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

[via llvm-commits]

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@@ -992,3 +996,98 @@ bool VPlanTransforms::handleMaxMinNumReductions(VPlan &Plan) {
   MiddleTerm->setOperand(0, NewCond);
   return true;
 }
+
+bool VPlanTransforms::handleMultiUseReductions(VPlan &Plan) {
+  for (auto &PhiR : make_early_inc_range(
+           Plan.getVectorLoopRegion()->getEntryBasicBlock()->phis())) {
+    auto *MinMaxPhiR = dyn_cast<VPReductionPHIRecipe>(&PhiR);
+    // TODO: check for multi-uses in VPlan directly.
+    if (!MinMaxPhiR || !MinMaxPhiR->hasLoopUsesOutsideReductionChain())
+      continue;
+
+    RecurKind RdxKind = MinMaxPhiR->getRecurrenceKind();
+    assert(
+        RecurrenceDescriptor::isIntMinMaxRecurrenceKind(RdxKind) &&
+        "only min/max recurrences support users outside the reduction chain");
+
+    // One user of MinMaxPhiR is MinMaxOp, the other user 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;
+
+    assert((isa<VPWidenIntrinsicRecipe>(MinMaxOp) ||
+            (isa<VPReplicateRecipe>(MinMaxOp) &&
+             isa<IntrinsicInst>(
+                 cast<VPReplicateRecipe>(MinMaxOp)->getUnderlyingValue()))) &&
+           "MinMaxOp must be a wide or scalar intrinsic");
+    VPValue *MinMaxOpA = MinMaxOp->getOperand(0);
+    VPValue *MinMaxOpB = MinMaxOp->getOperand(1);
+    if (MinMaxOpA != MinMaxPhiR)
+      std::swap(MinMaxOpA, MinMaxOpB);
+    if (MinMaxOpA != MinMaxPhiR)
+      return false;
+
+    VPValue *CmpOpA;
+    VPValue *CmpOpB;
+    CmpPredicate Pred;
+    auto *Cmp = dyn_cast_or_null<VPRecipeWithIRFlags>(findUserOf(
+        MinMaxPhiR, m_Cmp(Pred, m_VPValue(CmpOpA), m_VPValue(CmpOpB))));
+    if (!Cmp || Cmp->getNumUsers() != 1 ||
+        (CmpOpA != MinMaxOpB && CmpOpB != MinMaxOpB))
+      return false;
+
+    // TODO: Strict predicates need to find the first IV value for which the
+    // predicate holds, not the last.
+    if (Pred == CmpInst::ICMP_EQ || Pred == CmpInst::ICMP_NE ||
+        ICmpInst::isLT(Pred) || ICmpInst::isGT(Pred))
+      return false;
+
+    // Normalize the predicate so MinMaxPhiR is on the right side.
+    if (CmpOpA == MinMaxPhiR)
+      Pred = CmpInst::getSwappedPredicate(Pred);
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ayalz wrote:

Is it important to normalize so MinMaxPhiR is on the right - `Pred` appears to be unused?

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