[llvm] [AMDGPU][TTI] Add target hook for the custom instruction uniformity (PR #137639)

[Jay Foad via llvm-commits]

jayfoad wrote:

> The only way I see to achieve this is by passing the Uniformity of the operands to Targets and letting them define the custom rule.

Exactly, you would need some kind of target hook where the uniformity of the operands is passed in, or where the target hook can call back into UA to query the uniformity of operands.

> My concern is that it would be a bit expensive, also the decision of uniformity will be taken outside of UA.

I don't see why it would be too expensive. If it is then we should just give up on trying to implement this feature.

https://github.com/llvm/llvm-project/pull/137639