[llvm-branch-commits] [llvm] DAG: Avoid forming shufflevector from a single extract_vector_elt (PR #122672)

[Matt Arsenault via llvm-branch-commits]

arsenm wrote:

> > > * BuildVector w/one non-zero non-undef source, repeated 100 times (i.e. splat or select of two splats)
> > 
> > 
> > I don't follow, this is a 2 element vector, how can you have 100 variants?
> 
> Isn't the condition in code in terms of VecIn.size() == 2? I believe that VecIn is the _unique_ input elements, right? Which is distinct from the number of elements in the destination type? (Am I just misreading? I only skimmed this.)



> 
> > > If the target isn't optimally lowering the splat or select of splat case in the shuffle lowering, maybe we should just adjust the target lowering to do so?t
> > 
> > 
> > It's not a lowering issue, it's the effect on every other combine. We'd have to special case 1 element + 1 undef shuffles everywhere we handle extract_vector_elt now, which is just excessive complexity. #122671 is almost an alternative in one instance, but still shows expanding complexity of handling this edge case.
> 
> Honestly, #122671 (from the review description only) sounds like a worthwhile change. That's not a hugely compelling argument here. Let's settle the prior point, and then return to this. If I'm just misreading something, let's not waste time discussing this.




https://github.com/llvm/llvm-project/pull/122672