Fast hashtable

[Daniel Kozak via Digitalmars-d]

On Wednesday, 1 March 2017 at 06:44:34 UTC, Cecil Ward wrote:
>
> I liked that article. I didn't really understand the point 
> about implementation of modulo primes, maybe I missed 
> something. Given that our man is doing modulo a 'known' value 
> (he had a switch statement to get to them), why not do 
> something rather cheaper than a compiler-expanded constant 
> div/mod made up of multiplies and shifts
>
>     const uint power2 = 512; // say, some 1 << n anyway
>     const uint prime = 509; // some prime just below the power, 
> some prime > power2/2
>
>     static assert( power2 - 1 - prime < prime );
>
>     x = x & ( power2 - 1 );
>     x = ( x >= prime ) ? x - prime : x;
>
> which is good news on my x86 with GDC -O3 (only 3 operations, 
> and sub cmovx ) - all well provided you make sure that you are 
> getting CMOVx not branches. I could work out the power from the 
> prime using CTFE given a bit of thought. Maybe CTFE could even 
> do the reverse?
>
> Have I finally gone mad?

Yes :D, this is something compiler should do.

btw: https://github.com/dlang/phobos/pull/1452