aboutsummaryrefslogtreecommitdiffstats
path: root/contrib/libs/llvm12/include/llvm/Analysis/LazyCallGraph.h
blob: 2b0fa56f9d3a5d9c2d8fb6ccbfe1d3ceb4e62010 (plain) (blame)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
837
838
839
840
841
842
843
844
845
846
847
848
849
850
851
852
853
854
855
856
857
858
859
860
861
862
863
864
865
866
867
868
869
870
871
872
873
874
875
876
877
878
879
880
881
882
883
884
885
886
887
888
889
890
891
892
893
894
895
896
897
898
899
900
901
902
903
904
905
906
907
908
909
910
911
912
913
914
915
916
917
918
919
920
921
922
923
924
925
926
927
928
929
930
931
932
933
934
935
936
937
938
939
940
941
942
943
944
945
946
947
948
949
950
951
952
953
954
955
956
957
958
959
960
961
962
963
964
965
966
967
968
969
970
971
972
973
974
975
976
977
978
979
980
981
982
983
984
985
986
987
988
989
990
991
992
993
994
995
996
997
998
999
1000
1001
1002
1003
1004
1005
1006
1007
1008
1009
1010
1011
1012
1013
1014
1015
1016
1017
1018
1019
1020
1021
1022
1023
1024
1025
1026
1027
1028
1029
1030
1031
1032
1033
1034
1035
1036
1037
1038
1039
1040
1041
1042
1043
1044
1045
1046
1047
1048
1049
1050
1051
1052
1053
1054
1055
1056
1057
1058
1059
1060
1061
1062
1063
1064
1065
1066
1067
1068
1069
1070
1071
1072
1073
1074
1075
1076
1077
1078
1079
1080
1081
1082
1083
1084
1085
1086
1087
1088
1089
1090
1091
1092
1093
1094
1095
1096
1097
1098
1099
1100
1101
1102
1103
1104
1105
1106
1107
1108
1109
1110
1111
1112
1113
1114
1115
1116
1117
1118
1119
1120
1121
1122
1123
1124
1125
1126
1127
1128
1129
1130
1131
1132
1133
1134
1135
1136
1137
1138
1139
1140
1141
1142
1143
1144
1145
1146
1147
1148
1149
1150
1151
1152
1153
1154
1155
1156
1157
1158
1159
1160
1161
1162
1163
1164
1165
1166
1167
1168
1169
1170
1171
1172
1173
1174
1175
1176
1177
1178
1179
1180
1181
1182
1183
1184
1185
1186
1187
1188
1189
1190
1191
1192
1193
1194
1195
1196
1197
1198
1199
1200
1201
1202
1203
1204
1205
1206
1207
1208
1209
1210
1211
1212
1213
1214
1215
1216
1217
1218
1219
1220
1221
1222
1223
1224
1225
1226
1227
1228
1229
1230
1231
1232
1233
1234
1235
1236
1237
1238
1239
1240
1241
1242
1243
1244
1245
1246
1247
1248
1249
1250
1251
1252
1253
1254
1255
1256
1257
1258
1259
1260
1261
1262
1263
1264
1265
1266
1267
1268
1269
1270
1271
1272
1273
1274
1275
1276
1277
1278
1279
1280
1281
1282
1283
1284
1285
1286
1287
1288
1289
1290
1291
1292
1293
1294
1295
1296
1297
1298
1299
1300
1301
1302
1303
1304
1305
1306
1307
1308
1309
1310
1311
1312
1313
1314
1315
1316
1317
1318
1319
1320
1321
1322
1323
1324
1325
1326
1327
1328
1329
1330
1331
1332
1333
1334
1335
1336
1337
1338
1339
#pragma once

#ifdef __GNUC__
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-parameter"
#endif

//===- 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/Optional.h"
#include "llvm/ADT/PointerIntPair.h"
#include "llvm/ADT/STLExtras.h"
#include "llvm/ADT/SetVector.h"
#include "llvm/ADT/SmallPtrSet.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/Constant.h"
#include "llvm/IR/Constants.h"
#include "llvm/IR/Function.h"
#include "llvm/IR/PassManager.h"
#include "llvm/Support/Allocator.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/raw_ostream.h"
#include <cassert>
#include <iterator>
#include <string>
#include <utility>

namespace llvm {

class Module;
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;
  class edge_iterator;
  class call_edge_iterator;

