On Mon, Oct 18, 2021 at 4:03 PM Aldy Hernandez <al...@redhat.com> wrote:
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>
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> On 10/18/21 3:41 PM, Aldy Hernandez wrote:
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> > 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.
It was done because they were different threaders. Since the new threader
uses built-in VRP it shouldn't really matter whether it's before or after
VRP _for the threading_, but it might be that if threading runs before VRP
then VRP itself can do a better job on cleaning up the IL.
+ /* ?? Is this still needed. ?? */
/* Threading can leave many const/copy propagations in the IL.
Clean them up. Instead of just copy_prop, we use ccp to
compute alignment and nonzero bits. */
Yes, it's still needed but not for the stated reason - the VRP
substitution and folding stage should deal with copy/constant propagation
but we replaced the former copy propagation with CCP to re-compute
nonzero bits & alignment so I'd change the comment to
/* Run CCP to compute alignment and nonzero bits. */
Richard.
> Aldy
>
