lib/Transforms/IPO/MergeFunctions.cpp

   1 //===- MergeFunctions.cpp - Merge identical functions ---------------------===//
   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 pass looks for equivalent functions that are mergable and folds them.
  11 //
  12 // A hash is computed from the function, based on its type and number of
  13 // basic blocks.
  14 //
  15 // Once all hashes are computed, we perform an expensive equality comparison
  16 // on each function pair. This takes n^2/2 comparisons per bucket, so it's
  17 // important that the hash function be high quality. The equality comparison
  18 // iterates through each instruction in each basic block.
  19 //
  20 // When a match is found, the functions are folded. We can only fold two
  21 // functions when we know that the definition of one of them is not
  22 // overridable.
  23 //
  24 //===----------------------------------------------------------------------===//
  25 //
  26 // Future work:
  27 //
  28 // * fold vector<T*>::push_back and vector<S*>::push_back.
  29 //
  30 // These two functions have different types, but in a way that doesn't matter
  31 // to us. As long as we never see an S or T itself, using S* and S** is the
  32 // same as using a T* and T**.
  33 //
  34 // * virtual functions.
  35 //
  36 // Many functions have their address taken by the virtual function table for
  37 // the object they belong to. However, as long as it's only used for a lookup
  38 // and call, this is irrelevant, and we'd like to fold such implementations.
  39 //
  40 //===----------------------------------------------------------------------===//
  41
  42 #define DEBUG_TYPE "mergefunc"
  43 #include "llvm/Transforms/IPO.h"
  44 #include "llvm/ADT/DenseMap.h"
  45 #include "llvm/ADT/FoldingSet.h"
  46 #include "llvm/ADT/Statistic.h"
  47 #include "llvm/Constants.h"
  48 #include "llvm/InlineAsm.h"
  49 #include "llvm/Instructions.h"
  50 #include "llvm/LLVMContext.h"
  51 #include "llvm/Module.h"
  52 #include "llvm/Pass.h"
  53 #include "llvm/Support/CallSite.h"
  54 #include "llvm/Support/Compiler.h"
  55 #include "llvm/Support/Debug.h"
  56 #include "llvm/Support/ErrorHandling.h"
  57 #include "llvm/Support/raw_ostream.h"
  58 #include <map>
  59 #include <vector>
  60 using namespace llvm;
  61
  62 STATISTIC(NumFunctionsMerged, "Number of functions merged");
  63
  64 namespace {
  65   struct VISIBILITY_HIDDEN MergeFunctions : public ModulePass {
  66     static char ID; // Pass identification, replacement for typeid
  67     MergeFunctions() : ModulePass(&ID) {}
  68
  69     bool runOnModule(Module &M);
  70   };
  71 }
  72
  73 char MergeFunctions::ID = 0;
  74 static RegisterPass<MergeFunctions>
  75 X("mergefunc", "Merge Functions");
  76
  77 ModulePass *llvm::createMergeFunctionsPass() {
  78   return new MergeFunctions();
  79 }
  80
  81 // ===----------------------------------------------------------------------===
  82 // Comparison of functions
  83 // ===----------------------------------------------------------------------===
  84
  85 static unsigned long hash(const Function *F) {
  86   const FunctionType *FTy = F->getFunctionType();
  87
  88   FoldingSetNodeID ID;
  89   ID.AddInteger(F->size());
  90   ID.AddInteger(F->getCallingConv());
  91   ID.AddBoolean(F->hasGC());
  92   ID.AddBoolean(FTy->isVarArg());
  93   ID.AddInteger(FTy->getReturnType()->getTypeID());
  94   for (unsigned i = 0, e = FTy->getNumParams(); i != e; ++i)
  95     ID.AddInteger(FTy->getParamType(i)->getTypeID());
  96   return ID.ComputeHash();
  97 }
  98
  99 /// IgnoreBitcasts - given a bitcast, returns the first non-bitcast found by
 100 /// walking the chain of cast operands. Otherwise, returns the argument.
