[llvm] [AArch64][SME] Fix restoring callee-saves from FP with hazard padding (PR #143371)

[Sander de Smalen via llvm-commits]

================
@@ -2535,27 +2535,69 @@ void AArch64FrameLowering::emitEpilogue(MachineFunction &MF,
                     DeallocateAfter, TII, MachineInstr::FrameDestroy, false,
                     NeedsWinCFI, &HasWinCFI);
   } else if (SVEStackSize) {
-    // If we have stack realignment or variable sized objects on the stack,
-    // restore the stack pointer from the frame pointer prior to SVE CSR
-    // restoration.
-    if (AFI->isStackRealigned() || MFI.hasVarSizedObjects()) {
-      if (int64_t CalleeSavedSize = AFI->getSVECalleeSavedStackSize()) {
-        // Set SP to start of SVE callee-save area from which they can
-        // be reloaded. The code below will deallocate the stack space
-        // space by moving FP -> SP.
-        emitFrameOffset(MBB, RestoreBegin, DL, AArch64::SP, AArch64::FP,
-                        StackOffset::getScalable(-CalleeSavedSize), TII,
----------------
sdesmalen-arm wrote:

You mentioned offline that it may not make sense to update `emitFrameOffset` because it is specifically an issue when deallocating stack space (and not when allocating), so I agree that this not something that can be fixed in that function.

I did realise that the register scavenger runs after PEI/FrameLowering meaning that you can use virtual registers which will get allocated aftewards. That makes things simpler and means you could instead write:
```
if (int64_t HazardSize = getStackHazardSize(MF)) {
  Register TmpReg = MBB.getParent()->getRegInfo().createVirtualRegister(
      &AArch64::GPR64RegClass);
  emitFrameOffset(MBB, RestoreBegin, DL, TmpReg, AArch64::FP,
                  StackOffset::getFixed(-HazardSize),
                  TII, MachineInstr::FrameDestroy);
  emitFrameOffset(MBB, RestoreBegin, DL, AArch64::SP, TmpReg,
                  StackOffset::getScalable(-CalleeSavedSize), TII,
                  MachineInstr::FrameDestroy);
} else {
  // Set SP to start of SVE callee-save area from which they can
  // be reloaded. The code below will deallocate the stack space
  // space by moving FP -> SP.
  emitFrameOffset(MBB, RestoreBegin, DL, AArch64::SP, AArch64::FP,
                  StackOffset::getScalable(-CalleeSavedSize), TII,
                  MachineInstr::FrameDestroy);
}
```

https://github.com/llvm/llvm-project/pull/143371