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

[David Sherwood via llvm-commits]

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@@ -10377,6 +10380,36 @@ static void preparePlanForMainVectorLoop(VPlan &MainPlan, VPlan &EpiPlan) {
       VPInstruction::ResumePhi,
       {VectorTC, MainPlan.getCanonicalIV()->getStartValue()}, {},
       "vec.epilog.resume.val");
+
+  // When vectorizing the epilogue, FindLastIV reductions can introduce multiple
+  // uses of undef/poison. If the reduction start value is not guaranteed to be
+  // undef or poison, we need to freeze it and use the frozen start when
+  // computing the reduction result. We also need to use the frozen value in the
+  // resume phi generated by the main vector loop, as this is also used to
+  // compute the reduction result after the epilogue vector loop.
+  auto AddFreezeForFindLastIVReductions = [](VPlan &Plan,
+                                             bool UpdateResumePhis) {
+    for (VPRecipeBase &R : *Plan.getMiddleBlock()) {
+      auto *VPI = dyn_cast<VPInstruction>(&R);
+      if (!VPI || VPI->getOpcode() != VPInstruction::ComputeFindLastIVResult)
+        continue;
+      VPValue *OrigStart = VPI->getOperand(1);
+      if (isGuaranteedNotToBeUndefOrPoison(OrigStart->getLiveInIRValue()))
+        continue;
+      VPBuilder Builder(Plan.getEntry());
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david-arm wrote:

Maybe hoist this out of the loop?

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