How to add RVE to the LLVM

Now I’m doing something about rve. it seems like rve is not supported in llvm upstream.
How can I add the rve to the LLVM backend? I have try to learn the source code,but it seems really hard to find the specific position to add rve.
Did there have some useful examples or some specific patches which were in the repositories that I can learn or imitate? thanks
If the llvm could support the rve someday is great! :-)

The issue names no files, tests, or entry points. Start by locating the RISC-V backend and comparing existing extension implementations or relevant upstream patches; done would require a concrete, reviewed path for adding RVE support to LLVM.

Written by the indexing model from the issue text.