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

Wed Sep 22 04:50:32 PDT 2021

spatel added a comment.

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).

In D110170#3014232 <https://reviews.llvm.org/D110170#3014232>, @nemanjai wrote:

> 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.

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https://reviews.llvm.org/D110170