[llvm] [Codegen] Spill/Restore FP/BP under option (PR #114791)

[Reid Kleckner via llvm-commits]

rnk wrote:

Stack misalignment suggests there could be a bug in how we compute stack alignment here:
https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/X86/X86FrameLowering.cpp#L4240

If stack accesses still use RSP, that suggests that RBP (FP) is in use, but not RBX (BP). So, a possible reproducer would be a big inline asm blob that clobbers RBP and accesses a highly aligned vector alloca, like a __m512* variable.

https://github.com/llvm/llvm-project/pull/114791