path: root/include/llvm/Analysis/LoopInfo.h

//===- llvm/Analysis/LoopInfo.h - Natural Loop Calculator -------*- C++ -*-===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This file defines the LoopInfo class that is used to identify natural loops
// and determine the loop depth of various nodes of the CFG.  A natural loop
// has exactly one entry-point, which is called the header. Note that natural
// loops may actually be several loops that share the same header node.
//
// This analysis calculates the nesting structure of loops in a function.  For
// each natural loop identified, this analysis identifies natural loops
// contained entirely within the loop and the basic blocks the make up the loop.
//
// It can calculate on the fly various bits of information, for example:
//
//  * whether there is a preheader for the loop
//  * the number of back edges to the header
//  * whether or not a particular block branches out of the loop
//  * the successor blocks of the loop
//  * the loop depth
//  * etc...
//
//===----------------------------------------------------------------------===//

#ifndef LLVM_ANALYSIS_LOOPINFO_H
#define LLVM_ANALYSIS_LOOPINFO_H

#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/DepthFirstIterator.h"
#include "llvm/ADT/GraphTraits.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Pass.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <map>

namespace llvm {

template<typename T>
inline void RemoveFromVector(std::vector<T*> &V, T *N) {
  typename std::vector<T*>::iterator I = std::find(V.begin(), V.end(), N);
  assert(I != V.end() && "N is not in this list!");
  V.erase(I);
}

class DominatorTree;
class LoopInfo;
class Loop;
class PHINode;
template<class N, class M> class LoopInfoBase;
template<class N, class M> class LoopBase;

//===----------------------------------------------------------------------===//
/// LoopBase class - Instances of this class are used to represent loops that
/// are detected in the flow graph
///
template<class BlockT, class LoopT>
class LoopBase {
  LoopT *ParentLoop;
  // SubLoops - Loops contained entirely within this one.
  std::vector<LoopT *> SubLoops;

  // Blocks - The list of blocks in this loop.  First entry is the header node.
  std::vector<BlockT*> Blocks;

  LoopBase(const LoopBase<BlockT, LoopT> &) LLVM_DELETED_FUNCTION;
  const LoopBase<BlockT, LoopT>&
    operator=(const LoopBase<BlockT, LoopT> &) LLVM_DELETED_FUNCTION;
public:
  /// Loop ctor - This creates an empty loop.
  LoopBase() : ParentLoop(0) {}
  ~LoopBase() {
    for (size_t i = 0, e = SubLoops.size(); i != e; ++i)
      delete SubLoops[i];
  }

  /// getLoopDepth - Return the nesting level of this loop.  An outer-most
  /// loop has depth 1, for consistency with loop depth values used for basic
  /// blocks, where depth 0 is used for blocks not inside any loops.
  unsigned getLoopDepth() const {
    unsigned D = 1;
    for (const LoopT *CurLoop = ParentLoop; CurLoop;
         CurLoop = CurLoop->ParentLoop)
      ++D;
    return D;
  }
  BlockT *getHeader() const { return Blocks.front(); }
  LoopT *getParentLoop() const { return ParentLoop; }

  /// setParentLoop is a raw interface for bypassing addChildLoop.
  void setParentLoop(LoopT *L) { ParentLoop = L; }

  /// contains - Return true if the specified loop is contained within in
  /// this loop.
  ///
  bool contains(const LoopT *L) const {
    if (L == this) return true;
    if (L == 0)    return false;
    return contains(L->getParentLoop());
  }

  /// contains - Return true if the specified basic block is in this loop.
  ///
  bool contains(const BlockT *BB) const {
    return std::find(block_begin(), block_end(), BB) != block_end();
  }

  /// contains - Return true if the specified instruction is in this loop.
  ///
  template<class InstT>
  bool contains(const InstT *Inst) const {
    return contains(Inst->getParent());
  }

  /// iterator/begin/end - Return the loops contained entirely within this loop.
  ///
  const std::vector<LoopT *> &getSubLoops() const { return SubLoops; }
  std::vector<LoopT *> &getSubLoopsVector() { return SubLoops; }
  typedef typename std::vector<LoopT *>::const_iterator iterator;
  typedef typename std::vector<LoopT *>::const_reverse_iterator
    reverse_iterator;
  iterator begin() const { return SubLoops.begin(); }
  iterator end() const { return SubLoops.end(); }
  reverse_iterator rbegin() const { return SubLoops.rbegin(); }
  reverse_iterator rend() const { return SubLoops.rend(); }
  bool empty() const { return SubLoops.empty(); }

