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//===- BranchProbabilityInfo.h - Branch Probability Analysis ----*- 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

//

//===----------------------------------------------------------------------===//

//

// This pass is used to evaluate branch probabilties.

//

//===----------------------------------------------------------------------===//

#ifndef LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H

#define LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H

#include "llvm/ADT/DenseMap.h"

#include "llvm/ADT/DenseMapInfo.h"

#include "llvm/ADT/DenseSet.h"

#include "llvm/IR/BasicBlock.h"

#include "llvm/IR/CFG.h"

#include "llvm/IR/PassManager.h"

#include "llvm/IR/ValueHandle.h"

#include "llvm/Pass.h"

#include "llvm/Support/BranchProbability.h"

#include <algorithm>

#include <cassert>

#include <cstdint>

#include <memory>

#include <utility>

namespace llvm {

class Function;

class Loop;

class LoopInfo;

class raw_ostream;

class DominatorTree;

class PostDominatorTree;

class TargetLibraryInfo;

class Value;

/// Analysis providing branch probability information.

///

/// This is a function analysis which provides information on the relative

/// probabilities of each "edge" in the function's CFG where such an edge is

/// defined by a pair (PredBlock and an index in the successors). The

/// probability of an edge from one block is always relative to the

/// probabilities of other edges from the block. The probabilites of all edges

/// from a block sum to exactly one (100%).

/// We use a pair (PredBlock and an index in the successors) to uniquely

/// identify an edge, since we can have multiple edges from Src to Dst.

/// As an example, we can have a switch which jumps to Dst with value 0 and

/// value 10.

///

/// Process of computing branch probabilities can be logically viewed as three

/// step process:

///

///   First, if there is a profile information associated with the branch then

/// it is trivially translated to branch probabilities. There is one exception

/// from this rule though. Probabilities for edges leading to "unreachable"

/// blocks (blocks with the estimated weight not greater than

/// UNREACHABLE_WEIGHT) are evaluated according to static estimation and

/// override profile information. If no branch probabilities were calculated

/// on this step then take the next one.

///

///   Second, estimate absolute execution weights for each block based on

/// statically known information. Roots of such information are "cold",

/// "unreachable", "noreturn" and "unwind" blocks. Those blocks get their

/// weights set to BlockExecWeight::COLD, BlockExecWeight::UNREACHABLE,

/// BlockExecWeight::NORETURN and BlockExecWeight::UNWIND respectively. Then the

/// weights are propagated to the other blocks up the domination line. In

/// addition, if all successors have estimated weights set then maximum of these

/// weights assigned to the block itself (while this is not ideal heuristic in

/// theory it's simple and works reasonably well in most cases) and the process

/// repeats. Once the process of weights propagation converges branch

/// probabilities are set for all such branches that have at least one successor

/// with the weight set. Default execution weight (BlockExecWeight::DEFAULT) is

/// used for any successors which doesn't have its weight set. For loop back

/// branches we use their weights scaled by loop trip count equal to

/// 'LBH_TAKEN_WEIGHT/LBH_NOTTAKEN_WEIGHT'.

///

/// Here is a simple example demonstrating how the described algorithm works.

///

///          BB1

///         /   \

///        v     v

///      BB2     BB3

///     /   \

///    v     v

///  ColdBB  UnreachBB

///

/// Initially, ColdBB is associated with COLD_WEIGHT and UnreachBB with

/// UNREACHABLE_WEIGHT. COLD_WEIGHT is set to BB2 as maximum between its

/// successors. BB1 and BB3 has no explicit estimated weights and assumed to

/// have DEFAULT_WEIGHT. Based on assigned weights branches will have the

/// following probabilities:

/// P(BB1->BB2) = COLD_WEIGHT/(COLD_WEIGHT + DEFAULT_WEIGHT) =

///   0xffff / (0xffff + 0xfffff) = 0.0588(5.9%)

/// P(BB1->BB3) = DEFAULT_WEIGHT_WEIGHT/(COLD_WEIGHT + DEFAULT_WEIGHT) =

///          0xfffff / (0xffff + 0xfffff) = 0.941(94.1%)

/// P(BB2->ColdBB) = COLD_WEIGHT/(COLD_WEIGHT + UNREACHABLE_WEIGHT) = 1(100%)

/// P(BB2->UnreachBB) =

///   UNREACHABLE_WEIGHT/(COLD_WEIGHT+UNREACHABLE_WEIGHT) = 0(0%)

///

/// If no branch probabilities were calculated on this step then take the next

/// one.

