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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>
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 = 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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