[llvm-dev] Dataflow analysis regression in 3.7

[Davide Italiano via llvm-dev]

On Fri, Jul 7, 2017 at 1:47 PM, Chad Rosier <mcrosier at codeaurora.org> wrote:
> David/Johan,
>
> I would love to claim victory, but I don't think that D34901 catches this
> case.
>

Hi Chad, thanks for taking another look at this.
Maybe I didn't bisect correctly. Apologies. Anyway, more fun for us.

> However, I got interested and threw this together quickly:
> https://reviews.llvm.org/D35140.
>
> This does catch the below case.  If people are interested I can add test
> cases and submit for formal review.  FWIW, it does hit about 1/3 of all of
> the SPEC benchmarks.  I haven't done any performance analysis to see if it
> matters, however.
>

As I stated earlier, I think this calls for a slightly better VRP
[and/or improvements in VRP] & I'm not entirely sure `InstCombine` is
the best place for this transform.
OTOH, the fact this triggers a lot is a good thing, if your WIP patch
actually results in performance improvements, we should consider a
more general solution to address this kind of issues.

--
Davide