[PATCH] D29866: [PDSE] Add PDSE.

[Tobias Grosser via llvm-commits]

I think the PDT brokenness is something all of us want to see resolved.
I just had an interesting discussion with Hal how currently the PDT is
not usable to test the validity of liferange metadata.

I think it would be great if we collect the use case we have for
dominator trees to clearly see where they are "broken" today and which
solution could address this brokenness. With Jakub Kuderski pushing
non-trivial improvements to dominance relations in LLVM we seem to have
a unique chance to get this fixed and well documented in a way that
either satisfies all of us -- or at least clearly documents the
tradeoffs that have been taken in the final result.

I already started once a document discussing post-dominance literature,
but less so use cases. Maybe Jakub would like to collect the different
use cases if/when he is touching post-dominance.

Best,
Tobias

On Mon, Jun 19, 2017, at 09:00 AM, Davide Italiano via llvm-commits
wrote:
> On Sun, Jun 18, 2017 at 11:56 PM, Daniel Berlin <dberlin at dberlin.org>
> wrote:
> >>
> >>
> >> ================
> >> Comment at: lib/Transforms/Scalar/PDSE.cpp:920
> >> +    if (!PDT.getRootNode()) {
> >> +      DEBUG(dbgs() << "FIXME: ran into the PDT bug. nothing we can
> >> do.\n");
> >> +      return false;
> >> ----------------
> >> Can you please describe the `PDT` bug? I'm not really familiar with it (or
> >> I call it with another name :)
> >>
> >
> > This is because the postdominator tree is *badly* broken in a myriad of
> > ways.
> >
> > It's possible to hit this particular issue in a number of ways.
> >
> > In practice, we are going to have to fix post-dominators before we ever turn
> > on this pass.
> >
> 
> Oh. I'm aware of the PDT brokeness (I also have some examples) but I
> thought you're referring to something specifically :)
> 
> Thanks!
> 
> --
> Davide
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