Transforms/Scalar/GVNHoist.cpp

//===- GVNHoist.cpp - Hoist scalar and load expressions -------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This pass hoists expressions from branches to a common dominator. It uses
// GVN (global value numbering) to discover expressions computing the same
// values. The primary goal is to reduce the code size, and in some
// cases reduce critical path (by exposing more ILP).
// Hoisting may affect the performance in some cases. To mitigate that, hoisting
// is disabled in the following cases.
// 1. Scalars across calls.
// 2. geps when corresponding load/store cannot be hoisted.
//===----------------------------------------------------------------------===//

#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/Statistic.h"
#include "llvm/Analysis/ValueTracking.h"
#include "llvm/Transforms/Scalar.h"
#include "llvm/Transforms/Scalar/GVN.h"
#include "llvm/Transforms/Utils/MemorySSA.h"

using namespace llvm;

#define DEBUG_TYPE "gvn-hoist"

STATISTIC(NumHoisted, "Number of instructions hoisted");
STATISTIC(NumRemoved, "Number of instructions removed");
STATISTIC(NumLoadsHoisted, "Number of loads hoisted");
STATISTIC(NumLoadsRemoved, "Number of loads removed");
STATISTIC(NumStoresHoisted, "Number of stores hoisted");
STATISTIC(NumStoresRemoved, "Number of stores removed");
STATISTIC(NumCallsHoisted, "Number of calls hoisted");
STATISTIC(NumCallsRemoved, "Number of calls removed");

static cl::opt<int>
    MaxHoistedThreshold("gvn-max-hoisted", cl::Hidden, cl::init(-1),
                        cl::desc("Max number of instructions to hoist "
                                 "(default unlimited = -1)"));
static cl::opt<int> MaxNumberOfBBSInPath(
    "gvn-hoist-max-bbs", cl::Hidden, cl::init(4),
    cl::desc("Max number of basic blocks on the path between "
             "hoisting locations (default = 4, unlimited = -1)"));

namespace {

// Provides a sorting function based on the execution order of two instructions.
struct SortByDFSIn {
private:
  DenseMap<const BasicBlock *, unsigned> &DFSNumber;

public:
  SortByDFSIn(DenseMap<const BasicBlock *, unsigned> &D) : DFSNumber(D) {}

  // Returns true when A executes before B.
  bool operator()(const Instruction *A, const Instruction *B) const {
    // FIXME: libc++ has a std::sort() algorithm that will call the compare
    // function on the same element.  Once PR20837 is fixed and some more years
    // pass by and all the buildbots have moved to a corrected std::sort(),
    // enable the following assert:
    //
    // assert(A != B);

    const BasicBlock *BA = A->getParent();
    const BasicBlock *BB = B->getParent();
    unsigned NA = DFSNumber[BA];
    unsigned NB = DFSNumber[BB];
    if (NA < NB)
      return true;
    if (NA == NB) {
      // Sort them in the order they occur in the same basic block.
      BasicBlock::const_iterator AI(A), BI(B);
      return std::distance(AI, BI) < 0;
    }
    return false;
  }
};

// A map from a pair of VNs to all the instructions with those VNs.
typedef DenseMap<std::pair<unsigned, unsigned>, SmallVector<Instruction *, 4>>
    VNtoInsns;
// An invalid value number Used when inserting a single value number into
// VNtoInsns.
enum : unsigned { InvalidVN = ~2U };

// Records all scalar instructions candidate for code hoisting.
class InsnInfo {
  VNtoInsns VNtoScalars;

public:
  // Inserts I and its value number in VNtoScalars.
  void insert(Instruction *I, GVN::ValueTable &VN) {
    // Scalar instruction.
    unsigned V = VN.lookupOrAdd(I);
    VNtoScalars[{V, InvalidVN}].push_back(I);
  }

  const VNtoInsns &getVNTable() const { return VNtoScalars; }
};

