Revert "[SYCL] Honor dependencies of empty command groups" (#16190)

Reverts #16180

sycl/source/detail/scheduler/commands.cpp

@@ -3592,21 +3592,10 @@ ur_result_t ExecCGCommand::enqueueImpQueue() {
 
     return UR_RESULT_SUCCESS;
   }
-  case CGType::None: {
-    if (RawEvents.empty()) {
-      // urEnqueueEventsWait with zero events acts like a barrier which is NOT
-      // what we want here. On the other hand, there is nothing to wait for, so
-      // we don't need to enqueue anything.
-      return UR_RESULT_SUCCESS;
-    }
-    const detail::AdapterPtr &Adapter = MQueue->getAdapter();
-    ur_event_handle_t Event;
-    ur_result_t Result = Adapter->call_nocheck<UrApiKind::urEnqueueEventsWait>(
-        MQueue->getHandleRef(), RawEvents.size(),
-        RawEvents.size() ? &RawEvents[0] : nullptr, &Event);
-    MEvent->setHandle(Event);
-    return Result;
-  }
+  case CGType::None:
+    throw sycl::exception(sycl::make_error_code(sycl::errc::runtime),
+                          "CG type not implemented. " +
+                              codeToString(UR_RESULT_ERROR_INVALID_OPERATION));
   }
   return UR_RESULT_ERROR_INVALID_OPERATION;
 }

sycl/source/handler.cpp

@@ -496,8 +496,21 @@ event handler::finalize() {
         MCodeLoc));
     break;
   case detail::CGType::None:
-    CommandGroup.reset(new detail::CG(detail::CGType::None,
-                                      std::move(impl->CGData), MCodeLoc));
+    if (detail::ur::trace(detail::ur::TraceLevel::TRACE_ALL)) {
+      std::cout << "WARNING: An empty command group is submitted." << std::endl;
+    }
+
+    // Empty nodes are handled by Graph like standard nodes
+    // For Standard mode (non-graph),
+    // empty nodes are not sent to the scheduler to save time
+    if (impl->MGraph || (MQueue && MQueue->getCommandGraph())) {
+      CommandGroup.reset(new detail::CG(detail::CGType::None,
+                                        std::move(impl->CGData), MCodeLoc));
+    } else {
+      detail::EventImplPtr Event = std::make_shared<sycl::detail::event_impl>();
+      MLastEvent = detail::createSyclObjFromImpl<event>(Event);
+      return MLastEvent;
+    }
     break;
   }
 

sycl/test-e2e/Basic/empty_command.cpp

@@ -26,11 +26,18 @@ void test_host_task_dep() {
   auto empty_cg_event =
       q.submit([&](handler &cgh) { cgh.depends_on(host_event); });
 
+  // FIXME: This should deadlock, but the dependency is ignored currently.
+  empty_cg_event.wait();
+
   assert(x == 0);
   start_execution.count_down();
 
   empty_cg_event.wait();
-  assert(x == 42);
+  // FIXME: uncomment once the bug mentioned above is fixed.
+  // assert(x == 42);
+
+  // I'm seeing some weird hang without this:
+  host_event.wait();
 }
 
 void test_device_event_dep() {
@@ -46,12 +53,17 @@ void test_device_event_dep() {
   auto empty_cg_event =
       q.submit([&](handler &cgh) { cgh.depends_on(device_event); });
 
+  // FIXME: This should deadlock, but the dependency is ignored currently.
+  empty_cg_event.wait();
+
   assert(*p == 0);
   start_execution.count_down();
 
   empty_cg_event.wait();
-  assert(*p == 42);
+  // FIXME: uncomment once the bug mentioned above is fixed.
+  // assert(*p == 42);
 
+  q.wait();
   sycl::free(p, q);
 }
 
@@ -78,12 +90,17 @@ void test_accessor_dep() {
   auto empty_cg_event =
       q.submit([&](handler &cgh) { sycl::accessor a{b, cgh}; });
 
+  // FIXME: This should deadlock, but the dependency is ignored currently.
+  empty_cg_event.wait();
+
   assert(*p == 0);
   start_execution.count_down();
 
   empty_cg_event.wait();
-  assert(*p == 42);
+  // FIXME: uncomment once the bug mentioned above is fixed.
+  // assert(*p == 42);
 
+  q.wait();
   sycl::free(p, q);
 }