  /// 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;

    /// Returnss 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. 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.
  ///
  /// 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 `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.hasValue(); }

    /// 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;

    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 tranfsormations 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 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 descrtiption 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;

#ifndef NDEBUG
    /// 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 componet 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.
  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;

#ifndef NDEBUG
    /// 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.
    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);

    ///@}
  };

  /// A post-order depth-first RefSCC iterator over the call graph.
  ///
  /// This iterator walks the cached post-order sequence of RefSCCs. However,
  /// it trades stability for flexibility. It is restricted to a forward
  /// iterator but will survive mutations which insert new RefSCCs and continue
  /// to point to the same RefSCC even if it moves in the post-order sequence.
  class postorder_ref_scc_iterator
      : public iterator_facade_base<postorder_ref_scc_iterator,
                                    std::forward_iterator_tag, RefSCC> {
    friend class LazyCallGraph;
    friend class LazyCallGraph::Node;

    /// Nonce type to select the constructor for the end iterator.
    struct IsAtEndT {};

    LazyCallGraph *G;
    RefSCC *RC = nullptr;

    /// Build the begin iterator for a node.
    postorder_ref_scc_iterator(LazyCallGraph &G) : G(&G), RC(getRC(G, 0)) {}

    /// Build the end iterator for a node. This is selected purely by overload.
    postorder_ref_scc_iterator(LazyCallGraph &G, IsAtEndT /*Nonce*/) : G(&G) {}

    /// Get the post-order RefSCC at the given index of the postorder walk,
    /// populating it if necessary.
    static RefSCC *getRC(LazyCallGraph &G, int Index) {
      if (Index == (int)G.PostOrderRefSCCs.size())
        // We're at the end.
        return nullptr;

      return G.PostOrderRefSCCs[Index];
    }

  public:
    bool operator==(const postorder_ref_scc_iterator &Arg) const {
      return G == Arg.G && RC == Arg.RC;
    }

    reference operator*() const { return *RC; }

    using iterator_facade_base::operator++;
    postorder_ref_scc_iterator &operator++() {
      assert(RC && "Cannot increment the end iterator!");
      RC = getRC(*G, G->RefSCCIndices.find(RC)->second + 1);
      return *this;
    }
  };

  /// 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,
                function_ref<TargetLibraryInfo &(Function &)> GetTLI);

  LazyCallGraph(LazyCallGraph &&G);
  LazyCallGraph &operator=(LazyCallGraph &&RHS);

  bool invalidate(Module &, const PreservedAnalyses &PA,
                  ModuleAnalysisManager::Invalidator &);

  EdgeSequence::iterator begin() { return EntryEdges.begin(); }
  EdgeSequence::iterator end() { return EntryEdges.end(); }

  void buildRefSCCs();

  postorder_ref_scc_iterator postorder_ref_scc_begin() {
    if (!EntryEdges.empty())
      assert(!PostOrderRefSCCs.empty() &&
             "Must form RefSCCs before iterating them!");
    return postorder_ref_scc_iterator(*this);
  }
  postorder_ref_scc_iterator postorder_ref_scc_end() {
    if (!EntryEdges.empty())
      assert(!PostOrderRefSCCs.empty() &&
             "Must form RefSCCs before iterating them!");
    return postorder_ref_scc_iterator(*this,
                                      postorder_ref_scc_iterator::IsAtEndT());
  }

  iterator_range<postorder_ref_scc_iterator> postorder_ref_sccs() {
    return make_range(postorder_ref_scc_begin(), postorder_ref_scc_end());
  }

  /// Lookup a function in the graph which has already been scanned and added.
  Node *lookup(const Function &F) const { return NodeMap.lookup(&F); }

  /// Lookup a function's SCC in the graph.
  ///
  /// \returns null if the function hasn't been assigned an SCC via the RefSCC
  /// iterator walk.
  SCC *lookupSCC(Node &N) const { return SCCMap.lookup(&N); }