 101 static Value* IgnoreBitcasts(Value *V) {
 102   while (BitCastInst *BC = dyn_cast<BitCastInst>(V))
 103     V = BC->getOperand(0);
 104
 105   return V;
 106 }
 107
 108 /// isEquivalentType - any two pointers are equivalent. Otherwise, standard
 109 /// type equivalence rules apply.
 110 static bool isEquivalentType(const Type *Ty1, const Type *Ty2) {
 111   if (Ty1 == Ty2)
 112     return true;
 113   if (Ty1->getTypeID() != Ty2->getTypeID())
 114     return false;
 115
 116   switch(Ty1->getTypeID()) {
 117   case Type::VoidTyID:
 118   case Type::FloatTyID:
 119   case Type::DoubleTyID:
 120   case Type::X86_FP80TyID:
 121   case Type::FP128TyID:
 122   case Type::PPC_FP128TyID:
 123   case Type::LabelTyID:
 124   case Type::MetadataTyID:
 125     return true;
 126
 127   case Type::IntegerTyID:
 128   case Type::OpaqueTyID:
 129     // Ty1 == Ty2 would have returned true earlier.
 130     return false;
 131
 132   default:
 133     llvm_unreachable("Unknown type!");
 134     return false;
 135
 136   case Type::PointerTyID: {
 137     const PointerType *PTy1 = cast<PointerType>(Ty1);
 138     const PointerType *PTy2 = cast<PointerType>(Ty2);
 139     return PTy1->getAddressSpace() == PTy2->getAddressSpace();
 140   }
 141
 142   case Type::StructTyID: {
 143     const StructType *STy1 = cast<StructType>(Ty1);
 144     const StructType *STy2 = cast<StructType>(Ty2);
 145     if (STy1->getNumElements() != STy2->getNumElements())
 146       return false;
 147
 148     if (STy1->isPacked() != STy2->isPacked())
 149       return false;
 150
 151     for (unsigned i = 0, e = STy1->getNumElements(); i != e; ++i) {
 152       if (!isEquivalentType(STy1->getElementType(i), STy2->getElementType(i)))
 153         return false;
 154     }
 155     return true;
 156   }
 157
 158   case Type::FunctionTyID: {
 159     const FunctionType *FTy1 = cast<FunctionType>(Ty1);
 160     const FunctionType *FTy2 = cast<FunctionType>(Ty2);
 161     if (FTy1->getNumParams() != FTy2->getNumParams() ||
 162         FTy1->isVarArg() != FTy2->isVarArg())
 163       return false;
 164
 165     if (!isEquivalentType(FTy1->getReturnType(), FTy2->getReturnType()))
 166       return false;
 167
 168     for (unsigned i = 0, e = FTy1->getNumParams(); i != e; ++i) {
 169       if (!isEquivalentType(FTy1->getParamType(i), FTy2->getParamType(i)))
 170         return false;
 171     }
 172     return true;
 173   }
 174
 175   case Type::ArrayTyID:
 176   case Type::VectorTyID: {
 177     const SequentialType *STy1 = cast<SequentialType>(Ty1);
 178     const SequentialType *STy2 = cast<SequentialType>(Ty2);
 179     return isEquivalentType(STy1->getElementType(), STy2->getElementType());
 180   }
 181   }
 182 }
 183
 184 /// isEquivalentOperation - determine whether the two operations are the same
 185 /// except that pointer-to-A and pointer-to-B are equivalent. This should be
 186 /// kept in sync with Instruction::isSameOperationAs.