// Records all load instructions candidate for code hoisting.
class LoadInfo {
  VNtoInsns VNtoLoads;

public:
  // Insert Load and the value number of its memory address in VNtoLoads.
  void insert(LoadInst *Load, GVN::ValueTable &VN) {
    if (Load->isSimple()) {
      unsigned V = VN.lookupOrAdd(Load->getPointerOperand());
      VNtoLoads[{V, InvalidVN}].push_back(Load);
    }
  }

  const VNtoInsns &getVNTable() const { return VNtoLoads; }
};

// Records all store instructions candidate for code hoisting.
class StoreInfo {
  VNtoInsns VNtoStores;

public:
  // Insert the Store and a hash number of the store address and the stored
  // value in VNtoStores.
  void insert(StoreInst *Store, GVN::ValueTable &VN) {
    if (!Store->isSimple())
      return;
    // Hash the store address and the stored value.
    Value *Ptr = Store->getPointerOperand();
    Value *Val = Store->getValueOperand();
    VNtoStores[{VN.lookupOrAdd(Ptr), VN.lookupOrAdd(Val)}].push_back(Store);
  }

  const VNtoInsns &getVNTable() const { return VNtoStores; }
};

// Records all call instructions candidate for code hoisting.
class CallInfo {
  VNtoInsns VNtoCallsScalars;
  VNtoInsns VNtoCallsLoads;
  VNtoInsns VNtoCallsStores;

public:
  // Insert Call and its value numbering in one of the VNtoCalls* containers.
  void insert(CallInst *Call, GVN::ValueTable &VN) {
    // A call that doesNotAccessMemory is handled as a Scalar,
    // onlyReadsMemory will be handled as a Load instruction,
    // all other calls will be handled as stores.
    unsigned V = VN.lookupOrAdd(Call);
    auto Entry = std::make_pair(V, InvalidVN);

    if (Call->doesNotAccessMemory())
      VNtoCallsScalars[Entry].push_back(Call);
    else if (Call->onlyReadsMemory())
      VNtoCallsLoads[Entry].push_back(Call);
    else
      VNtoCallsStores[Entry].push_back(Call);
  }

  const VNtoInsns &getScalarVNTable() const { return VNtoCallsScalars; }

  const VNtoInsns &getLoadVNTable() const { return VNtoCallsLoads; }

  const VNtoInsns &getStoreVNTable() const { return VNtoCallsStores; }
};

typedef DenseMap<const BasicBlock *, bool> BBSideEffectsSet;
typedef SmallVector<Instruction *, 4> SmallVecInsn;
typedef SmallVectorImpl<Instruction *> SmallVecImplInsn;

// This pass hoists common computations across branches sharing common
// dominator. The primary goal is to reduce the code size, and in some
// cases reduce critical path (by exposing more ILP).
class GVNHoist {
public:
  GVN::ValueTable VN;
  DominatorTree *DT;
  AliasAnalysis *AA;
  MemoryDependenceResults *MD;
  const bool OptForMinSize;
  DenseMap<const BasicBlock *, unsigned> DFSNumber;
  BBSideEffectsSet BBSideEffects;
  MemorySSA *MSSA;
  int HoistedCtr;

  enum InsKind { Unknown, Scalar, Load, Store };

  GVNHoist(DominatorTree *Dt, AliasAnalysis *Aa, MemoryDependenceResults *Md,
           bool OptForMinSize)
      : DT(Dt), AA(Aa), MD(Md), OptForMinSize(OptForMinSize), HoistedCtr(0) {}

  // Return true when there are exception handling in BB.
  bool hasEH(const BasicBlock *BB) {
    auto It = BBSideEffects.find(BB);
    if (It != BBSideEffects.end())
      return It->second;

    if (BB->isEHPad() || BB->hasAddressTaken()) {
      BBSideEffects[BB] = true;
      return true;
    }

    if (BB->getTerminator()->mayThrow()) {
      BBSideEffects[BB] = true;
      return true;
    }

