llvm.org GIT mirror llvm / 85a14f0 lib / Analysis / CGSCCPassManager.cpp
85a14f0

Tree @85a14f0 (Download .tar.gz)

CGSCCPassManager.cpp @85a14f0raw · history · 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
//===- CGSCCPassManager.cpp - Managing & running CGSCC passes -------------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#include "llvm/Analysis/CGSCCPassManager.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/ADT/Optional.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_range.h"
#include "llvm/Analysis/LazyCallGraph.h"
#include "llvm/IR/CallSite.h"
#include "llvm/IR/Constant.h"
#include "llvm/IR/InstIterator.h"
#include "llvm/IR/Instruction.h"
#include "llvm/IR/PassManager.h"
#include "llvm/Support/Casting.h"
#include "llvm/Support/Debug.h"
#include "llvm/Support/raw_ostream.h"
#include <algorithm>
#include <cassert>
#include <iterator>

#define DEBUG_TYPE "cgscc"

using namespace llvm;

// Explicit template instantiations and specialization definitions for core
// template typedefs.
namespace llvm {

// Explicit instantiations for the core proxy templates.
template class AllAnalysesOn<LazyCallGraph::SCC>;
template class AnalysisManager<LazyCallGraph::SCC, LazyCallGraph &>;
template class PassManager<LazyCallGraph::SCC, CGSCCAnalysisManager,
                           LazyCallGraph &, CGSCCUpdateResult &>;
template class InnerAnalysisManagerProxy<CGSCCAnalysisManager, Module>;
template class OuterAnalysisManagerProxy<ModuleAnalysisManager,
                                         LazyCallGraph::SCC, LazyCallGraph &>;
template class OuterAnalysisManagerProxy<CGSCCAnalysisManager, Function>;

/// Explicitly specialize the pass manager run method to handle call graph
/// updates.
template <>
PreservedAnalyses
PassManager<LazyCallGraph::SCC, CGSCCAnalysisManager, LazyCallGraph &,
            CGSCCUpdateResult &>::run(LazyCallGraph::SCC &InitialC,
                                      CGSCCAnalysisManager &AM,
                                      LazyCallGraph &G, CGSCCUpdateResult &UR) {
  PreservedAnalyses PA = PreservedAnalyses::all();

  if (DebugLogging)
    dbgs() << "Starting CGSCC pass manager run.\n";

  // The SCC may be refined while we are running passes over it, so set up
  // a pointer that we can update.
  LazyCallGraph::SCC *C = &InitialC;

  for (auto &Pass : Passes) {
    if (DebugLogging)
      dbgs() << "Running pass: " << Pass->name() << " on " << *C << "\n";

    PreservedAnalyses PassPA = Pass->run(*C, AM, G, UR);

    // Update the SCC if necessary.
    C = UR.UpdatedC ? UR.UpdatedC : C;

    // If the CGSCC pass wasn't able to provide a valid updated SCC, the
    // current SCC may simply need to be skipped if invalid.
    if (UR.InvalidatedSCCs.count(C)) {
      LLVM_DEBUG(dbgs() << "Skipping invalidated root or island SCC!\n");
      break;
    }
    // Check that we didn't miss any update scenario.
    assert(C->begin() != C->end() && "Cannot have an empty SCC!");

    // Update the analysis manager as each pass runs and potentially
    // invalidates analyses.
    AM.invalidate(*C, PassPA);

    // Finally, we intersect the final preserved analyses to compute the
    // aggregate preserved set for this pass manager.
    PA.intersect(std::move(PassPA));

    // FIXME: Historically, the pass managers all called the LLVM context's
    // yield function here. We don't have a generic way to acquire the
    // context and it isn't yet clear what the right pattern is for yielding
    // in the new pass manager so it is currently omitted.
    // ...getContext().yield();
  }

  // Invalidation was handled after each pass in the above loop for the current
  // SCC. Therefore, the remaining analysis results in the AnalysisManager are
  // preserved. We mark this with a set so that we don't need to inspect each
  // one individually.
  PA.preserveSet<AllAnalysesOn<LazyCallGraph::SCC>>();

  if (DebugLogging)
    dbgs() << "Finished CGSCC pass manager run.\n";

  return PA;
}

bool CGSCCAnalysisManagerModuleProxy::Result::invalidate(
    Module &M, const PreservedAnalyses &PA,
    ModuleAnalysisManager::Invalidator &Inv) {
  // If literally everything is preserved, we're done.
  if (PA.areAllPreserved())
    return false; // This is still a valid proxy.

