Why is this D code slower than C++?

[Bill Baxter]

Bradley Smith 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.
> 
> Here are the changes I've made. Attached is the new code.
> 
>   Call RegisterClass outside of assert. (Broken if -release used)
>   Apply -release option. (Increases speed in an unknown way)
>   Converted templates to regular functions. (Templates not being inlined)
>   Manually inlined DOT function. (Function not being inlined)

You left out changing Intersect's Ray argument to be inout.  And 
generally all Ray (and possibly vector3 parameters) to be inout to avoid 
  the cost of copying them on the stack.

Also converting vector expressions like
       vector3 v = a_Ray.origin - m_Centre;
to
       vector3 v = a_Ray.origin;
       v -= m_Centre;

makes a difference.  Changing that one line in the Sphere.Intersect 
routine changes my runtime from 12.2 to 14.3 sec.

Interestingly the same sort of transformation to the C++ code didn't 
seem to make much difference.  It could be related in part to the C++ 
vector parameters on the operators all taking const vector& (references) 
vs the D ones being just plain vector3.  Chaging all the operators in 
the D version to inout may help speed too.

With those changes on my Intel Xeon 3.6GHz CPU the run times are about 
10.1 sec vs 12.2 sec.  D still not as fast as the C++, but close.

--bb