    BBSideEffects[BB] = false;
    return false;
  }

  // Return true when all paths from A to the end of the function pass through
  // either B or C.
  bool hoistingFromAllPaths(const BasicBlock *A, const BasicBlock *B,
                            const BasicBlock *C) {
    // We fully copy the WL in order to be able to remove items from it.
    SmallPtrSet<const BasicBlock *, 2> WL;
    WL.insert(B);
    WL.insert(C);

    for (auto It = df_begin(A), E = df_end(A); It != E;) {
      // There exists a path from A to the exit of the function if we are still
      // iterating in DF traversal and we removed all instructions from the work
      // list.
      if (WL.empty())
        return false;

      const BasicBlock *BB = *It;
      if (WL.erase(BB)) {
        // Stop DFS traversal when BB is in the work list.
        It.skipChildren();
        continue;
      }

      // Check for end of function, calls that do not return, etc.
      if (!isGuaranteedToTransferExecutionToSuccessor(BB->getTerminator()))
        return false;

      // Increment DFS traversal when not skipping children.
      ++It;
    }

    return true;
  }

  /* Return true when I1 appears before I2 in the instructions of BB.  */
  bool firstInBB(BasicBlock *BB, const Instruction *I1, const Instruction *I2) {
    for (Instruction &I : *BB) {
      if (&I == I1)
        return true;
      if (&I == I2)
        return false;
    }

    llvm_unreachable("I1 and I2 not found in BB");
  }
  // Return true when there are users of Def in BB.
  bool hasMemoryUseOnPath(MemoryAccess *Def, const BasicBlock *BB,
                          const Instruction *OldPt) {
    const BasicBlock *DefBB = Def->getBlock();
    const BasicBlock *OldBB = OldPt->getParent();

    for (User *U : Def->users())
      if (auto *MU = dyn_cast<MemoryUse>(U)) {
        BasicBlock *UBB = MU->getBlock();
        // Only analyze uses in BB.
        if (BB != UBB)
          continue;

        // A use in the same block as the Def is on the path.
        if (UBB == DefBB) {
          assert(MSSA->locallyDominates(Def, MU) && "def not dominating use");
          return true;
        }

        if (UBB != OldBB)
          return true;

        // It is only harmful to hoist when the use is before OldPt.
        if (firstInBB(UBB, MU->getMemoryInst(), OldPt))
          return true;
      }

    return false;
  }

  // Return true when there are exception handling or loads of memory Def
  // between OldPt and NewPt.

  // Decrement by 1 NBBsOnAllPaths for each block between HoistPt and BB, and
  // return true when the counter NBBsOnAllPaths reaces 0, except when it is
  // initialized to -1 which is unlimited.
  bool hasEHOrLoadsOnPath(const Instruction *NewPt, const Instruction *OldPt,
                          MemoryAccess *Def, int &NBBsOnAllPaths) {
    const BasicBlock *NewBB = NewPt->getParent();
    const BasicBlock *OldBB = OldPt->getParent();
    assert(DT->dominates(NewBB, OldBB) && "invalid path");
    assert(DT->dominates(Def->getBlock(), NewBB) &&
           "def does not dominate new hoisting point");

    // Walk all basic blocks reachable in depth-first iteration on the inverse
    // CFG from OldBB to NewBB. These blocks are all the blocks that may be
    // executed between the execution of NewBB and OldBB. Hoisting an expression
    // from OldBB into NewBB has to be safe on all execution paths.
    for (auto I = idf_begin(OldBB), E = idf_end(OldBB); I != E;) {
      if (*I == NewBB) {
        // Stop traversal when reaching HoistPt.
        I.skipChildren();
        continue;
      }

      // Impossible to hoist with exceptions on the path.
      if (hasEH(*I))
        return true;

      // Check that we do not move a store past loads.
      if (hasMemoryUseOnPath(Def, *I, OldPt))
        return true;

      // Stop walk once the limit is reached.
      if (NBBsOnAllPaths == 0)
        return true;

      // -1 is unlimited number of blocks on all paths.
      if (NBBsOnAllPaths != -1)
        --NBBsOnAllPaths;

      ++I;
    }

    return false;
  }

  // Return true when there are exception handling between HoistPt and BB.
  // Decrement by 1 NBBsOnAllPaths for each block between HoistPt and BB, and
  // return true when the counter NBBsOnAllPaths reaches 0, except when it is
  // initialized to -1 which is unlimited.
  bool hasEHOnPath(const BasicBlock *HoistPt, const BasicBlock *BB,
                   int &NBBsOnAllPaths) {
    assert(DT->dominates(HoistPt, BB) && "Invalid path");

