SampleProfileLoaderBaseImpl.h

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////===- SampleProfileLoadBaseImpl.h - Profile loader base impl --*- C++-*-===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
/// \file
/// This file provides the interface for the sampled PGO profile loader base
/// implementation.
//
//===----------------------------------------------------------------------===//
 
#ifndef LLVM_TRANSFORMS_UTILS_SAMPLEPROFILELOADERBASEIMPL_H
#define LLVM_TRANSFORMS_UTILS_SAMPLEPROFILELOADERBASEIMPL_H
 
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/LoopInfo.h"
#include "llvm/Analysis/OptimizationRemarkEmitter.h"
#include "llvm/Analysis/PostDominators.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/CFG.h"
#include "llvm/IR/DebugInfoMetadata.h"
#include "llvm/IR/DebugLoc.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/Instructions.h"
#include "llvm/IR/Module.h"
#include "llvm/ProfileData/SampleProf.h"
#include "llvm/ProfileData/SampleProfReader.h"
#include "llvm/Support/CommandLine.h"
#include "llvm/Support/GenericDomTree.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/SampleProfileLoaderBaseUtil.h"
 
namespace llvm {
using namespace sampleprof;
using namespace sampleprofutil;
using ProfileCount = Function::ProfileCount;
 
#define DEBUG_TYPE "sample-profile-impl"
 
namespace afdo_detail {
 
template <typename BlockT> struct IRTraits;
template <> struct IRTraits<BasicBlock> {
  using InstructionT = Instruction;
  using BasicBlockT = BasicBlock;
  using FunctionT = Function;
  using BlockFrequencyInfoT = BlockFrequencyInfo;
  using LoopT = Loop;
  using LoopInfoPtrT = std::unique_ptr<LoopInfo>;
  using DominatorTreePtrT = std::unique_ptr<DominatorTree>;
  using PostDominatorTreeT = PostDominatorTree;
  using PostDominatorTreePtrT = std::unique_ptr<PostDominatorTree>;
  using OptRemarkEmitterT = OptimizationRemarkEmitter;
  using OptRemarkAnalysisT = OptimizationRemarkAnalysis;
  using PredRangeT = pred_range;
  using SuccRangeT = succ_range;
  static Function &getFunction(Function &F) { return F; }
  static const BasicBlock *getEntryBB(const Function *F) {
    return &F->getEntryBlock();
  }
  static pred_range getPredecessors(BasicBlock *BB) { return predecessors(BB); }
  static succ_range getSuccessors(BasicBlock *BB) { return successors(BB); }
};
 
} // end namespace afdo_detail
 
extern cl::opt<unsigned> SampleProfileMaxPropagateIterations;
extern cl::opt<unsigned> SampleProfileRecordCoverage;
extern cl::opt<unsigned> SampleProfileSampleCoverage;
extern cl::opt<bool> NoWarnSampleUnused;
 
template <typename BT> class SampleProfileLoaderBaseImpl {
public:
  SampleProfileLoaderBaseImpl(std::string Name, std::string RemapName)
      : Filename(Name), RemappingFilename(RemapName) {}
  void dump() { Reader->dump(); }
 
  using InstructionT = typename afdo_detail::IRTraits<BT>::InstructionT;
  using BasicBlockT = typename afdo_detail::IRTraits<BT>::BasicBlockT;
  using BlockFrequencyInfoT =
      typename afdo_detail::IRTraits<BT>::BlockFrequencyInfoT;
  using FunctionT = typename afdo_detail::IRTraits<BT>::FunctionT;
  using LoopT = typename afdo_detail::IRTraits<BT>::LoopT;
  using LoopInfoPtrT = typename afdo_detail::IRTraits<BT>::LoopInfoPtrT;
  using DominatorTreePtrT =
      typename afdo_detail::IRTraits<BT>::DominatorTreePtrT;
  using PostDominatorTreePtrT =
      typename afdo_detail::IRTraits<BT>::PostDominatorTreePtrT;
  using PostDominatorTreeT =
      typename afdo_detail::IRTraits<BT>::PostDominatorTreeT;
  using OptRemarkEmitterT =
      typename afdo_detail::IRTraits<BT>::OptRemarkEmitterT;
  using OptRemarkAnalysisT =
      typename afdo_detail::IRTraits<BT>::OptRemarkAnalysisT;
  using PredRangeT = typename afdo_detail::IRTraits<BT>::PredRangeT;
  using SuccRangeT = typename afdo_detail::IRTraits<BT>::SuccRangeT;
 
  using BlockWeightMap = DenseMap<const BasicBlockT *, uint64_t>;
  using EquivalenceClassMap =
      DenseMap<const BasicBlockT *, const BasicBlockT *>;
  using Edge = std::pair<const BasicBlockT *, const BasicBlockT *>;
  using EdgeWeightMap = DenseMap<Edge, uint64_t>;
  using BlockEdgeMap =
      DenseMap<const BasicBlockT *, SmallVector<const BasicBlockT *, 8>>;
 
protected:
  ~SampleProfileLoaderBaseImpl() = default;
  friend class SampleCoverageTracker;
 
