Import LLVM, at r72732.

1 files changed, 600 insertions, 0 deletions

diff --git a/lib/Transforms/Utils/LoopSimplify.cpp b/lib/Transforms/Utils/LoopSimplify.cpp
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+//===- LoopSimplify.cpp - Loop Canonicalization Pass ----------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// This pass performs several transformations to transform natural loops into a
+// simpler form, which makes subsequent analyses and transformations simpler and
+// more effective.
+//
+// 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) only have predecessors from inside of the loop (and are thus dominated
+// by the loop header).  This simplifies transformations such as store-sinking
+// that are built into LICM.
+//
+// This pass also guarantees that loops will have exactly one backedge.
+//
+// Note that the simplifycfg pass will clean up blocks which are split out but
+// end up being unnecessary, so usage of this pass should not pessimize
+// generated code.
+//
+// This pass obviously modifies the CFG, but updates loop information and
+// dominator information.
+//
+//===----------------------------------------------------------------------===//
+
+#define DEBUG_TYPE "loopsimplify"
+#include "llvm/Transforms/Scalar.h"
+#include "llvm/Constants.h"
+#include "llvm/Instructions.h"
+#include "llvm/Function.h"
+#include "llvm/Type.h"
+#include "llvm/Analysis/AliasAnalysis.h"
+#include "llvm/Analysis/Dominators.h"
+#include "llvm/Analysis/LoopInfo.h"
+#include "llvm/Transforms/Utils/BasicBlockUtils.h"
+#include "llvm/Support/CFG.h"
+#include "llvm/Support/Compiler.h"
+#include "llvm/ADT/SetOperations.h"
+#include "llvm/ADT/SetVector.h"
+#include "llvm/ADT/Statistic.h"
+#include "llvm/ADT/DepthFirstIterator.h"
+using namespace llvm;
+
+STATISTIC(NumInserted, "Number of pre-header or exit blocks inserted");
+STATISTIC(NumNested  , "Number of nested loops split out");
+
+namespace {
+  struct VISIBILITY_HIDDEN LoopSimplify : public FunctionPass {
+    static char ID; // Pass identification, replacement for typeid
+    LoopSimplify() : FunctionPass(&ID) {}
+
+    // AA - If we have an alias analysis object to update, this is it, otherwise
+    // this is null.
+    AliasAnalysis *AA;
+    LoopInfo *LI;
+    DominatorTree *DT;
+    virtual bool runOnFunction(Function &F);
+
+    virtual void getAnalysisUsage(AnalysisUsage &AU) const {
+      // We need loop information to identify the loops...
+      AU.addRequired<LoopInfo>();
+      AU.addRequired<DominatorTree>();
+
+      AU.addPreserved<LoopInfo>();
+      AU.addPreserved<DominatorTree>();
+      AU.addPreserved<DominanceFrontier>();
+      AU.addPreserved<AliasAnalysis>();
+      AU.addPreservedID(BreakCriticalEdgesID);  // No critical edges added.
+    }
+
+    /// verifyAnalysis() - Verify loop nest.
+    void verifyAnalysis() const {
+#ifndef NDEBUG
+      LoopInfo *NLI = &getAnalysis<LoopInfo>();
+      for (LoopInfo::iterator I = NLI->begin(), E = NLI->end(); I != E; ++I) 
+        (*I)->verifyLoop();
+#endif  
+    }
+
+  private:
+    bool ProcessLoop(Loop *L);
+    BasicBlock *RewriteLoopExitBlock(Loop *L, BasicBlock *Exit);
+    void InsertPreheaderForLoop(Loop *L);
+    Loop *SeparateNestedLoop(Loop *L);
+    void InsertUniqueBackedgeBlock(Loop *L);
+    void PlaceSplitBlockCarefully(BasicBlock *NewBB,
+                                  SmallVectorImpl<BasicBlock*> &SplitPreds,
+                                  Loop *L);
+  };
+}
+
+char LoopSimplify::ID = 0;
+static RegisterPass<LoopSimplify>
+X("loopsimplify", "Canonicalize natural loops", true);
+
+// Publically exposed interface to pass...
