[PATCH] D122900: [x86] Disable optimization involved in an infinite loop inside DAG combiner.

Sanjay Patel via Phabricator via llvm-commits llvm-commits at lists.llvm.org
Fri Apr 1 07:15:19 PDT 2022


spatel added a comment.

In D122900#3422344 <https://reviews.llvm.org/D122900#3422344>, @pgousseau wrote:

> In D122900#3422266 <https://reviews.llvm.org/D122900#3422266>, @lebedev.ri wrote:
>
>> Do you know which commit introduced the problem, or which transforms conflict?
>> This looks like a pretty heavy hammer.
>
> Thank you Roman for reviewing!
> The regression appeared after https://github.com/llvm/llvm-project/commit/8a156d1c2795189389fadbf33702384f522f2506 (thank you @wristow  for finding it)
> As @RKSimon  found out, the IR does not get generated anymore from the original C++ test case but the issue is still present.
> I am hopeful we can reintroduce the optimization (at a later codegen stage maybe?) but I would like to do some performance analysis first to see if it is still a win.

That change might have exposed the bug from source/IR, but the backend loop has existed since at least the 10.0 release based on a godbolt check:
https://godbolt.org/z/Tfnaevboz


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  https://reviews.llvm.org/D122900/new/

https://reviews.llvm.org/D122900



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