Index: llvm/trunk/include/llvm/ADT/PostOrderIterator.h
===================================================================
--- llvm/trunk/include/llvm/ADT/PostOrderIterator.h
+++ llvm/trunk/include/llvm/ADT/PostOrderIterator.h
@@ -19,6 +19,7 @@
 #include "llvm/ADT/GraphTraits.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/iterator_range.h"
+#include "llvm/ADT/Optional.h"
 #include <set>
 #include <vector>
 
@@ -56,14 +57,13 @@
   SetType Visited;
 public:
   // Return true if edge destination should be visited.
-  template<typename NodeType>
-  bool insertEdge(NodeType *From, NodeType *To) {
+  template <typename NodeRef>
+  bool insertEdge(Optional<NodeRef> From, NodeRef To) {
     return Visited.insert(To).second;
   }
 
   // Called after all children of BB have been visited.
-  template<typename NodeType>
-  void finishPostorder(NodeType *BB) {}
+  template <typename NodeRef> void finishPostorder(NodeRef BB) {}
 };
 
 /// Specialization of po_iterator_storage that references an external set.
@@ -77,51 +77,49 @@
   // Return true if edge destination should be visited, called with From = 0 for
   // the root node.
   // Graph edges can be pruned by specializing this function.
-  template <class NodeType> bool insertEdge(NodeType *From, NodeType *To) {
+  template <class NodeRef> bool insertEdge(Optional<NodeRef> From, NodeRef To) {
     return Visited.insert(To).second;
   }
 
   // Called after all children of BB have been visited.
-  template<class NodeType>
-  void finishPostorder(NodeType *BB) {}
+  template <class NodeRef> void finishPostorder(NodeRef BB) {}
 };
 
-template<class GraphT,
-  class SetType = llvm::SmallPtrSet<typename GraphTraits<GraphT>::NodeType*, 8>,
-  bool ExtStorage = false,
-  class GT = GraphTraits<GraphT> >
-class po_iterator : public std::iterator<std::forward_iterator_tag,
-                                         typename GT::NodeType, ptrdiff_t>,
-                    public po_iterator_storage<SetType, ExtStorage> {
-  typedef std::iterator<std::forward_iterator_tag,
-                        typename GT::NodeType, ptrdiff_t> super;
-  typedef typename GT::NodeType          NodeType;
+template <class GraphT,
+          class SetType =
+              llvm::SmallPtrSet<typename GraphTraits<GraphT>::NodeRef, 8>,
+          bool ExtStorage = false, class GT = GraphTraits<GraphT>>
+class po_iterator
+    : public std::iterator<std::forward_iterator_tag, typename GT::NodeRef>,
+      public po_iterator_storage<SetType, ExtStorage> {
+  typedef std::iterator<std::forward_iterator_tag, typename GT::NodeRef> super;
+  typedef typename GT::NodeRef NodeRef;
   typedef typename GT::ChildIteratorType ChildItTy;
 
   // VisitStack - Used to maintain the ordering.  Top = current block
   // First element is basic block pointer, second is the 'next child' to visit
-  std::vector<std::pair<NodeType *, ChildItTy> > VisitStack;
+  std::vector<std::pair<NodeRef, ChildItTy>> VisitStack;
 
   void traverseChild() {
     while (VisitStack.back().second != GT::child_end(VisitStack.back().first)) {
-      NodeType *BB = *VisitStack.back().second++;
-      if (this->insertEdge(VisitStack.back().first, BB)) {
+      NodeRef BB = *VisitStack.back().second++;
+      if (this->insertEdge(Optional<NodeRef>(VisitStack.back().first), BB)) {
         // If the block is not visited...
         VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
       }
     }
   }
 
-  po_iterator(NodeType *BB) {
-    this->insertEdge((NodeType*)nullptr, BB);
+  po_iterator(NodeRef BB) {
+    this->insertEdge(Optional<NodeRef>(), BB);
     VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
     traverseChild();
   }
   po_iterator() {} // End is when stack is empty.
 
