[PATCH] D36107: [PostDom] document the current handling of infinite loops and unreachables

[Tobias Grosser via llvm-commits]

On Mon, Jul 31, 2017, at 22:39, Tobias Grosser via llvm-commits wrote:
> On Mon, Jul 31, 2017, at 22:29, Jakub Kuderski via Phabricator via
> llvm-commits wrote:
> > kuhar added a comment.
> > 
> > In https://reviews.llvm.org/D36107#826573, @grosser wrote:
> > 
> > > we have little tests that verify the update behavior of PDT after changes that introduce/remove virtual nodes. This is shown by the fact, that the first test case (construction of unreachables) illustrates a bug where we forget to add a new virtual exit node.
> > >
> > > I would like to commit this change, to have some more test coverage for https://reviews.llvm.org/D36107 and especially to make the impact of https://reviews.llvm.org/D36017 more clear.
> > 
> > 
> > Just to clarify a bit: the current behavior posdominator updates is just
> > broken when it comes to root handling -- it just happens that the
> > verifier is broken in an analogous way and blindly trusts the set of
> > roots inside a dominator tree. If for some reason we wanted keep not
> > including infinite loops in postdominators, then it would be still
> > possible to fix the root-finding behavior on updates. I mean, this issue
> > is a bit orthogonal to what https://reviews.llvm.org/D36017 tries to do.
> 
> Sure. The goal of this patch was nothing else but to show the general
> brokenness we see here. While looking through your patch closely and
> trying to explore the current and future behavior some of this
> brokenness popped up. I thought I better post these test cases as they
> might help to fix future bugs and as they also will document well the
> direction we are going.
> 
> I fully agree that the incorrect update behavior for unreachable
> instructions is mostly independent of the goal of connecting infinite
> loops to virtual exits.

Ah, the reason why https://reviews.llvm.org/D36017 is related is that it
provides a possible solution to consistently handle unreachables and
infinite loops.

Best,
Tobias