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//===- LazyCallGraph.h - Analysis of a Module's call graph ------*- 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

///

/// Implements a lazy call graph analysis and related passes for the new pass

/// manager.

///

/// NB: This is *not* a traditional call graph! It is a graph which models both

/// the current calls and potential calls. As a consequence there are many

/// edges in this call graph that do not correspond to a 'call' or 'invoke'

/// instruction.

///

/// The primary use cases of this graph analysis is to facilitate iterating

/// across the functions of a module in ways that ensure all callees are

/// visited prior to a caller (given any SCC constraints), or vice versa. As

/// such is it particularly well suited to organizing CGSCC optimizations such

/// as inlining, outlining, argument promotion, etc. That is its primary use

/// case and motivates the design. It may not be appropriate for other

/// purposes. The use graph of functions or some other conservative analysis of

/// call instructions may be interesting for optimizations and subsequent

/// analyses which don't work in the context of an overly specified

/// potential-call-edge graph.

///

/// To understand the specific rules and nature of this call graph analysis,

/// see the documentation of the \c LazyCallGraph below.

///

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

#ifndef LLVM_ANALYSIS_LAZYCALLGRAPH_H

#define LLVM_ANALYSIS_LAZYCALLGRAPH_H

#include "llvm/ADT/ArrayRef.h"

#include "llvm/ADT/DenseMap.h"

#include "llvm/ADT/PointerIntPair.h"

#include "llvm/ADT/SetVector.h"

#include "llvm/ADT/SmallVector.h"

#include "llvm/ADT/StringRef.h"

#include "llvm/ADT/iterator.h"

#include "llvm/ADT/iterator_range.h"

#include "llvm/Analysis/TargetLibraryInfo.h"

#include "llvm/IR/PassManager.h"

#include "llvm/Support/Allocator.h"

#include "llvm/Support/raw_ostream.h"

#include <cassert>

#include <iterator>

#include <optional>

#include <string>

#include <utility>

namespace llvm {

class Constant;

class Function;

template <class GraphType> struct GraphTraits;

class Module;

class TargetLibraryInfo;

class Value;

/// A lazily constructed view of the call graph of a module.

///

/// With the edges of this graph, the motivating constraint that we are

/// attempting to maintain is that function-local optimization, CGSCC-local

/// optimizations, and optimizations transforming a pair of functions connected

/// by an edge in the graph, do not invalidate a bottom-up traversal of the SCC

/// DAG. That is, no optimizations will delete, remove, or add an edge such

/// that functions already visited in a bottom-up order of the SCC DAG are no

/// longer valid to have visited, or such that functions not yet visited in

/// a bottom-up order of the SCC DAG are not required to have already been

/// visited.

///

/// Within this constraint, the desire is to minimize the merge points of the

/// SCC DAG. The greater the fanout of the SCC DAG and the fewer merge points

/// in the SCC DAG, the more independence there is in optimizing within it.

/// There is a strong desire to enable parallelization of optimizations over

/// the call graph, and both limited fanout and merge points will (artificially

/// in some cases) limit the scaling of such an effort.

///

/// To this end, graph represents both direct and any potential resolution to

/// an indirect call edge. Another way to think about it is that it represents

/// both the direct call edges and any direct call edges that might be formed

/// through static optimizations. Specifically, it considers taking the address

/// of a function to be an edge in the call graph because this might be

/// forwarded to become a direct call by some subsequent function-local

/// optimization. The result is that the graph closely follows the use-def

/// edges for functions. Walking "up" the graph can be done by looking at all

/// of the uses of a function.

///

/// The roots of the call graph are the external functions and functions

/// escaped into global variables. Those functions can be called from outside

/// of the module or via unknowable means in the IR -- we may not be able to

/// form even a potential call edge from a function body which may dynamically

/// load the function and call it.

///

/// This analysis still requires updates to remain valid after optimizations

/// which could potentially change the set of potential callees. The

/// constraints it operates under only make the traversal order remain valid.

///

/// The entire analysis must be re-computed if full interprocedural

/// optimizations run at any point. For example, globalopt completely

/// invalidates the information in this analysis.

