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

 //===- LoopPreheaders.cpp - Loop Preheader Insertion Pass -----------------===//
 //
 // Insert Loop pre-headers and exit blocks into the CFG for each function in the
 // module.  This pass updates loop information and dominator information.
 //
 // Loop pre-header insertion guarantees that there is a single, non-critical
 // entry edge from outside of the loop to the loop header.  This simplifies a
 // number of analyses and transformations, such as LICM.
 //
 // Loop exit-block insertion guarantees that all exit blocks from the loop
 // (blocks which are outside of the loop that have predecessors inside of the
 // loop) are dominated by the loop header.  This simplifies transformations such
 // as store-sinking that are built into LICM.
 //
 // Note that the simplifycfg pass will clean up blocks which are split out but
 // end up being unnecessary, so usage of this pass does not neccesarily
 // pessimize generated code.
 //
 //===----------------------------------------------------------------------===//

 #include "llvm/Transforms/Scalar.h"
 #include "llvm/Analysis/Dominators.h"
 #include "llvm/Analysis/LoopInfo.h"
 #include "llvm/Function.h"
 #include "llvm/iTerminators.h"
 #include "llvm/iPHINode.h"
 #include "llvm/Constant.h"
 #include "llvm/Support/CFG.h"
 #include "Support/SetOperations.h"
 #include "Support/Statistic.h"
 #include "Support/DepthFirstIterator.h"

 namespace {
   Statistic<> NumInserted("preheaders", "Number of pre-header nodes inserted");

   struct Preheaders : public FunctionPass {
     virtual bool runOnFunction(Function &F);

     virtual void getAnalysisUsage(AnalysisUsage &AU) const {
       // We need loop information to identify the loops...
       AU.addRequired<LoopInfo>();
       AU.addRequired<DominatorSet>();

       AU.addPreserved<LoopInfo>();
       AU.addPreserved<DominatorSet>();
       AU.addPreserved<ImmediateDominators>();
       AU.addPreserved<DominatorTree>();
       AU.addPreserved<DominanceFrontier>();
       AU.addPreservedID(BreakCriticalEdgesID);  // No crit edges added....
     }
   private:
     bool ProcessLoop(Loop *L);
     BasicBlock *SplitBlockPredecessors(BasicBlock *BB, const char *Suffix,
                                        const std::vector<BasicBlock*> &Preds);
     void RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
     void InsertPreheaderForLoop(Loop *L);
   };

   RegisterOpt<Preheaders> X("preheaders", "Natural loop pre-header insertion");
 }

 // Publically exposed interface to pass...
 const PassInfo *LoopPreheadersID = X.getPassInfo();
 Pass *createLoopPreheaderInsertionPass() { return new Preheaders(); }


 /// runOnFunction - Run down all loops in the CFG (recursively, but we could do
 /// it in any convenient order) inserting preheaders...
 ///
 bool Preheaders::runOnFunction(Function &F) {
   bool Changed = false;
   LoopInfo &LI = getAnalysis<LoopInfo>();

   for (unsigned i = 0, e = LI.getTopLevelLoops().size(); i != e; ++i)
     Changed |= ProcessLoop(LI.getTopLevelLoops()[i]);

   return Changed;
 }


 /// ProcessLoop - Walk the loop structure in depth first order, ensuring that
 /// all loops have preheaders.
 ///
 bool Preheaders::ProcessLoop(Loop *L) {
   bool Changed = false;

   // Does the loop already have a preheader?  If so, don't modify the loop...
   if (L->getLoopPreheader() == 0) {
     InsertPreheaderForLoop(L);
     NumInserted++;
     Changed = true;
   }

   // Regardless of whether or not we added a preheader to the loop we must
   // guarantee that the preheader dominates all exit nodes.  If there are any
   // exit nodes not dominated, split them now.
   DominatorSet &DS = getAnalysis<DominatorSet>();
   BasicBlock *Header = L->getHeader();
   for (unsigned i = 0, e = L->getExitBlocks().size(); i != e; ++i)
     if (!DS.dominates(Header, L->getExitBlocks()[i])) {
       RewriteLoopExitBlock(L, L->getExitBlocks()[i]);
       assert(DS.dominates(Header, L->getExitBlocks()[i]) &&
              "RewriteLoopExitBlock failed?");
       NumInserted++;
       Changed = true;
     }

   const std::vector<Loop*> &SubLoops = L->getSubLoops();
   for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
     Changed |= ProcessLoop(SubLoops[i]);
   return Changed;
 }

