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

[Reid Kleckner via llvm-commits]

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@@ -4231,3 +4231,325 @@ void X86FrameLowering::restoreWinEHStackPointersInParent(
                                   /*RestoreSP=*/IsSEH);
   }
 }
+
+static int computeSPAdjust4SpillFPBP(MachineFunction &MF,
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rnk wrote:

Please use a more descriptive name and add more comments about what this is computing. I came up with something like `computeFPBPAlignmentGap`, and then would describe the alignment gap as the difference between the current SP and the next properly aligned stack offset.

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