[SYCL] Add empty task type of commands

The command does nothing during enqueue. The task can be used to implement
locks in the graph, or to merge several nodes into one.

Signed-off-by: Vlad Romanov <[email protected]>

Author: romanovvlad

sycl/include/CL/sycl/detail/scheduler/commands.hpp

@@ -66,7 +66,8 @@ class Command {
     RELEASE,
     MAP_MEM_OBJ,
     UNMAP_MEM_OBJ,
-    UPDATE_REQUIREMENT
+    UPDATE_REQUIREMENT,
+    EMPTY_TASK
   };
 
   Command(CommandType Type, QueueImplPtr Queue, bool UseExclusiveQueue = false);
@@ -123,6 +124,23 @@ class Command {
   std::atomic<bool> MEnqueued;
 };
 
+// The command does nothing during enqueue. The task can be used to implement
+// lock in the graph, or to merge several nodes into one.
+class EmptyCommand : public Command {
+public:
+  EmptyCommand(QueueImplPtr Queue, Requirement *Req)
+      : Command(CommandType::EMPTY_TASK, std::move(Queue)),
+        MStoredRequirement(*Req) {}
+
+  Requirement *getStoredRequirement() { return &MStoredRequirement; }
+
+private:
+  cl_int enqueueImp() override { return CL_SUCCESS; }
+  void printDot(std::ostream &Stream) const override;
+
+  Requirement MStoredRequirement;
+};
+
 // The command enqueues release instance of memory allocated on Host or
 // underlying framework.
 class ReleaseCommand : public Command {

sycl/source/detail/scheduler/commands.cpp

@@ -467,6 +467,23 @@ cl_int MemCpyCommandHost::enqueueImp() {
   return CL_SUCCESS;
 }
 
+void EmptyCommand::printDot(std::ostream &Stream) const {
+  Stream << "\"" << this << "\" [style=filled, fillcolor=\"#8d8f29\", label=\"";
+
+  Stream << "ID = " << this << "\n";
+  Stream << "EMPTY NODE"
+         << "\\n";
+
+  Stream << "\"];" << std::endl;
+
+  for (const auto &Dep : MDeps) {
+    Stream << "  \"" << this << "\" -> \"" << Dep.MDepCommand << "\""
+           << " [ label = \"Access mode: "
+           << accessModeToString(Dep.MReq->MAccessMode) << "\\n"
+           << "MemObj: " << Dep.MReq->MSYCLMemObj << " \" ]" << std::endl;
+  }
+}
+
 void MemCpyCommandHost::printDot(std::ostream &Stream) const {
   Stream << "\"" << this << "\" [style=filled, fillcolor=\"#B6A2EB\", label=\"";