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//===- SparsePropagation.h - Sparse Conditional Property Propagation ------===//

//

// 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 file implements an abstract sparse conditional propagation algorithm,

// modeled after SCCP, but with a customizable lattice function.

//

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

#ifndef LLVM_ANALYSIS_SPARSEPROPAGATION_H

#define LLVM_ANALYSIS_SPARSEPROPAGATION_H

#include "llvm/ADT/SmallPtrSet.h"

#include "llvm/IR/Constants.h"

#include "llvm/IR/Instructions.h"

#include "llvm/Support/Debug.h"

#include <set>

#define DEBUG_TYPE "sparseprop"

namespace llvm {

/// A template for translating between LLVM Values and LatticeKeys. Clients must

/// provide a specialization of LatticeKeyInfo for their LatticeKey type.

template <class LatticeKey> struct LatticeKeyInfo {

  // static inline Value *getValueFromLatticeKey(LatticeKey Key);

  // static inline LatticeKey getLatticeKeyFromValue(Value *V);

};

template <class LatticeKey, class LatticeVal,

          class KeyInfo = LatticeKeyInfo<LatticeKey>>

class SparseSolver;

/// AbstractLatticeFunction - This class is implemented by the dataflow instance

/// to specify what the lattice values are and how they handle merges etc.  This

/// gives the client the power to compute lattice values from instructions,

/// constants, etc.  The current requirement is that lattice values must be

/// copyable.  At the moment, nothing tries to avoid copying.  Additionally,

/// lattice keys must be able to be used as keys of a mapping data structure.

/// Internally, the generic solver currently uses a DenseMap to map lattice keys

/// to lattice values.  If the lattice key is a non-standard type, a

/// specialization of DenseMapInfo must be provided.

template <class LatticeKey, class LatticeVal> class AbstractLatticeFunction {

private:

  LatticeVal UndefVal, OverdefinedVal, UntrackedVal;

public:

  AbstractLatticeFunction(LatticeVal undefVal, LatticeVal overdefinedVal,

                          LatticeVal untrackedVal) {

    UndefVal = undefVal;

    OverdefinedVal = overdefinedVal;

    UntrackedVal = untrackedVal;

  }

  virtual ~AbstractLatticeFunction() = default;

  LatticeVal getUndefVal()       const { return UndefVal; }

  LatticeVal getOverdefinedVal() const { return OverdefinedVal; }

  LatticeVal getUntrackedVal()   const { return UntrackedVal; }

  /// IsUntrackedValue - If the specified LatticeKey is obviously uninteresting

  /// to the analysis (i.e., it would always return UntrackedVal), this

  /// function can return true to avoid pointless work.

  virtual bool IsUntrackedValue(LatticeKey Key) { return false; }

  /// ComputeLatticeVal - Compute and return a LatticeVal corresponding to the

  /// given LatticeKey.

  virtual LatticeVal ComputeLatticeVal(LatticeKey Key) {

    return getOverdefinedVal();

  }

  /// IsSpecialCasedPHI - Given a PHI node, determine whether this PHI node is

  /// one that the we want to handle through ComputeInstructionState.

  virtual bool IsSpecialCasedPHI(PHINode *PN) { return false; }

  /// MergeValues - Compute and return the merge of the two specified lattice

  /// values.  Merging should only move one direction down the lattice to

  /// guarantee convergence (toward overdefined).

  virtual LatticeVal MergeValues(LatticeVal X, LatticeVal Y) {

    return getOverdefinedVal(); // always safe, never useful.

  }

  /// ComputeInstructionState - Compute the LatticeKeys that change as a result

  /// of executing instruction \p I. Their associated LatticeVals are store in

  /// \p ChangedValues.

  virtual void

  ComputeInstructionState(Instruction &I,

                          DenseMap<LatticeKey, LatticeVal> &ChangedValues,

                          SparseSolver<LatticeKey, LatticeVal> &SS) = 0;

  /// PrintLatticeVal - Render the given LatticeVal to the specified stream.

  virtual void PrintLatticeVal(LatticeVal LV, raw_ostream &OS);

  /// PrintLatticeKey - Render the given LatticeKey to the specified stream.