+const PassInfo *const llvm::LoopSimplifyID = &X;
+FunctionPass *llvm::createLoopSimplifyPass() { return new LoopSimplify(); }
+
+/// runOnFunction - Run down all loops in the CFG (recursively, but we could do
+/// it in any convenient order) inserting preheaders...
+///
+bool LoopSimplify::runOnFunction(Function &F) {
+  bool Changed = false;
+  LI = &getAnalysis<LoopInfo>();
+  AA = getAnalysisIfAvailable<AliasAnalysis>();
+  DT = &getAnalysis<DominatorTree>();
+
+  // Check to see that no blocks (other than the header) in loops have
+  // predecessors that are not in loops.  This is not valid for natural loops,
+  // but can occur if the blocks are unreachable.  Since they are unreachable we
+  // can just shamelessly destroy their terminators to make them not branch into
+  // the loop!
+  for (Function::iterator BB = F.begin(), E = F.end(); BB != E; ++BB) {
+    // This case can only occur for unreachable blocks.  Blocks that are
+    // unreachable can't be in loops, so filter those blocks out.
+    if (LI->getLoopFor(BB)) continue;
+    
+    bool BlockUnreachable = false;
+    TerminatorInst *TI = BB->getTerminator();
+
+    // Check to see if any successors of this block are non-loop-header loops
+    // that are not the header.
+    for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i) {
+      // If this successor is not in a loop, BB is clearly ok.
+      Loop *L = LI->getLoopFor(TI->getSuccessor(i));
+      if (!L) continue;
+      
+      // If the succ is the loop header, and if L is a top-level loop, then this
+      // is an entrance into a loop through the header, which is also ok.
+      if (L->getHeader() == TI->getSuccessor(i) && L->getParentLoop() == 0)
+        continue;
+      
+      // Otherwise, this is an entrance into a loop from some place invalid.
+      // Either the loop structure is invalid and this is not a natural loop (in
+      // which case the compiler is buggy somewhere else) or BB is unreachable.
+      BlockUnreachable = true;
+      break;
+    }
+    
+    // If this block is ok, check the next one.
+    if (!BlockUnreachable) continue;
+    
+    // Otherwise, this block is dead.  To clean up the CFG and to allow later
+    // loop transformations to ignore this case, we delete the edges into the
+    // loop by replacing the terminator.
+    
+    // Remove PHI entries from the successors.
+    for (unsigned i = 0, e = TI->getNumSuccessors(); i != e; ++i)
+      TI->getSuccessor(i)->removePredecessor(BB);
+   
+    // Add a new unreachable instruction before the old terminator.
+    new UnreachableInst(TI);
+    
+    // Delete the dead terminator.
+    if (AA) AA->deleteValue(TI);
+    if (!TI->use_empty())
+      TI->replaceAllUsesWith(UndefValue::get(TI->getType()));
+    TI->eraseFromParent();
+    Changed |= true;
+  }
+  
+  for (LoopInfo::iterator I = LI->begin(), E = LI->end(); I != E; ++I)
+    Changed |= ProcessLoop(*I);
+
+  return Changed;
+}
+
+/// ProcessLoop - Walk the loop structure in depth first order, ensuring that
+/// all loops have preheaders.
+///
+bool LoopSimplify::ProcessLoop(Loop *L) {
+  bool Changed = false;
+ReprocessLoop:
+  
+  // Canonicalize inner loops before outer loops.  Inner loop canonicalization
+  // can provide work for the outer loop to canonicalize.
+  for (Loop::iterator I = L->begin(), E = L->end(); I != E; ++I)
+    Changed |= ProcessLoop(*I);
+  
+  assert(L->getBlocks()[0] == L->getHeader() &&
+         "Header isn't first block in loop?");
+
+  // Does the loop already have a preheader?  If so, don't insert one.
+  if (L->getLoopPreheader() == 0) {
+    InsertPreheaderForLoop(L);
+    NumInserted++;
+    Changed = true;
+  }
+
+  // Next, check to make sure that all exit nodes of the loop only have
+  // predecessors that are inside of the loop.  This check guarantees that the
+  // loop preheader/header will dominate the exit blocks.  If the exit block has
+  // predecessors from outside of the loop, split the edge now.
