tools/llvm-diff/DifferenceEngine.cpp

   1 //===-- DifferenceEngine.cpp - Structural function/module comparison ------===//
   2 //
   3 //                     The LLVM Compiler Infrastructure
   4 //
   5 // This file is distributed under the University of Illinois Open Source
   6 // License. See LICENSE.TXT for details.
   7 //
   8 //===----------------------------------------------------------------------===//
   9 //
  10 // This header defines the implementation of the LLVM difference
  11 // engine, which structurally compares global values within a module.
  12 //
  13 //===----------------------------------------------------------------------===//
  14
  15 #include "DifferenceEngine.h"
  16
  17 #include "llvm/Constants.h"
  18 #include "llvm/Function.h"
  19 #include "llvm/Instructions.h"
  20 #include "llvm/Module.h"
  21 #include "llvm/ADT/DenseMap.h"
  22 #include "llvm/ADT/DenseSet.h"
  23 #include "llvm/ADT/SmallVector.h"
  24 #include "llvm/ADT/StringRef.h"
  25 #include "llvm/ADT/StringSet.h"
  26 #include "llvm/Support/CallSite.h"
  27 #include "llvm/Support/CFG.h"
  28 #include "llvm/Support/ErrorHandling.h"
  29 #include "llvm/Support/raw_ostream.h"
  30 #include "llvm/Support/type_traits.h"
  31
  32 #include <utility>
  33
  34 using namespace llvm;
  35
  36 namespace {
  37
  38 /// A priority queue, implemented as a heap.
  39 template <class T, class Sorter, unsigned InlineCapacity>
  40 class PriorityQueue {
  41   Sorter Precedes;
  42   llvm::SmallVector<T, InlineCapacity> Storage;
  43
  44 public:
  45   PriorityQueue(const Sorter &Precedes) : Precedes(Precedes) {}
  46
  47   /// Checks whether the heap is empty.
  48   bool empty() const { return Storage.empty(); }
  49
  50   /// Insert a new value on the heap.
  51   void insert(const T &V) {
  52     unsigned Index = Storage.size();
  53     Storage.push_back(V);
  54     if (Index == 0) return;
  55
  56     T *data = Storage.data();
  57     while (true) {
  58       unsigned Target = (Index + 1) / 2 - 1;
  59       if (!Precedes(data[Index], data[Target])) return;
  60       std::swap(data[Index], data[Target]);
  61       if (Target == 0) return;
  62       Index = Target;
  63     }
  64   }
  65
  66   /// Remove the minimum value in the heap.  Only valid on a non-empty heap.
  67   T remove_min() {
  68     assert(!empty());
  69     T tmp = Storage[0];
  70
  71     unsigned NewSize = Storage.size() - 1;
  72     if (NewSize) {
  73       // Move the slot at the end to the beginning.
  74       if (isPodLike<T>::value)
  75         Storage[0] = Storage[NewSize];
  76       else
  77         std::swap(Storage[0], Storage[NewSize]);
  78
  79       // Bubble the root up as necessary.
  80       unsigned Index = 0;
  81       while (true) {
  82         // With a 1-based index, the children would be Index*2 and Index*2+1.
  83         unsigned R = (Index + 1) * 2;
  84         unsigned L = R - 1;
  85
  86         // If R is out of bounds, we're done after this in any case.
  87         if (R >= NewSize) {
  88           // If L is also out of bounds, we're done immediately.
  89           if (L >= NewSize) break;
  90
  91           // Otherwise, test whether we should swap L and Index.
  92           if (Precedes(Storage[L], Storage[Index]))
  93             std::swap(Storage[L], Storage[Index]);
  94           break;
  95         }
  96
  97         // Otherwise, we need to compare with the smaller of L and R.
  98         // Prefer R because it's closer to the end of the array.
  99         unsigned IndexToTest = (Precedes(Storage[L], Storage[R]) ? L : R);
 100
 101         // If Index is >= the min of L and R, then heap ordering is restored.
 102         if (!Precedes(Storage[IndexToTest], Storage[Index]))
 103           break;
 104
 105         // Otherwise, keep bubbling up.
 106         std::swap(Storage[IndexToTest], Storage[Index]);
 107         Index = IndexToTest;
 108       }
 109     }
 110     Storage.pop_back();
 111
 112     return tmp;
 113   }
 114 };
 115
 116 /// A function-scope difference engine.
 117 class FunctionDifferenceEngine {
 118   DifferenceEngine &Engine;
 119
 120   /// The current mapping from old local values to new local values.
