[VPlan] Do not create dummy entry block (NFC).

[fhahn]

Summary

At the moment a dummy entry block is created at the beginning of VPlan
construction. This dummy block is later removed again.

This means it is not easy to identify the VPlan header block in a
general fashion, because during recipe creation it is the single
successor of the entry block, while later it is the entry block.

To make getting the header easier, just skip creating the dummy block.

[fhahn]

ping

[fhahn]

ping :)

[Ayal]

This is indeed better, thanks!
(Original version tried to simplify the loop by peeling setEntry(), but this does look better overall.)
Adding a minor nit.

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

9372–9373

Worth asserting that Plan Entry is set after the loop?
(Original version did this by calling getSingleSuccessor()).
(Perhaps also assert that this entry is not empty(?)).

[fhahn]

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

9372–9373

Thanks, committed with the suggested assert!

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

VPlanPtr LoopVectorizationPlanner::buildVPlanWithVPRecipes(
 
   // ---------------------------------------------------------------------------
   // Build initial VPlan: Scan the body of the loop in a topological order to
   // visit each basic block after having visited its predecessor basic blocks.
   // ---------------------------------------------------------------------------
 
   // Create a dummy pre-entry VPBasicBlock to start building the VPlan.
   auto Plan = std::make_unique<VPlan>();
-  VPBasicBlock *VPBB = new VPBasicBlock("Pre-Entry");
-  Plan->setEntry(VPBB);
 
   // Scan the body of the loop in a topological order to visit each basic block
   // after having visited its predecessor basic blocks.
   LoopBlocksDFS DFS(OrigLoop);
   DFS.perform(LI);
 
+  VPBasicBlock *VPBB = nullptr;
   for (BasicBlock *BB : make_range(DFS.beginRPO(), DFS.endRPO())) {
     // Relevant instructions from basic block BB will be grouped into VPRecipe
     // ingredients and fill a new VPBasicBlock.
     unsigned VPBBsForBB = 0;
     auto *FirstVPBBForBB = new VPBasicBlock(BB->getName());
+    if (VPBB)
       VPBlockUtils::insertBlockAfter(FirstVPBBForBB, VPBB);
+    else
+      Plan->setEntry(FirstVPBBForBB);
     VPBB = FirstVPBBForBB;
     Builder.setInsertPoint(VPBB);
 
     // Introduce each ingredient into VPlan.
     // TODO: Model and preserve debug instrinsics in VPlan.
     for (Instruction &I : BB->instructionsWithoutDebug()) {
       Instruction *Instr = &I;
 
for (Instruction &I : BB->instructionsWithoutDebug()) {
       if (NextVPBB != VPBB) {
         VPBB = NextVPBB;
         VPBB->setName(BB->hasName() ? BB->getName() + "." + Twine(VPBBsForBB++)
                                     : "");
       }
     }
   }
 
+  assert(isa<VPBasicBlock>(Plan->getEntry()) &&
+         !Plan->getEntry()->getEntryBasicBlock()->empty() &&
+         "entry block must be set to a non-empty VPBasicBlock");
   RecipeBuilder.fixHeaderPhis();
 
-  // Discard empty dummy pre-entry VPBasicBlock. Note that other VPBasicBlocks
-  // may also be empty, such as the last one VPBB, reflecting original
-  // basic-blocks with no recipes.
-  VPBasicBlock *PreEntry = cast<VPBasicBlock>(Plan->getEntry());
-  assert(PreEntry->empty() && "Expecting empty pre-entry block.");
-  VPBlockBase *Entry = Plan->setEntry(PreEntry->getSingleSuccessor());
-  VPBlockUtils::disconnectBlocks(PreEntry, Entry);
-  delete PreEntry;
-
   // ---------------------------------------------------------------------------
   // Transform initial VPlan: Apply previously taken decisions, in order, to
   // bring the VPlan to its final state.
   // ---------------------------------------------------------------------------
 
   // Apply Sink-After legal constraints.
   auto GetReplicateRegion = [](VPRecipeBase *R) -> VPRegionBlock * {
     auto *Region = dyn_cast_or_null<VPRegionBlock>(R->getParent()->getParent());
     if (Region && Region->isReplicator()) {