    // Walk all basic blocks reachable in depth-first iteration on
    // the inverse CFG from BBInsn to NewHoistPt. These blocks are all the
    // blocks that may be executed between the execution of NewHoistPt and
    // BBInsn. Hoisting an expression from BBInsn into NewHoistPt has to be safe
    // on all execution paths.
    for (auto I = idf_begin(BB), E = idf_end(BB); I != E;) {
      if (*I == HoistPt) {
        // Stop traversal when reaching NewHoistPt.
        I.skipChildren();
        continue;
      }

      // Impossible to hoist with exceptions on the path.
      if (hasEH(*I))
        return true;

      // Stop walk once the limit is reached.
      if (NBBsOnAllPaths == 0)
        return true;

      // -1 is unlimited number of blocks on all paths.
      if (NBBsOnAllPaths != -1)
        --NBBsOnAllPaths;

      ++I;
    }

    return false;
  }

  // Return true when it is safe to hoist a memory load or store U from OldPt
  // to NewPt.
  bool safeToHoistLdSt(const Instruction *NewPt, const Instruction *OldPt,
                       MemoryUseOrDef *U, InsKind K, int &NBBsOnAllPaths) {

    // In place hoisting is safe.
    if (NewPt == OldPt)
      return true;

    const BasicBlock *NewBB = NewPt->getParent();
    const BasicBlock *OldBB = OldPt->getParent();
    const BasicBlock *UBB = U->getBlock();

    // Check for dependences on the Memory SSA.
    MemoryAccess *D = U->getDefiningAccess();
    BasicBlock *DBB = D->getBlock();
    if (DT->properlyDominates(NewBB, DBB))
      // Cannot move the load or store to NewBB above its definition in DBB.
      return false;

    if (NewBB == DBB && !MSSA->isLiveOnEntryDef(D))
      if (auto *UD = dyn_cast<MemoryUseOrDef>(D))
        if (firstInBB(DBB, NewPt, UD->getMemoryInst()))
          // Cannot move the load or store to NewPt above its definition in D.
          return false;

    // Check for unsafe hoistings due to side effects.
    if (K == InsKind::Store) {
      if (hasEHOrLoadsOnPath(NewPt, OldPt, D, NBBsOnAllPaths))
        return false;
    } else if (hasEHOnPath(NewBB, OldBB, NBBsOnAllPaths))
      return false;

    if (UBB == NewBB) {
      if (DT->properlyDominates(DBB, NewBB))
        return true;
      assert(UBB == DBB);
      assert(MSSA->locallyDominates(D, U));
    }

    // No side effects: it is safe to hoist.
    return true;
  }

  // Return true when it is safe to hoist scalar instructions from BB1 and BB2
  // to HoistBB.
  bool safeToHoistScalar(const BasicBlock *HoistBB, const BasicBlock *BB1,
                         const BasicBlock *BB2, int &NBBsOnAllPaths) {
    // Check that the hoisted expression is needed on all paths.  When HoistBB
    // already contains an instruction to be hoisted, the expression is needed
    // on all paths.  Enable scalar hoisting at -Oz as it is safe to hoist
    // scalars to a place where they are partially needed.
    if (!OptForMinSize && BB1 != HoistBB &&
        !hoistingFromAllPaths(HoistBB, BB1, BB2))
      return false;

    if (hasEHOnPath(HoistBB, BB1, NBBsOnAllPaths) ||
        hasEHOnPath(HoistBB, BB2, NBBsOnAllPaths))
      return false;

    // Safe to hoist scalars from BB1 and BB2 to HoistBB.
    return true;
  }

  // Each element of a hoisting list contains the basic block where to hoist and
  // a list of instructions to be hoisted.
  typedef std::pair<BasicBlock *, SmallVecInsn> HoistingPointInfo;
  typedef SmallVector<HoistingPointInfo, 4> HoistingPointList;