///

/// FIXME: This class is named LazyCallGraph in a lame attempt to distinguish

/// it from the existing CallGraph. At some point, it is expected that this

/// will be the only call graph and it will be renamed accordingly.

class LazyCallGraph {

public:

  class Node;

  class EdgeSequence;

  class SCC;

  class RefSCC;

  /// A class used to represent edges in the call graph.

  ///

  /// The lazy call graph models both *call* edges and *reference* edges. Call

  /// edges are much what you would expect, and exist when there is a 'call' or

  /// 'invoke' instruction of some function. Reference edges are also tracked

  /// along side these, and exist whenever any instruction (transitively

  /// through its operands) references a function. All call edges are

  /// inherently reference edges, and so the reference graph forms a superset

  /// of the formal call graph.

  ///

  /// All of these forms of edges are fundamentally represented as outgoing

  /// edges. The edges are stored in the source node and point at the target

  /// node. This allows the edge structure itself to be a very compact data

  /// structure: essentially a tagged pointer.

  class Edge {

  public:

    /// The kind of edge in the graph.

    enum Kind : bool { Ref = false, Call = true };

    Edge();

    explicit Edge(Node &N, Kind K);

    /// Test whether the edge is null.

    ///

    /// This happens when an edge has been deleted. We leave the edge objects

    /// around but clear them.

    explicit operator bool() const;

    /// Returns the \c Kind of the edge.

    Kind getKind() const;

    /// Test whether the edge represents a direct call to a function.

    ///

    /// This requires that the edge is not null.

    bool isCall() const;

    /// Get the call graph node referenced by this edge.

    ///

    /// This requires that the edge is not null.

    Node &getNode() const;

    /// Get the function referenced by this edge.

    ///

    /// This requires that the edge is not null.

    Function &getFunction() const;

  private:

    friend class LazyCallGraph::EdgeSequence;

    friend class LazyCallGraph::RefSCC;

    PointerIntPair<Node *, 1, Kind> Value;

    void setKind(Kind K) { Value.setInt(K); }

  };

  /// The edge sequence object.

  ///

  /// This typically exists entirely within the node but is exposed as

  /// a separate type because a node doesn't initially have edges. An explicit

  /// population step is required to produce this sequence at first and it is

  /// then cached in the node. It is also used to represent edges entering the

  /// graph from outside the module to model the graph's roots.

  ///

  /// The sequence itself both iterable and indexable. The indexes remain

  /// stable even as the sequence mutates (including removal).

  class EdgeSequence {

    friend class LazyCallGraph;

    friend class LazyCallGraph::Node;

    friend class LazyCallGraph::RefSCC;

    using VectorT = SmallVector<Edge, 4>;

    using VectorImplT = SmallVectorImpl<Edge>;

  public:

    /// An iterator used for the edges to both entry nodes and child nodes.

    class iterator

        : public iterator_adaptor_base<iterator, VectorImplT::iterator,

                                       std::forward_iterator_tag> {

      friend class LazyCallGraph;

      friend class LazyCallGraph::Node;

      VectorImplT::iterator E;

      // Build the iterator for a specific position in the edge list.

      iterator(VectorImplT::iterator BaseI, VectorImplT::iterator E)

          : iterator_adaptor_base(BaseI), E(E) {

        while (I != E && !*I)

          ++I;

      }

    public:

      iterator() = default;

      using iterator_adaptor_base::operator++;

      iterator &operator++() {

        do {

          ++I;

        } while (I != E && !*I);

        return *this;

      }

    };

    /// An iterator over specifically call edges.

