Index: include/llvm/ADT/SmallPtrSet.h
===================================================================
--- include/llvm/ADT/SmallPtrSet.h
+++ include/llvm/ADT/SmallPtrSet.h
@@ -27,6 +27,12 @@
 #include <utility>
 
 namespace llvm {
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+extern bool ReverseIterate;
+#endif
+}
+
+namespace llvm {
 
 class SmallPtrSetIteratorImpl;
 
@@ -205,7 +211,12 @@
 public:
   explicit SmallPtrSetIteratorImpl(const void *const *BP, const void*const *E)
     : Bucket(BP), End(E) {
-    AdvanceIfNotValid();
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+    if (ReverseIterate)
+      RetreatIfNotValid();
+    else
+#endif
+      AdvanceIfNotValid();
   }
 
   bool operator==(const SmallPtrSetIteratorImpl &RHS) const {
@@ -226,6 +237,17 @@
             *Bucket == SmallPtrSetImplBase::getTombstoneMarker()))
       ++Bucket;
   }
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+  void RetreatIfNotValid() {
+    --Bucket;
+    assert(Bucket <= End);
+    while (Bucket != End &&
+           (*Bucket == SmallPtrSetImplBase::getEmptyMarker() ||
+            *Bucket == SmallPtrSetImplBase::getTombstoneMarker())) {
+      --Bucket;
+    }
+  }
+#endif
 };
 
 /// SmallPtrSetIterator - This implements a const_iterator for SmallPtrSet.
@@ -251,13 +273,29 @@
   }
 
   inline SmallPtrSetIterator& operator++() {          // Preincrement
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+    if (ReverseIterate)
+      RetreatIfNotValid();
+    else {
+      ++Bucket;
+      AdvanceIfNotValid();
+    }
+#else
     ++Bucket;
     AdvanceIfNotValid();
+#endif
     return *this;
   }
 
   SmallPtrSetIterator operator++(int) {        // Postincrement
-    SmallPtrSetIterator tmp = *this; ++*this; return tmp;
+    SmallPtrSetIterator tmp = *this;
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+    if (ReverseIterate)
+      --*this;
+    else
+#endif
+    ++*this;
+    return tmp;
   }
 };
 
@@ -342,9 +380,23 @@
   }
 
   inline iterator begin() const {
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+    if (ReverseIterate)
+      return endPtr();
+#endif
     return iterator(CurArray, EndPointer());
   }
+
   inline iterator end() const {
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+    if (ReverseIterate)
+      return iterator(CurArray, CurArray);
+#endif
+    return endPtr();
+  }
+
+private:
+  inline iterator endPtr() const {
     const void *const *End = EndPointer();
     return iterator(End, End);
   }
Index: lib/Support/CommandLine.cpp
===================================================================
--- lib/Support/CommandLine.cpp
+++ lib/Support/CommandLine.cpp
@@ -45,6 +45,18 @@
 
 #define DEBUG_TYPE "commandline"
 
+namespace llvm {
+// If LLVM_ENABLE_ABI_BREAKING_CHECKS is set the flag -mllvm -reverse-iterate
+// can be used to toggle forward/reverse iteration of unordered containers.
+// This will help uncover differences in codegen caused due to undefined
+// iteration order.
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+bool ReverseIterate;
+static cl::opt<bool, true> ReverseIteration("reverse-iterate",
+  cl::location(ReverseIterate), cl::init(true));
+#endif
+}
+
 //===----------------------------------------------------------------------===//
 // Template instantiations and anchors.
 //
Index: unittests/ADT/CMakeLists.txt
===================================================================
--- unittests/ADT/CMakeLists.txt
+++ unittests/ADT/CMakeLists.txt
@@ -41,6 +41,7 @@
   PostOrderIteratorTest.cpp
   PriorityWorklistTest.cpp
   RangeAdapterTest.cpp
+  ReverseIterationTest.cpp
   SCCIteratorTest.cpp
   STLExtrasTest.cpp
   ScopeExitTest.cpp
Index: unittests/ADT/ReverseIterationTest.cpp
===================================================================
--- /dev/null
+++ unittests/ADT/ReverseIterationTest.cpp
@@ -0,0 +1,41 @@
+//===- llvm/unittest/ADT/ReverseIterationTest.cpp ------------------------------===//
+//
+//                     The LLVM Compiler Infrastructure
+//
+// This file is distributed under the University of Illinois Open Source
+// License. See LICENSE.TXT for details.
+//
+//===----------------------------------------------------------------------===//
+//
+// ReverseIteration unit tests.
+//
+//===----------------------------------------------------------------------===//
+
+#include "gtest/gtest.h"
+#include "llvm/ADT/SmallPtrSet.h"
+
+using namespace llvm;
+
+#if LLVM_ENABLE_ABI_BREAKING_CHECKS
+
+TEST(ReverseIterationTest, SmallPtrSetTest) {
+
+  SmallPtrSet<void*, 4> Set;
+  void *Ptrs[] = { (void*)0x1, (void*)0x2, (void*)0x3, (void*)0x4 };
+  void *ReversePtrs[] = { (void*)0x4, (void*)0x3, (void*)0x2, (void*)0x1 };
+
+  for (auto *Ptr: Ptrs)
+    Set.insert(Ptr);
+
+  // Check forward iteration.
+  ReverseIterate = false;
+  for (const auto &Tuple : zip(Set, Ptrs))
+    ASSERT_EQ(std::get<0>(Tuple), std::get<1>(Tuple));
+
+  // Check reverse iteration.
+  ReverseIterate = true;
+  for (const auto &Tuple : zip(Set, ReversePtrs))
+    ASSERT_EQ(std::get<0>(Tuple), std::get<1>(Tuple));
+}
+
+#endif // LLVM_ENABLE_ABI_BREAKING_CHECKS