lib/Transforms/Utils/SSAUpdater.cpp

//===- SSAUpdater.cpp - Unstructured SSA Update Tool ----------------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file implements the SSAUpdater class.
//
//===----------------------------------------------------------------------===//

#define DEBUG_TYPE "ssaupdater"
#include "llvm/Instructions.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/Support/AlignOf.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Transforms/Utils/SSAUpdater.h"
#include "llvm/Transforms/Utils/SSAUpdaterImpl.h"
using namespace llvm;

typedef DenseMap<BasicBlock*, Value*> AvailableValsTy;
static AvailableValsTy &getAvailableVals(void *AV) {
  return *static_cast<AvailableValsTy*>(AV);
}

SSAUpdater::SSAUpdater(SmallVectorImpl<PHINode*> *NewPHI)
  : AV(0), PrototypeValue(0), InsertedPHIs(NewPHI) {}

SSAUpdater::~SSAUpdater() {
  delete &getAvailableVals(AV);
}

/// Initialize - Reset this object to get ready for a new set of SSA
/// updates.  ProtoValue is the value used to name PHI nodes.
void SSAUpdater::Initialize(Value *ProtoValue) {
  if (AV == 0)
    AV = new AvailableValsTy();
  else
    getAvailableVals(AV).clear();
  PrototypeValue = ProtoValue;
}

/// HasValueForBlock - Return true if the SSAUpdater already has a value for
/// the specified block.
bool SSAUpdater::HasValueForBlock(BasicBlock *BB) const {
  return getAvailableVals(AV).count(BB);
}

/// AddAvailableValue - Indicate that a rewritten value is available in the
/// specified block with the specified value.
void SSAUpdater::AddAvailableValue(BasicBlock *BB, Value *V) {
  assert(PrototypeValue != 0 && "Need to initialize SSAUpdater");
  assert(PrototypeValue->getType() == V->getType() &&
         "All rewritten values must have the same type");
  getAvailableVals(AV)[BB] = V;
}

/// IsEquivalentPHI - Check if PHI has the same incoming value as specified
/// in ValueMapping for each predecessor block.
static bool IsEquivalentPHI(PHINode *PHI,
                            DenseMap<BasicBlock*, Value*> &ValueMapping) {
  unsigned PHINumValues = PHI->getNumIncomingValues();
  if (PHINumValues != ValueMapping.size())
    return false;

  // Scan the phi to see if it matches.
  for (unsigned i = 0, e = PHINumValues; i != e; ++i)
    if (ValueMapping[PHI->getIncomingBlock(i)] !=
        PHI->getIncomingValue(i)) {
      return false;
    }

  return true;
}

/// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
/// live at the end of the specified block.
Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) {
  Value *Res = GetValueAtEndOfBlockInternal(BB);
  return Res;
}

/// GetValueInMiddleOfBlock - Construct SSA form, materializing a value that
/// is live in the middle of the specified block.
///
/// GetValueInMiddleOfBlock is the same as GetValueAtEndOfBlock except in one
/// important case: if there is a definition of the rewritten value after the
/// 'use' in BB.  Consider code like this:
///
///      X1 = ...
///   SomeBB:
///      use(X)
///      X2 = ...
///      br Cond, SomeBB, OutBB
///
/// In this case, there are two values (X1 and X2) added to the AvailableVals
/// set by the client of the rewriter, and those values are both live out of
/// their respective blocks.  However, the use of X happens in the *middle* of
/// a block.  Because of this, we need to insert a new PHI node in SomeBB to
/// merge the appropriate values, and this value isn't live out of the block.
///
Value *SSAUpdater::GetValueInMiddleOfBlock(BasicBlock *BB) {
  // If there is no definition of the renamed variable in this block, just use
  // GetValueAtEndOfBlock to do our work.
  if (!HasValueForBlock(BB))
    return GetValueAtEndOfBlock(BB);

  // Otherwise, we have the hard case.  Get the live-in values for each
  // predecessor.
  SmallVector<std::pair<BasicBlock*, Value*>, 8> PredValues;
  Value *SingularValue = 0;

