summaryrefslogtreecommitdiff
path: root/tools/llvm-diff/DifferenceEngine.cpp
blob: 9cf1f1271501b2c2aababcb5e9d8f97aa6f30728 (plain)
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
//===-- DifferenceEngine.cpp - Structural function/module comparison ------===//
//
//                     The LLVM Compiler Infrastructure
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// This header defines the implementation of the LLVM difference
// engine, which structurally compares global values within a module.
//
//===----------------------------------------------------------------------===//

#include "DifferenceEngine.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/ADT/DenseSet.h"
#include "llvm/ADT/SmallVector.h"
#include "llvm/ADT/StringRef.h"
#include "llvm/ADT/StringSet.h"
#include "llvm/Constants.h"
#include "llvm/Function.h"
#include "llvm/Instructions.h"
#include "llvm/Module.h"
#include "llvm/Support/CFG.h"
#include "llvm/Support/CallSite.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/raw_ostream.h"
#include "llvm/Support/type_traits.h"
#include <utility>

using namespace llvm;

namespace {

/// A priority queue, implemented as a heap.
template <class T, class Sorter, unsigned InlineCapacity>
class PriorityQueue {
  Sorter Precedes;
  llvm::SmallVector<T, InlineCapacity> Storage;

public:
  PriorityQueue(const Sorter &Precedes) : Precedes(Precedes) {}

  /// Checks whether the heap is empty.
  bool empty() const { return Storage.empty(); }

  /// Insert a new value on the heap.
  void insert(const T &V) {
    unsigned Index = Storage.size();
    Storage.push_back(V);
    if (Index == 0) return;

    T *data = Storage.data();
    while (true) {
      unsigned Target = (Index + 1) / 2 - 1;
      if (!Precedes(data[Index], data[Target])) return;
      std::swap(data[Index], data[Target]);
      if (Target == 0) return;
      Index = Target;
    }
  }

  /// Remove the minimum value in the heap.  Only valid on a non-empty heap.
  T remove_min() {
    assert(!empty());
    T tmp = Storage[0];
    
    unsigned NewSize = Storage.size() - 1;
    if (NewSize) {
      // Move the slot at the end to the beginning.
      if (isPodLike<T>::value)
        Storage[0] = Storage[NewSize];
      else
        std::swap(Storage[0], Storage[NewSize]);

      // Bubble the root up as necessary.
      unsigned Index = 0;
      while (true) {
        // With a 1-based index, the children would be Index*2 and Index*2+1.
        unsigned R = (Index + 1) * 2;
        unsigned L = R - 1;

        // If R is out of bounds, we're done after this in any case.
        if (R >= NewSize) {
          // If L is also out of bounds, we're done immediately.
          if (L >= NewSize) break;

          // Otherwise, test whether we should swap L and Index.
          if (Precedes(Storage[L], Storage[Index]))
            std::swap(Storage[L], Storage[Index]);
          break;
        }

        // Otherwise, we need to compare with the smaller of L and R.
        // Prefer R because it's closer to the end of the array.
        unsigned IndexToTest = (Precedes(Storage[L], Storage[R]) ? L : R);

        // If Index is >= the min of L and R, then heap ordering is restored.
        if (!Precedes(Storage[IndexToTest], Storage[Index]))
          break;

        // Otherwise, keep bubbling up.
        std::swap(Storage[IndexToTest], Storage[Index]);
        Index = IndexToTest;
      }
    }
    Storage.pop_back();

    return tmp;
  }
};

/// A function-scope difference engine.
class FunctionDifferenceEngine {
  DifferenceEngine &Engine;

  /// The current mapping from old local values to new local values.
  DenseMap<Value*, Value*> Values;

  /// The current mapping from old blocks to new blocks.
  DenseMap<BasicBlock*, BasicBlock*> Blocks;

  DenseSet<std::pair<Value*, Value*> > TentativeValues;

  unsigned getUnprocPredCount(BasicBlock *Block) const {
    unsigned Count = 0;
    for (pred_iterator I = pred_begin(Block), E = pred_end(Block); I != E; ++I)
      if (!Blocks.count(*I)) Count++;
    return Count;
  }

  typedef std::pair<BasicBlock*, BasicBlock*> BlockPair;

  /// A type which sorts a priority queue by the number of unprocessed
  /// predecessor blocks it has remaining.
  ///
  /// This is actually really expensive to calculate.
  struct QueueSorter {
    const FunctionDifferenceEngine &fde;
    explicit QueueSorter(const FunctionDifferenceEngine &fde) : fde(fde) {}

    bool operator()(const BlockPair &Old, const BlockPair &New) {
      return fde.getUnprocPredCount(Old.first)
           < fde.getUnprocPredCount(New.first);
    }
  };