    ///

    /// This has the same iteration properties as the \c iterator, but

    /// restricts itself to edges which represent actual calls.

    class call_iterator

        : public iterator_adaptor_base<call_iterator, VectorImplT::iterator,

                                       std::forward_iterator_tag> {

      friend class LazyCallGraph;

      friend class LazyCallGraph::Node;

      VectorImplT::iterator E;

      /// Advance the iterator to the next valid, call edge.

      void advanceToNextEdge() {

        while (I != E && (!*I || !I->isCall()))

          ++I;

      }

      // Build the iterator for a specific position in the edge list.

      call_iterator(VectorImplT::iterator BaseI, VectorImplT::iterator E)

          : iterator_adaptor_base(BaseI), E(E) {

        advanceToNextEdge();

      }

    public:

      call_iterator() = default;

      using iterator_adaptor_base::operator++;

      call_iterator &operator++() {

        ++I;

        advanceToNextEdge();

        return *this;

      }

    };

    iterator begin() { return iterator(Edges.begin(), Edges.end()); }

    iterator end() { return iterator(Edges.end(), Edges.end()); }

    Edge &operator[](Node &N) {

      assert(EdgeIndexMap.find(&N) != EdgeIndexMap.end() && "No such edge!");

      auto &E = Edges[EdgeIndexMap.find(&N)->second];

      assert(E && "Dead or null edge!");

      return E;

    }

    Edge *lookup(Node &N) {

      auto EI = EdgeIndexMap.find(&N);

      if (EI == EdgeIndexMap.end())

        return nullptr;

      auto &E = Edges[EI->second];

      return E ? &E : nullptr;

    }

    call_iterator call_begin() {

      return call_iterator(Edges.begin(), Edges.end());

    }

    call_iterator call_end() { return call_iterator(Edges.end(), Edges.end()); }

    iterator_range<call_iterator> calls() {

      return make_range(call_begin(), call_end());

    }

    bool empty() {

      for (auto &E : Edges)

        if (E)

          return false;

      return true;

    }

  private:

    VectorT Edges;

    DenseMap<Node *, int> EdgeIndexMap;

    EdgeSequence() = default;

    /// Internal helper to insert an edge to a node.

    void insertEdgeInternal(Node &ChildN, Edge::Kind EK);

    /// Internal helper to change an edge kind.

    void setEdgeKind(Node &ChildN, Edge::Kind EK);

    /// Internal helper to remove the edge to the given function.

    bool removeEdgeInternal(Node &ChildN);

  };

  /// A node in the call graph.

  ///

  /// This represents a single node. Its primary roles are to cache the list of

  /// callees, de-duplicate and provide fast testing of whether a function is a

  /// callee, and facilitate iteration of child nodes in the graph.

  ///

  /// The node works much like an optional in order to lazily populate the

  /// edges of each node. Until populated, there are no edges. Once populated,

  /// you can access the edges by dereferencing the node or using the `->`

  /// operator as if the node was an `std::optional<EdgeSequence>`.

  class Node {

    friend class LazyCallGraph;

    friend class LazyCallGraph::RefSCC;

  public:

    LazyCallGraph &getGraph() const { return *G; }

    Function &getFunction() const { return *F; }

    StringRef getName() const { return F->getName(); }

    /// Equality is defined as address equality.

    bool operator==(const Node &N) const { return this == &N; }

    bool operator!=(const Node &N) const { return !operator==(N); }

    /// Tests whether the node has been populated with edges.

    bool isPopulated() const { return Edges.has_value(); }

    /// Tests whether this is actually a dead node and no longer valid.

    ///

    /// Users rarely interact with nodes in this state and other methods are

    /// invalid. This is used to model a node in an edge list where the

    /// function has been completely removed.

    bool isDead() const {

      assert(!G == !F &&

             "Both graph and function pointers should be null or non-null.");

      return !G;

    }

    // We allow accessing the edges by dereferencing or using the arrow

    // operator, essentially wrapping the internal optional.

    EdgeSequence &operator*() const {

      // Rip const off because the node itself isn't changing here.

      return const_cast<EdgeSequence &>(*Edges);

    }

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

    /// Populate the edges of this node if necessary.

    ///

    /// The first time this is called it will populate the edges for this node

    /// in the graph. It does this by scanning the underlying function, so once

    /// this is done, any changes to that function must be explicitly reflected

    /// in updates to the graph.

    ///

    /// \returns the populated \c EdgeSequence to simplify walking it.