 187 static bool
 188 isEquivalentOperation(const Instruction *I1, const Instruction *I2) {
 189   if (I1->getOpcode() != I2->getOpcode() ||
 190       I1->getNumOperands() != I2->getNumOperands() ||
 191       !isEquivalentType(I1->getType(), I2->getType()) ||
 192       !I1->hasSameSubclassOptionalData(I2))
 193     return false;
 194
 195   // We have two instructions of identical opcode and #operands.  Check to see
 196   // if all operands are the same type
 197   for (unsigned i = 0, e = I1->getNumOperands(); i != e; ++i)
 198     if (!isEquivalentType(I1->getOperand(i)->getType(),
 199                           I2->getOperand(i)->getType()))
 200       return false;
 201
 202   // Check special state that is a part of some instructions.
 203   if (const LoadInst *LI = dyn_cast<LoadInst>(I1))
 204     return LI->isVolatile() == cast<LoadInst>(I2)->isVolatile() &&
 205            LI->getAlignment() == cast<LoadInst>(I2)->getAlignment();
 206   if (const StoreInst *SI = dyn_cast<StoreInst>(I1))
 207     return SI->isVolatile() == cast<StoreInst>(I2)->isVolatile() &&
 208            SI->getAlignment() == cast<StoreInst>(I2)->getAlignment();
 209   if (const CmpInst *CI = dyn_cast<CmpInst>(I1))
 210     return CI->getPredicate() == cast<CmpInst>(I2)->getPredicate();
 211   if (const CallInst *CI = dyn_cast<CallInst>(I1))
 212     return CI->isTailCall() == cast<CallInst>(I2)->isTailCall() &&
 213            CI->getCallingConv() == cast<CallInst>(I2)->getCallingConv() &&
 214            CI->getAttributes().getRawPointer() ==
 215              cast<CallInst>(I2)->getAttributes().getRawPointer();
 216   if (const InvokeInst *CI = dyn_cast<InvokeInst>(I1))
 217     return CI->getCallingConv() == cast<InvokeInst>(I2)->getCallingConv() &&
 218            CI->getAttributes().getRawPointer() ==
 219              cast<InvokeInst>(I2)->getAttributes().getRawPointer();
 220   if (const InsertValueInst *IVI = dyn_cast<InsertValueInst>(I1)) {
 221     if (IVI->getNumIndices() != cast<InsertValueInst>(I2)->getNumIndices())
 222       return false;
 223     for (unsigned i = 0, e = IVI->getNumIndices(); i != e; ++i)
 224       if (IVI->idx_begin()[i] != cast<InsertValueInst>(I2)->idx_begin()[i])
 225         return false;
 226     return true;
 227   }
 228   if (const ExtractValueInst *EVI = dyn_cast<ExtractValueInst>(I1)) {
 229     if (EVI->getNumIndices() != cast<ExtractValueInst>(I2)->getNumIndices())
 230       return false;
 231     for (unsigned i = 0, e = EVI->getNumIndices(); i != e; ++i)
 232       if (EVI->idx_begin()[i] != cast<ExtractValueInst>(I2)->idx_begin()[i])
 233         return false;
 234     return true;
 235   }
 236
 237   return true;
 238 }
 239
 240 static bool compare(const Value *V, const Value *U) {
 241   assert(!isa<BasicBlock>(V) && !isa<BasicBlock>(U) &&
 242          "Must not compare basic blocks.");
 243
 244   assert(isEquivalentType(V->getType(), U->getType()) &&
 245         "Two of the same operation have operands of different type.");
 246
 247   // TODO: If the constant is an expression of F, we should accept that it's
 248   // equal to the same expression in terms of G.
 249   if (isa<Constant>(V))
 250     return V == U;
 251
 252   // The caller has ensured that ValueMap[V] != U. Since Arguments are
 253   // pre-loaded into the ValueMap, and Instructions are added as we go, we know
 254   // that this can only be a mis-match.
