llvm/lib/Transforms/Utils/LoopUnroll.cpp - toolchain/llvm-project - Gitiles

 //===-- UnrollLoop.cpp - Loop unrolling utilities -------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // This file implements some loop unrolling utilities. It does not define any
 // actual pass or policy, but provides a single function to perform loop
 // unrolling.
 //
 // The process of unrolling can produce extraneous basic blocks linked with
 // unconditional branches.  This will be corrected in the future.
 //
 //===----------------------------------------------------------------------===//

 #define DEBUG_TYPE "loop-unroll"
 #include "llvm/Transforms/Utils/UnrollLoop.h"
 #include "llvm/BasicBlock.h"
 #include "llvm/ADT/Statistic.h"
 #include "llvm/Analysis/InstructionSimplify.h"
 #include "llvm/Analysis/LoopIterator.h"
 #include "llvm/Analysis/LoopPass.h"
 #include "llvm/Analysis/ScalarEvolution.h"
 #include "llvm/Support/Debug.h"
 #include "llvm/Support/raw_ostream.h"
 #include "llvm/Transforms/Utils/BasicBlockUtils.h"
 #include "llvm/Transforms/Utils/Cloning.h"
 #include "llvm/Transforms/Utils/Local.h"
 #include "llvm/Transforms/Utils/SimplifyIndVar.h"
 using namespace llvm;

 // TODO: Should these be here or in LoopUnroll?
 STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled");
 STATISTIC(NumUnrolled, "Number of loops unrolled (completely or otherwise)");

 /// RemapInstruction - Convert the instruction operands from referencing the
 /// current values into those specified by VMap.
 static inline void RemapInstruction(Instruction *I,
                                     ValueToValueMapTy &VMap) {
   for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
     Value *Op = I->getOperand(op);
     ValueToValueMapTy::iterator It = VMap.find(Op);
     if (It != VMap.end())
       I->setOperand(op, It->second);
   }

   if (PHINode *PN = dyn_cast<PHINode>(I)) {
     for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
       ValueToValueMapTy::iterator It = VMap.find(PN->getIncomingBlock(i));
       if (It != VMap.end())
         PN->setIncomingBlock(i, cast<BasicBlock>(It->second));
     }
   }
 }

 /// FoldBlockIntoPredecessor - Folds a basic block into its predecessor if it
 /// only has one predecessor, and that predecessor only has one successor.
 /// The LoopInfo Analysis that is passed will be kept consistent.
 /// Returns the new combined block.
 static BasicBlock *FoldBlockIntoPredecessor(BasicBlock *BB, LoopInfo* LI,
                                             LPPassManager *LPM) {
   // Merge basic blocks into their predecessor if there is only one distinct
   // pred, and if there is only one distinct successor of the predecessor, and
   // if there are no PHI nodes.
   BasicBlock *OnlyPred = BB->getSinglePredecessor();
   if (!OnlyPred) return 0;

   if (OnlyPred->getTerminator()->getNumSuccessors() != 1)
     return 0;

   DEBUG(dbgs() << "Merging: " << *BB << "into: " << *OnlyPred);

   // Resolve any PHI nodes at the start of the block.  They are all
   // guaranteed to have exactly one entry if they exist, unless there are
   // multiple duplicate (but guaranteed to be equal) entries for the
   // incoming edges.  This occurs when there are multiple edges from
   // OnlyPred to OnlySucc.
   FoldSingleEntryPHINodes(BB);

   // Delete the unconditional branch from the predecessor...
   OnlyPred->getInstList().pop_back();

   // Make all PHI nodes that referred to BB now refer to Pred as their
   // source...
   BB->replaceAllUsesWith(OnlyPred);

   // Move all definitions in the successor to the predecessor...
   OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());

   std::string OldName = BB->getName();

   // Erase basic block from the function...

   // ScalarEvolution holds references to loop exit blocks.
   if (LPM) {
     if (ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>()) {
       if (Loop *L = LI->getLoopFor(BB))
         SE->forgetLoop(L);
     }
   }
   LI->removeBlock(BB);
   BB->eraseFromParent();