  /// getBlocks - Get a list of the basic blocks which make up this loop.
  ///
  const std::vector<BlockT*> &getBlocks() const { return Blocks; }
  std::vector<BlockT*> &getBlocksVector() { return Blocks; }
  typedef typename std::vector<BlockT*>::const_iterator block_iterator;
  block_iterator block_begin() const { return Blocks.begin(); }
  block_iterator block_end() const { return Blocks.end(); }

  /// getNumBlocks - Get the number of blocks in this loop in constant time.
  unsigned getNumBlocks() const {
    return Blocks.size();
  }

  /// isLoopExiting - True if terminator in the block can branch to another
  /// block that is outside of the current loop.
  ///
  bool isLoopExiting(const BlockT *BB) const {
    typedef GraphTraits<BlockT*> BlockTraits;
    for (typename BlockTraits::ChildIteratorType SI =
         BlockTraits::child_begin(const_cast<BlockT*>(BB)),
         SE = BlockTraits::child_end(const_cast<BlockT*>(BB)); SI != SE; ++SI) {
      if (!contains(*SI))
        return true;
    }
    return false;
  }

  /// getNumBackEdges - Calculate the number of back edges to the loop header
  ///
  unsigned getNumBackEdges() const {
    unsigned NumBackEdges = 0;
    BlockT *H = getHeader();

    typedef GraphTraits<Inverse<BlockT*> > InvBlockTraits;
    for (typename InvBlockTraits::ChildIteratorType I =
         InvBlockTraits::child_begin(const_cast<BlockT*>(H)),
         E = InvBlockTraits::child_end(const_cast<BlockT*>(H)); I != E; ++I)
      if (contains(*I))
        ++NumBackEdges;

    return NumBackEdges;
  }

  //===--------------------------------------------------------------------===//
  // APIs for simple analysis of the loop.
  //
  // Note that all of these methods can fail on general loops (ie, there may not
  // be a preheader, etc).  For best success, the loop simplification and
  // induction variable canonicalization pass should be used to normalize loops
  // for easy analysis.  These methods assume canonical loops.

  /// getExitingBlocks - Return all blocks inside the loop that have successors
  /// outside of the loop.  These are the blocks _inside of the current loop_
  /// which branch out.  The returned list is always unique.
  ///
  void getExitingBlocks(SmallVectorImpl<BlockT *> &ExitingBlocks) const;

  /// getExitingBlock - If getExitingBlocks would return exactly one block,
  /// return that block. Otherwise return null.
  BlockT *getExitingBlock() const;

  /// getExitBlocks - Return all of the successor blocks of this loop.  These
  /// are the blocks _outside of the current loop_ which are branched to.
  ///
  void getExitBlocks(SmallVectorImpl<BlockT*> &ExitBlocks) const;

  /// getExitBlock - If getExitBlocks would return exactly one block,
  /// return that block. Otherwise return null.
  BlockT *getExitBlock() const;

  /// Edge type.
  typedef std::pair<const BlockT*, const BlockT*> Edge;

  /// getExitEdges - Return all pairs of (_inside_block_,_outside_block_).
  void getExitEdges(SmallVectorImpl<Edge> &ExitEdges) const;

  /// getLoopPreheader - If there is a preheader for this loop, return it.  A
  /// loop has a preheader if there is only one edge to the header of the loop
  /// from outside of the loop.  If this is the case, the block branching to the
  /// header of the loop is the preheader node.
  ///
  /// This method returns null if there is no preheader for the loop.
  ///
  BlockT *getLoopPreheader() const;

  /// getLoopPredecessor - If the given loop's header has exactly one unique
  /// predecessor outside the loop, return it. Otherwise return null.
  /// This is less strict that the loop "preheader" concept, which requires
  /// the predecessor to have exactly one successor.
  ///
  BlockT *getLoopPredecessor() const;

  /// getLoopLatch - If there is a single latch block for this loop, return it.
  /// A latch block is a block that contains a branch back to the header.
  BlockT *getLoopLatch() const;

  //===--------------------------------------------------------------------===//
  // APIs for updating loop information after changing the CFG
  //

  /// addBasicBlockToLoop - This method is used by other analyses to update loop
  /// information.  NewBB is set to be a new member of the current loop.
  /// Because of this, it is added as a member of all parent loops, and is added
  /// to the specified LoopInfo object as being in the current basic block.  It
  /// is not valid to replace the loop header with this method.
  ///
  void addBasicBlockToLoop(BlockT *NewBB, LoopInfoBase<BlockT, LoopT> &LI);

