A simple way to do compile time loop unrolling

Peter Alexander peter.alexander.au at gmail.com
Fri May 31 10:30:10 PDT 2013


On Friday, 31 May 2013 at 14:06:19 UTC, finalpatch wrote:
> Just want to share a new way I just discovered to do loop 
> unrolling.
>
> template Unroll(alias CODE, alias N)
> {
>     static if (N == 1)
>         enum Unroll = format(CODE, 0);
>     else
>         enum Unroll = Unroll!(CODE, N-1)~format(CODE, N-1);
> }
>
> after that you can write stuff like
>
> mixin(Unroll!("v[%1$d]"~op~"=rhs.v[%1$d];", 3));
>
> and it gets expanded to
>
> v[0]+=rhs.v[0];v[1]+=rhs.v[1];v[2]+=rhs.v[2];
>
> I find this method simpler than with foreach() and a tuple 
> range, and also faster because it's identical to hand unrolling.

Remember that in D, most side-effect free functions can be run at 
compile time. No need for recursive template trickery:

mixin(iota(3).map!(i => format("v[%1$d]+=rhs.v[%1$d];", 
i)).join());


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