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

[via llvm-commits]

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@@ -7467,23 +7467,31 @@ 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) {
+// create a merge phi node for it and add incoming values from the main vector
+// loop.
+static void updateAndCollectMergePhiForReductionForEpilogueVectorization(
+    VPInstruction *RedResult, VPTransformState &State, Loop *OrigLoop,
+    BasicBlock *LoopMiddleBlock, bool VectorizingEpilogue) {
   if (!RedResult ||
       RedResult->getOpcode() != VPInstruction::ComputeReductionResult)
     return;
 
+  using namespace VPlanPatternMatch;
+  VPValue *ResumePhiVPV =
+      cast<VPInstruction>(*find_if(RedResult->users(), [](VPUser *U) {
+        return match(U, m_VPInstruction<VPInstruction::ResumePhi>(m_VPValue(),
+                                                                  m_VPValue()));
+      }));
+  auto *BCBlockPhi = cast<PHINode>(State.get(ResumePhiVPV, true));
   auto *PhiR = cast<VPReductionPHIRecipe>(RedResult->getOperand(0));
   const RecurrenceDescriptor &RdxDesc = PhiR->getRecurrenceDescriptor();
+  if (!VectorizingEpilogue)
+    return;
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ayalz wrote:

nit: `assert((!VectorizingEpilogue || ResumePhi)` below can assert ResumePhi alone.

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