[PATCH] D75086: [AArch64][GlobalISel] Fixup <32b heterogeneous regbanks of G_PHIs just before selection.

[Amara Emerson via Phabricator via llvm-commits]

aemerson added a comment.

> I would expect G_PHI to be selected to PHI (i.e., just a opcode change) and then the phi elimination pass will insert the cross copies for you.
> 
> Where is this going wrong?

Thanks for taking a look. Yes the G_PHI gets selected to PHI. This is normally ok even in the case where we have operands with different regbanks. However on AArch64 this doesn't work when the type size is less than 32 bits. If this happens, the PHI operand with a GPR RegBank gets the gpr32 regclass assigned to it (we don't have anything smaller to represent types like s16 on GPR on AArch64). Then the FPR bank operand gets the appropriately sized FPR16 regclass assigned. These two registers have different sizes, and copyPhysReg doesn't know how to deal with copies between a gpr32 and an fpr16 or vice versa. I thought about maybe teaching copyPhysReg to deal with that, but the fact that we have to deal with subregisters (because of the differing sizes) makes me nervous and didn't seem like the best place to fix it.

I also tried to fix RegBankSelect to add fixups if we detect this case but this is an AArch64 specific issue, and it messed up a lot of existing PHI handling (which assumes we can just look at one operand to decide the RegBank).


Repository:
  rG LLVM Github Monorepo

CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D75086/new/

https://reviews.llvm.org/D75086