[VPlan] Add VPIRBasicBlock, use to model pre-preheader. (llvm#93398)

This patch adds a new special type of VPBasicBlock that wraps an
existing IR basic block. Recipes of the block get added before the
terminator of the wrapped IR basic block. Making it a subclass of
VPBasicBlock avoids duplicating various APIs to manage recipes in a
block, as well as makes sure the traversals filtering VPBasicBlocks
automatically apply as well.

Initially VPIRBasicBlock are only used for the pre-preheader (wrapping
the original preheader of the scalar loop).

As follow-up, this will be used to move more parts of the skeleton
inside VPlan, starting with the branch and condition in the middle
block.

Separated out of llvm#92651

PR: llvm#93398

Author: fhahn

llvm/lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -8608,7 +8608,7 @@ LoopVectorizationPlanner::tryToBuildVPlanWithVPRecipes(VFRange &Range) {
   // loop region contains a header and latch basic blocks.
   VPlanPtr Plan = VPlan::createInitialVPlan(
       createTripCountSCEV(Legal->getWidestInductionType(), PSE, OrigLoop),
-      *PSE.getSE());
+      *PSE.getSE(), OrigLoop->getLoopPreheader());
   VPBasicBlock *HeaderVPBB = new VPBasicBlock("vector.body");
   VPBasicBlock *LatchVPBB = new VPBasicBlock("vector.latch");
   VPBlockUtils::insertBlockAfter(LatchVPBB, HeaderVPBB);
@@ -8856,7 +8856,7 @@ VPlanPtr LoopVectorizationPlanner::buildVPlan(VFRange &Range) {
   // Create new empty VPlan
   auto Plan = VPlan::createInitialVPlan(
       createTripCountSCEV(Legal->getWidestInductionType(), PSE, OrigLoop),
-      *PSE.getSE());
+      *PSE.getSE(), OrigLoop->getLoopPreheader());
 
   // Build hierarchical CFG
   VPlanHCFGBuilder HCFGBuilder(OrigLoop, LI, *Plan);

llvm/lib/Transforms/Vectorize/VPlan.cpp

@@ -442,6 +442,16 @@ VPBasicBlock::createEmptyBasicBlock(VPTransformState::CFGState &CFG) {
   return NewBB;
 }
 
+void VPIRBasicBlock::execute(VPTransformState *State) {
+  assert(getHierarchicalPredecessors().empty() &&
+         "VPIRBasicBlock cannot have predecessors at the moment");
+  assert(getHierarchicalSuccessors().empty() &&
+         "VPIRBasicBlock cannot have successors at the moment");
+
+  State->Builder.SetInsertPoint(getIRBasicBlock()->getTerminator());
+  executeRecipes(State, getIRBasicBlock());
+}
+
 void VPBasicBlock::execute(VPTransformState *State) {
   bool Replica = State->Instance && !State->Instance->isFirstIteration();
   VPBasicBlock *PrevVPBB = State->CFG.PrevVPBB;
@@ -499,16 +509,7 @@ void VPBasicBlock::execute(VPTransformState *State) {
   }
 
   // 2. Fill the IR basic block with IR instructions.
-  LLVM_DEBUG(dbgs() << "LV: vectorizing VPBB:" << getName()
-                    << " in BB:" << NewBB->getName() << '\n');
-
-  State->CFG.VPBB2IRBB[this] = NewBB;
-  State->CFG.PrevVPBB = this;
-
-  for (VPRecipeBase &Recipe : Recipes)
-    Recipe.execute(*State);
-
-  LLVM_DEBUG(dbgs() << "LV: filled BB:" << *NewBB);
+  executeRecipes(State, NewBB);
 }
 
 void VPBasicBlock::dropAllReferences(VPValue *NewValue) {
@@ -521,6 +522,19 @@ void VPBasicBlock::dropAllReferences(VPValue *NewValue) {
   }
 }
 
+void VPBasicBlock::executeRecipes(VPTransformState *State, BasicBlock *BB) {
+  LLVM_DEBUG(dbgs() << "LV: vectorizing VPBB:" << getName()
+                    << " in BB:" << BB->getName() << '\n');
+
+  State->CFG.VPBB2IRBB[this] = BB;
+  State->CFG.PrevVPBB = this;
+
+  for (VPRecipeBase &Recipe : Recipes)
+    Recipe.execute(*State);
+
+  LLVM_DEBUG(dbgs() << "LV: filled BB:" << *BB);
+}
+
 VPBasicBlock *VPBasicBlock::splitAt(iterator SplitAt) {
   assert((SplitAt == end() || SplitAt->getParent() == this) &&
          "can only split at a position in the same block");
@@ -769,8 +783,9 @@ VPlan::~VPlan() {
     delete BackedgeTakenCount;
 }
 
