[SYCL] Add empty node implementation

sycl/include/sycl/ext/oneapi/experimental/graph.hpp

@@ -55,7 +55,7 @@ class __SYCL_EXPORT command_graph {
   command_graph(const property_list &propList = {});
 
   // Adding empty node with [0..n] predecessors:
-  node add(const std::vector<node> &dep = {});
+  node add(const std::vector<node> &dep = {}) { return add_impl(dep); }
 
   // Adding device node:
   template <typename T> node add(T cgf, const std::vector<node> &dep = {}) {
@@ -110,6 +110,8 @@ class __SYCL_EXPORT command_graph {
   node add_impl(std::function<void(handler &)> cgf,
                 const std::vector<node> &dep);
 
+  node add_impl(const std::vector<node> &dep);
+
   template <class Obj>
   friend decltype(Obj::impl)
   sycl::detail::getSyclObjImpl(const Obj &SyclObject);

sycl/source/detail/graph_impl.cpp

@@ -85,6 +85,26 @@ bool check_for_arg(const sycl::detail::ArgDesc &Arg,
   return SuccessorAddedDep;
 }
 
+std::shared_ptr<node_impl>
+graph_impl::add(const std::shared_ptr<graph_impl> &Impl,
+                const std::vector<std::shared_ptr<node_impl>> &Dep) {
+  const std::shared_ptr<node_impl> &NodeImpl =
+      std::make_shared<node_impl>(Impl);
+
+  // TODO: Encapsulate in separate function to avoid duplication
+  if (!Dep.empty()) {
+    for (auto N : Dep) {
+      N->register_successor(NodeImpl); // register successor
+      this->remove_root(NodeImpl);     // remove receiver from root node
+                                       // list
+    }
+  } else {
+    this->add_root(NodeImpl);
+  }
+
+  return NodeImpl;
+}
+
 std::shared_ptr<node_impl>
 graph_impl::add(const std::shared_ptr<graph_impl> &Impl,
                 std::function<void(handler &)> CGF,
@@ -193,6 +213,18 @@ command_graph<graph_state::modifiable>::command_graph(
     const sycl::property_list &)
     : impl(std::make_shared<detail::graph_impl>()) {}
 
+template <>
+node command_graph<graph_state::modifiable>::add_impl(
+    const std::vector<node> &Deps) {
+  std::vector<std::shared_ptr<detail::node_impl>> DepImpls;
+  for (auto &D : Deps) {
+    DepImpls.push_back(sycl::detail::getSyclObjImpl(D));
+  }
+
+  std::shared_ptr<detail::node_impl> NodeImpl = impl->add(impl, DepImpls);
+  return sycl::detail::createSyclObjFromImpl<node>(NodeImpl);
+}
+
 template <>
 node command_graph<graph_state::modifiable>::add_impl(
     std::function<void(handler &)> CGF, const std::vector<node> &Deps) {

sycl/source/detail/graph_impl.hpp

@@ -85,6 +85,9 @@ struct node_impl {
 
   sycl::event get_event(void) const { return MEvent; }
 
+  node_impl(const std::shared_ptr<graph_impl> &Graph)
+      : MScheduled(false), MGraph(Graph) {}
+
   node_impl(
       const std::shared_ptr<graph_impl> &Graph,
       std::shared_ptr<sycl::detail::kernel_impl> Kernel,
@@ -119,7 +122,8 @@ struct node_impl {
       if (!Next->MScheduled)
         Next->topology_sort(Schedule);
     }
-    Schedule.push_front(std::shared_ptr<node_impl>(this));
+    if (MKernel != nullptr)
+      Schedule.push_front(std::shared_ptr<node_impl>(this));
   }
 
   bool has_arg(const sycl::detail::ArgDesc &Arg) {
@@ -169,6 +173,10 @@ struct graph_impl {
       const std::vector<sycl::detail::ArgDesc> &Args,
       const std::vector<std::shared_ptr<node_impl>> &Dep = {});
 
+  std::shared_ptr<node_impl>
+  add(const std::shared_ptr<graph_impl> &Impl,
+      const std::vector<std::shared_ptr<node_impl>> &Dep = {});
+
   graph_impl() : MFirst(true) {}
 
   /// Add a queue to the set of queues which are currently recording to this

sycl/test/graph/graph-explicit-empty.cpp

@@ -0,0 +1,44 @@
+// RUN: %clangxx -fsycl -fsycl-targets=%sycl_triple %s -o %t.out
+#include <sycl/sycl.hpp>
+
+#include <sycl/ext/oneapi/experimental/graph.hpp>
+
+int main() {
+
+  sycl::property_list properties{
+      sycl::property::queue::in_order{},
+      sycl::ext::oneapi::property::queue::lazy_execution{}};
+
+  sycl::queue q{sycl::gpu_selector_v, properties};
+
+  sycl::ext::oneapi::experimental::command_graph g;
+
+  const size_t n = 10;
+  float *arr = sycl::malloc_device<float>(n, q);
+
+  auto start = g.add();
+
+  auto init = g.add([&](sycl::handler &h) {
+      h.parallel_for(sycl::range<1>{n}, [=](sycl::id<1> idx) {
+        size_t i = idx;
+        arr[i] = 0;
+      });
+  }, {start});
+
+  auto empty = g.add({init});
+
+  g.add([&](sycl::handler &h) {
+    h.parallel_for(sycl::range<1>{n}, [=](sycl::id<1> idx) {
+      size_t i = idx;
+      arr[i] = 1;
+    });
+  }, {empty});
+
+  auto executable_graph = g.finalize(q.get_context());
+
+  q.submit([&](sycl::handler &h) { h.ext_oneapi_graph(executable_graph); }).wait();
+
+  sycl::free(arr, q);
+
+  return 0;
+}