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//===- LazyCallGraph.h - Analysis of a Module's call graph ------*- C++ -*-===//
// The LLVM Compiler Infrastructure
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
/// \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.
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/PointerUnion.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/iterator.h"
#include "llvm/ADT/iterator_range.h"
#include "llvm/IR/BasicBlock.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/Module.h"
#include "llvm/Support/Allocator.h"
#include <iterator>
namespace llvm {
class ModuleAnalysisManager;
class PreservedAnalyses;
class raw_ostream;
/// \brief 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 {
class Node;
class SCC;
typedef SmallVector<PointerUnion<Function *, Node *>, 4> NodeVectorT;
typedef SmallVectorImpl<PointerUnion<Function *, Node *>> NodeVectorImplT;
/// \brief A lazy iterator used for both the entry nodes and child nodes.
/// When this iterator is dereferenced, if not yet available, a function will
/// be scanned for "calls" or uses of functions and its child information
/// will be constructed. All of these results are accumulated and cached in
/// the graph.
class iterator
: public iterator_adaptor_base<iterator, NodeVectorImplT::iterator,
std::forward_iterator_tag, Node> {
friend class LazyCallGraph;
friend class LazyCallGraph::Node;
LazyCallGraph *G;
NodeVectorImplT::iterator E;
// Build the iterator for a specific position in a node list.
iterator(LazyCallGraph &G, NodeVectorImplT::iterator NI,
NodeVectorImplT::iterator E)
: iterator_adaptor_base(NI), G(&G), E(E) {
while (I != E && I->isNull())
iterator() {}
using iterator_adaptor_base::operator++;
iterator &operator++() {
do {
} while (I != E && I->isNull());
return *this;
reference operator*() const {
if (I->is<Node *>())
return *I->get<Node *>();
Function *F = I->get<Function *>();
Node &ChildN = G->get(*F);
*I = &ChildN;
return ChildN;
/// \brief A node in the call graph.
/// This represents a single node. It's 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.
class Node {
friend class LazyCallGraph;
friend class LazyCallGraph::SCC;
LazyCallGraph *G;
Function &F;
// We provide for the DFS numbering and Tarjan walk lowlink numbers to be
// stored directly within the node.
int DFSNumber;
int LowLink;
mutable NodeVectorT Callees;
DenseMap<Function *, size_t> CalleeIndexMap;
/// \brief Basic constructor implements the scanning of F into Callees and
/// CalleeIndexMap.
Node(LazyCallGraph &G, Function &F);
/// \brief Internal helper to insert a callee.
void insertEdgeInternal(Function &Callee);
/// \brief Internal helper to insert a callee.
void insertEdgeInternal(Node &CalleeN);
/// \brief Internal helper to remove a callee from this node.
void removeEdgeInternal(Function &Callee);
typedef LazyCallGraph::iterator iterator;
Function &getFunction() const {
return F;
iterator begin() const {
return iterator(*G, Callees.begin(), Callees.end());
iterator end() const { return iterator(*G, Callees.end(), Callees.end()); }
/// Equality is defined as address equality.
bool operator==(const Node &N) const { return this == &N; }
bool operator!=(const Node &N) const { return !operator==(N); }
/// \brief An SCC of the call graph.
/// This represents a Strongly Connected Component of the call graph as
/// a collection of call graph nodes. While the order of nodes in the SCC is
/// stable, it is not any particular order.