  /// A queue of unified blocks to process.
  PriorityQueue<BlockPair, QueueSorter, 20> Queue;

  /// Try to unify the given two blocks.  Enqueues them for processing
  /// if they haven't already been processed.
  ///
  /// Returns true if there was a problem unifying them.
  bool tryUnify(BasicBlock *L, BasicBlock *R) {
    BasicBlock *&Ref = Blocks[L];

    if (Ref) {
      if (Ref == R) return false;

      Engine.logf("successor %l cannot be equivalent to %r; "
                  "it's already equivalent to %r")
        << L << R << Ref;
      return true;
    }

    Ref = R;
    Queue.insert(BlockPair(L, R));
    return false;
  }
  
  /// Unifies two instructions, given that they're known not to have
  /// structural differences.
  void unify(Instruction *L, Instruction *R) {
    DifferenceEngine::Context C(Engine, L, R);

    bool Result = diff(L, R, true, true);
    assert(!Result && "structural differences second time around?");
    (void) Result;
    if (!L->use_empty())
      Values[L] = R;
  }

  void processQueue() {
    while (!Queue.empty()) {
      BlockPair Pair = Queue.remove_min();
      diff(Pair.first, Pair.second);
    }
  }

  void diff(BasicBlock *L, BasicBlock *R) {
    DifferenceEngine::Context C(Engine, L, R);

    BasicBlock::iterator LI = L->begin(), LE = L->end();
    BasicBlock::iterator RI = R->begin();

    llvm::SmallVector<std::pair<Instruction*,Instruction*>, 20> TentativePairs;

    do {
      assert(LI != LE && RI != R->end());
      Instruction *LeftI = &*LI, *RightI = &*RI;

      // If the instructions differ, start the more sophisticated diff
      // algorithm at the start of the block.
      if (diff(LeftI, RightI, false, false)) {
        TentativeValues.clear();
        return runBlockDiff(L->begin(), R->begin());
      }

      // Otherwise, tentatively unify them.
      if (!LeftI->use_empty())
        TentativeValues.insert(std::make_pair(LeftI, RightI));

      ++LI, ++RI;
    } while (LI != LE); // This is sufficient: we can't get equality of
                        // terminators if there are residual instructions.

    // Unify everything in the block, non-tentatively this time.
    TentativeValues.clear();
    for (LI = L->begin(), RI = R->begin(); LI != LE; ++LI, ++RI)
      unify(&*LI, &*RI);
  }

  bool matchForBlockDiff(Instruction *L, Instruction *R);
  void runBlockDiff(BasicBlock::iterator LI, BasicBlock::iterator RI);

  bool diffCallSites(CallSite L, CallSite R, bool Complain) {
    // FIXME: call attributes
    if (!equivalentAsOperands(L.getCalledValue(), R.getCalledValue())) {
      if (Complain) Engine.log("called functions differ");
      return true;
    }
    if (L.arg_size() != R.arg_size()) {
      if (Complain) Engine.log("argument counts differ");
      return true;
    }
    for (unsigned I = 0, E = L.arg_size(); I != E; ++I)
      if (!equivalentAsOperands(L.getArgument(I), R.getArgument(I))) {
        if (Complain)
          Engine.logf("arguments %l and %r differ")
            << L.getArgument(I) << R.getArgument(I);
        return true;
      }
    return false;
  }

  bool diff(Instruction *L, Instruction *R, bool Complain, bool TryUnify) {
    // FIXME: metadata (if Complain is set)

    // Different opcodes always imply different operations.
    if (L->getOpcode() != R->getOpcode()) {
      if (Complain) Engine.log("different instruction types");
      return true;
    }

    if (isa<CmpInst>(L)) {
      if (cast<CmpInst>(L)->getPredicate()
            != cast<CmpInst>(R)->getPredicate()) {
        if (Complain) Engine.log("different predicates");
        return true;
      }
    } else if (isa<CallInst>(L)) {
      return diffCallSites(CallSite(L), CallSite(R), Complain);
    } else if (isa<PHINode>(L)) {
      // FIXME: implement.

