[llvm-dev] Spurious peeling in simple loop unrolling

[Thomas Preud'homme via llvm-dev]

On 10/03/2021 10:58, Florian Hahn wrote:


On Mar 9, 2021, at 13:13, Thomas Preud'homme <thomasp at graphcore.ai<mailto:thomasp at graphcore.ai>> wrote:

On 08/03/2021 16:50, Florian Hahn wrote:


On Mar 4, 2021, at 15:53, Thomas Preud'homme <thomasp at graphcore.ai<mailto:thomasp at graphcore.ai>> wrote:
2. Instcombine before peeling already simplifies the IR so that both incoming values are bit casts of the same value. It probably would be trivial to also have instcombine simplify pointer phis if the incoming values striped by pointer casts are equal. (There might be some other reason why we are not doing this at the moment though).


As mentioned above, it's not as simple as bitcast of the same pointer so this would not work here. One would have to go look at whether the loads are equivalent which is a more involved check.


That’s true, but I think instcombine already CSE’d the loads. So if we simplify such phis, the unnecessary peeling should not happen https://reviews.llvm.org/D98058


I tried the patch (thanks) but that did not remove any of the PHI (the 2 loads are still there and thus the bitcast don't appear to have the same source). I'll try tolook at InstCombine to see why loads are not CSE'd.



I’m not sure I follow here. For your example (spurious_loop_peeling.cpp), it looks like there’s no peeling happening any more after the patch landed, at least when building for ARM64: https://godbolt.org/z/q6d6Kn . Is there anything else that’s going wrong?


The testcase I sent is indeed fixed with your commit. However the code it is inspired from still shows unwanted peeling. I'm going to investigate what causes the difference.


Best regards,


Thomas
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