  Function &getFunction(FunctionT &F) {
    return afdo_detail::IRTraits<BT>::getFunction(F);
  }
  const BasicBlockT *getEntryBB(const FunctionT *F) {
    return afdo_detail::IRTraits<BT>::getEntryBB(F);
  }
  PredRangeT getPredecessors(BasicBlockT *BB) {
    return afdo_detail::IRTraits<BT>::getPredecessors(BB);
  }
  SuccRangeT getSuccessors(BasicBlockT *BB) {
    return afdo_detail::IRTraits<BT>::getSuccessors(BB);
  }
 
  unsigned getFunctionLoc(FunctionT &Func);
  virtual ErrorOr<uint64_t> getInstWeight(const InstructionT &Inst);
  ErrorOr<uint64_t> getInstWeightImpl(const InstructionT &Inst);
  ErrorOr<uint64_t> getBlockWeight(const BasicBlockT *BB);
  mutable DenseMap<const DILocation *, const FunctionSamples *>
      DILocation2SampleMap;
  virtual const FunctionSamples *
  findFunctionSamples(const InstructionT &I) const;
  void printEdgeWeight(raw_ostream &OS, Edge E);
  void printBlockWeight(raw_ostream &OS, const BasicBlockT *BB) const;
  void printBlockEquivalence(raw_ostream &OS, const BasicBlockT *BB);
  bool computeBlockWeights(FunctionT &F);
  void findEquivalenceClasses(FunctionT &F);
  void findEquivalencesFor(BasicBlockT *BB1,
                           ArrayRef<BasicBlockT *> Descendants,
                           PostDominatorTreeT *DomTree);
  void propagateWeights(FunctionT &F);
  uint64_t visitEdge(Edge E, unsigned *NumUnknownEdges, Edge *UnknownEdge);
  void buildEdges(FunctionT &F);
  bool propagateThroughEdges(FunctionT &F, bool UpdateBlockCount);
  void clearFunctionData(bool ResetDT = true);
  void computeDominanceAndLoopInfo(FunctionT &F);
  bool
  computeAndPropagateWeights(FunctionT &F,
                             const DenseSet<GlobalValue::GUID> &InlinedGUIDs);
  void emitCoverageRemarks(FunctionT &F);
 
  /// Map basic blocks to their computed weights.
  ///
  /// The weight of a basic block is defined to be the maximum
  /// of all the instruction weights in that block.
  BlockWeightMap BlockWeights;
 
  /// Map edges to their computed weights.
  ///
  /// Edge weights are computed by propagating basic block weights in
  /// SampleProfile::propagateWeights.
  EdgeWeightMap EdgeWeights;
 
  /// Set of visited blocks during propagation.
  SmallPtrSet<const BasicBlockT *, 32> VisitedBlocks;
 
  /// Set of visited edges during propagation.
  SmallSet<Edge, 32> VisitedEdges;
 
  /// Equivalence classes for block weights.
  ///
  /// Two blocks BB1 and BB2 are in the same equivalence class if they
  /// dominate and post-dominate each other, and they are in the same loop
  /// nest. When this happens, the two blocks are guaranteed to execute
  /// the same number of times.
  EquivalenceClassMap EquivalenceClass;
 
  /// Dominance, post-dominance and loop information.
  DominatorTreePtrT DT;
  PostDominatorTreePtrT PDT;
  LoopInfoPtrT LI;
 
  /// Predecessors for each basic block in the CFG.
  BlockEdgeMap Predecessors;
 
  /// Successors for each basic block in the CFG.
  BlockEdgeMap Successors;
 
  /// Profile coverage tracker.
  SampleCoverageTracker CoverageTracker;
 
  /// Profile reader object.
  std::unique_ptr<SampleProfileReader> Reader;
 
  /// Samples collected for the body of this function.
  FunctionSamples *Samples = nullptr;
 
  /// Name of the profile file to load.
  std::string Filename;
 
  /// Name of the profile remapping file to load.
  std::string RemappingFilename;
 
  /// Profile Summary Info computed from sample profile.
  ProfileSummaryInfo *PSI = nullptr;
 
  /// Optimization Remark Emitter used to emit diagnostic remarks.
  OptRemarkEmitterT *ORE = nullptr;
};
 
