[compiler-rt] [llvm] [RISCV] Allocate Feature Bits for Zilsd/Zclsd/Zcmp (PR #135197)

[Sam Elliott via llvm-commits]

lenary wrote:

> is it right to land this before kernel supports?

To provide some context I provided on the toolchains call, with assistance from Kito:

- Zilsd/Zclsd are 32-bit only, so may take some time to appear in linux, if they ever do.
- Zcmp is incompatible with Zcd, so may not appear in linux hwcaps if linux focuses on rv64gc (or the profiles that include C+D, which implicitly have Zcd)

These bitmaps have been proposed because the Hazard3 core wants to skip any heavyweight capabilites spec, and just directly implement the C API's bitmap layout in hardware. This core implements these three extensions, and it is perfectly possible for a SDK to expose something compatible with the C API without it being linux.

LLVM/GCC implementations are a prerequisite for allocating these bits in the C API, which is why we have this implementation.

https://github.com/llvm/llvm-project/pull/135197