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[ARM] Remove dead mov's in preheader of tail predicated loops
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Authored by dmgreen on Nov 20 2020, 3:37 AM.

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Summary

With t2DoLoopDec we can be left with some extra MOV's in the preheaders of tail predicated loops. This removes them, in the same way we remove other dead variables.

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Event Timeline

dmgreen created this revision.Nov 20 2020, 3:37 AM
dmgreen requested review of this revision.Nov 20 2020, 3:37 AM
samtebbs added inline comments.Nov 20 2020, 6:04 AM
llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp
800

Is it guaranteed that these extra instructions are unused (and so it's ok to remove them without any extra RDA checks)?

dmgreen added inline comments.Nov 20 2020, 6:52 AM
llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp
800

I am hoping that is what TryRemove will do. It checks for things like corrupting IT blocks too.

I'm not super confident about this code late-backend dead code removal not causing some problems, but hope it's at least as sane as the existing checks. I have not seen any problems so far at least.

samparker added inline comments.Nov 20 2020, 7:06 AM
llvm/lib/Target/ARM/ARMLowOverheadLoops.cpp
809

TryRemove is, relatively, robust :) I think you should just be able to pass ToRemove in place of ElementChain here, though I can't immediately remember why that isn't the case for the loop above.

dmgreen updated this revision to Diff 320547.Feb 1 2021, 12:24 PM

Rebase and cleanup the ToRemove array

samparker accepted this revision.Feb 10 2021, 1:20 AM

cheers!

This revision is now accepted and ready to land.Feb 10 2021, 1:20 AM