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

[Nemanja Ivanovic via cfe-commits]

nemanjai wrote:

> I have always been unconvinced that these are a good idea to have / add significant value over using inline assembly. IIRC Arm has them but nobody uses them?

Is this a comment about the general concept of builtins to produce specific instructions or about these specific ones?
For these ones, the patch isn't something I materialized out of thin air just because I think it's nice to have - it was in response to the very use case I described (from a library writer). They had something like:
```
void __attribute__((always_inline)) set_csr(unsigned CSRNum, unsigned Val) {
__asm__ volatile ...
```
Which works great if you're optimizing, but doesn't with -O0. Of course they could write it as a macro, but that has all the pitfalls of function-style macros.

Of course, if the consensus is to not provide these because of some reason, I'm fine with that - I would just like to understand the reason for the opposition to this.

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