[llvm] [LV] Vectorize selecting last IV of min/max element. (PR #141431)

[Mel Chen via llvm-commits]

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@@ -815,3 +815,148 @@ bool VPlanTransforms::handleMaxMinNumReductions(VPlan &Plan) {
   MiddleTerm->setOperand(0, NewCond);
   return true;
 }
+
+bool VPlanTransforms::legalizeUnclassifiedPhis(VPlan &Plan) {
+  using namespace VPlanPatternMatch;
+  for (auto &PhiR : make_early_inc_range(
+           Plan.getVectorLoopRegion()->getEntryBasicBlock()->phis())) {
+    if (!isa<VPWidenPHIRecipe>(&PhiR))
+      continue;
+
+    // Check if PhiR is a min/max reduction that has a user inside the loop
+    // outside the min/max reduction chain. The other user must be the compare
+    // of a FindLastIV reduction chain.
+    auto *MinMaxPhiR = cast<VPWidenPHIRecipe>(&PhiR);
+    auto *MinMaxOp = dyn_cast_or_null<VPSingleDefRecipe>(
+        MinMaxPhiR->getOperand(1)->getDefiningRecipe());
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Mel-Chen wrote:

 MinMaxPhiR->getIncomingValue(1)

https://github.com/llvm/llvm-project/pull/141431