lib/CodeGen/MachineSSAUpdater.cpp - fp2-dev/platform/external/llvm - Gitiles

 //===- MachineSSAUpdater.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 MachineSSAUpdater class. It's based on SSAUpdater
 // class in lib/Transforms/Utils.
 //
 //===----------------------------------------------------------------------===//

 #include "llvm/CodeGen/MachineSSAUpdater.h"
 #include "llvm/CodeGen/MachineInstr.h"
 #include "llvm/CodeGen/MachineInstrBuilder.h"
 #include "llvm/CodeGen/MachineRegisterInfo.h"
 #include "llvm/Target/TargetInstrInfo.h"
 #include "llvm/Target/TargetMachine.h"
 #include "llvm/Target/TargetRegisterInfo.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/ADT/SmallVector.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ErrorHandling.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;

 typedef DenseMap<MachineBasicBlock*, unsigned> AvailableValsTy;
 typedef std::vector<std::pair<MachineBasicBlock*, unsigned> >
                 IncomingPredInfoTy;

 static AvailableValsTy &getAvailableVals(void *AV) {
   return *static_cast<AvailableValsTy*>(AV);
 }

 static IncomingPredInfoTy &getIncomingPredInfo(void *IPI) {
   return *static_cast<IncomingPredInfoTy*>(IPI);
 }


 MachineSSAUpdater::MachineSSAUpdater(MachineFunction &MF,
                                      SmallVectorImpl<MachineInstr*> *NewPHI)
   : AV(0), IPI(0), InsertedPHIs(NewPHI) {
   TII = MF.getTarget().getInstrInfo();
   MRI = &MF.getRegInfo();
 }

 MachineSSAUpdater::~MachineSSAUpdater() {
   delete &getAvailableVals(AV);
   delete &getIncomingPredInfo(IPI);
 }

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

   if (IPI == 0)
     IPI = new IncomingPredInfoTy();
   else
     getIncomingPredInfo(IPI).clear();

   VR = V;
   VRC = MRI->getRegClass(VR);
 }

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

 /// AddAvailableValue - Indicate that a rewritten value is available in the
 /// specified block with the specified value.
 void MachineSSAUpdater::AddAvailableValue(MachineBasicBlock *BB, unsigned V) {
   getAvailableVals(AV)[BB] = V;
 }

 /// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
 /// live at the end of the specified block.
 unsigned MachineSSAUpdater::GetValueAtEndOfBlock(MachineBasicBlock *BB) {
   return GetValueAtEndOfBlockInternal(BB);
 }

 static
 unsigned LookForIdenticalPHI(MachineBasicBlock *BB,
           SmallVector<std::pair<MachineBasicBlock*, unsigned>, 8> &PredValues) {
   if (BB->empty())
     return 0;

   MachineBasicBlock::iterator I = BB->front();
   if (!I->isPHI())
     return 0;

   AvailableValsTy AVals;
   for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
     AVals[PredValues[i].first] = PredValues[i].second;
   while (I != BB->end() && I->isPHI()) {
     bool Same = true;
     for (unsigned i = 1, e = I->getNumOperands(); i != e; i += 2) {
       unsigned SrcReg = I->getOperand(i).getReg();
       MachineBasicBlock *SrcBB = I->getOperand(i+1).getMBB();
       if (AVals[SrcBB] != SrcReg) {
         Same = false;
         break;
       }
     }
     if (Same)
       return I->getOperand(0).getReg();
     ++I;
   }
   return 0;
 }

 /// InsertNewDef - Insert an empty PHI or IMPLICIT_DEF instruction which define
 /// a value of the given register class at the start of the specified basic
 /// block. It returns the virtual register defined by the instruction.
 static
 MachineInstr *InsertNewDef(unsigned Opcode,
                            MachineBasicBlock *BB, MachineBasicBlock::iterator I,
                            const TargetRegisterClass *RC,
                            MachineRegisterInfo *MRI, const TargetInstrInfo *TII) {
   unsigned NewVR = MRI->createVirtualRegister(RC);
   return BuildMI(*BB, I, DebugLoc(), TII->get(Opcode), NewVR);
 }

