[llvm-dev] Dataflow analysis regression in 3.7

Davide Italiano via llvm-dev llvm-dev at lists.llvm.org
Fri Jul 7 13:59:59 PDT 2017

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.


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