Index: llvm/trunk/unittests/ADT/SmallSetTest.cpp
===================================================================
--- llvm/trunk/unittests/ADT/SmallSetTest.cpp	(revision 336804)
+++ llvm/trunk/unittests/ADT/SmallSetTest.cpp	(revision 336805)
@@ -1,70 +1,125 @@
 //===- llvm/unittest/ADT/SmallSetTest.cpp ------------------------------===//
 //
 //                     The LLVM Compiler Infrastructure
 //
 // This file is distributed under the University of Illinois Open Source
 // License. See LICENSE.TXT for details.
 //
 //===----------------------------------------------------------------------===//
 //
 // SmallSet unit tests.
 //
 //===----------------------------------------------------------------------===//
 
 #include "llvm/ADT/SmallSet.h"
 #include "gtest/gtest.h"
+#include <string>
 
 using namespace llvm;
 
 TEST(SmallSetTest, Insert) {
 
   SmallSet<int, 4> s1;
 
   for (int i = 0; i < 4; i++)
     s1.insert(i);
 
   for (int i = 0; i < 4; i++)
     s1.insert(i);
 
   EXPECT_EQ(4u, s1.size());
 
   for (int i = 0; i < 4; i++)
     EXPECT_EQ(1u, s1.count(i));
 
   EXPECT_EQ(0u, s1.count(4));
 }
 
 TEST(SmallSetTest, Grow) {
   SmallSet<int, 4> s1;
 
   for (int i = 0; i < 8; i++)
     s1.insert(i);
 
   EXPECT_EQ(8u, s1.size());
 
   for (int i = 0; i < 8; i++)
     EXPECT_EQ(1u, s1.count(i));
 
   EXPECT_EQ(0u, s1.count(8));
 }
 
 TEST(SmallSetTest, Erase) {
   SmallSet<int, 4> s1;
 
   for (int i = 0; i < 8; i++)
     s1.insert(i);
 
   EXPECT_EQ(8u, s1.size());
 
   // Remove elements one by one and check if all other elements are still there.
   for (int i = 0; i < 8; i++) {
     EXPECT_EQ(1u, s1.count(i));
     EXPECT_TRUE(s1.erase(i));
     EXPECT_EQ(0u, s1.count(i));
     EXPECT_EQ(8u - i - 1, s1.size());
     for (int j = i + 1; j < 8; j++)
       EXPECT_EQ(1u, s1.count(j));
   }
 
   EXPECT_EQ(0u, s1.count(8));
 }
+
+TEST(SmallSetTest, IteratorInt) {
+  SmallSet<int, 4> s1;
+
+  // Test the 'small' case.
+  for (int i = 0; i < 3; i++)
+    s1.insert(i);
+
+  std::vector<int> V(s1.begin(), s1.end());
+  // Make sure the elements are in the expected order.
+  std::sort(V.begin(), V.end());
+  for (int i = 0; i < 3; i++)
+    EXPECT_EQ(i, V[i]);
+
+  // Test the 'big' case by adding a few more elements to switch to std::set
+  // internally.
+  for (int i = 3; i < 6; i++)
+    s1.insert(i);
+
+  V.assign(s1.begin(), s1.end());
+  // Make sure the elements are in the expected order.
+  std::sort(V.begin(), V.end());
+  for (int i = 0; i < 6; i++)
+    EXPECT_EQ(i, V[i]);
+}
+
+TEST(SmallSetTest, IteratorString) {
+  // Test SmallSetIterator for SmallSet with a type with non-trivial
+  // ctors/dtors.
+  SmallSet<std::string, 2> s1;
+
+  s1.insert("str 1");
+  s1.insert("str 2");
+  s1.insert("str 1");
+
+  std::vector<std::string> V(s1.begin(), s1.end());
+  std::sort(V.begin(), V.end());
+  EXPECT_EQ(2u, s1.size());
+  EXPECT_EQ("str 1", V[0]);
+  EXPECT_EQ("str 2", V[1]);
+
+  s1.insert("str 4");
+  s1.insert("str 0");
+  s1.insert("str 4");
+
+  V.assign(s1.begin(), s1.end());
+  // Make sure the elements are in the expected order.
+  std::sort(V.begin(), V.end());
+  EXPECT_EQ(4u, s1.size());
+  EXPECT_EQ("str 0", V[0]);
+  EXPECT_EQ("str 1", V[1]);
+  EXPECT_EQ("str 2", V[2]);
+  EXPECT_EQ("str 4", V[3]);
+}
Index: llvm/trunk/include/llvm/ADT/SmallSet.h
===================================================================
--- llvm/trunk/include/llvm/ADT/SmallSet.h	(revision 336804)
+++ llvm/trunk/include/llvm/ADT/SmallSet.h	(revision 336805)
@@ -1,142 +1,209 @@
 //===- llvm/ADT/SmallSet.h - 'Normally small' sets --------------*- 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 SmallSet class.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_ADT_SMALLSET_H
 #define LLVM_ADT_SMALLSET_H
 
