1 //===- LazyCallGraph.h - Analysis of a Module's call graph ------*- C++ -*-===//
2 //
3 //                     The LLVM Compiler Infrastructure
4 //
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
7 //
8 //===----------------------------------------------------------------------===//
9 /// \file
10 ///
11 /// Implements a lazy call graph analysis and related passes for the new pass
12 /// manager.
13 ///
14 /// NB: This is *not* a traditional call graph! It is a graph which models both
15 /// the current calls and potential calls. As a consequence there are many
16 /// edges in this call graph that do not correspond to a 'call' or 'invoke'
17 /// instruction.
18 ///
19 /// The primary use cases of this graph analysis is to facilitate iterating
20 /// across the functions of a module in ways that ensure all callees are
21 /// visited prior to a caller (given any SCC constraints), or vice versa. As
22 /// such is it particularly well suited to organizing CGSCC optimizations such
23 /// as inlining, outlining, argument promotion, etc. That is its primary use
24 /// case and motivates the design. It may not be appropriate for other
25 /// purposes. The use graph of functions or some other conservative analysis of
26 /// call instructions may be interesting for optimizations and subsequent
27 /// analyses which don't work in the context of an overly specified
28 /// potential-call-edge graph.
29 ///
30 /// To understand the specific rules and nature of this call graph analysis,
31 /// see the documentation of the \c LazyCallGraph below.
32 ///
33 //===----------------------------------------------------------------------===//
34 
35 #ifndef LLVM_ANALYSIS_LAZYCALLGRAPH_H
36 #define LLVM_ANALYSIS_LAZYCALLGRAPH_H
37 
38 #include "llvm/ADT/DenseMap.h"
39 #include "llvm/ADT/PointerUnion.h"
40 #include "llvm/ADT/STLExtras.h"
41 #include "llvm/ADT/SetVector.h"
42 #include "llvm/ADT/SmallPtrSet.h"
43 #include "llvm/ADT/SmallVector.h"
44 #include "llvm/ADT/iterator.h"
45 #include "llvm/ADT/iterator_range.h"
46 #include "llvm/IR/BasicBlock.h"
47 #include "llvm/IR/Function.h"
48 #include "llvm/IR/Module.h"
49 #include "llvm/IR/PassManager.h"
50 #include "llvm/Support/Allocator.h"
51 #include <iterator>
52 
53 namespace llvm {
54 class PreservedAnalyses;
55 class raw_ostream;
56 
57 /// \brief A lazily constructed view of the call graph of a module.
58 ///
59 /// With the edges of this graph, the motivating constraint that we are
60 /// attempting to maintain is that function-local optimization, CGSCC-local
61 /// optimizations, and optimizations transforming a pair of functions connected
62 /// by an edge in the graph, do not invalidate a bottom-up traversal of the SCC
63 /// DAG. That is, no optimizations will delete, remove, or add an edge such
64 /// that functions already visited in a bottom-up order of the SCC DAG are no
65 /// longer valid to have visited, or such that functions not yet visited in
66 /// a bottom-up order of the SCC DAG are not required to have already been
67 /// visited.
68 ///
69 /// Within this constraint, the desire is to minimize the merge points of the
70 /// SCC DAG. The greater the fanout of the SCC DAG and the fewer merge points
71 /// in the SCC DAG, the more independence there is in optimizing within it.
72 /// There is a strong desire to enable parallelization of optimizations over
73 /// the call graph, and both limited fanout and merge points will (artificially
74 /// in some cases) limit the scaling of such an effort.
75 ///
76 /// To this end, graph represents both direct and any potential resolution to
77 /// an indirect call edge. Another way to think about it is that it represents
78 /// both the direct call edges and any direct call edges that might be formed
79 /// through static optimizations. Specifically, it considers taking the address
80 /// of a function to be an edge in the call graph because this might be
81 /// forwarded to become a direct call by some subsequent function-local
82 /// optimization. The result is that the graph closely follows the use-def
83 /// edges for functions. Walking "up" the graph can be done by looking at all
84 /// of the uses of a function.