 /// 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.
 ///
 unsigned MachineSSAUpdater::GetValueInMiddleOfBlock(MachineBasicBlock *BB) {
   // If there is no definition of the renamed variable in this block, just use
   // GetValueAtEndOfBlock to do our work.
   if (!getAvailableVals(AV).count(BB))
     return GetValueAtEndOfBlockInternal(BB);

   // If there are no predecessors, just return undef.
   if (BB->pred_empty()) {
     // Insert an implicit_def to represent an undef value.
     MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF,
                                         BB, BB->getFirstTerminator(),
                                         VRC, MRI, TII);
     return NewDef->getOperand(0).getReg();
   }

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

   bool isFirstPred = true;
   for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
          E = BB->pred_end(); PI != E; ++PI) {
     MachineBasicBlock *PredBB = *PI;
     unsigned PredVal = GetValueAtEndOfBlockInternal(PredBB);
     PredValues.push_back(std::make_pair(PredBB, PredVal));

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

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

   // If an identical PHI is already in BB, just reuse it.
   unsigned DupPHI = LookForIdenticalPHI(BB, PredValues);
   if (DupPHI)
     return DupPHI;

   // Otherwise, we do need a PHI: insert one now.
   MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->front();
   MachineInstr *InsertedPHI = InsertNewDef(TargetOpcode::PHI, BB,
                                            Loc, VRC, MRI, TII);

   // Fill in all the predecessors of the PHI.
   MachineInstrBuilder MIB(InsertedPHI);
   for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
     MIB.addReg(PredValues[i].second).addMBB(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 (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) {
     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->getOperand(0).getReg();
 }

 static
 MachineBasicBlock *findCorrespondingPred(const MachineInstr *MI,
                                          MachineOperand *U) {
   for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
     if (&MI->getOperand(i) == U)
       return MI->getOperand(i+1).getMBB();
   }

   llvm_unreachable("MachineOperand::getParent() failure?");
   return 0;
 }

 /// RewriteUse - Rewrite a use of the symbolic value.  This handles PHI nodes,
 /// which use their value in the corresponding predecessor.
 void MachineSSAUpdater::RewriteUse(MachineOperand &U) {
   MachineInstr *UseMI = U.getParent();
   unsigned NewVR = 0;
   if (UseMI->isPHI()) {
     MachineBasicBlock *SourceBB = findCorrespondingPred(UseMI, &U);
     NewVR = GetValueAtEndOfBlockInternal(SourceBB);
   } else {
     NewVR = GetValueInMiddleOfBlock(UseMI->getParent());
   }

   U.setReg(NewVR);
 }

 void MachineSSAUpdater::ReplaceRegWith(unsigned OldReg, unsigned NewReg) {
   MRI->replaceRegWith(OldReg, NewReg);

   AvailableValsTy &AvailableVals = getAvailableVals(AV);
   for (DenseMap<MachineBasicBlock*, unsigned>::iterator
          I = AvailableVals.begin(), E = AvailableVals.end(); I != E; ++I)
     if (I->second == OldReg)
       I->second = NewReg;
 }

 /// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
 /// for the specified BB and if so, return it.  If not, construct SSA form by
 /// walking predecessors inserting PHI nodes as needed until we get to a block
 /// where the value is available.
 ///
 unsigned MachineSSAUpdater::GetValueAtEndOfBlockInternal(MachineBasicBlock *BB){
   AvailableValsTy &AvailableVals = getAvailableVals(AV);

   // Query AvailableVals by doing an insertion of null.
   std::pair<AvailableValsTy::iterator, bool> InsertRes =
     AvailableVals.insert(std::make_pair(BB, 0));

   // Handle the case when the insertion fails because we have already seen BB.
   if (!InsertRes.second) {
     // If the insertion failed, there are two cases.  The first case is that the
     // value is already available for the specified block.  If we get this, just
     // return the value.
     if (InsertRes.first->second != 0)
       return InsertRes.first->second;

     // Otherwise, if the value we find is null, then this is the value is not
     // known but it is being computed elsewhere in our recursion.  This means
     // that we have a cycle.  Handle this by inserting a PHI node and returning
     // it.  When we get back to the first instance of the recursion we will fill
     // in the PHI node.
     MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->front();
     MachineInstr *NewPHI = InsertNewDef(TargetOpcode::PHI, BB, Loc,
                                         VRC, MRI,TII);
     unsigned NewVR = NewPHI->getOperand(0).getReg();
     InsertRes.first->second = NewVR;
     return NewVR;
   }