/// Clear all the per-function data used to load samples and propagate weights.
template <typename BT>
void SampleProfileLoaderBaseImpl<BT>::clearFunctionData(bool ResetDT) {
  BlockWeights.clear();
  EdgeWeights.clear();
  VisitedBlocks.clear();
  VisitedEdges.clear();
  EquivalenceClass.clear();
  if (ResetDT) {
    DT = nullptr;
    PDT = nullptr;
    LI = nullptr;
  }
  Predecessors.clear();
  Successors.clear();
  CoverageTracker.clear();
}
 
#ifndef NDEBUG
/// Print the weight of edge \p E on stream \p OS.
///
/// \param OS  Stream to emit the output to.
/// \param E  Edge to print.
template <typename BT>
void SampleProfileLoaderBaseImpl<BT>::printEdgeWeight(raw_ostream &OS, Edge E) {
  OS << "weight[" << E.first->getName() << "->" << E.second->getName()
     << "]: " << EdgeWeights[E] << "\n";
}
 
/// Print the equivalence class of block \p BB on stream \p OS.
///
/// \param OS  Stream to emit the output to.
/// \param BB  Block to print.
template <typename BT>
void SampleProfileLoaderBaseImpl<BT>::printBlockEquivalence(
    raw_ostream &OS, const BasicBlockT *BB) {
  const BasicBlockT *Equiv = EquivalenceClass[BB];
  OS << "equivalence[" << BB->getName()
     << "]: " << ((Equiv) ? EquivalenceClass[BB]->getName() : "NONE") << "\n";
}
 
/// Print the weight of block \p BB on stream \p OS.
///
/// \param OS  Stream to emit the output to.
/// \param BB  Block to print.
template <typename BT>
void SampleProfileLoaderBaseImpl<BT>::printBlockWeight(
    raw_ostream &OS, const BasicBlockT *BB) const {
  const auto &I = BlockWeights.find(BB);
  uint64_t W = (I == BlockWeights.end() ? 0 : I->second);
  OS << "weight[" << BB->getName() << "]: " << W << "\n";
}
#endif
 
/// Get the weight for an instruction.
///
/// The "weight" of an instruction \p Inst is the number of samples
/// collected on that instruction at runtime. To retrieve it, we
/// need to compute the line number of \p Inst relative to the start of its
/// function. We use HeaderLineno to compute the offset. We then
/// look up the samples collected for \p Inst using BodySamples.
///
/// \param Inst Instruction to query.
///
/// \returns the weight of \p Inst.
template <typename BT>
ErrorOr<uint64_t>
SampleProfileLoaderBaseImpl<BT>::getInstWeight(const InstructionT &Inst) {
  return getInstWeightImpl(Inst);
}
 
template <typename BT>
ErrorOr<uint64_t>
SampleProfileLoaderBaseImpl<BT>::getInstWeightImpl(const InstructionT &Inst) {
  const FunctionSamples *FS = findFunctionSamples(Inst);
  if (!FS)
    return std::error_code();
 
  const DebugLoc &DLoc = Inst.getDebugLoc();
  if (!DLoc)
    return std::error_code();
 
  const DILocation *DIL = DLoc;
  uint32_t LineOffset = FunctionSamples::getOffset(DIL);
  uint32_t Discriminator;
  if (EnableFSDiscriminator)
    Discriminator = DIL->getDiscriminator();
  else
    Discriminator = DIL->getBaseDiscriminator();
 
  ErrorOr<uint64_t> R = FS->findSamplesAt(LineOffset, Discriminator);
  if (R) {
    bool FirstMark =
        CoverageTracker.markSamplesUsed(FS, LineOffset, Discriminator, R.get());
    if (FirstMark) {
      ORE->emit([&]() {
        OptRemarkAnalysisT Remark(DEBUG_TYPE, "AppliedSamples", &Inst);
        Remark << "Applied " << ore::NV("NumSamples", *R);
        Remark << " samples from profile (offset: ";
        Remark << ore::NV("LineOffset", LineOffset);
        if (Discriminator) {
          Remark << ".";
          Remark << ore::NV("Discriminator", Discriminator);
        }
        Remark << ")";
        return Remark;
      });
    }
    LLVM_DEBUG(dbgs() << "    " << DLoc.getLine() << "." << Discriminator << ":"
                      << Inst << " (line offset: " << LineOffset << "."
                      << Discriminator << " - weight: " << R.get() << ")\n");
  }
  return R;
}
 
/// Compute the weight of a basic block.
///
/// The weight of basic block \p BB is the maximum weight of all the
/// instructions in BB.
///
/// \param BB The basic block to query.
///
/// \returns the weight for \p BB.
template <typename BT>
ErrorOr<uint64_t>
SampleProfileLoaderBaseImpl<BT>::getBlockWeight(const BasicBlockT *BB) {
  uint64_t Max = 0;
  bool HasWeight = false;
  for (auto &I : *BB) {
    const ErrorOr<uint64_t> &R = getInstWeight(I);
    if (R) {
      Max = std::max(Max, R.get());
      HasWeight = true;
    }
  }
  return HasWeight ? ErrorOr<uint64_t>(Max) : std::error_code();
}
 
