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

[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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ayalz wrote:

This ResumePhi placed in the scalar preheader prepares for 2 values. When vectorizing an epilog loop, it should ideally prepare for 3: the ResumePhi after the main loop should feed the ResumePhi after the epilog loop (unless we're sure according to the trip count that if main and epilog are run they will take care of all iterations, w/o leaving any remainder for the scalar loop). But neither recipes nor blocks of the two VPlans can currently be connected.

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