   // Inherit predecessor's name if it exists...
   if (!OldName.empty() && !OnlyPred->hasName())
     OnlyPred->setName(OldName);

   return OnlyPred;
 }

 /// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true
 /// if unrolling was successful, or false if the loop was unmodified. Unrolling
 /// can only fail when the loop's latch block is not terminated by a conditional
 /// branch instruction. However, if the trip count (and multiple) are not known,
 /// loop unrolling will mostly produce more code that is no faster.
 ///
 /// TripCount is generally defined as the number of times the loop header
 /// executes. UnrollLoop relaxes the definition to permit early exits: here
 /// TripCount is the iteration on which control exits LatchBlock if no early
 /// exits were taken. Note that UnrollLoop assumes that the loop counter test
 /// terminates LatchBlock in order to remove unnecesssary instances of the
 /// test. In other words, control may exit the loop prior to TripCount
 /// iterations via an early branch, but control may not exit the loop from the
 /// LatchBlock's terminator prior to TripCount iterations.
 ///
 /// Similarly, TripMultiple divides the number of times that the LatchBlock may
 /// execute without exiting the loop.
 ///
 /// The LoopInfo Analysis that is passed will be kept consistent.
 ///
 /// If a LoopPassManager is passed in, and the loop is fully removed, it will be
 /// removed from the LoopPassManager as well. LPM can also be NULL.
 ///
 /// This utility preserves LoopInfo. If DominatorTree or ScalarEvolution are
 /// available it must also preserve those analyses.
 bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
                       bool AllowRuntime, unsigned TripMultiple,
                       LoopInfo *LI, LPPassManager *LPM) {
   BasicBlock *Preheader = L->getLoopPreheader();
   if (!Preheader) {
     DEBUG(dbgs() << "  Can't unroll; loop preheader-insertion failed.\n");
     return false;
   }

   BasicBlock *LatchBlock = L->getLoopLatch();
   if (!LatchBlock) {
     DEBUG(dbgs() << "  Can't unroll; loop exit-block-insertion failed.\n");
     return false;
   }

   // Loops with indirectbr cannot be cloned.
   if (!L->isSafeToClone()) {
     DEBUG(dbgs() << "  Can't unroll; Loop body cannot be cloned.\n");
     return false;
   }

   BasicBlock *Header = L->getHeader();
   BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator());

   if (!BI || BI->isUnconditional()) {
     // The loop-rotate pass can be helpful to avoid this in many cases.
     DEBUG(dbgs() <<
              "  Can't unroll; loop not terminated by a conditional branch.\n");
     return false;
   }

   if (Header->hasAddressTaken()) {
     // The loop-rotate pass can be helpful to avoid this in many cases.
     DEBUG(dbgs() <<
           "  Won't unroll loop: address of header block is taken.\n");
     return false;
   }

   if (TripCount != 0)
     DEBUG(dbgs() << "  Trip Count = " << TripCount << "\n");
   if (TripMultiple != 1)
     DEBUG(dbgs() << "  Trip Multiple = " << TripMultiple << "\n");

   // Effectively "DCE" unrolled iterations that are beyond the tripcount
   // and will never be executed.
   if (TripCount != 0 && Count > TripCount)
     Count = TripCount;

   // Don't enter the unroll code if there is nothing to do. This way we don't
   // need to support "partial unrolling by 1".
   if (TripCount == 0 && Count < 2)
     return false;

   assert(Count > 0);
   assert(TripMultiple > 0);
   assert(TripCount == 0 || TripCount % TripMultiple == 0);

   // Are we eliminating the loop control altogether?
   bool CompletelyUnroll = Count == TripCount;

   // We assume a run-time trip count if the compiler cannot
   // figure out the loop trip count and the unroll-runtime
   // flag is specified.
   bool RuntimeTripCount = (TripCount == 0 && Count > 0 && AllowRuntime);

   if (RuntimeTripCount && !UnrollRuntimeLoopProlog(L, Count, LI, LPM))
     return false;

   // Notify ScalarEvolution that the loop will be substantially changed,
   // if not outright eliminated.
   if (LPM) {
     ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>();
     if (SE)
       SE->forgetLoop(L);
   }

   // If we know the trip count, we know the multiple...
   unsigned BreakoutTrip = 0;
   if (TripCount != 0) {
     BreakoutTrip = TripCount % Count;
     TripMultiple = 0;
   } else {
     // Figure out what multiple to use.
     BreakoutTrip = TripMultiple =
       (unsigned)GreatestCommonDivisor64(Count, TripMultiple);
   }

   if (CompletelyUnroll) {
     DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName()
           << " with trip count " << TripCount << "!\n");
   } else {
     DEBUG(dbgs() << "UNROLLING loop %" << Header->getName()
           << " by " << Count);
     if (TripMultiple == 0 || BreakoutTrip != TripMultiple) {
       DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip);
     } else if (TripMultiple != 1) {
       DEBUG(dbgs() << " with " << TripMultiple << " trips per branch");
     } else if (RuntimeTripCount) {
       DEBUG(dbgs() << " with run-time trip count");
     }
     DEBUG(dbgs() << "!\n");
   }

   std::vector<BasicBlock*> LoopBlocks = L->getBlocks();

   bool ContinueOnTrue = L->contains(BI->getSuccessor(0));
   BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue);