 #include "llvm/ADT/None.h"
 #include "llvm/ADT/SmallPtrSet.h"
 #include "llvm/ADT/SmallVector.h"
+#include "llvm/ADT/iterator.h"
 #include "llvm/Support/Compiler.h"
+#include "llvm/Support/type_traits.h"
 #include <cstddef>
 #include <functional>
 #include <set>
+#include <type_traits>
 #include <utility>
 
 namespace llvm {
 
+/// SmallSetIterator - This class implements a const_iterator for SmallSet by
+/// delegating to the underlying SmallVector or Set iterators.
+template <typename T, unsigned N, typename C>
+class SmallSetIterator
+    : public iterator_facade_base<SmallSetIterator<T, N, C>,
+                                  std::forward_iterator_tag, T> {
+private:
+  using SetIterTy = typename std::set<T, C>::const_iterator;
+  using VecIterTy = typename SmallVector<T, N>::const_iterator;
+  using SelfTy = SmallSetIterator<T, N, C>;
+
+  /// Iterators to the parts of the SmallSet containing the data. They are set
+  /// depending on isSmall.
+  union {
+    SetIterTy SetIter;
+    VecIterTy VecIter;
+  };
+
+  bool isSmall;
+
+public:
+  SmallSetIterator(SetIterTy SetIter) : SetIter(SetIter), isSmall(false) {
+    // Use static_assert here, as the SmallSetIterator type is incomplete in the
+    // class scope.
+    static_assert(std::is_trivially_destructible<SelfTy>() &&
+                      llvm::is_trivially_copy_constructible<SelfTy>() &&
+                      llvm::is_trivially_move_constructible<SelfTy>(),
+                  "SelfTy needs to by trivially constructible and destructible");
+  }
+
+  SmallSetIterator(VecIterTy VecIter) : VecIter(VecIter), isSmall(true) {}
+
+  bool operator==(const SmallSetIterator &RHS) const {
+    if (isSmall != RHS.isSmall)
+      return false;
+    if (isSmall)
+      return VecIter == RHS.VecIter;
+    return SetIter == RHS.SetIter;
+  }
+
+  SmallSetIterator &operator++() { // Preincrement
+    if (isSmall)
+      VecIter++;
+    else
+      SetIter++;
+    return *this;
+  }
+
+  const T &operator*() const { return isSmall ? *VecIter : *SetIter; }
+};
+
 /// SmallSet - This maintains a set of unique values, optimizing for the case
 /// when the set is small (less than N).  In this case, the set can be
 /// maintained with no mallocs.  If the set gets large, we expand to using an
 /// std::set to maintain reasonable lookup times.
 ///
 /// Note that this set does not provide a way to iterate over members in the
 /// set.
 template <typename T, unsigned N, typename C = std::less<T>>
 class SmallSet {
   /// Use a SmallVector to hold the elements here (even though it will never
   /// reach its 'large' stage) to avoid calling the default ctors of elements
   /// we will never use.
   SmallVector<T, N> Vector;
   std::set<T, C> Set;
 
   using VIterator = typename SmallVector<T, N>::const_iterator;
   using mutable_iterator = typename SmallVector<T, N>::iterator;
 
   // In small mode SmallPtrSet uses linear search for the elements, so it is
   // not a good idea to choose this value too high. You may consider using a
   // DenseSet<> instead if you expect many elements in the set.
   static_assert(N <= 32, "N should be small");
 
 public:
   using size_type = size_t;
+  using const_iterator = SmallSetIterator<T, N, C>;
 
   SmallSet() = default;
 
   LLVM_NODISCARD bool empty() const {
     return Vector.empty() && Set.empty();
   }
 
   size_type size() const {
     return isSmall() ? Vector.size() : Set.size();
   }
 
   /// count - Return 1 if the element is in the set, 0 otherwise.
   size_type count(const T &V) const {
     if (isSmall()) {
       // Since the collection is small, just do a linear search.
       return vfind(V) == Vector.end() ? 0 : 1;
     } else {
       return Set.count(V);
     }
   }
 
   /// insert - Insert an element into the set if it isn't already there.
   /// Returns true if the element is inserted (it was not in the set before).
   /// The first value of the returned pair is unused and provided for
   /// partial compatibility with the standard library self-associative container
   /// concept.
   // FIXME: Add iterators that abstract over the small and large form, and then
   // return those here.
   std::pair<NoneType, bool> insert(const T &V) {
     if (!isSmall())
       return std::make_pair(None, Set.insert(V).second);
 
     VIterator I = vfind(V);
     if (I != Vector.end())    // Don't reinsert if it already exists.
       return std::make_pair(None, false);
     if (Vector.size() < N) {
       Vector.push_back(V);
       return std::make_pair(None, true);
     }
 
     // Otherwise, grow from vector to set.
     while (!Vector.empty()) {
       Set.insert(Vector.back());
       Vector.pop_back();
     }
     Set.insert(V);
     return std::make_pair(None, true);
   }
 
   template <typename IterT>
   void insert(IterT I, IterT E) {
     for (; I != E; ++I)
       insert(*I);
   }
 
   bool erase(const T &V) {
     if (!isSmall())
       return Set.erase(V);
     for (mutable_iterator I = Vector.begin(), E = Vector.end(); I != E; ++I)
       if (*I == V) {
         Vector.erase(I);
         return true;
       }
     return false;
   }
 
   void clear() {
     Vector.clear();
     Set.clear();
   }
 
+  const_iterator begin() const {
+    if (isSmall())
+      return {Vector.begin()};
+    return {Set.begin()};
+  }
+
+  const_iterator end() const {
+    if (isSmall())
+      return {Vector.end()};
+    return {Set.end()};
+  }
+
 private:
   bool isSmall() const { return Set.empty(); }
 
   VIterator vfind(const T &V) const {
     for (VIterator I = Vector.begin(), E = Vector.end(); I != E; ++I)
       if (*I == V)
         return I;
     return Vector.end();
   }
 };
 
 /// If this set is of pointer values, transparently switch over to using
 /// SmallPtrSet for performance.
 template <typename PointeeType, unsigned N>
 class SmallSet<PointeeType*, N> : public SmallPtrSet<PointeeType*, N> {};
 
 } // end namespace llvm
 
 #endif // LLVM_ADT_SMALLSET_H