[NFC][SYCL][Graph] Add successors/predecessors views + cleanup

Part of refactoring to get rid of most (all?) `std::weak_ptr<node_impl>`
and some of `std::shared_ptr<node_impl>` started in
#19295. Use `nodes_range` from that PR
to implement `successors`/`predecessors` views and update read-only
accesses to the successors/predecessors to go through them.

I'm not changing the data members `MSuccessors`/`MPredecessors` yet
because it would affect unittests. I'd prefer to refactor most of the
code in future PRs before making that change and updating unittests in
one go.

I'm updating some APIs to accept `node_impl &` instead of
`std::shared_ptr` where the change is mostly localized to the callers
iterating over successors/predecessors and doesn't spoil into other code
too much. For those that weren't updated here we (temporarily) use
`shared_from_this()` but I expect to eliminate those unnecessary copies
when those interfaces will be updated in the subsequent PRs.

Author: aelovikov-intel

sycl/source/detail/graph/graph_impl.cpp

@@ -100,17 +100,16 @@ void sortTopological(std::set<std::weak_ptr<node_impl>,
     Source.pop();
     SortedNodes.push_back(Node);
 
-    for (auto &SuccWP : Node->MSuccessors) {
-      auto Succ = SuccWP.lock();
+    for (node_impl &Succ : Node->successors()) {
 
-      if (PartitionBounded && (Succ->MPartitionNum != Node->MPartitionNum)) {
+      if (PartitionBounded && (Succ.MPartitionNum != Node->MPartitionNum)) {
         continue;
       }
 
-      auto &TotalVisitedEdges = Succ->MTotalVisitedEdges;
+      auto &TotalVisitedEdges = Succ.MTotalVisitedEdges;
       ++TotalVisitedEdges;
-      if (TotalVisitedEdges == Succ->MPredecessors.size()) {
-        Source.push(Succ);
+      if (TotalVisitedEdges == Succ.MPredecessors.size()) {
+        Source.push(Succ.weak_from_this());
       }
     }
   }
@@ -127,14 +126,14 @@ void sortTopological(std::set<std::weak_ptr<node_impl>,
 /// a node with a smaller partition number.
 /// @param Node Node to assign to the partition.
 /// @param PartitionNum Number to propagate.
-void propagatePartitionUp(std::shared_ptr<node_impl> Node, int PartitionNum) {
-  if (((Node->MPartitionNum != -1) && (Node->MPartitionNum <= PartitionNum)) ||
-      (Node->MCGType == sycl::detail::CGType::CodeplayHostTask)) {
+void propagatePartitionUp(node_impl &Node, int PartitionNum) {
+  if (((Node.MPartitionNum != -1) && (Node.MPartitionNum <= PartitionNum)) ||
+      (Node.MCGType == sycl::detail::CGType::CodeplayHostTask)) {
     return;
   }
-  Node->MPartitionNum = PartitionNum;
-  for (auto &Predecessor : Node->MPredecessors) {
-    propagatePartitionUp(Predecessor.lock(), PartitionNum);
+  Node.MPartitionNum = PartitionNum;
+  for (node_impl &Predecessor : Node.predecessors()) {
+    propagatePartitionUp(Predecessor, PartitionNum);
   }
 }
 
@@ -146,17 +145,17 @@ void propagatePartitionUp(std::shared_ptr<node_impl> Node, int PartitionNum) {
 /// @param HostTaskList List of host tasks that have already been processed and
 /// are encountered as successors to the node Node.
 void propagatePartitionDown(
-    const std::shared_ptr<node_impl> &Node, int PartitionNum,
+    node_impl &Node, int PartitionNum,
     std::list<std::shared_ptr<node_impl>> &HostTaskList) {
-  if (Node->MCGType == sycl::detail::CGType::CodeplayHostTask) {
-    if (Node->MPartitionNum != -1) {
-      HostTaskList.push_front(Node);
+  if (Node.MCGType == sycl::detail::CGType::CodeplayHostTask) {
+    if (Node.MPartitionNum != -1) {
+      HostTaskList.push_front(Node.shared_from_this());
     }
     return;
   }
-  Node->MPartitionNum = PartitionNum;
-  for (auto &Successor : Node->MSuccessors) {
-    propagatePartitionDown(Successor.lock(), PartitionNum, HostTaskList);
+  Node.MPartitionNum = PartitionNum;
+  for (node_impl &Successor : Node.successors()) {
+    propagatePartitionDown(Successor, PartitionNum, HostTaskList);
   }
 }
 
