GraphTraits.h

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//===- llvm/ADT/GraphTraits.h - Graph traits template -----------*- C++ -*-===//
//
// 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
//
//===----------------------------------------------------------------------===//
///
/// \file
/// This file defines the little GraphTraits<X> template class that should be
/// specialized by classes that want to be iteratable by generic graph
/// iterators.
///
/// This file also defines the marker class Inverse that is used to iterate over
/// graphs in a graph defined, inverse ordering...
///
//===----------------------------------------------------------------------===//
 
#ifndef LLVM_ADT_GRAPHTRAITS_H
#define LLVM_ADT_GRAPHTRAITS_H
 
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/iterator_range.h"
 
namespace llvm {
 
// GraphTraits - This class should be specialized by different graph types...
// which is why the default version is empty.
//
// This template evolved from supporting `BasicBlock` to also later supporting
// more complex types (e.g. CFG and DomTree).
//
// GraphTraits can be used to create a view over a graph interpreting it
// differently without requiring a copy of the original graph. This could
// be achieved by carrying more data in NodeRef.
template<class GraphType>
struct GraphTraits {
  // Elements to provide:
 
  // typedef NodeRef           - Type of Node token in the graph, which should
  //                             be cheap to copy.
  // typedef ChildIteratorType - Type used to iterate over children in graph,
  //                             dereference to a NodeRef.
 
  // static NodeRef getEntryNode(const GraphType &)
  //    Return the entry node of the graph
 
  // static ChildIteratorType child_begin(NodeRef)
  // static ChildIteratorType child_end  (NodeRef)
  //    Return iterators that point to the beginning and ending of the child
  //    node list for the specified node.
 
  // typedef  ...iterator nodes_iterator; - dereference to a NodeRef
  // static nodes_iterator nodes_begin(GraphType *G)
  // static nodes_iterator nodes_end  (GraphType *G)
  //    nodes_iterator/begin/end - Allow iteration over all nodes in the graph
 
  // typedef EdgeRef           - Type of Edge token in the graph, which should
  //                             be cheap to copy.
  // typedef ChildEdgeIteratorType - Type used to iterate over children edges in
  //                             graph, dereference to a EdgeRef.
 
  // static ChildEdgeIteratorType child_edge_begin(NodeRef)
  // static ChildEdgeIteratorType child_edge_end(NodeRef)
  //     Return iterators that point to the beginning and ending of the
  //     edge list for the given callgraph node.
  //
  // static NodeRef edge_dest(EdgeRef)
  //     Return the destination node of an edge.
 
  // static unsigned       size       (GraphType *G)
  //    Return total number of nodes in the graph
 
  // Optionally implement the following:
  // static unsigned getNumber(NodeRef)
  //    Return a unique number of a node. Numbers are ideally dense, these are
  //    used to store nodes in a vector.
  // static unsigned getMaxNumber(GraphType *G)
  //    Return the maximum number that getNumber() will return, or 0 if this is
  //    unknown. Intended for reserving large enough buffers.
  // static unsigned getNumberEpoch(GraphType *G)
  //    Return the "epoch" of the node numbers. Should return a different
  //    number after renumbering, so users can assert that the epoch didn't
  //    change => numbers are still valid. If renumberings are not tracked, it
  //    is always valid to return a constant value. This is solely for to ease
  //    debugging by having a way to detect use of outdated numbers.
 
  // If anyone tries to use this class without having an appropriate
  // specialization, make an error.  If you get this error, it's because you
  // need to include the appropriate specialization of GraphTraits<> for your
  // graph, or you need to define it for a new graph type. Either that or
  // your argument to XXX_begin(...) is unknown or needs to have the proper .h
  // file #include'd.
  using NodeRef = typename GraphType::UnknownGraphTypeError;
};
 
namespace detail {
template <typename T>
using has_number_t = decltype(GraphTraits<T>::getNumber(
    std::declval<typename GraphTraits<T>::NodeRef>()));
} // namespace detail
 
/// Indicate whether a GraphTraits<NodeT>::getNumber() is supported.
template <typename NodeT>
constexpr bool GraphHasNodeNumbers =
    is_detected<detail::has_number_t, NodeT>::value;
 
// Inverse - This class is used as a little marker class to tell the graph
// iterator to iterate over the graph in a graph defined "Inverse" ordering.
// Not all graphs define an inverse ordering, and if they do, it depends on
// the graph exactly what that is.  Here's an example of usage with the
// df_iterator:
//
// idf_iterator<Method*> I = idf_begin(M), E = idf_end(M);
// for (; I != E; ++I) { ... }
//
// Which is equivalent to:
// df_iterator<Inverse<Method*>> I = idf_begin(M), E = idf_end(M);
// for (; I != E; ++I) { ... }
//
template <class GraphType>
struct Inverse {
  const GraphType &Graph;
 
  inline Inverse(const GraphType &G) : Graph(G) {}
};
 
// Provide a partial specialization of GraphTraits so that the inverse of an
// inverse falls back to the original graph.
template <class T> struct GraphTraits<Inverse<Inverse<T>>> : GraphTraits<T> {};
 
// Provide iterator ranges for the graph traits nodes and children
template <class GraphType>
iterator_range<typename GraphTraits<GraphType>::nodes_iterator>
nodes(const GraphType &G) {
  return make_range(GraphTraits<GraphType>::nodes_begin(G),
                    GraphTraits<GraphType>::nodes_end(G));
}
template <class GraphType>
iterator_range<typename GraphTraits<Inverse<GraphType>>::nodes_iterator>
inverse_nodes(const GraphType &G) {
  return make_range(GraphTraits<Inverse<GraphType>>::nodes_begin(G),
                    GraphTraits<Inverse<GraphType>>::nodes_end(G));
}
 
template <class GraphType>
iterator_range<typename GraphTraits<GraphType>::ChildIteratorType>
children(const typename GraphTraits<GraphType>::NodeRef &G) {
  return make_range(GraphTraits<GraphType>::child_begin(G),
                    GraphTraits<GraphType>::child_end(G));
}
 
template <class GraphType>
iterator_range<typename GraphTraits<Inverse<GraphType>>::ChildIteratorType>
inverse_children(const typename GraphTraits<GraphType>::NodeRef &G) {
  return make_range(GraphTraits<Inverse<GraphType>>::child_begin(G),
                    GraphTraits<Inverse<GraphType>>::child_end(G));
}
 
template <class GraphType>
iterator_range<typename GraphTraits<GraphType>::ChildEdgeIteratorType>
children_edges(const typename GraphTraits<GraphType>::NodeRef &G) {
  return make_range(GraphTraits<GraphType>::child_edge_begin(G),
                    GraphTraits<GraphType>::child_edge_end(G));
}
 
} // end namespace llvm
 
#endif // LLVM_ADT_GRAPHTRAITS_H