 /// SplitBlockPredecessors - Split the specified block into two blocks.  We want
 /// to move the predecessors specified in the Preds list to point to the new
 /// block, leaving the remaining predecessors pointing to BB.  This method
 /// updates the SSA PHINode's, but no other analyses.
 ///
 BasicBlock *Preheaders::SplitBlockPredecessors(BasicBlock *BB,
                                                const char *Suffix,
                                        const std::vector<BasicBlock*> &Preds) {

   // Create new basic block, insert right before the original block...
   BasicBlock *NewBB = new BasicBlock(BB->getName()+Suffix, BB);

   // The preheader first gets an unconditional branch to the loop header...
   BranchInst *BI = new BranchInst(BB);
   NewBB->getInstList().push_back(BI);

   // For every PHI node in the block, insert a PHI node into NewBB where the
   // incoming values from the out of loop edges are moved to NewBB.  We have two
   // possible cases here.  If the loop is dead, we just insert dummy entries
   // into the PHI nodes for the new edge.  If the loop is not dead, we move the
   // incoming edges in BB into new PHI nodes in NewBB.
   //
   if (!Preds.empty()) {  // Is the loop not obviously dead?
     for (BasicBlock::iterator I = BB->begin();
          PHINode *PN = dyn_cast<PHINode>(I); ++I) {

       // Create the new PHI node, insert it into NewBB at the end of the block
       PHINode *NewPHI = new PHINode(PN->getType(), PN->getName()+".ph", BI);

       // Move all of the edges from blocks outside the loop to the new PHI
       for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
         Value *V = PN->removeIncomingValue(Preds[i]);
         NewPHI->addIncoming(V, Preds[i]);
       }

       // Add an incoming value to the PHI node in the loop for the preheader
       // edge
       PN->addIncoming(NewPHI, NewBB);
     }

     // Now that the PHI nodes are updated, actually move the edges from
     // Preds to point to NewBB instead of BB.
     //
     for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
       TerminatorInst *TI = Preds[i]->getTerminator();
       for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s)
         if (TI->getSuccessor(s) == BB)
           TI->setSuccessor(s, NewBB);
     }

   } else {                       // Otherwise the loop is dead...
     for (BasicBlock::iterator I = BB->begin();
          PHINode *PN = dyn_cast<PHINode>(I); ++I)
       // Insert dummy values as the incoming value...
       PN->addIncoming(Constant::getNullValue(PN->getType()), NewBB);
   }
   return NewBB;
 }

 // ChangeExitBlock - This recursive function is used to change any exit blocks
 // that use OldExit to use NewExit instead.  This is recursive because children
 // may need to be processed as well.
 //
 static void ChangeExitBlock(Loop *L, BasicBlock *OldExit, BasicBlock *NewExit) {
   if (L->hasExitBlock(OldExit)) {
     L->changeExitBlock(OldExit, NewExit);
     const std::vector<Loop*> &SubLoops = L->getSubLoops();
     for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
       ChangeExitBlock(SubLoops[i], OldExit, NewExit);
   }
 }


 /// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
 /// preheader, this method is called to insert one.  This method has two phases:
 /// preheader insertion and analysis updating.
 ///
 void Preheaders::InsertPreheaderForLoop(Loop *L) {
   BasicBlock *Header = L->getHeader();

   // Compute the set of predecessors of the loop that are not in the loop.
   std::vector<BasicBlock*> OutsideBlocks;
   for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
        PI != PE; ++PI)
       if (!L->contains(*PI))           // Coming in from outside the loop?
         OutsideBlocks.push_back(*PI);  // Keep track of it...

   // Split out the loop pre-header
   BasicBlock *NewBB =
     SplitBlockPredecessors(Header, ".preheader", OutsideBlocks);

   //===--------------------------------------------------------------------===//
   //  Update analysis results now that we have preformed the transformation
   //

   // We know that we have loop information to update... update it now.
   if (Loop *Parent = L->getParentLoop())
     Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());

   // If the header for the loop used to be an exit node for another loop, then
   // we need to update this to know that the loop-preheader is now the exit
   // node.  Note that the only loop that could have our header as an exit node
   // is a sibling loop, ie, one with the same parent loop, or one if it's
   // children.
   //
   const std::vector<Loop*> *ParentSubLoops;
   if (Loop *Parent = L->getParentLoop())
     ParentSubLoops = &Parent->getSubLoops();
   else       // Must check top-level loops...
     ParentSubLoops = &getAnalysis<LoopInfo>().getTopLevelLoops();

