[llvm] [AMDGPU] Avoid dangling SSAUpdater reference in PromoteAlloca full-vector store (PR #215686)

[Matt Arsenault via llvm-commits]

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@@ -729,9 +729,21 @@ static Value *promoteAllocaUserToVector(Instruction *Inst, const DataLayout &DL,
     // We're storing the full vector, we can handle this without knowing CurVal.
     Type *AccessTy = Val->getType();
     TypeSize AccessSize = DL.getTypeStoreSize(AccessTy);
-    if (Constant *CI = dyn_cast<Constant>(Index))
-      if (CI->isNullValue() && AccessSize == VecStoreSize)
-        return Builder.CreateBitPreservingCastChain(DL, Val, AA.Vector.Ty);
+    if (Constant *CI = dyn_cast<Constant>(Index)) {
+      if (CI->isNullValue() && AccessSize == VecStoreSize) {
+        Value *Result =
+            Builder.CreateBitPreservingCastChain(DL, Val, AA.Vector.Ty);
+        // If Result is an instruction in the worklist (e.g. a load from this
+        // alloca), it will later be RAUW'd and deleted. The SSAUpdater holds
+        // a raw Value* that RAUW doesn't update, leaving a dangling pointer.
+        // Wrap in a freeze to create a fresh value the SSAUpdater can safely
+        // hold; the freeze's operand is a proper IR use that RAUW does update.
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arsenm wrote:

Using freeze in a mechanical way doesn't sit right with me. Should this be using value handles or something 

https://github.com/llvm/llvm-project/pull/215686