lib/Transforms/Scalar/TailDuplication.cpp - llvm - Git at Google

 //===- TailDuplication.cpp - Simplify CFG through tail duplication --------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file was developed by the LLVM research group and is distributed under
 // the University of Illinois Open Source License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This pass performs a limited form of tail duplication, intended to simplify
 // CFGs by removing some unconditional branches.  This pass is necessary to
 // straighten out loops created by the C front-end, but also is capable of
 // making other code nicer.  After this pass is run, the CFG simplify pass
 // should be run to clean up the mess.
 //
 // This pass could be enhanced in the future to use profile information to be
 // more aggressive.
 //
 //===----------------------------------------------------------------------===//

 #include "llvm/Transforms/Scalar.h"
 #include "llvm/Constant.h"
 #include "llvm/Function.h"
 #include "llvm/iPHINode.h"
 #include "llvm/iTerminators.h"
 #include "llvm/Pass.h"
 #include "llvm/Type.h"
 #include "llvm/Support/CFG.h"
 #include "llvm/Support/ValueHolder.h"
 #include "llvm/Transforms/Utils/Local.h"
 #include "Support/Debug.h"
 #include "Support/Statistic.h"

 namespace {
   Statistic<> NumEliminated("tailduplicate",
                             "Number of unconditional branches eliminated");
   Statistic<> NumPHINodes("tailduplicate", "Number of phi nodes inserted");

   class TailDup : public FunctionPass {
     bool runOnFunction(Function &F);
   private:
     inline bool shouldEliminateUnconditionalBranch(TerminatorInst *TI);
     inline void eliminateUnconditionalBranch(BranchInst *BI);
     inline void InsertPHINodesIfNecessary(Instruction *OrigInst, Value *NewInst,
                                           BasicBlock *NewBlock);
     inline Value *GetValueInBlock(BasicBlock *BB, Value *OrigVal,
                                   std::map<BasicBlock*, ValueHolder> &ValueMap,
                               std::map<BasicBlock*, ValueHolder> &OutValueMap);
     inline Value *GetValueOutBlock(BasicBlock *BB, Value *OrigVal,
                                    std::map<BasicBlock*, ValueHolder> &ValueMap,
                                std::map<BasicBlock*, ValueHolder> &OutValueMap);
   };
   RegisterOpt<TailDup> X("tailduplicate", "Tail Duplication");
 }

 Pass *createTailDuplicationPass() { return new TailDup(); }

 /// runOnFunction - Top level algorithm - Loop over each unconditional branch in
 /// the function, eliminating it if it looks attractive enough.
 ///
 bool TailDup::runOnFunction(Function &F) {
   bool Changed = false;
   for (Function::iterator I = F.begin(), E = F.end(); I != E; )
     if (shouldEliminateUnconditionalBranch(I->getTerminator())) {
       eliminateUnconditionalBranch(cast<BranchInst>(I->getTerminator()));
       Changed = true;
     } else {
       ++I;
     }
   return Changed;
 }

 /// shouldEliminateUnconditionalBranch - Return true if this branch looks
 /// attractive to eliminate.  We eliminate the branch if the destination basic
 /// block has <= 5 instructions in it, not counting PHI nodes.  In practice,
 /// since one of these is a terminator instruction, this means that we will add
 /// up to 4 instructions to the new block.
 ///
 /// We don't count PHI nodes in the count since they will be removed when the
 /// contents of the block are copied over.
 ///
 bool TailDup::shouldEliminateUnconditionalBranch(TerminatorInst *TI) {
   BranchInst *BI = dyn_cast<BranchInst>(TI);
   if (!BI || !BI->isUnconditional()) return false;  // Not an uncond branch!

   BasicBlock *Dest = BI->getSuccessor(0);
   if (Dest == BI->getParent()) return false;        // Do not loop infinitely!

   // Do not inline a block if we will just get another branch to the same block!
   if (BranchInst *DBI = dyn_cast<BranchInst>(Dest->getTerminator()))
     if (DBI->isUnconditional() && DBI->getSuccessor(0) == Dest)
       return false;                                 // Do not loop infinitely!

   // Do not bother working on dead blocks...
   pred_iterator PI = pred_begin(Dest), PE = pred_end(Dest);
   if (PI == PE && Dest != Dest->getParent()->begin())
     return false;   // It's just a dead block, ignore it...