   // Loop over all sibling loops, performing the substitution (recursively to
   // include child loops)...
   for (unsigned i = 0, e = ParentSubLoops->size(); i != e; ++i)
     ChangeExitBlock((*ParentSubLoops)[i], Header, NewBB);

   DominatorSet &DS = getAnalysis<DominatorSet>();  // Update dominator info
   {
     // The blocks that dominate NewBB are the blocks that dominate Header,
     // minus Header, plus NewBB.
     DominatorSet::DomSetType DomSet = DS.getDominators(Header);
     DomSet.insert(NewBB);  // We dominate ourself
     DomSet.erase(Header);  // Header does not dominate us...
     DS.addBasicBlock(NewBB, DomSet);

     // The newly created basic block dominates all nodes dominated by Header.
     for (Function::iterator I = Header->getParent()->begin(),
            E = Header->getParent()->end(); I != E; ++I)
       if (DS.dominates(Header, I))
         DS.addDominator(I, NewBB);
   }

   // Update immediate dominator information if we have it...
   if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
     // Whatever i-dominated the header node now immediately dominates NewBB
     ID->addNewBlock(NewBB, ID->get(Header));

     // The preheader now is the immediate dominator for the header node...
     ID->setImmediateDominator(Header, NewBB);
   }

   // Update DominatorTree information if it is active.
   if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
     // The immediate dominator of the preheader is the immediate dominator of
     // the old header.
     //
     DominatorTree::Node *HeaderNode = DT->getNode(Header);
     DominatorTree::Node *PHNode = DT->createNewNode(NewBB,
                                                     HeaderNode->getIDom());

     // Change the header node so that PNHode is the new immediate dominator
     DT->changeImmediateDominator(HeaderNode, PHNode);
   }

   // Update dominance frontier information...
   if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
     // The DF(NewBB) is just (DF(Header)-Header), because NewBB dominates
     // everything that Header does, and it strictly dominates Header in
     // addition.
     assert(DF->find(Header) != DF->end() && "Header node doesn't have DF set?");
     DominanceFrontier::DomSetType NewDFSet = DF->find(Header)->second;
     NewDFSet.erase(Header);
     DF->addBasicBlock(NewBB, NewDFSet);

     // Now we must loop over all of the dominance frontiers in the function,
     // replacing occurrences of Header with NewBB in some cases.  If a block
     // dominates a (now) predecessor of NewBB, but did not strictly dominate
     // Header, it will have Header in it's DF set, but should now have NewBB in
     // its set.
     for (unsigned i = 0, e = OutsideBlocks.size(); i != e; ++i) {
       // Get all of the dominators of the predecessor...
       const DominatorSet::DomSetType &PredDoms =
         DS.getDominators(OutsideBlocks[i]);
       for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
              PDE = PredDoms.end(); PDI != PDE; ++PDI) {
         BasicBlock *PredDom = *PDI;
         // If the loop header is in DF(PredDom), then PredDom didn't dominate
         // the header but did dominate a predecessor outside of the loop.  Now
         // we change this entry to include the preheader in the DF instead of
         // the header.
         DominanceFrontier::iterator DFI = DF->find(PredDom);
         assert(DFI != DF->end() && "No dominance frontier for node?");
         if (DFI->second.count(Header)) {
           DF->removeFromFrontier(DFI, Header);
           DF->addToFrontier(DFI, NewBB);
         }
       }
     }
   }
 }

 void Preheaders::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
   DominatorSet &DS = getAnalysis<DominatorSet>();
   assert(!DS.dominates(L->getHeader(), Exit) &&
          "Loop already dominates exit block??");
   assert(std::find(L->getExitBlocks().begin(), L->getExitBlocks().end(), Exit)
          != L->getExitBlocks().end() && "Not a current exit block!");

   std::vector<BasicBlock*> LoopBlocks;
   for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I)
     if (L->contains(*I))
       LoopBlocks.push_back(*I);

   assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
   BasicBlock *NewBB = SplitBlockPredecessors(Exit, ".loopexit", LoopBlocks);

   // Update Loop Information - we know that the new block will be in the parent
   // loop of L.
   if (Loop *Parent = L->getParentLoop())
     Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());

