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

[via llvm-commits]

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@@ -965,3 +965,101 @@ bool VPlanTransforms::handleMaxMinNumReductions(VPlan &Plan) {
   MiddleTerm->setOperand(0, NewCond);
   return true;
 }
+
+bool VPlanTransforms::legalizeMultiUseReductions(VPlan &Plan) {
+  for (auto &PhiR : make_early_inc_range(
+           Plan.getVectorLoopRegion()->getEntryBasicBlock()->phis())) {
+    auto *MinMaxPhiR = dyn_cast<VPReductionPHIRecipe>(&PhiR);
+    if (!MinMaxPhiR)
+      continue;
+
+    RecurKind RdxKind = MinMaxPhiR->getRecurrenceKind();
+    // TODO: check for multi-uses in VPlan directly.
+    if (!RecurrenceDescriptor::isIntMinMaxRecurrenceKind(RdxKind) ||
+        !MinMaxPhiR->isPhiMultiUse())
+      continue;
+
+    // One user of MinMaxPhiR is MinMaxOp, the other users must be a compare
+    // that's part of a FindLastIV chain.
+    auto *MinMaxOp =
+        dyn_cast<VPRecipeWithIRFlags>(MinMaxPhiR->getBackedgeValue());
+    if (!MinMaxOp || MinMaxOp->getNumUsers() != 2)
+      return false;
+    auto MinMaxUsers = to_vector(MinMaxPhiR->users());
+    auto *Cmp = dyn_cast<VPRecipeWithIRFlags>(
+        MinMaxUsers[0] == MinMaxOp ? MinMaxUsers[1] : MinMaxUsers[0]);
+    VPValue *CmpOpA;
+    VPValue *CmpOpB;
+    if (!Cmp || Cmp->getNumUsers() != 1 ||
+        !match(Cmp, m_Binary<Instruction::ICmp>(m_VPValue(CmpOpA),
+                                                m_VPValue(CmpOpB))))
+      return false;
+
+    // Normalize the predicate so MinMaxPhiR is on the right side.
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ayalz wrote:

So that argmax corresponds to GE/GT and argmin corresponds to LE/LT (FindLastIV/FindFirstIV for both. respectively)

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