[PATCH] D103755: [DAG] Fold neg(bvsplat(neg(x)) -> bvsplat(x)

[Dave Green via Phabricator via llvm-commits]

dmgreen added a comment.

In D103755#2800920 <https://reviews.llvm.org/D103755#2800920>, @RKSimon wrote:

> Funnily enough I was wondering about this pattern the other day as a followup to D98778 <https://reviews.llvm.org/D98778>...
>
> Should we always be folding unaryop(splat(x)) -> splat(unaryop(x)) if the unaryop is legal/custom on the scalar type? And then maybe extend that to binop(splat(x),splat(y)) -> splat(binop(x,y)) as well?

Maybe. Not sure. I always find it difficult to see when optimizations like that would be universally beneficial over a wide range of very different architectures, considering how different they can be. I can see that it would make this more general, but it seems easy to think of cases where it would make things worse.

For this case I think it would need to work with the truncated type, not the scalar element type. For a v8i16 the scalar element type would not be legal under AArch64. If it was using the element type it would only handle half of the tests changed here.


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