@@ -165,8 +164,8 @@ void propagatePartitionDown(
 /// @param Node node to test
 /// @return True is `Node` is a root of its partition
 bool isPartitionRoot(std::shared_ptr<node_impl> Node) {
-  for (auto &Predecessor : Node->MPredecessors) {
-    if (Predecessor.lock()->MPartitionNum == Node->MPartitionNum) {
+  for (node_impl &Predecessor : Node->predecessors()) {
+    if (Predecessor.MPartitionNum == Node->MPartitionNum) {
       return false;
     }
   }
@@ -221,15 +220,15 @@ void exec_graph_impl::makePartitions() {
     auto Node = HostTaskList.front();
     HostTaskList.pop_front();
     CurrentPartition++;
-    for (auto &Predecessor : Node->MPredecessors) {
-      propagatePartitionUp(Predecessor.lock(), CurrentPartition);
+    for (node_impl &Predecessor : Node->predecessors()) {
+      propagatePartitionUp(Predecessor, CurrentPartition);
     }
     CurrentPartition++;
     Node->MPartitionNum = CurrentPartition;
     CurrentPartition++;
     auto TmpSize = HostTaskList.size();
-    for (auto &Successor : Node->MSuccessors) {
-      propagatePartitionDown(Successor.lock(), CurrentPartition, HostTaskList);
+    for (node_impl &Successor : Node->successors()) {
+      propagatePartitionDown(Successor, CurrentPartition, HostTaskList);
     }
     if (HostTaskList.size() > TmpSize) {
       // At least one HostTask has been re-numbered so group merge opportunities
@@ -290,9 +289,9 @@ void exec_graph_impl::makePartitions() {
   for (const auto &Partition : MPartitions) {
     for (auto const &Root : Partition->MRoots) {
       auto RootNode = Root.lock();
-      for (const auto &Dep : RootNode->MPredecessors) {
-        auto NodeDep = Dep.lock();
-        auto &Predecessor = MPartitions[MPartitionNodes[NodeDep]];
+      for (node_impl &NodeDep : RootNode->predecessors()) {
+        auto &Predecessor =
+            MPartitions[MPartitionNodes[NodeDep.shared_from_this()]];
         Partition->MPredecessors.push_back(Predecessor.get());
         Predecessor->MSuccessors.push_back(Partition.get());
       }
@@ -390,8 +389,8 @@ std::set<std::shared_ptr<node_impl>> graph_impl::getCGEdges(
         bool ShouldAddDep = true;
         // If any of this node's successors have this requirement then we skip
         // adding the current node as a dependency.
-        for (auto &Succ : Node->MSuccessors) {
-          if (Succ.lock()->hasRequirementDependency(Req)) {
+        for (node_impl &Succ : Node->successors()) {
+          if (Succ.hasRequirementDependency(Req)) {
             ShouldAddDep = false;
             break;
           }
@@ -721,17 +720,17 @@ void graph_impl::beginRecording(sycl::detail::queue_impl &Queue) {
 // predecessors until we find the real dependency.
 void exec_graph_impl::findRealDeps(
     std::vector<ur_exp_command_buffer_sync_point_t> &Deps,
-    std::shared_ptr<node_impl> CurrentNode, int ReferencePartitionNum) {
-  if (!CurrentNode->requiresEnqueue()) {
-    for (auto &N : CurrentNode->MPredecessors) {
-      auto NodeImpl = N.lock();
+    node_impl &CurrentNode, int ReferencePartitionNum) {
+  if (!CurrentNode.requiresEnqueue()) {
+    for (node_impl &NodeImpl : CurrentNode.predecessors()) {
       findRealDeps(Deps, NodeImpl, ReferencePartitionNum);
     }
   } else {
+    auto CurrentNodePtr = CurrentNode.shared_from_this();
     // Verify if CurrentNode belong the the same partition
-    if (MPartitionNodes[CurrentNode] == ReferencePartitionNum) {
+    if (MPartitionNodes[CurrentNodePtr] == ReferencePartitionNum) {
       // Verify that the sync point has actually been set for this node.
-      auto SyncPoint = MSyncPoints.find(CurrentNode);
+      auto SyncPoint = MSyncPoints.find(CurrentNodePtr);
       assert(SyncPoint != MSyncPoints.end() &&
              "No sync point has been set for node dependency.");
       // Check if the dependency has already been added.
@@ -749,8 +748,8 @@ exec_graph_impl::enqueueNodeDirect(const sycl::context &Ctx,
                                    ur_exp_command_buffer_handle_t CommandBuffer,
                                    std::shared_ptr<node_impl> Node) {
   std::vector<ur_exp_command_buffer_sync_point_t> Deps;
-  for (auto &N : Node->MPredecessors) {
-    findRealDeps(Deps, N.lock(), MPartitionNodes[Node]);
+  for (node_impl &N : Node->predecessors()) {
+    findRealDeps(Deps, N, MPartitionNodes[Node]);
   }
   ur_exp_command_buffer_sync_point_t NewSyncPoint;
   ur_exp_command_buffer_command_handle_t NewCommand = 0;
@@ -805,8 +804,8 @@ exec_graph_impl::enqueueNode(ur_exp_command_buffer_handle_t CommandBuffer,
                              std::shared_ptr<node_impl> Node) {
 
