[PATCH] D111299: [VPlan] Do not create dummy entry block (NFC).

[Florian Hahn via Phabricator via llvm-commits]

fhahn created this revision.
fhahn added reviewers: Ayal, gilr.
Herald added subscribers: tschuett, psnobl, rogfer01, bollu, hiraditya.
fhahn requested review of this revision.
Herald added a subscriber: vkmr.
Herald added a project: LLVM.

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.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D111299

Files:
  llvm/lib/Transforms/Vectorize/LoopVectorize.cpp


Index: llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
===================================================================
--- llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
+++ llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
@@ -9273,20 +9273,22 @@
 
   // 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());
-    VPBlockUtils::insertBlockAfter(FirstVPBBForBB, VPBB);
+    if (VPBB)
+      VPBlockUtils::insertBlockAfter(FirstVPBBForBB, VPBB);
+    else
+      Plan->setEntry(FirstVPBBForBB);
     VPBB = FirstVPBBForBB;
     Builder.setInsertPoint(VPBB);
 
@@ -9347,15 +9349,6 @@
 
   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.


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