   // Replace any instances of Exit with NewBB in this and any nested loops...
   for (df_iterator<Loop*> I = df_begin(L), E = df_end(L); I != E; ++I)
     if (I->hasExitBlock(Exit))
       I->changeExitBlock(Exit, NewBB);   // Update exit block information

   // Update dominator information...  The blocks that dominate NewBB are the
   // intersection of the dominators of predecessors, plus the block itself.
   // The newly created basic block does not dominate anything except itself.
   //
   DominatorSet::DomSetType NewBBDomSet = DS.getDominators(LoopBlocks[0]);
   for (unsigned i = 1, e = LoopBlocks.size(); i != e; ++i)
     set_intersect(NewBBDomSet, DS.getDominators(LoopBlocks[i]));
   NewBBDomSet.insert(NewBB);  // All blocks dominate themselves...
   DS.addBasicBlock(NewBB, NewBBDomSet);

   // Update immediate dominator information if we have it...
   BasicBlock *NewBBIDom = 0;
   if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
     // This block does not strictly dominate anything, so it is not an immediate
     // dominator.  To find the immediate dominator of the new exit node, we
     // trace up the immediate dominators of a predecessor until we find a basic
     // block that dominates the exit block.
     //
     BasicBlock *Dom = LoopBlocks[0];  // Some random predecessor...
     while (!NewBBDomSet.count(Dom)) {  // Loop until we find a dominator...
       assert(Dom != 0 && "No shared dominator found???");
       Dom = ID->get(Dom);
     }

     // Set the immediate dominator now...
     ID->addNewBlock(NewBB, Dom);
     NewBBIDom = Dom;   // Reuse this if calculating DominatorTree info...
   }

   // Update DominatorTree information if it is active.
   if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
     // NewBB doesn't dominate anything, so just create a node and link it into
     // its immediate dominator.  If we don't have ImmediateDominator info
     // around, calculate the idom as above.
     DominatorTree::Node *NewBBIDomNode;
     if (NewBBIDom) {
       NewBBIDomNode = DT->getNode(NewBBIDom);
     } else {
       NewBBIDomNode = DT->getNode(LoopBlocks[0]); // Random pred
       while (!NewBBDomSet.count(NewBBIDomNode->getBlock())) {
         NewBBIDomNode = NewBBIDomNode->getIDom();
         assert(NewBBIDomNode && "No shared dominator found??");
       }
     }

     // Create the new dominator tree node...
     DT->createNewNode(NewBB, NewBBIDomNode);
   }

   // Update dominance frontier information...
   if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
     // DF(NewBB) is {Exit} because NewBB does not strictly dominate Exit, but it
     // does dominate itself (and there is an edge (NewBB -> Exit)).
     DominanceFrontier::DomSetType NewDFSet;
     NewDFSet.insert(Exit);
     DF->addBasicBlock(NewBB, NewDFSet);

     // Now we must loop over all of the dominance frontiers in the function,
     // replacing occurrences of Exit with NewBB in some cases.  If a block
     // dominates a (now) predecessor of NewBB, but did not strictly dominate
     // Exit, it will have Exit in it's DF set, but should now have NewBB in its
     // set.
     for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
       // Get all of the dominators of the predecessor...
       const DominatorSet::DomSetType &PredDoms =DS.getDominators(LoopBlocks[i]);
       for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
              PDE = PredDoms.end(); PDI != PDE; ++PDI) {
         BasicBlock *PredDom = *PDI;
         // Make sure to only rewrite blocks that are part of the loop...
         if (L->contains(PredDom)) {
           // If the exit node is in DF(PredDom), then PredDom didn't dominate
           // Exit but did dominate a predecessor inside of the loop.  Now we
           // change this entry to include NewBB in the DF instead of Exit.
           DominanceFrontier::iterator DFI = DF->find(PredDom);
           assert(DFI != DF->end() && "No dominance frontier for node?");
           if (DFI->second.count(Exit)) {
             DF->removeFromFrontier(DFI, Exit);
             DF->addToFrontier(DFI, NewBB);
           }
         }
       }
     }
   }
 }
	//===- LoopPreheaders.cpp - Loop Preheader Insertion Pass -----------------===//
	//
	// Insert Loop pre-headers and exit blocks into the CFG for each function in the
	// module. This pass updates loop information and dominator information.
	//
	// Loop pre-header insertion guarantees that there is a single, non-critical
	// entry edge from outside of the loop to the loop header. This simplifies a
	// number of analyses and transformations, such as LICM.
	//
	// Loop exit-block insertion guarantees that all exit blocks from the loop
	// (blocks which are outside of the loop that have predecessors inside of the
	// loop) are dominated by the loop header. This simplifies transformations such
	// as store-sinking that are built into LICM.
	//
	// Note that the simplifycfg pass will clean up blocks which are split out but
	// end up being unnecessary, so usage of this pass does not neccesarily
	// pessimize generated code.
	//
	//===----------------------------------------------------------------------===//

