1 /* Copyright 2017 The TensorFlow Authors. All Rights Reserved. 2 3 Licensed under the Apache License, Version 2.0 (the "License"); 4 you may not use this file except in compliance with the License. 5 You may obtain a copy of the License at 6 7 http://www.apache.org/licenses/LICENSE-2.0 8 9 Unless required by applicable law or agreed to in writing, software 10 distributed under the License is distributed on an "AS IS" BASIS, 11 WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 12 See the License for the specific language governing permissions and 13 limitations under the License. 14 ==============================================================================*/ 15 16 #ifndef TENSORFLOW_COMPILER_JIT_GRAPHCYCLES_GRAPHCYCLES_H_ 17 #define TENSORFLOW_COMPILER_JIT_GRAPHCYCLES_GRAPHCYCLES_H_ 18 19 // GraphCycles detects the introduction of a cycle into a directed 20 // graph that is being built up incrementally. 21 // 22 // Nodes are identified by small integers. It is not possible to 23 // record multiple edges with the same (source, destination) pair; 24 // requests to add an edge where one already exists are silently 25 // ignored. 26 // 27 // It is also not possible to introduce a cycle; an attempt to insert 28 // an edge that would introduce a cycle fails and returns false. 29 // 30 // GraphCycles uses no internal locking; calls into it should be 31 // serialized externally. 32 33 // Performance considerations: 34 // Works well on sparse graphs, poorly on dense graphs. 35 // Extra information is maintained incrementally to detect cycles quickly. 36 // InsertEdge() is very fast when the edge already exists, and reasonably fast 37 // otherwise. 38 // FindPath() is linear in the size of the graph. 39 // The current implementation uses O(|V|+|E|) space. 40 41 #include <unordered_set> 42 43 #include "tensorflow/core/platform/macros.h" 44 #include "tensorflow/core/platform/types.h" 45 46 namespace tensorflow { 47 48 // NOTE!!! 49 // For now a copy of this is forked to net/plaque. If you 50 // find a bug or add a feature, please inform the owners of the 51 // net/plaque copy in case it should be integrated. 52 // NOTE!!! 53 class GraphCycles { 54 public: 55 GraphCycles(); 56 ~GraphCycles(); 57 58 // Allocate an unused node id and return it. 59 // The new node has a null pointer for its node data. 60 // All node identifiers passed to other routines in this interface 61 // must have been allocated by NewNode() and not yet deallocated 62 // by RemoveNode(). 63 int32 NewNode(); 64 65 // Remove "node" from the graph, deleting all edges to and from it. 66 // After this call the identifier "node" it may no longer be used 67 // as an argument to any routine until it has been reallocated with 68 // NewNode(). 69 void RemoveNode(int32 node); 70 71 // Attempt to insert an edge from source_node to dest_node. If the 72 // edge would introduce a cycle, return false without making any 73 // changes. Otherwise add the edge and return true. 74 bool InsertEdge(int32 source_node, int32 dest_node); 75 76 // Remove any edge that exists from source_node to dest_node. 77 void RemoveEdge(int32 source_node, int32 dest_node); 78 79 // Return whether there is an edge directly from source_node to dest_node. 80 bool HasEdge(int32 source_node, int32 dest_node) const; 81 82 // Contracts the edge from 'a' to node 'b', merging nodes 'a' and 'b'. 'b' is 83 // removed from the graph, and edges to/from 'b' are replaced with edges 84 // to/from 'a'. If contracting the edge would create a cycle, does nothing 85 // and returns false. 86 bool ContractEdge(int32 a, int32 b); 87 88 // Return true if can contract edge, otherwise return false. 89 bool CanContractEdge(int32 a, int32 b); 90 91 // Return whether dest_node is reachable from source_node 92 // by following edges. 93 bool IsReachable(int32 source_node, int32 dest_node) const; 94 95 // A faster non-thread-safe version of IsReachable. 96 bool IsReachableNonConst(int32 source_node, int32 dest_node); 97 98 // Return or set the node data for a node. This data is unused 99 // by the implementation. 100 void *GetNodeData(int32 node) const; 101 void SetNodeData(int32 node, void *data); 102 103 // Find a path from "source" to "dest". If such a path exists, place the 104 // node IDs of the nodes on the path in the array path[], and return the 105 // number of nodes on the path. If the path is longer than max_path_len 106 // nodes, only the first max_path_len nodes are placed in path[]. The client 107 // should compare the return value with max_path_len" to see when this 108 // occurs. If no path exists, return 0. Any valid path stored in path[] 109 // will start with "source" and end with "dest". There is no guarantee that 110 // the path is the shortest, but no node will appear twice in the path, 111 // except the source and destination node if they are identical; therefore, 112 // the return value is at most one greater than the number of nodes in the 113 // graph. 114 int FindPath(int32 source, int32 dest, int max_path_len, int32 path[]) const; 115 116 // Check internal invariants. Crashes on failure, returns true on success. 117 // Expensive: should only be called from graphcycles_test.cc. 118 bool CheckInvariants() const; 119 120 std::unordered_set<int32> Successors(int32 node); 121 std::unordered_set<int32> Predecessors(int32 node); 122 123 // ---------------------------------------------------- 124 struct Rep; 125 126 private: 127 Rep *rep_; // opaque representation 128 TF_DISALLOW_COPY_AND_ASSIGN(GraphCycles); 129 }; 130 131 } // namespace tensorflow 132 #endif // TENSORFLOW_COMPILER_JIT_GRAPHCYCLES_GRAPHCYCLES_H_ 133