 121   DenseMap<Value*, Value*> Values;
 122
 123   /// The current mapping from old blocks to new blocks.
 124   DenseMap<BasicBlock*, BasicBlock*> Blocks;
 125
 126   DenseSet<std::pair<Value*, Value*> > TentativeValues;
 127
 128   unsigned getUnprocPredCount(BasicBlock *Block) const {
 129     unsigned Count = 0;
 130     for (pred_iterator I = pred_begin(Block), E = pred_end(Block); I != E; ++I)
 131       if (!Blocks.count(*I)) Count++;
 132     return Count;
 133   }
 134
 135   typedef std::pair<BasicBlock*, BasicBlock*> BlockPair;
 136
 137   /// A type which sorts a priority queue by the number of unprocessed
 138   /// predecessor blocks it has remaining.
 139   ///
 140   /// This is actually really expensive to calculate.
 141   struct QueueSorter {
 142     const FunctionDifferenceEngine &fde;
 143     explicit QueueSorter(const FunctionDifferenceEngine &fde) : fde(fde) {}
 144
 145     bool operator()(const BlockPair &Old, const BlockPair &New) {
 146       return fde.getUnprocPredCount(Old.first)
 147            < fde.getUnprocPredCount(New.first);
 148     }
 149   };
 150
 151   /// A queue of unified blocks to process.
 152   PriorityQueue<BlockPair, QueueSorter, 20> Queue;
 153
 154   /// Try to unify the given two blocks.  Enqueues them for processing
 155   /// if they haven't already been processed.
 156   ///
 157   /// Returns true if there was a problem unifying them.
 158   bool tryUnify(BasicBlock *L, BasicBlock *R) {
 159     BasicBlock *&Ref = Blocks[L];
 160
 161     if (Ref) {
 162       if (Ref == R) return false;
 163
 164       Engine.logf("successor %l cannot be equivalent to %r; "
 165                   "it's already equivalent to %r")
 166         << L << R << Ref;
 167       return true;
 168     }
 169
 170     Ref = R;
 171     Queue.insert(BlockPair(L, R));
 172     return false;
 173   }
 174
 175   /// Unifies two instructions, given that they're known not to have
 176   /// structural differences.
 177   void unify(Instruction *L, Instruction *R) {
 178     DifferenceEngine::Context C(Engine, L, R);
 179
 180     bool Result = diff(L, R, true, true);
 181     assert(!Result && "structural differences second time around?");
 182     (void) Result;
 183     if (!L->use_empty())
 184       Values[L] = R;
 185   }
 186
 187   void processQueue() {
 188     while (!Queue.empty()) {
 189       BlockPair Pair = Queue.remove_min();
 190       diff(Pair.first, Pair.second);
 191     }
 192   }
 193
 194   void diff(BasicBlock *L, BasicBlock *R) {
 195     DifferenceEngine::Context C(Engine, L, R);
 196
 197     BasicBlock::iterator LI = L->begin(), LE = L->end();
 198     BasicBlock::iterator RI = R->begin(), RE = R->end();
 199
 200     llvm::SmallVector<std::pair<Instruction*,Instruction*>, 20> TentativePairs;
 201
 202     do {
 203       assert(LI != LE && RI != RE);
 204       Instruction *LeftI = &*LI, *RightI = &*RI;
 205
 206       // If the instructions differ, start the more sophisticated diff
 207       // algorithm at the start of the block.
 208       if (diff(LeftI, RightI, false, false)) {
 209         TentativeValues.clear();
 210         return runBlockDiff(L->begin(), R->begin());
 211       }
 212
 213       // Otherwise, tentatively unify them.
 214       if (!LeftI->use_empty())
 215         TentativeValues.insert(std::make_pair(LeftI, RightI));
 216
 217       ++LI, ++RI;
 218     } while (LI != LE); // This is sufficient: we can't get equality of
 219                         // terminators if there are residual instructions.
 220
 221     // Unify everything in the block, non-tentatively this time.
