diff --git a/include/llvm/Support/CrashRecoveryContext.h b/include/llvm/Support/CrashRecoveryContext.h
index c132373e91d..3869ebdc4a9 100644
--- a/include/llvm/Support/CrashRecoveryContext.h
+++ b/include/llvm/Support/CrashRecoveryContext.h
@@ -87,6 +87,9 @@ public:
   /// requested stack size).
   ///
   /// See RunSafely() and llvm_execute_on_thread().
+  ///
+  /// On Darwin, if PRIO_DARWIN_BG is set on the calling thread, it will be
+  /// propagated to the new thread as well.
   bool RunSafelyOnThread(function_ref<void()>, unsigned RequestedStackSize = 0);
   bool RunSafelyOnThread(void (*Fn)(void*), void *UserData,
                          unsigned RequestedStackSize = 0) {
diff --git a/lib/Support/CrashRecoveryContext.cpp b/lib/Support/CrashRecoveryContext.cpp
index 016c805cc7a..9b0e44339d8 100644
--- a/lib/Support/CrashRecoveryContext.cpp
+++ b/lib/Support/CrashRecoveryContext.cpp
@@ -332,12 +332,26 @@ const std::string &CrashRecoveryContext::getBacktrace() const {
   return CRC->Backtrace;
 }
 
-//
+// FIXME: Portability.
+static void setThreadBackgroundPriority() {
+#ifdef __APPLE__
+  setpriority(PRIO_DARWIN_THREAD, 0, PRIO_DARWIN_BG);
+#endif
+}
+
+static bool hasThreadBackgroundPriority() {
+#ifdef __APPLE__
+  return getpriority(PRIO_DARWIN_THREAD, 0) == 1;
+#else
+  return false;
+#endif
+}
 
 namespace {
 struct RunSafelyOnThreadInfo {
   function_ref<void()> Fn;
   CrashRecoveryContext *CRC;
+  bool UseBackgroundPriority;
   bool Result;
 };
 }
@@ -345,11 +359,16 @@ struct RunSafelyOnThreadInfo {
 static void RunSafelyOnThread_Dispatch(void *UserData) {
   RunSafelyOnThreadInfo *Info =
     reinterpret_cast<RunSafelyOnThreadInfo*>(UserData);
+
+  if (Info->UseBackgroundPriority)
+    setThreadBackgroundPriority();
+
   Info->Result = Info->CRC->RunSafely(Info->Fn);
 }
 bool CrashRecoveryContext::RunSafelyOnThread(function_ref<void()> Fn,
                                              unsigned RequestedStackSize) {
-  RunSafelyOnThreadInfo Info = { Fn, this, false };
+  bool UseBackgroundPriority = hasThreadBackgroundPriority();
+  RunSafelyOnThreadInfo Info = { Fn, this, UseBackgroundPriority, false };
   llvm_execute_on_thread(RunSafelyOnThread_Dispatch, &Info, RequestedStackSize);
   if (CrashRecoveryContextImpl *CRC = (CrashRecoveryContextImpl *)Impl)
     CRC->setSwitchedThread();