   // Also, do not bother with blocks with only a single predecessor: simplify
   // CFG will fold these two blocks together!
   ++PI;
   if (PI == PE) return false;  // Exactly one predecessor!

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

   for (unsigned Size = 0; I != Dest->end(); ++Size, ++I)
     if (Size == 6) return false;  // The block is too large...
   return true;
 }


 /// eliminateUnconditionalBranch - Clone the instructions from the destination
 /// block into the source block, eliminating the specified unconditional branch.
 /// If the destination block defines values used by successors of the dest
 /// block, we may need to insert PHI nodes.
 ///
 void TailDup::eliminateUnconditionalBranch(BranchInst *Branch) {
   BasicBlock *SourceBlock = Branch->getParent();
   BasicBlock *DestBlock = Branch->getSuccessor(0);
   assert(SourceBlock != DestBlock && "Our predicate is broken!");

   DEBUG(std::cerr << "TailDuplication[" << SourceBlock->getParent()->getName()
                   << "]: Eliminating branch: " << *Branch);

   // We are going to have to map operands from the original block B to the new
   // copy of the block B'.  If there are PHI nodes in the DestBlock, these PHI
   // nodes also define part of this mapping.  Loop over these PHI nodes, adding
   // them to our mapping.
   //
   std::map<Value*, Value*> ValueMapping;

   BasicBlock::iterator BI = DestBlock->begin();
   bool HadPHINodes = isa<PHINode>(BI);
   for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
     ValueMapping[PN] = PN->getIncomingValueForBlock(SourceBlock);

   // Clone the non-phi instructions of the dest block into the source block,
   // keeping track of the mapping...
   //
   for (; BI != DestBlock->end(); ++BI) {
     Instruction *New = BI->clone();
     New->setName(BI->getName());
     SourceBlock->getInstList().push_back(New);
     ValueMapping[BI] = New;
   }

   // Now that we have built the mapping information and cloned all of the
   // instructions (giving us a new terminator, among other things), walk the new
   // instructions, rewriting references of old instructions to use new
   // instructions.
   //
   BI = Branch; ++BI;  // Get an iterator to the first new instruction
   for (; BI != SourceBlock->end(); ++BI)
     for (unsigned i = 0, e = BI->getNumOperands(); i != e; ++i)
       if (Value *Remapped = ValueMapping[BI->getOperand(i)])
         BI->setOperand(i, Remapped);

   // Next we check to see if any of the successors of DestBlock had PHI nodes.
   // If so, we need to add entries to the PHI nodes for SourceBlock now.
   for (succ_iterator SI = succ_begin(DestBlock), SE = succ_end(DestBlock);
        SI != SE; ++SI) {
     BasicBlock *Succ = *SI;
     for (BasicBlock::iterator PNI = Succ->begin();
          PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
       // Ok, we have a PHI node.  Figure out what the incoming value was for the
       // DestBlock.
       Value *IV = PN->getIncomingValueForBlock(DestBlock);

       // Remap the value if necessary...
       if (Value *MappedIV = ValueMapping[IV])
         IV = MappedIV;
       PN->addIncoming(IV, SourceBlock);
     }
   }

   // Now that all of the instructions are correctly copied into the SourceBlock,
   // we have one more minor problem: the successors of the original DestBB may
   // use the values computed in DestBB either directly (if DestBB dominated the
   // block), or through a PHI node.  In either case, we need to insert PHI nodes
   // into any successors of DestBB (which are now our successors) for each value
   // that is computed in DestBB, but is used outside of it.  All of these uses
   // we have to rewrite with the new PHI node.
   //
   if (succ_begin(SourceBlock) != succ_end(SourceBlock)) // Avoid wasting time...
     for (BI = DestBlock->begin(); BI != DestBlock->end(); ++BI)
       if (BI->getType() != Type::VoidTy)
         InsertPHINodesIfNecessary(BI, ValueMapping[BI], SourceBlock);

   // Final step: now that we have finished everything up, walk the cloned
   // instructions one last time, constant propagating and DCE'ing them, because
   // they may not be needed anymore.
   //
   BI = Branch; ++BI;  // Get an iterator to the first new instruction
   if (HadPHINodes)
     while (BI != SourceBlock->end())
       if (!dceInstruction(BI) && !doConstantPropagation(BI))
         ++BI;

   DestBlock->removePredecessor(SourceBlock); // Remove entries in PHI nodes...
   SourceBlock->getInstList().erase(Branch);  // Destroy the uncond branch...