85 ///
86 /// The roots of the call graph are the external functions and functions
87 /// escaped into global variables. Those functions can be called from outside
88 /// of the module or via unknowable means in the IR -- we may not be able to
89 /// form even a potential call edge from a function body which may dynamically
90 /// load the function and call it.
91 ///
92 /// This analysis still requires updates to remain valid after optimizations
93 /// which could potentially change the set of potential callees. The
94 /// constraints it operates under only make the traversal order remain valid.
95 ///
96 /// The entire analysis must be re-computed if full interprocedural
97 /// optimizations run at any point. For example, globalopt completely
98 /// invalidates the information in this analysis.
99 ///
100 /// FIXME: This class is named LazyCallGraph in a lame attempt to distinguish
101 /// it from the existing CallGraph. At some point, it is expected that this
102 /// will be the only call graph and it will be renamed accordingly.
103 class LazyCallGraph {
104 public:
105   class Node;
106   class SCC;
107   typedef SmallVector<PointerUnion<Function *, Node *>, 4> NodeVectorT;
108   typedef SmallVectorImpl<PointerUnion<Function *, Node *>> NodeVectorImplT;
109 
110   /// \brief A lazy iterator used for both the entry nodes and child nodes.
111   ///
112   /// When this iterator is dereferenced, if not yet available, a function will
113   /// be scanned for "calls" or uses of functions and its child information
114   /// will be constructed. All of these results are accumulated and cached in
115   /// the graph.
116   class iterator
117       : public iterator_adaptor_base<iterator, NodeVectorImplT::iterator,
118                                      std::forward_iterator_tag, Node> {
119     friend class LazyCallGraph;
120     friend class LazyCallGraph::Node;
121 
122     LazyCallGraph *G;
123     NodeVectorImplT::iterator E;
124 
125     // Build the iterator for a specific position in a node list.
iterator(LazyCallGraph & G,NodeVectorImplT::iterator NI,NodeVectorImplT::iterator E)126     iterator(LazyCallGraph &G, NodeVectorImplT::iterator NI,
127              NodeVectorImplT::iterator E)
128         : iterator_adaptor_base(NI), G(&G), E(E) {
129       while (I != E && I->isNull())
130         ++I;
131     }
132 
133   public:
iterator()134     iterator() {}
135 
136     using iterator_adaptor_base::operator++;
137     iterator &operator++() {
138       do {
139         ++I;
140       } while (I != E && I->isNull());
141       return *this;
142     }
143 
144     reference operator*() const {
145       if (I->is<Node *>())
146         return *I->get<Node *>();
147 
148       Function *F = I->get<Function *>();
149       Node &ChildN = G->get(*F);
150       *I = &ChildN;
151       return ChildN;
152     }
153   };
154 
155   /// \brief A node in the call graph.
156   ///
157   /// This represents a single node. It's primary roles are to cache the list of
158   /// callees, de-duplicate and provide fast testing of whether a function is
159   /// a callee, and facilitate iteration of child nodes in the graph.
160   class Node {
161     friend class LazyCallGraph;
162     friend class LazyCallGraph::SCC;
163 
164     LazyCallGraph *G;
165     Function &F;
166 
167     // We provide for the DFS numbering and Tarjan walk lowlink numbers to be
168     // stored directly within the node.
169     int DFSNumber;
170     int LowLink;
171 
172     mutable NodeVectorT Callees;
173     DenseMap<Function *, size_t> CalleeIndexMap;
174 
175     /// \brief Basic constructor implements the scanning of F into Callees and
176     /// CalleeIndexMap.
177     Node(LazyCallGraph &G, Function &F);
178 
179     /// \brief Internal helper to insert a callee.
180     void insertEdgeInternal(Function &Callee);
181 
182     /// \brief Internal helper to insert a callee.
183     void insertEdgeInternal(Node &CalleeN);
184 
185     /// \brief Internal helper to remove a callee from this node.
186     void removeEdgeInternal(Function &Callee);
187 
188   public:
189     typedef LazyCallGraph::iterator iterator;
190 
getFunction()191     Function &getFunction() const {
192       return F;
193     };
194 
begin()195     iterator begin() const {
196       return iterator(*G, Callees.begin(), Callees.end());
197     }
end()198     iterator end() const { return iterator(*G, Callees.end(), Callees.end()); }
199 
200     /// Equality is defined as address equality.