/// Compute and store the weights of every basic block.
///
/// This populates the BlockWeights map by computing
/// the weights of every basic block in the CFG.
///
/// \param F The function to query.
template <typename BT>
bool SampleProfileLoaderBaseImpl<BT>::computeBlockWeights(FunctionT &F) {
  bool Changed = false;
  LLVM_DEBUG(dbgs() << "Block weights\n");
  for (const auto &BB : F) {
    ErrorOr<uint64_t> Weight = getBlockWeight(&BB);
    if (Weight) {
      BlockWeights[&BB] = Weight.get();
      VisitedBlocks.insert(&BB);
      Changed = true;
    }
    LLVM_DEBUG(printBlockWeight(dbgs(), &BB));
  }
 
  return Changed;
}
 
/// Get the FunctionSamples for an instruction.
///
/// The FunctionSamples of an instruction \p Inst is the inlined instance
/// in which that instruction is coming from. We traverse the inline stack
/// of that instruction, and match it with the tree nodes in the profile.
///
/// \param Inst Instruction to query.
///
/// \returns the FunctionSamples pointer to the inlined instance.
template <typename BT>
const FunctionSamples *SampleProfileLoaderBaseImpl<BT>::findFunctionSamples(
    const InstructionT &Inst) const {
  const DILocation *DIL = Inst.getDebugLoc();
  if (!DIL)
    return Samples;
 
  auto it = DILocation2SampleMap.try_emplace(DIL, nullptr);
  if (it.second) {
    it.first->second = Samples->findFunctionSamples(DIL, Reader->getRemapper());
  }
  return it.first->second;
}
 
/// Find equivalence classes for the given block.
///
/// This finds all the blocks that are guaranteed to execute the same
/// number of times as \p BB1. To do this, it traverses all the
/// descendants of \p BB1 in the dominator or post-dominator tree.
///
/// A block BB2 will be in the same equivalence class as \p BB1 if
/// the following holds:
///
/// 1- \p BB1 is a descendant of BB2 in the opposite tree. So, if BB2
///    is a descendant of \p BB1 in the dominator tree, then BB2 should
///    dominate BB1 in the post-dominator tree.
///
/// 2- Both BB2 and \p BB1 must be in the same loop.
///
/// For every block BB2 that meets those two requirements, we set BB2's
/// equivalence class to \p BB1.
///
/// \param BB1  Block to check.
/// \param Descendants  Descendants of \p BB1 in either the dom or pdom tree.
/// \param DomTree  Opposite dominator tree. If \p Descendants is filled
///                 with blocks from \p BB1's dominator tree, then
///                 this is the post-dominator tree, and vice versa.
template <typename BT>
void SampleProfileLoaderBaseImpl<BT>::findEquivalencesFor(
    BasicBlockT *BB1, ArrayRef<BasicBlockT *> Descendants,
    PostDominatorTreeT *DomTree) {
  const BasicBlockT *EC = EquivalenceClass[BB1];
  uint64_t Weight = BlockWeights[EC];
  for (const auto *BB2 : Descendants) {
    bool IsDomParent = DomTree->dominates(BB2, BB1);
    bool IsInSameLoop = LI->getLoopFor(BB1) == LI->getLoopFor(BB2);
    if (BB1 != BB2 && IsDomParent && IsInSameLoop) {
      EquivalenceClass[BB2] = EC;
      // If BB2 is visited, then the entire EC should be marked as visited.
      if (VisitedBlocks.count(BB2)) {
        VisitedBlocks.insert(EC);
      }
 
      // If BB2 is heavier than BB1, make BB2 have the same weight
      // as BB1.
      //
      // Note that we don't worry about the opposite situation here
      // (when BB2 is lighter than BB1). We will deal with this
      // during the propagation phase. Right now, we just want to
      // make sure that BB1 has the largest weight of all the
      // members of its equivalence set.
      Weight = std::max(Weight, BlockWeights[BB2]);
    }
  }
  const BasicBlockT *EntryBB = getEntryBB(EC->getParent());
  if (EC == EntryBB) {
    BlockWeights[EC] = Samples->getHeadSamples() + 1;
  } else {
    BlockWeights[EC] = Weight;
  }
}
 
/// Find equivalence classes.
///
/// Since samples may be missing from blocks, we can fill in the gaps by setting
/// the weights of all the blocks in the same equivalence class to the same
/// weight. To compute the concept of equivalence, we use dominance and loop
/// information. Two blocks B1 and B2 are in the same equivalence class if B1
/// dominates B2, B2 post-dominates B1 and both are in the same loop.
///
/// \param F The function to query.
template <typename BT>
void SampleProfileLoaderBaseImpl<BT>::findEquivalenceClasses(FunctionT &F) {
  SmallVector<BasicBlockT *, 8> DominatedBBs;
  LLVM_DEBUG(dbgs() << "\nBlock equivalence classes\n");
  // Find equivalence sets based on dominance and post-dominance information.
  for (auto &BB : F) {
    BasicBlockT *BB1 = &BB;
 
