[ADT][Expensive checks] Create a std::random_device seed only once when shuffling before sorting

This speeds up the build of compiler-rt with an expensive checks enabled clang by an order of
1 or 2 magnitudes on my machine. I was hoping this would also fix the 'large.test' libFuzzer
timeout on the expensive checks bot on green dragon http://lab.llvm.org:8080/green/job/clang-stage1-cmake-RA-expensive/,
but the fuzzer test still takes too long to compile because of other IR/MIR verification inefficiencies.

Differential Revision: https://reviews.llvm.org/D70288

1 files changed, 21 insertions, 8 deletions

diff --git a/llvm/include/llvm/ADT/STLExtras.h b/llvm/include/llvm/ADT/STLExtras.h
index 7c121238b69..32b052e976e 100644
--- a/llvm/include/llvm/ADT/STLExtras.h
+++ b/llvm/include/llvm/ADT/STLExtras.h
@@ -1053,6 +1053,23 @@ inline int (*get_array_pod_sort_comparator(const T &))
   return array_pod_sort_comparator<T>;
 }
 
+#ifdef EXPENSIVE_CHECKS
+namespace detail {
+
+inline unsigned presortShuffleEntropy() {
+  static unsigned Result(std::random_device{}());
+  return Result;
+}
+
+template <class IteratorTy>
+inline void presortShuffle(IteratorTy Start, IteratorTy End) {
+  std::mt19937 Generator(presortShuffleEntropy());
+  std::shuffle(Start, End, Generator);
+}
+
+} // end namespace detail
+#endif
+
 /// array_pod_sort - This sorts an array with the specified start and end
 /// extent.  This is just like std::sort, except that it calls qsort instead of
 /// using an inlined template.  qsort is slightly slower than std::sort, but
@@ -1074,8 +1091,7 @@ inline void array_pod_sort(IteratorTy Start, IteratorTy End) {
   auto NElts = End - Start;
   if (NElts <= 1) return;
 #ifdef EXPENSIVE_CHECKS
-  std::mt19937 Generator(std::random_device{}());
-  std::shuffle(Start, End, Generator);
+  detail::presortShuffle<IteratorTy>(Start, End);
 #endif
   qsort(&*Start, NElts, sizeof(*Start), get_array_pod_sort_comparator(*Start));
 }
@@ -1091,8 +1107,7 @@ inline void array_pod_sort(
   auto NElts = End - Start;
   if (NElts <= 1) return;
 #ifdef EXPENSIVE_CHECKS
-  std::mt19937 Generator(std::random_device{}());
-  std::shuffle(Start, End, Generator);
+  detail::presortShuffle<IteratorTy>(Start, End);
 #endif
   qsort(&*Start, NElts, sizeof(*Start),
         reinterpret_cast<int (*)(const void *, const void *)>(Compare));
@@ -1103,8 +1118,7 @@ inline void array_pod_sort(
 template <typename IteratorTy>
 inline void sort(IteratorTy Start, IteratorTy End) {
 #ifdef EXPENSIVE_CHECKS
-  std::mt19937 Generator(std::random_device{}());
-  std::shuffle(Start, End, Generator);
+  detail::presortShuffle<IteratorTy>(Start, End);
 #endif
   std::sort(Start, End);
 }
@@ -1116,8 +1130,7 @@ template <typename Container> inline void sort(Container &&C) {
 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);
+  detail::presortShuffle<IteratorTy>(Start, End);
 #endif
   std::sort(Start, End, Comp);
 }