201     bool operator==(const Node &N) const { return this == &N; }
202     bool operator!=(const Node &N) const { return !operator==(N); }
203   };
204 
205   /// \brief An SCC of the call graph.
206   ///
207   /// This represents a Strongly Connected Component of the call graph as
208   /// a collection of call graph nodes. While the order of nodes in the SCC is
209   /// stable, it is not any particular order.
210   class SCC {
211     friend class LazyCallGraph;
212     friend class LazyCallGraph::Node;
213 
214     LazyCallGraph *G;
215     SmallPtrSet<SCC *, 1> ParentSCCs;
216     SmallVector<Node *, 1> Nodes;
217 
SCC(LazyCallGraph & G)218     SCC(LazyCallGraph &G) : G(&G) {}
219 
220     void insert(Node &N);
221 
222     void
223     internalDFS(SmallVectorImpl<std::pair<Node *, Node::iterator>> &DFSStack,
224                 SmallVectorImpl<Node *> &PendingSCCStack, Node *N,
225                 SmallVectorImpl<SCC *> &ResultSCCs);
226 
227   public:
228     typedef SmallVectorImpl<Node *>::const_iterator iterator;
229     typedef pointee_iterator<SmallPtrSet<SCC *, 1>::const_iterator> parent_iterator;
230 
begin()231     iterator begin() const { return Nodes.begin(); }
end()232     iterator end() const { return Nodes.end(); }
233 
parent_begin()234     parent_iterator parent_begin() const { return ParentSCCs.begin(); }
parent_end()235     parent_iterator parent_end() const { return ParentSCCs.end(); }
236 
parents()237     iterator_range<parent_iterator> parents() const {
238       return iterator_range<parent_iterator>(parent_begin(), parent_end());
239     }
240 
241     /// \brief Test if this SCC is a parent of \a C.
isParentOf(const SCC & C)242     bool isParentOf(const SCC &C) const { return C.isChildOf(*this); }
243 
244     /// \brief Test if this SCC is an ancestor of \a C.
isAncestorOf(const SCC & C)245     bool isAncestorOf(const SCC &C) const { return C.isDescendantOf(*this); }
246 
247     /// \brief Test if this SCC is a child of \a C.
isChildOf(const SCC & C)248     bool isChildOf(const SCC &C) const {
249       return ParentSCCs.count(const_cast<SCC *>(&C));
250     }
251 
252     /// \brief Test if this SCC is a descendant of \a C.
253     bool isDescendantOf(const SCC &C) const;
254 
255     /// \brief Short name useful for debugging or logging.
256     ///
257     /// We use the name of the first function in the SCC to name the SCC for
258     /// the purposes of debugging and logging.
getName()259     StringRef getName() const { return (*begin())->getFunction().getName(); }
260 
261     ///@{
262     /// \name Mutation API
263     ///
264     /// These methods provide the core API for updating the call graph in the
265     /// presence of a (potentially still in-flight) DFS-found SCCs.
266     ///
267     /// Note that these methods sometimes have complex runtimes, so be careful
268     /// how you call them.
269 
270     /// \brief Insert an edge from one node in this SCC to another in this SCC.
271     ///
272     /// By the definition of an SCC, this does not change the nature or make-up
273     /// of any SCCs.
274     void insertIntraSCCEdge(Node &CallerN, Node &CalleeN);
275 
276     /// \brief Insert an edge whose tail is in this SCC and head is in some
277     /// child SCC.
278     ///
279     /// There must be an existing path from the caller to the callee. This
280     /// operation is inexpensive and does not change the set of SCCs in the
281     /// graph.
282     void insertOutgoingEdge(Node &CallerN, Node &CalleeN);
283 
284     /// \brief Insert an edge whose tail is in a descendant SCC and head is in
285     /// this SCC.
