DMD compilation speed

[Ola Fosheim Grøstad via Digitalmars-d]

On Thursday, 11 February 2016 at 08:57:22 UTC, Andrea Fontana 
wrote:
> Zooming on graph you can see that the improvement was due to 
> this:
> https://github.com/D-Programming-Language/dmd/pull/4923

But why? Is it using the GC, or what?