  // If this proxy or the call graph is going to be invalidated, we also need
  // to clear all the keys coming from that analysis.
  //
  // We also directly invalidate the FAM's module proxy if necessary, and if
  // that proxy isn't preserved we can't preserve this proxy either. We rely on
  // it to handle module -> function analysis invalidation in the face of
  // structural changes and so if it's unavailable we conservatively clear the
  // entire SCC layer as well rather than trying to do invalidation ourselves.
  auto PAC = PA.getChecker<CGSCCAnalysisManagerModuleProxy>();
  if (!(PAC.preserved() || PAC.preservedSet<AllAnalysesOn<Module>>()) ||
      Inv.invalidate<LazyCallGraphAnalysis>(M, PA) ||
      Inv.invalidate<FunctionAnalysisManagerModuleProxy>(M, PA)) {
    InnerAM->clear();

    // And the proxy itself should be marked as invalid so that we can observe
    // the new call graph. This isn't strictly necessary because we cheat
    // above, but is still useful.
    return true;
  }

  // Directly check if the relevant set is preserved so we can short circuit
  // invalidating SCCs below.
  bool AreSCCAnalysesPreserved =
      PA.allAnalysesInSetPreserved<AllAnalysesOn<LazyCallGraph::SCC>>();

  // Ok, we have a graph, so we can propagate the invalidation down into it.
  G->buildRefSCCs();
  for (auto &RC : G->postorder_ref_sccs())
    for (auto &C : RC) {
      Optional<PreservedAnalyses> InnerPA;

      // Check to see whether the preserved set needs to be adjusted based on
      // module-level analysis invalidation triggering deferred invalidation
      // for this SCC.
      if (auto *OuterProxy =
              InnerAM->getCachedResult<ModuleAnalysisManagerCGSCCProxy>(C))
        for (const auto &OuterInvalidationPair :
             OuterProxy->getOuterInvalidations()) {
          AnalysisKey *OuterAnalysisID = OuterInvalidationPair.first;
          const auto &InnerAnalysisIDs = OuterInvalidationPair.second;
          if (Inv.invalidate(OuterAnalysisID, M, PA)) {
            if (!InnerPA)
              InnerPA = PA;
            for (AnalysisKey *InnerAnalysisID : InnerAnalysisIDs)
              InnerPA->abandon(InnerAnalysisID);
          }
        }

      // Check if we needed a custom PA set. If so we'll need to run the inner
      // invalidation.
      if (InnerPA) {
        InnerAM->invalidate(C, *InnerPA);
        continue;
      }

      // Otherwise we only need to do invalidation if the original PA set didn't
      // preserve all SCC analyses.
      if (!AreSCCAnalysesPreserved)
        InnerAM->invalidate(C, PA);
    }

  // Return false to indicate that this result is still a valid proxy.
  return false;
}

template <>
CGSCCAnalysisManagerModuleProxy::Result
CGSCCAnalysisManagerModuleProxy::run(Module &M, ModuleAnalysisManager &AM) {
  // Force the Function analysis manager to also be available so that it can
  // be accessed in an SCC analysis and proxied onward to function passes.
  // FIXME: It is pretty awkward to just drop the result here and assert that
  // we can find it again later.
  (void)AM.getResult<FunctionAnalysisManagerModuleProxy>(M);

  return Result(*InnerAM, AM.getResult<LazyCallGraphAnalysis>(M));
}

AnalysisKey FunctionAnalysisManagerCGSCCProxy::Key;

FunctionAnalysisManagerCGSCCProxy::Result
FunctionAnalysisManagerCGSCCProxy::run(LazyCallGraph::SCC &C,
                                       CGSCCAnalysisManager &AM,
                                       LazyCallGraph &CG) {
  // Collect the FunctionAnalysisManager from the Module layer and use that to
  // build the proxy result.
  //
  // This allows us to rely on the FunctionAnalysisMangaerModuleProxy to
  // invalidate the function analyses.
  auto &MAM = AM.getResult<ModuleAnalysisManagerCGSCCProxy>(C, CG).getManager();
  Module &M = *C.begin()->getFunction().getParent();
  auto *FAMProxy = MAM.getCachedResult<FunctionAnalysisManagerModuleProxy>(M);
  assert(FAMProxy && "The CGSCC pass manager requires that the FAM module "
                     "proxy is run on the module prior to entering the CGSCC "
                     "walk.");

