[llvm] [RegAllocBase] Produce IMPLICIT_DEF instead of COPY undef during cleanupFailedVReg (PR #147392)

[Matt Arsenault via llvm-commits]

arsenm wrote:

Option 2:
```
diff --git a/llvm/lib/CodeGen/TargetInstrInfo.cpp b/llvm/lib/CodeGen/TargetInstrInfo.cpp
index 8b82deb2a9d8..8c23dff565ab 100644
--- a/llvm/lib/CodeGen/TargetInstrInfo.cpp
+++ b/llvm/lib/CodeGen/TargetInstrInfo.cpp
@@ -773,10 +773,14 @@ MachineInstr *TargetInstrInfo::foldMemoryOperand(MachineInstr &MI,
   const MachineOperand &MO = MI.getOperand(1 - Ops[0]);
   MachineBasicBlock::iterator Pos = MI;
 
-  if (Flags == MachineMemOperand::MOStore)
-    storeRegToStackSlot(*MBB, Pos, MO.getReg(), MO.isKill(), FI, RC, TRI,
-                        Register());
-  else
+  if (Flags == MachineMemOperand::MOStore) {
+    if (MO.isUndef()) {
+      BuildMI(*MBB, Pos, MI.getDebugLoc(), get(TargetOpcode::KILL)).add(MO);
+    } else {
+      storeRegToStackSlot(*MBB, Pos, MO.getReg(), MO.isKill(), FI, RC, TRI,
+                          Register());
+    }
+  } else
     loadRegFromStackSlot(*MBB, Pos, MO.getReg(), FI, RC, TRI, Register());
   return &*--Pos;
 }
```

https://github.com/llvm/llvm-project/pull/147392