	#include "llvm/Transforms/Scalar.h"
	#include "llvm/Analysis/Dominators.h"
	#include "llvm/Analysis/LoopInfo.h"
	#include "llvm/Function.h"
	#include "llvm/iTerminators.h"
	#include "llvm/iPHINode.h"
	#include "llvm/Constant.h"
	#include "llvm/Support/CFG.h"
	#include "Support/SetOperations.h"
	#include "Support/Statistic.h"
	#include "Support/DepthFirstIterator.h"

	namespace {
	Statistic<> NumInserted("preheaders", "Number of pre-header nodes inserted");

	struct Preheaders : public FunctionPass {
	virtual bool runOnFunction(Function &F);

	virtual void getAnalysisUsage(AnalysisUsage &AU) const {
	// We need loop information to identify the loops...
	AU.addRequired<LoopInfo>();
	AU.addRequired<DominatorSet>();

	AU.addPreserved<LoopInfo>();
	AU.addPreserved<DominatorSet>();
	AU.addPreserved<ImmediateDominators>();
	AU.addPreserved<DominatorTree>();
	AU.addPreserved<DominanceFrontier>();
	AU.addPreservedID(BreakCriticalEdgesID); // No crit edges added....
	}
	private:
	bool ProcessLoop(Loop *L);
	BasicBlock SplitBlockPredecessors(BasicBlock BB, const char *Suffix,
	const std::vector<BasicBlock*> &Preds);
	void RewriteLoopExitBlock(Loop L, BasicBlock Exit);
	void InsertPreheaderForLoop(Loop *L);
	};

	RegisterOpt<Preheaders> X("preheaders", "Natural loop pre-header insertion");
	}

	// Publically exposed interface to pass...
	const PassInfo *LoopPreheadersID = X.getPassInfo();
	Pass *createLoopPreheaderInsertionPass() { return new Preheaders(); }


	/// runOnFunction - Run down all loops in the CFG (recursively, but we could do
	/// it in any convenient order) inserting preheaders...
	///
	bool Preheaders::runOnFunction(Function &F) {
	bool Changed = false;
	LoopInfo &LI = getAnalysis<LoopInfo>();

	for (unsigned i = 0, e = LI.getTopLevelLoops().size(); i != e; ++i)
	Changed \|= ProcessLoop(LI.getTopLevelLoops()[i]);

	return Changed;
	}


	/// ProcessLoop - Walk the loop structure in depth first order, ensuring that
	/// all loops have preheaders.
	///
	bool Preheaders::ProcessLoop(Loop *L) {
	bool Changed = false;

	// Does the loop already have a preheader? If so, don't modify the loop...
	if (L->getLoopPreheader() == 0) {
	InsertPreheaderForLoop(L);
	NumInserted++;
	Changed = true;
	}

	// Regardless of whether or not we added a preheader to the loop we must
	// guarantee that the preheader dominates all exit nodes. If there are any
	// exit nodes not dominated, split them now.
	DominatorSet &DS = getAnalysis<DominatorSet>();
	BasicBlock *Header = L->getHeader();
	for (unsigned i = 0, e = L->getExitBlocks().size(); i != e; ++i)
	if (!DS.dominates(Header, L->getExitBlocks()[i])) {
	RewriteLoopExitBlock(L, L->getExitBlocks()[i]);
	assert(DS.dominates(Header, L->getExitBlocks()[i]) &&
	"RewriteLoopExitBlock failed?");
	NumInserted++;
	Changed = true;
	}

	const std::vector<Loop*> &SubLoops = L->getSubLoops();
	for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
	Changed \|= ProcessLoop(SubLoops[i]);
	return Changed;
	}

	/// SplitBlockPredecessors - Split the specified block into two blocks. We want
	/// to move the predecessors specified in the Preds list to point to the new
	/// block, leaving the remaining predecessors pointing to BB. This method
	/// updates the SSA PHINode's, but no other analyses.
	///
	BasicBlock Preheaders::SplitBlockPredecessors(BasicBlock BB,
	const char *Suffix,
	const std::vector<BasicBlock*> &Preds) {

