[llvm] [LV] Use frozen start value for FindLastIV if needed. (PR #132691)

[David Sherwood via llvm-commits]

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@@ -9866,14 +9869,31 @@ void LoopVectorizationPlanner::adjustRecipesForReductions(
     // also modeled in VPlan.
     VPInstruction *FinalReductionResult;
     VPBuilder::InsertPointGuard Guard(Builder);
-    Builder.setInsertPoint(MiddleVPBB, IP);
     if (RecurrenceDescriptor::isFindLastIVRecurrenceKind(
             RdxDesc.getRecurrenceKind())) {
       VPValue *Start = PhiR->getStartValue();
+      if (!isGuaranteedNotToBeUndefOrPoison(
+              PhiR->getStartValue()->getLiveInIRValue())) {
+        Builder.setInsertPoint(cast<VPBasicBlock>(Plan->getEntry()));
+        Start = Builder.createNaryOp(Instruction::Freeze, {Start}, {}, "fr");
+      }
+      Builder.setInsertPoint(MiddleVPBB, IP);
       FinalReductionResult =
           Builder.createNaryOp(VPInstruction::ComputeFindLastIVResult,
                                {PhiR, Start, NewExitingVPV}, ExitDL);
+      // Update all users outside the vector region.
+      for (VPUser *U : to_vector(OrigExitingVPV->users())) {
+        auto *R = cast<VPRecipeBase>(U);
+        if (R->getParent() && R->getParent()->getParent())
+          continue;
+
+        for (unsigned Idx = 0; Idx != R->getNumOperands(); ++Idx) {
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david-arm wrote:

This looks like a commonly-used idiom, something we probably have an equivalent IR helper for. Is it worth adding a helper in VPUser?

https://github.com/llvm/llvm-project/pull/132691