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

[Nemanja Ivanovic via Phabricator via llvm-commits]

nemanjai added a comment.

In D110170#3014861 <https://reviews.llvm.org/D110170#3014861>, @spatel wrote:

> In D110170#3014407 <https://reviews.llvm.org/D110170#3014407>, @mstorsjo wrote:
>
>> I also run into infinite loops caused by this commit. It's reproducible with https://martin.st/temp/qsettings-preproc.cpp with `clang -target x86_64-w64-mingw32 -c qsettings-preproc.cpp -O3 -std=c++17`. But that's not minimzed/reduced at all (and that source file takes a pretty significant amount of time to compile even to begin with).
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> In D110170#3014232 <https://reviews.llvm.org/D110170#3014232>, @nemanjai wrote:
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>> This is causing timeouts on a number of multistage builds. I have seen it at least on PPC, SystemZ and AArch64:
>> PPC: https://lab.llvm.org/buildbot/#/builders/121/builds/11680
>> PPC: https://lab.llvm.org/buildbot/#/builders/36/builds/12596
>> SystemZ: https://lab.llvm.org/buildbot/#/builders/8/builds/1821
>> AArch64: https://lab.llvm.org/buildbot/#/builders/179/builds/1073
>>
>> The timeout happens when the stage 1 compiler is building `lib/Transforms/Coroutines/CoroFrame.cpp`
>> I'll see if I can track down what is causing the infinite loop/recursion.
>
> Thanks for letting me know. I reverted the patch (and the follow-up that fixed a clang test failure at 52832cd917af0 <https://reviews.llvm.org/rG52832cd917af00e2b9c6a9d1476ba79754dcabff>), so I'm not holding up those bots while we investigate. I'm trying to reduce Martin's file now, but it is taking a while! If anyone finds a smaller example, please do post it here.

I am running a reducer on the CoroFrame.cpp one and it is pretty small so far and still reducing. I'll provide it once it is done in case it turns out to be useful.


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  https://reviews.llvm.org/D110170/new/

https://reviews.llvm.org/D110170