llvm-project/llvm/lib/Transforms/Vectorize/VPlan.cpp

//===- VPlan.cpp - Vectorizer Plan ----------------------------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This is the LLVM vectorization plan. It represents a candidate for
/// vectorization, allowing to plan and optimize how to vectorize a given loop
/// before generating LLVM-IR.
/// The vectorizer uses vectorization plans to estimate the costs of potential
/// candidates and if profitable to execute the desired plan, generating vector
/// LLVM-IR code.
///
//===----------------------------------------------------------------------===//

#include "VPlan.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/Twine.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Type.h"
#include "llvm/IR/Value.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/GraphWriter.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/BasicBlockUtils.h"
#include <cassert>
#include <iterator>
#include <string>
#include <vector>

using namespace llvm;

#define DEBUG_TYPE "vplan"

/// \return the VPBasicBlock that is the entry of Block, possibly indirectly.
const VPBasicBlock *VPBlockBase::getEntryBasicBlock() const {
  const VPBlockBase *Block = this;
  while (const VPRegionBlock *Region = dyn_cast<VPRegionBlock>(Block))
    Block = Region->getEntry();
  return cast<VPBasicBlock>(Block);
}

VPBasicBlock *VPBlockBase::getEntryBasicBlock() {
  VPBlockBase *Block = this;
  while (VPRegionBlock *Region = dyn_cast<VPRegionBlock>(Block))
    Block = Region->getEntry();
  return cast<VPBasicBlock>(Block);
}

/// \return the VPBasicBlock that is the exit of Block, possibly indirectly.
const VPBasicBlock *VPBlockBase::getExitBasicBlock() const {
  const VPBlockBase *Block = this;
  while (const VPRegionBlock *Region = dyn_cast<VPRegionBlock>(Block))
    Block = Region->getExit();
  return cast<VPBasicBlock>(Block);
}

VPBasicBlock *VPBlockBase::getExitBasicBlock() {
  VPBlockBase *Block = this;
  while (VPRegionBlock *Region = dyn_cast<VPRegionBlock>(Block))
    Block = Region->getExit();
  return cast<VPBasicBlock>(Block);
}

VPBlockBase *VPBlockBase::getEnclosingBlockWithSuccessors() {
  if (!Successors.empty() || !Parent)
    return this;
  assert(Parent->getExit() == this &&
         "Block w/o successors not the exit of its parent.");
  return Parent->getEnclosingBlockWithSuccessors();
}

VPBlockBase *VPBlockBase::getEnclosingBlockWithPredecessors() {
  if (!Predecessors.empty() || !Parent)
    return this;
  assert(Parent->getEntry() == this &&
         "Block w/o predecessors not the entry of its parent.");
  return Parent->getEnclosingBlockWithPredecessors();
}

void VPBlockBase::deleteCFG(VPBlockBase *Entry) {
  SmallVector<VPBlockBase *, 8> Blocks;
  for (VPBlockBase *Block : depth_first(Entry))
    Blocks.push_back(Block);

  for (VPBlockBase *Block : Blocks)
    delete Block;
}

BasicBlock *
VPBasicBlock::createEmptyBasicBlock(VPTransformState::CFGState &CFG) {
  // BB stands for IR BasicBlocks. VPBB stands for VPlan VPBasicBlocks.
  // Pred stands for Predessor. Prev stands for Previous - last visited/created.
  BasicBlock *PrevBB = CFG.PrevBB;
  BasicBlock *NewBB = BasicBlock::Create(PrevBB->getContext(), getName(),
                                         PrevBB->getParent(), CFG.LastBB);
  DEBUG(dbgs() << "LV: created " << NewBB->getName() << '\n');

  // Hook up the new basic block to its predecessors.
  for (VPBlockBase *PredVPBlock : getHierarchicalPredecessors()) {
    VPBasicBlock *PredVPBB = PredVPBlock->getExitBasicBlock();
    auto &PredVPSuccessors = PredVPBB->getSuccessors();
    BasicBlock *PredBB = CFG.VPBB2IRBB[PredVPBB];
    assert(PredBB && "Predecessor basic-block not found building successor.");
    auto *PredBBTerminator = PredBB->getTerminator();
    DEBUG(dbgs() << "LV: draw edge from" << PredBB->getName() << '\n');
    if (isa<UnreachableInst>(PredBBTerminator)) {
      assert(PredVPSuccessors.size() == 1 &&
             "Predecessor ending w/o branch must have single successor.");
      PredBBTerminator->eraseFromParent();
      BranchInst::Create(NewBB, PredBB);
    } else {
      assert(PredVPSuccessors.size() == 2 &&
             "Predecessor ending with branch must have two successors.");
      unsigned idx = PredVPSuccessors.front() == this ? 0 : 1;
      assert(!PredBBTerminator->getSuccessor(idx) &&
             "Trying to reset an existing successor block.");
      PredBBTerminator->setSuccessor(idx, NewBB);
    }
  }
  return NewBB;
}

