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. If both functions are
  21 // overridable, we move the functionality into a new internal function and
  22 // leave two overridable thunks to it.
  23 //
  24 //===----------------------------------------------------------------------===//
  25 //
  26 // Future work:
  27 //
  28 // * virtual functions.
  29 //
  30 // Many functions have their address taken by the virtual function table for
  31 // the object they belong to. However, as long as it's only used for a lookup
  32 // and call, this is irrelevant, and we'd like to fold such functions.
  33 //
  34 // * switch from n^2 pair-wise comparisons to an n-way comparison for each
  35 // bucket.
  36 //
  37 // * be smarter about bitcasts.
  38 //
  39 // In order to fold functions, we will sometimes add either bitcast instructions
  40 // or bitcast constant expressions. Unfortunately, this can confound further
  41 // analysis since the two functions differ where one has a bitcast and the
  42 // other doesn't. We should learn to look through bitcasts.
  43 //
  44 //===----------------------------------------------------------------------===//
  45
  46 #define DEBUG_TYPE "mergefunc"
  47 #include "llvm/Transforms/IPO.h"
  48 #include "llvm/ADT/DenseMap.h"
  49 #include "llvm/ADT/FoldingSet.h"
  50 #include "llvm/ADT/SmallSet.h"
  51 #include "llvm/ADT/Statistic.h"
  52 #include "llvm/Constants.h"
  53 #include "llvm/InlineAsm.h"
  54 #include "llvm/Instructions.h"
  55 #include "llvm/LLVMContext.h"
  56 #include "llvm/Module.h"
  57 #include "llvm/Pass.h"
  58 #include "llvm/Support/CallSite.h"
  59 #include "llvm/Support/Debug.h"
  60 #include "llvm/Support/ErrorHandling.h"
  61 #include "llvm/Support/IRBuilder.h"
  62 #include "llvm/Support/raw_ostream.h"
  63 #include "llvm/Target/TargetData.h"
  64 #include <map>
  65 #include <vector>
  66 using namespace llvm;
  67
  68 STATISTIC(NumFunctionsMerged, "Number of functions merged");
  69
  70 namespace {
  71   /// MergeFunctions finds functions which will generate identical machine code,
  72   /// by considering all pointer types to be equivalent. Once identified,
  73   /// MergeFunctions will fold them by replacing a call to one to a call to a
  74   /// bitcast of the other.
  75   ///
  76   class MergeFunctions : public ModulePass {
  77   public:
  78     static char ID;
  79     MergeFunctions() : ModulePass(ID) {}
  80
  81     bool runOnModule(Module &M);
  82
  83   private:
  84     /// PairwiseCompareAndMerge - Given a list of functions, compare each pair
  85     /// and merge the pairs of equivalent functions.
  86     bool PairwiseCompareAndMerge(std::vector<Function *> &FnVec);
  87
  88     /// MergeTwoFunctions - Merge two equivalent functions. Upon completion,
  89     /// FnVec[j] should never be visited again.
  90     void MergeTwoFunctions(std::vector<Function *> &FnVec,
  91                            unsigned i, unsigned j) const;
  92
  93     /// WriteThunk - Replace G with a simple tail call to bitcast(F). Also
  94     /// replace direct uses of G with bitcast(F).
  95     void WriteThunk(Function *F, Function *G) const;
  96
  97     TargetData *TD;
  98   };
  99 }
 100
 101 char MergeFunctions::ID = 0;
 102 INITIALIZE_PASS(MergeFunctions, "mergefunc", "Merge Functions", false, false);
 103
 104 ModulePass *llvm::createMergeFunctionsPass() {
 105   return new MergeFunctions();
 106 }
 107
 108 namespace {
 109 /// FunctionComparator - Compares two functions to determine whether or not
 110 /// they will generate machine code with the same behaviour. TargetData is
 111 /// used if available. The comparator always fails conservatively (erring on the
 112 /// side of claiming that two functions are different).
 113 class FunctionComparator {
 114 public:
 115   FunctionComparator(TargetData *TD, Function *F1, Function *F2)
 116     : F1(F1), F2(F2), TD(TD), IDMap1Count(0), IDMap2Count(0) {}
 117
 118   /// Compare - test whether the two functions have equivalent behaviour.