class SCC {
friend class LazyCallGraph;
friend class LazyCallGraph::Node;
LazyCallGraph *G;
SmallPtrSet<SCC *, 1> ParentSCCs;
SmallVector<Node *, 1> Nodes;
SCC(LazyCallGraph &G) : G(&G) {}
void insert(Node &N);
internalDFS(SmallVectorImpl<std::pair<Node *, Node::iterator>> &DFSStack,
SmallVectorImpl<Node *> &PendingSCCStack, Node *N,
SmallVectorImpl<SCC *> &ResultSCCs);
typedef SmallVectorImpl<Node *>::const_iterator iterator;
typedef pointee_iterator<SmallPtrSet<SCC *, 1>::const_iterator> parent_iterator;
iterator begin() const { return Nodes.begin(); }
iterator end() const { return Nodes.end(); }
parent_iterator parent_begin() const { return ParentSCCs.begin(); }
parent_iterator parent_end() const { return ParentSCCs.end(); }
iterator_range<parent_iterator> parents() const {
return iterator_range<parent_iterator>(parent_begin(), parent_end());
/// \brief Test if this SCC is a parent of \a C.
bool isParentOf(const SCC &C) const { return C.isChildOf(*this); }
/// \brief Test if this SCC is an ancestor of \a C.
bool isAncestorOf(const SCC &C) const { return C.isDescendantOf(*this); }
/// \brief Test if this SCC is a child of \a C.
bool isChildOf(const SCC &C) const {
return ParentSCCs.count(const_cast<SCC *>(&C));
/// \brief Test if this SCC is a descendant of \a C.
bool isDescendantOf(const SCC &C) const;
/// \name Mutation API
/// These methods provide the core API for updating the call graph in the
/// presence of a (potentially still in-flight) DFS-found SCCs.
/// Note that these methods sometimes have complex runtimes, so be careful
/// how you call them.
/// \brief Insert an edge from one node in this SCC to another in this SCC.
/// By the definition of an SCC, this does not change the nature or make-up
/// of any SCCs.
void insertIntraSCCEdge(Node &CallerN, Node &CalleeN);
/// \brief Insert an edge whose tail is in this SCC and head is in some
/// child SCC.
/// There must be an existing path from the caller to the callee. This
/// operation is inexpensive and does not change the set of SCCs in the
/// graph.
void insertOutgoingEdge(Node &CallerN, Node &CalleeN);
/// \brief Insert an edge whose tail is in a descendant SCC and head is in
/// this SCC.
/// There must be an existing path from the callee to the caller in this
/// case. NB! This is has the potential to be a very expensive function. It
/// inherently forms a cycle in the prior SCC DAG and we have to merge SCCs
/// to resolve that cycle. But finding all of the SCCs which participate in
/// the cycle can in the worst case require traversing every SCC in the
/// graph. Every attempt is made to avoid that, but passes must still
/// exercise caution calling this routine repeatedly.
/// 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<SCC *, 1> insertIncomingEdge(Node &CallerN, Node &CalleeN);
/// \brief Remove an edge whose source is in this SCC and target is *not*.
/// This removes an inter-SCC edge. All inter-SCC edges originating from
/// this SCC have been fully explored by any in-flight DFS SCC formation,
/// so this is always safe to call once you have the source SCC.
/// This operation does not change the set of SCCs or the members of the
/// SCCs 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 removeInterSCCEdge(Node &CallerN, Node &CalleeN);
/// \brief Remove an edge which is entirely within this SCC.
/// Both the \a Caller and the \a Callee must be within this SCC. Removing
/// such an edge make break cycles that form this SCC and thus this
/// operation may change the SCC graph significantly. In particular, this
/// operation will re-form new SCCs based on the remaining connectivity of
/// the graph. The following invariants are guaranteed to hold after
/// calling this method:
/// 1) This SCC is still an SCC in the graph.
/// 2) This SCC will be the parent of any new SCCs. Thus, this SCC is
/// preserved as the root of any new SCC directed graph formed.
/// 3) No SCC other than this SCC has its member set changed (this is
/// inherent in the definition of removing such an edge).
/// 4) All of the parent links of the SCC graph will be updated to reflect
/// the new SCC structure.
/// 5) All SCCs formed out of this SCC, excluding this SCC, will be
/// returned in a vector.
/// 6) The order of the SCCs in the vector will be a valid postorder
/// traversal of the new SCCs.
