diff --git a/compiler-rt/lib/asan/asan_interceptors.cpp b/compiler-rt/lib/asan/asan_interceptors.cpp
--- a/compiler-rt/lib/asan/asan_interceptors.cpp
+++ b/compiler-rt/lib/asan/asan_interceptors.cpp
@@ -222,7 +222,8 @@
   SetCurrentThread(t);
   auto self = GetThreadSelf();
   auto args = asanThreadArgRetval().GetArgs(self);
-  thread_return_t retval = t->ThreadStart(GetTid());
+  t->ThreadStart(GetTid());
+  thread_return_t retval = t->RunThread();
   asanThreadArgRetval().Finish(self, retval);
   CHECK_EQ(args.arg_retval, t->get_arg());
   return retval;
diff --git a/compiler-rt/lib/asan/asan_thread.h b/compiler-rt/lib/asan/asan_thread.h
--- a/compiler-rt/lib/asan/asan_thread.h
+++ b/compiler-rt/lib/asan/asan_thread.h
@@ -67,7 +67,8 @@
   struct InitOptions;
   void Init(const InitOptions *options = nullptr);
 
-  thread_return_t ThreadStart(tid_t os_id);
+  void ThreadStart(tid_t os_id);
+  thread_return_t RunThread();
 
   uptr stack_top();
   uptr stack_bottom();
diff --git a/compiler-rt/lib/asan/asan_thread.cpp b/compiler-rt/lib/asan/asan_thread.cpp
--- a/compiler-rt/lib/asan/asan_thread.cpp
+++ b/compiler-rt/lib/asan/asan_thread.cpp
@@ -273,24 +273,16 @@
 // asan_fuchsia.c definies CreateMainThread and SetThreadStackAndTls.
 #if !SANITIZER_FUCHSIA
 
-thread_return_t AsanThread::ThreadStart(tid_t os_id) {
+void AsanThread::ThreadStart(tid_t os_id) {
   Init();
   asanThreadRegistry().StartThread(tid(), os_id, ThreadType::Regular, nullptr);
 
   if (common_flags()->use_sigaltstack)
     SetAlternateSignalStack();
-
-  if (!start_routine_) {
-    // start_routine_ == 0 if we're on the main thread or on one of the
-    // OS X libdispatch worker threads. But nobody is supposed to call
-    // ThreadStart() for the worker threads.
-    CHECK_EQ(tid(), 0);
-    return 0;
-  }
-
-  return start_routine_(arg_);
 }
 
+thread_return_t AsanThread::RunThread() { return start_routine_(arg_); }
+
 AsanThread *CreateMainThread() {
   AsanThread *main_thread = AsanThread::Create(
       /* start_routine */ nullptr, /* arg */ nullptr, /* parent_tid */ kMainTid,
diff --git a/compiler-rt/lib/asan/asan_win.cpp b/compiler-rt/lib/asan/asan_win.cpp
--- a/compiler-rt/lib/asan/asan_win.cpp
+++ b/compiler-rt/lib/asan/asan_win.cpp
@@ -134,7 +134,8 @@
 static thread_return_t THREAD_CALLING_CONV asan_thread_start(void *arg) {
   AsanThread *t = (AsanThread *)arg;
   SetCurrentThread(t);
-  auto res = t->ThreadStart(GetTid());
+  t->ThreadStart(GetTid());
+  auto res = t->RunThread();
   t->Destroy();  // POSIX calls this from TSD destructor.
   return res;
 }