[PATCH] D41605: StructurizeCFG: Fix broken backedge detection

[Matt Arsenault via Phabricator via llvm-commits]

arsenm added a comment.

In https://reviews.llvm.org/D41605#964650, @dberlin wrote:

>




> You would currently detect this as some form of loop, but again, most of the algorithms used here do not look like the ones i'm familiar with (I feel like i can find a ton of edge cases in it).

There are definitely a lot of strange things going on here and I don't really understand how this manages to work at all. I think part of the problem was using RegionInfo for this was a mistake, and this should probably be rewritten to not depend on it. For example because the pass itself only sees one region at a time, it won't even see the example you posted as something it can try to process. There isn't a well formed single exit region, so I think the pass will just see the function top level region and bail out. I've variants of the same problem pop up from time to time. Fundamentally using an analysis to look at well formed regions probably doesn't make sense for a pass which is trying to produce well formed regions. I've also been surprised by some of the visitation order decisions for the various graph iterators with RegionNode compared to the same orders over the entire function. The various places that need to differently handle subregion nodes vs. individual blocks also seems more confusing to me.


https://reviews.llvm.org/D41605