	// Create new basic block, insert right before the original block...
	BasicBlock *NewBB = new BasicBlock(BB->getName()+Suffix, BB);

	// The preheader first gets an unconditional branch to the loop header...
	BranchInst *BI = new BranchInst(BB);
	NewBB->getInstList().push_back(BI);

	// For every PHI node in the block, insert a PHI node into NewBB where the
	// incoming values from the out of loop edges are moved to NewBB. We have two
	// possible cases here. If the loop is dead, we just insert dummy entries
	// into the PHI nodes for the new edge. If the loop is not dead, we move the
	// incoming edges in BB into new PHI nodes in NewBB.
	//
	if (!Preds.empty()) { // Is the loop not obviously dead?
	for (BasicBlock::iterator I = BB->begin();
	PHINode *PN = dyn_cast<PHINode>(I); ++I) {

	// Create the new PHI node, insert it into NewBB at the end of the block
	PHINode *NewPHI = new PHINode(PN->getType(), PN->getName()+".ph", BI);

	// Move all of the edges from blocks outside the loop to the new PHI
	for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
	Value *V = PN->removeIncomingValue(Preds[i]);
	NewPHI->addIncoming(V, Preds[i]);
	}

	// Add an incoming value to the PHI node in the loop for the preheader
	// edge
	PN->addIncoming(NewPHI, NewBB);
	}

	// Now that the PHI nodes are updated, actually move the edges from
	// Preds to point to NewBB instead of BB.
	//
	for (unsigned i = 0, e = Preds.size(); i != e; ++i) {
	TerminatorInst *TI = Preds[i]->getTerminator();
	for (unsigned s = 0, e = TI->getNumSuccessors(); s != e; ++s)
	if (TI->getSuccessor(s) == BB)
	TI->setSuccessor(s, NewBB);
	}

	} else { // Otherwise the loop is dead...
	for (BasicBlock::iterator I = BB->begin();
	PHINode *PN = dyn_cast<PHINode>(I); ++I)
	// Insert dummy values as the incoming value...
	PN->addIncoming(Constant::getNullValue(PN->getType()), NewBB);
	}
	return NewBB;
	}

	// ChangeExitBlock - This recursive function is used to change any exit blocks
	// that use OldExit to use NewExit instead. This is recursive because children
	// may need to be processed as well.
	//
	static void ChangeExitBlock(Loop L, BasicBlock OldExit, BasicBlock *NewExit) {
	if (L->hasExitBlock(OldExit)) {
	L->changeExitBlock(OldExit, NewExit);
	const std::vector<Loop*> &SubLoops = L->getSubLoops();
	for (unsigned i = 0, e = SubLoops.size(); i != e; ++i)
	ChangeExitBlock(SubLoops[i], OldExit, NewExit);
	}
	}


	/// InsertPreheaderForLoop - Once we discover that a loop doesn't have a
	/// preheader, this method is called to insert one. This method has two phases:
	/// preheader insertion and analysis updating.
	///
	void Preheaders::InsertPreheaderForLoop(Loop *L) {
	BasicBlock *Header = L->getHeader();

	// Compute the set of predecessors of the loop that are not in the loop.
	std::vector<BasicBlock*> OutsideBlocks;
	for (pred_iterator PI = pred_begin(Header), PE = pred_end(Header);
	PI != PE; ++PI)
	if (!L->contains(*PI)) // Coming in from outside the loop?
	OutsideBlocks.push_back(*PI); // Keep track of it...

	// Split out the loop pre-header
	BasicBlock *NewBB =
	SplitBlockPredecessors(Header, ".preheader", OutsideBlocks);

	//===--------------------------------------------------------------------===//
	// Update analysis results now that we have preformed the transformation
	//

	// We know that we have loop information to update... update it now.
	if (Loop *Parent = L->getParentLoop())
	Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());

	// If the header for the loop used to be an exit node for another loop, then
	// we need to update this to know that the loop-preheader is now the exit
	// node. Note that the only loop that could have our header as an exit node
	// is a sibling loop, ie, one with the same parent loop, or one if it's
	// children.
	//
	const std::vector<Loop> ParentSubLoops;
	if (Loop *Parent = L->getParentLoop())
	ParentSubLoops = &Parent->getSubLoops();
	else // Must check top-level loops...
	ParentSubLoops = &getAnalysis<LoopInfo>().getTopLevelLoops();

