[llvm] [VPlan] Add ReductionStartVector VPInstruction. (PR #142290)

[Sam Tebbs via llvm-commits]

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@@ -9311,6 +9315,28 @@ void LoopVectorizationPlanner::adjustRecipesForReductions(
       // start value.
       PhiR->setOperand(0, Plan->getOrAddLiveIn(RdxDesc.getSentinelValue()));
     }
+    RecurKind RK = RdxDesc.getRecurrenceKind();
+    if (PhiR->isOrdered() || PhiR->isInLoop() ||
+        (!RecurrenceDescriptor::isAnyOfRecurrenceKind(RK) &&
+         !RecurrenceDescriptor::isFindLastIVRecurrenceKind(RK) &&
+         !RecurrenceDescriptor::isMinMaxRecurrenceKind(RK))) {
+      VPBuilder PHBuilder(Plan->getVectorPreheader());
+      VPValue *Iden = Plan->getOrAddLiveIn(
+          getRecurrenceIdentity(RK, PhiTy, RdxDesc.getFastMathFlags()));
+      // If the PHI is used by a partial reduction, set the scale factor.
+      unsigned ScaleFactor =
+          RecipeBuilder.getScalingForReduction(RdxDesc.getLoopExitInstr())
+              .value_or(1);
+      Type *I32Ty = IntegerType::getInt32Ty(PhiTy->getContext());
+      auto *ScalarFactorVPV =
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SamTebbs33 wrote:

Scalar -> Scale

https://github.com/llvm/llvm-project/pull/142290