  // We can get our predecessor info by walking the pred_iterator list, but it
  // is relatively slow.  If we already have PHI nodes in this block, walk one
  // of them to get the predecessor list instead.
  if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
    for (unsigned i = 0, e = SomePhi->getNumIncomingValues(); i != e; ++i) {
      BasicBlock *PredBB = SomePhi->getIncomingBlock(i);
      Value *PredVal = GetValueAtEndOfBlock(PredBB);
      PredValues.push_back(std::make_pair(PredBB, PredVal));

      // Compute SingularValue.
      if (i == 0)
        SingularValue = PredVal;
      else if (PredVal != SingularValue)
        SingularValue = 0;
    }
  } else {
    bool isFirstPred = true;
    for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI) {
      BasicBlock *PredBB = *PI;
      Value *PredVal = GetValueAtEndOfBlock(PredBB);
      PredValues.push_back(std::make_pair(PredBB, PredVal));

      // Compute SingularValue.
      if (isFirstPred) {
        SingularValue = PredVal;
        isFirstPred = false;
      } else if (PredVal != SingularValue)
        SingularValue = 0;
    }
  }

  // If there are no predecessors, just return undef.
  if (PredValues.empty())
    return UndefValue::get(PrototypeValue->getType());

  // Otherwise, if all the merged values are the same, just use it.
  if (SingularValue != 0)
    return SingularValue;

  // Otherwise, we do need a PHI: check to see if we already have one available
  // in this block that produces the right value.
  if (isa<PHINode>(BB->begin())) {
    DenseMap<BasicBlock*, Value*> ValueMapping(PredValues.begin(),
                                               PredValues.end());
    PHINode *SomePHI;
    for (BasicBlock::iterator It = BB->begin();
         (SomePHI = dyn_cast<PHINode>(It)); ++It) {
      if (IsEquivalentPHI(SomePHI, ValueMapping))
        return SomePHI;
    }
  }

  // Ok, we have no way out, insert a new one now.
  PHINode *InsertedPHI = PHINode::Create(PrototypeValue->getType(),
                                         PrototypeValue->getName(),
                                         &BB->front());
  InsertedPHI->reserveOperandSpace(PredValues.size());

  // Fill in all the predecessors of the PHI.
  for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
    InsertedPHI->addIncoming(PredValues[i].second, PredValues[i].first);

  // See if the PHI node can be merged to a single value.  This can happen in
  // loop cases when we get a PHI of itself and one other value.
  if (Value *ConstVal = InsertedPHI->hasConstantValue()) {
    InsertedPHI->eraseFromParent();
    return ConstVal;
  }

  // If the client wants to know about all new instructions, tell it.
  if (InsertedPHIs) InsertedPHIs->push_back(InsertedPHI);

  DEBUG(dbgs() << "  Inserted PHI: " << *InsertedPHI << "\n");
  return InsertedPHI;
}

/// RewriteUse - Rewrite a use of the symbolic value.  This handles PHI nodes,
/// which use their value in the corresponding predecessor.
void SSAUpdater::RewriteUse(Use &U) {
  Instruction *User = cast<Instruction>(U.getUser());

  Value *V;
  if (PHINode *UserPN = dyn_cast<PHINode>(User))
    V = GetValueAtEndOfBlock(UserPN->getIncomingBlock(U));
  else
    V = GetValueInMiddleOfBlock(User->getParent());

  U.set(V);
}

/// PHIiter - Iterator for PHI operands.  This is used for the PHI_iterator
/// in the SSAUpdaterImpl template.
namespace {
  class PHIiter {
  private:
    PHINode *PHI;
    unsigned idx;

  public:
    explicit PHIiter(PHINode *P) // begin iterator
      : PHI(P), idx(0) {}
    PHIiter(PHINode *P, bool) // end iterator
      : PHI(P), idx(PHI->getNumIncomingValues()) {}

    PHIiter &operator++() { ++idx; return *this; } 
    bool operator==(const PHIiter& x) const { return idx == x.idx; }
    bool operator!=(const PHIiter& x) const { return !operator==(x); }
    Value *getIncomingValue() { return PHI->getIncomingValue(idx); }
    BasicBlock *getIncomingBlock() { return PHI->getIncomingBlock(idx); }
  };
}