  virtual void PrintLatticeKey(LatticeKey Key, raw_ostream &OS);

  /// GetValueFromLatticeVal - If the given LatticeVal is representable as an

  /// LLVM value, return it; otherwise, return nullptr. If a type is given, the

  /// returned value must have the same type. This function is used by the

  /// generic solver in attempting to resolve branch and switch conditions.

  virtual Value *GetValueFromLatticeVal(LatticeVal LV, Type *Ty = nullptr) {

    return nullptr;

  }

};

/// SparseSolver - This class is a general purpose solver for Sparse Conditional

/// Propagation with a programmable lattice function.

template <class LatticeKey, class LatticeVal, class KeyInfo>

class SparseSolver {

  /// LatticeFunc - This is the object that knows the lattice and how to

  /// compute transfer functions.

  AbstractLatticeFunction<LatticeKey, LatticeVal> *LatticeFunc;

  /// ValueState - Holds the LatticeVals associated with LatticeKeys.

  DenseMap<LatticeKey, LatticeVal> ValueState;

  /// BBExecutable - Holds the basic blocks that are executable.

  SmallPtrSet<BasicBlock *, 16> BBExecutable;

  /// ValueWorkList - Holds values that should be processed.

  SmallVector<Value *, 64> ValueWorkList;

  /// BBWorkList - Holds basic blocks that should be processed.

  SmallVector<BasicBlock *, 64> BBWorkList;

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

  /// KnownFeasibleEdges - Entries in this set are edges which have already had

  /// PHI nodes retriggered.

  std::set<Edge> KnownFeasibleEdges;

public:

  explicit SparseSolver(

      AbstractLatticeFunction<LatticeKey, LatticeVal> *Lattice)

      : LatticeFunc(Lattice) {}

  SparseSolver(const SparseSolver &) = delete;

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

  /// Solve - Solve for constants and executable blocks.

  void Solve();

  void Print(raw_ostream &OS) const;

  /// getExistingValueState - Return the LatticeVal object corresponding to the

  /// given value from the ValueState map. If the value is not in the map,

  /// UntrackedVal is returned, unlike the getValueState method.

  LatticeVal getExistingValueState(LatticeKey Key) const {

    auto I = ValueState.find(Key);

    return I != ValueState.end() ? I->second : LatticeFunc->getUntrackedVal();

  }

  /// getValueState - Return the LatticeVal object corresponding to the given

  /// value from the ValueState map. If the value is not in the map, its state

  /// is initialized.

  LatticeVal getValueState(LatticeKey Key);

  /// isEdgeFeasible - Return true if the control flow edge from the 'From'

  /// basic block to the 'To' basic block is currently feasible.  If

  /// AggressiveUndef is true, then this treats values with unknown lattice

  /// values as undefined.  This is generally only useful when solving the

  /// lattice, not when querying it.

  bool isEdgeFeasible(BasicBlock *From, BasicBlock *To,

                      bool AggressiveUndef = false);

  /// isBlockExecutable - Return true if there are any known feasible

  /// edges into the basic block.  This is generally only useful when

  /// querying the lattice.

  bool isBlockExecutable(BasicBlock *BB) const {

    return BBExecutable.count(BB);

  }

  /// MarkBlockExecutable - This method can be used by clients to mark all of

  /// the blocks that are known to be intrinsically live in the processed unit.

  void MarkBlockExecutable(BasicBlock *BB);

private:

  /// UpdateState - When the state of some LatticeKey is potentially updated to

  /// the given LatticeVal, this function notices and adds the LLVM value

  /// corresponding the key to the work list, if needed.

  void UpdateState(LatticeKey Key, LatticeVal LV);

  /// markEdgeExecutable - Mark a basic block as executable, adding it to the BB

  /// work list if it is not already executable.

  void markEdgeExecutable(BasicBlock *Source, BasicBlock *Dest);

  /// getFeasibleSuccessors - Return a vector of booleans to indicate which

  /// successors are reachable from a given terminator instruction.

  void getFeasibleSuccessors(Instruction &TI, SmallVectorImpl<bool> &Succs,

                             bool AggressiveUndef);

  void visitInst(Instruction &I);

  void visitPHINode(PHINode &I);

  void visitTerminator(Instruction &TI);

