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

[Julian Nagele 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)) {
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juliannagele wrote:

Yes, thanks! I added checks that the extracts are in the same block as the extend and that they are executed, and tests showing the freeze will still be inserted if those checks fail.

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