  // Note that we special-case invalidation handling of this proxy in the CGSCC
  // analysis manager's Module proxy. This avoids the need to do anything
  // special here to recompute all of this if ever the FAM's module proxy goes
  // away.
  return Result(FAMProxy->getManager());
}

bool FunctionAnalysisManagerCGSCCProxy::Result::invalidate(
    LazyCallGraph::SCC &C, const PreservedAnalyses &PA,
    CGSCCAnalysisManager::Invalidator &Inv) {
  // If literally everything is preserved, we're done.
  if (PA.areAllPreserved())
    return false; // This is still a valid proxy.

  // If this proxy isn't marked as preserved, then even if the result remains
  // valid, the key itself may no longer be valid, so we clear everything.
  //
  // Note that in order to preserve this proxy, a module pass must ensure that
  // the FAM has been completely updated to handle the deletion of functions.
  // Specifically, any FAM-cached results for those functions need to have been
  // forcibly cleared. When preserved, this proxy will only invalidate results
  // cached on functions *still in the module* at the end of the module pass.
  auto PAC = PA.getChecker<FunctionAnalysisManagerCGSCCProxy>();
  if (!PAC.preserved() && !PAC.preservedSet<AllAnalysesOn<LazyCallGraph::SCC>>()) {
    for (LazyCallGraph::Node &N : C)
      FAM->clear(N.getFunction(), N.getFunction().getName());

    return true;
  }

  // Directly check if the relevant set is preserved.
  bool AreFunctionAnalysesPreserved =
      PA.allAnalysesInSetPreserved<AllAnalysesOn<Function>>();

  // Now walk all the functions to see if any inner analysis invalidation is
  // necessary.
  for (LazyCallGraph::Node &N : C) {
    Function &F = N.getFunction();
    Optional<PreservedAnalyses> FunctionPA;

    // Check to see whether the preserved set needs to be pruned based on
    // SCC-level analysis invalidation that triggers deferred invalidation
    // registered with the outer analysis manager proxy for this function.
    if (auto *OuterProxy =
            FAM->getCachedResult<CGSCCAnalysisManagerFunctionProxy>(F))
      for (const auto &OuterInvalidationPair :
           OuterProxy->getOuterInvalidations()) {
        AnalysisKey *OuterAnalysisID = OuterInvalidationPair.first;
        const auto &InnerAnalysisIDs = OuterInvalidationPair.second;
        if (Inv.invalidate(OuterAnalysisID, C, PA)) {
          if (!FunctionPA)
            FunctionPA = PA;
          for (AnalysisKey *InnerAnalysisID : InnerAnalysisIDs)
            FunctionPA->abandon(InnerAnalysisID);
        }
      }

    // Check if we needed a custom PA set, and if so we'll need to run the
    // inner invalidation.
    if (FunctionPA) {
      FAM->invalidate(F, *FunctionPA);
      continue;
    }

    // Otherwise we only need to do invalidation if the original PA set didn't
    // preserve all function analyses.
    if (!AreFunctionAnalysesPreserved)
      FAM->invalidate(F, PA);
  }

  // Return false to indicate that this result is still a valid proxy.
  return false;
}

} // end namespace llvm

/// When a new SCC is created for the graph and there might be function
/// analysis results cached for the functions now in that SCC two forms of
/// updates are required.
///
/// First, a proxy from the SCC to the FunctionAnalysisManager needs to be
/// created so that any subsequent invalidation events to the SCC are
/// propagated to the function analysis results cached for functions within it.
///
/// Second, if any of the functions within the SCC have analysis results with
/// outer analysis dependencies, then those dependencies would point to the
/// *wrong* SCC's analysis result. We forcibly invalidate the necessary
/// function analyses so that they don't retain stale handles.
static void updateNewSCCFunctionAnalyses(LazyCallGraph::SCC &C,
                                         LazyCallGraph &G,
                                         CGSCCAnalysisManager &AM) {
  // Get the relevant function analysis manager.
  auto &FAM =
      AM.getResult<FunctionAnalysisManagerCGSCCProxy>(C, G).getManager();