    // Compute BB1's equivalence class once.
    if (EquivalenceClass.count(BB1)) {
      LLVM_DEBUG(printBlockEquivalence(dbgs(), BB1));
      continue;
    }
 
    // By default, blocks are in their own equivalence class.
    EquivalenceClass[BB1] = BB1;
 
    // Traverse all the blocks dominated by BB1. We are looking for
    // every basic block BB2 such that:
    //
    // 1- BB1 dominates BB2.
    // 2- BB2 post-dominates BB1.
    // 3- BB1 and BB2 are in the same loop nest.
    //
    // If all those conditions hold, it means that BB2 is executed
    // as many times as BB1, so they are placed in the same equivalence
    // class by making BB2's equivalence class be BB1.
    DominatedBBs.clear();
    DT->getDescendants(BB1, DominatedBBs);
    findEquivalencesFor(BB1, DominatedBBs, &*PDT);
 
    LLVM_DEBUG(printBlockEquivalence(dbgs(), BB1));
  }
 
  // Assign weights to equivalence classes.
  //
  // All the basic blocks in the same equivalence class will execute
  // the same number of times. Since we know that the head block in
  // each equivalence class has the largest weight, assign that weight
  // to all the blocks in that equivalence class.
  LLVM_DEBUG(
      dbgs() << "\nAssign the same weight to all blocks in the same class\n");
  for (auto &BI : F) {
    const BasicBlockT *BB = &BI;
    const BasicBlockT *EquivBB = EquivalenceClass[BB];
    if (BB != EquivBB)
      BlockWeights[BB] = BlockWeights[EquivBB];
    LLVM_DEBUG(printBlockWeight(dbgs(), BB));
  }
}
 
/// Visit the given edge to decide if it has a valid weight.
///
/// If \p E has not been visited before, we copy to \p UnknownEdge
/// and increment the count of unknown edges.
///
/// \param E  Edge to visit.
/// \param NumUnknownEdges  Current number of unknown edges.
/// \param UnknownEdge  Set if E has not been visited before.
///
/// \returns E's weight, if known. Otherwise, return 0.
template <typename BT>
uint64_t SampleProfileLoaderBaseImpl<BT>::visitEdge(Edge E,
                                                    unsigned *NumUnknownEdges,
                                                    Edge *UnknownEdge) {
  if (!VisitedEdges.count(E)) {
    (*NumUnknownEdges)++;
    *UnknownEdge = E;
    return 0;
  }
 
  return EdgeWeights[E];
}
 
/// Propagate weights through incoming/outgoing edges.
///
/// If the weight of a basic block is known, and there is only one edge
/// with an unknown weight, we can calculate the weight of that edge.
///
/// Similarly, if all the edges have a known count, we can calculate the
/// count of the basic block, if needed.
///
/// \param F  Function to process.
/// \param UpdateBlockCount  Whether we should update basic block counts that
///                          has already been annotated.
///
/// \returns  True if new weights were assigned to edges or blocks.
template <typename BT>
bool SampleProfileLoaderBaseImpl<BT>::propagateThroughEdges(
    FunctionT &F, bool UpdateBlockCount) {
  bool Changed = false;
  LLVM_DEBUG(dbgs() << "\nPropagation through edges\n");
  for (const auto &BI : F) {
    const BasicBlockT *BB = &BI;
    const BasicBlockT *EC = EquivalenceClass[BB];
 
    // Visit all the predecessor and successor edges to determine
    // which ones have a weight assigned already. Note that it doesn't
    // matter that we only keep track of a single unknown edge. The
    // only case we are interested in handling is when only a single
    // edge is unknown (see setEdgeOrBlockWeight).
    for (unsigned i = 0; i < 2; i++) {
      uint64_t TotalWeight = 0;
      unsigned NumUnknownEdges = 0, NumTotalEdges = 0;
      Edge UnknownEdge, SelfReferentialEdge, SingleEdge;
 
      if (i == 0) {
        // First, visit all predecessor edges.
        NumTotalEdges = Predecessors[BB].size();
        for (auto *Pred : Predecessors[BB]) {
          Edge E = std::make_pair(Pred, BB);
          TotalWeight += visitEdge(E, &NumUnknownEdges, &UnknownEdge);
          if (E.first == E.second)
            SelfReferentialEdge = E;
        }
        if (NumTotalEdges == 1) {
          SingleEdge = std::make_pair(Predecessors[BB][0], BB);
        }
      } else {
        // On the second round, visit all successor edges.
        NumTotalEdges = Successors[BB].size();
        for (auto *Succ : Successors[BB]) {
          Edge E = std::make_pair(BB, Succ);
          TotalWeight += visitEdge(E, &NumUnknownEdges, &UnknownEdge);
        }
        if (NumTotalEdges == 1) {
          SingleEdge = std::make_pair(BB, Successors[BB][0]);
        }
      }
 