+  SmallVector<BasicBlock*, 8> ExitBlocks;
+  L->getExitBlocks(ExitBlocks);
+    
+  SetVector<BasicBlock*> ExitBlockSet(ExitBlocks.begin(), ExitBlocks.end());
+  for (SetVector<BasicBlock*>::iterator I = ExitBlockSet.begin(),
+         E = ExitBlockSet.end(); I != E; ++I) {
+    BasicBlock *ExitBlock = *I;
+    for (pred_iterator PI = pred_begin(ExitBlock), PE = pred_end(ExitBlock);
+         PI != PE; ++PI)
+      // Must be exactly this loop: no subloops, parent loops, or non-loop preds
+      // allowed.
+      if (!L->contains(*PI)) {
+        RewriteLoopExitBlock(L, ExitBlock);
+        NumInserted++;
+        Changed = true;
+        break;
+      }
+  }
+
+  // If the header has more than two predecessors at this point (from the
+  // preheader and from multiple backedges), we must adjust the loop.
+  unsigned NumBackedges = L->getNumBackEdges();
+  if (NumBackedges != 1) {
+    // If this is really a nested loop, rip it out into a child loop.  Don't do
+    // this for loops with a giant number of backedges, just factor them into a
+    // common backedge instead.
+    if (NumBackedges < 8) {
+      if (Loop *NL = SeparateNestedLoop(L)) {
+        ++NumNested;
+        // This is a big restructuring change, reprocess the whole loop.
+        ProcessLoop(NL);
+        Changed = true;
+        // GCC doesn't tail recursion eliminate this.
+        goto ReprocessLoop;
+      }
+    }
+
+    // If we either couldn't, or didn't want to, identify nesting of the loops,
+    // insert a new block that all backedges target, then make it jump to the
+    // loop header.
+    InsertUniqueBackedgeBlock(L);
+    NumInserted++;
+    Changed = true;
+  }
+
+  // Scan over the PHI nodes in the loop header.  Since they now have only two
+  // incoming values (the loop is canonicalized), we may have simplified the PHI
+  // down to 'X = phi [X, Y]', which should be replaced with 'Y'.
+  PHINode *PN;
+  for (BasicBlock::iterator I = L->getHeader()->begin();
+       (PN = dyn_cast<PHINode>(I++)); )
+    if (Value *V = PN->hasConstantValue()) {
+      if (AA) AA->deleteValue(PN);
+      PN->replaceAllUsesWith(V);
+      PN->eraseFromParent();
+    }
+
+  return Changed;
+}
+
+/// 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 LoopSimplify::InsertPreheaderForLoop(Loop *L) {
+  BasicBlock *Header = L->getHeader();
+
+  // Compute the set of predecessors of the loop that are not in the loop.
+  SmallVector<BasicBlock*, 8> 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, &OutsideBlocks[0], OutsideBlocks.size(),
+                           ".preheader", this);
+  
+
+  //===--------------------------------------------------------------------===//
+  //  Update analysis results now that we have performed the transformation
+  //
+
+  // We know that we have loop information to update... update it now.
+  if (Loop *Parent = L->getParentLoop())
+    Parent->addBasicBlockToLoop(NewBB, LI->getBase());
+
+  // Make sure that NewBB is put someplace intelligent, which doesn't mess up
+  // code layout too horribly.
+  PlaceSplitBlockCarefully(NewBB, OutsideBlocks, L);
+}
+
+/// RewriteLoopExitBlock - Ensure that the loop preheader dominates all exit
+/// blocks.  This method is used to split exit blocks that have predecessors
+/// outside of the loop.
+BasicBlock *LoopSimplify::RewriteLoopExitBlock(Loop *L, BasicBlock *Exit) {
+  SmallVector<BasicBlock*, 8> 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, &LoopBlocks[0], 
+                                             LoopBlocks.size(), ".loopexit",
+                                             this);
+
+  // Update Loop Information - we know that the new block will be in whichever
+  // loop the Exit block is in.  Note that it may not be in that immediate loop,
+  // if the successor is some other loop header.  In that case, we continue 
+  // walking up the loop tree to find a loop that contains both the successor
+  // block and the predecessor block.