    ///

    /// This will not update or re-scan anything if called repeatedly. Instead,

    /// the edge sequence is cached and returned immediately on subsequent

    /// calls.

    EdgeSequence &populate() {

      if (Edges)

        return *Edges;

      return populateSlow();

    }

  private:

    LazyCallGraph *G;

    Function *F;

    // We provide for the DFS numbering and Tarjan walk lowlink numbers to be

    // stored directly within the node. These are both '-1' when nodes are part

    // of an SCC (or RefSCC), or '0' when not yet reached in a DFS walk.

    int DFSNumber = 0;

    int LowLink = 0;

    std::optional<EdgeSequence> Edges;

    /// Basic constructor implements the scanning of F into Edges and

    /// EdgeIndexMap.

    Node(LazyCallGraph &G, Function &F) : G(&G), F(&F) {}

    /// Implementation of the scan when populating.

    EdgeSequence &populateSlow();

    /// Internal helper to directly replace the function with a new one.

    ///

    /// This is used to facilitate transformations which need to replace the

    /// formal Function object but directly move the body and users from one to

    /// the other.

    void replaceFunction(Function &NewF);

    void clear() { Edges.reset(); }

    /// Print the name of this node's function.

    friend raw_ostream &operator<<(raw_ostream &OS, const Node &N) {

      return OS << N.F->getName();

    }

    /// Dump the name of this node's function to stderr.

    void dump() const;

  };

  /// An SCC of the call graph.

  ///

  /// This represents a Strongly Connected Component of the direct call graph

  /// -- ignoring indirect calls and function references. It stores this as

  /// a collection of call graph nodes. While the order of nodes in the SCC is

  /// stable, it is not any particular order.

  ///

  /// The SCCs are nested within a \c RefSCC, see below for details about that

  /// outer structure. SCCs do not support mutation of the call graph, that

  /// must be done through the containing \c RefSCC in order to fully reason

  /// about the ordering and connections of the graph.

  class LLVM_EXTERNAL_VISIBILITY SCC {

    friend class LazyCallGraph;

    friend class LazyCallGraph::Node;

    RefSCC *OuterRefSCC;

    SmallVector<Node *, 1> Nodes;

    template <typename NodeRangeT>

    SCC(RefSCC &OuterRefSCC, NodeRangeT &&Nodes)

        : OuterRefSCC(&OuterRefSCC), Nodes(std::forward<NodeRangeT>(Nodes)) {}

    void clear() {

      OuterRefSCC = nullptr;

      Nodes.clear();

    }

    /// Print a short description useful for debugging or logging.

    ///

    /// We print the function names in the SCC wrapped in '()'s and skipping

    /// the middle functions if there are a large number.

    //

    // Note: this is defined inline to dodge issues with GCC's interpretation

    // of enclosing namespaces for friend function declarations.

    friend raw_ostream &operator<<(raw_ostream &OS, const SCC &C) {

      OS << '(';

      int I = 0;

      for (LazyCallGraph::Node &N : C) {

        if (I > 0)

          OS << ", ";

        // Elide the inner elements if there are too many.

        if (I > 8) {

          OS << "..., " << *C.Nodes.back();

          break;

        }

        OS << N;

        ++I;

      }

      OS << ')';

      return OS;

    }

    /// Dump a short description of this SCC to stderr.

    void dump() const;

#if !defined(NDEBUG) || defined(EXPENSIVE_CHECKS)

    /// Verify invariants about the SCC.

    ///

    /// This will attempt to validate all of the basic invariants within an

    /// SCC, but not that it is a strongly connected component per se.

    /// Primarily useful while building and updating the graph to check that

    /// basic properties are in place rather than having inexplicable crashes

    /// later.

    void verify();

#endif

  public:

    using iterator = pointee_iterator<SmallVectorImpl<Node *>::const_iterator>;

    iterator begin() const { return Nodes.begin(); }

    iterator end() const { return Nodes.end(); }

    int size() const { return Nodes.size(); }

    RefSCC &getOuterRefSCC() const { return *OuterRefSCC; }

    /// Test if this SCC is a parent of \a C.

