[llvm] [RISCV] Add software pipeliner support (PR #117546)

[Pengcheng Wang via llvm-commits]

wangpc-pp wrote:

> Out of curiosity, have you seen any benefits of adding and enabling this support?

I haven't tried current implementation, but we did implement it and saw very small performance gain on an OoO core before (that was about 3 years ago, on LLVM 13).
Situation may have changed during these years and AAch64 added this support recently (about one year ago, not that *recently*) and disabled by default. It seems some ARM cores can benefit from it.
Currently, this implementation is focusing on correctness, not performance. There are some rooms to improve and we may enable it via subtarget feature some day.

https://github.com/llvm/llvm-project/pull/117546