 255   if (isa<Instruction>(V) || isa<Argument>(V))
 256     return false;
 257
 258   if (isa<InlineAsm>(V) && isa<InlineAsm>(U)) {
 259     const InlineAsm *IAF = cast<InlineAsm>(V);
 260     const InlineAsm *IAG = cast<InlineAsm>(U);
 261     return IAF->getAsmString() == IAG->getAsmString() &&
 262            IAF->getConstraintString() == IAG->getConstraintString();
 263   }
 264
 265   return false;
 266 }
 267
 268 static bool equals(const BasicBlock *BB1, const BasicBlock *BB2,
 269                    DenseMap<const Value *, const Value *> &ValueMap,
 270                    DenseMap<const Value *, const Value *> &SpeculationMap) {
 271   // Speculatively add it anyways. If it's false, we'll notice a difference
 272   // later, and this won't matter.
 273   ValueMap[BB1] = BB2;
 274
 275   BasicBlock::const_iterator FI = BB1->begin(), FE = BB1->end();
 276   BasicBlock::const_iterator GI = BB2->begin(), GE = BB2->end();
 277
 278   do {
 279     if (isa<BitCastInst>(FI)) {
 280       ++FI;
 281       continue;
 282     }
 283     if (isa<BitCastInst>(GI)) {
 284       ++GI;
 285       continue;
 286     }
 287
 288     if (!isEquivalentOperation(FI, GI))
 289       return false;
 290
 291     if (isa<GetElementPtrInst>(FI)) {
 292       const GetElementPtrInst *GEPF = cast<GetElementPtrInst>(FI);
 293       const GetElementPtrInst *GEPG = cast<GetElementPtrInst>(GI);
 294       if (GEPF->hasAllZeroIndices() && GEPG->hasAllZeroIndices()) {
 295         // It's effectively a bitcast.
 296         ++FI, ++GI;
 297         continue;
 298       }
 299
 300       // TODO: we only really care about the elements before the index
 301       if (FI->getOperand(0)->getType() != GI->getOperand(0)->getType())
 302         return false;
 303     }
 304
 305     if (ValueMap[FI] == GI) {
 306       ++FI, ++GI;
 307       continue;
 308     }
 309
 310     if (ValueMap[FI] != NULL)
 311       return false;
 312
 313     for (unsigned i = 0, e = FI->getNumOperands(); i != e; ++i) {
 314       Value *OpF = IgnoreBitcasts(FI->getOperand(i));
 315       Value *OpG = IgnoreBitcasts(GI->getOperand(i));
 316
 317       if (ValueMap[OpF] == OpG)
 318         continue;
 319
 320       if (ValueMap[OpF] != NULL)
 321         return false;
 322
 323       if (OpF->getValueID() != OpG->getValueID() ||
 324           !isEquivalentType(OpF->getType(), OpG->getType()))
 325         return false;
 326
 327       if (isa<PHINode>(FI)) {
 328         if (SpeculationMap[OpF] == NULL)
 329           SpeculationMap[OpF] = OpG;
 330         else if (SpeculationMap[OpF] != OpG)
 331           return false;
 332         continue;
 333       } else if (isa<BasicBlock>(OpF)) {
 334         assert(isa<TerminatorInst>(FI) &&
 335                "BasicBlock referenced by non-Terminator non-PHI");
 336         // This call changes the ValueMap, hence we can't use
 337         // Value *& = ValueMap[...]
 338         if (!equals(cast<BasicBlock>(OpF), cast<BasicBlock>(OpG), ValueMap,
 339                     SpeculationMap))
 340           return false;
 341       } else {
 342         if (!compare(OpF, OpG))
 343           return false;
 344       }
 345
 346       ValueMap[OpF] = OpG;
 347     }
 348
 349     ValueMap[FI] = GI;
 350     ++FI, ++GI;
 351   } while (FI != FE && GI != GE);
 352
 353   return FI == FE && GI == GE;
 354 }
 355
 356 static bool equals(const Function *F, const Function *G) {
 357   // We need to recheck everything, but check the things that weren't included
 358   // in the hash first.