  // Partition InstructionsToHoist into a set of candidates which can share a
  // common hoisting point. The partitions are collected in HPL. IsScalar is
  // true when the instructions in InstructionsToHoist are scalars. IsLoad is
  // true when the InstructionsToHoist are loads, false when they are stores.
  void partitionCandidates(SmallVecImplInsn &InstructionsToHoist,
                           HoistingPointList &HPL, InsKind K) {
    // No need to sort for two instructions.
    if (InstructionsToHoist.size() > 2) {
      SortByDFSIn Pred(DFSNumber);
      std::sort(InstructionsToHoist.begin(), InstructionsToHoist.end(), Pred);
    }

    int NBBsOnAllPaths = MaxNumberOfBBSInPath;

    SmallVecImplInsn::iterator II = InstructionsToHoist.begin();
    SmallVecImplInsn::iterator Start = II;
    Instruction *HoistPt = *II;
    BasicBlock *HoistBB = HoistPt->getParent();
    MemoryUseOrDef *UD;
    if (K != InsKind::Scalar)
      UD = cast<MemoryUseOrDef>(MSSA->getMemoryAccess(HoistPt));

    for (++II; II != InstructionsToHoist.end(); ++II) {
      Instruction *Insn = *II;
      BasicBlock *BB = Insn->getParent();
      BasicBlock *NewHoistBB;
      Instruction *NewHoistPt;

      if (BB == HoistBB) {
        NewHoistBB = HoistBB;
        NewHoistPt = firstInBB(BB, Insn, HoistPt) ? Insn : HoistPt;
      } else {
        NewHoistBB = DT->findNearestCommonDominator(HoistBB, BB);
        if (NewHoistBB == BB)
          NewHoistPt = Insn;
        else if (NewHoistBB == HoistBB)
          NewHoistPt = HoistPt;
        else
          NewHoistPt = NewHoistBB->getTerminator();
      }

      if (K == InsKind::Scalar) {
        if (safeToHoistScalar(NewHoistBB, HoistBB, BB, NBBsOnAllPaths)) {
          // Extend HoistPt to NewHoistPt.
          HoistPt = NewHoistPt;
          HoistBB = NewHoistBB;
          continue;
        }
      } else {
        // When NewBB already contains an instruction to be hoisted, the
        // expression is needed on all paths.
        // Check that the hoisted expression is needed on all paths: it is
        // unsafe to hoist loads to a place where there may be a path not
        // loading from the same address: for instance there may be a branch on
        // which the address of the load may not be initialized.
        if ((HoistBB == NewHoistBB || BB == NewHoistBB ||
             hoistingFromAllPaths(NewHoistBB, HoistBB, BB)) &&
            // Also check that it is safe to move the load or store from HoistPt
            // to NewHoistPt, and from Insn to NewHoistPt.
            safeToHoistLdSt(NewHoistPt, HoistPt, UD, K, NBBsOnAllPaths) &&
            safeToHoistLdSt(NewHoistPt, Insn,
                            cast<MemoryUseOrDef>(MSSA->getMemoryAccess(Insn)),
                            K, NBBsOnAllPaths)) {
          // Extend HoistPt to NewHoistPt.
          HoistPt = NewHoistPt;
          HoistBB = NewHoistBB;
          continue;
        }
      }

      // At this point it is not safe to extend the current hoisting to
      // NewHoistPt: save the hoisting list so far.
      if (std::distance(Start, II) > 1)
        HPL.push_back({HoistBB, SmallVecInsn(Start, II)});

      // Start over from BB.
      Start = II;
      if (K != InsKind::Scalar)
        UD = cast<MemoryUseOrDef>(MSSA->getMemoryAccess(*Start));
      HoistPt = Insn;
      HoistBB = BB;
      NBBsOnAllPaths = MaxNumberOfBBSInPath;
    }

    // Save the last partition.
    if (std::distance(Start, II) > 1)
      HPL.push_back({HoistBB, SmallVecInsn(Start, II)});
  }

  // Initialize HPL from Map.
  void computeInsertionPoints(const VNtoInsns &Map, HoistingPointList &HPL,
                              InsKind K) {
    for (const auto &Entry : Map) {
      if (MaxHoistedThreshold != -1 && ++HoistedCtr > MaxHoistedThreshold)
        return;

      const SmallVecInsn &V = Entry.second;
      if (V.size() < 2)
        continue;

