lib/Transforms/Utils/SSAUpdater.cpp - platform/external/llvm - Gitiles

 //===- 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.
 //
 //===----------------------------------------------------------------------===//

 #include "llvm/Transforms/Utils/SSAUpdater.h"
 #include "llvm/Instructions.h"
 #include "llvm/ADT/DenseMap.h"
 #include "llvm/Support/CFG.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/ValueHandle.h"
 #include "llvm/Support/raw_ostream.h"
 using namespace llvm;

 typedef DenseMap<BasicBlock*, TrackingVH<Value> > AvailableValsTy;
 typedef std::vector<std::pair<BasicBlock*, TrackingVH<Value> > >
                 IncomingPredInfoTy;

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

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


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

 SSAUpdater::~SSAUpdater() {
   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 SSAUpdater::Initialize(Value *ProtoValue) {
   if (AV == 0)
     AV = new AvailableValsTy();
   else
     getAvailableVals(AV).clear();

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

 /// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
 /// live at the end of the specified block.
 Value *SSAUpdater::GetValueAtEndOfBlock(BasicBlock *BB) {
   assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
   Value *Res = GetValueAtEndOfBlockInternal(BB);
   assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
   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 (!getAvailableVals(AV).count(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) {
       // Scan this phi to see if it is what we need.
       bool Equal = true;
       for (unsigned i = 0, e = SomePHI->getNumIncomingValues(); i != e; ++i)
         if (ValueMapping[SomePHI->getIncomingBlock(i)] !=
             SomePHI->getIncomingValue(i)) {
           Equal = false;
           break;
         }

       if (Equal)
         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);
 }


 /// 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.
 ///
 Value *SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock *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, TrackingVH<Value>()));

   // 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.
     return InsertRes.first->second =
       PHINode::Create(PrototypeValue->getType(), PrototypeValue->getName(),
                       &BB->front());
   }

   // 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.
   TrackingVH<Value> SingularValue;

   // 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 = GetValueAtEndOfBlockInternal(PredBB);
       IncomingPredInfo.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 = GetValueAtEndOfBlockInternal(PredBB);
       IncomingPredInfo.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, then we must have found an unreachable block
   // just return 'undef'.  Since there are no predecessors, InsertRes must not
   // be invalidated.
   if (IncomingPredInfo.size() == FirstPredInfoEntry)
     return InsertRes.first->second = UndefValue::get(PrototypeValue->getType());

   /// 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.
   TrackingVH<Value> &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) {
       PHINode *OldVal = cast<PHINode>(InsertedVal);
       // Be careful about dead loops.  These RAUW's also update InsertedVal.
       if (InsertedVal != SingularValue)
         OldVal->replaceAllUsesWith(SingularValue);
       else
         OldVal->replaceAllUsesWith(UndefValue::get(InsertedVal->getType()));
       OldVal->eraseFromParent();
     } else {
       InsertedVal = SingularValue;
     }

     // Either path through the 'if' should have set insertedVal -> SingularVal.
     assert((InsertedVal == SingularValue || isa<UndefValue>(InsertedVal)) &&
            "RAUW didn't change InsertedVal to be SingularVal");

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

   // Otherwise, we do need a PHI: insert one now if we don't already have one.
   if (InsertedVal == 0)
     InsertedVal = PHINode::Create(PrototypeValue->getType(),
                                   PrototypeValue->getName(), &BB->front());

   PHINode *InsertedPHI = cast<PHINode>(InsertedVal);
   InsertedPHI->reserveOperandSpace(IncomingPredInfo.size()-FirstPredInfoEntry);

   // Fill in all the predecessors of the PHI.
   for (IncomingPredInfoTy::iterator I =
          IncomingPredInfo.begin()+FirstPredInfoEntry,
        E = IncomingPredInfo.end(); I != E; ++I)
     InsertedPHI->addIncoming(I->second, 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 (Value *ConstVal = InsertedPHI->hasConstantValue()) {
     InsertedPHI->replaceAllUsesWith(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;
 }
	//===- 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.
	//
	//===----------------------------------------------------------------------===//