   ++NumEliminated;  // We just killed a branch!
 }

 /// InsertPHINodesIfNecessary - So at this point, we cloned the OrigInst
 /// instruction into the NewBlock with the value of NewInst.  If OrigInst was
 /// used outside of its defining basic block, we need to insert a PHI nodes into
 /// the successors.
 ///
 void TailDup::InsertPHINodesIfNecessary(Instruction *OrigInst, Value *NewInst,
                                         BasicBlock *NewBlock) {
   // Loop over all of the uses of OrigInst, rewriting them to be newly inserted
   // PHI nodes, unless they are in the same basic block as OrigInst.
   BasicBlock *OrigBlock = OrigInst->getParent();
   std::vector<Instruction*> Users;
   Users.reserve(OrigInst->use_size());
   for (Value::use_iterator I = OrigInst->use_begin(), E = OrigInst->use_end();
        I != E; ++I) {
     Instruction *In = cast<Instruction>(*I);
     if (In->getParent() != OrigBlock ||  // Don't modify uses in the orig block!
         isa<PHINode>(In))
       Users.push_back(In);
   }

   // The common case is that the instruction is only used within the block that
   // defines it.  If we have this case, quick exit.
   //
   if (Users.empty()) return;

   // Otherwise, we have a more complex case, handle it now.  This requires the
   // construction of a mapping between a basic block and the value to use when
   // in the scope of that basic block.  This map will map to the original and
   // new values when in the original or new block, but will map to inserted PHI
   // nodes when in other blocks.
   //
   std::map<BasicBlock*, ValueHolder> ValueMap;
   std::map<BasicBlock*, ValueHolder> OutValueMap;   // The outgoing value map
   OutValueMap[OrigBlock] = OrigInst;
   OutValueMap[NewBlock ] = NewInst;    // Seed the initial values...

   DEBUG(std::cerr << "  ** Inserting PHI nodes for " << OrigInst);
   while (!Users.empty()) {
     Instruction *User = Users.back(); Users.pop_back();

     if (PHINode *PN = dyn_cast<PHINode>(User)) {
       // PHI nodes must be handled specially here, because their operands are
       // actually defined in predecessor basic blocks, NOT in the block that the
       // PHI node lives in.  Note that we have already added entries to PHI nods
       // which are in blocks that are immediate successors of OrigBlock, so
       // don't modify them again.
       for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
         if (PN->getIncomingValue(i) == OrigInst &&
             PN->getIncomingBlock(i) != OrigBlock) {
           Value *V = GetValueOutBlock(PN->getIncomingBlock(i), OrigInst,
                                       ValueMap, OutValueMap);
           PN->setIncomingValue(i, V);
         }

     } else {
       // Any other user of the instruction can just replace any uses with the
       // new value defined in the block it resides in.
       Value *V = GetValueInBlock(User->getParent(), OrigInst, ValueMap,
                                  OutValueMap);
       User->replaceUsesOfWith(OrigInst, V);
     }
   }
 }

 /// GetValueInBlock - This is a recursive method which inserts PHI nodes into
 /// the function until there is a value available in basic block BB.
 ///
 Value *TailDup::GetValueInBlock(BasicBlock *BB, Value *OrigVal,
                                 std::map<BasicBlock*, ValueHolder> &ValueMap,
                                 std::map<BasicBlock*,ValueHolder> &OutValueMap){
   ValueHolder &BBVal = ValueMap[BB];
   if (BBVal) return BBVal;       // Value already computed for this block?

   // If this block has no predecessors, then it must be unreachable, thus, it
   // doesn't matter which value we use.
   if (pred_begin(BB) == pred_end(BB))
     return BBVal = Constant::getNullValue(OrigVal->getType());

   // If there is no value already available in this basic block, we need to
   // either reuse a value from an incoming, dominating, basic block, or we need
   // to create a new PHI node to merge in different incoming values.  Because we
   // don't know if we're part of a loop at this point or not, we create a PHI
   // node, even if we will ultimately eliminate it.
   PHINode *PN = new PHINode(OrigVal->getType(), OrigVal->getName()+".pn",
                             BB->begin());
   BBVal = PN;   // Insert this into the BBVal slot in case of cycles...

