[llvm] [VPlan] Use ResumePhi to create reduction resume phis. (PR #110004)

[via llvm-commits]

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@@ -7561,67 +7561,53 @@ static void addRuntimeUnrollDisableMetaData(Loop *L) {
   }
 }
 
-// Check if \p RedResult is a ComputeReductionResult instruction, and if it is
-// create a merge phi node for it.
-static void createAndCollectMergePhiForReduction(
-    VPInstruction *RedResult,
-    VPTransformState &State, Loop *OrigLoop, BasicBlock *LoopMiddleBlock,
-    bool VectorizingEpilogue) {
+// If \p R is a ComputeReductionResult when vectorizing the epilog loop,
+// update the reduction's scalar PHI node by adding the incoming value from the
+// main vector loop.
+static void updateMergePhiForReductionForEpilogueVectorization(
+    VPRecipeBase *R, VPTransformState &State, Loop *OrigLoop,
+    BasicBlock *LoopMiddleBlock) {
+  auto *RedResult = dyn_cast<VPInstruction>(R);
   if (!RedResult ||
       RedResult->getOpcode() != VPInstruction::ComputeReductionResult)
     return;
 
   auto *PhiR = cast<VPReductionPHIRecipe>(RedResult->getOperand(0));
   const RecurrenceDescriptor &RdxDesc = PhiR->getRecurrenceDescriptor();
-
-  Value *FinalValue = State.get(RedResult, VPLane(VPLane::getFirstLane()));
-  auto *ResumePhi =
-      dyn_cast<PHINode>(PhiR->getStartValue()->getUnderlyingValue());
-  if (VectorizingEpilogue && RecurrenceDescriptor::isAnyOfRecurrenceKind(
-                                 RdxDesc.getRecurrenceKind())) {
+  PHINode *MainResumePhi;
+  if (RecurrenceDescriptor::isAnyOfRecurrenceKind(
+          RdxDesc.getRecurrenceKind())) {
     auto *Cmp = cast<ICmpInst>(PhiR->getStartValue()->getUnderlyingValue());
     assert(Cmp->getPredicate() == CmpInst::ICMP_NE);
     assert(Cmp->getOperand(1) == RdxDesc.getRecurrenceStartValue());
-    ResumePhi = cast<PHINode>(Cmp->getOperand(0));
+    MainResumePhi = cast<PHINode>(Cmp->getOperand(0));
+  } else {
+    MainResumePhi = cast<PHINode>(PhiR->getStartValue()->getUnderlyingValue());
   }
-  assert((!VectorizingEpilogue || ResumePhi) &&
-         "when vectorizing the epilogue loop, we need a resume phi from main "
-         "vector loop");
 
-  // TODO: bc.merge.rdx should not be created here, instead it should be
-  // modeled in VPlan.
+  // When fixing reductions in the epilogue loop we should already have
+  // created a bc.merge.rdx Phi after the main vector body. Ensure that we carry
+  // over the incoming values correctly.
+  using namespace VPlanPatternMatch;
+  auto IsResumePhi = [](VPUser *U) {
+    return match(
+        U, m_VPInstruction<VPInstruction::ResumePhi>(m_VPValue(), m_VPValue()));
+  };
+  auto *EpiResumePhiVPI =
+      cast<VPInstruction>(*find_if(RedResult->users(), IsResumePhi));
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ayalz wrote:

Simpler to iterate over recipes R of scalar preheader, which should be ResumePhi's (of epilog), fed by ComputeResult, fed by HeaderPhi, fed by ResumePhi (of main)? Rather than iterating over recipes R of middle-block (of epilog), then find its ResumePhi in scalar preheader.

https://github.com/llvm/llvm-project/pull/110004