 222     TentativeValues.clear();
 223     for (LI = L->begin(), RI = R->begin(); LI != LE; ++LI, ++RI)
 224       unify(&*LI, &*RI);
 225   }
 226
 227   bool matchForBlockDiff(Instruction *L, Instruction *R);
 228   void runBlockDiff(BasicBlock::iterator LI, BasicBlock::iterator RI);
 229
 230   bool diffCallSites(CallSite L, CallSite R, bool Complain) {
 231     // FIXME: call attributes
 232     if (!equivalentAsOperands(L.getCalledValue(), R.getCalledValue())) {
 233       if (Complain) Engine.log("called functions differ");
 234       return true;
 235     }
 236     if (L.arg_size() != R.arg_size()) {
 237       if (Complain) Engine.log("argument counts differ");
 238       return true;
 239     }
 240     for (unsigned I = 0, E = L.arg_size(); I != E; ++I)
 241       if (!equivalentAsOperands(L.getArgument(I), R.getArgument(I))) {
 242         if (Complain)
 243           Engine.logf("arguments %l and %r differ")
 244             << L.getArgument(I) << R.getArgument(I);
 245         return true;
 246       }
 247     return false;
 248   }
 249
 250   bool diff(Instruction *L, Instruction *R, bool Complain, bool TryUnify) {
 251     // FIXME: metadata (if Complain is set)
 252
 253     // Different opcodes always imply different operations.
 254     if (L->getOpcode() != R->getOpcode()) {
 255       if (Complain) Engine.log("different instruction types");
 256       return true;
 257     }
 258
 259     if (isa<CmpInst>(L)) {
 260       if (cast<CmpInst>(L)->getPredicate()
 261             != cast<CmpInst>(R)->getPredicate()) {
 262         if (Complain) Engine.log("different predicates");
 263         return true;
 264       }
 265     } else if (isa<CallInst>(L)) {
 266       return diffCallSites(CallSite(L), CallSite(R), Complain);
 267     } else if (isa<PHINode>(L)) {
 268       // FIXME: implement.
 269
 270       // This is really weird;  type uniquing is broken?
 271       if (L->getType() != R->getType()) {
 272         if (!L->getType()->isPointerTy() || !R->getType()->isPointerTy()) {
 273           if (Complain) Engine.log("different phi types");
 274           return true;
 275         }
 276       }
 277       return false;
 278
 279     // Terminators.
 280     } else if (isa<InvokeInst>(L)) {
 281       InvokeInst *LI = cast<InvokeInst>(L);
 282       InvokeInst *RI = cast<InvokeInst>(R);
 283       if (diffCallSites(CallSite(LI), CallSite(RI), Complain))
 284         return true;
 285
 286       if (TryUnify) {
 287         tryUnify(LI->getNormalDest(), RI->getNormalDest());
 288         tryUnify(LI->getUnwindDest(), RI->getUnwindDest());
 289       }
 290       return false;
 291
 292     } else if (isa<BranchInst>(L)) {
 293       BranchInst *LI = cast<BranchInst>(L);
 294       BranchInst *RI = cast<BranchInst>(R);
 295       if (LI->isConditional() != RI->isConditional()) {
 296         if (Complain) Engine.log("branch conditionality differs");
 297         return true;
 298       }
 299
 300       if (LI->isConditional()) {
 301         if (!equivalentAsOperands(LI->getCondition(), RI->getCondition())) {
 302           if (Complain) Engine.log("branch conditions differ");
 303           return true;
 304         }
 305         if (TryUnify) tryUnify(LI->getSuccessor(1), RI->getSuccessor(1));
 306       }
 307       if (TryUnify) tryUnify(LI->getSuccessor(0), RI->getSuccessor(0));
 308       return false;
 309
 310     } else if (isa<SwitchInst>(L)) {
 311       SwitchInst *LI = cast<SwitchInst>(L);
 312       SwitchInst *RI = cast<SwitchInst>(R);
 313       if (!equivalentAsOperands(LI->getCondition(), RI->getCondition())) {
 314         if (Complain) Engine.log("switch conditions differ");
 315         return true;
 316       }
 317       if (TryUnify) tryUnify(LI->getDefaultDest(), RI->getDefaultDest());
 318
 319       bool Difference = false;
 320
 321       DenseMap<ConstantInt*,BasicBlock*> LCases;
 322       for (unsigned I = 1, E = LI->getNumCases(); I != E; ++I)
 323         LCases[LI->getCaseValue(I)] = LI->getSuccessor(I);
 324       for (unsigned I = 1, E = RI->getNumCases(); I != E; ++I) {
 325         ConstantInt *CaseValue = RI->getCaseValue(I);
 326         BasicBlock *LCase = LCases[CaseValue];
 327         if (LCase) {
 328           if (TryUnify) tryUnify(LCase, RI->getSuccessor(I));
 329           LCases.erase(CaseValue);
 330         } else if (!Difference) {
 331           if (Complain)
 332             Engine.logf("right switch has extra case %r") << CaseValue;
 333           Difference = true;
 334         }
 335       }
 336       if (!Difference)
 337         for (DenseMap<ConstantInt*,BasicBlock*>::iterator
 338                I = LCases.begin(), E = LCases.end(); I != E; ++I) {
 339           if (Complain)
 340             Engine.logf("left switch has extra case %l") << I->first;
 341           Difference = true;
 342         }
 343       return Difference;
 344     } else if (isa<UnreachableInst>(L)) {
 345       return false;
 346     }
 347
 348     if (L->getNumOperands() != R->getNumOperands()) {
 349       if (Complain) Engine.log("instructions have different operand counts");
 350       return true;
 351     }
 352
 353     for (unsigned I = 0, E = L->getNumOperands(); I != E; ++I) {
 354       Value *LO = L->getOperand(I), *RO = R->getOperand(I);
 355       if (!equivalentAsOperands(LO, RO)) {
 356         if (Complain) Engine.logf("operands %l and %r differ") << LO << RO;
 357         return true;
 358       }
 359     }
 360
 361     return false;
 362   }
 363
 364   bool equivalentAsOperands(Constant *L, Constant *R) {
 365     // Use equality as a preliminary filter.
