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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 <type_traits>
 #include <utility>
 
 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:
   // When External storage is used we are not multi-pass safe.
   using iterator_category =
       std::conditional_t<ExtStorage, std::input_iterator_tag,
                          std::forward_iterator_tag>;
   using value_type = typename GT::NodeRef;
   using difference_type = std::ptrdiff_t;
   using pointer = value_type *;
   using reference = const value_type &;
 
 private:
   using NodeRef = typename GT::NodeRef;
   using ChildItTy = typename GT::ChildIteratorType;
 
   /// Used to maintain the ordering.
   /// First element is basic block pointer, second is iterator for the next
   /// child to visit, third is the end iterator.
   SmallVector<std::tuple<NodeRef, ChildItTy, ChildItTy>, 8> VisitStack;
 
   po_iterator(NodeRef BB) {
     this->insertEdge(std::optional<NodeRef>(), BB);
     VisitStack.emplace_back(BB, GT::child_begin(BB), GT::child_end(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.emplace_back(BB, GT::child_begin(BB), GT::child_end(BB));
       traverseChild();
     }
   }
 
   po_iterator(SetType &S)
       : po_iterator_storage<SetType, ExtStorage>(S) {
   } // End is when stack is empty.
 
   void traverseChild() {
     while (true) {
       auto &Entry = VisitStack.back();
       if (std::get<1>(Entry) == std::get<2>(Entry))
         break;
       NodeRef BB = *std::get<1>(Entry)++;
       if (this->insertEdge(std::optional<NodeRef>(std::get<0>(Entry)), BB)) {
         // If the block is not visited...
         VisitStack.emplace_back(BB, GT::child_begin(BB), GT::child_end(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); }
 
   reference operator*() const { return std::get<0>(VisitStack.back()); }
 
   // 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(std::get<0>(VisitStack.back()));
     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;
 
   using VecTy = SmallVector<NodeRef, 8>;
   VecTy 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 VecTy::reverse_iterator;
   using const_rpo_iterator = typename VecTy::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.rbegin(); }
   rpo_iterator end() { return Blocks.rend(); }
   const_rpo_iterator end() const { return Blocks.rend(); }
 };
 
 } // end namespace llvm
 
 #endif // LLVM_ADT_POSTORDERITERATOR_H

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