[llvm] [PPC][AIX] Save/restore r31 when using base pointer (PR #100182)

[Sean Fertile via llvm-commits]

mandlebug wrote:

Thanks for the insight Zheng - I never considered that we would be wasting an allocatable register. I think the new direction alleviates my concerns as well.

https://github.com/llvm/llvm-project/pull/100182