   ValueHolder &BBOutVal = OutValueMap[BB];
   if (BBOutVal == 0) BBOutVal = PN;

   // Now that we have created the PHI node, loop over all of the predecessors of
   // this block, computing an incoming value for the predecessor.
   std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
   for (unsigned i = 0, e = Preds.size(); i != e; ++i)
     PN->addIncoming(GetValueOutBlock(Preds[i], OrigVal, ValueMap, OutValueMap),
                     Preds[i]);

   // The PHI node is complete.  In many cases, however the PHI node was
   // ultimately unnecessary: we could have just reused a dominating incoming
   // value.  If this is the case, nuke the PHI node and replace the map entry
   // with the dominating value.
   //
   assert(PN->getNumIncomingValues() > 0 && "No predecessors?");

   // Check to see if all of the elements in the PHI node are either the PHI node
   // itself or ONE particular value.
   unsigned i = 0;
   Value *ReplVal = PN->getIncomingValue(i);
   for (; ReplVal == PN && i != PN->getNumIncomingValues(); ++i)
     ReplVal = PN->getIncomingValue(i);  // Skip values equal to the PN

   for (; i != PN->getNumIncomingValues(); ++i)
     if (PN->getIncomingValue(i) != PN && PN->getIncomingValue(i) != ReplVal) {
       ReplVal = 0;
       break;
     }

   // Found a value to replace the PHI node with?
   if (ReplVal && ReplVal != PN) {
     PN->replaceAllUsesWith(ReplVal);
     BB->getInstList().erase(PN);   // Erase the PHI node...
   } else {
     ++NumPHINodes;
   }

   return BBVal;
 }

 Value *TailDup::GetValueOutBlock(BasicBlock *BB, Value *OrigVal,
                                  std::map<BasicBlock*, ValueHolder> &ValueMap,
                               std::map<BasicBlock*, ValueHolder> &OutValueMap) {
   ValueHolder &BBVal = OutValueMap[BB];
   if (BBVal) return BBVal;       // Value already computed for this block?

   return GetValueInBlock(BB, OrigVal, ValueMap, OutValueMap);
 }
	//===- TailDuplication.cpp - Simplify CFG through tail duplication --------===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file was developed by the LLVM research group and is distributed under
	// the University of Illinois Open Source License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	//
	// This pass performs a limited form of tail duplication, intended to simplify
	// CFGs by removing some unconditional branches. This pass is necessary to
	// straighten out loops created by the C front-end, but also is capable of
	// making other code nicer. After this pass is run, the CFG simplify pass
	// should be run to clean up the mess.
	//
	// This pass could be enhanced in the future to use profile information to be
	// more aggressive.
	//
	//===----------------------------------------------------------------------===//

	#include "llvm/Transforms/Scalar.h"
	#include "llvm/Constant.h"
	#include "llvm/Function.h"
	#include "llvm/iPHINode.h"
	#include "llvm/iTerminators.h"
	#include "llvm/Pass.h"
	#include "llvm/Type.h"
	#include "llvm/Support/CFG.h"
	#include "llvm/Support/ValueHolder.h"
	#include "llvm/Transforms/Utils/Local.h"
	#include "Support/Debug.h"
	#include "Support/Statistic.h"

	namespace {
	Statistic<> NumEliminated("tailduplicate",
	"Number of unconditional branches eliminated");
	Statistic<> NumPHINodes("tailduplicate", "Number of phi nodes inserted");

	class TailDup : public FunctionPass {
	bool runOnFunction(Function &F);
	private:
	inline bool shouldEliminateUnconditionalBranch(TerminatorInst *TI);
	inline void eliminateUnconditionalBranch(BranchInst *BI);
	inline void InsertPHINodesIfNecessary(Instruction OrigInst, Value NewInst,
	BasicBlock *NewBlock);
	inline Value GetValueInBlock(BasicBlock BB, Value *OrigVal,
	std::map<BasicBlock*, ValueHolder> &ValueMap,
	std::map<BasicBlock*, ValueHolder> &OutValueMap);
	inline Value GetValueOutBlock(BasicBlock BB, Value *OrigVal,
	std::map<BasicBlock*, ValueHolder> &ValueMap,
	std::map<BasicBlock*, ValueHolder> &OutValueMap);
	};
	RegisterOpt<TailDup> X("tailduplicate", "Tail Duplication");
	}