};

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

//                  AbstractLatticeFunction Implementation

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

template <class LatticeKey, class LatticeVal>

void AbstractLatticeFunction<LatticeKey, LatticeVal>::PrintLatticeVal(

    LatticeVal V, raw_ostream &OS) {

  if (V == UndefVal)

    OS << "undefined";

  else if (V == OverdefinedVal)

    OS << "overdefined";

  else if (V == UntrackedVal)

    OS << "untracked";

  else

    OS << "unknown lattice value";

}

template <class LatticeKey, class LatticeVal>

void AbstractLatticeFunction<LatticeKey, LatticeVal>::PrintLatticeKey(

    LatticeKey Key, raw_ostream &OS) {

  OS << "unknown lattice key";

}

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

//                          SparseSolver Implementation

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

template <class LatticeKey, class LatticeVal, class KeyInfo>

LatticeVal

SparseSolver<LatticeKey, LatticeVal, KeyInfo>::getValueState(LatticeKey Key) {

  auto I = ValueState.find(Key);

  if (I != ValueState.end())

    return I->second; // Common case, in the map

  if (LatticeFunc->IsUntrackedValue(Key))

    return LatticeFunc->getUntrackedVal();

  LatticeVal LV = LatticeFunc->ComputeLatticeVal(Key);

  // If this value is untracked, don't add it to the map.

  if (LV == LatticeFunc->getUntrackedVal())

    return LV;

  return ValueState[Key] = std::move(LV);

}

template <class LatticeKey, class LatticeVal, class KeyInfo>

void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::UpdateState(LatticeKey Key,

                                                                LatticeVal LV) {

  auto I = ValueState.find(Key);

  if (I != ValueState.end() && I->second == LV)

    return; // No change.

  // Update the state of the given LatticeKey and add its corresponding LLVM

  // value to the work list.

  ValueState[Key] = std::move(LV);

  if (Value *V = KeyInfo::getValueFromLatticeKey(Key))

    ValueWorkList.push_back(V);

}

template <class LatticeKey, class LatticeVal, class KeyInfo>

void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::MarkBlockExecutable(

    BasicBlock *BB) {

  if (!BBExecutable.insert(BB).second)

    return;

  LLVM_DEBUG(dbgs() << "Marking Block Executable: " << BB->getName() << "\n");

  BBWorkList.push_back(BB); // Add the block to the work list!

}

template <class LatticeKey, class LatticeVal, class KeyInfo>

void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::markEdgeExecutable(

    BasicBlock *Source, BasicBlock *Dest) {

  if (!KnownFeasibleEdges.insert(Edge(Source, Dest)).second)

    return; // This edge is already known to be executable!

  LLVM_DEBUG(dbgs() << "Marking Edge Executable: " << Source->getName()

                    << " -> " << Dest->getName() << "\n");

  if (BBExecutable.count(Dest)) {

    // The destination is already executable, but we just made an edge

    // feasible that wasn't before.  Revisit the PHI nodes in the block

    // because they have potentially new operands.

    for (BasicBlock::iterator I = Dest->begin(); isa<PHINode>(I); ++I)

      visitPHINode(*cast<PHINode>(I));

  } else {

    MarkBlockExecutable(Dest);

  }

}

template <class LatticeKey, class LatticeVal, class KeyInfo>

void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::getFeasibleSuccessors(

    Instruction &TI, SmallVectorImpl<bool> &Succs, bool AggressiveUndef) {

  Succs.resize(TI.getNumSuccessors());

  if (TI.getNumSuccessors() == 0)

    return;

  if (BranchInst *BI = dyn_cast<BranchInst>(&TI)) {

    if (BI->isUnconditional()) {

      Succs[0] = true;

      return;

    }

    LatticeVal BCValue;

    if (AggressiveUndef)

      BCValue =

          getValueState(KeyInfo::getLatticeKeyFromValue(BI->getCondition()));

    else

      BCValue = getExistingValueState(

          KeyInfo::getLatticeKeyFromValue(BI->getCondition()));

    if (BCValue == LatticeFunc->getOverdefinedVal() ||

        BCValue == LatticeFunc->getUntrackedVal()) {

      // Overdefined condition variables can branch either way.

