[llvm] [RISCV] Add SiFive X390 scheduling model (PR #143938)

[Pengcheng Wang via llvm-commits]

https://github.com/wangpc-pp commented:

Just a general comment: would the abstraction make the code hard to maintain compared to put X380 model into a standalone file?

https://github.com/llvm/llvm-project/pull/143938