///

///   Third, apply different kinds of local heuristics for each individual

/// branch until first match. For example probability of a pointer to be null is

/// estimated as PH_TAKEN_WEIGHT/(PH_TAKEN_WEIGHT + PH_NONTAKEN_WEIGHT). If

/// no local heuristic has been matched then branch is left with no explicit

/// probability set and assumed to have default probability.

class BranchProbabilityInfo {

public:

  BranchProbabilityInfo() = default;

  BranchProbabilityInfo(const Function &F, const LoopInfo &LI,

                        const TargetLibraryInfo *TLI = nullptr,

                        DominatorTree *DT = nullptr,

                        PostDominatorTree *PDT = nullptr) {

    calculate(F, LI, TLI, DT, PDT);

  }

  BranchProbabilityInfo(BranchProbabilityInfo &&Arg)

      : Probs(std::move(Arg.Probs)), LastF(Arg.LastF),

        EstimatedBlockWeight(std::move(Arg.EstimatedBlockWeight)) {}

  BranchProbabilityInfo(const BranchProbabilityInfo &) = delete;

  BranchProbabilityInfo &operator=(const BranchProbabilityInfo &) = delete;

  BranchProbabilityInfo &operator=(BranchProbabilityInfo &&RHS) {

    releaseMemory();

    Probs = std::move(RHS.Probs);

    EstimatedBlockWeight = std::move(RHS.EstimatedBlockWeight);

    return *this;

  }

  bool invalidate(Function &, const PreservedAnalyses &PA,

                  FunctionAnalysisManager::Invalidator &);

  void releaseMemory();

  void print(raw_ostream &OS) const;

  /// Get an edge's probability, relative to other out-edges of the Src.

  ///

  /// This routine provides access to the fractional probability between zero

  /// (0%) and one (100%) of this edge executing, relative to other edges

  /// leaving the 'Src' block. The returned probability is never zero, and can

  /// only be one if the source block has only one successor.

  BranchProbability getEdgeProbability(const BasicBlock *Src,

                                       unsigned IndexInSuccessors) const;

  /// Get the probability of going from Src to Dst.

  ///

  /// It returns the sum of all probabilities for edges from Src to Dst.

  BranchProbability getEdgeProbability(const BasicBlock *Src,

                                       const BasicBlock *Dst) const;

  BranchProbability getEdgeProbability(const BasicBlock *Src,

                                       const_succ_iterator Dst) const;

  /// Test if an edge is hot relative to other out-edges of the Src.

  ///

  /// Check whether this edge out of the source block is 'hot'. We define hot

  /// as having a relative probability >= 80%.

  bool isEdgeHot(const BasicBlock *Src, const BasicBlock *Dst) const;

  /// Print an edge's probability.

  ///

  /// Retrieves an edge's probability similarly to \see getEdgeProbability, but

  /// then prints that probability to the provided stream. That stream is then

  /// returned.

  raw_ostream &printEdgeProbability(raw_ostream &OS, const BasicBlock *Src,

                                    const BasicBlock *Dst) const;

public:

  /// Set the raw probabilities for all edges from the given block.

  ///

  /// This allows a pass to explicitly set edge probabilities for a block. It

  /// can be used when updating the CFG to update the branch probability

  /// information.

  void setEdgeProbability(const BasicBlock *Src,

                          const SmallVectorImpl<BranchProbability> &Probs);

  /// Copy outgoing edge probabilities from \p Src to \p Dst.