  /// replaceChildLoopWith - This is used when splitting loops up.  It replaces
  /// the OldChild entry in our children list with NewChild, and updates the
  /// parent pointer of OldChild to be null and the NewChild to be this loop.
  /// This updates the loop depth of the new child.
  void replaceChildLoopWith(LoopT *OldChild, LoopT *NewChild);

  /// addChildLoop - Add the specified loop to be a child of this loop.  This
  /// updates the loop depth of the new child.
  ///
  void addChildLoop(LoopT *NewChild) {
    assert(NewChild->ParentLoop == 0 && "NewChild already has a parent!");
    NewChild->ParentLoop = static_cast<LoopT *>(this);
    SubLoops.push_back(NewChild);
  }

  /// removeChildLoop - This removes the specified child from being a subloop of
  /// this loop.  The loop is not deleted, as it will presumably be inserted
  /// into another loop.
  LoopT *removeChildLoop(iterator I) {
    assert(I != SubLoops.end() && "Cannot remove end iterator!");
    LoopT *Child = *I;
    assert(Child->ParentLoop == this && "Child is not a child of this loop!");
    SubLoops.erase(SubLoops.begin()+(I-begin()));
    Child->ParentLoop = 0;
    return Child;
  }

  /// addBlockEntry - This adds a basic block directly to the basic block list.
  /// This should only be used by transformations that create new loops.  Other
  /// transformations should use addBasicBlockToLoop.
  void addBlockEntry(BlockT *BB) {
    Blocks.push_back(BB);
  }

  /// moveToHeader - This method is used to move BB (which must be part of this
  /// loop) to be the loop header of the loop (the block that dominates all
  /// others).
  void moveToHeader(BlockT *BB) {
    if (Blocks[0] == BB) return;
    for (unsigned i = 0; ; ++i) {
      assert(i != Blocks.size() && "Loop does not contain BB!");
      if (Blocks[i] == BB) {
        Blocks[i] = Blocks[0];
        Blocks[0] = BB;
        return;
      }
    }
  }

  /// removeBlockFromLoop - This removes the specified basic block from the
  /// current loop, updating the Blocks as appropriate.  This does not update
  /// the mapping in the LoopInfo class.
  void removeBlockFromLoop(BlockT *BB) {
    RemoveFromVector(Blocks, BB);
  }

  /// verifyLoop - Verify loop structure
  void verifyLoop() const;

  /// verifyLoop - Verify loop structure of this loop and all nested loops.
  void verifyLoopNest(DenseSet<const LoopT*> *Loops) const;

  void print(raw_ostream &OS, unsigned Depth = 0) const;

protected:
  friend class LoopInfoBase<BlockT, LoopT>;
  explicit LoopBase(BlockT *BB) : ParentLoop(0) {
    Blocks.push_back(BB);
  }
};

template<class BlockT, class LoopT>
raw_ostream& operator<<(raw_ostream &OS, const LoopBase<BlockT, LoopT> &Loop) {
  Loop.print(OS);
  return OS;
}

// Implementation in LoopInfoImpl.h
#ifdef __GNUC__
__extension__ extern template class LoopBase<BasicBlock, Loop>;
#endif

class Loop : public LoopBase<BasicBlock, Loop> {
public:
  Loop() {}

  /// isLoopInvariant - Return true if the specified value is loop invariant
  ///
  bool isLoopInvariant(Value *V) const;

  /// hasLoopInvariantOperands - Return true if all the operands of the
  /// specified instruction are loop invariant.
  bool hasLoopInvariantOperands(Instruction *I) const;

  /// makeLoopInvariant - If the given value is an instruction inside of the
  /// loop and it can be hoisted, do so to make it trivially loop-invariant.
  /// Return true if the value after any hoisting is loop invariant. This
  /// function can be used as a slightly more aggressive replacement for
  /// isLoopInvariant.
  ///
  /// If InsertPt is specified, it is the point to hoist instructions to.
  /// If null, the terminator of the loop preheader is used.
  ///
  bool makeLoopInvariant(Value *V, bool &Changed,
                         Instruction *InsertPt = 0) const;