void VPBasicBlock::execute(VPTransformState *State) {
  bool Replica = State->Instance &&
                 !(State->Instance->Part == 0 && State->Instance->Lane == 0);
  VPBasicBlock *PrevVPBB = State->CFG.PrevVPBB;
  VPBlockBase *SingleHPred = nullptr;
  BasicBlock *NewBB = State->CFG.PrevBB; // Reuse it if possible.

  // 1. Create an IR basic block, or reuse the last one if possible.
  // The last IR basic block is reused, as an optimization, in three cases:
  // A. the first VPBB reuses the loop header BB - when PrevVPBB is null;
  // B. when the current VPBB has a single (hierarchical) predecessor which
  //    is PrevVPBB and the latter has a single (hierarchical) successor; and
  // C. when the current VPBB is an entry of a region replica - where PrevVPBB
  //    is the exit of this region from a previous instance, or the predecessor
  //    of this region.
  if (PrevVPBB && /* A */
      !((SingleHPred = getSingleHierarchicalPredecessor()) &&
        SingleHPred->getExitBasicBlock() == PrevVPBB &&
        PrevVPBB->getSingleHierarchicalSuccessor()) && /* B */
      !(Replica && getPredecessors().empty())) {       /* C */
    NewBB = createEmptyBasicBlock(State->CFG);
    State->Builder.SetInsertPoint(NewBB);
    // Temporarily terminate with unreachable until CFG is rewired.
    UnreachableInst *Terminator = State->Builder.CreateUnreachable();
    State->Builder.SetInsertPoint(Terminator);
    // Register NewBB in its loop. In innermost loops its the same for all BB's.
    Loop *L = State->LI->getLoopFor(State->CFG.LastBB);
    L->addBasicBlockToLoop(NewBB, *State->LI);
    State->CFG.PrevBB = NewBB;
  }

  // 2. Fill the IR basic block with IR instructions.
  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);

  DEBUG(dbgs() << "LV: filled BB:" << *NewBB);
}

void VPRegionBlock::execute(VPTransformState *State) {
  ReversePostOrderTraversal<VPBlockBase *> RPOT(Entry);

  if (!isReplicator()) {
    // Visit the VPBlocks connected to "this", starting from it.
    for (VPBlockBase *Block : RPOT) {
      DEBUG(dbgs() << "LV: VPBlock in RPO " << Block->getName() << '\n');
      Block->execute(State);
    }
    return;
  }

  assert(!State->Instance && "Replicating a Region with non-null instance.");

  // Enter replicating mode.
  State->Instance = {0, 0};

  for (unsigned Part = 0, UF = State->UF; Part < UF; ++Part) {
    State->Instance->Part = Part;
    for (unsigned Lane = 0, VF = State->VF; Lane < VF; ++Lane) {
      State->Instance->Lane = Lane;
      // Visit the VPBlocks connected to \p this, starting from it.
      for (VPBlockBase *Block : RPOT) {
        DEBUG(dbgs() << "LV: VPBlock in RPO " << Block->getName() << '\n');
        Block->execute(State);
      }
    }
  }

  // Exit replicating mode.
  State->Instance.reset();
}

/// Generate the code inside the body of the vectorized loop. Assumes a single
/// LoopVectorBody basic-block was created for this. Introduce additional
/// basic-blocks as needed, and fill them all.
void VPlan::execute(VPTransformState *State) {
  BasicBlock *VectorPreHeaderBB = State->CFG.PrevBB;
  BasicBlock *VectorHeaderBB = VectorPreHeaderBB->getSingleSuccessor();
  assert(VectorHeaderBB && "Loop preheader does not have a single successor.");
  BasicBlock *VectorLatchBB = VectorHeaderBB;