      Succs[0] = Succs[1] = true;

      return;

    }

    // If undefined, neither is feasible yet.

    if (BCValue == LatticeFunc->getUndefVal())

      return;

    Constant *C =

        dyn_cast_or_null<Constant>(LatticeFunc->GetValueFromLatticeVal(

            std::move(BCValue), BI->getCondition()->getType()));

    if (!C || !isa<ConstantInt>(C)) {

      // Non-constant values can go either way.

      Succs[0] = Succs[1] = true;

      return;

    }

    // Constant condition variables mean the branch can only go a single way

    Succs[C->isNullValue()] = true;

    return;

  }

  if (!isa<SwitchInst>(TI)) {

    // Unknown termintor, assume all successors are feasible.

    Succs.assign(Succs.size(), true);

    return;

  }

  SwitchInst &SI = cast<SwitchInst>(TI);

  LatticeVal SCValue;

  if (AggressiveUndef)

    SCValue = getValueState(KeyInfo::getLatticeKeyFromValue(SI.getCondition()));

  else

    SCValue = getExistingValueState(

        KeyInfo::getLatticeKeyFromValue(SI.getCondition()));

  if (SCValue == LatticeFunc->getOverdefinedVal() ||

      SCValue == LatticeFunc->getUntrackedVal()) {

    // All destinations are executable!

    Succs.assign(TI.getNumSuccessors(), true);

    return;

  }

  // If undefined, neither is feasible yet.

  if (SCValue == LatticeFunc->getUndefVal())

    return;

  Constant *C = dyn_cast_or_null<Constant>(LatticeFunc->GetValueFromLatticeVal(

      std::move(SCValue), SI.getCondition()->getType()));

  if (!C || !isa<ConstantInt>(C)) {

    // All destinations are executable!

    Succs.assign(TI.getNumSuccessors(), true);

    return;

  }

  SwitchInst::CaseHandle Case = *SI.findCaseValue(cast<ConstantInt>(C));

  Succs[Case.getSuccessorIndex()] = true;

}

template <class LatticeKey, class LatticeVal, class KeyInfo>

bool SparseSolver<LatticeKey, LatticeVal, KeyInfo>::isEdgeFeasible(

    BasicBlock *From, BasicBlock *To, bool AggressiveUndef) {

  SmallVector<bool, 16> SuccFeasible;

  Instruction *TI = From->getTerminator();

  getFeasibleSuccessors(*TI, SuccFeasible, AggressiveUndef);

  for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)

    if (TI->getSuccessor(i) == To && SuccFeasible[i])

      return true;

  return false;

}

template <class LatticeKey, class LatticeVal, class KeyInfo>

void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::visitTerminator(

    Instruction &TI) {

  SmallVector<bool, 16> SuccFeasible;

  getFeasibleSuccessors(TI, SuccFeasible, true);

  BasicBlock *BB = TI.getParent();

  // Mark all feasible successors executable...

  for (unsigned i = 0, e = SuccFeasible.size(); i != e; ++i)

    if (SuccFeasible[i])

      markEdgeExecutable(BB, TI.getSuccessor(i));

}

template <class LatticeKey, class LatticeVal, class KeyInfo>

void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::visitPHINode(PHINode &PN) {

  // The lattice function may store more information on a PHINode than could be

  // computed from its incoming values.  For example, SSI form stores its sigma

  // functions as PHINodes with a single incoming value.

  if (LatticeFunc->IsSpecialCasedPHI(&PN)) {

    DenseMap<LatticeKey, LatticeVal> ChangedValues;

    LatticeFunc->ComputeInstructionState(PN, ChangedValues, *this);

    for (auto &ChangedValue : ChangedValues)

      if (ChangedValue.second != LatticeFunc->getUntrackedVal())

        UpdateState(std::move(ChangedValue.first),

                    std::move(ChangedValue.second));

    return;

  }

  LatticeKey Key = KeyInfo::getLatticeKeyFromValue(&PN);

  LatticeVal PNIV = getValueState(Key);

