[llvm] [PATCH] [Xtensa] Implement FrameLowering methods and stack operation lowering. (PR #92960)

[Matt Arsenault via llvm-commits]

================
@@ -51,9 +195,34 @@ MachineBasicBlock::iterator XtensaFrameLowering::eliminateCallFramePseudoInstr(
     if (I->getOpcode() == Xtensa::ADJCALLSTACKDOWN)
       Amount = -Amount;
 
-    unsigned SP = Xtensa::SP;
-    TII.adjustStackPtr(SP, Amount, MBB, I);
+    TII.adjustStackPtr(Xtensa::SP, Amount, MBB, I);
   }
 
   return MBB.erase(I);
 }
+
+void XtensaFrameLowering::determineCalleeSaves(MachineFunction &MF,
+                                               BitVector &SavedRegs,
+                                               RegScavenger *RS) const {
+  MachineFrameInfo &MFI = MF.getFrameInfo();
+  unsigned FP = TRI->getFrameRegister(MF);
+
+  TargetFrameLowering::determineCalleeSaves(MF, SavedRegs, RS);
+
+  // Mark $fp as used if function has dedicated frame pointer.
+  if (hasFP(MF))
+    SavedRegs.set(FP);
+
+  // Set scavenging frame index if necessary.
+  uint64_t MaxSPOffset = MFI.estimateStackSize(MF);
+
+  if (isInt<12>(MaxSPOffset))
+    return;
+
+  const TargetRegisterClass &RC = Xtensa::ARRegClass;
+  unsigned Size = TRI->getSpillSize(RC);
+  Align Alignment = TRI->getSpillAlign(RC);
+  int FI = MF.getFrameInfo().CreateStackObject(Size, Alignment, false);
+
+  RS->addScavengingFrameIndex(FI);
----------------
arsenm wrote:

I'm not sure it's safe to do this in determineCalleeSaves. Can you create the emergency scavenging index in processFunctionBeforeFrameFinalized instead? 

https://github.com/llvm/llvm-project/pull/92960