      // This is really weird;  type uniquing is broken?
      if (L->getType() != R->getType()) {
        if (!L->getType()->isPointerTy() || !R->getType()->isPointerTy()) {
          if (Complain) Engine.log("different phi types");
          return true;
        }
      }
      return false;

    // Terminators.
    } else if (isa<InvokeInst>(L)) {
      InvokeInst *LI = cast<InvokeInst>(L);
      InvokeInst *RI = cast<InvokeInst>(R);
      if (diffCallSites(CallSite(LI), CallSite(RI), Complain))
        return true;

      if (TryUnify) {
        tryUnify(LI->getNormalDest(), RI->getNormalDest());
        tryUnify(LI->getUnwindDest(), RI->getUnwindDest());
      }
      return false;

    } else if (isa<BranchInst>(L)) {
      BranchInst *LI = cast<BranchInst>(L);
      BranchInst *RI = cast<BranchInst>(R);
      if (LI->isConditional() != RI->isConditional()) {
        if (Complain) Engine.log("branch conditionality differs");
        return true;
      }

      if (LI->isConditional()) {
        if (!equivalentAsOperands(LI->getCondition(), RI->getCondition())) {
          if (Complain) Engine.log("branch conditions differ");
          return true;
        }
        if (TryUnify) tryUnify(LI->getSuccessor(1), RI->getSuccessor(1));
      }
      if (TryUnify) tryUnify(LI->getSuccessor(0), RI->getSuccessor(0));
      return false;

    } else if (isa<SwitchInst>(L)) {
      SwitchInst *LI = cast<SwitchInst>(L);
      SwitchInst *RI = cast<SwitchInst>(R);
      if (!equivalentAsOperands(LI->getCondition(), RI->getCondition())) {
        if (Complain) Engine.log("switch conditions differ");
        return true;
      }
      if (TryUnify) tryUnify(LI->getDefaultDest(), RI->getDefaultDest());

      bool Difference = false;

      DenseMap<Constant*, BasicBlock*> LCases;
      
      for (SwitchInst::CaseIt I = LI->case_begin(), E = LI->case_end();
           I != E; ++I)
        LCases[I.getCaseValueEx()] = I.getCaseSuccessor();
        
      for (SwitchInst::CaseIt I = RI->case_begin(), E = RI->case_end();
           I != E; ++I) {
        IntegersSubset CaseValue = I.getCaseValueEx();
        BasicBlock *LCase = LCases[CaseValue];
        if (LCase) {
          if (TryUnify) tryUnify(LCase, I.getCaseSuccessor());
          LCases.erase(CaseValue);
        } else if (Complain || !Difference) {
          if (Complain)
            Engine.logf("right switch has extra case %r") << CaseValue;
          Difference = true;
        }
      }
      if (!Difference)
        for (DenseMap<Constant*, BasicBlock*>::iterator
               I = LCases.begin(), E = LCases.end(); I != E; ++I) {
          if (Complain)
            Engine.logf("left switch has extra case %l") << I->first;
          Difference = true;
        }
      return Difference;
    } else if (isa<UnreachableInst>(L)) {
      return false;
    }

    if (L->getNumOperands() != R->getNumOperands()) {
      if (Complain) Engine.log("instructions have different operand counts");
      return true;
    }

    for (unsigned I = 0, E = L->getNumOperands(); I != E; ++I) {
      Value *LO = L->getOperand(I), *RO = R->getOperand(I);
      if (!equivalentAsOperands(LO, RO)) {
        if (Complain) Engine.logf("operands %l and %r differ") << LO << RO;
        return true;
      }
    }

    return false;
  }

  bool equivalentAsOperands(Constant *L, Constant *R) {
    // Use equality as a preliminary filter.
    if (L == R)
      return true;

    if (L->getValueID() != R->getValueID())
      return false;
    
    // Ask the engine about global values.
    if (isa<GlobalValue>(L))
      return Engine.equivalentAsOperands(cast<GlobalValue>(L),
                                         cast<GlobalValue>(R));

    // Compare constant expressions structurally.
    if (isa<ConstantExpr>(L))
      return equivalentAsOperands(cast<ConstantExpr>(L),
                                  cast<ConstantExpr>(R));

    // Nulls of the "same type" don't always actually have the same
    // type; I don't know why.  Just white-list them.
    if (isa<ConstantPointerNull>(L))
      return true;

    // Block addresses only match if we've already encountered the
    // block.  FIXME: tentative matches?
    if (isa<BlockAddress>(L))
      return Blocks[cast<BlockAddress>(L)->getBasicBlock()]
                 == cast<BlockAddress>(R)->getBasicBlock();

    return false;
  }

  bool equivalentAsOperands(ConstantExpr *L, ConstantExpr *R) {
    if (L == R)
      return true;
    if (L->getOpcode() != R->getOpcode())
      return false;

    switch (L->getOpcode()) {
    case Instruction::ICmp:
    case Instruction::FCmp:
      if (L->getPredicate() != R->getPredicate())
        return false;
      break;

    case Instruction::GetElementPtr:
      // FIXME: inbounds?
      break;

    default:
      break;
    }

    if (L->getNumOperands() != R->getNumOperands())
      return false;

    for (unsigned I = 0, E = L->getNumOperands(); I != E; ++I)
      if (!equivalentAsOperands(L->getOperand(I), R->getOperand(I)))
        return false;

    return true;
  }

  bool equivalentAsOperands(Value *L, Value *R) {
    // Fall out if the values have different kind.
    // This possibly shouldn't take priority over oracles.
    if (L->getValueID() != R->getValueID())
      return false;