   // For the first iteration of the loop, we should use the precloned values for
   // PHI nodes.  Insert associations now.
   ValueToValueMapTy LastValueMap;
   std::vector<PHINode*> OrigPHINode;
   for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
     OrigPHINode.push_back(cast<PHINode>(I));
   }

   std::vector<BasicBlock*> Headers;
   std::vector<BasicBlock*> Latches;
   Headers.push_back(Header);
   Latches.push_back(LatchBlock);

   // The current on-the-fly SSA update requires blocks to be processed in
   // reverse postorder so that LastValueMap contains the correct value at each
   // exit.
   LoopBlocksDFS DFS(L);
   DFS.perform(LI);

   // Stash the DFS iterators before adding blocks to the loop.
   LoopBlocksDFS::RPOIterator BlockBegin = DFS.beginRPO();
   LoopBlocksDFS::RPOIterator BlockEnd = DFS.endRPO();

   for (unsigned It = 1; It != Count; ++It) {
     std::vector<BasicBlock*> NewBlocks;

     for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
       ValueToValueMapTy VMap;
       BasicBlock *New = CloneBasicBlock(*BB, VMap, "." + Twine(It));
       Header->getParent()->getBasicBlockList().push_back(New);

       // Loop over all of the PHI nodes in the block, changing them to use the
       // incoming values from the previous block.
       if (*BB == Header)
         for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
           PHINode *NewPHI = cast<PHINode>(VMap[OrigPHINode[i]]);
           Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock);
           if (Instruction *InValI = dyn_cast<Instruction>(InVal))
             if (It > 1 && L->contains(InValI))
               InVal = LastValueMap[InValI];
           VMap[OrigPHINode[i]] = InVal;
           New->getInstList().erase(NewPHI);
         }

       // Update our running map of newest clones
       LastValueMap[*BB] = New;
       for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end();
            VI != VE; ++VI)
         LastValueMap[VI->first] = VI->second;

       L->addBasicBlockToLoop(New, LI->getBase());

       // Add phi entries for newly created values to all exit blocks.
       for (succ_iterator SI = succ_begin(*BB), SE = succ_end(*BB);
            SI != SE; ++SI) {
         if (L->contains(*SI))
           continue;
         for (BasicBlock::iterator BBI = (*SI)->begin();
              PHINode *phi = dyn_cast<PHINode>(BBI); ++BBI) {
           Value *Incoming = phi->getIncomingValueForBlock(*BB);
           ValueToValueMapTy::iterator It = LastValueMap.find(Incoming);
           if (It != LastValueMap.end())
             Incoming = It->second;
           phi->addIncoming(Incoming, New);
         }
       }
       // Keep track of new headers and latches as we create them, so that
       // we can insert the proper branches later.
       if (*BB == Header)
         Headers.push_back(New);
       if (*BB == LatchBlock)
         Latches.push_back(New);

       NewBlocks.push_back(New);
     }

     // Remap all instructions in the most recent iteration
     for (unsigned i = 0; i < NewBlocks.size(); ++i)
       for (BasicBlock::iterator I = NewBlocks[i]->begin(),
            E = NewBlocks[i]->end(); I != E; ++I)
         ::RemapInstruction(I, LastValueMap);
   }

   // Loop over the PHI nodes in the original block, setting incoming values.
   for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
     PHINode *PN = OrigPHINode[i];
     if (CompletelyUnroll) {
       PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader));
       Header->getInstList().erase(PN);
     }
     else if (Count > 1) {
       Value *InVal = PN->removeIncomingValue(LatchBlock, false);
       // If this value was defined in the loop, take the value defined by the
       // last iteration of the loop.
       if (Instruction *InValI = dyn_cast<Instruction>(InVal)) {
         if (L->contains(InValI))
           InVal = LastValueMap[InVal];
       }
       assert(Latches.back() == LastValueMap[LatchBlock] && "bad last latch");
       PN->addIncoming(InVal, Latches.back());
     }
   }

   // Now that all the basic blocks for the unrolled iterations are in place,
   // set up the branches to connect them.
   for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
     // The original branch was replicated in each unrolled iteration.
     BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator());