 119   bool Compare();
 120
 121 private:
 122   /// Compare - test whether two basic blocks have equivalent behaviour.
 123   bool Compare(const BasicBlock *BB1, const BasicBlock *BB2);
 124
 125   /// Enumerate - Assign or look up previously assigned numbers for the two
 126   /// values, and return whether the numbers are equal. Numbers are assigned in
 127   /// the order visited.
 128   bool Enumerate(const Value *V1, const Value *V2);
 129
 130   /// isEquivalentOperation - Compare two Instructions for equivalence, similar
 131   /// to Instruction::isSameOperationAs but with modifications to the type
 132   /// comparison.
 133   bool isEquivalentOperation(const Instruction *I1,
 134                              const Instruction *I2) const;
 135
 136   /// isEquivalentGEP - Compare two GEPs for equivalent pointer arithmetic.
 137   bool isEquivalentGEP(const GEPOperator *GEP1, const GEPOperator *GEP2);
 138   bool isEquivalentGEP(const GetElementPtrInst *GEP1,
 139                        const GetElementPtrInst *GEP2) {
 140     return isEquivalentGEP(cast<GEPOperator>(GEP1), cast<GEPOperator>(GEP2));
 141   }
 142
 143   /// isEquivalentType - Compare two Types, treating all pointer types as equal.
 144   bool isEquivalentType(const Type *Ty1, const Type *Ty2) const;
 145
 146   // The two functions undergoing comparison.
 147   Function *F1, *F2;
 148
 149   TargetData *TD;
 150
 151   typedef DenseMap<const Value *, unsigned long> IDMap;
 152   IDMap Map1, Map2;
 153   unsigned long IDMap1Count, IDMap2Count;
 154 };
 155 }
 156
 157 /// Compute a hash guaranteed to be equal for two equivalent functions, but
 158 /// very likely to be different for different functions.
 159 static unsigned long ProfileFunction(const Function *F) {
 160   const FunctionType *FTy = F->getFunctionType();
 161
 162   FoldingSetNodeID ID;
 163   ID.AddInteger(F->size());
 164   ID.AddInteger(F->getCallingConv());
 165   ID.AddBoolean(F->hasGC());
 166   ID.AddBoolean(FTy->isVarArg());
 167   ID.AddInteger(FTy->getReturnType()->getTypeID());
 168   for (unsigned i = 0, e = FTy->getNumParams(); i != e; ++i)
 169     ID.AddInteger(FTy->getParamType(i)->getTypeID());
 170   return ID.ComputeHash();
 171 }
 172
 173 /// isEquivalentType - any two pointers in the same address space are
 174 /// equivalent. Otherwise, standard type equivalence rules apply.
 175 bool FunctionComparator::isEquivalentType(const Type *Ty1,
 176                                           const Type *Ty2) const {
 177   if (Ty1 == Ty2)
 178     return true;
 179   if (Ty1->getTypeID() != Ty2->getTypeID())
 180     return false;
 181
 182   switch(Ty1->getTypeID()) {
 183   default:
 184     llvm_unreachable("Unknown type!");
 185     // Fall through in Release mode.
 186   case Type::IntegerTyID:
 187   case Type::OpaqueTyID:
 188     // Ty1 == Ty2 would have returned true earlier.