    // Value subtypes:  Argument, Constant, Instruction, BasicBlock,
    //                  InlineAsm, MDNode, MDString, PseudoSourceValue

    if (isa<Constant>(L))
      return equivalentAsOperands(cast<Constant>(L), cast<Constant>(R));

    if (isa<Instruction>(L))
      return Values[L] == R || TentativeValues.count(std::make_pair(L, R));

    if (isa<Argument>(L))
      return Values[L] == R;

    if (isa<BasicBlock>(L))
      return Blocks[cast<BasicBlock>(L)] != R;

    // Pretend everything else is identical.
    return true;
  }

  // Avoid a gcc warning about accessing 'this' in an initializer.
  FunctionDifferenceEngine *this_() { return this; }

public:
  FunctionDifferenceEngine(DifferenceEngine &Engine) :
    Engine(Engine), Queue(QueueSorter(*this_())) {}

  void diff(Function *L, Function *R) {
    if (L->arg_size() != R->arg_size())
      Engine.log("different argument counts");

    // Map the arguments.
    for (Function::arg_iterator
           LI = L->arg_begin(), LE = L->arg_end(),
           RI = R->arg_begin(), RE = R->arg_end();
         LI != LE && RI != RE; ++LI, ++RI)
      Values[&*LI] = &*RI;

    tryUnify(&*L->begin(), &*R->begin());
    processQueue();
  }
};

struct DiffEntry {
  DiffEntry() : Cost(0) {}

  unsigned Cost;
  llvm::SmallVector<char, 8> Path; // actually of DifferenceEngine::DiffChange
};

bool FunctionDifferenceEngine::matchForBlockDiff(Instruction *L,
                                                 Instruction *R) {
  return !diff(L, R, false, false);
}

void FunctionDifferenceEngine::runBlockDiff(BasicBlock::iterator LStart,
                                            BasicBlock::iterator RStart) {
  BasicBlock::iterator LE = LStart->getParent()->end();
  BasicBlock::iterator RE = RStart->getParent()->end();

  unsigned NL = std::distance(LStart, LE);

  SmallVector<DiffEntry, 20> Paths1(NL+1);
  SmallVector<DiffEntry, 20> Paths2(NL+1);

  DiffEntry *Cur = Paths1.data();
  DiffEntry *Next = Paths2.data();

  const unsigned LeftCost = 2;
  const unsigned RightCost = 2;
  const unsigned MatchCost = 0;

  assert(TentativeValues.empty());

  // Initialize the first column.
  for (unsigned I = 0; I != NL+1; ++I) {
    Cur[I].Cost = I * LeftCost;
    for (unsigned J = 0; J != I; ++J)
      Cur[I].Path.push_back(DC_left);
  }

  for (BasicBlock::iterator RI = RStart; RI != RE; ++RI) {
    // Initialize the first row.
    Next[0] = Cur[0];
    Next[0].Cost += RightCost;
    Next[0].Path.push_back(DC_right);

    unsigned Index = 1;
    for (BasicBlock::iterator LI = LStart; LI != LE; ++LI, ++Index) {
      if (matchForBlockDiff(&*LI, &*RI)) {
        Next[Index] = Cur[Index-1];
        Next[Index].Cost += MatchCost;
        Next[Index].Path.push_back(DC_match);
        TentativeValues.insert(std::make_pair(&*LI, &*RI));
      } else if (Next[Index-1].Cost <= Cur[Index].Cost) {
        Next[Index] = Next[Index-1];
        Next[Index].Cost += LeftCost;
        Next[Index].Path.push_back(DC_left);
      } else {
        Next[Index] = Cur[Index];
        Next[Index].Cost += RightCost;
        Next[Index].Path.push_back(DC_right);
      }
    }

    std::swap(Cur, Next);
  }

  // We don't need the tentative values anymore; everything from here
  // on out should be non-tentative.
  TentativeValues.clear();

  SmallVectorImpl<char> &Path = Cur[NL].Path;
  BasicBlock::iterator LI = LStart, RI = RStart;

  DiffLogBuilder Diff(Engine.getConsumer());