  /// Lookup a function's RefSCC in the graph.
  ///
  /// \returns null if the function hasn't been assigned a RefSCC via the
  /// RefSCC iterator walk.
  RefSCC *lookupRefSCC(Node &N) const {
    if (SCC *C = lookupSCC(N))
      return &C->getOuterRefSCC();

    return nullptr;
  }

  /// 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);
  }

  /// Get the sequence of known and defined library functions.
  ///
  /// These functions, because they are known to LLVM, can have calls
  /// introduced out of thin air from arbitrary IR.
  ArrayRef<Function *> getLibFunctions() const {
    return LibFunctions.getArrayRef();
  }

  /// Test whether a function is a known and defined library function tracked by
  /// the call graph.
  ///
  /// Because these functions are known to LLVM they are specially modeled in
  /// the call graph and even when all IR-level references have been removed
  /// remain active and reachable.
  bool isLibFunction(Function &F) const { return LibFunctions.count(&F); }

  ///@{
  /// \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, most mutation of the
  /// graph must be performed via a RefSCC method. There are some exceptions
  /// below.

  /// Update the call graph after inserting a new edge.
  void insertEdge(Node &SourceN, Node &TargetN, Edge::Kind EK);

  /// Update the call graph after inserting a new edge.
  void insertEdge(Function &Source, Function &Target, Edge::Kind EK) {
    return insertEdge(get(Source), get(Target), EK);
  }

  /// Update the call graph after deleting an edge.
  void removeEdge(Node &SourceN, Node &TargetN);

  /// Update the call graph after deleting an edge.
  void removeEdge(Function &Source, Function &Target) {
    return removeEdge(get(Source), get(Target));
  }

  ///@}

  ///@{
  /// \name General Mutation API
  ///
  /// There are a very limited set of mutations allowed on the graph as a whole
  /// once SCCs have started to be formed. These routines have strict contracts
  /// but may be called at any point.

  /// Remove a dead function from the call graph (typically to delete it).
  ///
  /// Note that the function must have an empty use list, and the call graph
  /// must be up-to-date prior to calling this. That means it is by itself in
  /// a maximal SCC which is by itself in a maximal RefSCC, etc. No structural
  /// changes result from calling this routine other than potentially removing
  /// entry points into the call graph.
  ///
  /// If SCC formation has begun, this function must not be part of the current
  /// DFS in order to call this safely. Typically, the function will have been
  /// fully visited by the DFS prior to calling this routine.
  void removeDeadFunction(Function &F);

  /// Add a new function split/outlined from an existing function.
  ///
  /// The new function may only reference other functions that the original
  /// function did.
  ///
  /// The original function must reference (either directly or indirectly) the
  /// new function.
  ///
  /// The new function may also reference the original function.
  /// It may end up in a parent SCC in the case that the original function's
  /// edge to the new function is a ref edge, and the edge back is a call edge.
  void addSplitFunction(Function &OriginalFunction, Function &NewFunction);

  /// Add new ref-recursive functions split/outlined from an existing function.
  ///
  /// The new functions may only reference other functions that the original
  /// function did. The new functions may reference (not call) the original
  /// function.
  ///
  /// The original function must reference (not call) all new functions.
  /// All new functions must reference (not call) each other.
  void addSplitRefRecursiveFunctions(Function &OriginalFunction,
                                     ArrayRef<Function *> NewFunctions);

  ///@}

  ///@{
  /// \name Static helpers for code doing updates to the call graph.
  ///
  /// These helpers are used to implement parts of the call graph but are also
  /// useful to code doing updates or otherwise wanting to walk the IR in the
  /// same patterns as when we build the call graph.

  /// Recursively visits the defined functions whose address is reachable from
  /// every constant in the \p Worklist.
  ///
  /// Doesn't recurse through any constants already in the \p Visited set, and
  /// updates that set with every constant visited.
  ///
  /// For each defined function, calls \p Callback with that function.
  template <typename CallbackT>
  static void visitReferences(SmallVectorImpl<Constant *> &Worklist,
                              SmallPtrSetImpl<Constant *> &Visited,
                              CallbackT Callback) {
    while (!Worklist.empty()) {
      Constant *C = Worklist.pop_back_val();

      if (Function *F = dyn_cast<Function>(C)) {
        if (!F->isDeclaration())
          Callback(*F);
        continue;
      }

      // The blockaddress constant expression is a weird special case, we can't
      // generically walk its operands the way we do for all other constants.
      if (BlockAddress *BA = dyn_cast<BlockAddress>(C)) {
        // If we've already visited the function referred to by the block
        // address, we don't need to revisit it.
        if (Visited.count(BA->getFunction()))
          continue;