-  po_iterator(NodeType *BB, SetType &S)
+  po_iterator(NodeRef BB, SetType &S)
       : po_iterator_storage<SetType, ExtStorage>(S) {
-    if (this->insertEdge((NodeType*)nullptr, BB)) {
+    if (this->insertEdge(Optional<NodeRef>(), BB)) {
       VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));
       traverseChild();
     }
@@ -149,13 +147,13 @@
   }
   bool operator!=(const po_iterator &x) const { return !(*this == x); }
 
-  pointer operator*() const { return VisitStack.back().first; }
+  const NodeRef &operator*() const { return VisitStack.back().first; }
 
   // This is a nonstandard operator-> that dereferences the pointer an extra
   // time... so that you can actually call methods ON the BasicBlock, because
   // the contained type is a pointer.  This allows BBIt->getTerminator() f.e.
   //
-  NodeType *operator->() const { return **this; }
+  NodeRef operator->() const { return **this; }
 
   po_iterator &operator++() { // Preincrement
     this->finishPostorder(VisitStack.back().first);
@@ -184,7 +182,7 @@
 }
 
 // Provide global definitions of external postorder iterators...
-template<class T, class SetType=std::set<typename GraphTraits<T>::NodeType*> >
+template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
 struct po_ext_iterator : public po_iterator<T, SetType, true> {
   po_ext_iterator(const po_iterator<T, SetType, true> &V) :
   po_iterator<T, SetType, true>(V) {}
@@ -206,10 +204,9 @@
 }
 
 // Provide global definitions of inverse post order iterators...
-template <class T,
-          class SetType = std::set<typename GraphTraits<T>::NodeType*>,
+template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>,
           bool External = false>
-struct ipo_iterator : public po_iterator<Inverse<T>, SetType, External > {
+struct ipo_iterator : public po_iterator<Inverse<T>, SetType, External> {
   ipo_iterator(const po_iterator<Inverse<T>, SetType, External> &V) :
      po_iterator<Inverse<T>, SetType, External> (V) {}
 };
@@ -230,8 +227,7 @@
 }
 
 // Provide global definitions of external inverse postorder iterators...
-template <class T,
-          class SetType = std::set<typename GraphTraits<T>::NodeType*> >
+template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>
 struct ipo_ext_iterator : public ipo_iterator<T, SetType, true> {
   ipo_ext_iterator(const ipo_iterator<T, SetType, true> &V) :
     ipo_iterator<T, SetType, true>(V) {}
@@ -280,13 +276,13 @@
 
 template<class GraphT, class GT = GraphTraits<GraphT> >
 class ReversePostOrderTraversal {
-  typedef typename GT::NodeType NodeType;
-  std::vector<NodeType*> Blocks;       // Block list in normal PO order
-  void Initialize(NodeType *BB) {
+  typedef typename GT::NodeRef NodeRef;
+  std::vector<NodeRef> Blocks; // Block list in normal PO order
+  void Initialize(NodeRef BB) {
     std::copy(po_begin(BB), po_end(BB), std::back_inserter(Blocks));
   }
 public:
-  typedef typename std::vector<NodeType*>::reverse_iterator rpo_iterator;
+  typedef typename std::vector<NodeRef>::reverse_iterator rpo_iterator;
 
   ReversePostOrderTraversal(GraphT G) { Initialize(GT::getEntryNode(G)); }
 
Index: llvm/trunk/include/llvm/Analysis/LoopIterator.h
===================================================================
--- llvm/trunk/include/llvm/Analysis/LoopIterator.h
+++ llvm/trunk/include/llvm/Analysis/LoopIterator.h
@@ -114,7 +114,7 @@
 public:
   po_iterator_storage(LoopBlocksTraversal &lbs) : LBT(lbs) {}
   // These functions are defined below.
-  bool insertEdge(BasicBlock *From, BasicBlock *To);
+  bool insertEdge(Optional<BasicBlock *> From, BasicBlock *To);
   void finishPostorder(BasicBlock *BB);
 };
 