-VPlanPtr VPlan::createInitialVPlan(const SCEV *TripCount, ScalarEvolution &SE) {
-  VPBasicBlock *Preheader = new VPBasicBlock("ph");
+VPlanPtr VPlan::createInitialVPlan(const SCEV *TripCount, ScalarEvolution &SE,
+                                   BasicBlock *PH) {
+  VPIRBasicBlock *Preheader = new VPIRBasicBlock(PH);
   VPBasicBlock *VecPreheader = new VPBasicBlock("vector.ph");
   auto Plan = std::make_unique<VPlan>(Preheader, VecPreheader);
   Plan->TripCount =

llvm/lib/Transforms/Vectorize/VPlan.h

@@ -473,7 +473,7 @@ class VPBlockBase {
   /// that are actually instantiated. Values of this enumeration are kept in the
   /// SubclassID field of the VPBlockBase objects. They are used for concrete
   /// type identification.
-  using VPBlockTy = enum { VPBasicBlockSC, VPRegionBlockSC };
+  using VPBlockTy = enum { VPRegionBlockSC, VPBasicBlockSC, VPIRBasicBlockSC };
 
   using VPBlocksTy = SmallVectorImpl<VPBlockBase *>;
 
@@ -2833,10 +2833,13 @@ class VPBasicBlock : public VPBlockBase {
 public:
   using RecipeListTy = iplist<VPRecipeBase>;
 
-private:
+protected:
   /// The VPRecipes held in the order of output instructions to generate.
   RecipeListTy Recipes;
 
+  VPBasicBlock(const unsigned char BlockSC, const Twine &Name = "")
+      : VPBlockBase(BlockSC, Name.str()) {}
+
 public:
   VPBasicBlock(const Twine &Name = "", VPRecipeBase *Recipe = nullptr)
       : VPBlockBase(VPBasicBlockSC, Name.str()) {
@@ -2885,7 +2888,8 @@ class VPBasicBlock : public VPBlockBase {
 
   /// Method to support type inquiry through isa, cast, and dyn_cast.
   static inline bool classof(const VPBlockBase *V) {
-    return V->getVPBlockID() == VPBlockBase::VPBasicBlockSC;
+    return V->getVPBlockID() == VPBlockBase::VPBasicBlockSC ||
+           V->getVPBlockID() == VPBlockBase::VPIRBasicBlockSC;
   }
 
   void insert(VPRecipeBase *Recipe, iterator InsertPt) {
@@ -2948,12 +2952,48 @@ class VPBasicBlock : public VPBlockBase {
     return NewBlock;
   }
 
+protected:
+  /// Execute the recipes in the IR basic block \p BB.
+  void executeRecipes(VPTransformState *State, BasicBlock *BB);
+
 private:
   /// Create an IR BasicBlock to hold the output instructions generated by this
   /// VPBasicBlock, and return it. Update the CFGState accordingly.
   BasicBlock *createEmptyBasicBlock(VPTransformState::CFGState &CFG);
 };
 
+/// A special type of VPBasicBlock that wraps an existing IR basic block.
+/// Recipes of the block get added before the first non-phi instruction in the
+/// wrapped block.
+/// Note: At the moment, VPIRBasicBlock can only be used to wrap VPlan's
+/// preheader block.
+class VPIRBasicBlock : public VPBasicBlock {
+  BasicBlock *IRBB;
+
+public:
+  VPIRBasicBlock(BasicBlock *IRBB)
+      : VPBasicBlock(VPIRBasicBlockSC, "ph"), IRBB(IRBB) {}
+
+  ~VPIRBasicBlock() override {}
+
+  static inline bool classof(const VPBlockBase *V) {
+    return V->getVPBlockID() == VPBlockBase::VPIRBasicBlockSC;
+  }
+
+  /// The method which generates the output IR instructions that correspond to
+  /// this VPBasicBlock, thereby "executing" the VPlan.
+  void execute(VPTransformState *State) override;
+
+  VPIRBasicBlock *clone() override {
+    auto *NewBlock = new VPIRBasicBlock(IRBB);
+    for (VPRecipeBase &R : Recipes)
+      NewBlock->appendRecipe(R.clone());
+    return NewBlock;
+  }
+
+  BasicBlock *getIRBasicBlock() const { return IRBB; }
+};
+
 /// VPRegionBlock represents a collection of VPBasicBlocks and VPRegionBlocks
 /// which form a Single-Entry-Single-Exiting subgraph of the output IR CFG.
 /// A VPRegionBlock may indicate that its contents are to be replicated several
@@ -3142,12 +3182,12 @@ class VPlan {
   ~VPlan();
 