	Pass *createTailDuplicationPass() { return new TailDup(); }

	/// runOnFunction - Top level algorithm - Loop over each unconditional branch in
	/// the function, eliminating it if it looks attractive enough.
	///
	bool TailDup::runOnFunction(Function &F) {
	bool Changed = false;
	for (Function::iterator I = F.begin(), E = F.end(); I != E; )
	if (shouldEliminateUnconditionalBranch(I->getTerminator())) {
	eliminateUnconditionalBranch(cast<BranchInst>(I->getTerminator()));
	Changed = true;
	} else {
	++I;
	}
	return Changed;
	}

	/// shouldEliminateUnconditionalBranch - Return true if this branch looks
	/// attractive to eliminate. We eliminate the branch if the destination basic
	/// block has <= 5 instructions in it, not counting PHI nodes. In practice,
	/// since one of these is a terminator instruction, this means that we will add
	/// up to 4 instructions to the new block.
	///
	/// We don't count PHI nodes in the count since they will be removed when the
	/// contents of the block are copied over.
	///
	bool TailDup::shouldEliminateUnconditionalBranch(TerminatorInst *TI) {
	BranchInst *BI = dyn_cast<BranchInst>(TI);
	if (!BI \|\| !BI->isUnconditional()) return false; // Not an uncond branch!

	BasicBlock *Dest = BI->getSuccessor(0);
	if (Dest == BI->getParent()) return false; // Do not loop infinitely!

	// Do not inline a block if we will just get another branch to the same block!
	if (BranchInst *DBI = dyn_cast<BranchInst>(Dest->getTerminator()))
	if (DBI->isUnconditional() && DBI->getSuccessor(0) == Dest)
	return false; // Do not loop infinitely!

	// Do not bother working on dead blocks...
	pred_iterator PI = pred_begin(Dest), PE = pred_end(Dest);
	if (PI == PE && Dest != Dest->getParent()->begin())
	return false; // It's just a dead block, ignore it...

	// Also, do not bother with blocks with only a single predecessor: simplify
	// CFG will fold these two blocks together!
	++PI;
	if (PI == PE) return false; // Exactly one predecessor!

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

	for (unsigned Size = 0; I != Dest->end(); ++Size, ++I)
	if (Size == 6) return false; // The block is too large...
	return true;
	}


	/// eliminateUnconditionalBranch - Clone the instructions from the destination
	/// block into the source block, eliminating the specified unconditional branch.
	/// If the destination block defines values used by successors of the dest
	/// block, we may need to insert PHI nodes.
	///
	void TailDup::eliminateUnconditionalBranch(BranchInst *Branch) {
	BasicBlock *SourceBlock = Branch->getParent();
	BasicBlock *DestBlock = Branch->getSuccessor(0);
	assert(SourceBlock != DestBlock && "Our predicate is broken!");

	DEBUG(std::cerr << "TailDuplication[" << SourceBlock->getParent()->getName()
	<< "]: Eliminating branch: " << *Branch);

	// We are going to have to map operands from the original block B to the new
	// copy of the block B'. If there are PHI nodes in the DestBlock, these PHI
	// nodes also define part of this mapping. Loop over these PHI nodes, adding
	// them to our mapping.
	//
	std::map<Value, Value> ValueMapping;

	BasicBlock::iterator BI = DestBlock->begin();
	bool HadPHINodes = isa<PHINode>(BI);
	for (; PHINode *PN = dyn_cast<PHINode>(BI); ++BI)
	ValueMapping[PN] = PN->getIncomingValueForBlock(SourceBlock);

	// Clone the non-phi instructions of the dest block into the source block,
	// keeping track of the mapping...
	//
	for (; BI != DestBlock->end(); ++BI) {
	Instruction *New = BI->clone();
	New->setName(BI->getName());
	SourceBlock->getInstList().push_back(New);
	ValueMapping[BI] = New;
	}

	// Now that we have built the mapping information and cloned all of the
	// instructions (giving us a new terminator, among other things), walk the new
	// instructions, rewriting references of old instructions to use new
	// instructions.
	//
	BI = Branch; ++BI; // Get an iterator to the first new instruction
	for (; BI != SourceBlock->end(); ++BI)
	for (unsigned i = 0, e = BI->getNumOperands(); i != e; ++i)
	if (Value *Remapped = ValueMapping[BI->getOperand(i)])
	BI->setOperand(i, Remapped);

