Why is this D code slower than C++?

[Steve Horne]

On Wed, 17 Jan 2007 11:18:10 -0800, Bradley Smith
<digitalmars-com at baysmith.com> wrote:

>Thanks for all the suggestions. It helps, but not enough to make the D 
>code faster than the C++. It is now 2.6 times slower. The render times 
>are now approx. 13 sec for raytracer_d and approx. 5 sec for raytracer_cpp.

...

>
>Any other suggestions?

I haven't actually looked at the code, but I'll take a guess anyway.

Raytracing is heavy on the floating point math. As Walter Bright
acknowledges, the DMD compiler does not handle the optimisation of
float arithmetic as well as some C++ compilers.

You could try the GNU D compiler - GDC. Since it is using the standard
GNU compiler suite backend code generator, it will probably handle the
optimisation better.

A second option is to split out some key inner-loop calculations and
handle them in C, using D for the less performance-sensitive code.
Calling C code from D is easy enough, though calling C++ is more of a
hassle. This hack could be considered temporary, as the D float
performance will no doubt be improved in time.

Alternatively, if you don't mind losing portability, you could try
using inline assembler for those key inner-loop calculations. If
you're a real speed freak, you might even try using SIMD instructions
to get 4 float calculations per instruction (and IIRC most SIMD
instructions complete in a single clock cycle these days). The down
side to that would be lower floating point precision, but for
raytracing I wouldn't expect that to be a big deal.

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