    ///

    /// Note that this is linear in the number of edges departing the current

    /// SCC.

    bool isParentOf(const SCC &C) const;

    /// Test if this SCC is an ancestor of \a C.

    ///

    /// Note that in the worst case this is linear in the number of edges

    /// departing the current SCC and every SCC in the entire graph reachable

    /// from this SCC. Thus this very well may walk every edge in the entire

    /// call graph! Do not call this in a tight loop!

    bool isAncestorOf(const SCC &C) const;

    /// Test if this SCC is a child of \a C.

    ///

    /// See the comments for \c isParentOf for detailed notes about the

    /// complexity of this routine.

    bool isChildOf(const SCC &C) const { return C.isParentOf(*this); }

    /// Test if this SCC is a descendant of \a C.

    ///

    /// See the comments for \c isParentOf for detailed notes about the

    /// complexity of this routine.

    bool isDescendantOf(const SCC &C) const { return C.isAncestorOf(*this); }

    /// Provide a short name by printing this SCC to a std::string.

    ///

    /// This copes with the fact that we don't have a name per se for an SCC

    /// while still making the use of this in debugging and logging useful.

    std::string getName() const {

      std::string Name;

      raw_string_ostream OS(Name);

      OS << *this;

      OS.flush();

      return Name;

    }

  };

  /// A RefSCC of the call graph.

  ///

  /// This models a Strongly Connected Component of function reference edges in

  /// the call graph. As opposed to actual SCCs, these can be used to scope

  /// subgraphs of the module which are independent from other subgraphs of the

  /// module because they do not reference it in any way. This is also the unit

  /// where we do mutation of the graph in order to restrict mutations to those

  /// which don't violate this independence.

  ///

  /// A RefSCC contains a DAG of actual SCCs. All the nodes within the RefSCC

  /// are necessarily within some actual SCC that nests within it. Since

  /// a direct call *is* a reference, there will always be at least one RefSCC

  /// around any SCC.

  ///

  /// Spurious ref edges, meaning ref edges that still exist in the call graph

  /// even though the corresponding IR reference no longer exists, are allowed.

  /// This is mostly to support argument promotion, which can modify a caller to

  /// no longer pass a function. The only place that needs to specially handle

  /// this is deleting a dead function/node, otherwise the dead ref edges are

  /// automatically removed when visiting the function/node no longer containing

  /// the ref edge.

  class RefSCC {

    friend class LazyCallGraph;

    friend class LazyCallGraph::Node;

    LazyCallGraph *G;

    /// A postorder list of the inner SCCs.

    SmallVector<SCC *, 4> SCCs;

    /// A map from SCC to index in the postorder list.

    SmallDenseMap<SCC *, int, 4> SCCIndices;

    /// Fast-path constructor. RefSCCs should instead be constructed by calling

    /// formRefSCCFast on the graph itself.

    RefSCC(LazyCallGraph &G);

    void clear() {

      SCCs.clear();

      SCCIndices.clear();

    }

    /// Print a short description useful for debugging or logging.

    ///

    /// We print the SCCs wrapped in '[]'s and skipping the middle SCCs if

    /// there are a large number.

    //

    // Note: this is defined inline to dodge issues with GCC's interpretation

    // of enclosing namespaces for friend function declarations.

    friend raw_ostream &operator<<(raw_ostream &OS, const RefSCC &RC) {

      OS << '[';

      int I = 0;

      for (LazyCallGraph::SCC &C : RC) {

        if (I > 0)

          OS << ", ";

        // Elide the inner elements if there are too many.

        if (I > 4) {

          OS << "..., " << *RC.SCCs.back();

          break;

        }

        OS << C;

        ++I;

      }

      OS << ']';

      return OS;

    }

    /// Dump a short description of this RefSCC to stderr.

    void dump() const;

#if !defined(NDEBUG) || defined(EXPENSIVE_CHECKS)

    /// Verify invariants about the RefSCC and all its SCCs.