	#include "llvm/Transforms/Utils/SSAUpdater.h"
	#include "llvm/Instructions.h"
	#include "llvm/ADT/DenseMap.h"
	#include "llvm/Support/CFG.h"
	#include "llvm/Support/Debug.h"
	#include "llvm/Support/ValueHandle.h"
	#include "llvm/Support/raw_ostream.h"
	using namespace llvm;

	typedef DenseMap<BasicBlock*, TrackingVH<Value> > AvailableValsTy;
	typedef std::vector<std::pair<BasicBlock*, TrackingVH<Value> > >
	IncomingPredInfoTy;

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

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


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

	SSAUpdater::~SSAUpdater() {
	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 SSAUpdater::Initialize(Value *ProtoValue) {
	if (AV == 0)
	AV = new AvailableValsTy();
	else
	getAvailableVals(AV).clear();

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

	/// GetValueAtEndOfBlock - Construct SSA form, materializing a value that is
	/// live at the end of the specified block.
	Value SSAUpdater::GetValueAtEndOfBlock(BasicBlock BB) {
	assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
	Value *Res = GetValueAtEndOfBlockInternal(BB);
	assert(getIncomingPredInfo(IPI).empty() && "Unexpected Internal State");
	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 (!getAvailableVals(AV).count(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) {
	// Scan this phi to see if it is what we need.
	bool Equal = true;
	for (unsigned i = 0, e = SomePHI->getNumIncomingValues(); i != e; ++i)
	if (ValueMapping[SomePHI->getIncomingBlock(i)] !=
	SomePHI->getIncomingValue(i)) {
	Equal = false;
	break;
	}

	if (Equal)
	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);
	}


	/// 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.
	///
	Value SSAUpdater::GetValueAtEndOfBlockInternal(BasicBlock 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, TrackingVH<Value>()));

	// 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.
	return InsertRes.first->second =
	PHINode::Create(PrototypeValue->getType(), PrototypeValue->getName(),
	&BB->front());
	}

	// 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.
	TrackingVH<Value> SingularValue;

	// 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 = GetValueAtEndOfBlockInternal(PredBB);
	IncomingPredInfo.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 = GetValueAtEndOfBlockInternal(PredBB);
	IncomingPredInfo.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, then we must have found an unreachable block
	// just return 'undef'. Since there are no predecessors, InsertRes must not
	// be invalidated.
	if (IncomingPredInfo.size() == FirstPredInfoEntry)
	return InsertRes.first->second = UndefValue::get(PrototypeValue->getType());

	/// 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.
	TrackingVH<Value> &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) {
	PHINode *OldVal = cast<PHINode>(InsertedVal);
	// Be careful about dead loops. These RAUW's also update InsertedVal.
	if (InsertedVal != SingularValue)
	OldVal->replaceAllUsesWith(SingularValue);
	else
	OldVal->replaceAllUsesWith(UndefValue::get(InsertedVal->getType()));
	OldVal->eraseFromParent();
	} else {
	InsertedVal = SingularValue;
	}

	// Either path through the 'if' should have set insertedVal -> SingularVal.
	assert((InsertedVal == SingularValue \|\| isa<UndefValue>(InsertedVal)) &&
	"RAUW didn't change InsertedVal to be SingularVal");

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

	// Otherwise, we do need a PHI: insert one now if we don't already have one.
	if (InsertedVal == 0)
	InsertedVal = PHINode::Create(PrototypeValue->getType(),
	PrototypeValue->getName(), &BB->front());

	PHINode *InsertedPHI = cast<PHINode>(InsertedVal);
	InsertedPHI->reserveOperandSpace(IncomingPredInfo.size()-FirstPredInfoEntry);

	// Fill in all the predecessors of the PHI.
	for (IncomingPredInfoTy::iterator I =
	IncomingPredInfo.begin()+FirstPredInfoEntry,
	E = IncomingPredInfo.end(); I != E; ++I)
	InsertedPHI->addIncoming(I->second, 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 (Value *ConstVal = InsertedPHI->hasConstantValue()) {
	InsertedPHI->replaceAllUsesWith(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;
	}