Index: llvm/trunk/include/llvm/ADT/STLExtras.h
===================================================================
--- llvm/trunk/include/llvm/ADT/STLExtras.h
+++ llvm/trunk/include/llvm/ADT/STLExtras.h
@@ -36,6 +36,10 @@
 #include <type_traits>
 #include <utility>
 
+#ifdef EXPENSIVE_CHECKS
+#include <random> // for std::mt19937
+#endif
+
 namespace llvm {
 
 // Only used by compiler if both template types are the same.  Useful when
@@ -762,6 +766,10 @@
   // behavior with an empty sequence.
   auto NElts = End - Start;
   if (NElts <= 1) return;
+#ifdef EXPENSIVE_CHECKS
+  std::mt19937 Generator(std::random_device{}());
+  std::shuffle(Start, End, Generator);
+#endif
   qsort(&*Start, NElts, sizeof(*Start), get_array_pod_sort_comparator(*Start));
 }
 
@@ -775,10 +783,34 @@
   // behavior with an empty sequence.
   auto NElts = End - Start;
   if (NElts <= 1) return;
+#ifdef EXPENSIVE_CHECKS
+  std::mt19937 Generator(std::random_device{}());
+  std::shuffle(Start, End, Generator);
+#endif
   qsort(&*Start, NElts, sizeof(*Start),
         reinterpret_cast<int (*)(const void *, const void *)>(Compare));
 }
 
+// Provide wrappers to std::sort which shuffle the elements before sorting
+// to help uncover non-deterministic behavior (PR35135).
+template <typename IteratorTy>
+inline void sort(IteratorTy Start, IteratorTy End) {
+#ifdef EXPENSIVE_CHECKS
+  std::mt19937 Generator(std::random_device{}());
+  std::shuffle(Start, End, Generator);
+#endif
+  std::sort(Start, End);
+}
+
+template <typename IteratorTy, typename Compare>
+inline void sort(IteratorTy Start, IteratorTy End, Compare Comp) {
+#ifdef EXPENSIVE_CHECKS
+  std::mt19937 Generator(std::random_device{}());
+  std::shuffle(Start, End, Generator);
+#endif
+  std::sort(Start, End, Comp);
+}
+
 //===----------------------------------------------------------------------===//
 //     Extra additions to <algorithm>
 //===----------------------------------------------------------------------===//