1 files changed, 4 insertions, 0 deletions
diff --git a/test/tsan/Darwin/gcd-apply-race.mm b/test/tsan/Darwin/gcd-apply-race.mm
index 13b24e0fdb90e..028be1ac5b1cc 100644
--- a/test/tsan/Darwin/gcd-apply-race.mm
+++ b/test/tsan/Darwin/gcd-apply-race.mm
@@ -10,6 +10,10 @@ long global;
 int main(int argc, const char *argv[]) {
   barrier_init(&barrier, 2);
   fprintf(stderr, "start\n");
+
+  // Warm up GCD (workaround for macOS Sierra where dispatch_apply might run single-threaded).
+  dispatch_sync(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{ });
+
   dispatch_queue_t q = dispatch_queue_create("my.queue", DISPATCH_QUEUE_CONCURRENT);
   dispatch_apply(2, q, ^(size_t i) {
     global = i;