   // If there are no predecessors, then we must have found an unreachable block
   // just return 'undef'.  Since there are no predecessors, InsertRes must not
   // be invalidated.
   if (BB->pred_empty()) {
     // Insert an implicit_def to represent an undef value.
     MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF,
                                         BB, BB->getFirstTerminator(),
                                         VRC, MRI, TII);
     return InsertRes.first->second = NewDef->getOperand(0).getReg();
   }

   // Okay, the value isn't in the map and we just inserted a null in the entry
   // to indicate that we're processing the block.  Since we have no idea what
   // value is in this block, we have to recurse through our predecessors.
   //
   // While we're walking our predecessors, we keep track of them in a vector,
   // then insert a PHI node in the end if we actually need one.  We could use a
   // smallvector here, but that would take a lot of stack space for every level
   // of the recursion, just use IncomingPredInfo as an explicit stack.
   IncomingPredInfoTy &IncomingPredInfo = getIncomingPredInfo(IPI);
   unsigned FirstPredInfoEntry = IncomingPredInfo.size();

   // As we're walking the predecessors, keep track of whether they are all
   // producing the same value.  If so, this value will capture it, if not, it
   // will get reset to null.  We distinguish the no-predecessor case explicitly
   // below.
   unsigned SingularValue = 0;
   bool isFirstPred = true;
   for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
          E = BB->pred_end(); PI != E; ++PI) {
     MachineBasicBlock *PredBB = *PI;
     unsigned PredVal = GetValueAtEndOfBlockInternal(PredBB);
     IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal));

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

   /// Look up BB's entry in AvailableVals.  'InsertRes' may be invalidated.  If
   /// this block is involved in a loop, a no-entry PHI node will have been
   /// inserted as InsertedVal.  Otherwise, we'll still have the null we inserted
   /// above.
   unsigned &InsertedVal = AvailableVals[BB];

   // If all the predecessor values are the same then we don't need to insert a
   // PHI.  This is the simple and common case.
   if (SingularValue) {
     // If a PHI node got inserted, replace it with the singlar value and delete
     // it.
     if (InsertedVal) {
       MachineInstr *OldVal = MRI->getVRegDef(InsertedVal);
       // Be careful about dead loops.  These RAUW's also update InsertedVal.
       assert(InsertedVal != SingularValue && "Dead loop?");
       ReplaceRegWith(InsertedVal, SingularValue);
       OldVal->eraseFromParent();
     }

     InsertedVal = SingularValue;

     // Drop the entries we added in IncomingPredInfo to restore the stack.
     IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
                            IncomingPredInfo.end());
     return InsertedVal;
   }


   // Otherwise, we do need a PHI: insert one now if we don't already have one.
   MachineInstr *InsertedPHI;
   if (InsertedVal == 0) {
     MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->front();
     InsertedPHI = InsertNewDef(TargetOpcode::PHI, BB, Loc,
                                VRC, MRI, TII);
     InsertedVal = InsertedPHI->getOperand(0).getReg();
   } else {
     InsertedPHI = MRI->getVRegDef(InsertedVal);
   }

   // Fill in all the predecessors of the PHI.
   MachineInstrBuilder MIB(InsertedPHI);
   for (IncomingPredInfoTy::iterator I =
          IncomingPredInfo.begin()+FirstPredInfoEntry,
          E = IncomingPredInfo.end(); I != E; ++I)
     MIB.addReg(I->second).addMBB(I->first);

   // Drop the entries we added in IncomingPredInfo to restore the stack.
   IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
                          IncomingPredInfo.end());

   // 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 (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) {
     MRI->replaceRegWith(InsertedVal, ConstVal);
     InsertedPHI->eraseFromParent();
     InsertedVal = ConstVal;
   } else {
     DEBUG(dbgs() << "  Inserted PHI: " << *InsertedPHI << "\n");

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

   return InsertedVal;
 }
	//===- MachineSSAUpdater.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 MachineSSAUpdater class. It's based on SSAUpdater
	// class in lib/Transforms/Utils.
	//
	//===----------------------------------------------------------------------===//

