[PATCH] D25347: [VirtRegRewriter] Eliminate COPYs before re-writing by renaming.

[Geoff Berry via llvm-commits]

gberry added a comment.

In https://reviews.llvm.org/D25347#563996, @qcolombet wrote:

> Hi Geoff,
>
> Disclaimer: I haven't looked at the patch, just a comment.
>  I would rather fix the MachineCopyPropagation pass than adding this logic to this pass.
>
> Why aren't we catching those cases in the MachineCopyPropagation pass?


Hi Quentin,

I believe this was discussed before here: https://reviews.llvm.org/D20531.  In this change, the COPYs only become removable as the result of renaming/recoloring their destination registers.  I thought the consensus was that this recoloring was only safe to do with virtual registers to avoid violating ABI/other register constraints.  The COPYs that this change catches are mostly only removable after register allocation since they are only removable once live range splitting/spilling has occurred.

In https://reviews.llvm.org/D25347#563996, @qcolombet wrote:

> Hi Geoff,
>
> Disclaimer: I haven't looked at the patch, just a comment.
>  I would rather fix the MachineCopyPropagation pass than adding this logic to this pass.
>
> Why aren't we catching those cases in the MachineCopyPropagation pass?
>
> Cheers,
> -Quentin





https://reviews.llvm.org/D25347