Index: llvm/include/llvm/ADT/ilist_iterator.h
===================================================================
--- llvm/include/llvm/ADT/ilist_iterator.h
+++ llvm/include/llvm/ADT/ilist_iterator.h
@@ -102,10 +102,23 @@
     return *this;
   }
 
-  /// Convert from an iterator to its reverse.
+  /// Explicit conversion between forward/reverse iterators.
   ///
-  /// TODO: Roll this into the implicit constructor once we're sure that no one
-  /// is relying on the std::reverse_iterator off-by-one semantics.
+  /// Translate between forward and reverse iterators without changing range
+  /// boundaries.  The resulting iterator will dereference (and have a handle)
+  /// to the previous node, which is somewhat unexpected; but converting the
+  /// two endpoints in a range will give the same range in reverse.
+  ///
+  /// This matches std::reverse_iterator conversions.
+  explicit ilist_iterator(
+      const ilist_iterator<OptionsT, !IsReverse, IsConst> &RHS)
+      : ilist_iterator(++RHS.getReverse()) {}
+
+  /// Get a reverse iterator to the same node.
+  ///
+  /// Gives a reverse iterator that will dereference (and have a handle) to the
+  /// same node.  Converting the endpoint iterators in a range will give a
+  /// different range; for range operations, use the explicit conversions.
   ilist_iterator<OptionsT, !IsReverse, IsConst> getReverse() const {
     if (NodePtr)
       return ilist_iterator<OptionsT, !IsReverse, IsConst>(*NodePtr);
Index: llvm/include/llvm/CodeGen/MachineInstrBundleIterator.h
===================================================================
--- llvm/include/llvm/CodeGen/MachineInstrBundleIterator.h
+++ llvm/include/llvm/CodeGen/MachineInstrBundleIterator.h
@@ -153,6 +153,18 @@
       : MII(I.getInstrIterator()) {}
   MachineInstrBundleIterator() : MII(nullptr) {}
 
+  /// Explicit conversion between forward/reverse iterators.
+  ///
+  /// Translate between forward and reverse iterators without changing range
+  /// boundaries.  The resulting iterator will dereference (and have a handle)
+  /// to the previous node, which is somewhat unexpected; but converting the
+  /// two endpoints in a range will give the same range in reverse.
+  ///
+  /// This matches std::reverse_iterator conversions.
+  explicit MachineInstrBundleIterator(
+      const MachineInstrBundleIterator<Ty, !IsReverse> &I)
+      : MachineInstrBundleIterator(++I.getReverse()) {}
+
   /// Get the bundle iterator for the given instruction's bundle.
   static MachineInstrBundleIterator getAtBundleBegin(instr_iterator MI) {
     return MachineInstrBundleIteratorHelper<IsReverse>::getBundleBegin(MI);
@@ -258,6 +270,11 @@
 
   nonconst_iterator getNonConstIterator() const { return MII.getNonConst(); }
 
+  /// Get a reverse iterator to the same node.
+  ///
+  /// Gives a reverse iterator that will dereference (and have a handle) to the
+  /// same node.  Converting the endpoint iterators in a range will give a
+  /// different range; for range operations, use the explicit conversions.
   reverse_iterator getReverse() const { return MII.getReverse(); }
 };
 
Index: llvm/unittests/ADT/IListIteratorTest.cpp
===================================================================
--- llvm/unittests/ADT/IListIteratorTest.cpp
+++ llvm/unittests/ADT/IListIteratorTest.cpp
@@ -131,4 +131,46 @@
   EXPECT_EQ(L.rend(), RI);
 }
 
+TEST(IListIteratorTest, ReverseConstructor) {
+  simple_ilist<Node> L;
+  const simple_ilist<Node> &CL = L;
+  Node A, B;
+  L.insert(L.end(), A);
+  L.insert(L.end(), B);
+
+  // Save typing.
+  typedef simple_ilist<Node>::iterator iterator;
+  typedef simple_ilist<Node>::reverse_iterator reverse_iterator;
+  typedef simple_ilist<Node>::const_iterator const_iterator;
+  typedef simple_ilist<Node>::const_reverse_iterator const_reverse_iterator;
+
+  // Check explicit conversions.
+  EXPECT_EQ(++CL.begin(), const_iterator(++CL.rbegin()));
+  EXPECT_EQ(++CL.begin(), const_iterator(++L.rbegin()));
+  EXPECT_EQ(++CL.rbegin(), const_reverse_iterator(++CL.begin()));
+  EXPECT_EQ(++CL.rbegin(), const_reverse_iterator(++L.begin()));
+  EXPECT_EQ(++L.begin(), iterator(++L.rbegin()));
+  EXPECT_EQ(++L.rbegin(), reverse_iterator(++L.begin()));
+  EXPECT_EQ(CL.begin(), const_iterator(CL.rend()));
+  EXPECT_EQ(CL.begin(), const_iterator(L.rend()));
+  EXPECT_EQ(CL.end(), const_iterator(CL.rbegin()));
+  EXPECT_EQ(CL.end(), const_iterator(L.rbegin()));
+  EXPECT_EQ(CL.rbegin(), const_reverse_iterator(CL.end()));
+  EXPECT_EQ(CL.rbegin(), const_reverse_iterator(L.end()));
+  EXPECT_EQ(CL.rend(), const_reverse_iterator(CL.begin()));
+  EXPECT_EQ(CL.rend(), const_reverse_iterator(L.begin()));
+  EXPECT_EQ(L.begin(), iterator(L.rend()));
+  EXPECT_EQ(L.end(), iterator(L.rbegin()));
+  EXPECT_EQ(L.rbegin(), reverse_iterator(L.end()));
+  EXPECT_EQ(L.rend(), reverse_iterator(L.begin()));
+
+  // Confirm lack of implicit conversions.
+  EXPECT_FALSE((std::is_convertible<iterator, reverse_iterator>::value));
+  EXPECT_FALSE((std::is_convertible<reverse_iterator, iterator>::value));
+  EXPECT_FALSE(
+      (std::is_convertible<const_iterator, const_reverse_iterator>::value));
+  EXPECT_FALSE(
+      (std::is_convertible<const_reverse_iterator, const_iterator>::value));
+}
+
 } // end namespace
Index: llvm/unittests/CodeGen/MachineInstrBundleIteratorTest.cpp
===================================================================
--- llvm/unittests/CodeGen/MachineInstrBundleIteratorTest.cpp
+++ llvm/unittests/CodeGen/MachineInstrBundleIteratorTest.cpp
@@ -130,4 +130,70 @@
   ASSERT_TRUE(CI != CMBI.getIterator());
 }
 