@@ -166,8 +166,8 @@
   }
 };
 
-inline bool po_iterator_storage<LoopBlocksTraversal, true>::
-insertEdge(BasicBlock *From, BasicBlock *To) {
+inline bool po_iterator_storage<LoopBlocksTraversal, true>::insertEdge(
+    Optional<BasicBlock *> From, BasicBlock *To) {
   return LBT.visitPreorder(To);
 }
 
Index: llvm/trunk/include/llvm/IR/Type.h
===================================================================
--- llvm/trunk/include/llvm/IR/Type.h
+++ llvm/trunk/include/llvm/IR/Type.h
@@ -430,6 +430,7 @@
 
 template <> struct GraphTraits<Type *> {
   typedef Type NodeType;
+  typedef Type *NodeRef;
   typedef Type::subtype_iterator ChildIteratorType;
 
   static inline NodeType *getEntryNode(Type *T) { return T; }
@@ -443,6 +444,7 @@
 
 template <> struct GraphTraits<const Type*> {
   typedef const Type NodeType;
+  typedef const Type *NodeRef;
   typedef Type::subtype_iterator ChildIteratorType;
 
   static inline NodeType *getEntryNode(NodeType *T) { return T; }
Index: llvm/trunk/lib/CodeGen/MachineTraceMetrics.cpp
===================================================================
--- llvm/trunk/lib/CodeGen/MachineTraceMetrics.cpp
+++ llvm/trunk/lib/CodeGen/MachineTraceMetrics.cpp
@@ -430,16 +430,17 @@
   po_iterator_storage(LoopBounds &lb) : LB(lb) {}
   void finishPostorder(const MachineBasicBlock*) {}
 
-  bool insertEdge(const MachineBasicBlock *From, const MachineBasicBlock *To) {
+  bool insertEdge(Optional<const MachineBasicBlock *> From,
+                  const MachineBasicBlock *To) {
     // Skip already visited To blocks.
     MachineTraceMetrics::TraceBlockInfo &TBI = LB.Blocks[To->getNumber()];
     if (LB.Downward ? TBI.hasValidHeight() : TBI.hasValidDepth())
       return false;
     // From is null once when To is the trace center block.
     if (From) {
-      if (const MachineLoop *FromLoop = LB.Loops->getLoopFor(From)) {
+      if (const MachineLoop *FromLoop = LB.Loops->getLoopFor(*From)) {
         // Don't follow backedges, don't leave FromLoop when going upwards.
-        if ((LB.Downward ? To : From) == FromLoop->getHeader())
+        if ((LB.Downward ? To : *From) == FromLoop->getHeader())
           return false;
         // Don't leave FromLoop.
         if (isExitingLoop(FromLoop, LB.Loops->getLoopFor(To)))
Index: llvm/trunk/unittests/ADT/PostOrderIteratorTest.cpp
===================================================================
--- llvm/trunk/unittests/ADT/PostOrderIteratorTest.cpp
+++ llvm/trunk/unittests/ADT/PostOrderIteratorTest.cpp
@@ -21,17 +21,17 @@
   // Tests that template specializations are kept up to date
   void *Null = nullptr;
   po_iterator_storage<std::set<void *>, false> PIS;
-  PIS.insertEdge(Null, Null);
+  PIS.insertEdge(Optional<void *>(), Null);
   ExtSetTy Ext;
   po_iterator_storage<ExtSetTy, true> PISExt(Ext);
-  PIS.insertEdge(Null, Null);
+  PIS.insertEdge(Optional<void *>(), Null);
 
   // Test above, but going through po_iterator (which inherits from template
   // base)
   BasicBlock *NullBB = nullptr;
   auto PI = po_end(NullBB);
-  PI.insertEdge(NullBB, NullBB);
+  PI.insertEdge(Optional<BasicBlock *>(), NullBB);
   auto PIExt = po_ext_end(NullBB, Ext);
-  PIExt.insertEdge(NullBB, NullBB);
+  PIExt.insertEdge(Optional<BasicBlock *>(), NullBB);
 }
 }