      // After visiting all the edges, there are three cases that we
      // can handle immediately:
      //
      // - All the edge weights are known (i.e., NumUnknownEdges == 0).
      //   In this case, we simply check that the sum of all the edges
      //   is the same as BB's weight. If not, we change BB's weight
      //   to match. Additionally, if BB had not been visited before,
      //   we mark it visited.
      //
      // - Only one edge is unknown and BB has already been visited.
      //   In this case, we can compute the weight of the edge by
      //   subtracting the total block weight from all the known
      //   edge weights. If the edges weight more than BB, then the
      //   edge of the last remaining edge is set to zero.
      //
      // - There exists a self-referential edge and the weight of BB is
      //   known. In this case, this edge can be based on BB's weight.
      //   We add up all the other known edges and set the weight on
      //   the self-referential edge as we did in the previous case.
      //
      // In any other case, we must continue iterating. Eventually,
      // all edges will get a weight, or iteration will stop when
      // it reaches SampleProfileMaxPropagateIterations.
      if (NumUnknownEdges <= 1) {
        uint64_t &BBWeight = BlockWeights[EC];
        if (NumUnknownEdges == 0) {
          if (!VisitedBlocks.count(EC)) {
            // If we already know the weight of all edges, the weight of the
            // basic block can be computed. It should be no larger than the sum
            // of all edge weights.
            if (TotalWeight > BBWeight) {
              BBWeight = TotalWeight;
              Changed = true;
              LLVM_DEBUG(dbgs() << "All edge weights for " << BB->getName()
                                << " known. Set weight for block: ";
                         printBlockWeight(dbgs(), BB););
            }
          } else if (NumTotalEdges == 1 &&
                     EdgeWeights[SingleEdge] < BlockWeights[EC]) {
            // If there is only one edge for the visited basic block, use the
            // block weight to adjust edge weight if edge weight is smaller.
            EdgeWeights[SingleEdge] = BlockWeights[EC];
            Changed = true;
          }
        } else if (NumUnknownEdges == 1 && VisitedBlocks.count(EC)) {
          // If there is a single unknown edge and the block has been
          // visited, then we can compute E's weight.
          if (BBWeight >= TotalWeight)
            EdgeWeights[UnknownEdge] = BBWeight - TotalWeight;
          else
            EdgeWeights[UnknownEdge] = 0;
          const BasicBlockT *OtherEC;
          if (i == 0)
            OtherEC = EquivalenceClass[UnknownEdge.first];
          else
            OtherEC = EquivalenceClass[UnknownEdge.second];
          // Edge weights should never exceed the BB weights it connects.
          if (VisitedBlocks.count(OtherEC) &&
              EdgeWeights[UnknownEdge] > BlockWeights[OtherEC])
            EdgeWeights[UnknownEdge] = BlockWeights[OtherEC];
          VisitedEdges.insert(UnknownEdge);
          Changed = true;
          LLVM_DEBUG(dbgs() << "Set weight for edge: ";
                     printEdgeWeight(dbgs(), UnknownEdge));
        }
      } else if (VisitedBlocks.count(EC) && BlockWeights[EC] == 0) {
        // If a block Weights 0, all its in/out edges should weight 0.
        if (i == 0) {
          for (auto *Pred : Predecessors[BB]) {
            Edge E = std::make_pair(Pred, BB);
            EdgeWeights[E] = 0;
            VisitedEdges.insert(E);
          }
        } else {
          for (auto *Succ : Successors[BB]) {
            Edge E = std::make_pair(BB, Succ);
            EdgeWeights[E] = 0;
            VisitedEdges.insert(E);
          }
        }
      } else if (SelfReferentialEdge.first && VisitedBlocks.count(EC)) {
        uint64_t &BBWeight = BlockWeights[BB];
        // We have a self-referential edge and the weight of BB is known.
        if (BBWeight >= TotalWeight)
          EdgeWeights[SelfReferentialEdge] = BBWeight - TotalWeight;
        else
          EdgeWeights[SelfReferentialEdge] = 0;
        VisitedEdges.insert(SelfReferentialEdge);
        Changed = true;
        LLVM_DEBUG(dbgs() << "Set self-referential edge weight to: ";
                   printEdgeWeight(dbgs(), SelfReferentialEdge));
      }
      if (UpdateBlockCount && !VisitedBlocks.count(EC) && TotalWeight > 0) {
        BlockWeights[EC] = TotalWeight;
        VisitedBlocks.insert(EC);
        Changed = true;
      }
    }
  }
 
  return Changed;
}
 
/// Build in/out edge lists for each basic block in the CFG.
///
/// We are interested in unique edges. If a block B1 has multiple
/// edges to another block B2, we only add a single B1->B2 edge.
template <typename BT>
void SampleProfileLoaderBaseImpl<BT>::buildEdges(FunctionT &F) {
  for (auto &BI : F) {
    BasicBlockT *B1 = &BI;
 
