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

Wed Sep 22 13:41:38 PDT 2021

spatel added a comment.

In D110170#3016261 <https://reviews.llvm.org/D110170#3016261>, @spatel wrote:

> In D110170#3015960 <https://reviews.llvm.org/D110170#3015960>, @nemanjai wrote:
>
>> Reduced test case at https://pastebin.com/NVYbsRfD
>>
>> Run `opt -O3 -mtriple=powerpc64le-- -disable-output file.ll` and it doesn't terminate.
>
> Thanks! It should be easy to find the opposing transform now. I got that test down to an infinite loop with -instcombine and:

That was easy (I hope)...
We had a transform that was creating extra instructions without checking uses correctly, so we'd end up ping-pong'ing between that and this one. I don't see infinite looping on either of the failure examples posted here after:
1cd6b44f267b <https://reviews.llvm.org/rG1cd6b44f267b256051e6106d9cf665f3bae2fd22>

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https://reviews.llvm.org/D110170