Lars Kyllingstad wrote:
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.
My experience is that using indexing is faster in DMD. I don't know why.
To compare D and C, compile your C code with DMC. Then you'll get the
same code generation capability. DMD and DMC are extremely weak on
floating-point optimisation. You shouldn't see any difference between them.