	#include "llvm/CodeGen/MachineSSAUpdater.h"
	#include "llvm/CodeGen/MachineInstr.h"
	#include "llvm/CodeGen/MachineInstrBuilder.h"
	#include "llvm/CodeGen/MachineRegisterInfo.h"
	#include "llvm/Target/TargetInstrInfo.h"
	#include "llvm/Target/TargetMachine.h"
	#include "llvm/Target/TargetRegisterInfo.h"
	#include "llvm/ADT/DenseMap.h"
	#include "llvm/ADT/SmallVector.h"
	#include "llvm/Support/Debug.h"
	#include "llvm/Support/ErrorHandling.h"
	#include "llvm/Support/raw_ostream.h"
	using namespace llvm;

	typedef DenseMap<MachineBasicBlock*, unsigned> AvailableValsTy;
	typedef std::vector<std::pair<MachineBasicBlock*, unsigned> >
	IncomingPredInfoTy;

	static AvailableValsTy &getAvailableVals(void *AV) {
	return static_cast<AvailableValsTy>(AV);
	}

	static IncomingPredInfoTy &getIncomingPredInfo(void *IPI) {
	return static_cast<IncomingPredInfoTy>(IPI);
	}


	MachineSSAUpdater::MachineSSAUpdater(MachineFunction &MF,
	SmallVectorImpl<MachineInstr> NewPHI)
	: AV(0), IPI(0), InsertedPHIs(NewPHI) {
	TII = MF.getTarget().getInstrInfo();
	MRI = &MF.getRegInfo();
	}

	MachineSSAUpdater::~MachineSSAUpdater() {
	delete &getAvailableVals(AV);
	delete &getIncomingPredInfo(IPI);
	}

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

	if (IPI == 0)
	IPI = new IncomingPredInfoTy();
	else
	getIncomingPredInfo(IPI).clear();

	VR = V;
	VRC = MRI->getRegClass(VR);
	}

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

	/// AddAvailableValue - Indicate that a rewritten value is available in the
	/// specified block with the specified value.
	void MachineSSAUpdater::AddAvailableValue(MachineBasicBlock *BB, unsigned V) {
	getAvailableVals(AV)[BB] = V;
	}

	/// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
	/// live at the end of the specified block.
	unsigned MachineSSAUpdater::GetValueAtEndOfBlock(MachineBasicBlock *BB) {
	return GetValueAtEndOfBlockInternal(BB);
	}

	static
	unsigned LookForIdenticalPHI(MachineBasicBlock *BB,
	SmallVector<std::pair<MachineBasicBlock*, unsigned>, 8> &PredValues) {
	if (BB->empty())
	return 0;

	MachineBasicBlock::iterator I = BB->front();
	if (!I->isPHI())
	return 0;

	AvailableValsTy AVals;
	for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
	AVals[PredValues[i].first] = PredValues[i].second;
	while (I != BB->end() && I->isPHI()) {
	bool Same = true;
	for (unsigned i = 1, e = I->getNumOperands(); i != e; i += 2) {
	unsigned SrcReg = I->getOperand(i).getReg();
	MachineBasicBlock *SrcBB = I->getOperand(i+1).getMBB();
	if (AVals[SrcBB] != SrcReg) {
	Same = false;
	break;
	}
	}
	if (Same)
	return I->getOperand(0).getReg();
	++I;
	}
	return 0;
	}

	/// InsertNewDef - Insert an empty PHI or IMPLICIT_DEF instruction which define
	/// a value of the given register class at the start of the specified basic
	/// block. It returns the virtual register defined by the instruction.
	static
	MachineInstr *InsertNewDef(unsigned Opcode,
	MachineBasicBlock *BB, MachineBasicBlock::iterator I,
	const TargetRegisterClass *RC,
	MachineRegisterInfo MRI, const TargetInstrInfo TII) {
	unsigned NewVR = MRI->createVirtualRegister(RC);
	return BuildMI(*BB, I, DebugLoc(), TII->get(Opcode), NewVR);
	}

	/// 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.
	///
	unsigned MachineSSAUpdater::GetValueInMiddleOfBlock(MachineBasicBlock *BB) {
	// If there is no definition of the renamed variable in this block, just use
	// GetValueAtEndOfBlock to do our work.
	if (!getAvailableVals(AV).count(BB))
	return GetValueAtEndOfBlockInternal(BB);

