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

[Alexis Engelke via llvm-commits]

aengelke wrote:

This incurs a substantial compile-time regression at O0. http://llvm-compile-time-tracker.com/compare.php?from=ce2a3d9042c95630f12b790bf201c4daf8941afb&to=0d471b3f64d3116bd57c79d872f7384fff80daa5&stat=instructions:u

Can we come up with a no-overhead solution for this, e.g. check during call lowering whether there's a call that clobbers FP/BP and then selectively look at these calls instead of iterating over the entire IR? (IR iteration is expensive. And >99% of the code doesn't need this.) If this takes longer, we might want to temporarily revert this.

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