      // Compute the insertion point and the list of expressions to be hoisted.
      SmallVecInsn InstructionsToHoist;
      for (auto I : V)
        if (!hasEH(I->getParent()))
          InstructionsToHoist.push_back(I);

      if (!InstructionsToHoist.empty())
        partitionCandidates(InstructionsToHoist, HPL, K);
    }
  }

  // Return true when all operands of Instr are available at insertion point
  // HoistPt. When limiting the number of hoisted expressions, one could hoist
  // a load without hoisting its access function. So before hoisting any
  // expression, make sure that all its operands are available at insert point.
  bool allOperandsAvailable(const Instruction *I,
                            const BasicBlock *HoistPt) const {
    for (const Use &Op : I->operands())
      if (const auto *Inst = dyn_cast<Instruction>(&Op))
        if (!DT->dominates(Inst->getParent(), HoistPt))
          return false;

    return true;
  }

  Instruction *firstOfTwo(Instruction *I, Instruction *J) const {
    for (Instruction &I1 : *I->getParent())
      if (&I1 == I || &I1 == J)
        return &I1;
    llvm_unreachable("Both I and J must be from same BB");
  }

  bool makeOperandsAvailable(Instruction *Repl, BasicBlock *HoistPt,
                             const SmallVecInsn &InstructionsToHoist) const {
    // Check whether the GEP of a ld/st can be synthesized at HoistPt.
    GetElementPtrInst *Gep = nullptr;
    Instruction *Val = nullptr;
    if (auto *Ld = dyn_cast<LoadInst>(Repl))
      Gep = dyn_cast<GetElementPtrInst>(Ld->getPointerOperand());
    if (auto *St = dyn_cast<StoreInst>(Repl)) {
      Gep = dyn_cast<GetElementPtrInst>(St->getPointerOperand());
      Val = dyn_cast<Instruction>(St->getValueOperand());
      // Check that the stored value is available.
      if (Val) {
        if (isa<GetElementPtrInst>(Val)) {
          // Check whether we can compute the GEP at HoistPt.
          if (!allOperandsAvailable(Val, HoistPt))
            return false;
        } else if (!DT->dominates(Val->getParent(), HoistPt))
          return false;
      }
    }

    // Check whether we can compute the Gep at HoistPt.
    if (!Gep || !allOperandsAvailable(Gep, HoistPt))
      return false;

    // Copy the gep before moving the ld/st.
    Instruction *ClonedGep = Gep->clone();
    ClonedGep->insertBefore(HoistPt->getTerminator());
    // Conservatively discard any optimization hints, they may differ on the
    // other paths.
    ClonedGep->dropUnknownNonDebugMetadata();
    for (const Instruction *OtherInst : InstructionsToHoist) {
      const GetElementPtrInst *OtherGep;
      if (auto *OtherLd = dyn_cast<LoadInst>(OtherInst))
        OtherGep = cast<GetElementPtrInst>(OtherLd->getPointerOperand());
      else
        OtherGep = cast<GetElementPtrInst>(
            cast<StoreInst>(OtherInst)->getPointerOperand());
      ClonedGep->intersectOptionalDataWith(OtherGep);
    }
    Repl->replaceUsesOfWith(Gep, ClonedGep);

    // Also copy Val when it is a GEP.
    if (Val && isa<GetElementPtrInst>(Val)) {
      Instruction *ClonedVal = Val->clone();
      ClonedVal->insertBefore(HoistPt->getTerminator());
      // Conservatively discard any optimization hints, they may differ on the
      // other paths.
      ClonedVal->dropUnknownNonDebugMetadata();
      for (const Instruction *OtherInst : InstructionsToHoist) {
        const auto *OtherVal =
            cast<Instruction>(cast<StoreInst>(OtherInst)->getValueOperand());
        ClonedVal->intersectOptionalDataWith(OtherVal);
      }
      ClonedVal->clearSubclassOptionalData();
      Repl->replaceUsesOfWith(Val, ClonedVal);
    }