  // 1. Make room to generate basic-blocks inside loop body if needed.
  VectorLatchBB = VectorHeaderBB->splitBasicBlock(
      VectorHeaderBB->getFirstInsertionPt(), "vector.body.latch");
  Loop *L = State->LI->getLoopFor(VectorHeaderBB);
  L->addBasicBlockToLoop(VectorLatchBB, *State->LI);
  // Remove the edge between Header and Latch to allow other connections.
  // Temporarily terminate with unreachable until CFG is rewired.
  // Note: this asserts the generated code's assumption that
  // getFirstInsertionPt() can be dereferenced into an Instruction.
  VectorHeaderBB->getTerminator()->eraseFromParent();
  State->Builder.SetInsertPoint(VectorHeaderBB);
  UnreachableInst *Terminator = State->Builder.CreateUnreachable();
  State->Builder.SetInsertPoint(Terminator);

  // 2. Generate code in loop body.
  State->CFG.PrevVPBB = nullptr;
  State->CFG.PrevBB = VectorHeaderBB;
  State->CFG.LastBB = VectorLatchBB;

  for (VPBlockBase *Block : depth_first(Entry))
    Block->execute(State);

  // 3. Merge the temporary latch created with the last basic-block filled.
  BasicBlock *LastBB = State->CFG.PrevBB;
  // Connect LastBB to VectorLatchBB to facilitate their merge.
  assert(isa<UnreachableInst>(LastBB->getTerminator()) &&
         "Expected VPlan CFG to terminate with unreachable");
  LastBB->getTerminator()->eraseFromParent();
  BranchInst::Create(VectorLatchBB, LastBB);

  // Merge LastBB with Latch.
  bool Merged = MergeBlockIntoPredecessor(VectorLatchBB, nullptr, State->LI);
  (void)Merged;
  assert(Merged && "Could not merge last basic block with latch.");
  VectorLatchBB = LastBB;

  updateDominatorTree(State->DT, VectorPreHeaderBB, VectorLatchBB);
}

void VPlan::updateDominatorTree(DominatorTree *DT, BasicBlock *LoopPreHeaderBB,
                                BasicBlock *LoopLatchBB) {
  BasicBlock *LoopHeaderBB = LoopPreHeaderBB->getSingleSuccessor();
  assert(LoopHeaderBB && "Loop preheader does not have a single successor.");
  DT->addNewBlock(LoopHeaderBB, LoopPreHeaderBB);
  // The vector body may be more than a single basic-block by this point.
  // Update the dominator tree information inside the vector body by propagating
  // it from header to latch, expecting only triangular control-flow, if any.
  BasicBlock *PostDomSucc = nullptr;
  for (auto *BB = LoopHeaderBB; BB != LoopLatchBB; BB = PostDomSucc) {
    // Get the list of successors of this block.
    std::vector<BasicBlock *> Succs(succ_begin(BB), succ_end(BB));
    assert(Succs.size() <= 2 &&
           "Basic block in vector loop has more than 2 successors.");
    PostDomSucc = Succs[0];
    if (Succs.size() == 1) {
      assert(PostDomSucc->getSinglePredecessor() &&
             "PostDom successor has more than one predecessor.");
      DT->addNewBlock(PostDomSucc, BB);
      continue;
    }
    BasicBlock *InterimSucc = Succs[1];
    if (PostDomSucc->getSingleSuccessor() == InterimSucc) {
      PostDomSucc = Succs[1];
      InterimSucc = Succs[0];
    }
    assert(InterimSucc->getSingleSuccessor() == PostDomSucc &&
           "One successor of a basic block does not lead to the other.");
    assert(InterimSucc->getSinglePredecessor() &&
           "Interim successor has more than one predecessor.");
    assert(std::distance(pred_begin(PostDomSucc), pred_end(PostDomSucc)) == 2 &&
           "PostDom successor has more than two predecessors.");
    DT->addNewBlock(InterimSucc, BB);
    DT->addNewBlock(PostDomSucc, BB);
  }
}

const Twine VPlanPrinter::getUID(const VPBlockBase *Block) {
  return (isa<VPRegionBlock>(Block) ? "cluster_N" : "N") +
         Twine(getOrCreateBID(Block));
}

const Twine VPlanPrinter::getOrCreateName(const VPBlockBase *Block) {
  const std::string &Name = Block->getName();
  if (!Name.empty())
    return Name;
  return "VPB" + Twine(getOrCreateBID(Block));
}

void VPlanPrinter::dump() {
  Depth = 1;
  bumpIndent(0);
  OS << "digraph VPlan {\n";
  OS << "graph [labelloc=t, fontsize=30; label=\"Vectorization Plan";
  if (!Plan.getName().empty())
    OS << "\\n" << DOT::EscapeString(Plan.getName());
  OS << "\"]\n";
  OS << "node [shape=rect, fontname=Courier, fontsize=30]\n";
  OS << "edge [fontname=Courier, fontsize=30]\n";
  OS << "compound=true\n";

  for (VPBlockBase *Block : depth_first(Plan.getEntry()))
    dumpBlock(Block);