+  Loop *SuccLoop = LI->getLoopFor(Exit);
+  while (SuccLoop && !SuccLoop->contains(L->getHeader()))
+    SuccLoop = SuccLoop->getParentLoop();
+  if (SuccLoop)
+    SuccLoop->addBasicBlockToLoop(NewBB, LI->getBase());
+
+  return NewBB;
+}
+
+/// AddBlockAndPredsToSet - Add the specified block, and all of its
+/// predecessors, to the specified set, if it's not already in there.  Stop
+/// predecessor traversal when we reach StopBlock.
+static void AddBlockAndPredsToSet(BasicBlock *InputBB, BasicBlock *StopBlock,
+                                  std::set<BasicBlock*> &Blocks) {
+  std::vector<BasicBlock *> WorkList;
+  WorkList.push_back(InputBB);
+  do {
+    BasicBlock *BB = WorkList.back(); WorkList.pop_back();
+    if (Blocks.insert(BB).second && BB != StopBlock)
+      // If BB is not already processed and it is not a stop block then
+      // insert its predecessor in the work list
+      for (pred_iterator I = pred_begin(BB), E = pred_end(BB); I != E; ++I) {
+        BasicBlock *WBB = *I;
+        WorkList.push_back(WBB);
+      }
+  } while(!WorkList.empty());
+}
+
+/// FindPHIToPartitionLoops - The first part of loop-nestification is to find a
+/// PHI node that tells us how to partition the loops.
+static PHINode *FindPHIToPartitionLoops(Loop *L, DominatorTree *DT,
+                                        AliasAnalysis *AA) {
+  for (BasicBlock::iterator I = L->getHeader()->begin(); isa<PHINode>(I); ) {
+    PHINode *PN = cast<PHINode>(I);
+    ++I;
+    if (Value *V = PN->hasConstantValue())
+      if (!isa<Instruction>(V) || DT->dominates(cast<Instruction>(V), PN)) {
+        // This is a degenerate PHI already, don't modify it!
+        PN->replaceAllUsesWith(V);
+        if (AA) AA->deleteValue(PN);
+        PN->eraseFromParent();
+        continue;
+      }
+
+    // Scan this PHI node looking for a use of the PHI node by itself.
+    for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
+      if (PN->getIncomingValue(i) == PN &&
+          L->contains(PN->getIncomingBlock(i)))
+        // We found something tasty to remove.
+        return PN;
+  }
+  return 0;
+}
+
+// PlaceSplitBlockCarefully - If the block isn't already, move the new block to
+// right after some 'outside block' block.  This prevents the preheader from
+// being placed inside the loop body, e.g. when the loop hasn't been rotated.
+void LoopSimplify::PlaceSplitBlockCarefully(BasicBlock *NewBB,
+                                       SmallVectorImpl<BasicBlock*> &SplitPreds,
+                                            Loop *L) {
+  // Check to see if NewBB is already well placed.
+  Function::iterator BBI = NewBB; --BBI;
+  for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
+    if (&*BBI == SplitPreds[i])
+      return;
+  }
+  
+  // If it isn't already after an outside block, move it after one.  This is
+  // always good as it makes the uncond branch from the outside block into a
+  // fall-through.
+  
+  // Figure out *which* outside block to put this after.  Prefer an outside
+  // block that neighbors a BB actually in the loop.
+  BasicBlock *FoundBB = 0;
+  for (unsigned i = 0, e = SplitPreds.size(); i != e; ++i) {
+    Function::iterator BBI = SplitPreds[i];
+    if (++BBI != NewBB->getParent()->end() && 
+        L->contains(BBI)) {
+      FoundBB = SplitPreds[i];
+      break;
+    }
+  }
+  
+  // If our heuristic for a *good* bb to place this after doesn't find
+  // anything, just pick something.  It's likely better than leaving it within
+  // the loop.
+  if (!FoundBB)
+    FoundBB = SplitPreds[0];
+  NewBB->moveAfter(FoundBB);
+}
+
+
+/// SeparateNestedLoop - If this loop has multiple backedges, try to pull one of
+/// them out into a nested loop.  This is important for code that looks like
+/// this:
+///
+///  Loop:
+///     ...
+///     br cond, Loop, Next
+///     ...
+///     br cond2, Loop, Out
+///
+/// To identify this common case, we look at the PHI nodes in the header of the
+/// loop.  PHI nodes with unchanging values on one backedge correspond to values
+/// that change in the "outer" loop, but not in the "inner" loop.