 189     return false;
 190
 191   case Type::VoidTyID:
 192   case Type::FloatTyID:
 193   case Type::DoubleTyID:
 194   case Type::X86_FP80TyID:
 195   case Type::FP128TyID:
 196   case Type::PPC_FP128TyID:
 197   case Type::LabelTyID:
 198   case Type::MetadataTyID:
 199     return true;
 200
 201   case Type::PointerTyID: {
 202     const PointerType *PTy1 = cast<PointerType>(Ty1);
 203     const PointerType *PTy2 = cast<PointerType>(Ty2);
 204     return PTy1->getAddressSpace() == PTy2->getAddressSpace();
 205   }
 206
 207   case Type::StructTyID: {
 208     const StructType *STy1 = cast<StructType>(Ty1);
 209     const StructType *STy2 = cast<StructType>(Ty2);
 210     if (STy1->getNumElements() != STy2->getNumElements())
 211       return false;
 212
 213     if (STy1->isPacked() != STy2->isPacked())
 214       return false;
 215
 216     for (unsigned i = 0, e = STy1->getNumElements(); i != e; ++i) {
 217       if (!isEquivalentType(STy1->getElementType(i), STy2->getElementType(i)))
 218         return false;
 219     }
 220     return true;
 221   }
 222
 223   case Type::UnionTyID: {
 224     const UnionType *UTy1 = cast<UnionType>(Ty1);
 225     const UnionType *UTy2 = cast<UnionType>(Ty2);
 226
 227     if (UTy1->getNumElements() != UTy2->getNumElements())
 228       return false;
 229
 230     for (unsigned i = 0, e = UTy1->getNumElements(); i != e; ++i) {
 231       if (!isEquivalentType(UTy1->getElementType(i), UTy2->getElementType(i)))
 232         return false;
 233     }
 234     return true;
 235   }
 236
 237   case Type::FunctionTyID: {
 238     const FunctionType *FTy1 = cast<FunctionType>(Ty1);
 239     const FunctionType *FTy2 = cast<FunctionType>(Ty2);
 240     if (FTy1->getNumParams() != FTy2->getNumParams() ||
 241         FTy1->isVarArg() != FTy2->isVarArg())
 242       return false;
 243
 244     if (!isEquivalentType(FTy1->getReturnType(), FTy2->getReturnType()))
 245       return false;
 246
 247     for (unsigned i = 0, e = FTy1->getNumParams(); i != e; ++i) {
 248       if (!isEquivalentType(FTy1->getParamType(i), FTy2->getParamType(i)))
 249         return false;
 250     }
 251     return true;
 252   }
 253
 254   case Type::ArrayTyID: {
 255     const ArrayType *ATy1 = cast<ArrayType>(Ty1);
 256     const ArrayType *ATy2 = cast<ArrayType>(Ty2);
 257     return ATy1->getNumElements() == ATy2->getNumElements() &&
 258            isEquivalentType(ATy1->getElementType(), ATy2->getElementType());
 259   }
 260
 261   case Type::VectorTyID: {
 262     const VectorType *VTy1 = cast<VectorType>(Ty1);
 263     const VectorType *VTy2 = cast<VectorType>(Ty2);
 264     return VTy1->getNumElements() == VTy2->getNumElements() &&
 265            isEquivalentType(VTy1->getElementType(), VTy2->getElementType());
 266   }
 267   }
 268 }
 269
 270 /// isEquivalentOperation - determine whether the two operations are the same
 271 /// except that pointer-to-A and pointer-to-B are equivalent. This should be
 272 /// kept in sync with Instruction::isSameOperationAs.
 273 bool FunctionComparator::isEquivalentOperation(const Instruction *I1,
 274                                                const Instruction *I2) const {
 275   if (I1->getOpcode() != I2->getOpcode() ||
 276       I1->getNumOperands() != I2->getNumOperands() ||
 277       !isEquivalentType(I1->getType(), I2->getType()) ||
 278       !I1->hasSameSubclassOptionalData(I2))
 279     return false;
 280
 281   // We have two instructions of identical opcode and #operands.  Check to see
 282   // if all operands are the same type
 283   for (unsigned i = 0, e = I1->getNumOperands(); i != e; ++i)
 284     if (!isEquivalentType(I1->getOperand(i)->getType(),
 285                           I2->getOperand(i)->getType()))
 286       return false;
 287
 288   // Check special state that is a part of some instructions.
