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//===- llvm/ADT/PostOrderIterator.h - PostOrder iterator --------*- 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 builds on the ADT/GraphTraits.h file to build a generic graph

/// post order iterator.  This should work over any graph type that has a

/// GraphTraits specialization.

///

//===----------------------------------------------------------------------===//

#ifndef LLVM_ADT_POSTORDERITERATOR_H

#define LLVM_ADT_POSTORDERITERATOR_H

#include "llvm/ADT/GraphTraits.h"

#include "llvm/ADT/SmallPtrSet.h"

#include "llvm/ADT/SmallVector.h"

#include "llvm/ADT/iterator_range.h"

#include <iterator>

#include <optional>

#include <set>

#include <utility>

#include <vector>

namespace llvm {

// The po_iterator_storage template provides access to the set of already

// visited nodes during the po_iterator's depth-first traversal.

//

// The default implementation simply contains a set of visited nodes, while

// the External=true version uses a reference to an external set.

//

// It is possible to prune the depth-first traversal in several ways:

//

// - When providing an external set that already contains some graph nodes,

//   those nodes won't be visited again. This is useful for restarting a

//   post-order traversal on a graph with nodes that aren't dominated by a

//   single node.

//

// - By providing a custom SetType class, unwanted graph nodes can be excluded

//   by having the insert() function return false. This could for example

//   confine a CFG traversal to blocks in a specific loop.

//

// - Finally, by specializing the po_iterator_storage template itself, graph

//   edges can be pruned by returning false in the insertEdge() function. This

//   could be used to remove loop back-edges from the CFG seen by po_iterator.

//

// A specialized po_iterator_storage class can observe both the pre-order and

// the post-order. The insertEdge() function is called in a pre-order, while

// the finishPostorder() function is called just before the po_iterator moves

// on to the next node.

/// Default po_iterator_storage implementation with an internal set object.

template<class SetType, bool External>

class po_iterator_storage {

  SetType Visited;

public:

  // Return true if edge destination should be visited.

  template <typename NodeRef>

  bool insertEdge(std::optional<NodeRef> From, NodeRef To) {

    return Visited.insert(To).second;

  }

  // Called after all children of BB have been visited.

  template <typename NodeRef> void finishPostorder(NodeRef BB) {}

};

/// Specialization of po_iterator_storage that references an external set.

template<class SetType>

class po_iterator_storage<SetType, true> {

  SetType &Visited;

public:

  po_iterator_storage(SetType &VSet) : Visited(VSet) {}

  po_iterator_storage(const po_iterator_storage &S) : Visited(S.Visited) {}

  // Return true if edge destination should be visited, called with From = 0 for

  // the root node.

  // Graph edges can be pruned by specializing this function.

  template <class NodeRef>

  bool insertEdge(std::optional<NodeRef> From, NodeRef To) {

    return Visited.insert(To).second;

  }

  // Called after all children of BB have been visited.

  template <class NodeRef> void finishPostorder(NodeRef BB) {}

};

template <class GraphT,

          class SetType = SmallPtrSet<typename GraphTraits<GraphT>::NodeRef, 8>,

          bool ExtStorage = false, class GT = GraphTraits<GraphT>>

class po_iterator : public po_iterator_storage<SetType, ExtStorage> {

public:

  using iterator_category = std::forward_iterator_tag;

  using value_type = typename GT::NodeRef;

  using difference_type = std::ptrdiff_t;

  using pointer = value_type *;

  using reference = value_type &;

private:

  using NodeRef = typename GT::NodeRef;

  using ChildItTy = typename GT::ChildIteratorType;

  // VisitStack - Used to maintain the ordering.  Top = current block

  // First element is basic block pointer, second is the 'next child' to visit

  SmallVector<std::pair<NodeRef, ChildItTy>, 8> VisitStack;

  po_iterator(NodeRef BB) {

    this->insertEdge(std::optional<NodeRef>(), BB);

    VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));

    traverseChild();

  }

  po_iterator() = default; // End is when stack is empty.

  po_iterator(NodeRef BB, SetType &S)

      : po_iterator_storage<SetType, ExtStorage>(S) {

    if (this->insertEdge(std::optional<NodeRef>(), BB)) {

      VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));

      traverseChild();

    }

  }

  po_iterator(SetType &S)

      : po_iterator_storage<SetType, ExtStorage>(S) {

  } // End is when stack is empty.

  void traverseChild() {

    while (VisitStack.back().second != GT::child_end(VisitStack.back().first)) {

      NodeRef BB = *VisitStack.back().second++;

      if (this->insertEdge(std::optional<NodeRef>(VisitStack.back().first),

                           BB)) {

        // If the block is not visited...

        VisitStack.push_back(std::make_pair(BB, GT::child_begin(BB)));

      }

    }

  }

public:

  // Provide static "constructors"...

  static po_iterator begin(const GraphT &G) {

    return po_iterator(GT::getEntryNode(G));

  }

  static po_iterator end(const GraphT &G) { return po_iterator(); }

  static po_iterator begin(const GraphT &G, SetType &S) {

    return po_iterator(GT::getEntryNode(G), S);

  }

  static po_iterator end(const GraphT &G, SetType &S) { return po_iterator(S); }

  bool operator==(const po_iterator &x) const {

    return VisitStack == x.VisitStack;

  }

  bool operator!=(const po_iterator &x) const { return !(*this == x); }

  const NodeRef &operator*() const { return VisitStack.back().first; }

  // This is a nonstandard operator-> that dereferences the pointer an extra

  // time... so that you can actually call methods ON the BasicBlock, because

  // the contained type is a pointer.  This allows BBIt->getTerminator() f.e.