   std::vector<ur_exp_command_buffer_sync_point_t> Deps;
-  for (auto &N : Node->MPredecessors) {
-    findRealDeps(Deps, N.lock(), MPartitionNodes[Node]);
+  for (node_impl &N : Node->predecessors()) {
+    findRealDeps(Deps, N, MPartitionNodes[Node]);
   }
 
   sycl::detail::EventImplPtr Event =
@@ -1275,8 +1274,8 @@ void exec_graph_impl::duplicateNodes() {
     auto NodeCopy = NewNodes[i];
     // Look through all the original node successors, find their copies and
     // register those as successors with the current copied node
-    for (auto &NextNode : OriginalNode->MSuccessors) {
-      auto Successor = NodesMap.at(NextNode.lock());
+    for (node_impl &NextNode : OriginalNode->successors()) {
+      auto Successor = NodesMap.at(NextNode.shared_from_this());
       NodeCopy->registerSuccessor(Successor);
     }
   }
@@ -1317,8 +1316,8 @@ void exec_graph_impl::duplicateNodes() {
       auto SubgraphNode = SubgraphNodes[i];
       auto NodeCopy = NewSubgraphNodes[i];
 
-      for (auto &NextNode : SubgraphNode->MSuccessors) {
-        auto Successor = SubgraphNodesMap.at(NextNode.lock());
+      for (node_impl &NextNode : SubgraphNode->successors()) {
+        auto Successor = SubgraphNodesMap.at(NextNode.shared_from_this());
         NodeCopy->registerSuccessor(Successor);
       }
     }
@@ -1339,9 +1338,8 @@ void exec_graph_impl::duplicateNodes() {
     // original subgraph node
 
     // Predecessors
-    for (auto &PredNodeWeak : NewNode->MPredecessors) {
-      auto PredNode = PredNodeWeak.lock();
-      auto &Successors = PredNode->MSuccessors;
+    for (node_impl &PredNode : NewNode->predecessors()) {
+      auto &Successors = PredNode.MSuccessors;
 
       // Remove the subgraph node from this nodes successors
       Successors.erase(std::remove_if(Successors.begin(), Successors.end(),
@@ -1353,14 +1351,13 @@ void exec_graph_impl::duplicateNodes() {
       // Add all input nodes from the subgraph as successors for this node
       // instead
       for (auto &Input : Inputs) {
-        PredNode->registerSuccessor(Input);
+        PredNode.registerSuccessor(Input);
       }
     }
 
     // Successors
-    for (auto &SuccNodeWeak : NewNode->MSuccessors) {
-      auto SuccNode = SuccNodeWeak.lock();
-      auto &Predecessors = SuccNode->MPredecessors;
+    for (node_impl &SuccNode : NewNode->successors()) {
+      auto &Predecessors = SuccNode.MPredecessors;
 