  /// makeLoopInvariant - If the given instruction is inside of the
  /// loop and it can be hoisted, do so to make it trivially loop-invariant.
  /// Return true if the instruction after any hoisting is loop invariant. This
  /// function can be used as a slightly more aggressive replacement for
  /// isLoopInvariant.
  ///
  /// If InsertPt is specified, it is the point to hoist instructions to.
  /// If null, the terminator of the loop preheader is used.
  ///
  bool makeLoopInvariant(Instruction *I, bool &Changed,
                         Instruction *InsertPt = 0) const;

  /// getCanonicalInductionVariable - Check to see if the loop has a canonical
  /// induction variable: an integer recurrence that starts at 0 and increments
  /// by one each time through the loop.  If so, return the phi node that
  /// corresponds to it.
  ///
  /// The IndVarSimplify pass transforms loops to have a canonical induction
  /// variable.
  ///
  PHINode *getCanonicalInductionVariable() const;

  /// isLCSSAForm - Return true if the Loop is in LCSSA form
  bool isLCSSAForm(DominatorTree &DT) const;

  /// isLoopSimplifyForm - Return true if the Loop is in the form that
  /// the LoopSimplify form transforms loops to, which is sometimes called
  /// normal form.
  bool isLoopSimplifyForm() const;

  /// isSafeToClone - Return true if the loop body is safe to clone in practice.
  bool isSafeToClone() const;

  /// hasDedicatedExits - Return true if no exit block for the loop
  /// has a predecessor that is outside the loop.
  bool hasDedicatedExits() const;

  /// getUniqueExitBlocks - Return all unique successor blocks of this loop.
  /// These are the blocks _outside of the current loop_ which are branched to.
  /// This assumes that loop exits are in canonical form.
  ///
  void getUniqueExitBlocks(SmallVectorImpl<BasicBlock *> &ExitBlocks) const;

  /// getUniqueExitBlock - If getUniqueExitBlocks would return exactly one
  /// block, return that block. Otherwise return null.
  BasicBlock *getUniqueExitBlock() const;

  void dump() const;

private:
  friend class LoopInfoBase<BasicBlock, Loop>;
  explicit Loop(BasicBlock *BB) : LoopBase<BasicBlock, Loop>(BB) {}
};

//===----------------------------------------------------------------------===//
/// LoopInfo - This class builds and contains all of the top level loop
/// structures in the specified function.
///

template<class BlockT, class LoopT>
class LoopInfoBase {
  // BBMap - Mapping of basic blocks to the inner most loop they occur in
  DenseMap<BlockT *, LoopT *> BBMap;
  std::vector<LoopT *> TopLevelLoops;
  friend class LoopBase<BlockT, LoopT>;
  friend class LoopInfo;

  void operator=(const LoopInfoBase &) LLVM_DELETED_FUNCTION;
  LoopInfoBase(const LoopInfo &) LLVM_DELETED_FUNCTION;
public:
  LoopInfoBase() { }
  ~LoopInfoBase() { releaseMemory(); }

  void releaseMemory() {
    for (typename std::vector<LoopT *>::iterator I =
         TopLevelLoops.begin(), E = TopLevelLoops.end(); I != E; ++I)
      delete *I;   // Delete all of the loops...

    BBMap.clear();                           // Reset internal state of analysis
    TopLevelLoops.clear();
  }

  /// iterator/begin/end - The interface to the top-level loops in the current
  /// function.
  ///
  typedef typename std::vector<LoopT *>::const_iterator iterator;
  typedef typename std::vector<LoopT *>::const_reverse_iterator
    reverse_iterator;
  iterator begin() const { return TopLevelLoops.begin(); }
  iterator end() const { return TopLevelLoops.end(); }
  reverse_iterator rbegin() const { return TopLevelLoops.rbegin(); }
  reverse_iterator rend() const { return TopLevelLoops.rend(); }
  bool empty() const { return TopLevelLoops.empty(); }

  /// getLoopFor - Return the inner most loop that BB lives in.  If a basic
  /// block is in no loop (for example the entry node), null is returned.
  ///
  LoopT *getLoopFor(const BlockT *BB) const {
    return BBMap.lookup(const_cast<BlockT*>(BB));
  }

  /// operator[] - same as getLoopFor...
  ///
  const LoopT *operator[](const BlockT *BB) const {
    return getLoopFor(BB);
  }

  /// getLoopDepth - Return the loop nesting level of the specified block.  A
  /// depth of 0 means the block is not inside any loop.
  ///
  unsigned getLoopDepth(const BlockT *BB) const {
    const LoopT *L = getLoopFor(BB);
    return L ? L->getLoopDepth() : 0;
  }