+///
+/// If we are able to separate out a loop, return the new outer loop that was
+/// created.
+///
+Loop *LoopSimplify::SeparateNestedLoop(Loop *L) {
+  PHINode *PN = FindPHIToPartitionLoops(L, DT, AA);
+  if (PN == 0) return 0;  // No known way to partition.
+
+  // Pull out all predecessors that have varying values in the loop.  This
+  // handles the case when a PHI node has multiple instances of itself as
+  // arguments.
+  SmallVector<BasicBlock*, 8> OuterLoopPreds;
+  for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i)
+    if (PN->getIncomingValue(i) != PN ||
+        !L->contains(PN->getIncomingBlock(i)))
+      OuterLoopPreds.push_back(PN->getIncomingBlock(i));
+
+  BasicBlock *Header = L->getHeader();
+  BasicBlock *NewBB = SplitBlockPredecessors(Header, &OuterLoopPreds[0],
+                                             OuterLoopPreds.size(),
+                                             ".outer", this);
+
+  // Make sure that NewBB is put someplace intelligent, which doesn't mess up
+  // code layout too horribly.
+  PlaceSplitBlockCarefully(NewBB, OuterLoopPreds, L);
+  
+  // Create the new outer loop.
+  Loop *NewOuter = new Loop();
+
+  // Change the parent loop to use the outer loop as its child now.
+  if (Loop *Parent = L->getParentLoop())
+    Parent->replaceChildLoopWith(L, NewOuter);
+  else
+    LI->changeTopLevelLoop(L, NewOuter);
+
+  // This block is going to be our new header block: add it to this loop and all
+  // parent loops.
+  NewOuter->addBasicBlockToLoop(NewBB, LI->getBase());
+
+  // L is now a subloop of our outer loop.
+  NewOuter->addChildLoop(L);
+
+  for (Loop::block_iterator I = L->block_begin(), E = L->block_end();
+       I != E; ++I)
+    NewOuter->addBlockEntry(*I);
+
+  // Determine which blocks should stay in L and which should be moved out to
+  // the Outer loop now.
+  std::set<BasicBlock*> BlocksInL;
+  for (pred_iterator PI = pred_begin(Header), E = pred_end(Header); PI!=E; ++PI)
+    if (DT->dominates(Header, *PI))
+      AddBlockAndPredsToSet(*PI, Header, BlocksInL);
+
+
+  // Scan all of the loop children of L, moving them to OuterLoop if they are
+  // not part of the inner loop.
+  const std::vector<Loop*> &SubLoops = L->getSubLoops();
+  for (size_t I = 0; I != SubLoops.size(); )
+    if (BlocksInL.count(SubLoops[I]->getHeader()))
+      ++I;   // Loop remains in L
+    else
+      NewOuter->addChildLoop(L->removeChildLoop(SubLoops.begin() + I));
+
+  // Now that we know which blocks are in L and which need to be moved to
+  // OuterLoop, move any blocks that need it.
+  for (unsigned i = 0; i != L->getBlocks().size(); ++i) {
+    BasicBlock *BB = L->getBlocks()[i];
+    if (!BlocksInL.count(BB)) {
+      // Move this block to the parent, updating the exit blocks sets
+      L->removeBlockFromLoop(BB);
+      if ((*LI)[BB] == L)
+        LI->changeLoopFor(BB, NewOuter);
+      --i;
+    }
+  }
+
+  return NewOuter;
+}
+
+
+
+/// InsertUniqueBackedgeBlock - This method is called when the specified loop
+/// has more than one backedge in it.  If this occurs, revector all of these
+/// backedges to target a new basic block and have that block branch to the loop
+/// header.  This ensures that loops have exactly one backedge.
+///
+void LoopSimplify::InsertUniqueBackedgeBlock(Loop *L) {
+  assert(L->getNumBackEdges() > 1 && "Must have > 1 backedge!");
+
+  // Get information about the loop
+  BasicBlock *Preheader = L->getLoopPreheader();
+  BasicBlock *Header = L->getHeader();
+  Function *F = Header->getParent();
+
+  // Figure out which basic blocks contain back-edges to the loop header.
