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

[via llvm-commits]

================
@@ -1001,3 +1005,138 @@ 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;
+
+    // MinMaxPhiR has users outside the reduction cycle in the loop. Check if
+    // the only other user is a FindLastIV reduction. MinMaxPhiR must have
+    // exactly 3 users: 1) the min/max operation, the compare of a FindLastIV
+    // reduction and ComputeReductionResult. The comparisom must compare
+    // MinMaxPhiR against the min/max operand used for the min/max reduction
+    // and only be used by the select of the FindLastIV reduction.
+    RecurKind RdxKind = MinMaxPhiR->getRecurrenceKind();
+    assert(
+        RecurrenceDescriptor::isIntMinMaxRecurrenceKind(RdxKind) &&
+        "only min/max recurrences support users outside the reduction chain");
+
+    auto *MinMaxOp =
+        dyn_cast<VPRecipeWithIRFlags>(MinMaxPhiR->getBackedgeValue());
+    if (!MinMaxOp)
+      return false;
+
+    // MinMaxOp must have 2 users: 1) MinMaxPhiR and 2) ComputeReductionResult
+    // (asserted below).
+    assert(MinMaxOp->getNumUsers() == 2 &&
+           "MinMaxOp must have exactly 2 users");
+
+    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);
+    assert(MinMaxOpA == MinMaxPhiR &&
+           "one of MinMaxOp's operands must be the phi");
----------------
ayalz wrote:

Would this be clearer:
```suggestion
    VPValue *TempValueBeingReduced = MinMaxOp->getOperand(0);
    if (TempValueBeingReduced == MinMaxPhiR)
      TempValueBeingReduced = MinMaxOp->getOperand(1);
```
"one of MinMaxOp's operands must be the phi" - is asserted later by making sure MinMaxOp uses MinMaxPhiR.


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