[clang] [X86] Emit Warnings for frontend options to enable knl/knm. (PR #75580)

[Simon Pilgrim via cfe-commits]

RKSimon wrote:

I think if we have an approach that allows people to emulate a very basic KNL/KNM implementation with the equivalent of "-march=x86-64-v3 -mavx512f -mavx512cd" then that would be sufficient. I do think we should be keeping -march/tune support though for the knl/knm cpu model names, but we shouldn't need to support the xeon phi specific ISAs.

We should keep asm handling for avx512er/avx512pf/etc but no need for attributes/intrinsics handling for them - if somebody needs to write assembly for them we shouldn't prevent it.

We should retain -march=native detection if we can, but not mandatory.

I also think we need a policy regarding what test coverage we need for various avx512 features (when should we assume avx512vl etc.)

Does that sound OK?

https://github.com/llvm/llvm-project/pull/75580