    // Add predecessors for B1.
    SmallPtrSet<BasicBlockT *, 16> Visited;
    if (!Predecessors[B1].empty())
      llvm_unreachable("Found a stale predecessors list in a basic block.");
    for (auto *B2 : getPredecessors(B1))
      if (Visited.insert(B2).second)
        Predecessors[B1].push_back(B2);
 
    // Add successors for B1.
    Visited.clear();
    if (!Successors[B1].empty())
      llvm_unreachable("Found a stale successors list in a basic block.");
    for (auto *B2 : getSuccessors(B1))
      if (Visited.insert(B2).second)
        Successors[B1].push_back(B2);
  }
}
 
/// Propagate weights into edges
///
/// The following rules are applied to every block BB in the CFG:
///
/// - If BB has a single predecessor/successor, then the weight
///   of that edge is the weight of the block.
///
/// - If all incoming or outgoing edges are known except one, and the
///   weight of the block is already known, the weight of the unknown
///   edge will be the weight of the block minus the sum of all the known
///   edges. If the sum of all the known edges is larger than BB's weight,
///   we set the unknown edge weight to zero.
///
/// - If there is a self-referential edge, and the weight of the block is
///   known, the weight for that edge is set to the weight of the block
///   minus the weight of the other incoming edges to that block (if
///   known).
template <typename BT>
void SampleProfileLoaderBaseImpl<BT>::propagateWeights(FunctionT &F) {
  bool Changed = true;
  unsigned I = 0;
 
  // If BB weight is larger than its corresponding loop's header BB weight,
  // use the BB weight to replace the loop header BB weight.
  for (auto &BI : F) {
    BasicBlockT *BB = &BI;
    LoopT *L = LI->getLoopFor(BB);
    if (!L) {
      continue;
    }
    BasicBlockT *Header = L->getHeader();
    if (Header && BlockWeights[BB] > BlockWeights[Header]) {
      BlockWeights[Header] = BlockWeights[BB];
    }
  }
 
  // Before propagation starts, build, for each block, a list of
  // unique predecessors and successors. This is necessary to handle
  // identical edges in multiway branches. Since we visit all blocks and all
  // edges of the CFG, it is cleaner to build these lists once at the start
  // of the pass.
  buildEdges(F);
 
  // Propagate until we converge or we go past the iteration limit.
  while (Changed && I++ < SampleProfileMaxPropagateIterations) {
    Changed = propagateThroughEdges(F, false);
  }
 
  // The first propagation propagates BB counts from annotated BBs to unknown
  // BBs. The 2nd propagation pass resets edges weights, and use all BB weights
  // to propagate edge weights.
  VisitedEdges.clear();
  Changed = true;
  while (Changed && I++ < SampleProfileMaxPropagateIterations) {
    Changed = propagateThroughEdges(F, false);
  }
 
  // The 3rd propagation pass allows adjust annotated BB weights that are
  // obviously wrong.
  Changed = true;
  while (Changed && I++ < SampleProfileMaxPropagateIterations) {
    Changed = propagateThroughEdges(F, true);
  }
}
 
/// Generate branch weight metadata for all branches in \p F.
///
/// Branch weights are computed out of instruction samples using a
/// propagation heuristic. Propagation proceeds in 3 phases:
///
/// 1- Assignment of block weights. All the basic blocks in the function
///    are initial assigned the same weight as their most frequently
///    executed instruction.
///
/// 2- Creation of equivalence classes. Since samples may be missing from
///    blocks, we can fill in the gaps by setting the weights of all the
///    blocks in the same equivalence class to the same weight. To compute
///    the concept of equivalence, we use dominance and loop information.
///    Two blocks B1 and B2 are in the same equivalence class if B1
///    dominates B2, B2 post-dominates B1 and both are in the same loop.
///
/// 3- Propagation of block weights into edges. This uses a simple
///    propagation heuristic. The following rules are applied to every
///    block BB in the CFG:
///
///    - If BB has a single predecessor/successor, then the weight
///      of that edge is the weight of the block.
///
///    - If all the edges are known except one, and the weight of the
///      block is already known, the weight of the unknown edge will
///      be the weight of the block minus the sum of all the known
///      edges. If the sum of all the known edges is larger than BB's weight,
///      we set the unknown edge weight to zero.
///
///    - If there is a self-referential edge, and the weight of the block is
///      known, the weight for that edge is set to the weight of the block
///      minus the weight of the other incoming edges to that block (if
///      known).
///
/// Since this propagation is not guaranteed to finalize for every CFG, we
/// only allow it to proceed for a limited number of iterations (controlled
/// by -sample-profile-max-propagate-iterations).
///
/// FIXME: Try to replace this propagation heuristic with a scheme
/// that is guaranteed to finalize. A work-list approach similar to
/// the standard value propagation algorithm used by SSA-CCP might
/// work here.
///
/// \param F The function to query.
///
/// \returns true if \p F was modified. Returns false, otherwise.
template <typename BT>
bool SampleProfileLoaderBaseImpl<BT>::computeAndPropagateWeights(
    FunctionT &F, const DenseSet<GlobalValue::GUID> &InlinedGUIDs) {
  bool Changed = (InlinedGUIDs.size() != 0);
 