  ///

  /// This allows to keep probabilities unset for the destination if they were

  /// unset for source.

  void copyEdgeProbabilities(BasicBlock *Src, BasicBlock *Dst);

  static BranchProbability getBranchProbStackProtector(bool IsLikely) {

    static const BranchProbability LikelyProb((1u << 20) - 1, 1u << 20);

    return IsLikely ? LikelyProb : LikelyProb.getCompl();

  }

  void calculate(const Function &F, const LoopInfo &LI,

                 const TargetLibraryInfo *TLI, DominatorTree *DT,

                 PostDominatorTree *PDT);

  /// Forget analysis results for the given basic block.

  void eraseBlock(const BasicBlock *BB);

  // Data structure to track SCCs for handling irreducible loops.

  class SccInfo {

    // Enum of types to classify basic blocks in SCC. Basic block belonging to

    // SCC is 'Inner' until it is either 'Header' or 'Exiting'. Note that a

    // basic block can be 'Header' and 'Exiting' at the same time.

    enum SccBlockType {

      Inner = 0x0,

      Header = 0x1,

      Exiting = 0x2,

    };

    // Map of basic blocks to SCC IDs they belong to. If basic block doesn't

    // belong to any SCC it is not in the map.

    using SccMap = DenseMap<const BasicBlock *, int>;

    // Each basic block in SCC is attributed with one or several types from

    // SccBlockType. Map value has uint32_t type (instead of SccBlockType)

    // since basic block may be for example "Header" and "Exiting" at the same

    // time and we need to be able to keep more than one value from

    // SccBlockType.

    using SccBlockTypeMap = DenseMap<const BasicBlock *, uint32_t>;

    // Vector containing classification of basic blocks for all  SCCs where i'th

    // vector element corresponds to SCC with ID equal to i.

    using SccBlockTypeMaps = std::vector<SccBlockTypeMap>;

    SccMap SccNums;

    SccBlockTypeMaps SccBlocks;

  public:

    explicit SccInfo(const Function &F);

    /// If \p BB belongs to some SCC then ID of that SCC is returned, otherwise

    /// -1 is returned. If \p BB belongs to more than one SCC at the same time

    /// result is undefined.

    int getSCCNum(const BasicBlock *BB) const;

    /// Returns true if \p BB is a 'header' block in SCC with \p SccNum ID,

    /// false otherwise.

    bool isSCCHeader(const BasicBlock *BB, int SccNum) const {

      return getSccBlockType(BB, SccNum) & Header;

    }

    /// Returns true if \p BB is an 'exiting' block in SCC with \p SccNum ID,

    /// false otherwise.

    bool isSCCExitingBlock(const BasicBlock *BB, int SccNum) const {

      return getSccBlockType(BB, SccNum) & Exiting;

    }

    /// Fills in \p Enters vector with all such blocks that don't belong to

    /// SCC with \p SccNum ID but there is an edge to a block belonging to the

    /// SCC.

    void getSccEnterBlocks(int SccNum,

                           SmallVectorImpl<BasicBlock *> &Enters) const;

    /// Fills in \p Exits vector with all such blocks that don't belong to

    /// SCC with \p SccNum ID but there is an edge from a block belonging to the

    /// SCC.

    void getSccExitBlocks(int SccNum,

                          SmallVectorImpl<BasicBlock *> &Exits) const;

  private:

    /// Returns \p BB's type according to classification given by SccBlockType

    /// enum. Please note that \p BB must belong to SSC with \p SccNum ID.

    uint32_t getSccBlockType(const BasicBlock *BB, int SccNum) const;

    /// Calculates \p BB's type and stores it in internal data structures for

    /// future use. Please note that \p BB must belong to SSC with \p SccNum ID.

    void calculateSccBlockType(const BasicBlock *BB, int SccNum);

  };

private:

  // We need to store CallbackVH's in order to correctly handle basic block

  // removal.

  class BasicBlockCallbackVH final : public CallbackVH {

    BranchProbabilityInfo *BPI;

    void deleted() override {

      assert(BPI != nullptr);

      BPI->eraseBlock(cast<BasicBlock>(getValPtr()));

    }

  public:

    BasicBlockCallbackVH(const Value *V, BranchProbabilityInfo *BPI = nullptr)