 359
 360   if (F->getAttributes() != G->getAttributes())
 361     return false;
 362
 363   if (F->hasGC() != G->hasGC())
 364     return false;
 365
 366   if (F->hasGC() && F->getGC() != G->getGC())
 367     return false;
 368
 369   if (F->hasSection() != G->hasSection())
 370     return false;
 371
 372   if (F->hasSection() && F->getSection() != G->getSection())
 373     return false;
 374
 375   if (F->isVarArg() != G->isVarArg())
 376     return false;
 377
 378   // TODO: if it's internal and only used in direct calls, we could handle this
 379   // case too.
 380   if (F->getCallingConv() != G->getCallingConv())
 381     return false;
 382
 383   if (!isEquivalentType(F->getFunctionType(), G->getFunctionType()))
 384     return false;
 385
 386   DenseMap<const Value *, const Value *> ValueMap;
 387   DenseMap<const Value *, const Value *> SpeculationMap;
 388   ValueMap[F] = G;
 389
 390   assert(F->arg_size() == G->arg_size() &&
 391          "Identical functions have a different number of args.");
 392
 393   for (Function::const_arg_iterator fi = F->arg_begin(), gi = G->arg_begin(),
 394          fe = F->arg_end(); fi != fe; ++fi, ++gi)
 395     ValueMap[fi] = gi;
 396
 397   if (!equals(&F->getEntryBlock(), &G->getEntryBlock(), ValueMap,
 398               SpeculationMap))
 399     return false;
 400
 401   for (DenseMap<const Value *, const Value *>::iterator
 402          I = SpeculationMap.begin(), E = SpeculationMap.end(); I != E; ++I) {
 403     if (ValueMap[I->first] != I->second)
 404       return false;
 405   }
 406
 407   return true;
 408 }
 409
 410 // ===----------------------------------------------------------------------===
 411 // Folding of functions
 412 // ===----------------------------------------------------------------------===
 413
 414 // Cases:
 415 // * F is external strong, G is external strong:
 416 //   turn G into a thunk to F    (1)
 417 // * F is external strong, G is external weak:
 418 //   turn G into a thunk to F    (1)
 419 // * F is external weak, G is external weak:
 420 //   unfoldable
 421 // * F is external strong, G is internal:
 422 //   address of G taken:
 423 //     turn G into a thunk to F  (1)
 424 //   address of G not taken:
 425 //     make G an alias to F      (2)
 426 // * F is internal, G is external weak
 427 //   address of F is taken:
 428 //     turn G into a thunk to F  (1)
 429 //   address of F is not taken:
 430 //     make G an alias of F      (2)
 431 // * F is internal, G is internal:
 432 //   address of F and G are taken:
 433 //     turn G into a thunk to F  (1)
 434 //   address of G is not taken:
 435 //     make G an alias to F      (2)
 436 //
 437 // alias requires linkage == (external,local,weak) fallback to creating a thunk
 438 // external means 'externally visible' linkage != (internal,private)
 439 // internal means linkage == (internal,private)
 440 // weak means linkage mayBeOverridable
 441 // being external implies that the address is taken
 442 //
 443 // 1. turn G into a thunk to F
 444 // 2. make G an alias to F
 445
 446 enum LinkageCategory {
 447   ExternalStrong,
 448   ExternalWeak,
 449   Internal
 450 };
 451
 452 static LinkageCategory categorize(const Function *F) {
 453   switch (F->getLinkage()) {
 454   case GlobalValue::InternalLinkage:
 455   case GlobalValue::PrivateLinkage:
 456   case GlobalValue::LinkerPrivateLinkage:
 457     return Internal;
 458
 459   case GlobalValue::WeakAnyLinkage:
 460   case GlobalValue::WeakODRLinkage:
 461   case GlobalValue::ExternalWeakLinkage:
 462     return ExternalWeak;
 463