	// If there are no predecessors, just return undef.
	if (BB->pred_empty()) {
	// Insert an implicit_def to represent an undef value.
	MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF,
	BB, BB->getFirstTerminator(),
	VRC, MRI, TII);
	return NewDef->getOperand(0).getReg();
	}

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

	bool isFirstPred = true;
	for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
	E = BB->pred_end(); PI != E; ++PI) {
	MachineBasicBlock PredBB = PI;
	unsigned PredVal = GetValueAtEndOfBlockInternal(PredBB);
	PredValues.push_back(std::make_pair(PredBB, PredVal));

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

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

	// If an identical PHI is already in BB, just reuse it.
	unsigned DupPHI = LookForIdenticalPHI(BB, PredValues);
	if (DupPHI)
	return DupPHI;

	// Otherwise, we do need a PHI: insert one now.
	MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->front();
	MachineInstr *InsertedPHI = InsertNewDef(TargetOpcode::PHI, BB,
	Loc, VRC, MRI, TII);

	// Fill in all the predecessors of the PHI.
	MachineInstrBuilder MIB(InsertedPHI);
	for (unsigned i = 0, e = PredValues.size(); i != e; ++i)
	MIB.addReg(PredValues[i].second).addMBB(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 (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) {
	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->getOperand(0).getReg();
	}

	static
	MachineBasicBlock findCorrespondingPred(const MachineInstr MI,
	MachineOperand *U) {
	for (unsigned i = 1, e = MI->getNumOperands(); i != e; i += 2) {
	if (&MI->getOperand(i) == U)
	return MI->getOperand(i+1).getMBB();
	}

	llvm_unreachable("MachineOperand::getParent() failure?");
	return 0;
	}

	/// RewriteUse - Rewrite a use of the symbolic value. This handles PHI nodes,
	/// which use their value in the corresponding predecessor.
	void MachineSSAUpdater::RewriteUse(MachineOperand &U) {
	MachineInstr *UseMI = U.getParent();
	unsigned NewVR = 0;
	if (UseMI->isPHI()) {
	MachineBasicBlock *SourceBB = findCorrespondingPred(UseMI, &U);
	NewVR = GetValueAtEndOfBlockInternal(SourceBB);
	} else {
	NewVR = GetValueInMiddleOfBlock(UseMI->getParent());
	}

	U.setReg(NewVR);
	}

	void MachineSSAUpdater::ReplaceRegWith(unsigned OldReg, unsigned NewReg) {
	MRI->replaceRegWith(OldReg, NewReg);

	AvailableValsTy &AvailableVals = getAvailableVals(AV);
	for (DenseMap<MachineBasicBlock*, unsigned>::iterator
	I = AvailableVals.begin(), E = AvailableVals.end(); I != E; ++I)
	if (I->second == OldReg)
	I->second = NewReg;
	}

	/// GetValueAtEndOfBlockInternal - Check to see if AvailableVals has an entry
	/// for the specified BB and if so, return it. If not, construct SSA form by
	/// walking predecessors inserting PHI nodes as needed until we get to a block
	/// where the value is available.
	///
	unsigned MachineSSAUpdater::GetValueAtEndOfBlockInternal(MachineBasicBlock *BB){
	AvailableValsTy &AvailableVals = getAvailableVals(AV);

	// Query AvailableVals by doing an insertion of null.
	std::pair<AvailableValsTy::iterator, bool> InsertRes =
	AvailableVals.insert(std::make_pair(BB, 0));

	// Handle the case when the insertion fails because we have already seen BB.
	if (!InsertRes.second) {
	// If the insertion failed, there are two cases. The first case is that the
	// value is already available for the specified block. If we get this, just
	// return the value.
	if (InsertRes.first->second != 0)
	return InsertRes.first->second;

	// Otherwise, if the value we find is null, then this is the value is not
	// known but it is being computed elsewhere in our recursion. This means
	// that we have a cycle. Handle this by inserting a PHI node and returning
	// it. When we get back to the first instance of the recursion we will fill
	// in the PHI node.
	MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->front();
	MachineInstr *NewPHI = InsertNewDef(TargetOpcode::PHI, BB, Loc,
	VRC, MRI,TII);
	unsigned NewVR = NewPHI->getOperand(0).getReg();
	InsertRes.first->second = NewVR;
	return NewVR;
	}