        : CallbackVH(const_cast<Value *>(V)), BPI(BPI) {}

  };

  /// Pair of Loop and SCC ID number. Used to unify handling of normal and

  /// SCC based loop representations.

  using LoopData = std::pair<Loop *, int>;

  /// Helper class to keep basic block along with its loop data information.

  class LoopBlock {

  public:

    explicit LoopBlock(const BasicBlock *BB, const LoopInfo &LI,

                       const SccInfo &SccI);

    const BasicBlock *getBlock() const { return BB; }

    BasicBlock *getBlock() { return const_cast<BasicBlock *>(BB); }

    LoopData getLoopData() const { return LD; }

    Loop *getLoop() const { return LD.first; }

    int getSccNum() const { return LD.second; }

    bool belongsToLoop() const { return getLoop() || getSccNum() != -1; }

    bool belongsToSameLoop(const LoopBlock &LB) const {

      return (LB.getLoop() && getLoop() == LB.getLoop()) ||

             (LB.getSccNum() != -1 && getSccNum() == LB.getSccNum());

    }

  private:

    const BasicBlock *const BB = nullptr;

    LoopData LD = {nullptr, -1};

  };

  // Pair of LoopBlocks representing an edge from first to second block.

  using LoopEdge = std::pair<const LoopBlock &, const LoopBlock &>;

  DenseSet<BasicBlockCallbackVH, DenseMapInfo<Value*>> Handles;

  // Since we allow duplicate edges from one basic block to another, we use

  // a pair (PredBlock and an index in the successors) to specify an edge.

  using Edge = std::pair<const BasicBlock *, unsigned>;

  DenseMap<Edge, BranchProbability> Probs;

  /// Track the last function we run over for printing.

  const Function *LastF = nullptr;

  const LoopInfo *LI = nullptr;

  /// Keeps information about all SCCs in a function.

  std::unique_ptr<const SccInfo> SccI;

  /// Keeps mapping of a basic block to its estimated weight.

  SmallDenseMap<const BasicBlock *, uint32_t> EstimatedBlockWeight;

  /// Keeps mapping of a loop to estimated weight to enter the loop.

  SmallDenseMap<LoopData, uint32_t> EstimatedLoopWeight;

  /// Helper to construct LoopBlock for \p BB.

  LoopBlock getLoopBlock(const BasicBlock *BB) const {

    return LoopBlock(BB, *LI, *SccI.get());

  }

  /// Returns true if destination block belongs to some loop and source block is

  /// either doesn't belong to any loop or belongs to a loop which is not inner

  /// relative to the destination block.

  bool isLoopEnteringEdge(const LoopEdge &Edge) const;

  /// Returns true if source block belongs to some loop and destination block is

  /// either doesn't belong to any loop or belongs to a loop which is not inner

  /// relative to the source block.

  bool isLoopExitingEdge(const LoopEdge &Edge) const;

  /// Returns true if \p Edge is either enters to or exits from some loop, false

  /// in all other cases.

  bool isLoopEnteringExitingEdge(const LoopEdge &Edge) const;

  /// Returns true if source and destination blocks belongs to the same loop and

  /// destination block is loop header.

  bool isLoopBackEdge(const LoopEdge &Edge) const;

  // Fills in \p Enters vector with all "enter" blocks to a loop \LB belongs to.

  void getLoopEnterBlocks(const LoopBlock &LB,

                          SmallVectorImpl<BasicBlock *> &Enters) const;

  // Fills in \p Exits vector with all "exit" blocks from a loop \LB belongs to.

  void getLoopExitBlocks(const LoopBlock &LB,

                         SmallVectorImpl<BasicBlock *> &Exits) const;

  /// Returns estimated weight for \p BB. std::nullopt if \p BB has no estimated

  /// weight.

  std::optional<uint32_t> getEstimatedBlockWeight(const BasicBlock *BB) const;

  /// Returns estimated weight to enter \p L. In other words it is weight of

  /// loop's header block not scaled by trip count. Returns std::nullopt if \p L

  /// has no no estimated weight.