  // Now walk the functions in this SCC and invalidate any function analysis
  // results that might have outer dependencies on an SCC analysis.
  for (LazyCallGraph::Node &N : C) {
    Function &F = N.getFunction();

    auto *OuterProxy =
        FAM.getCachedResult<CGSCCAnalysisManagerFunctionProxy>(F);
    if (!OuterProxy)
      // No outer analyses were queried, nothing to do.
      continue;

    // Forcibly abandon all the inner analyses with dependencies, but
    // invalidate nothing else.
    auto PA = PreservedAnalyses::all();
    for (const auto &OuterInvalidationPair :
         OuterProxy->getOuterInvalidations()) {
      const auto &InnerAnalysisIDs = OuterInvalidationPair.second;
      for (AnalysisKey *InnerAnalysisID : InnerAnalysisIDs)
        PA.abandon(InnerAnalysisID);
    }

    // Now invalidate anything we found.
    FAM.invalidate(F, PA);
  }
}

/// Helper function to update both the \c CGSCCAnalysisManager \p AM and the \c
/// CGSCCPassManager's \c CGSCCUpdateResult \p UR based on a range of newly
/// added SCCs.
///
/// The range of new SCCs must be in postorder already. The SCC they were split
/// out of must be provided as \p C. The current node being mutated and
/// triggering updates must be passed as \p N.
///
/// This function returns the SCC containing \p N. This will be either \p C if
/// no new SCCs have been split out, or it will be the new SCC containing \p N.
template <typename SCCRangeT>
static LazyCallGraph::SCC *
incorporateNewSCCRange(const SCCRangeT &NewSCCRange, LazyCallGraph &G,
                       LazyCallGraph::Node &N, LazyCallGraph::SCC *C,
                       CGSCCAnalysisManager &AM, CGSCCUpdateResult &UR) {
  using SCC = LazyCallGraph::SCC;

  if (NewSCCRange.begin() == NewSCCRange.end())
    return C;

  // Add the current SCC to the worklist as its shape has changed.
  UR.CWorklist.insert(C);
  LLVM_DEBUG(dbgs() << "Enqueuing the existing SCC in the worklist:" << *C
                    << "\n");

  SCC *OldC = C;

  // Update the current SCC. Note that if we have new SCCs, this must actually
  // change the SCC.
  assert(C != &*NewSCCRange.begin() &&
         "Cannot insert new SCCs without changing current SCC!");
  C = &*NewSCCRange.begin();
  assert(G.lookupSCC(N) == C && "Failed to update current SCC!");

  // If we had a cached FAM proxy originally, we will want to create more of
  // them for each SCC that was split off.
  bool NeedFAMProxy =
      AM.getCachedResult<FunctionAnalysisManagerCGSCCProxy>(*OldC) != nullptr;

  // We need to propagate an invalidation call to all but the newly current SCC
  // because the outer pass manager won't do that for us after splitting them.
  // FIXME: We should accept a PreservedAnalysis from the CG updater so that if
  // there are preserved analysis we can avoid invalidating them here for
  // split-off SCCs.
  // We know however that this will preserve any FAM proxy so go ahead and mark
  // that.
  PreservedAnalyses PA;
  PA.preserve<FunctionAnalysisManagerCGSCCProxy>();
  AM.invalidate(*OldC, PA);

  // Ensure the now-current SCC's function analyses are updated.
  if (NeedFAMProxy)
    updateNewSCCFunctionAnalyses(*C, G, AM);

  for (SCC &NewC : llvm::reverse(make_range(std::next(NewSCCRange.begin()),
                                            NewSCCRange.end()))) {
    assert(C != &NewC && "No need to re-visit the current SCC!");
    assert(OldC != &NewC && "Already handled the original SCC!");
    UR.CWorklist.insert(&NewC);
    LLVM_DEBUG(dbgs() << "Enqueuing a newly formed SCC:" << NewC << "\n");

    // Ensure new SCCs' function analyses are updated.
    if (NeedFAMProxy)
      updateNewSCCFunctionAnalyses(NewC, G, AM);

    // Also propagate a normal invalidation to the new SCC as only the current
    // will get one from the pass manager infrastructure.
    AM.invalidate(NewC, PA);
  }
  return C;
}