286     ///
287     /// There must be an existing path from the callee to the caller in this
288     /// case. NB! This is has the potential to be a very expensive function. It
289     /// inherently forms a cycle in the prior SCC DAG and we have to merge SCCs
290     /// to resolve that cycle. But finding all of the SCCs which participate in
291     /// the cycle can in the worst case require traversing every SCC in the
292     /// graph. Every attempt is made to avoid that, but passes must still
293     /// exercise caution calling this routine repeatedly.
294     ///
295     /// FIXME: We could possibly optimize this quite a bit for cases where the
296     /// caller and callee are very nearby in the graph. See comments in the
297     /// implementation for details, but that use case might impact users.
298     SmallVector<SCC *, 1> insertIncomingEdge(Node &CallerN, Node &CalleeN);
299 
300     /// \brief Remove an edge whose source is in this SCC and target is *not*.
301     ///
302     /// This removes an inter-SCC edge. All inter-SCC edges originating from
303     /// this SCC have been fully explored by any in-flight DFS SCC formation,
304     /// so this is always safe to call once you have the source SCC.
305     ///
306     /// This operation does not change the set of SCCs or the members of the
307     /// SCCs and so is very inexpensive. It may change the connectivity graph
308     /// of the SCCs though, so be careful calling this while iterating over
309     /// them.
310     void removeInterSCCEdge(Node &CallerN, Node &CalleeN);
311 
312     /// \brief Remove an edge which is entirely within this SCC.
313     ///
314     /// Both the \a Caller and the \a Callee must be within this SCC. Removing
315     /// such an edge make break cycles that form this SCC and thus this
316     /// operation may change the SCC graph significantly. In particular, this
317     /// operation will re-form new SCCs based on the remaining connectivity of
318     /// the graph. The following invariants are guaranteed to hold after
319     /// calling this method:
320     ///
321     /// 1) This SCC is still an SCC in the graph.
322     /// 2) This SCC will be the parent of any new SCCs. Thus, this SCC is
323     ///    preserved as the root of any new SCC directed graph formed.
324     /// 3) No SCC other than this SCC has its member set changed (this is
325     ///    inherent in the definition of removing such an edge).
326     /// 4) All of the parent links of the SCC graph will be updated to reflect
327     ///    the new SCC structure.
328     /// 5) All SCCs formed out of this SCC, excluding this SCC, will be
329     ///    returned in a vector.
330     /// 6) The order of the SCCs in the vector will be a valid postorder
331     ///    traversal of the new SCCs.
332     ///
333     /// These invariants are very important to ensure that we can build
334     /// optimization pipeliens on top of the CGSCC pass manager which
335     /// intelligently update the SCC graph without invalidating other parts of
336     /// the SCC graph.
337     ///
338     /// The runtime complexity of this method is, in the worst case, O(V+E)
339     /// where V is the number of nodes in this SCC and E is the number of edges
340     /// leaving the nodes in this SCC. Note that E includes both edges within
341     /// this SCC and edges from this SCC to child SCCs. Some effort has been
342     /// made to minimize the overhead of common cases such as self-edges and
343     /// edge removals which result in a spanning tree with no more cycles.
344     SmallVector<SCC *, 1> removeIntraSCCEdge(Node &CallerN, Node &CalleeN);
345 
346     ///@}
347   };
348 
349   /// \brief A post-order depth-first SCC iterator over the call graph.
350   ///
351   /// This iterator triggers the Tarjan DFS-based formation of the SCC DAG for
352   /// the call graph, walking it lazily in depth-first post-order. That is, it
353   /// always visits SCCs for a callee prior to visiting the SCC for a caller
354   /// (when they are in different SCCs).
355   class postorder_scc_iterator
356       : public iterator_facade_base<postorder_scc_iterator,
357                                     std::forward_iterator_tag, SCC> {
358     friend class LazyCallGraph;
359     friend class LazyCallGraph::Node;
360 
361     /// \brief Nonce type to select the constructor for the end iterator.