  // Drop trailing matches.
  while (Path.back() == DC_match)
    Path.pop_back();

  // Skip leading matches.
  SmallVectorImpl<char>::iterator
    PI = Path.begin(), PE = Path.end();
  while (PI != PE && *PI == DC_match) {
    unify(&*LI, &*RI);
    ++PI, ++LI, ++RI;
  }

  for (; PI != PE; ++PI) {
    switch (static_cast<DiffChange>(*PI)) {
    case DC_match:
      assert(LI != LE && RI != RE);
      {
        Instruction *L = &*LI, *R = &*RI;
        unify(L, R);
        Diff.addMatch(L, R);
      }
      ++LI; ++RI;
      break;

    case DC_left:
      assert(LI != LE);
      Diff.addLeft(&*LI);
      ++LI;
      break;

    case DC_right:
      assert(RI != RE);
      Diff.addRight(&*RI);
      ++RI;
      break;
    }
  }

  // Finishing unifying and complaining about the tails of the block,
  // which should be matches all the way through.
  while (LI != LE) {
    assert(RI != RE);
    unify(&*LI, &*RI);
    ++LI, ++RI;
  }

  // If the terminators have different kinds, but one is an invoke and the
  // other is an unconditional branch immediately following a call, unify
  // the results and the destinations.
  TerminatorInst *LTerm = LStart->getParent()->getTerminator();
  TerminatorInst *RTerm = RStart->getParent()->getTerminator();
  if (isa<BranchInst>(LTerm) && isa<InvokeInst>(RTerm)) {
    if (cast<BranchInst>(LTerm)->isConditional()) return;
    BasicBlock::iterator I = LTerm;
    if (I == LStart->getParent()->begin()) return;
    --I;
    if (!isa<CallInst>(*I)) return;
    CallInst *LCall = cast<CallInst>(&*I);
    InvokeInst *RInvoke = cast<InvokeInst>(RTerm);
    if (!equivalentAsOperands(LCall->getCalledValue(), RInvoke->getCalledValue()))
      return;
    if (!LCall->use_empty())
      Values[LCall] = RInvoke;
    tryUnify(LTerm->getSuccessor(0), RInvoke->getNormalDest());
  } else if (isa<InvokeInst>(LTerm) && isa<BranchInst>(RTerm)) {
    if (cast<BranchInst>(RTerm)->isConditional()) return;
    BasicBlock::iterator I = RTerm;
    if (I == RStart->getParent()->begin()) return;
    --I;
    if (!isa<CallInst>(*I)) return;
    CallInst *RCall = cast<CallInst>(I);
    InvokeInst *LInvoke = cast<InvokeInst>(LTerm);
    if (!equivalentAsOperands(LInvoke->getCalledValue(), RCall->getCalledValue()))
      return;
    if (!LInvoke->use_empty())
      Values[LInvoke] = RCall;
    tryUnify(LInvoke->getNormalDest(), RTerm->getSuccessor(0));
  }
}

}

void DifferenceEngine::Oracle::anchor() { }

void DifferenceEngine::diff(Function *L, Function *R) {
  Context C(*this, L, R);

  // FIXME: types
  // FIXME: attributes and CC
  // FIXME: parameter attributes
  
  // If both are declarations, we're done.
  if (L->empty() && R->empty())
    return;
  else if (L->empty())
    log("left function is declaration, right function is definition");
  else if (R->empty())
    log("right function is declaration, left function is definition");
  else
    FunctionDifferenceEngine(*this).diff(L, R);
}

void DifferenceEngine::diff(Module *L, Module *R) {
  StringSet<> LNames;
  SmallVector<std::pair<Function*,Function*>, 20> Queue;

  for (Module::iterator I = L->begin(), E = L->end(); I != E; ++I) {
    Function *LFn = &*I;
    LNames.insert(LFn->getName());

    if (Function *RFn = R->getFunction(LFn->getName()))
      Queue.push_back(std::make_pair(LFn, RFn));
    else
      logf("function %l exists only in left module") << LFn;
  }

  for (Module::iterator I = R->begin(), E = R->end(); I != E; ++I) {
    Function *RFn = &*I;
    if (!LNames.count(RFn->getName()))
      logf("function %r exists only in right module") << RFn;
  }

  for (SmallVectorImpl<std::pair<Function*,Function*> >::iterator
         I = Queue.begin(), E = Queue.end(); I != E; ++I)
    diff(I->first, I->second);
}

bool DifferenceEngine::equivalentAsOperands(GlobalValue *L, GlobalValue *R) {
  if (globalValueOracle) return (*globalValueOracle)(L, R);
  return L->getName() == R->getName();
}