[RFC] Remove VRP threader passes in exchange for better threading pre-VRP.

[Aldy Hernandez]

On 10/18/21 3:41 PM, Aldy Hernandez wrote:

> I've been experimenting with reducing the total number of threading
> passes, and I'd like to see if there's consensus/stomach for altering
> the pipeline.  Note, that the goal is to remove forward threader clients,
> not the other way around.  So, we should prefer to remove a VRP threader
> instance over a *.thread one immediately before VRP.
> 
> After some playing, it looks like if we enable fully-resolving mode in
> the *.thread passes immediately preceeding VRP, we can remove the VRP
> threading passes altogether, thus removing 2 threading passes (and
> forward threading passes at that!).

It occurs to me that we could also remove the threading before VRP 
passes, and enable a fully-resolving backward threader after VRP.  I 
haven't played with this scenario, but it should be just as good.  That 
being said, I don't know the intricacies of why we had both pre and post 
VRP threading passes, and if one is ideally better than the other.

Aldy