     // The branch destination.
     unsigned j = (i + 1) % e;
     BasicBlock *Dest = Headers[j];
     bool NeedConditional = true;

     if (RuntimeTripCount && j != 0) {
       NeedConditional = false;
     }

     // For a complete unroll, make the last iteration end with a branch
     // to the exit block.
     if (CompletelyUnroll && j == 0) {
       Dest = LoopExit;
       NeedConditional = false;
     }

     // If we know the trip count or a multiple of it, we can safely use an
     // unconditional branch for some iterations.
     if (j != BreakoutTrip && (TripMultiple == 0 || j % TripMultiple != 0)) {
       NeedConditional = false;
     }

     if (NeedConditional) {
       // Update the conditional branch's successor for the following
       // iteration.
       Term->setSuccessor(!ContinueOnTrue, Dest);
     } else {
       // Remove phi operands at this loop exit
       if (Dest != LoopExit) {
         BasicBlock *BB = Latches[i];
         for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB);
              SI != SE; ++SI) {
           if (*SI == Headers[i])
             continue;
           for (BasicBlock::iterator BBI = (*SI)->begin();
                PHINode *Phi = dyn_cast<PHINode>(BBI); ++BBI) {
             Phi->removeIncomingValue(BB, false);
           }
         }
       }
       // Replace the conditional branch with an unconditional one.
       BranchInst::Create(Dest, Term);
       Term->eraseFromParent();
     }
   }

   // Merge adjacent basic blocks, if possible.
   for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
     BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator());
     if (Term->isUnconditional()) {
       BasicBlock *Dest = Term->getSuccessor(0);
       if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest, LI, LPM))
         std::replace(Latches.begin(), Latches.end(), Dest, Fold);
     }
   }

   if (LPM) {
     // FIXME: Reconstruct dom info, because it is not preserved properly.
     // Incrementally updating domtree after loop unrolling would be easy.
     if (DominatorTree *DT = LPM->getAnalysisIfAvailable<DominatorTree>())
       DT->runOnFunction(*L->getHeader()->getParent());

     // Simplify any new induction variables in the partially unrolled loop.
     ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>();
     if (SE && !CompletelyUnroll) {
       SmallVector<WeakVH, 16> DeadInsts;
       simplifyLoopIVs(L, SE, LPM, DeadInsts);

       // Aggressively clean up dead instructions that simplifyLoopIVs already
       // identified. Any remaining should be cleaned up below.
       while (!DeadInsts.empty())
         if (Instruction *Inst =
             dyn_cast_or_null<Instruction>(&*DeadInsts.pop_back_val()))
           RecursivelyDeleteTriviallyDeadInstructions(Inst);
     }
   }
   // At this point, the code is well formed.  We now do a quick sweep over the
   // inserted code, doing constant propagation and dead code elimination as we
   // go.
   const std::vector<BasicBlock*> &NewLoopBlocks = L->getBlocks();
   for (std::vector<BasicBlock*>::const_iterator BB = NewLoopBlocks.begin(),
        BBE = NewLoopBlocks.end(); BB != BBE; ++BB)
     for (BasicBlock::iterator I = (*BB)->begin(), E = (*BB)->end(); I != E; ) {
       Instruction *Inst = I++;

       if (isInstructionTriviallyDead(Inst))
         (*BB)->getInstList().erase(Inst);
       else if (Value *V = SimplifyInstruction(Inst))
         if (LI->replacementPreservesLCSSAForm(Inst, V)) {
           Inst->replaceAllUsesWith(V);
           (*BB)->getInstList().erase(Inst);
         }
     }

   NumCompletelyUnrolled += CompletelyUnroll;
   ++NumUnrolled;
   // Remove the loop from the LoopPassManager if it's completely removed.
   if (CompletelyUnroll && LPM != NULL)
     LPM->deleteLoopFromQueue(L);

   return true;
 }
	//===-- UnrollLoop.cpp - Loop unrolling utilities -------------------------===//
	//
	// The LLVM Compiler Infrastructure
	//
	// This file is distributed under the University of Illinois Open Source
	// License. See LICENSE.TXT for details.
	//
	//===----------------------------------------------------------------------===//
	//
	// This file implements some loop unrolling utilities. It does not define any
	// actual pass or policy, but provides a single function to perform loop
	// unrolling.
	//
	// The process of unrolling can produce extraneous basic blocks linked with
	// unconditional branches. This will be corrected in the future.
	//
	//===----------------------------------------------------------------------===//