LazyCallGraph::SCC &llvm::updateCGAndAnalysisManagerForFunctionPass(
    LazyCallGraph &G, LazyCallGraph::SCC &InitialC, LazyCallGraph::Node &N,
    CGSCCAnalysisManager &AM, CGSCCUpdateResult &UR) {
  using Node = LazyCallGraph::Node;
  using Edge = LazyCallGraph::Edge;
  using SCC = LazyCallGraph::SCC;
  using RefSCC = LazyCallGraph::RefSCC;

  RefSCC &InitialRC = InitialC.getOuterRefSCC();
  SCC *C = &InitialC;
  RefSCC *RC = &InitialRC;
  Function &F = N.getFunction();

  // Walk the function body and build up the set of retained, promoted, and
  // demoted edges.
  SmallVector<Constant *, 16> Worklist;
  SmallPtrSet<Constant *, 16> Visited;
  SmallPtrSet<Node *, 16> RetainedEdges;
  SmallSetVector<Node *, 4> PromotedRefTargets;
  SmallSetVector<Node *, 4> DemotedCallTargets;

  // First walk the function and handle all called functions. We do this first
  // because if there is a single call edge, whether there are ref edges is
  // irrelevant.
  for (Instruction &I : instructions(F))
    if (auto CS = CallSite(&I))
      if (Function *Callee = CS.getCalledFunction())
        if (Visited.insert(Callee).second && !Callee->isDeclaration()) {
          Node &CalleeN = *G.lookup(*Callee);
          Edge *E = N->lookup(CalleeN);
          // FIXME: We should really handle adding new calls. While it will
          // make downstream usage more complex, there is no fundamental
          // limitation and it will allow passes within the CGSCC to be a bit
          // more flexible in what transforms they can do. Until then, we
          // verify that new calls haven't been introduced.
          assert(E && "No function transformations should introduce *new* "
                      "call edges! Any new calls should be modeled as "
                      "promoted existing ref edges!");
          bool Inserted = RetainedEdges.insert(&CalleeN).second;
          (void)Inserted;
          assert(Inserted && "We should never visit a function twice.");
          if (!E->isCall())
            PromotedRefTargets.insert(&CalleeN);
        }

  // Now walk all references.
  for (Instruction &I : instructions(F))
    for (Value *Op : I.operand_values())
      if (auto *C = dyn_cast<Constant>(Op))
        if (Visited.insert(C).second)
          Worklist.push_back(C);

  auto VisitRef = [&](Function &Referee) {
    Node &RefereeN = *G.lookup(Referee);
    Edge *E = N->lookup(RefereeN);
    // FIXME: Similarly to new calls, we also currently preclude
    // introducing new references. See above for details.
    assert(E && "No function transformations should introduce *new* ref "
                "edges! Any new ref edges would require IPO which "
                "function passes aren't allowed to do!");
    bool Inserted = RetainedEdges.insert(&RefereeN).second;
    (void)Inserted;
    assert(Inserted && "We should never visit a function twice.");
    if (E->isCall())
      DemotedCallTargets.insert(&RefereeN);
  };
  LazyCallGraph::visitReferences(Worklist, Visited, VisitRef);

  // Include synthetic reference edges to known, defined lib functions.
  for (auto *F : G.getLibFunctions())
    // While the list of lib functions doesn't have repeats, don't re-visit
    // anything handled above.
    if (!Visited.count(F))
      VisitRef(*F);

  // First remove all of the edges that are no longer present in this function.
  // The first step makes these edges uniformly ref edges and accumulates them
  // into a separate data structure so removal doesn't invalidate anything.
  SmallVector<Node *, 4> DeadTargets;
  for (Edge &E : *N) {
    if (RetainedEdges.count(&E.getNode()))
      continue;

    SCC &TargetC = *G.lookupSCC(E.getNode());
    RefSCC &TargetRC = TargetC.getOuterRefSCC();
    if (&TargetRC == RC && E.isCall()) {
      if (C != &TargetC) {
        // For separate SCCs this is trivial.
        RC->switchTrivialInternalEdgeToRef(N, E.getNode());
      } else {
        // Now update the call graph.
        C = incorporateNewSCCRange(RC->switchInternalEdgeToRef(N, E.getNode()),
                                   G, N, C, AM, UR);
      }
    }