/// These invariants are very important to ensure that we can build
/// optimization pipeliens on top of the CGSCC pass manager which
/// intelligently update the SCC graph without invalidating other parts of
/// the SCC graph.
/// The runtime complexity of this method is, in the worst case, O(V+E)
/// where V is the number of nodes in this SCC and E is the number of edges
/// leaving the nodes in this SCC. Note that E includes both edges within
/// this SCC and edges from this SCC to child SCCs. 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.
SmallVector<SCC *, 1> removeIntraSCCEdge(Node &CallerN, Node &CalleeN);
/// \brief A post-order depth-first SCC iterator over the call graph.
/// This iterator triggers the Tarjan DFS-based formation of the SCC DAG for
/// the call graph, walking it lazily in depth-first post-order. That is, it
/// always visits SCCs for a callee prior to visiting the SCC for a caller
/// (when they are in different SCCs).
class postorder_scc_iterator
: public iterator_facade_base<postorder_scc_iterator,
std::forward_iterator_tag, SCC> {
friend class LazyCallGraph;
friend class LazyCallGraph::Node;
/// \brief Nonce type to select the constructor for the end iterator.
struct IsAtEndT {};
LazyCallGraph *G;
// Build the begin iterator for a node.
postorder_scc_iterator(LazyCallGraph &G) : G(&G) {
C = G.getNextSCCInPostOrder();
// Build the end iterator for a node. This is selected purely by overload.
postorder_scc_iterator(LazyCallGraph &G, IsAtEndT /*Nonce*/)
: G(&G), C(nullptr) {}
bool operator==(const postorder_scc_iterator &Arg) const {
return G == Arg.G && C == Arg.C;
reference operator*() const { return *C; }
using iterator_facade_base::operator++;
postorder_scc_iterator &operator++() {
C = G->getNextSCCInPostOrder();
return *this;
/// \brief Construct a graph for the given module.
/// This sets up the graph and computes all of the entry points of the graph.
/// No function definitions are scanned until their nodes in the graph are
/// requested during traversal.
LazyCallGraph(Module &M);
LazyCallGraph(LazyCallGraph &&G);
LazyCallGraph &operator=(LazyCallGraph &&RHS);
iterator begin() {
return iterator(*this, EntryNodes.begin(), EntryNodes.end());
iterator end() { return iterator(*this, EntryNodes.end(), EntryNodes.end()); }
postorder_scc_iterator postorder_scc_begin() {
return postorder_scc_iterator(*this);
postorder_scc_iterator postorder_scc_end() {
return postorder_scc_iterator(*this, postorder_scc_iterator::IsAtEndT());
iterator_range<postorder_scc_iterator> postorder_sccs() {
return iterator_range<postorder_scc_iterator>(postorder_scc_begin(),
/// \brief Lookup a function in the graph which has already been scanned and
/// added.
Node *lookup(const Function &F) const { return NodeMap.lookup(&F); }
/// \brief Lookup a function's SCC in the graph.
/// \returns null if the function hasn't been assigned an SCC via the SCC
/// iterator walk.
SCC *lookupSCC(Node &N) const { return SCCMap.lookup(&N); }
/// \brief Get a graph node for a given function, scanning it to populate the
/// graph data as necessary.
Node &get(Function &F) {
Node *&N = NodeMap[&F];
if (N)
return *N;
return insertInto(F, N);
/// \name Pre-SCC Mutation API
/// These methods are only valid to call prior to forming any SCCs for this
/// call graph. They can be used to update the core node-graph during
/// a node-based inorder traversal that precedes any SCC-based traversal.
/// Once you begin manipulating a call graph's SCCs, you must perform all
/// mutation of the graph via the SCC methods.