 464   case GlobalValue::ExternalLinkage:
 465   case GlobalValue::AvailableExternallyLinkage:
 466   case GlobalValue::LinkOnceAnyLinkage:
 467   case GlobalValue::LinkOnceODRLinkage:
 468   case GlobalValue::AppendingLinkage:
 469   case GlobalValue::DLLImportLinkage:
 470   case GlobalValue::DLLExportLinkage:
 471   case GlobalValue::GhostLinkage:
 472   case GlobalValue::CommonLinkage:
 473     return ExternalStrong;
 474   }
 475
 476   llvm_unreachable("Unknown LinkageType.");
 477   return ExternalWeak;
 478 }
 479
 480 static void ThunkGToF(Function *F, Function *G) {
 481   Function *NewG = Function::Create(G->getFunctionType(), G->getLinkage(), "",
 482                                     G->getParent());
 483   BasicBlock *BB = BasicBlock::Create(F->getContext(), "", NewG);
 484
 485   std::vector<Value *> Args;
 486   unsigned i = 0;
 487   const FunctionType *FFTy = F->getFunctionType();
 488   for (Function::arg_iterator AI = NewG->arg_begin(), AE = NewG->arg_end();
 489        AI != AE; ++AI) {
 490     if (FFTy->getParamType(i) == AI->getType())
 491       Args.push_back(AI);
 492     else {
 493       Value *BCI = new BitCastInst(AI, FFTy->getParamType(i), "", BB);
 494       Args.push_back(BCI);
 495     }
 496     ++i;
 497   }
 498
 499   CallInst *CI = CallInst::Create(F, Args.begin(), Args.end(), "", BB);
 500   CI->setTailCall();
 501   CI->setCallingConv(F->getCallingConv());
 502   if (NewG->getReturnType() == Type::getVoidTy(F->getContext())) {
 503     ReturnInst::Create(F->getContext(), BB);
 504   } else if (CI->getType() != NewG->getReturnType()) {
 505     Value *BCI = new BitCastInst(CI, NewG->getReturnType(), "", BB);
 506     ReturnInst::Create(F->getContext(), BCI, BB);
 507   } else {
 508     ReturnInst::Create(F->getContext(), CI, BB);
 509   }
 510
 511   NewG->copyAttributesFrom(G);
 512   NewG->takeName(G);
 513   G->replaceAllUsesWith(NewG);
 514   G->eraseFromParent();
 515
 516   // TODO: look at direct callers to G and make them all direct callers to F.
 517 }
 518
 519 static void AliasGToF(Function *F, Function *G) {
 520   if (!G->hasExternalLinkage() && !G->hasLocalLinkage() && !G->hasWeakLinkage())
 521     return ThunkGToF(F, G);
 522
 523   GlobalAlias *GA = new GlobalAlias(
 524     G->getType(), G->getLinkage(), "",
 525     ConstantExpr::getBitCast(F, G->getType()), G->getParent());
 526   F->setAlignment(std::max(F->getAlignment(), G->getAlignment()));
 527   GA->takeName(G);
 528   GA->setVisibility(G->getVisibility());
 529   G->replaceAllUsesWith(GA);
 530   G->eraseFromParent();
 531 }
 532
 533 static bool fold(std::vector<Function *> &FnVec, unsigned i, unsigned j) {
 534   Function *F = FnVec[i];
 535   Function *G = FnVec[j];
 536
 537   LinkageCategory catF = categorize(F);
 538   LinkageCategory catG = categorize(G);
 539
 540   if (catF == ExternalWeak || (catF == Internal && catG == ExternalStrong)) {
 541     std::swap(FnVec[i], FnVec[j]);
 542     std::swap(F, G);
 543     std::swap(catF, catG);
 544   }
 545
 546   switch (catF) {
 547     case ExternalStrong:
 548       switch (catG) {
 549         case ExternalStrong:
 550         case ExternalWeak:
 551           ThunkGToF(F, G);
 552           break;
 553         case Internal:
 554           if (G->hasAddressTaken())
 555             ThunkGToF(F, G);
 556           else
 557             AliasGToF(F, G);
 558           break;
 559       }
 560       break;
 561
 562     case ExternalWeak: {
 563       assert(catG == ExternalWeak);
 564
 565       // Make them both thunks to the same internal function.
