[llvm] [RISCV] Initial ISel support for the experimental zacas extension (PR #67918)

[Yingwei Zheng via llvm-commits]

dtcxzyw wrote:

> Please hold off on working on this patch. I'd like to experiment with a different way to do this.

@topperc Can I continue to work on this patch after https://github.com/llvm/llvm-project/pull/77418 is landed?

https://github.com/llvm/llvm-project/pull/67918