	#define DEBUG_TYPE "loop-unroll"
	#include "llvm/Transforms/Utils/UnrollLoop.h"
	#include "llvm/BasicBlock.h"
	#include "llvm/ADT/Statistic.h"
	#include "llvm/Analysis/InstructionSimplify.h"
	#include "llvm/Analysis/LoopIterator.h"
	#include "llvm/Analysis/LoopPass.h"
	#include "llvm/Analysis/ScalarEvolution.h"
	#include "llvm/Support/Debug.h"
	#include "llvm/Support/raw_ostream.h"
	#include "llvm/Transforms/Utils/BasicBlockUtils.h"
	#include "llvm/Transforms/Utils/Cloning.h"
	#include "llvm/Transforms/Utils/Local.h"
	#include "llvm/Transforms/Utils/SimplifyIndVar.h"
	using namespace llvm;

	// TODO: Should these be here or in LoopUnroll?
	STATISTIC(NumCompletelyUnrolled, "Number of loops completely unrolled");
	STATISTIC(NumUnrolled, "Number of loops unrolled (completely or otherwise)");

	/// RemapInstruction - Convert the instruction operands from referencing the
	/// current values into those specified by VMap.
	static inline void RemapInstruction(Instruction *I,
	ValueToValueMapTy &VMap) {
	for (unsigned op = 0, E = I->getNumOperands(); op != E; ++op) {
	Value *Op = I->getOperand(op);
	ValueToValueMapTy::iterator It = VMap.find(Op);
	if (It != VMap.end())
	I->setOperand(op, It->second);
	}

	if (PHINode *PN = dyn_cast<PHINode>(I)) {
	for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
	ValueToValueMapTy::iterator It = VMap.find(PN->getIncomingBlock(i));
	if (It != VMap.end())
	PN->setIncomingBlock(i, cast<BasicBlock>(It->second));
	}
	}
	}

	/// FoldBlockIntoPredecessor - Folds a basic block into its predecessor if it
	/// only has one predecessor, and that predecessor only has one successor.
	/// The LoopInfo Analysis that is passed will be kept consistent.
	/// Returns the new combined block.
	static BasicBlock FoldBlockIntoPredecessor(BasicBlock BB, LoopInfo* LI,
	LPPassManager *LPM) {
	// Merge basic blocks into their predecessor if there is only one distinct
	// pred, and if there is only one distinct successor of the predecessor, and
	// if there are no PHI nodes.
	BasicBlock *OnlyPred = BB->getSinglePredecessor();
	if (!OnlyPred) return 0;

	if (OnlyPred->getTerminator()->getNumSuccessors() != 1)
	return 0;

	DEBUG(dbgs() << "Merging: " << BB << "into: " << OnlyPred);

	// Resolve any PHI nodes at the start of the block. They are all
	// guaranteed to have exactly one entry if they exist, unless there are
	// multiple duplicate (but guaranteed to be equal) entries for the
	// incoming edges. This occurs when there are multiple edges from
	// OnlyPred to OnlySucc.
	FoldSingleEntryPHINodes(BB);

	// Delete the unconditional branch from the predecessor...
	OnlyPred->getInstList().pop_back();

	// Make all PHI nodes that referred to BB now refer to Pred as their
	// source...
	BB->replaceAllUsesWith(OnlyPred);

	// Move all definitions in the successor to the predecessor...
	OnlyPred->getInstList().splice(OnlyPred->end(), BB->getInstList());

	std::string OldName = BB->getName();

	// Erase basic block from the function...

	// ScalarEvolution holds references to loop exit blocks.
	if (LPM) {
	if (ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>()) {
	if (Loop *L = LI->getLoopFor(BB))
	SE->forgetLoop(L);
	}
	}
	LI->removeBlock(BB);
	BB->eraseFromParent();

	// Inherit predecessor's name if it exists...
	if (!OldName.empty() && !OnlyPred->hasName())
	OnlyPred->setName(OldName);