  //

  NodeRef operator->() const { return **this; }

  po_iterator &operator++() { // Preincrement

    this->finishPostorder(VisitStack.back().first);

    VisitStack.pop_back();

    if (!VisitStack.empty())

      traverseChild();

    return *this;

  }

  po_iterator operator++(int) { // Postincrement

    po_iterator tmp = *this;

    ++*this;

    return tmp;

  }

};

// Provide global constructors that automatically figure out correct types...

//

template <class T>

po_iterator<T> po_begin(const T &G) { return po_iterator<T>::begin(G); }

template <class T>

po_iterator<T> po_end  (const T &G) { return po_iterator<T>::end(G); }

template <class T> iterator_range<po_iterator<T>> post_order(const T &G) {

  return make_range(po_begin(G), po_end(G));

}

// Provide global definitions of external postorder iterators...

template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>

struct po_ext_iterator : public po_iterator<T, SetType, true> {

  po_ext_iterator(const po_iterator<T, SetType, true> &V) :

  po_iterator<T, SetType, true>(V) {}

};

template<class T, class SetType>

po_ext_iterator<T, SetType> po_ext_begin(T G, SetType &S) {

  return po_ext_iterator<T, SetType>::begin(G, S);

}

template<class T, class SetType>

po_ext_iterator<T, SetType> po_ext_end(T G, SetType &S) {

  return po_ext_iterator<T, SetType>::end(G, S);

}

template <class T, class SetType>

iterator_range<po_ext_iterator<T, SetType>> post_order_ext(const T &G, SetType &S) {

  return make_range(po_ext_begin(G, S), po_ext_end(G, S));

}

// Provide global definitions of inverse post order iterators...

template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>,

          bool External = false>

struct ipo_iterator : public po_iterator<Inverse<T>, SetType, External> {

  ipo_iterator(const po_iterator<Inverse<T>, SetType, External> &V) :

     po_iterator<Inverse<T>, SetType, External> (V) {}

};

template <class T>

ipo_iterator<T> ipo_begin(const T &G) {

  return ipo_iterator<T>::begin(G);

}

template <class T>

ipo_iterator<T> ipo_end(const T &G){

  return ipo_iterator<T>::end(G);

}

template <class T>

iterator_range<ipo_iterator<T>> inverse_post_order(const T &G) {

  return make_range(ipo_begin(G), ipo_end(G));

}

// Provide global definitions of external inverse postorder iterators...

template <class T, class SetType = std::set<typename GraphTraits<T>::NodeRef>>

struct ipo_ext_iterator : public ipo_iterator<T, SetType, true> {

  ipo_ext_iterator(const ipo_iterator<T, SetType, true> &V) :

    ipo_iterator<T, SetType, true>(V) {}

  ipo_ext_iterator(const po_iterator<Inverse<T>, SetType, true> &V) :

    ipo_iterator<T, SetType, true>(V) {}

};

template <class T, class SetType>

ipo_ext_iterator<T, SetType> ipo_ext_begin(const T &G, SetType &S) {

  return ipo_ext_iterator<T, SetType>::begin(G, S);

}

template <class T, class SetType>

ipo_ext_iterator<T, SetType> ipo_ext_end(const T &G, SetType &S) {

  return ipo_ext_iterator<T, SetType>::end(G, S);

}

template <class T, class SetType>

iterator_range<ipo_ext_iterator<T, SetType>>

inverse_post_order_ext(const T &G, SetType &S) {

  return make_range(ipo_ext_begin(G, S), ipo_ext_end(G, S));

}

//===--------------------------------------------------------------------===//

// Reverse Post Order CFG iterator code

//===--------------------------------------------------------------------===//

//

// This is used to visit basic blocks in a method in reverse post order.  This

// class is awkward to use because I don't know a good incremental algorithm to

// computer RPO from a graph.  Because of this, the construction of the

// ReversePostOrderTraversal object is expensive (it must walk the entire graph

// with a postorder iterator to build the data structures).  The moral of this

// story is: Don't create more ReversePostOrderTraversal classes than necessary.

//

// Because it does the traversal in its constructor, it won't invalidate when

// BasicBlocks are removed, *but* it may contain erased blocks. Some places

// rely on this behavior (i.e. GVN).

//

// This class should be used like this:

// {

//   ReversePostOrderTraversal<Function*> RPOT(FuncPtr); // Expensive to create

//   for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) {

//      ...

//   }

//   for (rpo_iterator I = RPOT.begin(); I != RPOT.end(); ++I) {

//      ...

//   }

// }

//

template<class GraphT, class GT = GraphTraits<GraphT>>

class ReversePostOrderTraversal {

  using NodeRef = typename GT::NodeRef;

  std::vector<NodeRef> Blocks; // Block list in normal PO order

  void Initialize(const GraphT &G) {

    std::copy(po_begin(G), po_end(G), std::back_inserter(Blocks));

  }

public:

  using rpo_iterator = typename std::vector<NodeRef>::reverse_iterator;

  using const_rpo_iterator = typename std::vector<NodeRef>::const_reverse_iterator;

  ReversePostOrderTraversal(const GraphT &G) { Initialize(G); }

  // Because we want a reverse post order, use reverse iterators from the vector

  rpo_iterator begin() { return Blocks.rbegin(); }

  const_rpo_iterator begin() const { return Blocks.crbegin(); }

  rpo_iterator end() { return Blocks.rend(); }

  const_rpo_iterator end() const { return Blocks.crend(); }

};

} // end namespace llvm

#endif // LLVM_ADT_POSTORDERITERATOR_H