[ADT] Shuffle containers before sorting to uncover non-deterministic …

…behavior

Summary:
std::sort and array_pod_sort both use non-stable sorting algorithms.
This means that the relative order of elements with the same key is
undefined. This patch is an attempt to uncover such scenarios by
randomly shuffling all containers before sorting, if EXPENSIVE_CHECKS
is enabled.

Here's the bugzilla for this: https://bugs.llvm.org/show_bug.cgi?id=35135

Reviewers: dblaikie, dexonsmith, chandlerc, efriedma, RKSimon

Reviewed By: RKSimon

Subscribers: fhahn, davide, RKSimon, vsk, mgorny, llvm-commits

Differential Revision: https://reviews.llvm.org/D39245

llvm-svn: 327219

llvm/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 @@ inline void array_pod_sort(IteratorTy Start, IteratorTy End) {
   // 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 @@ inline void array_pod_sort(
   // 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>
 //===----------------------------------------------------------------------===//