        // If all of the blockaddress' users are instructions within the
        // referred to function, we don't need to insert a cycle.
        if (llvm::all_of(BA->users(), [&](User *U) {
              if (Instruction *I = dyn_cast<Instruction>(U))
                return I->getFunction() == BA->getFunction();
              return false;
            }))
          continue;

        // Otherwise we should go visit the referred to function.
        Visited.insert(BA->getFunction());
        Worklist.push_back(BA->getFunction());
        continue;
      }

      for (Value *Op : C->operand_values())
        if (Visited.insert(cast<Constant>(Op)).second)
          Worklist.push_back(cast<Constant>(Op));
    }
  }

  ///@}

private:
  using node_stack_iterator = SmallVectorImpl<Node *>::reverse_iterator;
  using node_stack_range = iterator_range<node_stack_iterator>;

  /// Allocator that holds all the call graph nodes.
  SpecificBumpPtrAllocator<Node> BPA;

  /// Maps function->node for fast lookup.
  DenseMap<const Function *, Node *> NodeMap;

  /// The entry edges into the graph.
  ///
  /// These edges are from "external" sources. Put another way, they
  /// escape at the module scope.
  EdgeSequence EntryEdges;

  /// Allocator that holds all the call graph SCCs.
  SpecificBumpPtrAllocator<SCC> SCCBPA;

  /// Maps Function -> SCC for fast lookup.
  DenseMap<Node *, SCC *> SCCMap;

  /// Allocator that holds all the call graph RefSCCs.
  SpecificBumpPtrAllocator<RefSCC> RefSCCBPA;

  /// The post-order sequence of RefSCCs.
  ///
  /// This list is lazily formed the first time we walk the graph.
  SmallVector<RefSCC *, 16> PostOrderRefSCCs;

  /// A map from RefSCC to the index for it in the postorder sequence of
  /// RefSCCs.
  DenseMap<RefSCC *, int> RefSCCIndices;

  /// Defined functions that are also known library functions which the
  /// optimizer can reason about and therefore might introduce calls to out of
  /// thin air.
  SmallSetVector<Function *, 4> LibFunctions;

  /// Helper to insert a new function, with an already looked-up entry in
  /// the NodeMap.
  Node &insertInto(Function &F, Node *&MappedN);

  /// Helper to initialize a new node created outside of creating SCCs and add
  /// it to the NodeMap if necessary. For example, useful when a function is
  /// split.
  Node &initNode(Function &F);

  /// Helper to update pointers back to the graph object during moves.
  void updateGraphPtrs();

  /// Allocates an SCC and constructs it using the graph allocator.
  ///
  /// The arguments are forwarded to the constructor.
  template <typename... Ts> SCC *createSCC(Ts &&... Args) {
    return new (SCCBPA.Allocate()) SCC(std::forward<Ts>(Args)...);
  }

  /// Allocates a RefSCC and constructs it using the graph allocator.
  ///
  /// The arguments are forwarded to the constructor.
  template <typename... Ts> RefSCC *createRefSCC(Ts &&... Args) {
    return new (RefSCCBPA.Allocate()) RefSCC(std::forward<Ts>(Args)...);
  }

  /// Common logic for building SCCs from a sequence of roots.
  ///
  /// This is a very generic implementation of the depth-first walk and SCC
  /// formation algorithm. It uses a generic sequence of roots and generic
  /// callbacks for each step. This is designed to be used to implement both
  /// the RefSCC formation and SCC formation with shared logic.
  ///
  /// Currently this is a relatively naive implementation of Tarjan's DFS
  /// algorithm to form the SCCs.
  ///
  /// FIXME: We should consider newer variants such as Nuutila.
  template <typename RootsT, typename GetBeginT, typename GetEndT,
            typename GetNodeT, typename FormSCCCallbackT>
  static void buildGenericSCCs(RootsT &&Roots, GetBeginT &&GetBegin,
                               GetEndT &&GetEnd, GetNodeT &&GetNode,
                               FormSCCCallbackT &&FormSCC);

  /// Build the SCCs for a RefSCC out of a list of nodes.
  void buildSCCs(RefSCC &RC, node_stack_range Nodes);

