[clang] [llvm] [RISC-V] Add CSR read/write builtins (PR #85091)

[Jessica Clarke via cfe-commits]

jrtc27 wrote:

I guess my underlying point is that Arm's ACLE provides functions like __arm_rsr, but I'm not aware of them really being used, with inline assembly being the far more common alternative, so what's the real point of providing an interface that developers have already demonstrated they won't generally use?

https://github.com/llvm/llvm-project/pull/85091