[llvm] [VPlan] Model branch cond to enter scalar epilogue in VPlan. (PR #92651)

[via llvm-commits]

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@@ -879,15 +922,41 @@ void VPlan::execute(VPTransformState *State) {
   State->CFG.ExitBB = State->CFG.PrevBB->getSingleSuccessor();
   BasicBlock *VectorPreHeader = State->CFG.PrevBB;
   State->Builder.SetInsertPoint(VectorPreHeader->getTerminator());
-  replaceVPBBWithIRVPBB(
-      cast<VPBasicBlock>(getVectorLoopRegion()->getSingleSuccessor()),
-      State->CFG.ExitBB);
 
   // Disconnect VectorPreHeader from ExitBB in both the CFG and DT.
   cast<BranchInst>(VectorPreHeader->getTerminator())->setSuccessor(0, nullptr);
   State->CFG.DTU.applyUpdates(
       {{DominatorTree::Delete, VectorPreHeader, State->CFG.ExitBB}});
 
+  // Replace regular VPBB's for the middle and scalar preheader blocks with
+  // VPIRBasicBlocks with VPIRBasicBlocks wrapping their IR blocks. The IR
+  // blocks are created during skeleton creation, so we can only create the
+  // VPIRBasicBlocks after legacy skeleton creation.
+  BasicBlock *MiddleBB = State->CFG.ExitBB;
+  VPBasicBlock *MiddleVPBB =
+      cast<VPBasicBlock>(getVectorLoopRegion()->getSingleSuccessor());
+  // Find the VPBB for the scalar preheader, relying on the current structure
+  // when creating the middle block and its successrs: if there's a single
+  // predecessor, it must be the scalar preheader. Otherwise, the second
+  // successor is the scalar preheader.
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ayalz wrote:

Follow-up, potentially: if according to roadmap the scalar loop is wrapped in a VPIRRegionBlock, retrievable via getScalarLoopRegion(), then ScalarPhVPBB could be found by its getSinglePredecessor, analogous to how MiddleVPBB is found. MiddleVPBB may have the exit block as its only successor - when tail folding or having no tail.

https://github.com/llvm/llvm-project/pull/92651