GVN.h

Go to the documentation of this file.

//===- GVN.h - Eliminate redundant values and loads -------------*- 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 LLVM's Global Value Numbering pass
/// which eliminates fully redundant instructions. It also does somewhat Ad-Hoc
/// PRE and dead load elimination.
///
//===----------------------------------------------------------------------===//
 
#ifndef LLVM_TRANSFORMS_SCALAR_GVN_H
#define LLVM_TRANSFORMS_SCALAR_GVN_H
 
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/MapVector.h"
#include "llvm/ADT/PostOrderIterator.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/InstructionPrecedenceTracking.h"
#include "llvm/IR/Dominators.h"
#include "llvm/IR/InstrTypes.h"
#include "llvm/IR/PassManager.h"
#include "llvm/IR/ValueHandle.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/Compiler.h"
#include <cstdint>
#include <utility>
#include <vector>
 
namespace llvm {
 
class AAResults;
class AssumeInst;
class AssumptionCache;
class BasicBlock;
class BranchInst;
class CallInst;
class Constant;
class ExtractValueInst;
class Function;
class FunctionPass;
class IntrinsicInst;
class LoadInst;
class LoopInfo;
class MemDepResult;
class MemoryDependenceResults;
class MemorySSA;
class MemorySSAUpdater;
class NonLocalDepResult;
class OptimizationRemarkEmitter;
class PHINode;
class TargetLibraryInfo;
class Value;
/// A private "module" namespace for types and utilities used by GVN. These
/// are implementation details and should not be used by clients.
namespace gvn LLVM_LIBRARY_VISIBILITY {
 
struct AvailableValue;
struct AvailableValueInBlock;
class GVNLegacyPass;
 
} // end namespace gvn
 
/// A set of parameters to control various transforms performed by GVN pass.
//  Each of the optional boolean parameters can be set to:
///      true - enabling the transformation.
///      false - disabling the transformation.
///      None - relying on a global default.
/// Intended use is to create a default object, modify parameters with
/// additional setters and then pass it to GVN.
struct GVNOptions {
  Optional<bool> AllowPRE = None;
  Optional<bool> AllowLoadPRE = None;
  Optional<bool> AllowLoadInLoopPRE = None;
  Optional<bool> AllowLoadPRESplitBackedge = None;
  Optional<bool> AllowMemDep = None;
 
  GVNOptions() = default;
 
  /// Enables or disables PRE in GVN.
  GVNOptions &setPRE(bool PRE) {
    AllowPRE = PRE;
    return *this;
  }
 
  /// Enables or disables PRE of loads in GVN.
  GVNOptions &setLoadPRE(bool LoadPRE) {
    AllowLoadPRE = LoadPRE;
    return *this;
  }
 
  GVNOptions &setLoadInLoopPRE(bool LoadInLoopPRE) {
    AllowLoadInLoopPRE = LoadInLoopPRE;
    return *this;
  }
 
  /// Enables or disables PRE of loads in GVN.
  GVNOptions &setLoadPRESplitBackedge(bool LoadPRESplitBackedge) {
    AllowLoadPRESplitBackedge = LoadPRESplitBackedge;
    return *this;
  }
 
  /// Enables or disables use of MemDepAnalysis.
  GVNOptions &setMemDep(bool MemDep) {
    AllowMemDep = MemDep;
    return *this;
  }
};
 
/// The core GVN pass object.
///
/// FIXME: We should have a good summary of the GVN algorithm implemented by
/// this particular pass here.
class GVN : public PassInfoMixin<GVN> {
  GVNOptions Options;
 
public:
  struct Expression;
 
  GVN(GVNOptions Options = {}) : Options(Options) {}
 
  /// Run the pass over the function.
  PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
 
  /// This removes the specified instruction from
  /// our various maps and marks it for deletion.
  void markInstructionForDeletion(Instruction *I) {
    VN.erase(I);
    InstrsToErase.push_back(I);
  }
 
  DominatorTree &getDominatorTree() const { return *DT; }
  AAResults *getAliasAnalysis() const { return VN.getAliasAnalysis(); }
  MemoryDependenceResults &getMemDep() const { return *MD; }
 
  bool isPREEnabled() const;
  bool isLoadPREEnabled() const;
  bool isLoadInLoopPREEnabled() const;
  bool isLoadPRESplitBackedgeEnabled() const;
  bool isMemDepEnabled() const;
 
  /// This class holds the mapping between values and value numbers.  It is used
  /// as an efficient mechanism to determine the expression-wise equivalence of
  /// two values.
  class ValueTable {
    DenseMap<Value *, uint32_t> valueNumbering;
    DenseMap<Expression, uint32_t> expressionNumbering;
 
    // Expressions is the vector of Expression. ExprIdx is the mapping from
    // value number to the index of Expression in Expressions. We use it
    // instead of a DenseMap because filling such mapping is faster than
    // filling a DenseMap and the compile time is a little better.
    uint32_t nextExprNumber = 0;
 
    std::vector<Expression> Expressions;
    std::vector<uint32_t> ExprIdx;
 