362     struct IsAtEndT {};
363 
364     LazyCallGraph *G;
365     SCC *C;
366 
367     // Build the begin iterator for a node.
postorder_scc_iterator(LazyCallGraph & G)368     postorder_scc_iterator(LazyCallGraph &G) : G(&G) {
369       C = G.getNextSCCInPostOrder();
370     }
371 
372     // Build the end iterator for a node. This is selected purely by overload.
postorder_scc_iterator(LazyCallGraph & G,IsAtEndT)373     postorder_scc_iterator(LazyCallGraph &G, IsAtEndT /*Nonce*/)
374         : G(&G), C(nullptr) {}
375 
376   public:
377     bool operator==(const postorder_scc_iterator &Arg) const {
378       return G == Arg.G && C == Arg.C;
379     }
380 
381     reference operator*() const { return *C; }
382 
383     using iterator_facade_base::operator++;
384     postorder_scc_iterator &operator++() {
385       C = G->getNextSCCInPostOrder();
386       return *this;
387     }
388   };
389 
390   /// \brief Construct a graph for the given module.
391   ///
392   /// This sets up the graph and computes all of the entry points of the graph.
393   /// No function definitions are scanned until their nodes in the graph are
394   /// requested during traversal.
395   LazyCallGraph(Module &M);
396 
397   LazyCallGraph(LazyCallGraph &&G);
398   LazyCallGraph &operator=(LazyCallGraph &&RHS);
399 
begin()400   iterator begin() {
401     return iterator(*this, EntryNodes.begin(), EntryNodes.end());
402   }
end()403   iterator end() { return iterator(*this, EntryNodes.end(), EntryNodes.end()); }
404 
postorder_scc_begin()405   postorder_scc_iterator postorder_scc_begin() {
406     return postorder_scc_iterator(*this);
407   }
postorder_scc_end()408   postorder_scc_iterator postorder_scc_end() {
409     return postorder_scc_iterator(*this, postorder_scc_iterator::IsAtEndT());
410   }
411 
postorder_sccs()412   iterator_range<postorder_scc_iterator> postorder_sccs() {
413     return iterator_range<postorder_scc_iterator>(postorder_scc_begin(),
414                                                   postorder_scc_end());
415   }
416 
417   /// \brief Lookup a function in the graph which has already been scanned and
418   /// added.
lookup(const Function & F)419   Node *lookup(const Function &F) const { return NodeMap.lookup(&F); }
420 
421   /// \brief Lookup a function's SCC in the graph.
422   ///
423   /// \returns null if the function hasn't been assigned an SCC via the SCC
424   /// iterator walk.
lookupSCC(Node & N)425   SCC *lookupSCC(Node &N) const { return SCCMap.lookup(&N); }
426 
427   /// \brief Get a graph node for a given function, scanning it to populate the
428   /// graph data as necessary.
get(Function & F)429   Node &get(Function &F) {
430     Node *&N = NodeMap[&F];
431     if (N)
432       return *N;
433 
434     return insertInto(F, N);
435   }
436 
437   ///@{
438   /// \name Pre-SCC Mutation API
439   ///
440   /// These methods are only valid to call prior to forming any SCCs for this
441   /// call graph. They can be used to update the core node-graph during
442   /// a node-based inorder traversal that precedes any SCC-based traversal.
443   ///
444   /// Once you begin manipulating a call graph's SCCs, you must perform all
445   /// mutation of the graph via the SCC methods.
446 
447   /// \brief Update the call graph after inserting a new edge.
448   void insertEdge(Node &Caller, Function &Callee);
449 
450   /// \brief Update the call graph after inserting a new edge.
insertEdge(Function & Caller,Function & Callee)451   void insertEdge(Function &Caller, Function &Callee) {
452     return insertEdge(get(Caller), Callee);
453   }
454 
455   /// \brief Update the call graph after deleting an edge.
456   void removeEdge(Node &Caller, Function &Callee);
457 
458   /// \brief Update the call graph after deleting an edge.
removeEdge(Function & Caller,Function & Callee)459   void removeEdge(Function &Caller, Function &Callee) {
460     return removeEdge(get(Caller), Callee);
461   }
462 
463   ///@}
464 
465 private:
466   /// \brief Allocator that holds all the call graph nodes.
467   SpecificBumpPtrAllocator<Node> BPA;
468 
469   /// \brief Maps function->node for fast lookup.
470   DenseMap<const Function *, Node *> NodeMap;
471 
472   /// \brief The entry nodes to the graph.
