[llvm] [RISCV][WIP] Let RA do the CSR saves. (PR #90819)

[Francis Visoiu Mistrih via llvm-commits]

francisvm wrote:

> I fought some old post about improving shrink wrapping. https://lists.llvm.org/pipermail/llvm-dev/2017-August/116137.html not sure how much of it was implemented.
> 
> CC @francisvm

Not much of it was implemented in the end. It turned out that for us, the compile-time impact, the unwinding impact and the various regressions were not worth the speedup in the few benchmarks. I wish I had the old data to justify this, but that was a while ago. The unwinder impact was more significant than I initially thought at that time, since Apple platforms rely heavily on compact unwinding, therefore missing some frames and falling back to eh/debug_frame CFI is a huge penalty, and in some memory/permission-constrained environments, very very hard, or impossible for those that only look at prologues/epilogues to quickly unwind (some crash reporters, for example). That made enabling it by default a hard task, and we never pursued an attribute/flag-based approach.

https://github.com/llvm/llvm-project/pull/90819