[PATCH] D36404: Disable jump threading into loop headers

[Balaram Makam via Phabricator via llvm-commits]

bmakam added a comment.

In https://reviews.llvm.org/D36404#870092, @hfinkel wrote:

> In https://reviews.llvm.org/D36404#869695, @bmakam wrote:
>
> > In https://reviews.llvm.org/D36404#869657, @kparzysz wrote:
> >
> > > Have you tried it?  Changing jump threading to have early/late versions is trivial, the question is when should the late one run, and if that would help in the first place.
> >
> >
> > I reverted this change and it recovered the regression. I was going to try to re-run the whole jumpthreading pass after latesimplifycfg to see if it would help, but I don't know if it is a good place because LSR that runs after, will not have the loop in canonical form and loopsimplify will again turn it into irreducible loop. Another place I was thinking was to try before CGP.
>
>
> If it'll work right before CGP, that's probably best. That way targets can run IR-level passes that want to look at loops before that.


I tried right after latesimplifycfg and it almost recovers the regression. However, running it right before CGP made it even worse for performance of spec2017/perlbench. It seems to be due to some kind of bad interaction with LSR, I haven't digged deeper. Would running right before LSR be reasonable?


Repository:
  rL LLVM

https://reviews.llvm.org/D36404