  // isLoopHeader - True if the block is a loop header node
  bool isLoopHeader(BlockT *BB) const {
    const LoopT *L = getLoopFor(BB);
    return L && L->getHeader() == BB;
  }

  /// removeLoop - This removes the specified top-level loop from this loop info
  /// object.  The loop is not deleted, as it will presumably be inserted into
  /// another loop.
  LoopT *removeLoop(iterator I) {
    assert(I != end() && "Cannot remove end iterator!");
    LoopT *L = *I;
    assert(L->getParentLoop() == 0 && "Not a top-level loop!");
    TopLevelLoops.erase(TopLevelLoops.begin() + (I-begin()));
    return L;
  }

  /// changeLoopFor - Change the top-level loop that contains BB to the
  /// specified loop.  This should be used by transformations that restructure
  /// the loop hierarchy tree.
  void changeLoopFor(BlockT *BB, LoopT *L) {
    if (!L) {
      BBMap.erase(BB);
      return;
    }
    BBMap[BB] = L;
  }

  /// changeTopLevelLoop - Replace the specified loop in the top-level loops
  /// list with the indicated loop.
  void changeTopLevelLoop(LoopT *OldLoop,
                          LoopT *NewLoop) {
    typename std::vector<LoopT *>::iterator I =
                 std::find(TopLevelLoops.begin(), TopLevelLoops.end(), OldLoop);
    assert(I != TopLevelLoops.end() && "Old loop not at top level!");
    *I = NewLoop;
    assert(NewLoop->ParentLoop == 0 && OldLoop->ParentLoop == 0 &&
           "Loops already embedded into a subloop!");
  }

  /// addTopLevelLoop - This adds the specified loop to the collection of
  /// top-level loops.
  void addTopLevelLoop(LoopT *New) {
    assert(New->getParentLoop() == 0 && "Loop already in subloop!");
    TopLevelLoops.push_back(New);
  }

  /// removeBlock - This method completely removes BB from all data structures,
  /// including all of the Loop objects it is nested in and our mapping from
  /// BasicBlocks to loops.
  void removeBlock(BlockT *BB) {
    typename DenseMap<BlockT *, LoopT *>::iterator I = BBMap.find(BB);
    if (I != BBMap.end()) {
      for (LoopT *L = I->second; L; L = L->getParentLoop())
        L->removeBlockFromLoop(BB);

      BBMap.erase(I);
    }
  }

  // Internals

  static bool isNotAlreadyContainedIn(const LoopT *SubLoop,
                                      const LoopT *ParentLoop) {
    if (SubLoop == 0) return true;
    if (SubLoop == ParentLoop) return false;
    return isNotAlreadyContainedIn(SubLoop->getParentLoop(), ParentLoop);
  }

  /// Create the loop forest using a stable algorithm.
  void Analyze(DominatorTreeBase<BlockT> &DomTree);

  // Debugging

  void print(raw_ostream &OS) const;
};

// Implementation in LoopInfoImpl.h
#ifdef __GNUC__
__extension__ extern template class LoopInfoBase<BasicBlock, Loop>;
#endif

class LoopInfo : public FunctionPass {
  LoopInfoBase<BasicBlock, Loop> LI;
  friend class LoopBase<BasicBlock, Loop>;

  void operator=(const LoopInfo &) LLVM_DELETED_FUNCTION;
  LoopInfo(const LoopInfo &) LLVM_DELETED_FUNCTION;
public:
  static char ID; // Pass identification, replacement for typeid

  LoopInfo() : FunctionPass(ID) {
    initializeLoopInfoPass(*PassRegistry::getPassRegistry());
  }

  LoopInfoBase<BasicBlock, Loop>& getBase() { return LI; }

  /// iterator/begin/end - The interface to the top-level loops in the current
  /// function.
  ///
  typedef LoopInfoBase<BasicBlock, Loop>::iterator iterator;
  typedef LoopInfoBase<BasicBlock, Loop>::reverse_iterator reverse_iterator;
  inline iterator begin() const { return LI.begin(); }
  inline iterator end() const { return LI.end(); }
  inline reverse_iterator rbegin() const { return LI.rbegin(); }
  inline reverse_iterator rend() const { return LI.rend(); }
  bool empty() const { return LI.empty(); }

  /// getLoopFor - Return the inner most loop that BB lives in.  If a basic
  /// block is in no loop (for example the entry node), null is returned.
  ///
  inline Loop *getLoopFor(const BasicBlock *BB) const {
    return LI.getLoopFor(BB);
  }