  /// Get the index of a RefSCC within the postorder traversal.
  ///
  /// Requires that this RefSCC is a valid one in the (perhaps partial)
  /// postorder traversed part of the graph.
  int getRefSCCIndex(RefSCC &RC) {
    auto IndexIt = RefSCCIndices.find(&RC);
    assert(IndexIt != RefSCCIndices.end() && "RefSCC doesn't have an index!");
    assert(PostOrderRefSCCs[IndexIt->second] == &RC &&
           "Index does not point back at RC!");
    return IndexIt->second;
  }
};

inline LazyCallGraph::Edge::Edge() : Value() {}
inline LazyCallGraph::Edge::Edge(Node &N, Kind K) : Value(&N, K) {}

inline LazyCallGraph::Edge::operator bool() const {
  return Value.getPointer() && !Value.getPointer()->isDead();
}

inline LazyCallGraph::Edge::Kind LazyCallGraph::Edge::getKind() const {
  assert(*this && "Queried a null edge!");
  return Value.getInt();
}

inline bool LazyCallGraph::Edge::isCall() const {
  assert(*this && "Queried a null edge!");
  return getKind() == Call;
}

inline LazyCallGraph::Node &LazyCallGraph::Edge::getNode() const {
  assert(*this && "Queried a null edge!");
  return *Value.getPointer();
}

inline Function &LazyCallGraph::Edge::getFunction() const {
  assert(*this && "Queried a null edge!");
  return getNode().getFunction();
}

// Provide GraphTraits specializations for call graphs.
template <> struct GraphTraits<LazyCallGraph::Node *> {
  using NodeRef = LazyCallGraph::Node *;
  using ChildIteratorType = LazyCallGraph::EdgeSequence::iterator;

  static NodeRef getEntryNode(NodeRef N) { return N; }
  static ChildIteratorType child_begin(NodeRef N) { return (*N)->begin(); }
  static ChildIteratorType child_end(NodeRef N) { return (*N)->end(); }
};
template <> struct GraphTraits<LazyCallGraph *> {
  using NodeRef = LazyCallGraph::Node *;
  using ChildIteratorType = LazyCallGraph::EdgeSequence::iterator;

  static NodeRef getEntryNode(NodeRef N) { return N; }
  static ChildIteratorType child_begin(NodeRef N) { return (*N)->begin(); }
  static ChildIteratorType child_end(NodeRef N) { return (*N)->end(); }
};

/// An analysis pass which computes the call graph for a module.
class LazyCallGraphAnalysis : public AnalysisInfoMixin<LazyCallGraphAnalysis> {
  friend AnalysisInfoMixin<LazyCallGraphAnalysis>;

  static AnalysisKey Key;

public:
  /// Inform generic clients of the result type.
  using Result = LazyCallGraph;

  /// Compute the \c LazyCallGraph for 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, ModuleAnalysisManager &AM) {
    FunctionAnalysisManager &FAM =
        AM.getResult<FunctionAnalysisManagerModuleProxy>(M).getManager();
    auto GetTLI = [&FAM](Function &F) -> TargetLibraryInfo & {
      return FAM.getResult<TargetLibraryAnalysis>(F);
    };
    return LazyCallGraph(M, GetTLI);
  }
};

/// A pass which prints the call graph to a \c raw_ostream.
///
/// This is primarily useful for testing the analysis.
class LazyCallGraphPrinterPass
    : public PassInfoMixin<LazyCallGraphPrinterPass> {
  raw_ostream &OS;

public:
  explicit LazyCallGraphPrinterPass(raw_ostream &OS);

  PreservedAnalyses run(Module &M, ModuleAnalysisManager &AM);
};

/// A pass which prints the call graph as a DOT file to a \c raw_ostream.
///
/// This is primarily useful for visualization purposes.
class LazyCallGraphDOTPrinterPass
    : public PassInfoMixin<LazyCallGraphDOTPrinterPass> {
  raw_ostream &OS;

public:
  explicit LazyCallGraphDOTPrinterPass(raw_ostream &OS);

  PreservedAnalyses run(Module &M, ModuleAnalysisManager &AM);
};

} // end namespace llvm

#endif // LLVM_ANALYSIS_LAZYCALLGRAPH_H

#ifdef __GNUC__
#pragma GCC diagnostic pop
#endif