  // Compute basic block weights.
  Changed |= computeBlockWeights(F);
 
  if (Changed) {
    // Add an entry count to the function using the samples gathered at the
    // function entry.
    // Sets the GUIDs that are inlined in the profiled binary. This is used
    // for ThinLink to make correct liveness analysis, and also make the IR
    // match the profiled binary before annotation.
    getFunction(F).setEntryCount(
        ProfileCount(Samples->getHeadSamples() + 1, Function::PCT_Real),
        &InlinedGUIDs);
 
    // Compute dominance and loop info needed for propagation.
    computeDominanceAndLoopInfo(F);
 
    // Find equivalence classes.
    findEquivalenceClasses(F);
 
    // Propagate weights to all edges.
    propagateWeights(F);
  }
 
  return Changed;
}
 
template <typename BT>
void SampleProfileLoaderBaseImpl<BT>::emitCoverageRemarks(FunctionT &F) {
  // If coverage checking was requested, compute it now.
  const Function &Func = getFunction(F);
  if (SampleProfileRecordCoverage) {
    unsigned Used = CoverageTracker.countUsedRecords(Samples, PSI);
    unsigned Total = CoverageTracker.countBodyRecords(Samples, PSI);
    unsigned Coverage = CoverageTracker.computeCoverage(Used, Total);
    if (Coverage < SampleProfileRecordCoverage) {
      Func.getContext().diagnose(DiagnosticInfoSampleProfile(
          Func.getSubprogram()->getFilename(), getFunctionLoc(F),
          Twine(Used) + " of " + Twine(Total) + " available profile records (" +
              Twine(Coverage) + "%) were applied",
          DS_Warning));
    }
  }
 
  if (SampleProfileSampleCoverage) {
    uint64_t Used = CoverageTracker.getTotalUsedSamples();
    uint64_t Total = CoverageTracker.countBodySamples(Samples, PSI);
    unsigned Coverage = CoverageTracker.computeCoverage(Used, Total);
    if (Coverage < SampleProfileSampleCoverage) {
      Func.getContext().diagnose(DiagnosticInfoSampleProfile(
          Func.getSubprogram()->getFilename(), getFunctionLoc(F),
          Twine(Used) + " of " + Twine(Total) + " available profile samples (" +
              Twine(Coverage) + "%) were applied",
          DS_Warning));
    }
  }
}
 
/// Get the line number for the function header.
///
/// This looks up function \p F in the current compilation unit and
/// retrieves the line number where the function is defined. This is
/// line 0 for all the samples read from the profile file. Every line
/// number is relative to this line.
///
/// \param F  Function object to query.
///
/// \returns the line number where \p F is defined. If it returns 0,
///          it means that there is no debug information available for \p F.
template <typename BT>
unsigned SampleProfileLoaderBaseImpl<BT>::getFunctionLoc(FunctionT &F) {
  const Function &Func = getFunction(F);
  if (DISubprogram *S = Func.getSubprogram())
    return S->getLine();
 
  if (NoWarnSampleUnused)
    return 0;
 
  // If the start of \p F is missing, emit a diagnostic to inform the user
  // about the missed opportunity.
  Func.getContext().diagnose(DiagnosticInfoSampleProfile(
      "No debug information found in function " + Func.getName() +
          ": Function profile not used",
      DS_Warning));
  return 0;
}
 
template <typename BT>
void SampleProfileLoaderBaseImpl<BT>::computeDominanceAndLoopInfo(
    FunctionT &F) {
  DT.reset(new DominatorTree);
  DT->recalculate(F);
 
  PDT.reset(new PostDominatorTree(F));
 
  LI.reset(new LoopInfo);
  LI->analyze(*DT);
}
 
#undef DEBUG_TYPE
 
} // namespace llvm
#endif // LLVM_TRANSFORMS_UTILS_SAMPLEPROFILELOADERBASEIMPL_H