[SYCL] Reintroduce barrier trivial event for empty in-order queue (#20263)

The changes in #20159 removed a test
case checking that barriers on empty queues would be considered complete
immediately. This commit reintroduces it with a fix for the case.

---------

Signed-off-by: Larsen, Steffen <steffen.larsen@intel.com>

Author: steffenlarsen

sycl/source/queue.cpp

@@ -366,6 +366,14 @@ event queue::ext_oneapi_submit_barrier(const std::vector<event> &WaitList,
                !EventImpl.hasCommandGraph();
       });
 
+  // If we have an empty in-order queue and no dependencies, we can just return
+  // a trivially finished event.
+  if (is_in_order() && !impl->hasCommandGraph() && !impl->MIsProfilingEnabled &&
+      AllEventsEmptyOrNop && ext_oneapi_empty()) {
+    return detail::createSyclObjFromImpl<event>(
+        detail::event_impl::create_default_event());
+  }
+
   if (WaitList.empty() || AllEventsEmptyOrNop)
     return submit([=](handler &CGH) { CGH.ext_oneapi_barrier(); }, CodeLoc);
   else

sycl/test-e2e/InorderQueue/in_order_ext_oneapi_submit_barrier.cpp

@@ -60,6 +60,16 @@ int main() {
     Q.wait();
     assert(*Res == 10);
   }
+  {
+    // Test cast 3 - empty queue.
+    std::cout << "Test 3" << std::endl;
+    sycl::queue EmptyQ({sycl::property::queue::in_order{}});
+    auto BarrierEvent = EmptyQ.ext_oneapi_submit_barrier();
+    assert(
+        BarrierEvent.get_info<sycl::info::event::command_execution_status>() ==
+        sycl::info::event_command_status::complete);
+    BarrierEvent.wait();
+  }
   {
     // Test cast 4 - graph.
     sycl::queue GQueue{sycl::property::queue::in_order{}};

sycl/unittests/Extensions/ExtOneapiBarrierOpt.cpp

@@ -6,6 +6,7 @@
 //
 //===----------------------------------------------------------------------===//
 
+#include <detail/queue_impl.hpp>
 #include <gtest/gtest.h>
 #include <helpers/ScopedEnvVar.hpp>
 #include <helpers/UrMock.hpp>
@@ -39,6 +40,11 @@ class ExtOneapiBarrierOptTest : public ::testing::Test {
 TEST_F(ExtOneapiBarrierOptTest, EmptyEventTest) {
   sycl::queue q1{{sycl::property::queue::in_order()}};
 
+  // To avoid current optimizations for empty queues, we trick q1 into thinking
+  // that it isn't empty.
+  int dummyInt = 0;
+  q1.prefetch(&dummyInt, sizeof(int));
+
   mock::getCallbacks().set_after_callback(
       "urEnqueueEventsWaitWithBarrierExt",
       &redefinedEnqueueEventsWaitWithBarrierExt);