[SYCL] Avoid iteration through empty wait list in GraphProcessor (#432)

Signed-off-by: Viktoria Maksimova <[email protected]>

Author: mlychkov

sycl/source/detail/scheduler/graph_processor.cpp

@@ -23,8 +23,12 @@ static Command *getCommand(const EventImplPtr &Event) {
 
 std::vector<EventImplPtr>
 Scheduler::GraphProcessor::getWaitList(EventImplPtr Event) {
-  std::vector<EventImplPtr> Result;
   Command *Cmd = getCommand(Event);
+  // Command can be nullptr if user creates cl::sycl::event explicitly,
+  // as such event is not mapped to any SYCL task.
+  if (!Cmd)
+    return {};
+  std::vector<EventImplPtr> Result;
   for (const DepDesc &Dep : Cmd->MDeps) {
     if (Dep.MDepCommand)
       Result.push_back(Dep.MDepCommand->getEvent());

sycl/test/scheduler/GetWaitList.cpp

@@ -0,0 +1,25 @@
+// RUN: %clangxx -fsycl %s -o %t.out -lOpenCL
+// RUN: %t.out
+//==------------------- GetWaitList.cpp ----------------------------==//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+
+#include <CL/sycl.hpp>
+
+void foo() {
+  cl::sycl::event start;
+  start.wait_and_throw();
+  return;
+}
+
+int main() {
+  cl::sycl::queue Q;
+  Q.submit([&](cl::sycl::handler &CGH) {
+    foo();
+  });
+  return 0;
+}