[PATCH] D29866: [PDSE] Add PDSE.

Daniel Berlin via llvm-commits llvm-commits at lists.llvm.org
Tue Jun 20 04:03:51 PDT 2017


On Tue, Jun 20, 2017 at 12:26 AM, Tobias Grosser <tobias at grosser.es> wrote:

> 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.


I'm not sure why you put broken in quotes?


> 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.
>

Jakub has a verifier he's about to post that will verify the structural
properties of the trees are valid.
If you can make your version/tradeoffs verify, have at it.


>
> 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
> > _______________________________________________
> > llvm-commits mailing list
> > llvm-commits at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-commits
>
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