  OS << "}\n";
}

void VPlanPrinter::dumpBlock(const VPBlockBase *Block) {
  if (const VPBasicBlock *BasicBlock = dyn_cast<VPBasicBlock>(Block))
    dumpBasicBlock(BasicBlock);
  else if (const VPRegionBlock *Region = dyn_cast<VPRegionBlock>(Block))
    dumpRegion(Region);
  else
    llvm_unreachable("Unsupported kind of VPBlock.");
}

void VPlanPrinter::drawEdge(const VPBlockBase *From, const VPBlockBase *To,
                            bool Hidden, const Twine &Label) {
  // Due to "dot" we print an edge between two regions as an edge between the
  // exit basic block and the entry basic of the respective regions.
  const VPBlockBase *Tail = From->getExitBasicBlock();
  const VPBlockBase *Head = To->getEntryBasicBlock();
  OS << Indent << getUID(Tail) << " -> " << getUID(Head);
  OS << " [ label=\"" << Label << '\"';
  if (Tail != From)
    OS << " ltail=" << getUID(From);
  if (Head != To)
    OS << " lhead=" << getUID(To);
  if (Hidden)
    OS << "; splines=none";
  OS << "]\n";
}

void VPlanPrinter::dumpEdges(const VPBlockBase *Block) {
  auto &Successors = Block->getSuccessors();
  if (Successors.size() == 1)
    drawEdge(Block, Successors.front(), false, "");
  else if (Successors.size() == 2) {
    drawEdge(Block, Successors.front(), false, "T");
    drawEdge(Block, Successors.back(), false, "F");
  } else {
    unsigned SuccessorNumber = 0;
    for (auto *Successor : Successors)
      drawEdge(Block, Successor, false, Twine(SuccessorNumber++));
  }
}

void VPlanPrinter::dumpBasicBlock(const VPBasicBlock *BasicBlock) {
  OS << Indent << getUID(BasicBlock) << " [label =\n";
  bumpIndent(1);
  OS << Indent << "\"" << DOT::EscapeString(BasicBlock->getName()) << ":\\n\"";
  bumpIndent(1);
  for (const VPRecipeBase &Recipe : *BasicBlock)
    Recipe.print(OS, Indent);
  bumpIndent(-2);
  OS << "\n" << Indent << "]\n";
  dumpEdges(BasicBlock);
}

void VPlanPrinter::dumpRegion(const VPRegionBlock *Region) {
  OS << Indent << "subgraph " << getUID(Region) << " {\n";
  bumpIndent(1);
  OS << Indent << "fontname=Courier\n"
     << Indent << "label=\""
     << DOT::EscapeString(Region->isReplicator() ? "<xVFxUF> " : "<x1> ")
     << DOT::EscapeString(Region->getName()) << "\"\n";
  // Dump the blocks of the region.
  assert(Region->getEntry() && "Region contains no inner blocks.");
  for (const VPBlockBase *Block : depth_first(Region->getEntry()))
    dumpBlock(Block);
  bumpIndent(-1);
  OS << Indent << "}\n";
  dumpEdges(Region);
}

void VPlanPrinter::printAsIngredient(raw_ostream &O, Value *V) {
  std::string IngredientString;
  raw_string_ostream RSO(IngredientString);
  if (auto *Inst = dyn_cast<Instruction>(V)) {
    if (!Inst->getType()->isVoidTy()) {
      Inst->printAsOperand(RSO, false);
      RSO << " = ";
    }
    RSO << Inst->getOpcodeName() << " ";
    unsigned E = Inst->getNumOperands();
    if (E > 0) {
      Inst->getOperand(0)->printAsOperand(RSO, false);
      for (unsigned I = 1; I < E; ++I)
        Inst->getOperand(I)->printAsOperand(RSO << ", ", false);
    }
  } else // !Inst
    V->printAsOperand(RSO, false);
  RSO.flush();
  O << DOT::EscapeString(IngredientString);
}