/// SSAUpdaterTraits<SSAUpdater> - Traits for the SSAUpdaterImpl template,
/// specialized for SSAUpdater.
namespace llvm {
template<>
class SSAUpdaterTraits<SSAUpdater> {
public:
  typedef BasicBlock BlkT;
  typedef Value *ValT;
  typedef PHINode PhiT;

  typedef succ_iterator BlkSucc_iterator;
  static BlkSucc_iterator BlkSucc_begin(BlkT *BB) { return succ_begin(BB); }
  static BlkSucc_iterator BlkSucc_end(BlkT *BB) { return succ_end(BB); }

  typedef PHIiter PHI_iterator;
  static inline PHI_iterator PHI_begin(PhiT *PHI) { return PHI_iterator(PHI); }
  static inline PHI_iterator PHI_end(PhiT *PHI) {
    return PHI_iterator(PHI, true);
  }

  /// FindPredecessorBlocks - Put the predecessors of Info->BB into the Preds
  /// vector, set Info->NumPreds, and allocate space in Info->Preds.
  static void FindPredecessorBlocks(BasicBlock *BB,
                                    SmallVectorImpl<BasicBlock*> *Preds) {
    // We can get our predecessor info by walking the pred_iterator list,
    // but it is relatively slow.  If we already have PHI nodes in this
    // block, walk one of them to get the predecessor list instead.
    if (PHINode *SomePhi = dyn_cast<PHINode>(BB->begin())) {
      for (unsigned PI = 0, E = SomePhi->getNumIncomingValues(); PI != E; ++PI)
        Preds->push_back(SomePhi->getIncomingBlock(PI));
    } else {
      for (pred_iterator PI = pred_begin(BB), E = pred_end(BB); PI != E; ++PI)
        Preds->push_back(*PI);
    }
  }

  /// GetUndefVal - Get an undefined value of the same type as the value
  /// being handled.
  static Value *GetUndefVal(BasicBlock *BB, SSAUpdater *Updater) {
    return UndefValue::get(Updater->PrototypeValue->getType());
  }

  /// CreateEmptyPHI - Create a new PHI instruction in the specified block.
  /// Reserve space for the operands but do not fill them in yet.
  static Value *CreateEmptyPHI(BasicBlock *BB, unsigned NumPreds,
                               SSAUpdater *Updater) {
    PHINode *PHI = PHINode::Create(Updater->PrototypeValue->getType(),
                                   Updater->PrototypeValue->getName(),
                                   &BB->front());
    PHI->reserveOperandSpace(NumPreds);
    return PHI;
  }

  /// AddPHIOperand - Add the specified value as an operand of the PHI for
  /// the specified predecessor block.
  static void AddPHIOperand(PHINode *PHI, Value *Val, BasicBlock *Pred) {
    PHI->addIncoming(Val, Pred);
  }

  /// InstrIsPHI - Check if an instruction is a PHI.
  ///
  static PHINode *InstrIsPHI(Instruction *I) {
    return dyn_cast<PHINode>(I);
  }

  /// ValueIsPHI - Check if a value is a PHI.
  ///
  static PHINode *ValueIsPHI(Value *Val, SSAUpdater *Updater) {
    return dyn_cast<PHINode>(Val);
  }

  /// ValueIsNewPHI - Like ValueIsPHI but also check if the PHI has no source
  /// operands, i.e., it was just added.
  static PHINode *ValueIsNewPHI(Value *Val, SSAUpdater *Updater) {
    PHINode *PHI = ValueIsPHI(Val, Updater);
    if (PHI && PHI->getNumIncomingValues() == 0)
      return PHI;
    return 0;
  }

  /// GetPHIValue - For the specified PHI instruction, return the value
  /// that it defines.
  static Value *GetPHIValue(PHINode *PHI) {
    return PHI;
  }
};

} // End llvm namespace

/// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
/// for the specified BB and if so, return it.  If not, construct SSA form by
/// first calculating the required placement of PHIs and then inserting new
/// PHIs where needed.
Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *BB) {
  AvailableValsTy &AvailableVals = getAvailableVals(AV);
  if (Value *V = AvailableVals[BB])
    return V;

  SSAUpdaterImpl<SSAUpdater> Impl(this, &AvailableVals, InsertedPHIs);
  return Impl.GetValue(BB);
}