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

[Florian Hahn via llvm-commits]

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@@ -9511,6 +9497,22 @@ void LoopVectorizationPlanner::adjustRecipesForReductions(
         });
     FinalReductionResult->insertBefore(*MiddleVPBB, IP);
 
+    VPBasicBlock *ScalarPHVPBB = nullptr;
+    if (MiddleVPBB->getNumSuccessors() == 2) {
+      // Order is strict: first is the exit block, second is the scalar
+      // preheader.
+      ScalarPHVPBB = cast<VPBasicBlock>(MiddleVPBB->getSuccessors()[1]);
+    } else {
+      ScalarPHVPBB = cast<VPBasicBlock>(MiddleVPBB->getSingleSuccessor());
+    }
+
+    VPBuilder ScalarPHBuilder(ScalarPHVPBB);
+    auto *ResumePhiRecipe = ScalarPHBuilder.createNaryOp(
+        VPInstruction::ResumePhi, {FinalReductionResult, PhiR->getStartValue()},
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fhahn wrote:

Yes, unfortunately there is no way to determine if the plan will be used for epilogue vectorization at this point I think. Once we model it explicitly in VPlan, the update should be connected here 

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