   /// Create initial VPlan skeleton, having an "entry" VPBasicBlock (wrapping
-  /// original scalar pre-header) which contains SCEV expansions that need to
-  /// happen before the CFG is modified; a VPBasicBlock for the vector
+  /// original scalar pre-header \p PH) which contains SCEV expansions that need
+  /// to happen before the CFG is modified; a VPBasicBlock for the vector
   /// pre-header, followed by a region for the vector loop, followed by the
   /// middle VPBasicBlock.
   static VPlanPtr createInitialVPlan(const SCEV *TripCount,
-                                     ScalarEvolution &PSE);
+                                     ScalarEvolution &PSE, BasicBlock *PH);
 
   /// Prepare the plan for execution, setting up the required live-in values.
   void prepareToExecute(Value *TripCount, Value *VectorTripCount,

llvm/lib/Transforms/Vectorize/VPlanVerifier.cpp

@@ -17,6 +17,7 @@
 #include "VPlanCFG.h"
 #include "VPlanDominatorTree.h"
 #include "llvm/ADT/DepthFirstIterator.h"
+#include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/Support/CommandLine.h"
 
 #define DEBUG_TYPE "loop-vectorize"
@@ -27,6 +28,8 @@ namespace {
 class VPlanVerifier {
   const VPDominatorTree &VPDT;
 
+  SmallPtrSet<BasicBlock *, 8> WrappedIRBBs;
+
   // Verify that phi-like recipes are at the beginning of \p VPBB, with no
   // other recipes in between. Also check that only header blocks contain
   // VPHeaderPHIRecipes.
@@ -148,6 +151,19 @@ bool VPlanVerifier::verifyVPBasicBlock(const VPBasicBlock *VPBB) {
       }
     }
   }
+
+  auto *IRBB = dyn_cast<VPIRBasicBlock>(VPBB);
+  if (!IRBB)
+    return true;
+
+  if (!WrappedIRBBs.insert(IRBB->getIRBasicBlock()).second) {
+    errs() << "Same IR basic block used by multiple wrapper blocks!\n";
+    return false;
+  }
+  if (IRBB != IRBB->getPlan()->getPreheader()) {
+    errs() << "VPIRBasicBlock can only be used as pre-header at the moment!\n";
+    return false;
+  }
   return true;
 }
 

llvm/unittests/Transforms/Vectorize/VPlanTestBase.h

@@ -67,9 +67,10 @@ class VPlanTestBase : public testing::Test {
     assert(!verifyFunction(F) && "input function must be valid");
     doAnalysis(F);
 
-    auto Plan = VPlan::createInitialVPlan(
-        SE->getBackedgeTakenCount(LI->getLoopFor(LoopHeader)), *SE);
-    VPlanHCFGBuilder HCFGBuilder(LI->getLoopFor(LoopHeader), LI.get(), *Plan);
+    Loop *L = LI->getLoopFor(LoopHeader);
+    auto Plan = VPlan::createInitialVPlan(SE->getBackedgeTakenCount(L), *SE,
+                                          L->getLoopPreheader());
+    VPlanHCFGBuilder HCFGBuilder(L, LI.get(), *Plan);
     HCFGBuilder.buildHierarchicalCFG();
     return Plan;
   }
@@ -80,9 +81,10 @@ class VPlanTestBase : public testing::Test {
     assert(!verifyFunction(F) && "input function must be valid");
     doAnalysis(F);
 
-    auto Plan = VPlan::createInitialVPlan(
-        SE->getBackedgeTakenCount(LI->getLoopFor(LoopHeader)), *SE);
-    VPlanHCFGBuilder HCFGBuilder(LI->getLoopFor(LoopHeader), LI.get(), *Plan);
+    Loop *L = LI->getLoopFor(LoopHeader);
+    auto Plan = VPlan::createInitialVPlan(SE->getBackedgeTakenCount(L), *SE,
+                                          L->getLoopPreheader());
+    VPlanHCFGBuilder HCFGBuilder(L, LI.get(), *Plan);
     HCFGBuilder.buildPlainCFG();
     return Plan;
   }