    ///

    /// This will attempt to validate all of the invariants *within* the

    /// RefSCC, but not that it is a strongly connected component of the larger

    /// graph. This makes it useful even when partially through an update.

    ///

    /// Invariants checked:

    /// - SCCs and their indices match.

    /// - The SCCs list is in fact in post-order.

    void verify();

#endif

  public:

    using iterator = pointee_iterator<SmallVectorImpl<SCC *>::const_iterator>;

    using range = iterator_range<iterator>;

    using parent_iterator =

        pointee_iterator<SmallPtrSetImpl<RefSCC *>::const_iterator>;

    iterator begin() const { return SCCs.begin(); }

    iterator end() const { return SCCs.end(); }

    ssize_t size() const { return SCCs.size(); }

    SCC &operator[](int Idx) { return *SCCs[Idx]; }

    iterator find(SCC &C) const {

      return SCCs.begin() + SCCIndices.find(&C)->second;

    }

    /// Test if this RefSCC is a parent of \a RC.

    ///

    /// CAUTION: This method walks every edge in the \c RefSCC, it can be very

    /// expensive.

    bool isParentOf(const RefSCC &RC) const;

    /// Test if this RefSCC is an ancestor of \a RC.

    ///

    /// CAUTION: This method walks the directed graph of edges as far as

    /// necessary to find a possible path to the argument. In the worst case

    /// this may walk the entire graph and can be extremely expensive.

    bool isAncestorOf(const RefSCC &RC) const;

    /// Test if this RefSCC is a child of \a RC.

    ///

    /// CAUTION: This method walks every edge in the argument \c RefSCC, it can

    /// be very expensive.

    bool isChildOf(const RefSCC &RC) const { return RC.isParentOf(*this); }

    /// Test if this RefSCC is a descendant of \a RC.

    ///

    /// CAUTION: This method walks the directed graph of edges as far as

    /// necessary to find a possible path from the argument. In the worst case

    /// this may walk the entire graph and can be extremely expensive.

    bool isDescendantOf(const RefSCC &RC) const {

      return RC.isAncestorOf(*this);

    }

    /// Provide a short name by printing this RefSCC to a std::string.

    ///

    /// This copes with the fact that we don't have a name per se for an RefSCC

    /// while still making the use of this in debugging and logging useful.

    std::string getName() const {

      std::string Name;

      raw_string_ostream OS(Name);

      OS << *this;

      OS.flush();

      return Name;

    }

    ///@{

    /// \name Mutation API

    ///

    /// These methods provide the core API for updating the call graph in the

    /// presence of (potentially still in-flight) DFS-found RefSCCs and SCCs.

    ///

    /// Note that these methods sometimes have complex runtimes, so be careful

    /// how you call them.

    /// Make an existing internal ref edge into a call edge.

    ///

    /// This may form a larger cycle and thus collapse SCCs into TargetN's SCC.

    /// If that happens, the optional callback \p MergedCB will be invoked (if

    /// provided) on the SCCs being merged away prior to actually performing

    /// the merge. Note that this will never include the target SCC as that

    /// will be the SCC functions are merged into to resolve the cycle. Once

    /// this function returns, these merged SCCs are not in a valid state but

    /// the pointers will remain valid until destruction of the parent graph

    /// instance for the purpose of clearing cached information. This function

    /// also returns 'true' if a cycle was formed and some SCCs merged away as

    /// a convenience.

    ///

    /// After this operation, both SourceN's SCC and TargetN's SCC may move

    /// position within this RefSCC's postorder list. Any SCCs merged are

    /// merged into the TargetN's SCC in order to preserve reachability analyses

    /// which took place on that SCC.

    bool switchInternalEdgeToCall(

        Node &SourceN, Node &TargetN,

        function_ref<void(ArrayRef<SCC *> MergedSCCs)> MergeCB = {});

    /// Make an existing internal call edge between separate SCCs into a ref

    /// edge.

    ///

    /// If SourceN and TargetN in separate SCCs within this RefSCC, changing

    /// the call edge between them to a ref edge is a trivial operation that

    /// does not require any structural changes to the call graph.

    void switchTrivialInternalEdgeToRef(Node &SourceN, Node &TargetN);

    /// Make an existing internal call edge within a single SCC into a ref

    /// edge.

