[llvm] [AArch64] Stop reserved registers from being saved in prolog/epilog (PR #138448)

[Peter Smith via llvm-commits]

smithp35 wrote:

I'd like to check with our GCC team about the behaviour of -ffixed particularly for a Callee-saved register. The GCC docs leave a lot of room for interpretation. 

At one level -ffixed-xN can be seen as "I'm globally reserving xN for the whole program" In which case it would make sense to exclude a registers from the callee saved set. An alternative interpretation is "I'm globally reserving xN for the functions in this translation unit" for example I am using binary library code that I don't control so I can't globally guarantee that xN is reserved. In that case it would make sense to still callee save the register, although it would mean that only temporary registers (like x18) can be used if we require no callee saves. 



https://github.com/llvm/llvm-project/pull/138448