 289   if (const LoadInst *LI = dyn_cast<LoadInst>(I1))
 290     return LI->isVolatile() == cast<LoadInst>(I2)->isVolatile() &&
 291            LI->getAlignment() == cast<LoadInst>(I2)->getAlignment();
 292   if (const StoreInst *SI = dyn_cast<StoreInst>(I1))
 293     return SI->isVolatile() == cast<StoreInst>(I2)->isVolatile() &&
 294            SI->getAlignment() == cast<StoreInst>(I2)->getAlignment();
 295   if (const CmpInst *CI = dyn_cast<CmpInst>(I1))
 296     return CI->getPredicate() == cast<CmpInst>(I2)->getPredicate();
 297   if (const CallInst *CI = dyn_cast<CallInst>(I1))
 298     return CI->isTailCall() == cast<CallInst>(I2)->isTailCall() &&
 299            CI->getCallingConv() == cast<CallInst>(I2)->getCallingConv() &&
 300            CI->getAttributes().getRawPointer() ==
 301              cast<CallInst>(I2)->getAttributes().getRawPointer();
 302   if (const InvokeInst *CI = dyn_cast<InvokeInst>(I1))
 303     return CI->getCallingConv() == cast<InvokeInst>(I2)->getCallingConv() &&
 304            CI->getAttributes().getRawPointer() ==
 305              cast<InvokeInst>(I2)->getAttributes().getRawPointer();
 306   if (const InsertValueInst *IVI = dyn_cast<InsertValueInst>(I1)) {
 307     if (IVI->getNumIndices() != cast<InsertValueInst>(I2)->getNumIndices())
 308       return false;
 309     for (unsigned i = 0, e = IVI->getNumIndices(); i != e; ++i)
 310       if (IVI->idx_begin()[i] != cast<InsertValueInst>(I2)->idx_begin()[i])
 311         return false;
 312     return true;
 313   }
 314   if (const ExtractValueInst *EVI = dyn_cast<ExtractValueInst>(I1)) {
 315     if (EVI->getNumIndices() != cast<ExtractValueInst>(I2)->getNumIndices())
 316       return false;
 317     for (unsigned i = 0, e = EVI->getNumIndices(); i != e; ++i)
 318       if (EVI->idx_begin()[i] != cast<ExtractValueInst>(I2)->idx_begin()[i])
 319         return false;
 320     return true;
 321   }
 322
 323   return true;
 324 }
 325
 326 /// isEquivalentGEP - determine whether two GEP operations perform the same
 327 /// underlying arithmetic.
 328 bool FunctionComparator::isEquivalentGEP(const GEPOperator *GEP1,
 329                                          const GEPOperator *GEP2) {
 330   // When we have target data, we can reduce the GEP down to the value in bytes
 331   // added to the address.
 332   if (TD && GEP1->hasAllConstantIndices() && GEP2->hasAllConstantIndices()) {
 333     SmallVector<Value *, 8> Indices1(GEP1->idx_begin(), GEP1->idx_end());
 334     SmallVector<Value *, 8> Indices2(GEP2->idx_begin(), GEP2->idx_end());
 335     uint64_t Offset1 = TD->getIndexedOffset(GEP1->getPointerOperandType(),
 336                                             Indices1.data(), Indices1.size());
 337     uint64_t Offset2 = TD->getIndexedOffset(GEP2->getPointerOperandType(),
 338                                             Indices2.data(), Indices2.size());
 339     return Offset1 == Offset2;
 340   }
 341
 342   if (GEP1->getPointerOperand()->getType() !=
 343       GEP2->getPointerOperand()->getType())
 344     return false;
 345
 346   if (GEP1->getNumOperands() != GEP2->getNumOperands())
 347     return false;
 348
 349   for (unsigned i = 0, e = GEP1->getNumOperands(); i != e; ++i) {
 350     if (!Enumerate(GEP1->getOperand(i), GEP2->getOperand(i)))
 351       return false;
 352   }
 353
 354   return true;
 355 }
 356
 357 /// Enumerate - Compare two values used by the two functions under pair-wise
 358 /// comparison. If this is the first time the values are seen, they're added to
 359 /// the mapping so that we will detect mismatches on next use.
 360 bool FunctionComparator::Enumerate(const Value *V1, const Value *V2) {
 361   // Check for function @f1 referring to itself and function @f2 referring to
 362   // itself, or referring to each other, or both referring to either of them.
 363   // They're all equivalent if the two functions are otherwise equivalent.
 364   if (V1 == F1 && V2 == F2)
 365     return true;
 366   if (V1 == F2 && V2 == F1)
 367     return true;
 368
 369   // TODO: constant expressions with GEP or references to F1 or F2.