/// \brief Update the call graph after inserting a new edge.
void insertEdge(Node &Caller, Function &Callee);
/// \brief Update the call graph after inserting a new edge.
void insertEdge(Function &Caller, Function &Callee) {
return insertEdge(get(Caller), Callee);
/// \brief Update the call graph after deleting an edge.
void removeEdge(Node &Caller, Function &Callee);
/// \brief Update the call graph after deleting an edge.
void removeEdge(Function &Caller, Function &Callee) {
return removeEdge(get(Caller), Callee);
/// \brief Allocator that holds all the call graph nodes.
SpecificBumpPtrAllocator<Node> BPA;
/// \brief Maps function->node for fast lookup.
DenseMap<const Function *, Node *> NodeMap;
/// \brief The entry nodes to the graph.
/// These nodes are reachable through "external" means. Put another way, they
/// escape at the module scope.
NodeVectorT EntryNodes;
/// \brief Map of the entry nodes in the graph to their indices in
/// \c EntryNodes.
DenseMap<Function *, size_t> EntryIndexMap;
/// \brief Allocator that holds all the call graph SCCs.
SpecificBumpPtrAllocator<SCC> SCCBPA;
/// \brief Maps Function -> SCC for fast lookup.
DenseMap<Node *, SCC *> SCCMap;
/// \brief The leaf SCCs of the graph.
/// These are all of the SCCs which have no children.
SmallVector<SCC *, 4> LeafSCCs;
/// \brief Stack of nodes in the DFS walk.
SmallVector<std::pair<Node *, iterator>, 4> DFSStack;
/// \brief Set of entry nodes not-yet-processed into SCCs.
SmallVector<Function *, 4> SCCEntryNodes;
/// \brief Stack of nodes the DFS has walked but not yet put into a SCC.
SmallVector<Node *, 4> PendingSCCStack;
/// \brief Counter for the next DFS number to assign.
int NextDFSNumber;
/// \brief Helper to insert a new function, with an already looked-up entry in
/// the NodeMap.
Node &insertInto(Function &F, Node *&MappedN);
/// \brief Helper to update pointers back to the graph object during moves.
void updateGraphPtrs();
/// \brief Helper to form a new SCC out of the top of a DFSStack-like
/// structure.
SCC *formSCC(Node *RootN, SmallVectorImpl<Node *> &NodeStack);
/// \brief Retrieve the next node in the post-order SCC walk of the call graph.
SCC *getNextSCCInPostOrder();
// Provide GraphTraits specializations for call graphs.
template <> struct GraphTraits<LazyCallGraph::Node *> {
typedef LazyCallGraph::Node NodeType;
typedef LazyCallGraph::iterator ChildIteratorType;
static NodeType *getEntryNode(NodeType *N) { return N; }
static ChildIteratorType child_begin(NodeType *N) { return N->begin(); }
static ChildIteratorType child_end(NodeType *N) { return N->end(); }
template <> struct GraphTraits<LazyCallGraph *> {
typedef LazyCallGraph::Node NodeType;
typedef LazyCallGraph::iterator ChildIteratorType;
static NodeType *getEntryNode(NodeType *N) { return N; }
static ChildIteratorType child_begin(NodeType *N) { return N->begin(); }
static ChildIteratorType child_end(NodeType *N) { return N->end(); }
/// \brief An analysis pass which computes the call graph for a module.
class LazyCallGraphAnalysis {
/// \brief Inform generic clients of the result type.
typedef LazyCallGraph Result;
static void *ID() { return (void *)&PassID; }
/// \brief Compute the \c LazyCallGraph for a the module \c M.
/// This just builds the set of entry points to the call graph. The rest is
/// built lazily as it is walked.
LazyCallGraph run(Module *M) { return LazyCallGraph(*M); }
static char PassID;
/// \brief A pass which prints the call graph to a \c raw_ostream.
/// This is primarily useful for testing the analysis.
class LazyCallGraphPrinterPass {
raw_ostream &OS;
explicit LazyCallGraphPrinterPass(raw_ostream &OS);
PreservedAnalyses run(Module *M, ModuleAnalysisManager *AM);
static StringRef name() { return "LazyCallGraphPrinterPass"; }