    // Value number to PHINode mapping. Used for phi-translate in scalarpre.
    DenseMap<uint32_t, PHINode *> NumberingPhi;
 
    // Cache for phi-translate in scalarpre.
    using PhiTranslateMap =
        DenseMap<std::pair<uint32_t, const BasicBlock *>, uint32_t>;
    PhiTranslateMap PhiTranslateTable;
 
    AAResults *AA = nullptr;
    MemoryDependenceResults *MD = nullptr;
    DominatorTree *DT = nullptr;
 
    uint32_t nextValueNumber = 1;
 
    Expression createExpr(Instruction *I);
    Expression createCmpExpr(unsigned Opcode, CmpInst::Predicate Predicate,
                             Value *LHS, Value *RHS);
    Expression createExtractvalueExpr(ExtractValueInst *EI);
    uint32_t lookupOrAddCall(CallInst *C);
    uint32_t phiTranslateImpl(const BasicBlock *BB, const BasicBlock *PhiBlock,
                              uint32_t Num, GVN &Gvn);
    bool areCallValsEqual(uint32_t Num, uint32_t NewNum, const BasicBlock *Pred,
                          const BasicBlock *PhiBlock, GVN &Gvn);
    std::pair<uint32_t, bool> assignExpNewValueNum(Expression &exp);
    bool areAllValsInBB(uint32_t num, const BasicBlock *BB, GVN &Gvn);
 
  public:
    ValueTable();
    ValueTable(const ValueTable &Arg);
    ValueTable(ValueTable &&Arg);
    ~ValueTable();
    ValueTable &operator=(const ValueTable &Arg);
 
    uint32_t lookupOrAdd(Value *V);
    uint32_t lookup(Value *V, bool Verify = true) const;
    uint32_t lookupOrAddCmp(unsigned Opcode, CmpInst::Predicate Pred,
                            Value *LHS, Value *RHS);
    uint32_t phiTranslate(const BasicBlock *BB, const BasicBlock *PhiBlock,
                          uint32_t Num, GVN &Gvn);
    void eraseTranslateCacheEntry(uint32_t Num, const BasicBlock &CurrBlock);
    bool exists(Value *V) const;
    void add(Value *V, uint32_t num);
    void clear();
    void erase(Value *v);
    void setAliasAnalysis(AAResults *A) { AA = A; }
    AAResults *getAliasAnalysis() const { return AA; }
    void setMemDep(MemoryDependenceResults *M) { MD = M; }
    void setDomTree(DominatorTree *D) { DT = D; }
    uint32_t getNextUnusedValueNumber() { return nextValueNumber; }
    void verifyRemoved(const Value *) const;
  };
 
private:
  friend class gvn::GVNLegacyPass;
  friend struct DenseMapInfo<Expression>;
 
  MemoryDependenceResults *MD = nullptr;
  DominatorTree *DT = nullptr;
  const TargetLibraryInfo *TLI = nullptr;
  AssumptionCache *AC = nullptr;
  SetVector<BasicBlock *> DeadBlocks;
  OptimizationRemarkEmitter *ORE = nullptr;
  ImplicitControlFlowTracking *ICF = nullptr;
  LoopInfo *LI = nullptr;
  MemorySSAUpdater *MSSAU = nullptr;
 
  ValueTable VN;
 
  /// A mapping from value numbers to lists of Value*'s that
  /// have that value number.  Use findLeader to query it.
  struct LeaderTableEntry {
    Value *Val;
    const BasicBlock *BB;
    LeaderTableEntry *Next;
  };
  DenseMap<uint32_t, LeaderTableEntry> LeaderTable;
  BumpPtrAllocator TableAllocator;
 
  // Block-local map of equivalent values to their leader, does not
  // propagate to any successors. Entries added mid-block are applied
  // to the remaining instructions in the block.
  SmallMapVector<Value *, Value *, 4> ReplaceOperandsWithMap;
  SmallVector<Instruction *, 8> InstrsToErase;
 
  // Map the block to reversed postorder traversal number. It is used to
  // find back edge easily.
  DenseMap<AssertingVH<BasicBlock>, uint32_t> BlockRPONumber;
 
  // This is set 'true' initially and also when new blocks have been added to
  // the function being analyzed. This boolean is used to control the updating
  // of BlockRPONumber prior to accessing the contents of BlockRPONumber.
  bool InvalidBlockRPONumbers = true;
 
  using LoadDepVect = SmallVector<NonLocalDepResult, 64>;
  using AvailValInBlkVect = SmallVector<gvn::AvailableValueInBlock, 64>;
  using UnavailBlkVect = SmallVector<BasicBlock *, 64>;
 
  bool runImpl(Function &F, AssumptionCache &RunAC, DominatorTree &RunDT,
               const TargetLibraryInfo &RunTLI, AAResults &RunAA,
               MemoryDependenceResults *RunMD, LoopInfo *LI,
               OptimizationRemarkEmitter *ORE, MemorySSA *MSSA = nullptr);
 
  /// Push a new Value to the LeaderTable onto the list for its value number.
  void addToLeaderTable(uint32_t N, Value *V, const BasicBlock *BB) {
    LeaderTableEntry &Curr = LeaderTable[N];
    if (!Curr.Val) {
      Curr.Val = V;
      Curr.BB = BB;
      return;
    }
 