  LatticeVal Overdefined = LatticeFunc->getOverdefinedVal();

  // If this value is already overdefined (common) just return.

  if (PNIV == Overdefined || PNIV == LatticeFunc->getUntrackedVal())

    return; // Quick exit

  // Super-extra-high-degree PHI nodes are unlikely to ever be interesting,

  // and slow us down a lot.  Just mark them overdefined.

  if (PN.getNumIncomingValues() > 64) {

    UpdateState(Key, Overdefined);

    return;

  }

  // Look at all of the executable operands of the PHI node.  If any of them

  // are overdefined, the PHI becomes overdefined as well.  Otherwise, ask the

  // transfer function to give us the merge of the incoming values.

  for (unsigned i = 0, e = PN.getNumIncomingValues(); i != e; ++i) {

    // If the edge is not yet known to be feasible, it doesn't impact the PHI.

    if (!isEdgeFeasible(PN.getIncomingBlock(i), PN.getParent(), true))

      continue;

    // Merge in this value.

    LatticeVal OpVal =

        getValueState(KeyInfo::getLatticeKeyFromValue(PN.getIncomingValue(i)));

    if (OpVal != PNIV)

      PNIV = LatticeFunc->MergeValues(PNIV, OpVal);

    if (PNIV == Overdefined)

      break; // Rest of input values don't matter.

  }

  // Update the PHI with the compute value, which is the merge of the inputs.

  UpdateState(Key, PNIV);

}

template <class LatticeKey, class LatticeVal, class KeyInfo>

void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::visitInst(Instruction &I) {

  // PHIs are handled by the propagation logic, they are never passed into the

  // transfer functions.

  if (PHINode *PN = dyn_cast<PHINode>(&I))

    return visitPHINode(*PN);

  // Otherwise, ask the transfer function what the result is.  If this is

  // something that we care about, remember it.

  DenseMap<LatticeKey, LatticeVal> ChangedValues;

  LatticeFunc->ComputeInstructionState(I, ChangedValues, *this);

  for (auto &ChangedValue : ChangedValues)

    if (ChangedValue.second != LatticeFunc->getUntrackedVal())

      UpdateState(ChangedValue.first, ChangedValue.second);

  if (I.isTerminator())

    visitTerminator(I);

}

template <class LatticeKey, class LatticeVal, class KeyInfo>

void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::Solve() {

  // Process the work lists until they are empty!

  while (!BBWorkList.empty() || !ValueWorkList.empty()) {

    // Process the value work list.

    while (!ValueWorkList.empty()) {

      Value *V = ValueWorkList.pop_back_val();

      LLVM_DEBUG(dbgs() << "\nPopped off V-WL: " << *V << "\n");

      // "V" got into the work list because it made a transition. See if any

      // users are both live and in need of updating.

      for (User *U : V->users())

        if (Instruction *Inst = dyn_cast<Instruction>(U))

          if (BBExecutable.count(Inst->getParent())) // Inst is executable?

            visitInst(*Inst);

    }

    // Process the basic block work list.

    while (!BBWorkList.empty()) {

      BasicBlock *BB = BBWorkList.pop_back_val();

      LLVM_DEBUG(dbgs() << "\nPopped off BBWL: " << *BB);

      // Notify all instructions in this basic block that they are newly

      // executable.

      for (Instruction &I : *BB)

        visitInst(I);

    }

  }

}

template <class LatticeKey, class LatticeVal, class KeyInfo>

void SparseSolver<LatticeKey, LatticeVal, KeyInfo>::Print(

    raw_ostream &OS) const {

  if (ValueState.empty())

    return;

  LatticeKey Key;

  LatticeVal LV;

  OS << "ValueState:\n";

  for (auto &Entry : ValueState) {

    std::tie(Key, LV) = Entry;

    if (LV == LatticeFunc->getUntrackedVal())

      continue;

    OS << "\t";

    LatticeFunc->PrintLatticeVal(LV, OS);

    OS << ": ";

    LatticeFunc->PrintLatticeKey(Key, OS);

    OS << "\n";

  }

}

} // end namespace llvm

#undef DEBUG_TYPE

#endif // LLVM_ANALYSIS_SPARSEPROPAGATION_H