How are 2D static arrays allocated?

[Lars Kyllingstad]

Lars Kyllingstad wrote:
> Jarrett Billingsley wrote:
>> Performance of dynamic arrays is the same no matter where their data
>> is.  Fixed-size 2D arrays are not faster _because_ they are on the
>> stack, they just happen to be allocated on the stack.  They are faster
>> (usually) because they don't need two pointer dereferences.  You can
>> allocated a fixed-size 2D array on the heap (well.. inside a struct or
>> class anyway) and it will be just as fast.
> 
> Your "usually" interests me. I was under the impression, and it seems 
> quite logical, that static arrays are faster than dynamic ones. However, 
> recently I did some simple experiments with matrix operations 
> (multiplication, etc.), in which performance was actually a little bit 
> better for dynamic arrays.
> 
> Is there a general "rule" for when dynamic arrays are faster than static 
> ones, and vice versa?
> 
> Also, I tried multiplying large matrices both using arrays in D and 
> using a C BLAS implementation. The BLAS algorithm is the same as the one 
> I've written in D, and additionally includes a lot of runtime checks. 
> Still, multiplication using BLAS was about four times faster. Is this 
> simply because C compilers are a lot better at optimisation than the D 
> compilers?

I just remembered that D also does bounds checking on arrays. To work 
around this, I tried accessing the array elements through pointers. To 
my surprise, this actually didn't affect the results very much.

Therefore my question still stands.

-Lars