    LeaderTableEntry *Node = TableAllocator.Allocate<LeaderTableEntry>();
    Node->Val = V;
    Node->BB = BB;
    Node->Next = Curr.Next;
    Curr.Next = Node;
  }
 
  /// Scan the list of values corresponding to a given
  /// value number, and remove the given instruction if encountered.
  void removeFromLeaderTable(uint32_t N, Instruction *I, BasicBlock *BB) {
    LeaderTableEntry *Prev = nullptr;
    LeaderTableEntry *Curr = &LeaderTable[N];
 
    while (Curr && (Curr->Val != I || Curr->BB != BB)) {
      Prev = Curr;
      Curr = Curr->Next;
    }
 
    if (!Curr)
      return;
 
    if (Prev) {
      Prev->Next = Curr->Next;
    } else {
      if (!Curr->Next) {
        Curr->Val = nullptr;
        Curr->BB = nullptr;
      } else {
        LeaderTableEntry *Next = Curr->Next;
        Curr->Val = Next->Val;
        Curr->BB = Next->BB;
        Curr->Next = Next->Next;
      }
    }
  }
 
  // List of critical edges to be split between iterations.
  SmallVector<std::pair<Instruction *, unsigned>, 4> toSplit;
 
  // Helper functions of redundant load elimination
  bool processLoad(LoadInst *L);
  bool processNonLocalLoad(LoadInst *L);
  bool processAssumeIntrinsic(AssumeInst *II);
 
  /// Given a local dependency (Def or Clobber) determine if a value is
  /// available for the load.  Returns true if an value is known to be
  /// available and populates Res.  Returns false otherwise.
  bool AnalyzeLoadAvailability(LoadInst *Load, MemDepResult DepInfo,
                               Value *Address, gvn::AvailableValue &Res);
 
  /// Given a list of non-local dependencies, determine if a value is
  /// available for the load in each specified block.  If it is, add it to
  /// ValuesPerBlock.  If not, add it to UnavailableBlocks.
  void AnalyzeLoadAvailability(LoadInst *Load, LoadDepVect &Deps,
                               AvailValInBlkVect &ValuesPerBlock,
                               UnavailBlkVect &UnavailableBlocks);
 
  bool PerformLoadPRE(LoadInst *Load, AvailValInBlkVect &ValuesPerBlock,
                      UnavailBlkVect &UnavailableBlocks);
 
  /// Try to replace a load which executes on each loop iteraiton with Phi
  /// translation of load in preheader and load(s) in conditionally executed
  /// paths.
  bool performLoopLoadPRE(LoadInst *Load, AvailValInBlkVect &ValuesPerBlock,
                          UnavailBlkVect &UnavailableBlocks);
 
  /// Eliminates partially redundant \p Load, replacing it with \p
  /// AvailableLoads (connected by Phis if needed).
  void eliminatePartiallyRedundantLoad(
      LoadInst *Load, AvailValInBlkVect &ValuesPerBlock,
      MapVector<BasicBlock *, Value *> &AvailableLoads);
 
  // Other helper routines
  bool processInstruction(Instruction *I);
  bool processBlock(BasicBlock *BB);
  void dump(DenseMap<uint32_t, Value *> &d) const;
  bool iterateOnFunction(Function &F);
  bool performPRE(Function &F);
  bool performScalarPRE(Instruction *I);
  bool performScalarPREInsertion(Instruction *Instr, BasicBlock *Pred,
                                 BasicBlock *Curr, unsigned int ValNo);
  Value *findLeader(const BasicBlock *BB, uint32_t num);
  void cleanupGlobalSets();
  void verifyRemoved(const Instruction *I) const;
  bool splitCriticalEdges();
  BasicBlock *splitCriticalEdges(BasicBlock *Pred, BasicBlock *Succ);
  bool replaceOperandsForInBlockEquality(Instruction *I) const;
  bool propagateEquality(Value *LHS, Value *RHS, const BasicBlockEdge &Root,
                         bool DominatesByEdge);
  bool processFoldableCondBr(BranchInst *BI);
  void addDeadBlock(BasicBlock *BB);
  void assignValNumForDeadCode();
  void assignBlockRPONumber(Function &F);
};
 
/// Create a legacy GVN pass. This also allows parameterizing whether or not
/// MemDep is enabled.
FunctionPass *createGVNPass(bool NoMemDepAnalysis = false);
 
/// A simple and fast domtree-based GVN pass to hoist common expressions
/// from sibling branches.
struct GVNHoistPass : PassInfoMixin<GVNHoistPass> {
  /// Run the pass over the function.
  PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
};
 
/// Uses an "inverted" value numbering to decide the similarity of
/// expressions and sinks similar expressions into successors.
struct GVNSinkPass : PassInfoMixin<GVNSinkPass> {
  /// Run the pass over the function.
  PreservedAnalyses run(Function &F, FunctionAnalysisManager &AM);
};
 
} // end namespace llvm
 
#endif // LLVM_TRANSFORMS_SCALAR_GVN_H