[llvm] [AArch64][SVE] Avoid extra pop of "FixedObject" with FPAfterSVECalleeSaves (PR #156452)

[Benjamin Maxwell via llvm-commits]

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@@ -2587,15 +2595,22 @@ void AArch64FrameLowering::emitEpilogue(MachineFunction &MF,
     // may clobber), convert it to a post-index ldp.
     if (OffsetOp.getImm() == 0 && AfterCSRPopSize >= 0) {
       convertCalleeSaveRestoreToSPPrePostIncDec(
-          MBB, Pop, DL, TII, PrologueSaveSize, NeedsWinCFI, &HasWinCFI, EmitCFI,
-          MachineInstr::FrameDestroy, PrologueSaveSize);
+          MBB, Pop, DL, TII, ProloguePopSize, NeedsWinCFI, &HasWinCFI, EmitCFI,
+          MachineInstr::FrameDestroy, ProloguePopSize);
+    } else if (FPAfterSVECalleeSaves) {
+      // If not, and FPAfterSVECalleeSaves is enabled, deallocate callee-save
+      // non-SVE registers to move the stack pointer to the start of the SVE
+      // area.
+      emitFrameOffset(MBB, std::next(Pop), DL, AArch64::SP, AArch64::SP,
+                      StackOffset::getFixed(ProloguePopSize), TII,
+                      MachineInstr::FrameDestroy, false, NeedsWinCFI,
+                      &HasWinCFI);
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MacDue wrote:

I've been unable to come up with a reasonable test for this case. With `!FPAfterSVECalleeSaves` we only get here if:
- We have hazard padding (not applicable)
- Or use some somewhat niche `swiftcc` or `tailcc` functionality, which I'm not sure works with SVE.

I wonder if it'd be okay to just do `if (FPAfterSVECalleeSaves) reportFatalInternalError(...)` instead?

https://github.com/llvm/llvm-project/pull/156452