Slow performance compared to C++, ideas?

[bearophile]

finalpatch:

> I really like the features offered by D but it's the raw 
> performance that's worrying me.

 From my experience if you know what you are doing, you are able 
to write that kind of numerical D code that LDC compiles with a 
performance very close to C++, and sometimes higher. But you need 
to be careful about some things.

Don't do this:
foreach (y; (iota(height)))

Use this, because those abstractions are not for free:
foreach (y;  0 .. height)

Be careful with foreach on arrays of structs, because it perform 
copies that are slow if the structs aren't very small.

Be careful with classes, because on default their methods are 
virtual. Sometimes in D you want to use structs for performance 
reasons.

Sometimes in inner loops it's better to use a classic for instead 
of a foreach.

LDC needs far more flags to compile a raytracer well. LDC even 
support link time optimization, but you need even more obscure 
flags.

Also the ending brace of classes and structs doesn't need a 
semicolon in D.

Bye,
bearophile