    // Now that this is ready for actual removal, put it into our list.
    DeadTargets.push_back(&E.getNode());
  }
  // Remove the easy cases quickly and actually pull them out of our list.
  DeadTargets.erase(
      llvm::remove_if(DeadTargets,
                      [&](Node *TargetN) {
                        SCC &TargetC = *G.lookupSCC(*TargetN);
                        RefSCC &TargetRC = TargetC.getOuterRefSCC();

                        // We can't trivially remove internal targets, so skip
                        // those.
                        if (&TargetRC == RC)
                          return false;

                        RC->removeOutgoingEdge(N, *TargetN);
                        LLVM_DEBUG(dbgs() << "Deleting outgoing edge from '"
                                          << N << "' to '" << TargetN << "'\n");
                        return true;
                      }),
      DeadTargets.end());

  // Now do a batch removal of the internal ref edges left.
  auto NewRefSCCs = RC->removeInternalRefEdge(N, DeadTargets);
  if (!NewRefSCCs.empty()) {
    // The old RefSCC is dead, mark it as such.
    UR.InvalidatedRefSCCs.insert(RC);

    // Note that we don't bother to invalidate analyses as ref-edge
    // connectivity is not really observable in any way and is intended
    // exclusively to be used for ordering of transforms rather than for
    // analysis conclusions.

    // Update RC to the "bottom".
    assert(G.lookupSCC(N) == C && "Changed the SCC when splitting RefSCCs!");
    RC = &C->getOuterRefSCC();
    assert(G.lookupRefSCC(N) == RC && "Failed to update current RefSCC!");

    // The RC worklist is in reverse postorder, so we enqueue the new ones in
    // RPO except for the one which contains the source node as that is the
    // "bottom" we will continue processing in the bottom-up walk.
    assert(NewRefSCCs.front() == RC &&
           "New current RefSCC not first in the returned list!");
    for (RefSCC *NewRC : llvm::reverse(make_range(std::next(NewRefSCCs.begin()),
                                                  NewRefSCCs.end()))) {
      assert(NewRC != RC && "Should not encounter the current RefSCC further "
                            "in the postorder list of new RefSCCs.");
      UR.RCWorklist.insert(NewRC);
      LLVM_DEBUG(dbgs() << "Enqueuing a new RefSCC in the update worklist: "
                        << *NewRC << "\n");
    }
  }

  // Next demote all the call edges that are now ref edges. This helps make
  // the SCCs small which should minimize the work below as we don't want to
  // form cycles that this would break.
  for (Node *RefTarget : DemotedCallTargets) {
    SCC &TargetC = *G.lookupSCC(*RefTarget);
    RefSCC &TargetRC = TargetC.getOuterRefSCC();

    // The easy case is when the target RefSCC is not this RefSCC. This is
    // only supported when the target RefSCC is a child of this RefSCC.
    if (&TargetRC != RC) {
      assert(RC->isAncestorOf(TargetRC) &&
             "Cannot potentially form RefSCC cycles here!");
      RC->switchOutgoingEdgeToRef(N, *RefTarget);
      LLVM_DEBUG(dbgs() << "Switch outgoing call edge to a ref edge from '" << N
                        << "' to '" << *RefTarget << "'\n");
      continue;
    }

    // We are switching an internal call edge to a ref edge. This may split up
    // some SCCs.
    if (C != &TargetC) {
      // For separate SCCs this is trivial.
      RC->switchTrivialInternalEdgeToRef(N, *RefTarget);
      continue;
    }

    // Now update the call graph.
    C = incorporateNewSCCRange(RC->switchInternalEdgeToRef(N, *RefTarget), G, N,
                               C, AM, UR);
  }

  // Now promote ref edges into call edges.
  for (Node *CallTarget : PromotedRefTargets) {
    SCC &TargetC = *G.lookupSCC(*CallTarget);
    RefSCC &TargetRC = TargetC.getOuterRefSCC();

    // The easy case is when the target RefSCC is not this RefSCC. This is
    // only supported when the target RefSCC is a child of this RefSCC.
    if (&TargetRC != RC) {
      assert(RC->isAncestorOf(TargetRC) &&
             "Cannot potentially form RefSCC cycles here!");
      RC->switchOutgoingEdgeToCall(N, *CallTarget);
      LLVM_DEBUG(dbgs() << "Switch outgoing ref edge to a call edge from '" << N
                        << "' to '" << *CallTarget << "'\n");
      continue;
    }
    LLVM_DEBUG(dbgs() << "Switch an internal ref edge to a call edge from '"
                      << N << "' to '" << *CallTarget << "'\n");

