Performance of loops

[Chris via Digitalmars-d]

On Friday, 24 April 2015 at 11:53:56 UTC, Marc Schütz wrote:
> Most of the time is taken up by the array's allocation. The 
> optimizers of both DMD and LDC evidently doesn't optimize it 
> away when you use `ref`, even though it could in theory.
>
> Remove the `enum` and just use a normal global variable, and 
> you will get more or less identical times for all three loops. 
> LDC even turns then into empty functions (didn't check for DMD).

I've tried it, and the loops indeed show similar results. What is 
it about `enum` that slows it down?