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[AArch64] Mark all instructions that read/write FPCR as doing so
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Authored by john.brawn on Mar 17 2022, 10:26 AM.

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Summary

All instructions that can raise fp exceptions also read FPCR, with the only other instructions that interact with it being the MSR/MRS to write/read FPCR.

Introducing an FPCR register also requires adjusting invalidateWindowsRegisterPairing in AArch64FrameLowering.cpp to use the encoded value of registers instead of their enum value, as the enum value is based on the alphabetical order of register names and now FPCR is placed between FP and LR.

This change unfortunately means a large number of mir tests need to be adjusted due to instructions now requiring an implicit fpcr operand to be present.

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Event Timeline

john.brawn created this revision.Mar 17 2022, 10:26 AM
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john.brawn requested review of this revision.Mar 17 2022, 10:26 AM
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simon_tatham added inline comments.Mar 18 2022, 3:27 AM
llvm/lib/Target/AArch64/AArch64FrameLowering.cpp
2473

I don't know this code well, but just looking at this if statement in isolation, in the case where Reg1==FP, the previous version of the condition would have required Reg2==LR, whereas this one will accept either Reg2==LR or Reg2=FPCR.

Or is there some condition at the call sites which means nothing but GPRs will ever be passed to this function in the first place, so that the (FP,FPCR) pair will never trigger that false positive?

john.brawn planned changes to this revision.Apr 6 2022, 7:44 AM

This is interacting badly with MachineCSE, as the presence of a physical register prevents it from optimising in some cases. I think the fix is in MachineCSE, but removing this patch from review while I look at it.

john.brawn edited the summary of this revision. (Show Details)

Rebased. Also adjust invalidateWindowsRegisterPairing to use encoded register numbers which should be more robust.

john.brawn updated this revision to Diff 472938.Nov 3 2022, 9:01 AM
john.brawn set the repository for this revision to rG LLVM Github Monorepo.

Rebased again.

simon_tatham accepted this revision.Nov 15 2022, 3:48 AM
This revision is now accepted and ready to land.Nov 15 2022, 3:48 AM