       // Remove the subgraph node from this nodes successors
       Predecessors.erase(std::remove_if(Predecessors.begin(),
@@ -1373,7 +1370,7 @@ void exec_graph_impl::duplicateNodes() {
       // Add all Output nodes from the subgraph as predecessors for this node
       // instead
       for (auto &Output : Outputs) {
-        Output->registerSuccessor(SuccNode);
+        Output->registerSuccessor(SuccNode.shared_from_this());
       }
     }
 

sycl/source/detail/graph/graph_impl.hpp

@@ -346,19 +346,17 @@ class graph_impl : public std::enable_shared_from_this<graph_impl> {
   /// @param NodeA pointer to the first node for comparison
   /// @param NodeB pointer to the second node for comparison
   /// @return true is same structure found, false otherwise
-  static bool checkNodeRecursive(const std::shared_ptr<node_impl> &NodeA,
-                                 const std::shared_ptr<node_impl> &NodeB) {
+  static bool checkNodeRecursive(node_impl &NodeA, node_impl &NodeB) {
     size_t FoundCnt = 0;
-    for (std::weak_ptr<node_impl> &SuccA : NodeA->MSuccessors) {
-      for (std::weak_ptr<node_impl> &SuccB : NodeB->MSuccessors) {
-        if (NodeA->isSimilar(*NodeB) &&
-            checkNodeRecursive(SuccA.lock(), SuccB.lock())) {
+    for (node_impl &SuccA : NodeA.successors()) {
+      for (node_impl &SuccB : NodeB.successors()) {
+        if (NodeA.isSimilar(NodeB) && checkNodeRecursive(SuccA, SuccB)) {
           FoundCnt++;
           break;
         }
       }
     }
-    if (FoundCnt != NodeA->MSuccessors.size()) {
+    if (FoundCnt != NodeA.MSuccessors.size()) {
       return false;
     }
 
@@ -428,7 +426,7 @@ class graph_impl : public std::enable_shared_from_this<graph_impl> {
         auto NodeBLocked = NodeB.lock();
 
         if (NodeALocked->isSimilar(*NodeBLocked)) {
-          if (checkNodeRecursive(NodeALocked, NodeBLocked)) {
+          if (checkNodeRecursive(*NodeALocked, *NodeBLocked)) {
             RootsFound++;
             break;
           }
@@ -817,8 +815,7 @@ class exec_graph_impl {
   /// SyncPoint for CurrentNode, otherwise we need to
   /// synchronize on the host with the completion of previous partitions.
   void findRealDeps(std::vector<ur_exp_command_buffer_sync_point_t> &Deps,
-                    std::shared_ptr<node_impl> CurrentNode,
-                    int ReferencePartitionNum);
+                    node_impl &CurrentNode, int ReferencePartitionNum);
 
   /// Duplicate nodes from the modifiable graph associated with this executable
   /// graph and store them locally. Any subgraph nodes in the modifiable graph

sycl/source/detail/graph/memory_pool.cpp

@@ -116,49 +116,44 @@ graph_mem_pool::tryReuseExistingAllocation(
   // free nodes. We do this in a breadth-first approach because we want to find
   // the shortest path to a reusable allocation.
 
-  std::queue<std::weak_ptr<node_impl>> NodesToCheck;
+  std::queue<node_impl *> NodesToCheck;
 
   // Add all the dependent nodes to the queue, they will be popped first
   for (auto &Dep : DepNodes) {
-    NodesToCheck.push(Dep);
+    NodesToCheck.push(&*Dep);
   }
 
   // Called when traversing over nodes to check if the current node is a free
   // node for one of the available allocations. If it is we populate AllocInfo
   // with the allocation to be reused.
   auto CheckNodeEqual =
-      [&CompatibleAllocs](const std::shared_ptr<node_impl> &CurrentNode)
-      -> std::optional<alloc_info> {
+      [&CompatibleAllocs](node_impl &CurrentNode) -> std::optional<alloc_info> {
     for (auto &Alloc : CompatibleAllocs) {
-      const auto &AllocFreeNode = Alloc.LastFreeNode;
-      // Compare control blocks without having to lock AllocFreeNode to check
-      // for node equality
-      if (!CurrentNode.owner_before(AllocFreeNode) &&
-          !AllocFreeNode.owner_before(CurrentNode)) {
+      if (&CurrentNode == Alloc.LastFreeNode) {
         return Alloc;
       }
     }
     return std::nullopt;
   };
 
   while (!NodesToCheck.empty()) {
-    auto CurrentNode = NodesToCheck.front().lock();
+    node_impl &CurrentNode = *NodesToCheck.front();
 