473   ///
474   /// These nodes are reachable through "external" means. Put another way, they
475   /// escape at the module scope.
476   NodeVectorT EntryNodes;
477 
478   /// \brief Map of the entry nodes in the graph to their indices in
479   /// \c EntryNodes.
480   DenseMap<Function *, size_t> EntryIndexMap;
481 
482   /// \brief Allocator that holds all the call graph SCCs.
483   SpecificBumpPtrAllocator<SCC> SCCBPA;
484 
485   /// \brief Maps Function -> SCC for fast lookup.
486   DenseMap<Node *, SCC *> SCCMap;
487 
488   /// \brief The leaf SCCs of the graph.
489   ///
490   /// These are all of the SCCs which have no children.
491   SmallVector<SCC *, 4> LeafSCCs;
492 
493   /// \brief Stack of nodes in the DFS walk.
494   SmallVector<std::pair<Node *, iterator>, 4> DFSStack;
495 
496   /// \brief Set of entry nodes not-yet-processed into SCCs.
497   SmallVector<Function *, 4> SCCEntryNodes;
498 
499   /// \brief Stack of nodes the DFS has walked but not yet put into a SCC.
500   SmallVector<Node *, 4> PendingSCCStack;
501 
502   /// \brief Counter for the next DFS number to assign.
503   int NextDFSNumber;
504 
505   /// \brief Helper to insert a new function, with an already looked-up entry in
506   /// the NodeMap.
507   Node &insertInto(Function &F, Node *&MappedN);
508 
509   /// \brief Helper to update pointers back to the graph object during moves.
510   void updateGraphPtrs();
511 
512   /// \brief Helper to form a new SCC out of the top of a DFSStack-like
513   /// structure.
514   SCC *formSCC(Node *RootN, SmallVectorImpl<Node *> &NodeStack);
515 
516   /// \brief Retrieve the next node in the post-order SCC walk of the call graph.
517   SCC *getNextSCCInPostOrder();
518 };
519 
520 // Provide GraphTraits specializations for call graphs.
521 template <> struct GraphTraits<LazyCallGraph::Node *> {
522   typedef LazyCallGraph::Node NodeType;
523   typedef LazyCallGraph::iterator ChildIteratorType;
524 
525   static NodeType *getEntryNode(NodeType *N) { return N; }
526   static ChildIteratorType child_begin(NodeType *N) { return N->begin(); }
527   static ChildIteratorType child_end(NodeType *N) { return N->end(); }
528 };
529 template <> struct GraphTraits<LazyCallGraph *> {
530   typedef LazyCallGraph::Node NodeType;
531   typedef LazyCallGraph::iterator ChildIteratorType;
532 
533   static NodeType *getEntryNode(NodeType *N) { return N; }
534   static ChildIteratorType child_begin(NodeType *N) { return N->begin(); }
535   static ChildIteratorType child_end(NodeType *N) { return N->end(); }
536 };
537 
538 /// \brief An analysis pass which computes the call graph for a module.
539 class LazyCallGraphAnalysis {
540 public:
541   /// \brief Inform generic clients of the result type.
542   typedef LazyCallGraph Result;
543 
544   static void *ID() { return (void *)&PassID; }
545 
546   static StringRef name() { return "Lazy CallGraph Analysis"; }
547 
548   /// \brief Compute the \c LazyCallGraph for the module \c M.
549   ///
550   /// This just builds the set of entry points to the call graph. The rest is
551   /// built lazily as it is walked.
552   LazyCallGraph run(Module &M) { return LazyCallGraph(M); }
553 
554 private:
555   static char PassID;
556 };
557 
558 /// \brief A pass which prints the call graph to a \c raw_ostream.
559 ///
560 /// This is primarily useful for testing the analysis.
561 class LazyCallGraphPrinterPass {
562   raw_ostream &OS;
563 
564 public:
565   explicit LazyCallGraphPrinterPass(raw_ostream &OS);
566 
567   PreservedAnalyses run(Module &M, ModuleAnalysisManager *AM);
568 
569   static StringRef name() { return "LazyCallGraphPrinterPass"; }
570 };
571 
572 }
573 
574 #endif
575