[llvm] [VPlan] Introduce child regions as VPlan transform. (PR #129402)

[via llvm-commits]

================
@@ -14,26 +14,95 @@
 #include "LoopVectorizationPlanner.h"
 #include "VPlan.h"
 #include "VPlanCFG.h"
+#include "VPlanDominatorTree.h"
 #include "VPlanTransforms.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Analysis/ScalarEvolution.h"
 
 using namespace llvm;
 
-void VPlanTransforms::introduceTopLevelVectorLoopRegion(
-    VPlan &Plan, Type *InductionTy, PredicatedScalarEvolution &PSE,
-    bool RequiresScalarEpilogueCheck, bool TailFolded, Loop *TheLoop) {
-  // TODO: Generalize to introduce all loop regions.
-  auto *HeaderVPBB = cast<VPBasicBlock>(Plan.getEntry()->getSingleSuccessor());
-  VPBlockUtils::disconnectBlocks(Plan.getEntry(), HeaderVPBB);
+/// Checks if \p HeaderVPB is a loop header block in the plain CFG; that is, it
+/// has exactly 2 predecessors (preheader and latch), where the block
+/// dominates the latch and the preheader dominates the block. If it is a
+/// header block, returns a pair with the corresponding preheader and latch
+/// blocks. Otherwise return std::nullopt.
+static std::optional<std::pair<VPBasicBlock *, VPBasicBlock *>>
+getPreheaderAndLatch(VPBlockBase *HeaderVPB, const VPDominatorTree &VPDT) {
+  ArrayRef<VPBlockBase *> Preds = HeaderVPB->getPredecessors();
+  if (Preds.size() != 2)
+    return std::nullopt;
 
-  VPBasicBlock *OriginalLatch =
-      cast<VPBasicBlock>(HeaderVPBB->getSinglePredecessor());
-  VPBlockUtils::disconnectBlocks(OriginalLatch, HeaderVPBB);
-  VPBasicBlock *VecPreheader = Plan.createVPBasicBlock("vector.ph");
-  VPBlockUtils::connectBlocks(Plan.getEntry(), VecPreheader);
-  assert(OriginalLatch->getNumSuccessors() == 0 &&
-         "Plan should end at top level latch");
+  auto *PreheaderVPBB = cast<VPBasicBlock>(Preds[0]);
+  auto *LatchVPBB = cast<VPBasicBlock>(Preds[1]);
----------------
ayalz wrote:

```suggestion
  auto *PreheaderVPBB = Preds[0];
  auto *LatchVPBB = Preds[1];
```
or are the casts needed?

https://github.com/llvm/llvm-project/pull/129402