[PATCH] D110170: [InstCombine] fold cast of right-shift if high bits are not demanded

[Bjorn Pettersson via Phabricator via llvm-commits]

bjope added a comment.

Hi @spatel,

I noticed a regression in a downstream benchmark, that at least partly seem to be caused by it. Here is a reduced example: https://godbolt.org/z/M9MKjcYPG

>From what I can see there is a quite early run of InstCombine in the O3 <https://reviews.llvm.org/owners/package/3/> pipeline, which basically happens directly after GlobalOpt without any CSE in between. So in such an early run of InstCombine we do trigger transforms based on "one use", which wouldn't have happened if running CSE before InstCombine. I figure that might be a more general problem and not only specific to the rewrites introduced in this patch.

We'll analyse the regression a bit more (maybe there are other things that happens that contributes to the regression). But wanted to mention the above. And it makes me a bit curious if it is a general problem with that early instcombine run that "one use" checks might be fooled by not having done CSE after GlobalOpt.


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https://reviews.llvm.org/D110170