+  std::vector<BasicBlock*> BackedgeBlocks;
+  for (pred_iterator I = pred_begin(Header), E = pred_end(Header); I != E; ++I)
+    if (*I != Preheader) BackedgeBlocks.push_back(*I);
+
+  // Create and insert the new backedge block...
+  BasicBlock *BEBlock = BasicBlock::Create(Header->getName()+".backedge", F);
+  BranchInst *BETerminator = BranchInst::Create(Header, BEBlock);
+
+  // Move the new backedge block to right after the last backedge block.
+  Function::iterator InsertPos = BackedgeBlocks.back(); ++InsertPos;
+  F->getBasicBlockList().splice(InsertPos, F->getBasicBlockList(), BEBlock);
+
+  // Now that the block has been inserted into the function, create PHI nodes in
+  // the backedge block which correspond to any PHI nodes in the header block.
+  for (BasicBlock::iterator I = Header->begin(); isa<PHINode>(I); ++I) {
+    PHINode *PN = cast<PHINode>(I);
+    PHINode *NewPN = PHINode::Create(PN->getType(), PN->getName()+".be",
+                                     BETerminator);
+    NewPN->reserveOperandSpace(BackedgeBlocks.size());
+    if (AA) AA->copyValue(PN, NewPN);
+
+    // Loop over the PHI node, moving all entries except the one for the
+    // preheader over to the new PHI node.
+    unsigned PreheaderIdx = ~0U;
+    bool HasUniqueIncomingValue = true;
+    Value *UniqueValue = 0;
+    for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
+      BasicBlock *IBB = PN->getIncomingBlock(i);
+      Value *IV = PN->getIncomingValue(i);
+      if (IBB == Preheader) {
+        PreheaderIdx = i;
+      } else {
+        NewPN->addIncoming(IV, IBB);
+        if (HasUniqueIncomingValue) {
+          if (UniqueValue == 0)
+            UniqueValue = IV;
+          else if (UniqueValue != IV)
+            HasUniqueIncomingValue = false;
+        }
+      }
+    }
+
+    // Delete all of the incoming values from the old PN except the preheader's
+    assert(PreheaderIdx != ~0U && "PHI has no preheader entry??");
+    if (PreheaderIdx != 0) {
+      PN->setIncomingValue(0, PN->getIncomingValue(PreheaderIdx));
+      PN->setIncomingBlock(0, PN->getIncomingBlock(PreheaderIdx));
+    }
+    // Nuke all entries except the zero'th.
+    for (unsigned i = 0, e = PN->getNumIncomingValues()-1; i != e; ++i)
+      PN->removeIncomingValue(e-i, false);
+
+    // Finally, add the newly constructed PHI node as the entry for the BEBlock.
+    PN->addIncoming(NewPN, BEBlock);
+
+    // As an optimization, if all incoming values in the new PhiNode (which is a
+    // subset of the incoming values of the old PHI node) have the same value,
+    // eliminate the PHI Node.
+    if (HasUniqueIncomingValue) {
+      NewPN->replaceAllUsesWith(UniqueValue);
+      if (AA) AA->deleteValue(NewPN);
+      BEBlock->getInstList().erase(NewPN);
+    }
+  }
+
+  // Now that all of the PHI nodes have been inserted and adjusted, modify the
+  // backedge blocks to just to the BEBlock instead of the header.
+  for (unsigned i = 0, e = BackedgeBlocks.size(); i != e; ++i) {
+    TerminatorInst *TI = BackedgeBlocks[i]->getTerminator();
+    for (unsigned Op = 0, e = TI->getNumSuccessors(); Op != e; ++Op)
+      if (TI->getSuccessor(Op) == Header)
+        TI->setSuccessor(Op, BEBlock);
+  }
+
+  //===--- Update all analyses which we must preserve now -----------------===//
+
+  // Update Loop Information - we know that this block is now in the current
+  // loop and all parent loops.
+  L->addBasicBlockToLoop(BEBlock, LI->getBase());
+
+  // Update dominator information
+  DT->splitBlock(BEBlock);
+  if (DominanceFrontier *DF = getAnalysisIfAvailable<DominanceFrontier>())
+    DF->splitBlock(BEBlock);
+}