[llvm] [VectorCombine] Avoid inserting freeze when scalarizing extend-extract if all extracts would lead to UB on poison. (PR #164683)

[Simon Pilgrim via llvm-commits]

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@@ -2017,8 +2017,24 @@ bool VectorCombine::scalarizeExtExtract(Instruction &I) {
 
   Value *ScalarV = Ext->getOperand(0);
   if (!isGuaranteedNotToBePoison(ScalarV, &AC, dyn_cast<Instruction>(ScalarV),
-                                 &DT))
-    ScalarV = Builder.CreateFreeze(ScalarV);
+                                 &DT)) {
+    // Check if all lanes are extracted and all extracts trigger UB on poison.
+    // If so, we do not need to insert a freeze.
+    SmallDenseSet<uint64_t, 8> ExtractedLanes;
+    bool AllExtractsHaveUB = true;
+    for (User *U : Ext->users()) {
+      auto *Extract = cast<ExtractElementInst>(U);
+      uint64_t Idx =
+          cast<ConstantInt>(Extract->getIndexOperand())->getZExtValue();
+      ExtractedLanes.insert(Idx);
+      if (!programUndefinedIfPoison(Extract)) {
+        AllExtractsHaveUB = false;
+        break;
+      }
+    }
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RKSimon wrote:

+1

https://github.com/llvm/llvm-project/pull/164683