[llvm-dev] Spurious peeling in simple loop unrolling

Thomas Preud'homme via llvm-dev
2021-03-10T14:30:35Z

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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