[llvm] Expanding the Histogram Intrinsic (PR #127399)

[Graham Hunter via llvm-commits]

huntergr-arm wrote:

> > What will the strategy to lower those intrinsics? Doe the default lowering need updating?
> > It would be good to add the intrinsics together with initial lowering as separate patch, including codegen tests.
> > Then extend LV to use them, which will likely also need updating to the cost model?.
> 
> Hey Florian,
> 
> I'm working on out of tree target, so there will not be upstream targets that can lower these intrinsics. About the cost-model I didn't change anything related to the vectorization of histogram so it uses the same one as before. I prefer to upstream these changes in case any other will benefit from this histogram variations.

Hi,

I think the default lowering (`scalarizeMaskedVectorHistogram`, in ScalarizeMaskedMemIntrin.cpp) should be extended to support these new cases. It just performs scalarization of the vector code, so would need updating to check for the new intrinsics and plant the appropriate scalar update operation. No need to have dedicated target support.

Right now that won't be used outside of unit tests, but I will implement the target-independent cost model to enable it at some point.

https://github.com/llvm/llvm-project/pull/127399