	// Loop over all sibling loops, performing the substitution (recursively to
	// include child loops)...
	for (unsigned i = 0, e = ParentSubLoops->size(); i != e; ++i)
	ChangeExitBlock((*ParentSubLoops)[i], Header, NewBB);

	DominatorSet &DS = getAnalysis<DominatorSet>(); // Update dominator info
	{
	// The blocks that dominate NewBB are the blocks that dominate Header,
	// minus Header, plus NewBB.
	DominatorSet::DomSetType DomSet = DS.getDominators(Header);
	DomSet.insert(NewBB); // We dominate ourself
	DomSet.erase(Header); // Header does not dominate us...
	DS.addBasicBlock(NewBB, DomSet);

	// The newly created basic block dominates all nodes dominated by Header.
	for (Function::iterator I = Header->getParent()->begin(),
	E = Header->getParent()->end(); I != E; ++I)
	if (DS.dominates(Header, I))
	DS.addDominator(I, NewBB);
	}

	// Update immediate dominator information if we have it...
	if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
	// Whatever i-dominated the header node now immediately dominates NewBB
	ID->addNewBlock(NewBB, ID->get(Header));

	// The preheader now is the immediate dominator for the header node...
	ID->setImmediateDominator(Header, NewBB);
	}

	// Update DominatorTree information if it is active.
	if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
	// The immediate dominator of the preheader is the immediate dominator of
	// the old header.
	//
	DominatorTree::Node *HeaderNode = DT->getNode(Header);
	DominatorTree::Node *PHNode = DT->createNewNode(NewBB,
	HeaderNode->getIDom());

	// Change the header node so that PNHode is the new immediate dominator
	DT->changeImmediateDominator(HeaderNode, PHNode);
	}

	// Update dominance frontier information...
	if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
	// The DF(NewBB) is just (DF(Header)-Header), because NewBB dominates
	// everything that Header does, and it strictly dominates Header in
	// addition.
	assert(DF->find(Header) != DF->end() && "Header node doesn't have DF set?");
	DominanceFrontier::DomSetType NewDFSet = DF->find(Header)->second;
	NewDFSet.erase(Header);
	DF->addBasicBlock(NewBB, NewDFSet);

	// Now we must loop over all of the dominance frontiers in the function,
	// replacing occurrences of Header with NewBB in some cases. If a block
	// dominates a (now) predecessor of NewBB, but did not strictly dominate
	// Header, it will have Header in it's DF set, but should now have NewBB in
	// its set.
	for (unsigned i = 0, e = OutsideBlocks.size(); i != e; ++i) {
	// Get all of the dominators of the predecessor...
	const DominatorSet::DomSetType &PredDoms =
	DS.getDominators(OutsideBlocks[i]);
	for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
	PDE = PredDoms.end(); PDI != PDE; ++PDI) {
	BasicBlock PredDom = PDI;
	// If the loop header is in DF(PredDom), then PredDom didn't dominate
	// the header but did dominate a predecessor outside of the loop. Now
	// we change this entry to include the preheader in the DF instead of
	// the header.
	DominanceFrontier::iterator DFI = DF->find(PredDom);
	assert(DFI != DF->end() && "No dominance frontier for node?");
	if (DFI->second.count(Header)) {
	DF->removeFromFrontier(DFI, Header);
	DF->addToFrontier(DFI, NewBB);
	}
	}
	}
	}
	}

	void Preheaders::RewriteLoopExitBlock(Loop L, BasicBlock Exit) {
	DominatorSet &DS = getAnalysis<DominatorSet>();
	assert(!DS.dominates(L->getHeader(), Exit) &&
	"Loop already dominates exit block??");
	assert(std::find(L->getExitBlocks().begin(), L->getExitBlocks().end(), Exit)
	!= L->getExitBlocks().end() && "Not a current exit block!");

	std::vector<BasicBlock*> LoopBlocks;
	for (pred_iterator I = pred_begin(Exit), E = pred_end(Exit); I != E; ++I)
	if (L->contains(*I))
	LoopBlocks.push_back(*I);

	assert(!LoopBlocks.empty() && "No edges coming in from outside the loop?");
	BasicBlock *NewBB = SplitBlockPredecessors(Exit, ".loopexit", LoopBlocks);

	// Update Loop Information - we know that the new block will be in the parent
	// loop of L.
	if (Loop *Parent = L->getParentLoop())
	Parent->addBasicBlockToLoop(NewBB, getAnalysis<LoopInfo>());

