
program.