[PATCH] D62625: LFTR for multiple exit loops

[Philip Reames via Phabricator via llvm-commits]

reames added a comment.

In D62625#1552433 <https://reviews.llvm.org/D62625#1552433>, @SjoerdMeijer wrote:

> Hello,
>  I am going to be a bit unhelpful here. We see some regressions caused by this change, but I didn't have a chance to look into it today and so I don't have a reproducer. And here's my unhelpful question: are you aware or have you seen regressions, or are you still working on this? I am not really familiar with this pass, but from reading the description, it should do good things. Taking a wild guess, I see changes in the tests and they change comparisons from ult to ne, could it for example be that this confuses other passes/analysis?
>  Anyway, I will try to look into this tomorrow.
>  Cheers.


Just to be clear, you mean performance regressions correct?  Not correctness ones?

I won't be surprised if this exposes weaknesses in other passes.  If you can find me examples, I'm happy to go about trying to fix them.  If there's too many, or the regressions are too large, we can also disable this behind a flag while we work through issues.

One case I know of is that you have a hot loop which runs very few iterations, this could end up being a regression.  I have ideas on how to address that, but since they're involved, I'd really like to confirm a test case first.

Look forward to more details when you have them.


Repository:
  rL LLVM

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D62625/new/

https://reviews.llvm.org/D62625