[llvm] [VPlan] Introduce ExitPhi VPInstruction, use to create phi for FOR. (PR #94760)

[Florian Hahn via llvm-commits]

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@@ -193,11 +193,18 @@ void VPLiveOut::fixPhi(VPlan &Plan, VPTransformState &State) {
   auto Lane = vputils::isUniformAfterVectorization(ExitValue)
                   ? VPLane::getFirstLane()
                   : VPLane::getLastLaneForVF(State.VF);
-  VPBasicBlock *MiddleVPBB =
+  VPBasicBlock *PredVPBB =
       cast<VPBasicBlock>(Plan.getVectorLoopRegion()->getSingleSuccessor());
-  BasicBlock *MiddleBB = State.CFG.VPBB2IRBB[MiddleVPBB];
-  Phi->addIncoming(State.get(ExitValue, VPIteration(State.UF - 1, Lane)),
-                   MiddleBB);
+  VPRecipeBase *DefRecipe = ExitValue->getDefiningRecipe();
+  if (DefRecipe && !DefRecipe->getParent()->getParent())
+    PredVPBB = DefRecipe->getParent();
+  BasicBlock *PredBB = State.CFG.VPBB2IRBB[PredVPBB];
+  State.Builder.SetInsertPoint(PredBB, PredBB->getFirstNonPHIIt());
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fhahn wrote:

Added a comment + TODO, thanks!

>Is this potential post-extract why ResumePhi is considered (maybe!) vector-to-scalar? These extracts best materialize into recipes.
 
Yes, we need to select the last lane, which may require creating a new extract, if the VPValue is a vector. Added a TODO to model extracts explicitly.

https://github.com/llvm/llvm-project/pull/94760