    ///

    /// Since SourceN and TargetN are part of a single SCC, this SCC may be

    /// split up due to breaking a cycle in the call edges that formed it. If

    /// that happens, then this routine will insert new SCCs into the postorder

    /// list *before* the SCC of TargetN (previously the SCC of both). This

    /// preserves postorder as the TargetN can reach all of the other nodes by

    /// definition of previously being in a single SCC formed by the cycle from

    /// SourceN to TargetN.

    ///

    /// The newly added SCCs are added *immediately* and contiguously

    /// prior to the TargetN SCC and return the range covering the new SCCs in

    /// the RefSCC's postorder sequence. You can directly iterate the returned

    /// range to observe all of the new SCCs in postorder.

    ///

    /// Note that if SourceN and TargetN are in separate SCCs, the simpler

    /// routine `switchTrivialInternalEdgeToRef` should be used instead.

    iterator_range<iterator> switchInternalEdgeToRef(Node &SourceN,

                                                     Node &TargetN);

    /// Make an existing outgoing ref edge into a call edge.

    ///

    /// Note that this is trivial as there are no cyclic impacts and there

    /// remains a reference edge.

    void switchOutgoingEdgeToCall(Node &SourceN, Node &TargetN);

    /// Make an existing outgoing call edge into a ref edge.

    ///

    /// This is trivial as there are no cyclic impacts and there remains

    /// a reference edge.

    void switchOutgoingEdgeToRef(Node &SourceN, Node &TargetN);

    /// Insert a ref edge from one node in this RefSCC to another in this

    /// RefSCC.

    ///

    /// This is always a trivial operation as it doesn't change any part of the

    /// graph structure besides connecting the two nodes.

    ///

    /// Note that we don't support directly inserting internal *call* edges

    /// because that could change the graph structure and requires returning

    /// information about what became invalid. As a consequence, the pattern

    /// should be to first insert the necessary ref edge, and then to switch it

    /// to a call edge if needed and handle any invalidation that results. See

    /// the \c switchInternalEdgeToCall routine for details.

    void insertInternalRefEdge(Node &SourceN, Node &TargetN);

    /// Insert an edge whose parent is in this RefSCC and child is in some

    /// child RefSCC.

    ///

    /// There must be an existing path from the \p SourceN to the \p TargetN.

    /// This operation is inexpensive and does not change the set of SCCs and

    /// RefSCCs in the graph.

    void insertOutgoingEdge(Node &SourceN, Node &TargetN, Edge::Kind EK);

    /// Insert an edge whose source is in a descendant RefSCC and target is in

    /// this RefSCC.

    ///

    /// There must be an existing path from the target to the source in this

    /// case.

    ///

    /// NB! This is has the potential to be a very expensive function. It

    /// inherently forms a cycle in the prior RefSCC DAG and we have to merge

    /// RefSCCs to resolve that cycle. But finding all of the RefSCCs which

    /// participate in the cycle can in the worst case require traversing every

    /// RefSCC in the graph. Every attempt is made to avoid that, but passes

    /// must still exercise caution calling this routine repeatedly.

    ///

    /// Also note that this can only insert ref edges. In order to insert

    /// a call edge, first insert a ref edge and then switch it to a call edge.

    /// These are intentionally kept as separate interfaces because each step

    /// of the operation invalidates a different set of data structures.

    ///

    /// This returns all the RefSCCs which were merged into the this RefSCC

    /// (the target's). This allows callers to invalidate any cached

    /// information.

    ///

    /// FIXME: We could possibly optimize this quite a bit for cases where the

    /// caller and callee are very nearby in the graph. See comments in the

    /// implementation for details, but that use case might impact users.

    SmallVector<RefSCC *, 1> insertIncomingRefEdge(Node &SourceN,

                                                   Node &TargetN);

    /// Remove an edge whose source is in this RefSCC and target is *not*.