  std::optional<uint32_t> getEstimatedLoopWeight(const LoopData &L) const;

  /// Return estimated weight for \p Edge. Returns std::nullopt if estimated

  /// weight is unknown.

  std::optional<uint32_t> getEstimatedEdgeWeight(const LoopEdge &Edge) const;

  /// Iterates over all edges leading from \p SrcBB to \p Successors and

  /// returns maximum of all estimated weights. If at least one edge has unknown

  /// estimated weight std::nullopt is returned.

  template <class IterT>

  std::optional<uint32_t>

  getMaxEstimatedEdgeWeight(const LoopBlock &SrcBB,

                            iterator_range<IterT> Successors) const;

  /// If \p LoopBB has no estimated weight then set it to \p BBWeight and

  /// return true. Otherwise \p BB's weight remains unchanged and false is

  /// returned. In addition all blocks/loops that might need their weight to be

  /// re-estimated are put into BlockWorkList/LoopWorkList.

  bool updateEstimatedBlockWeight(LoopBlock &LoopBB, uint32_t BBWeight,

                                  SmallVectorImpl<BasicBlock *> &BlockWorkList,

                                  SmallVectorImpl<LoopBlock> &LoopWorkList);

  /// Starting from \p LoopBB (including \p LoopBB itself) propagate \p BBWeight

  /// up the domination tree.

  void propagateEstimatedBlockWeight(const LoopBlock &LoopBB, DominatorTree *DT,

                                     PostDominatorTree *PDT, uint32_t BBWeight,

                                     SmallVectorImpl<BasicBlock *> &WorkList,

                                     SmallVectorImpl<LoopBlock> &LoopWorkList);

  /// Returns block's weight encoded in the IR.

  std::optional<uint32_t> getInitialEstimatedBlockWeight(const BasicBlock *BB);

  // Computes estimated weights for all blocks in \p F.

  void computeEestimateBlockWeight(const Function &F, DominatorTree *DT,

                                   PostDominatorTree *PDT);

  /// Based on computed weights by \p computeEstimatedBlockWeight set

  /// probabilities on branches.

  bool calcEstimatedHeuristics(const BasicBlock *BB);

  bool calcMetadataWeights(const BasicBlock *BB);

  bool calcPointerHeuristics(const BasicBlock *BB);

  bool calcZeroHeuristics(const BasicBlock *BB, const TargetLibraryInfo *TLI);

  bool calcFloatingPointHeuristics(const BasicBlock *BB);

};

/// Analysis pass which computes \c BranchProbabilityInfo.

class BranchProbabilityAnalysis

    : public AnalysisInfoMixin<BranchProbabilityAnalysis> {

  friend AnalysisInfoMixin<BranchProbabilityAnalysis>;

  static AnalysisKey Key;

public:

  /// Provide the result type for this analysis pass.

  using Result = BranchProbabilityInfo;

  /// Run the analysis pass over a function and produce BPI.

  BranchProbabilityInfo run(Function &F, FunctionAnalysisManager &AM);

};

/// Printer pass for the \c BranchProbabilityAnalysis results.

class BranchProbabilityPrinterPass

    : public PassInfoMixin<BranchProbabilityPrinterPass> {

  raw_ostream &OS;

public:

  explicit BranchProbabilityPrinterPass(raw_ostream &OS) : OS(OS) {}

  PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);

};

/// Legacy analysis pass which computes \c BranchProbabilityInfo.

class BranchProbabilityInfoWrapperPass : public FunctionPass {

  BranchProbabilityInfo BPI;

public:

  static char ID;

  BranchProbabilityInfoWrapperPass();

  BranchProbabilityInfo &getBPI() { return BPI; }

  const BranchProbabilityInfo &getBPI() const { return BPI; }

  void getAnalysisUsage(AnalysisUsage &AU) const override;

  bool runOnFunction(Function &F) override;

  void releaseMemory() override;

  void print(raw_ostream &OS, const Module *M = nullptr) const override;

};

} // end namespace llvm

#endif // LLVM_ANALYSIS_BRANCHPROBABILITYINFO_H