[llvm] [GlobalIsel][AArch64] Add ADDO combine to the postlegalizer combiner (PR #101327)

[via llvm-commits]

KRM7 wrote:

> > > > > This sounds like an interesting optimization. Why doesn't it happen already after legalizing the add?
> > > > 
> > > > 
> > > > Where/why would you expect this to already happen?
> > > 
> > > 
> > > The legalizer will split the ADD into G_UADDO and G_UADDE. A post legalizer combiner should then be able to see that the G_UADDO created for the lower half has an all 0 input. That's how it's handed in SelectionDAG. Maybe such a post legalizer combine is missing for G_UADDO?
> > 
> > 
> > We rely on
> > https://github.com/llvm/llvm-project/blob/5dfdac74cadd9483a66eb17e51dc632b554cccb1/llvm/lib/CodeGen/GlobalISel/CombinerHelper.cpp#L7215
> 
> So we need this change?
> 
> ```
> diff --git a/llvm/lib/Target/AArch64/AArch64Combine.td b/llvm/lib/Target/AArch64/AArch64Combine.td
> index 3f717c8a6005..90cb004dcebd 100644
> --- a/llvm/lib/Target/AArch64/AArch64Combine.td
> +++ b/llvm/lib/Target/AArch64/AArch64Combine.td
> @@ -322,5 +322,5 @@ def AArch64PostLegalizerCombiner
>                          select_to_minmax, or_to_bsp, combine_concat_vector,
>                          commute_constant_to_rhs,
>                          push_freeze_to_prevent_poison_from_propagating,
> -                        combine_mul_cmlt]> {
> +                        combine_mul_cmlt, match_addos]> {
>  }
> ```

Yep, adding this was enough.

https://github.com/llvm/llvm-project/pull/101327