[llvm] [VPlan] Introduce scalar loop header in plan, remove VPLiveOut. (PR #109975)

[Florian Hahn via llvm-commits]

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@@ -8790,6 +8787,31 @@ static void addCanonicalIVRecipes(VPlan &Plan, Type *IdxTy, bool HasNUW,
                        {CanonicalIVIncrement, &Plan.getVectorTripCount()}, DL);
 }
 
+/// Create resume phis in the scalar preheader for first-order recurrences and
+/// reductions and update the VPIRInstructions wrapping the original phis in the
+/// scalar header.
+static void addScalarResumePhis(VPRecipeBuilder &Builder, VPlan &Plan) {
+  for (VPRecipeBase &R : *Plan.getScalarHeader()) {
+    auto *IRI = cast<VPIRInstruction>(&R);
+    if (!isa<PHINode>(IRI->getInstruction()))
+      break;
+
+    VPBuilder ScalarPHBuilder(Plan.getScalarPreheader());
+    auto *VectorR =
+        dyn_cast<VPHeaderPHIRecipe>(Builder.getRecipe(&IRI->getInstruction()));
+    if (isa<VPFirstOrderRecurrencePHIRecipe, VPReductionPHIRecipe>(VectorR)) {
+      StringRef Name = isa<VPFirstOrderRecurrencePHIRecipe>(VectorR)
+                           ? "scalar.recur.init"
+                           : "bc.merge.rdx";
+      auto *ResumePhiRecipe = ScalarPHBuilder.createNaryOp(
+          VPInstruction::ResumePhi,
+          {VectorR->getBackedgeValue(), VectorR->getStartValue()}, {}, Name);
+
+      IRI->addOperand(ResumePhiRecipe);
+    }
+  }
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fhahn wrote:

Adjusted, thanks!

https://github.com/llvm/llvm-project/pull/109975