	return OnlyPred;
	}

	/// Unroll the given loop by Count. The loop must be in LCSSA form. Returns true
	/// if unrolling was successful, or false if the loop was unmodified. Unrolling
	/// can only fail when the loop's latch block is not terminated by a conditional
	/// branch instruction. However, if the trip count (and multiple) are not known,
	/// loop unrolling will mostly produce more code that is no faster.
	///
	/// TripCount is generally defined as the number of times the loop header
	/// executes. UnrollLoop relaxes the definition to permit early exits: here
	/// TripCount is the iteration on which control exits LatchBlock if no early
	/// exits were taken. Note that UnrollLoop assumes that the loop counter test
	/// terminates LatchBlock in order to remove unnecesssary instances of the
	/// test. In other words, control may exit the loop prior to TripCount
	/// iterations via an early branch, but control may not exit the loop from the
	/// LatchBlock's terminator prior to TripCount iterations.
	///
	/// Similarly, TripMultiple divides the number of times that the LatchBlock may
	/// execute without exiting the loop.
	///
	/// The LoopInfo Analysis that is passed will be kept consistent.
	///
	/// If a LoopPassManager is passed in, and the loop is fully removed, it will be
	/// removed from the LoopPassManager as well. LPM can also be NULL.
	///
	/// This utility preserves LoopInfo. If DominatorTree or ScalarEvolution are
	/// available it must also preserve those analyses.
	bool llvm::UnrollLoop(Loop *L, unsigned Count, unsigned TripCount,
	bool AllowRuntime, unsigned TripMultiple,
	LoopInfo LI, LPPassManager LPM) {
	BasicBlock *Preheader = L->getLoopPreheader();
	if (!Preheader) {
	DEBUG(dbgs() << " Can't unroll; loop preheader-insertion failed.\n");
	return false;
	}

	BasicBlock *LatchBlock = L->getLoopLatch();
	if (!LatchBlock) {
	DEBUG(dbgs() << " Can't unroll; loop exit-block-insertion failed.\n");
	return false;
	}

	// Loops with indirectbr cannot be cloned.
	if (!L->isSafeToClone()) {
	DEBUG(dbgs() << " Can't unroll; Loop body cannot be cloned.\n");
	return false;
	}

	BasicBlock *Header = L->getHeader();
	BranchInst *BI = dyn_cast<BranchInst>(LatchBlock->getTerminator());

	if (!BI \|\| BI->isUnconditional()) {
	// The loop-rotate pass can be helpful to avoid this in many cases.
	DEBUG(dbgs() <<
	" Can't unroll; loop not terminated by a conditional branch.\n");
	return false;
	}

	if (Header->hasAddressTaken()) {
	// The loop-rotate pass can be helpful to avoid this in many cases.
	DEBUG(dbgs() <<
	" Won't unroll loop: address of header block is taken.\n");
	return false;
	}

	if (TripCount != 0)
	DEBUG(dbgs() << " Trip Count = " << TripCount << "\n");
	if (TripMultiple != 1)
	DEBUG(dbgs() << " Trip Multiple = " << TripMultiple << "\n");

	// Effectively "DCE" unrolled iterations that are beyond the tripcount
	// and will never be executed.
	if (TripCount != 0 && Count > TripCount)
	Count = TripCount;

	// Don't enter the unroll code if there is nothing to do. This way we don't
	// need to support "partial unrolling by 1".
	if (TripCount == 0 && Count < 2)
	return false;

	assert(Count > 0);
	assert(TripMultiple > 0);
	assert(TripCount == 0 \|\| TripCount % TripMultiple == 0);

	// Are we eliminating the loop control altogether?
	bool CompletelyUnroll = Count == TripCount;

	// We assume a run-time trip count if the compiler cannot
	// figure out the loop trip count and the unroll-runtime
	// flag is specified.
	bool RuntimeTripCount = (TripCount == 0 && Count > 0 && AllowRuntime);

	if (RuntimeTripCount && !UnrollRuntimeLoopProlog(L, Count, LI, LPM))
	return false;

	// Notify ScalarEvolution that the loop will be substantially changed,
	// if not outright eliminated.
	if (LPM) {
	ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>();
	if (SE)
	SE->forgetLoop(L);
	}

	// If we know the trip count, we know the multiple...
	unsigned BreakoutTrip = 0;
	if (TripCount != 0) {
	BreakoutTrip = TripCount % Count;
	TripMultiple = 0;
	} else {
	// Figure out what multiple to use.
	BreakoutTrip = TripMultiple =
	(unsigned)GreatestCommonDivisor64(Count, TripMultiple);
	}

	if (CompletelyUnroll) {
	DEBUG(dbgs() << "COMPLETELY UNROLLING loop %" << Header->getName()
	<< " with trip count " << TripCount << "!\n");
	} else {
	DEBUG(dbgs() << "UNROLLING loop %" << Header->getName()
	<< " by " << Count);
	if (TripMultiple == 0 \|\| BreakoutTrip != TripMultiple) {
	DEBUG(dbgs() << " with a breakout at trip " << BreakoutTrip);
	} else if (TripMultiple != 1) {
	DEBUG(dbgs() << " with " << TripMultiple << " trips per branch");
	} else if (RuntimeTripCount) {
	DEBUG(dbgs() << " with run-time trip count");
	}
	DEBUG(dbgs() << "!\n");
	}

	std::vector<BasicBlock*> LoopBlocks = L->getBlocks();

	bool ContinueOnTrue = L->contains(BI->getSuccessor(0));
	BasicBlock *LoopExit = BI->getSuccessor(ContinueOnTrue);

