[mlir] [llvm] [compiler-rt] [clang] [clang-tools-extra] [libcxxabi] [flang] [openmp] [lldb] [libcxx] [MachineCopyPropagation] When the source of PreviousCopy is undef, we cannot replace sub register (PR #74682)

[via cfe-commits]

DianQK wrote:

> I don't believe the undef is the issue - I think the issue is that AArch64InstrInfo::isCopyInstrImpl is saying that a W-reg orr is a copy, even if it is really a zextend because the entire X output register is depended upon.

Thanks for the explanation.

> Can you try and add something to isCopyInstrImpl instead, that says: If the register is virtual then it should not be a subreg, and if the register is physical then there should not be an implicit def of the X reg. Would that solve your issue, and would it cause other problems in AArch64 codegen? Thanks

I'm not sure if I'm misunderstanding anything. But I tried to file the #75184 PR.

It breaks the original commit https://github.com/llvm/llvm-project/commit/53726588f672a915c6f907569356091552500f23. It looks like it's because we're generating the mov instruction for the ORRWsr. (At this point this is a copy instruction?) I'm not sure exactly what is going wrong. Maybe we need to add a new method to TargetRegisterInfo?

Or are you going to fix this issue? I'm not familiar with MIR.
Alternatively, we can add implicit-def to MachineCopyPropagation? But that doesn't seem right either.


https://github.com/llvm/llvm-project/pull/74682