    return true;
  }

  std::pair<unsigned, unsigned> hoist(HoistingPointList &HPL) {
    unsigned NI = 0, NL = 0, NS = 0, NC = 0, NR = 0;
    for (const HoistingPointInfo &HP : HPL) {
      // Find out whether we already have one of the instructions in HoistPt,
      // in which case we do not have to move it.
      BasicBlock *HoistPt = HP.first;
      const SmallVecInsn &InstructionsToHoist = HP.second;
      Instruction *Repl = nullptr;
      for (Instruction *I : InstructionsToHoist)
        if (I->getParent() == HoistPt) {
          // If there are two instructions in HoistPt to be hoisted in place:
          // update Repl to be the first one, such that we can rename the uses
          // of the second based on the first.
          Repl = !Repl ? I : firstOfTwo(Repl, I);
        }

      if (Repl) {
        // Repl is already in HoistPt: it remains in place.
        assert(allOperandsAvailable(Repl, HoistPt) &&
               "instruction depends on operands that are not available");
      } else {
        // When we do not find Repl in HoistPt, select the first in the list
        // and move it to HoistPt.
        Repl = InstructionsToHoist.front();

        // We can move Repl in HoistPt only when all operands are available.
        // The order in which hoistings are done may influence the availability
        // of operands.
        if (!allOperandsAvailable(Repl, HoistPt) &&
            !makeOperandsAvailable(Repl, HoistPt, InstructionsToHoist))
          continue;
        Repl->moveBefore(HoistPt->getTerminator());
        // TBAA may differ on one of the other paths, we need to get rid of
        // anything which might conflict.
        Repl->dropUnknownNonDebugMetadata();
      }

      if (isa<LoadInst>(Repl))
        ++NL;
      else if (isa<StoreInst>(Repl))
        ++NS;
      else if (isa<CallInst>(Repl))
        ++NC;
      else // Scalar
        ++NI;

      // Remove and rename all other instructions.
      for (Instruction *I : InstructionsToHoist)
        if (I != Repl) {
          ++NR;
          if (isa<LoadInst>(Repl))
            ++NumLoadsRemoved;
          else if (isa<StoreInst>(Repl))
            ++NumStoresRemoved;
          else if (isa<CallInst>(Repl))
            ++NumCallsRemoved;
          Repl->intersectOptionalDataWith(I);
          I->replaceAllUsesWith(Repl);
          I->eraseFromParent();
        }
    }

    NumHoisted += NL + NS + NC + NI;
    NumRemoved += NR;
    NumLoadsHoisted += NL;
    NumStoresHoisted += NS;
    NumCallsHoisted += NC;
    return {NI, NL + NC + NS};
  }

  // Hoist all expressions. Returns Number of scalars hoisted
  // and number of non-scalars hoisted.
  std::pair<unsigned, unsigned> hoistExpressions(Function &F) {
    InsnInfo II;
    LoadInfo LI;
    StoreInfo SI;
    CallInfo CI;
    for (BasicBlock *BB : depth_first(&F.getEntryBlock())) {
      for (Instruction &I1 : *BB) {
        if (auto *Load = dyn_cast<LoadInst>(&I1))
          LI.insert(Load, VN);
        else if (auto *Store = dyn_cast<StoreInst>(&I1))
          SI.insert(Store, VN);
        else if (auto *Call = dyn_cast<CallInst>(&I1)) {
          if (auto *Intr = dyn_cast<IntrinsicInst>(Call)) {
            if (isa<DbgInfoIntrinsic>(Intr) ||
                Intr->getIntrinsicID() == Intrinsic::assume)
              continue;
          }
          if (Call->mayHaveSideEffects()) {
            if (!OptForMinSize)
              break;
            // We may continue hoisting across calls which write to memory.
            if (Call->mayThrow())
              break;
          }
          CI.insert(Call, VN);
        } else if (OptForMinSize || !isa<GetElementPtrInst>(&I1))
          // Do not hoist scalars past calls that may write to memory because
          // that could result in spills later. geps are handled separately.
          // TODO: We can relax this for targets like AArch64 as they have more
          // registers than X86.
          II.insert(&I1, VN);
      }
    }