-    if (CurrentNode->MTotalVisitedEdges > 0) {
+    if (CurrentNode.MTotalVisitedEdges > 0) {
       continue;
     }
 
     // Check if the node is a free node and, if so, check if it is a free node
     // for any of the allocations which are free for reuse. We should not bother
     // checking nodes that are not free nodes, so we continue and check their
     // predecessors.
-    if (CurrentNode->MNodeType == node_type::async_free) {
+    if (CurrentNode.MNodeType == node_type::async_free) {
       std::optional<alloc_info> AllocFound = CheckNodeEqual(CurrentNode);
       if (AllocFound) {
         // Reset visited nodes tracking
         MGraph.resetNodeVisitedEdges();
         // Reset last free node for allocation
-        MAllocations.at(AllocFound.value().Ptr).LastFreeNode.reset();
+        MAllocations.at(AllocFound.value().Ptr).LastFreeNode = nullptr;
         // Remove found allocation from the free list
         MFreeAllocations.erase(std::find(MFreeAllocations.begin(),
                                          MFreeAllocations.end(),
@@ -168,12 +163,12 @@ graph_mem_pool::tryReuseExistingAllocation(
     }
 
     // Add CurrentNode predecessors to queue
-    for (auto &Pred : CurrentNode->MPredecessors) {
-      NodesToCheck.push(Pred);
+    for (node_impl &Pred : CurrentNode.predecessors()) {
+      NodesToCheck.push(&Pred);
     }
 
     // Mark node as visited
-    CurrentNode->MTotalVisitedEdges = 1;
+    CurrentNode.MTotalVisitedEdges = 1;
     NodesToCheck.pop();
   }
 
@@ -183,7 +178,7 @@ graph_mem_pool::tryReuseExistingAllocation(
 void graph_mem_pool::markAllocationAsAvailable(
     void *Ptr, const std::shared_ptr<node_impl> &FreeNode) {
   MFreeAllocations.push_back(Ptr);
-  MAllocations.at(Ptr).LastFreeNode = FreeNode;
+  MAllocations.at(Ptr).LastFreeNode = FreeNode.get();
 }
 
 } // namespace detail

sycl/source/detail/graph/memory_pool.hpp

@@ -44,7 +44,7 @@ class graph_mem_pool {
     // Should the allocation be zero initialized during initial allocation
     bool ZeroInit = false;
     // Last free node for this allocation in the graph
-    std::weak_ptr<node_impl> LastFreeNode = {};
+    node_impl *LastFreeNode = nullptr;
   };
 
 public:

sycl/source/detail/graph/node_impl.hpp

@@ -31,6 +31,7 @@ class node;
 namespace detail {
 // Forward declarations
 class node_impl;
+class nodes_range;
 class exec_graph_impl;
 
 /// Takes a vector of weak_ptrs to node_impls and returns a vector of node
@@ -116,6 +117,10 @@ class node_impl : public std::enable_shared_from_this<node_impl> {
   /// cannot be used to find out the partion of a node outside of this process.
   int MPartitionNum = -1;
 
+  // Out-of-class as need "complete" `nodes_range`:
+  inline nodes_range successors() const;
+  inline nodes_range predecessors() const;
+
   /// Add successor to the node.
   /// @param Node Node to add as a successor.
   void registerSuccessor(const std::shared_ptr<node_impl> &Node) {
@@ -830,6 +835,10 @@ class nodes_range {
   size_t size() const { return Size; }
   bool empty() const { return Size == 0; }
 };
+
+inline nodes_range node_impl::successors() const { return MSuccessors; }
+inline nodes_range node_impl::predecessors() const { return MPredecessors; }
+
 } // namespace detail
 } // namespace experimental
 } // namespace oneapi