Array assigns performance

[Robert Jacques]

On Thu, 02 Jun 2011 10:36:54 -0400, bearophile <bearophileHUGS at lycos.com>  
wrote:
> In some (not benchmark) D2 code I have this (variable names changed):
>
> int[2][8] array;
> ...
> array[i] = [x, y];
>
>
> Such lines of code are not inside the inner loop, that's in the "..."  
> part. Despite they are not in the inner loop I've seen that replacing  
> this line:
> array[i] = [x, y];
>
> with:
> array[i][0] = x;
> array[i][1] = y;
>
> reduces the run time from about 1.8 seconds to about 1.05 seconds :-)
>
> If I change the matrix type like this:
> int[][8] array;
>
> The runtime goes to about 1.55 seconds, because using dynamic arrays  
> just for two integers is not so efficient for other parts of the code.
>
> (I have also tried to replace the int[2] with a Tuple!(int,int) but this  
> seems to give small problems elsewhere in my code, I have not tried this  
> hard enough yet.)
>
> I'd like DMD to compile and perform this very simple line of code in a  
> much more efficient way:
> array[i] = [x, y];
>
> Bye,
> bearophile

bearophile, array literals default to dynamic arrays, which require an  
allocation, etc. IIRC, originally, literals specified fixed sized arrays,  
but was considered unexpected behavior and reduced usability and was  
changed.