 366     if (L == R)
 367       return true;
 368
 369     if (L->getValueID() != R->getValueID())
 370       return false;
 371
 372     // Ask the engine about global values.
 373     if (isa<GlobalValue>(L))
 374       return Engine.equivalentAsOperands(cast<GlobalValue>(L),
 375                                          cast<GlobalValue>(R));
 376
 377     // Compare constant expressions structurally.
 378     if (isa<ConstantExpr>(L))
 379       return equivalentAsOperands(cast<ConstantExpr>(L),
 380                                   cast<ConstantExpr>(R));
 381
 382     // Nulls of the "same type" don't always actually have the same
 383     // type; I don't know why.  Just white-list them.
 384     if (isa<ConstantPointerNull>(L))
 385       return true;
 386
 387     // Block addresses only match if we've already encountered the
 388     // block.  FIXME: tentative matches?
 389     if (isa<BlockAddress>(L))
 390       return Blocks[cast<BlockAddress>(L)->getBasicBlock()]
 391                  == cast<BlockAddress>(R)->getBasicBlock();
 392
 393     return false;
 394   }
 395
 396   bool equivalentAsOperands(ConstantExpr *L, ConstantExpr *R) {
 397     if (L == R)
 398       return true;
 399     if (L->getOpcode() != R->getOpcode())
 400       return false;
 401
 402     switch (L->getOpcode()) {
 403     case Instruction::ICmp:
 404     case Instruction::FCmp:
 405       if (L->getPredicate() != R->getPredicate())
 406         return false;
 407       break;
 408
 409     case Instruction::GetElementPtr:
 410       // FIXME: inbounds?
 411       break;
 412
 413     default:
 414       break;
 415     }
 416
 417     if (L->getNumOperands() != R->getNumOperands())
 418       return false;
 419
 420     for (unsigned I = 0, E = L->getNumOperands(); I != E; ++I)
 421       if (!equivalentAsOperands(L->getOperand(I), R->getOperand(I)))
 422         return false;
 423
 424     return true;
 425   }
 426
 427   bool equivalentAsOperands(Value *L, Value *R) {
 428     // Fall out if the values have different kind.
 429     // This possibly shouldn't take priority over oracles.
 430     if (L->getValueID() != R->getValueID())
 431       return false;
 432
 433     // Value subtypes:  Argument, Constant, Instruction, BasicBlock,
 434     //                  InlineAsm, MDNode, MDString, PseudoSourceValue
 435
 436     if (isa<Constant>(L))
 437       return equivalentAsOperands(cast<Constant>(L), cast<Constant>(R));
 438
 439     if (isa<Instruction>(L))
 440       return Values[L] == R || TentativeValues.count(std::make_pair(L, R));
 441
 442     if (isa<Argument>(L))
 443       return Values[L] == R;
 444
 445     if (isa<BasicBlock>(L))
 446       return Blocks[cast<BasicBlock>(L)] != R;
 447
 448     // Pretend everything else is identical.
 449     return true;
 450   }
 451
 452   // Avoid a gcc warning about accessing 'this' in an initializer.
 453   FunctionDifferenceEngine *this_() { return this; }
 454
 455 public:
 456   FunctionDifferenceEngine(DifferenceEngine &Engine) :
 457     Engine(Engine), Queue(QueueSorter(*this_())) {}
 458
 459   void diff(Function *L, Function *R) {
 460     if (L->arg_size() != R->arg_size())
 461       Engine.log("different argument counts");
 462
 463     // Map the arguments.