    HoistingPointList HPL;
    computeInsertionPoints(II.getVNTable(), HPL, InsKind::Scalar);
    computeInsertionPoints(LI.getVNTable(), HPL, InsKind::Load);
    computeInsertionPoints(SI.getVNTable(), HPL, InsKind::Store);
    computeInsertionPoints(CI.getScalarVNTable(), HPL, InsKind::Scalar);
    computeInsertionPoints(CI.getLoadVNTable(), HPL, InsKind::Load);
    computeInsertionPoints(CI.getStoreVNTable(), HPL, InsKind::Store);
    return hoist(HPL);
  }

  bool run(Function &F) {
    VN.setDomTree(DT);
    VN.setAliasAnalysis(AA);
    VN.setMemDep(MD);
    bool Res = false;

    unsigned I = 0;
    for (const BasicBlock *BB : depth_first(&F.getEntryBlock()))
      DFSNumber.insert({BB, ++I});

    // FIXME: use lazy evaluation of VN to avoid the fix-point computation.
    while (1) {
      // FIXME: only compute MemorySSA once. We need to update the analysis in
      // the same time as transforming the code.
      MemorySSA M(F, AA, DT);
      MSSA = &M;

      auto HoistStat = hoistExpressions(F);
      if (HoistStat.first + HoistStat.second == 0) {
        return Res;
      }
      if (HoistStat.second > 0) {
        // To address a limitation of the current GVN, we need to rerun the
        // hoisting after we hoisted loads in order to be able to hoist all
        // scalars dependent on the hoisted loads. Same for stores.
        VN.clear();
      }
      Res = true;
    }

    return Res;
  }
};

class GVNHoistLegacyPass : public FunctionPass {
public:
  static char ID;

  GVNHoistLegacyPass() : FunctionPass(ID) {
    initializeGVNHoistLegacyPassPass(*PassRegistry::getPassRegistry());
  }

  bool runOnFunction(Function &F) override {
    if (skipFunction(F))
      return false;
    auto &DT = getAnalysis<DominatorTreeWrapperPass>().getDomTree();
    auto &AA = getAnalysis<AAResultsWrapperPass>().getAAResults();
    auto &MD = getAnalysis<MemoryDependenceWrapperPass>().getMemDep();

    GVNHoist G(&DT, &AA, &MD, F.optForMinSize());
    return G.run(F);
  }

  void getAnalysisUsage(AnalysisUsage &AU) const override {
    AU.addRequired<DominatorTreeWrapperPass>();
    AU.addRequired<AAResultsWrapperPass>();
    AU.addRequired<MemoryDependenceWrapperPass>();
    AU.addPreserved<DominatorTreeWrapperPass>();
  }
};
} // namespace

PreservedAnalyses GVNHoistPass::run(Function &F,
                                    AnalysisManager<Function> &AM) {
  DominatorTree &DT = AM.getResult<DominatorTreeAnalysis>(F);
  AliasAnalysis &AA = AM.getResult<AAManager>(F);
  MemoryDependenceResults &MD = AM.getResult<MemoryDependenceAnalysis>(F);

  GVNHoist G(&DT, &AA, &MD, F.optForMinSize());
  if (!G.run(F))
    return PreservedAnalyses::all();

  PreservedAnalyses PA;
  PA.preserve<DominatorTreeAnalysis>();
  return PA;
}

char GVNHoistLegacyPass::ID = 0;
INITIALIZE_PASS_BEGIN(GVNHoistLegacyPass, "gvn-hoist",
                      "Early GVN Hoisting of Expressions", false, false)
INITIALIZE_PASS_DEPENDENCY(MemoryDependenceWrapperPass)
INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
INITIALIZE_PASS_DEPENDENCY(AAResultsWrapperPass)
INITIALIZE_PASS_END(GVNHoistLegacyPass, "gvn-hoist",
                    "Early GVN Hoisting of Expressions", false, false)

FunctionPass *llvm::createGVNHoistPass() { return new GVNHoistLegacyPass(); }