 566       F->setAlignment(std::max(F->getAlignment(), G->getAlignment()));
 567       Function *H = Function::Create(F->getFunctionType(), F->getLinkage(), "",
 568                                      F->getParent());
 569       H->copyAttributesFrom(F);
 570       H->takeName(F);
 571       F->replaceAllUsesWith(H);
 572
 573       ThunkGToF(F, G);
 574       ThunkGToF(F, H);
 575
 576       F->setLinkage(GlobalValue::InternalLinkage);
 577     } break;
 578
 579     case Internal:
 580       switch (catG) {
 581         case ExternalStrong:
 582           llvm_unreachable(0);
 583           // fall-through
 584         case ExternalWeak:
 585           if (F->hasAddressTaken())
 586             ThunkGToF(F, G);
 587           else
 588             AliasGToF(F, G);
 589           break;
 590         case Internal: {
 591           bool addrTakenF = F->hasAddressTaken();
 592           bool addrTakenG = G->hasAddressTaken();
 593           if (!addrTakenF && addrTakenG) {
 594             std::swap(FnVec[i], FnVec[j]);
 595             std::swap(F, G);
 596             std::swap(addrTakenF, addrTakenG);
 597           }
 598
 599           if (addrTakenF && addrTakenG) {
 600             ThunkGToF(F, G);
 601           } else {
 602             assert(!addrTakenG);
 603             AliasGToF(F, G);
 604           }
 605         } break;
 606       }
 607       break;
 608   }
 609
 610   ++NumFunctionsMerged;
 611   return true;
 612 }
 613
 614 // ===----------------------------------------------------------------------===
 615 // Pass definition
 616 // ===----------------------------------------------------------------------===
 617
 618 bool MergeFunctions::runOnModule(Module &M) {
 619   bool Changed = false;
 620
 621   std::map<unsigned long, std::vector<Function *> > FnMap;
 622
 623   for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
 624     if (F->isDeclaration() || F->isIntrinsic())
 625       continue;
 626
 627     FnMap[hash(F)].push_back(F);
 628   }
 629
 630   // TODO: instead of running in a loop, we could also fold functions in
 631   // callgraph order. Constructing the CFG probably isn't cheaper than just
 632   // running in a loop, unless it happened to already be available.
 633
 634   bool LocalChanged;
 635   do {
 636     LocalChanged = false;
 637     DEBUG(errs() << "size: " << FnMap.size() << "\n");
 638     for (std::map<unsigned long, std::vector<Function *> >::iterator
 639          I = FnMap.begin(), E = FnMap.end(); I != E; ++I) {
 640       std::vector<Function *> &FnVec = I->second;
 641       DEBUG(errs() << "hash (" << I->first << "): " << FnVec.size() << "\n");
 642
 643       for (int i = 0, e = FnVec.size(); i != e; ++i) {
 644         for (int j = i + 1; j != e; ++j) {
 645           bool isEqual = equals(FnVec[i], FnVec[j]);
 646
 647           DEBUG(errs() << "  " << FnVec[i]->getName()
 648                 << (isEqual ? " == " : " != ")
 649                 << FnVec[j]->getName() << "\n");
 650
 651           if (isEqual) {
 652             if (fold(FnVec, i, j)) {
 653               LocalChanged = true;
 654               FnVec.erase(FnVec.begin() + j);
 655               --j, --e;
 656             }
 657           }
 658         }
 659       }
 660
 661     }
 662     Changed |= LocalChanged;
 663   } while (LocalChanged);
 664
 665   return Changed;
 666 }