 464     for (Function::arg_iterator
 465            LI = L->arg_begin(), LE = L->arg_end(),
 466            RI = R->arg_begin(), RE = R->arg_end();
 467          LI != LE && RI != RE; ++LI, ++RI)
 468       Values[&*LI] = &*RI;
 469
 470     tryUnify(&*L->begin(), &*R->begin());
 471     processQueue();
 472   }
 473 };
 474
 475 struct DiffEntry {
 476   DiffEntry() : Cost(0) {}
 477
 478   unsigned Cost;
 479   llvm::SmallVector<char, 8> Path; // actually of DifferenceEngine::DiffChange
 480 };
 481
 482 bool FunctionDifferenceEngine::matchForBlockDiff(Instruction *L,
 483                                                  Instruction *R) {
 484   return !diff(L, R, false, false);
 485 }
 486
 487 void FunctionDifferenceEngine::runBlockDiff(BasicBlock::iterator LStart,
 488                                             BasicBlock::iterator RStart) {
 489   BasicBlock::iterator LE = LStart->getParent()->end();
 490   BasicBlock::iterator RE = RStart->getParent()->end();
 491
 492   unsigned NL = std::distance(LStart, LE);
 493
 494   SmallVector<DiffEntry, 20> Paths1(NL+1);
 495   SmallVector<DiffEntry, 20> Paths2(NL+1);
 496
 497   DiffEntry *Cur = Paths1.data();
 498   DiffEntry *Next = Paths2.data();
 499
 500   const unsigned LeftCost = 2;
 501   const unsigned RightCost = 2;
 502   const unsigned MatchCost = 0;
 503
 504   assert(TentativeValues.empty());
 505
 506   // Initialize the first column.
 507   for (unsigned I = 0; I != NL+1; ++I) {
 508     Cur[I].Cost = I * LeftCost;
 509     for (unsigned J = 0; J != I; ++J)
 510       Cur[I].Path.push_back(DC_left);
 511   }
 512
 513   for (BasicBlock::iterator RI = RStart; RI != RE; ++RI) {
 514     // Initialize the first row.
 515     Next[0] = Cur[0];
 516     Next[0].Cost += RightCost;
 517     Next[0].Path.push_back(DC_right);
 518
 519     unsigned Index = 1;
 520     for (BasicBlock::iterator LI = LStart; LI != LE; ++LI, ++Index) {
 521       if (matchForBlockDiff(&*LI, &*RI)) {
 522         Next[Index] = Cur[Index-1];
 523         Next[Index].Cost += MatchCost;
 524         Next[Index].Path.push_back(DC_match);
 525         TentativeValues.insert(std::make_pair(&*LI, &*RI));
 526       } else if (Next[Index-1].Cost <= Cur[Index].Cost) {
 527         Next[Index] = Next[Index-1];
 528         Next[Index].Cost += LeftCost;
 529         Next[Index].Path.push_back(DC_left);
 530       } else {
 531         Next[Index] = Cur[Index];
 532         Next[Index].Cost += RightCost;
 533         Next[Index].Path.push_back(DC_right);
 534       }
 535     }
 536
 537     std::swap(Cur, Next);
 538   }
 539
 540   // We don't need the tentative values anymore; everything from here
 541   // on out should be non-tentative.
 542   TentativeValues.clear();
 543
 544   SmallVectorImpl<char> &Path = Cur[NL].Path;
 545   BasicBlock::iterator LI = LStart, RI = RStart;
 546
 547   DiffLogBuilder Diff(Engine.getConsumer());
 548
 549   // Drop trailing matches.
 550   while (Path.back() == DC_match)
 551     Path.pop_back();
 552
 553   // Skip leading matches.
 554   SmallVectorImpl<char>::iterator
 555     PI = Path.begin(), PE = Path.end();
 556   while (PI != PE && *PI == DC_match) {
 557     unify(&*LI, &*RI);
 558     ++PI, ++LI, ++RI;
 559   }
 560
 561   for (; PI != PE; ++PI) {
 562     switch (static_cast<DiffChange>(*PI)) {
 563     case DC_match:
 564       assert(LI != LE && RI != RE);
 565       {
 566         Instruction *L = &*LI, *R = &*RI;
 567         unify(L, R);
 568         Diff.addMatch(L, R);
 569       }
 570       ++LI; ++RI;
 571       break;
 572
 573     case DC_left:
 574       assert(LI != LE);
 575       Diff.addLeft(&*LI);
 576       ++LI;
 577       break;
 578
 579     case DC_right:
 580       assert(RI != RE);
 581       Diff.addRight(&*RI);
 582       ++RI;
 583       break;
 584     }
 585   }
 586
 587   // Finishing unifying and complaining about the tails of the block,
 588   // which should be matches all the way through.
