[llvm] [AMDGPU] Promote allocas used by phi/select (PR #221870)

[Matt Arsenault via llvm-commits]

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@@ -464,17 +549,52 @@ static bool isSupportedMemset(MemSetInst *I, AllocaInst *AI,
 }
 
 static Value *calculateVectorIndex(Value *Ptr, AllocaAnalysis &AA) {
-  IRBuilder<> B(Ptr->getContext());
-
+  LLVMContext &Ctx = Ptr->getContext();
   Ptr = Ptr->stripPointerCasts();
-  if (Ptr == AA.Alloca)
-    return B.getInt32(0);
+
+  // Already computed (also breaks cycles for pointer phis).
+  auto Cached = AA.Vector.IndexCache.find(Ptr);
+  if (Cached != AA.Vector.IndexCache.end())
+    return Cached->second;
+
+  // A pointer that is directly one of the member allocas indexes lane
+  // BaseLane[member].
+  if (auto *AI = dyn_cast<AllocaInst>(Ptr)) {
+    Value *Idx =
+        ConstantInt::get(Type::getInt32Ty(Ctx), AA.Vector.BaseLane.lookup(AI));
+    AA.Vector.IndexCache[Ptr] = Idx;
+    return Idx;
+  }
+
+  // Pointer phi: build a parallel phi of lane indices. Insert and cache it
+  // before recursing so self-referential phis terminate.
+  if (auto *Phi = dyn_cast<PHINode>(Ptr)) {
+    IRBuilder<> B(Phi);
+    PHINode *IdxPhi = B.CreatePHI(B.getInt32Ty(), Phi->getNumIncomingValues(),
+                                  "promotealloca.idx");
+    AA.Vector.IndexCache[Ptr] = IdxPhi;
+    for (unsigned I = 0, E = Phi->getNumIncomingValues(); I != E; ++I)
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arsenm wrote:

Braces 

https://github.com/llvm/llvm-project/pull/221870