  /// operator[] - same as getLoopFor...
  ///
  inline const Loop *operator[](const BasicBlock *BB) const {
    return LI.getLoopFor(BB);
  }

  /// getLoopDepth - Return the loop nesting level of the specified block.  A
  /// depth of 0 means the block is not inside any loop.
  ///
  inline unsigned getLoopDepth(const BasicBlock *BB) const {
    return LI.getLoopDepth(BB);
  }

  // isLoopHeader - True if the block is a loop header node
  inline bool isLoopHeader(BasicBlock *BB) const {
    return LI.isLoopHeader(BB);
  }

  /// runOnFunction - Calculate the natural loop information.
  ///
  virtual bool runOnFunction(Function &F);

  virtual void verifyAnalysis() const;

  virtual void releaseMemory() { LI.releaseMemory(); }

  virtual void print(raw_ostream &O, const Module* M = 0) const;

  virtual void getAnalysisUsage(AnalysisUsage &AU) const;

  /// removeLoop - This removes the specified top-level loop from this loop info
  /// object.  The loop is not deleted, as it will presumably be inserted into
  /// another loop.
  inline Loop *removeLoop(iterator I) { return LI.removeLoop(I); }

  /// changeLoopFor - Change the top-level loop that contains BB to the
  /// specified loop.  This should be used by transformations that restructure
  /// the loop hierarchy tree.
  inline void changeLoopFor(BasicBlock *BB, Loop *L) {
    LI.changeLoopFor(BB, L);
  }

  /// changeTopLevelLoop - Replace the specified loop in the top-level loops
  /// list with the indicated loop.
  inline void changeTopLevelLoop(Loop *OldLoop, Loop *NewLoop) {
    LI.changeTopLevelLoop(OldLoop, NewLoop);
  }

  /// addTopLevelLoop - This adds the specified loop to the collection of
  /// top-level loops.
  inline void addTopLevelLoop(Loop *New) {
    LI.addTopLevelLoop(New);
  }

  /// removeBlock - This method completely removes BB from all data structures,
  /// including all of the Loop objects it is nested in and our mapping from
  /// BasicBlocks to loops.
  void removeBlock(BasicBlock *BB) {
    LI.removeBlock(BB);
  }

  /// updateUnloop - Update LoopInfo after removing the last backedge from a
  /// loop--now the "unloop". This updates the loop forest and parent loops for
  /// each block so that Unloop is no longer referenced, but the caller must
  /// actually delete the Unloop object.
  void updateUnloop(Loop *Unloop);

  /// replacementPreservesLCSSAForm - Returns true if replacing From with To
  /// everywhere is guaranteed to preserve LCSSA form.
  bool replacementPreservesLCSSAForm(Instruction *From, Value *To) {
    // Preserving LCSSA form is only problematic if the replacing value is an
    // instruction.
    Instruction *I = dyn_cast<Instruction>(To);
    if (!I) return true;
    // If both instructions are defined in the same basic block then replacement
    // cannot break LCSSA form.
    if (I->getParent() == From->getParent())
      return true;
    // If the instruction is not defined in a loop then it can safely replace
    // anything.
    Loop *ToLoop = getLoopFor(I->getParent());
    if (!ToLoop) return true;
    // If the replacing instruction is defined in the same loop as the original
    // instruction, or in a loop that contains it as an inner loop, then using
    // it as a replacement will not break LCSSA form.
    return ToLoop->contains(getLoopFor(From->getParent()));
  }
};


// Allow clients to walk the list of nested loops...
template <> struct GraphTraits<const Loop*> {
  typedef const Loop NodeType;
  typedef LoopInfo::iterator ChildIteratorType;

  static NodeType *getEntryNode(const Loop *L) { return L; }
  static inline ChildIteratorType child_begin(NodeType *N) {
    return N->begin();
  }
  static inline ChildIteratorType child_end(NodeType *N) {
    return N->end();
  }
};

template <> struct GraphTraits<Loop*> {
  typedef Loop NodeType;
  typedef LoopInfo::iterator ChildIteratorType;

  static NodeType *getEntryNode(Loop *L) { return L; }
  static inline ChildIteratorType child_begin(NodeType *N) {
    return N->begin();
  }
  static inline ChildIteratorType child_end(NodeType *N) {
    return N->end();
  }
};

} // End llvm namespace

#endif