	// Replace any instances of Exit with NewBB in this and any nested loops...
	for (df_iterator<Loop*> I = df_begin(L), E = df_end(L); I != E; ++I)
	if (I->hasExitBlock(Exit))
	I->changeExitBlock(Exit, NewBB); // Update exit block information

	// Update dominator information... The blocks that dominate NewBB are the
	// intersection of the dominators of predecessors, plus the block itself.
	// The newly created basic block does not dominate anything except itself.
	//
	DominatorSet::DomSetType NewBBDomSet = DS.getDominators(LoopBlocks[0]);
	for (unsigned i = 1, e = LoopBlocks.size(); i != e; ++i)
	set_intersect(NewBBDomSet, DS.getDominators(LoopBlocks[i]));
	NewBBDomSet.insert(NewBB); // All blocks dominate themselves...
	DS.addBasicBlock(NewBB, NewBBDomSet);

	// Update immediate dominator information if we have it...
	BasicBlock *NewBBIDom = 0;
	if (ImmediateDominators *ID = getAnalysisToUpdate<ImmediateDominators>()) {
	// This block does not strictly dominate anything, so it is not an immediate
	// dominator. To find the immediate dominator of the new exit node, we
	// trace up the immediate dominators of a predecessor until we find a basic
	// block that dominates the exit block.
	//
	BasicBlock *Dom = LoopBlocks[0]; // Some random predecessor...
	while (!NewBBDomSet.count(Dom)) { // Loop until we find a dominator...
	assert(Dom != 0 && "No shared dominator found???");
	Dom = ID->get(Dom);
	}

	// Set the immediate dominator now...
	ID->addNewBlock(NewBB, Dom);
	NewBBIDom = Dom; // Reuse this if calculating DominatorTree info...
	}

	// Update DominatorTree information if it is active.
	if (DominatorTree *DT = getAnalysisToUpdate<DominatorTree>()) {
	// NewBB doesn't dominate anything, so just create a node and link it into
	// its immediate dominator. If we don't have ImmediateDominator info
	// around, calculate the idom as above.
	DominatorTree::Node *NewBBIDomNode;
	if (NewBBIDom) {
	NewBBIDomNode = DT->getNode(NewBBIDom);
	} else {
	NewBBIDomNode = DT->getNode(LoopBlocks[0]); // Random pred
	while (!NewBBDomSet.count(NewBBIDomNode->getBlock())) {
	NewBBIDomNode = NewBBIDomNode->getIDom();
	assert(NewBBIDomNode && "No shared dominator found??");
	}
	}

	// Create the new dominator tree node...
	DT->createNewNode(NewBB, NewBBIDomNode);
	}

	// Update dominance frontier information...
	if (DominanceFrontier *DF = getAnalysisToUpdate<DominanceFrontier>()) {
	// DF(NewBB) is {Exit} because NewBB does not strictly dominate Exit, but it
	// does dominate itself (and there is an edge (NewBB -> Exit)).
	DominanceFrontier::DomSetType NewDFSet;
	NewDFSet.insert(Exit);
	DF->addBasicBlock(NewBB, NewDFSet);

	// Now we must loop over all of the dominance frontiers in the function,
	// replacing occurrences of Exit with NewBB in some cases. If a block
	// dominates a (now) predecessor of NewBB, but did not strictly dominate
	// Exit, it will have Exit in it's DF set, but should now have NewBB in its
	// set.
	for (unsigned i = 0, e = LoopBlocks.size(); i != e; ++i) {
	// Get all of the dominators of the predecessor...
	const DominatorSet::DomSetType &PredDoms =DS.getDominators(LoopBlocks[i]);
	for (DominatorSet::DomSetType::const_iterator PDI = PredDoms.begin(),
	PDE = PredDoms.end(); PDI != PDE; ++PDI) {
	BasicBlock PredDom = PDI;
	// Make sure to only rewrite blocks that are part of the loop...
	if (L->contains(PredDom)) {
	// If the exit node is in DF(PredDom), then PredDom didn't dominate
	// Exit but did dominate a predecessor inside of the loop. Now we
	// change this entry to include NewBB in the DF instead of Exit.
	DominanceFrontier::iterator DFI = DF->find(PredDom);
	assert(DFI != DF->end() && "No dominance frontier for node?");
	if (DFI->second.count(Exit)) {
	DF->removeFromFrontier(DFI, Exit);
	DF->addToFrontier(DFI, NewBB);
	}
	}
	}
	}
	}
	}