	// If there are no predecessors, then we must have found an unreachable block
	// just return 'undef'. Since there are no predecessors, InsertRes must not
	// be invalidated.
	if (BB->pred_empty()) {
	// Insert an implicit_def to represent an undef value.
	MachineInstr *NewDef = InsertNewDef(TargetOpcode::IMPLICIT_DEF,
	BB, BB->getFirstTerminator(),
	VRC, MRI, TII);
	return InsertRes.first->second = NewDef->getOperand(0).getReg();
	}

	// Okay, the value isn't in the map and we just inserted a null in the entry
	// to indicate that we're processing the block. Since we have no idea what
	// value is in this block, we have to recurse through our predecessors.
	//
	// While we're walking our predecessors, we keep track of them in a vector,
	// then insert a PHI node in the end if we actually need one. We could use a
	// smallvector here, but that would take a lot of stack space for every level
	// of the recursion, just use IncomingPredInfo as an explicit stack.
	IncomingPredInfoTy &IncomingPredInfo = getIncomingPredInfo(IPI);
	unsigned FirstPredInfoEntry = IncomingPredInfo.size();

	// As we're walking the predecessors, keep track of whether they are all
	// producing the same value. If so, this value will capture it, if not, it
	// will get reset to null. We distinguish the no-predecessor case explicitly
	// below.
	unsigned SingularValue = 0;
	bool isFirstPred = true;
	for (MachineBasicBlock::pred_iterator PI = BB->pred_begin(),
	E = BB->pred_end(); PI != E; ++PI) {
	MachineBasicBlock PredBB = PI;
	unsigned PredVal = GetValueAtEndOfBlockInternal(PredBB);
	IncomingPredInfo.push_back(std::make_pair(PredBB, PredVal));

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

	/// Look up BB's entry in AvailableVals. 'InsertRes' may be invalidated. If
	/// this block is involved in a loop, a no-entry PHI node will have been
	/// inserted as InsertedVal. Otherwise, we'll still have the null we inserted
	/// above.
	unsigned &InsertedVal = AvailableVals[BB];

	// If all the predecessor values are the same then we don't need to insert a
	// PHI. This is the simple and common case.
	if (SingularValue) {
	// If a PHI node got inserted, replace it with the singlar value and delete
	// it.
	if (InsertedVal) {
	MachineInstr *OldVal = MRI->getVRegDef(InsertedVal);
	// Be careful about dead loops. These RAUW's also update InsertedVal.
	assert(InsertedVal != SingularValue && "Dead loop?");
	ReplaceRegWith(InsertedVal, SingularValue);
	OldVal->eraseFromParent();
	}

	InsertedVal = SingularValue;

	// Drop the entries we added in IncomingPredInfo to restore the stack.
	IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
	IncomingPredInfo.end());
	return InsertedVal;
	}


	// Otherwise, we do need a PHI: insert one now if we don't already have one.
	MachineInstr *InsertedPHI;
	if (InsertedVal == 0) {
	MachineBasicBlock::iterator Loc = BB->empty() ? BB->end() : BB->front();
	InsertedPHI = InsertNewDef(TargetOpcode::PHI, BB, Loc,
	VRC, MRI, TII);
	InsertedVal = InsertedPHI->getOperand(0).getReg();
	} else {
	InsertedPHI = MRI->getVRegDef(InsertedVal);
	}

	// Fill in all the predecessors of the PHI.
	MachineInstrBuilder MIB(InsertedPHI);
	for (IncomingPredInfoTy::iterator I =
	IncomingPredInfo.begin()+FirstPredInfoEntry,
	E = IncomingPredInfo.end(); I != E; ++I)
	MIB.addReg(I->second).addMBB(I->first);

	// Drop the entries we added in IncomingPredInfo to restore the stack.
	IncomingPredInfo.erase(IncomingPredInfo.begin()+FirstPredInfoEntry,
	IncomingPredInfo.end());

	// 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 (unsigned ConstVal = InsertedPHI->isConstantValuePHI()) {
	MRI->replaceRegWith(InsertedVal, ConstVal);
	InsertedPHI->eraseFromParent();
	InsertedVal = ConstVal;
	} else {
	DEBUG(dbgs() << " Inserted PHI: " << *InsertedPHI << "\n");

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

	return InsertedVal;
	}