+struct MyUnbundledInstr
+    : ilist_node<MyUnbundledInstr, ilist_sentinel_tracking<true>> {
+  bool isBundledWithPred() const { return false; }
+  bool isBundledWithSucc() const { return false; }
+};
+typedef MachineInstrBundleIterator<MyUnbundledInstr> unbundled_iterator;
+typedef MachineInstrBundleIterator<const MyUnbundledInstr>
+    const_unbundled_iterator;
+typedef MachineInstrBundleIterator<MyUnbundledInstr, true>
+    reverse_unbundled_iterator;
+typedef MachineInstrBundleIterator<const MyUnbundledInstr, true>
+    const_reverse_unbundled_iterator;
+
+TEST(MachineInstrBundleIteratorTest, ReverseConstructor) {
+  simple_ilist<MyUnbundledInstr, ilist_sentinel_tracking<true>> L;
+  const auto &CL = L;
+  MyUnbundledInstr A, B;
+  L.insert(L.end(), A);
+  L.insert(L.end(), B);
+
+  // Save typing.
+  typedef MachineInstrBundleIterator<MyUnbundledInstr> iterator;
+  typedef MachineInstrBundleIterator<MyUnbundledInstr, true> reverse_iterator;
+  typedef MachineInstrBundleIterator<const MyUnbundledInstr> const_iterator;
+  typedef MachineInstrBundleIterator<const MyUnbundledInstr, true>
+      const_reverse_iterator;
+
+  // Convert to bundle iterators.
+  auto begin = [&]() -> iterator { return L.begin(); };
+  auto end = [&]() -> iterator { return L.end(); };
+  auto rbegin = [&]() -> reverse_iterator { return L.rbegin(); };
+  auto rend = [&]() -> reverse_iterator { return L.rend(); };
+  auto cbegin = [&]() -> const_iterator { return CL.begin(); };
+  auto cend = [&]() -> const_iterator { return CL.end(); };
+  auto crbegin = [&]() -> const_reverse_iterator { return CL.rbegin(); };
+  auto crend = [&]() -> const_reverse_iterator { return CL.rend(); };
+
+  // Check explicit conversions.
+  EXPECT_EQ(++cbegin(), const_iterator(++crbegin()));
+  EXPECT_EQ(++cbegin(), const_iterator(++rbegin()));
+  EXPECT_EQ(++crbegin(), const_reverse_iterator(++cbegin()));
+  EXPECT_EQ(++crbegin(), const_reverse_iterator(++begin()));
+  EXPECT_EQ(++begin(), iterator(++rbegin()));
+  EXPECT_EQ(++rbegin(), reverse_iterator(++begin()));
+  EXPECT_EQ(cbegin(), const_iterator(crend()));
+  EXPECT_EQ(cbegin(), const_iterator(rend()));
+  EXPECT_EQ(cend(), const_iterator(crbegin()));
+  EXPECT_EQ(cend(), const_iterator(rbegin()));
+  EXPECT_EQ(crbegin(), const_reverse_iterator(cend()));
+  EXPECT_EQ(crbegin(), const_reverse_iterator(end()));
+  EXPECT_EQ(crend(), const_reverse_iterator(cbegin()));
+  EXPECT_EQ(crend(), const_reverse_iterator(begin()));
+  EXPECT_EQ(begin(), iterator(rend()));
+  EXPECT_EQ(end(), iterator(rbegin()));
+  EXPECT_EQ(rbegin(), reverse_iterator(end()));
+  EXPECT_EQ(rend(), reverse_iterator(begin()));
+
+  // Confirm lack of implicit conversions.
+  EXPECT_FALSE((std::is_convertible<iterator, reverse_iterator>::value));
+  EXPECT_FALSE((std::is_convertible<reverse_iterator, iterator>::value));
+  EXPECT_FALSE(
+      (std::is_convertible<const_iterator, const_reverse_iterator>::value));
+  EXPECT_FALSE(
+      (std::is_convertible<const_reverse_iterator, const_iterator>::value));
+}
+
 } // end namespace