[llvm] [VPlan] Transform VPFirstOrderRecurrencePHIRecipe into concrete recipes (PR #172009)

[Ramkumar Ramachandra via llvm-commits]

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@@ -3695,6 +3695,34 @@ void VPlanTransforms::convertToConcreteRecipes(VPlan &Plan) {
         continue;
       }
 
+      if (auto *FORPhiR = dyn_cast<VPFirstOrderRecurrencePHIRecipe>(&R)) {
+        VPValue *InitVec = FORPhiR->getStartValue();
+        DebugLoc DL = FORPhiR->getDebugLoc();
+        if (VF.isVector()) {
+          VPBuilder PHBuilder(Plan.getVectorPreheader());
+          VPValue *Poison = Plan.getOrAddLiveIn(
+              PoisonValue::get(TypeInfo.inferScalarType(InitVec)));
+          Type *IdxTy = Type::getInt32Ty(Plan.getContext());
+          VPValue *RuntimeVF = PHBuilder.createScalarZExtOrTrunc(
+              &Plan.getVF(), IdxTy, TypeInfo.inferScalarType(&Plan.getVF()),
+              DL);
+          VPValue *LastIdx = PHBuilder.createOverflowingOp(
+              Instruction::Sub, {RuntimeVF, Plan.getConstantInt(IdxTy, 1)},
+              {false, false}, DL);
+          InitVec = PHBuilder.createNaryOp(Instruction::InsertElement,
+                                           {Poison, InitVec, LastIdx}, DL,
+                                           "vector.recur.init");
+        }
+        auto *WidenPhi =
+            new VPWidenPHIRecipe(cast<PHINode>(FORPhiR->getUnderlyingInstr()),
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artagnon wrote:

Would be good to introduce VPFirstOrderRecurrencePHIRecipe::getPHINode() like in VPWidenInductionRecipe::getPHINode()?

https://github.com/llvm/llvm-project/pull/172009