    ///

    /// This removes an inter-RefSCC edge. All inter-RefSCC edges originating

    /// from this SCC have been fully explored by any in-flight DFS graph

    /// formation, so this is always safe to call once you have the source

    /// RefSCC.

    ///

    /// This operation does not change the cyclic structure of the graph and so

    /// is very inexpensive. It may change the connectivity graph of the SCCs

    /// though, so be careful calling this while iterating over them.

    void removeOutgoingEdge(Node &SourceN, Node &TargetN);

    /// Remove a list of ref edges which are entirely within this RefSCC.

    ///

    /// Both the \a SourceN and all of the \a TargetNs must be within this

    /// RefSCC. Removing these edges may break cycles that form this RefSCC and

    /// thus this operation may change the RefSCC graph significantly. In

    /// particular, this operation will re-form new RefSCCs based on the

    /// remaining connectivity of the graph. The following invariants are

    /// guaranteed to hold after calling this method:

    ///

    /// 1) If a ref-cycle remains after removal, it leaves this RefSCC intact

    ///    and in the graph. No new RefSCCs are built.

    /// 2) Otherwise, this RefSCC will be dead after this call and no longer in

    ///    the graph or the postorder traversal of the call graph. Any iterator

    ///    pointing at this RefSCC will become invalid.

    /// 3) All newly formed RefSCCs will be returned and the order of the

    ///    RefSCCs returned will be a valid postorder traversal of the new

    ///    RefSCCs.

    /// 4) No RefSCC other than this RefSCC has its member set changed (this is

    ///    inherent in the definition of removing such an edge).

    ///

    /// These invariants are very important to ensure that we can build

    /// optimization pipelines on top of the CGSCC pass manager which

    /// intelligently update the RefSCC graph without invalidating other parts

    /// of the RefSCC graph.

    ///

    /// Note that we provide no routine to remove a *call* edge. Instead, you

    /// must first switch it to a ref edge using \c switchInternalEdgeToRef.

    /// This split API is intentional as each of these two steps can invalidate

    /// a different aspect of the graph structure and needs to have the

    /// invalidation handled independently.

    ///

    /// The runtime complexity of this method is, in the worst case, O(V+E)

    /// where V is the number of nodes in this RefSCC and E is the number of

    /// edges leaving the nodes in this RefSCC. Note that E includes both edges

    /// within this RefSCC and edges from this RefSCC to child RefSCCs. Some

    /// effort has been made to minimize the overhead of common cases such as

    /// self-edges and edge removals which result in a spanning tree with no

    /// more cycles.

    [[nodiscard]] SmallVector<RefSCC *, 1>

    removeInternalRefEdge(Node &SourceN, ArrayRef<Node *> TargetNs);

    /// A convenience wrapper around the above to handle trivial cases of

    /// inserting a new call edge.

    ///

    /// This is trivial whenever the target is in the same SCC as the source or

    /// the edge is an outgoing edge to some descendant SCC. In these cases

    /// there is no change to the cyclic structure of SCCs or RefSCCs.

    ///

    /// To further make calling this convenient, it also handles inserting

    /// already existing edges.

    void insertTrivialCallEdge(Node &SourceN, Node &TargetN);

    /// A convenience wrapper around the above to handle trivial cases of

    /// inserting a new ref edge.

    ///

    /// This is trivial whenever the target is in the same RefSCC as the source

    /// or the edge is an outgoing edge to some descendant RefSCC. In these

    /// cases there is no change to the cyclic structure of the RefSCCs.

    ///

    /// To further make calling this convenient, it also handles inserting

    /// already existing edges.

    void insertTrivialRefEdge(Node &SourceN, Node &TargetN);

    /// Directly replace a node's function with a new function.

    ///

    /// This should be used when moving the body and users of a function to

    /// a new formal function object but not otherwise changing the call graph

    /// structure in any way.

    ///

    /// It requires that the old function in the provided node have zero uses

    /// and the new function must have calls and references to it establishing

    /// an equivalent graph.

    void replaceNodeFunction(Node &N, Function &NewF);