	// Next we check to see if any of the successors of DestBlock had PHI nodes.
	// If so, we need to add entries to the PHI nodes for SourceBlock now.
	for (succ_iterator SI = succ_begin(DestBlock), SE = succ_end(DestBlock);
	SI != SE; ++SI) {
	BasicBlock Succ = SI;
	for (BasicBlock::iterator PNI = Succ->begin();
	PHINode *PN = dyn_cast<PHINode>(PNI); ++PNI) {
	// Ok, we have a PHI node. Figure out what the incoming value was for the
	// DestBlock.
	Value *IV = PN->getIncomingValueForBlock(DestBlock);

	// Remap the value if necessary...
	if (Value *MappedIV = ValueMapping[IV])
	IV = MappedIV;
	PN->addIncoming(IV, SourceBlock);
	}
	}

	// Now that all of the instructions are correctly copied into the SourceBlock,
	// we have one more minor problem: the successors of the original DestBB may
	// use the values computed in DestBB either directly (if DestBB dominated the
	// block), or through a PHI node. In either case, we need to insert PHI nodes
	// into any successors of DestBB (which are now our successors) for each value
	// that is computed in DestBB, but is used outside of it. All of these uses
	// we have to rewrite with the new PHI node.
	//
	if (succ_begin(SourceBlock) != succ_end(SourceBlock)) // Avoid wasting time...
	for (BI = DestBlock->begin(); BI != DestBlock->end(); ++BI)
	if (BI->getType() != Type::VoidTy)
	InsertPHINodesIfNecessary(BI, ValueMapping[BI], SourceBlock);

	// Final step: now that we have finished everything up, walk the cloned
	// instructions one last time, constant propagating and DCE'ing them, because
	// they may not be needed anymore.
	//
	BI = Branch; ++BI; // Get an iterator to the first new instruction
	if (HadPHINodes)
	while (BI != SourceBlock->end())
	if (!dceInstruction(BI) && !doConstantPropagation(BI))
	++BI;

	DestBlock->removePredecessor(SourceBlock); // Remove entries in PHI nodes...
	SourceBlock->getInstList().erase(Branch); // Destroy the uncond branch...

	++NumEliminated; // We just killed a branch!
	}

	/// InsertPHINodesIfNecessary - So at this point, we cloned the OrigInst
	/// instruction into the NewBlock with the value of NewInst. If OrigInst was
	/// used outside of its defining basic block, we need to insert a PHI nodes into
	/// the successors.
	///
	void TailDup::InsertPHINodesIfNecessary(Instruction OrigInst, Value NewInst,
	BasicBlock *NewBlock) {
	// Loop over all of the uses of OrigInst, rewriting them to be newly inserted
	// PHI nodes, unless they are in the same basic block as OrigInst.
	BasicBlock *OrigBlock = OrigInst->getParent();
	std::vector<Instruction*> Users;
	Users.reserve(OrigInst->use_size());
	for (Value::use_iterator I = OrigInst->use_begin(), E = OrigInst->use_end();
	I != E; ++I) {
	Instruction In = cast<Instruction>(I);
	if (In->getParent() != OrigBlock \|\| // Don't modify uses in the orig block!
	isa<PHINode>(In))
	Users.push_back(In);
	}

	// The common case is that the instruction is only used within the block that
	// defines it. If we have this case, quick exit.
	//
	if (Users.empty()) return;

	// Otherwise, we have a more complex case, handle it now. This requires the
	// construction of a mapping between a basic block and the value to use when
	// in the scope of that basic block. This map will map to the original and
	// new values when in the original or new block, but will map to inserted PHI
	// nodes when in other blocks.
	//
	std::map<BasicBlock*, ValueHolder> ValueMap;
	std::map<BasicBlock*, ValueHolder> OutValueMap; // The outgoing value map
	OutValueMap[OrigBlock] = OrigInst;
	OutValueMap[NewBlock ] = NewInst; // Seed the initial values...