	// For the first iteration of the loop, we should use the precloned values for
	// PHI nodes. Insert associations now.
	ValueToValueMapTy LastValueMap;
	std::vector<PHINode*> OrigPHINode;
	for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
	OrigPHINode.push_back(cast<PHINode>(I));
	}

	std::vector<BasicBlock*> Headers;
	std::vector<BasicBlock*> Latches;
	Headers.push_back(Header);
	Latches.push_back(LatchBlock);

	// The current on-the-fly SSA update requires blocks to be processed in
	// reverse postorder so that LastValueMap contains the correct value at each
	// exit.
	LoopBlocksDFS DFS(L);
	DFS.perform(LI);

	// Stash the DFS iterators before adding blocks to the loop.
	LoopBlocksDFS::RPOIterator BlockBegin = DFS.beginRPO();
	LoopBlocksDFS::RPOIterator BlockEnd = DFS.endRPO();

	for (unsigned It = 1; It != Count; ++It) {
	std::vector<BasicBlock*> NewBlocks;

	for (LoopBlocksDFS::RPOIterator BB = BlockBegin; BB != BlockEnd; ++BB) {
	ValueToValueMapTy VMap;
	BasicBlock New = CloneBasicBlock(BB, VMap, "." + Twine(It));
	Header->getParent()->getBasicBlockList().push_back(New);

	// Loop over all of the PHI nodes in the block, changing them to use the
	// incoming values from the previous block.
	if (*BB == Header)
	for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
	PHINode *NewPHI = cast<PHINode>(VMap[OrigPHINode[i]]);
	Value *InVal = NewPHI->getIncomingValueForBlock(LatchBlock);
	if (Instruction *InValI = dyn_cast<Instruction>(InVal))
	if (It > 1 && L->contains(InValI))
	InVal = LastValueMap[InValI];
	VMap[OrigPHINode[i]] = InVal;
	New->getInstList().erase(NewPHI);
	}

	// Update our running map of newest clones
	LastValueMap[*BB] = New;
	for (ValueToValueMapTy::iterator VI = VMap.begin(), VE = VMap.end();
	VI != VE; ++VI)
	LastValueMap[VI->first] = VI->second;

	L->addBasicBlockToLoop(New, LI->getBase());

	// Add phi entries for newly created values to all exit blocks.
	for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB);
	SI != SE; ++SI) {
	if (L->contains(*SI))
	continue;
	for (BasicBlock::iterator BBI = (*SI)->begin();
	PHINode *phi = dyn_cast<PHINode>(BBI); ++BBI) {
	Value Incoming = phi->getIncomingValueForBlock(BB);
	ValueToValueMapTy::iterator It = LastValueMap.find(Incoming);
	if (It != LastValueMap.end())
	Incoming = It->second;
	phi->addIncoming(Incoming, New);
	}
	}
	// Keep track of new headers and latches as we create them, so that
	// we can insert the proper branches later.
	if (*BB == Header)
	Headers.push_back(New);
	if (*BB == LatchBlock)
	Latches.push_back(New);

	NewBlocks.push_back(New);
	}

	// Remap all instructions in the most recent iteration
	for (unsigned i = 0; i < NewBlocks.size(); ++i)
	for (BasicBlock::iterator I = NewBlocks[i]->begin(),
	E = NewBlocks[i]->end(); I != E; ++I)
	::RemapInstruction(I, LastValueMap);
	}

	// Loop over the PHI nodes in the original block, setting incoming values.
	for (unsigned i = 0, e = OrigPHINode.size(); i != e; ++i) {
	PHINode *PN = OrigPHINode[i];
	if (CompletelyUnroll) {
	PN->replaceAllUsesWith(PN->getIncomingValueForBlock(Preheader));
	Header->getInstList().erase(PN);
	}
	else if (Count > 1) {
	Value *InVal = PN->removeIncomingValue(LatchBlock, false);
	// If this value was defined in the loop, take the value defined by the
	// last iteration of the loop.
	if (Instruction *InValI = dyn_cast<Instruction>(InVal)) {
	if (L->contains(InValI))
	InVal = LastValueMap[InVal];
	}
	assert(Latches.back() == LastValueMap[LatchBlock] && "bad last latch");
	PN->addIncoming(InVal, Latches.back());
	}
	}