 589   while (LI != LE) {
 590     assert(RI != RE);
 591     unify(&*LI, &*RI);
 592     ++LI, ++RI;
 593   }
 594
 595   // If the terminators have different kinds, but one is an invoke and the
 596   // other is an unconditional branch immediately following a call, unify
 597   // the results and the destinations.
 598   TerminatorInst *LTerm = LStart->getParent()->getTerminator();
 599   TerminatorInst *RTerm = RStart->getParent()->getTerminator();
 600   if (isa<BranchInst>(LTerm) && isa<InvokeInst>(RTerm)) {
 601     if (cast<BranchInst>(LTerm)->isConditional()) return;
 602     BasicBlock::iterator I = LTerm;
 603     if (I == LStart->getParent()->begin()) return;
 604     --I;
 605     if (!isa<CallInst>(*I)) return;
 606     CallInst *LCall = cast<CallInst>(&*I);
 607     InvokeInst *RInvoke = cast<InvokeInst>(RTerm);
 608     if (!equivalentAsOperands(LCall->getCalledValue(), RInvoke->getCalledValue()))
 609       return;
 610     if (!LCall->use_empty())
 611       Values[LCall] = RInvoke;
 612     tryUnify(LTerm->getSuccessor(0), RInvoke->getNormalDest());
 613   } else if (isa<InvokeInst>(LTerm) && isa<BranchInst>(RTerm)) {
 614     if (cast<BranchInst>(RTerm)->isConditional()) return;
 615     BasicBlock::iterator I = RTerm;
 616     if (I == RStart->getParent()->begin()) return;
 617     --I;
 618     if (!isa<CallInst>(*I)) return;
 619     CallInst *RCall = cast<CallInst>(I);
 620     InvokeInst *LInvoke = cast<InvokeInst>(LTerm);
 621     if (!equivalentAsOperands(LInvoke->getCalledValue(), RCall->getCalledValue()))
 622       return;
 623     if (!LInvoke->use_empty())
 624       Values[LInvoke] = RCall;
 625     tryUnify(LInvoke->getNormalDest(), RTerm->getSuccessor(0));
 626   }
 627 }
 628
 629 }
 630
 631 void DifferenceEngine::diff(Function *L, Function *R) {
 632   Context C(*this, L, R);
 633
 634   // FIXME: types
 635   // FIXME: attributes and CC
 636   // FIXME: parameter attributes
 637
 638   // If both are declarations, we're done.
 639   if (L->empty() && R->empty())
 640     return;
 641   else if (L->empty())
 642     log("left function is declaration, right function is definition");
 643   else if (R->empty())
 644     log("right function is declaration, left function is definition");
 645   else
 646     FunctionDifferenceEngine(*this).diff(L, R);
 647 }
 648
 649 void DifferenceEngine::diff(Module *L, Module *R) {
 650   StringSet<> LNames;
 651   SmallVector<std::pair<Function*,Function*>, 20> Queue;
 652
 653   for (Module::iterator I = L->begin(), E = L->end(); I != E; ++I) {
 654     Function *LFn = &*I;
 655     LNames.insert(LFn->getName());
 656
 657     if (Function *RFn = R->getFunction(LFn->getName()))
 658       Queue.push_back(std::make_pair(LFn, RFn));
 659     else
 660       logf("function %l exists only in left module") << LFn;
 661   }
 662
 663   for (Module::iterator I = R->begin(), E = R->end(); I != E; ++I) {
 664     Function *RFn = &*I;
 665     if (!LNames.count(RFn->getName()))
 666       logf("function %r exists only in right module") << RFn;
 667   }
 668
 669   for (SmallVectorImpl<std::pair<Function*,Function*> >::iterator
 670          I = Queue.begin(), E = Queue.end(); I != E; ++I)
 671     diff(I->first, I->second);
 672 }
 673
 674 bool DifferenceEngine::equivalentAsOperands(GlobalValue *L, GlobalValue *R) {
 675   if (globalValueOracle) return (*globalValueOracle)(L, R);
 676   return L->getName() == R->getName();
 677 }