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

[Benjamin Maxwell via llvm-commits]

================
@@ -2535,24 +2535,70 @@ 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,
+    const AArch64RegisterInfo *RegInfo = Subtarget.getRegisterInfo();
+    int64_t SVECalleeSavedSize = AFI->getSVECalleeSavedStackSize();
+    Register BaseForSVEDealloc = [&]() -> Register {
+      // With stack realignment we must use the FP to restore SVE CSRs (as both
+      // the SP and BP can't be used due to the unknown alignment padding).
+      if (AFI->isStackRealigned())
+        return AArch64::FP;
+      // With variable sized objects on the stack, we can use the BP or FP to
+      // restore the SVE callee saves.
+      if (MFI.hasVarSizedObjects()) {
+        if (DeallocateBefore && !AFI->hasStackHazardSlotIndex()) {
+          // If there's SVE locals and no hazard padding we can do:
+          //  ADDVL SP, FP, #(-SVECalleeSavedSize)
+          return AArch64::FP;
+        }
+        // Otherwise, we can choose between:
+        //  SUB TMP, FP, #(-CalleeSaveBaseOffset)
----------------
MacDue wrote:

```suggestion
        //  SUB TMP, FP, #CalleeSaveBaseOffset
```

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