    // Otherwise we are switching an internal ref edge to a call edge. This
    // may merge away some SCCs, and we add those to the UpdateResult. We also
    // need to make sure to update the worklist in the event SCCs have moved
    // before the current one in the post-order sequence
    bool HasFunctionAnalysisProxy = false;
    auto InitialSCCIndex = RC->find(*C) - RC->begin();
    bool FormedCycle = RC->switchInternalEdgeToCall(
        N, *CallTarget, [&](ArrayRef<SCC *> MergedSCCs) {
          for (SCC *MergedC : MergedSCCs) {
            assert(MergedC != &TargetC && "Cannot merge away the target SCC!");

            HasFunctionAnalysisProxy |=
                AM.getCachedResult<FunctionAnalysisManagerCGSCCProxy>(
                    *MergedC) != nullptr;

            // Mark that this SCC will no longer be valid.
            UR.InvalidatedSCCs.insert(MergedC);

            // FIXME: We should really do a 'clear' here to forcibly release
            // memory, but we don't have a good way of doing that and
            // preserving the function analyses.
            auto PA = PreservedAnalyses::allInSet<AllAnalysesOn<Function>>();
            PA.preserve<FunctionAnalysisManagerCGSCCProxy>();
            AM.invalidate(*MergedC, PA);
          }
        });

    // If we formed a cycle by creating this call, we need to update more data
    // structures.
    if (FormedCycle) {
      C = &TargetC;
      assert(G.lookupSCC(N) == C && "Failed to update current SCC!");

      // If one of the invalidated SCCs had a cached proxy to a function
      // analysis manager, we need to create a proxy in the new current SCC as
      // the invalidated SCCs had their functions moved.
      if (HasFunctionAnalysisProxy)
        AM.getResult<FunctionAnalysisManagerCGSCCProxy>(*C, G);

      // Any analyses cached for this SCC are no longer precise as the shape
      // has changed by introducing this cycle. However, we have taken care to
      // update the proxies so it remains valide.
      auto PA = PreservedAnalyses::allInSet<AllAnalysesOn<Function>>();
      PA.preserve<FunctionAnalysisManagerCGSCCProxy>();
      AM.invalidate(*C, PA);
    }
    auto NewSCCIndex = RC->find(*C) - RC->begin();
    // If we have actually moved an SCC to be topologically "below" the current
    // one due to merging, we will need to revisit the current SCC after
    // visiting those moved SCCs.
    //
    // It is critical that we *do not* revisit the current SCC unless we
    // actually move SCCs in the process of merging because otherwise we may
    // form a cycle where an SCC is split apart, merged, split, merged and so
    // on infinitely.
    if (InitialSCCIndex < NewSCCIndex) {
      // Put our current SCC back onto the worklist as we'll visit other SCCs
      // that are now definitively ordered prior to the current one in the
      // post-order sequence, and may end up observing more precise context to
      // optimize the current SCC.
      UR.CWorklist.insert(C);
      LLVM_DEBUG(dbgs() << "Enqueuing the existing SCC in the worklist: " << *C
                        << "\n");
      // Enqueue in reverse order as we pop off the back of the worklist.
      for (SCC &MovedC : llvm::reverse(make_range(RC->begin() + InitialSCCIndex,
                                                  RC->begin() + NewSCCIndex))) {
        UR.CWorklist.insert(&MovedC);
        LLVM_DEBUG(dbgs() << "Enqueuing a newly earlier in post-order SCC: "
                          << MovedC << "\n");
      }
    }
  }

  assert(!UR.InvalidatedSCCs.count(C) && "Invalidated the current SCC!");
  assert(!UR.InvalidatedRefSCCs.count(RC) && "Invalidated the current RefSCC!");
  assert(&C->getOuterRefSCC() == RC && "Current SCC not in current RefSCC!");

  // Record the current RefSCC and SCC for higher layers of the CGSCC pass
  // manager now that all the updates have been applied.
  if (RC != &InitialRC)
    UR.UpdatedRC = RC;
  if (C != &InitialC)
    UR.UpdatedC = C;

  return *C;
}