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

[Florian Hahn via llvm-commits]

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@@ -7458,6 +7403,21 @@ LoopVectorizationPlanner::executePlan(
   std::tie(State.CFG.PrevBB, CanonicalIVStartValue) =
       ILV.createVectorizedLoopSkeleton(ExpandedSCEVs ? *ExpandedSCEVs
                                                      : State.ExpandedSCEVs);
+#ifdef EXPENSIVE_CHECKS
+  assert(DT->verify(DominatorTree::VerificationLevel::Fast));
+#endif
+
+  VPBasicBlock *MiddleVPBB =
+      cast<VPBasicBlock>(BestVPlan.getVectorLoopRegion()->getSingleSuccessor());
+
+  using namespace llvm::VPlanPatternMatch;
+  if (MiddleVPBB->begin() != MiddleVPBB->end() &&
+      match(&MiddleVPBB->back(), m_BranchOnCond(m_VPValue()))) {
+    cast<VPIRBasicBlock>(MiddleVPBB->getSuccessors()[1])
+        ->resetBlock(OrigLoop->getLoopPreheader());
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fhahn wrote:

I m not sure if there's a better way at the moment. For now, we rely on the structure of the created middle block and it's successors: single successor must be to the scalar ph,  2 successors meas the second one is the scalar ph; if tail folding it will still have 2 successors, but <true> as condition. Added a comment.

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