	// Now that all the basic blocks for the unrolled iterations are in place,
	// set up the branches to connect them.
	for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
	// The original branch was replicated in each unrolled iteration.
	BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator());

	// The branch destination.
	unsigned j = (i + 1) % e;
	BasicBlock *Dest = Headers[j];
	bool NeedConditional = true;

	if (RuntimeTripCount && j != 0) {
	NeedConditional = false;
	}

	// For a complete unroll, make the last iteration end with a branch
	// to the exit block.
	if (CompletelyUnroll && j == 0) {
	Dest = LoopExit;
	NeedConditional = false;
	}

	// If we know the trip count or a multiple of it, we can safely use an
	// unconditional branch for some iterations.
	if (j != BreakoutTrip && (TripMultiple == 0 \|\| j % TripMultiple != 0)) {
	NeedConditional = false;
	}

	if (NeedConditional) {
	// Update the conditional branch's successor for the following
	// iteration.
	Term->setSuccessor(!ContinueOnTrue, Dest);
	} else {
	// Remove phi operands at this loop exit
	if (Dest != LoopExit) {
	BasicBlock *BB = Latches[i];
	for (succ_iterator SI = succ_begin(BB), SE = succ_end(BB);
	SI != SE; ++SI) {
	if (*SI == Headers[i])
	continue;
	for (BasicBlock::iterator BBI = (*SI)->begin();
	PHINode *Phi = dyn_cast<PHINode>(BBI); ++BBI) {
	Phi->removeIncomingValue(BB, false);
	}
	}
	}
	// Replace the conditional branch with an unconditional one.
	BranchInst::Create(Dest, Term);
	Term->eraseFromParent();
	}
	}

	// Merge adjacent basic blocks, if possible.
	for (unsigned i = 0, e = Latches.size(); i != e; ++i) {
	BranchInst *Term = cast<BranchInst>(Latches[i]->getTerminator());
	if (Term->isUnconditional()) {
	BasicBlock *Dest = Term->getSuccessor(0);
	if (BasicBlock *Fold = FoldBlockIntoPredecessor(Dest, LI, LPM))
	std::replace(Latches.begin(), Latches.end(), Dest, Fold);
	}
	}

	if (LPM) {
	// FIXME: Reconstruct dom info, because it is not preserved properly.
	// Incrementally updating domtree after loop unrolling would be easy.
	if (DominatorTree *DT = LPM->getAnalysisIfAvailable<DominatorTree>())
	DT->runOnFunction(*L->getHeader()->getParent());

	// Simplify any new induction variables in the partially unrolled loop.
	ScalarEvolution *SE = LPM->getAnalysisIfAvailable<ScalarEvolution>();
	if (SE && !CompletelyUnroll) {
	SmallVector<WeakVH, 16> DeadInsts;
	simplifyLoopIVs(L, SE, LPM, DeadInsts);

	// Aggressively clean up dead instructions that simplifyLoopIVs already
	// identified. Any remaining should be cleaned up below.
	while (!DeadInsts.empty())
	if (Instruction *Inst =
	dyn_cast_or_null<Instruction>(&*DeadInsts.pop_back_val()))
	RecursivelyDeleteTriviallyDeadInstructions(Inst);
	}
	}
	// At this point, the code is well formed. We now do a quick sweep over the
	// inserted code, doing constant propagation and dead code elimination as we
	// go.
	const std::vector<BasicBlock*> &NewLoopBlocks = L->getBlocks();
	for (std::vector<BasicBlock*>::const_iterator BB = NewLoopBlocks.begin(),
	BBE = NewLoopBlocks.end(); BB != BBE; ++BB)
	for (BasicBlock::iterator I = (BB)->begin(), E = (BB)->end(); I != E; ) {
	Instruction *Inst = I++;

	if (isInstructionTriviallyDead(Inst))
	(*BB)->getInstList().erase(Inst);
	else if (Value *V = SimplifyInstruction(Inst))
	if (LI->replacementPreservesLCSSAForm(Inst, V)) {
	Inst->replaceAllUsesWith(V);
	(*BB)->getInstList().erase(Inst);
	}
	}

	NumCompletelyUnrolled += CompletelyUnroll;
	++NumUnrolled;
	// Remove the loop from the LoopPassManager if it's completely removed.
	if (CompletelyUnroll && LPM != NULL)
	LPM->deleteLoopFromQueue(L);

	return true;
	}