	DEBUG(std::cerr << " ** Inserting PHI nodes for " << OrigInst);
	while (!Users.empty()) {
	Instruction *User = Users.back(); Users.pop_back();

	if (PHINode *PN = dyn_cast<PHINode>(User)) {
	// PHI nodes must be handled specially here, because their operands are
	// actually defined in predecessor basic blocks, NOT in the block that the
	// PHI node lives in. Note that we have already added entries to PHI nods
	// which are in blocks that are immediate successors of OrigBlock, so
	// don't modify them again.
	for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
	if (PN->getIncomingValue(i) == OrigInst &&
	PN->getIncomingBlock(i) != OrigBlock) {
	Value *V = GetValueOutBlock(PN->getIncomingBlock(i), OrigInst,
	ValueMap, OutValueMap);
	PN->setIncomingValue(i, V);
	}

	} else {
	// Any other user of the instruction can just replace any uses with the
	// new value defined in the block it resides in.
	Value *V = GetValueInBlock(User->getParent(), OrigInst, ValueMap,
	OutValueMap);
	User->replaceUsesOfWith(OrigInst, V);
	}
	}
	}

	/// GetValueInBlock - This is a recursive method which inserts PHI nodes into
	/// the function until there is a value available in basic block BB.
	///
	Value TailDup::GetValueInBlock(BasicBlock BB, Value *OrigVal,
	std::map<BasicBlock*, ValueHolder> &ValueMap,
	std::map<BasicBlock*,ValueHolder> &OutValueMap){
	ValueHolder &BBVal = ValueMap[BB];
	if (BBVal) return BBVal; // Value already computed for this block?

	// If this block has no predecessors, then it must be unreachable, thus, it
	// doesn't matter which value we use.
	if (pred_begin(BB) == pred_end(BB))
	return BBVal = Constant::getNullValue(OrigVal->getType());

	// If there is no value already available in this basic block, we need to
	// either reuse a value from an incoming, dominating, basic block, or we need
	// to create a new PHI node to merge in different incoming values. Because we
	// don't know if we're part of a loop at this point or not, we create a PHI
	// node, even if we will ultimately eliminate it.
	PHINode *PN = new PHINode(OrigVal->getType(), OrigVal->getName()+".pn",
	BB->begin());
	BBVal = PN; // Insert this into the BBVal slot in case of cycles...

	ValueHolder &BBOutVal = OutValueMap[BB];
	if (BBOutVal == 0) BBOutVal = PN;

	// Now that we have created the PHI node, loop over all of the predecessors of
	// this block, computing an incoming value for the predecessor.
	std::vector<BasicBlock*> Preds(pred_begin(BB), pred_end(BB));
	for (unsigned i = 0, e = Preds.size(); i != e; ++i)
	PN->addIncoming(GetValueOutBlock(Preds[i], OrigVal, ValueMap, OutValueMap),
	Preds[i]);

	// The PHI node is complete. In many cases, however the PHI node was
	// ultimately unnecessary: we could have just reused a dominating incoming
	// value. If this is the case, nuke the PHI node and replace the map entry
	// with the dominating value.
	//
	assert(PN->getNumIncomingValues() > 0 && "No predecessors?");

	// Check to see if all of the elements in the PHI node are either the PHI node
	// itself or ONE particular value.
	unsigned i = 0;
	Value *ReplVal = PN->getIncomingValue(i);
	for (; ReplVal == PN && i != PN->getNumIncomingValues(); ++i)
	ReplVal = PN->getIncomingValue(i); // Skip values equal to the PN

	for (; i != PN->getNumIncomingValues(); ++i)
	if (PN->getIncomingValue(i) != PN && PN->getIncomingValue(i) != ReplVal) {
	ReplVal = 0;
	break;
	}

	// Found a value to replace the PHI node with?
	if (ReplVal && ReplVal != PN) {
	PN->replaceAllUsesWith(ReplVal);
	BB->getInstList().erase(PN); // Erase the PHI node...
	} else {
	++NumPHINodes;
	}

	return BBVal;
	}

	Value TailDup::GetValueOutBlock(BasicBlock BB, Value *OrigVal,
	std::map<BasicBlock*, ValueHolder> &ValueMap,
	std::map<BasicBlock*, ValueHolder> &OutValueMap) {
	ValueHolder &BBVal = OutValueMap[BB];
	if (BBVal) return BBVal; // Value already computed for this block?

	return GetValueInBlock(BB, OrigVal, ValueMap, OutValueMap);
	}