[llvm] Spill/restore FP/BP around instructions in which they are clobbered (PR #81048)

[via llvm-commits]

aengelke wrote:

Still, the CFI info looks wrong once the frame pointer is clobbered (updates to rsp *and* rbp should be accompanied by matching CFI directives) and I have concerns that this will break unwinding (not just asynchronous unwinding but also exceptions thrown from the called function). Could you test that unwinding through such a function call works as expected?

(fwiw, I'm not a maintainer and I'd personally like to see support for this use case, but I have concerns that this will break other things.)

https://github.com/llvm/llvm-project/pull/81048