 370   if (isa<Constant>(V1))
 371     return V1 == V2;
 372
 373   if (isa<InlineAsm>(V1) && isa<InlineAsm>(V2)) {
 374     const InlineAsm *IA1 = cast<InlineAsm>(V1);
 375     const InlineAsm *IA2 = cast<InlineAsm>(V2);
 376     return IA1->getAsmString() == IA2->getAsmString() &&
 377            IA1->getConstraintString() == IA2->getConstraintString();
 378   }
 379
 380   unsigned long &ID1 = Map1[V1];
 381   if (!ID1)
 382     ID1 = ++IDMap1Count;
 383
 384   unsigned long &ID2 = Map2[V2];
 385   if (!ID2)
 386     ID2 = ++IDMap2Count;
 387
 388   return ID1 == ID2;
 389 }
 390
 391 /// Compare - test whether two basic blocks have equivalent behaviour.
 392 bool FunctionComparator::Compare(const BasicBlock *BB1, const BasicBlock *BB2) {
 393   BasicBlock::const_iterator F1I = BB1->begin(), F1E = BB1->end();
 394   BasicBlock::const_iterator F2I = BB2->begin(), F2E = BB2->end();
 395
 396   do {
 397     if (!Enumerate(F1I, F2I))
 398       return false;
 399
 400     if (const GetElementPtrInst *GEP1 = dyn_cast<GetElementPtrInst>(F1I)) {
 401       const GetElementPtrInst *GEP2 = dyn_cast<GetElementPtrInst>(F2I);
 402       if (!GEP2)
 403         return false;
 404
 405       if (!Enumerate(GEP1->getPointerOperand(), GEP2->getPointerOperand()))
 406         return false;
 407
 408       if (!isEquivalentGEP(GEP1, GEP2))
 409         return false;
 410     } else {
 411       if (!isEquivalentOperation(F1I, F2I))
 412         return false;
 413
 414       assert(F1I->getNumOperands() == F2I->getNumOperands());
 415       for (unsigned i = 0, e = F1I->getNumOperands(); i != e; ++i) {
 416         Value *OpF1 = F1I->getOperand(i);
 417         Value *OpF2 = F2I->getOperand(i);
 418
 419         if (!Enumerate(OpF1, OpF2))
 420           return false;
 421
 422         if (OpF1->getValueID() != OpF2->getValueID() ||
 423             !isEquivalentType(OpF1->getType(), OpF2->getType()))
 424           return false;
 425       }
 426     }
 427
 428     ++F1I, ++F2I;
 429   } while (F1I != F1E && F2I != F2E);
 430
 431   return F1I == F1E && F2I == F2E;
 432 }
 433
 434 /// Compare - test whether the two functions have equivalent behaviour.
 435 bool FunctionComparator::Compare() {
 436   // We need to recheck everything, but check the things that weren't included
 437   // in the hash first.
 438
 439   if (F1->getAttributes() != F2->getAttributes())
 440     return false;
 441
 442   if (F1->hasGC() != F2->hasGC())
 443     return false;
 444
 445   if (F1->hasGC() && F1->getGC() != F2->getGC())
 446     return false;
 447
 448   if (F1->hasSection() != F2->hasSection())
 449     return false;
 450
 451   if (F1->hasSection() && F1->getSection() != F2->getSection())
 452     return false;
 453
 454   if (F1->isVarArg() != F2->isVarArg())
 455     return false;
 456
 457   // TODO: if it's internal and only used in direct calls, we could handle this
 458   // case too.
 459   if (F1->getCallingConv() != F2->getCallingConv())
 460     return false;
 461
 462   if (!isEquivalentType(F1->getFunctionType(), F2->getFunctionType()))
 463     return false;
 464
 465   assert(F1->arg_size() == F2->arg_size() &&
 466          "Identical functions have a different number of args.");
 467
 468   // Visit the arguments so that they get enumerated in the order they're
 469   // passed in.
 470   for (Function::const_arg_iterator f1i = F1->arg_begin(),
 471          f2i = F2->arg_begin(), f1e = F1->arg_end(); f1i != f1e; ++f1i, ++f2i) {
 472     if (!Enumerate(f1i, f2i))
 473       llvm_unreachable("Arguments repeat");
 474   }
 475
 476   // We do a CFG-ordered walk since the actual ordering of the blocks in the
 477   // linked list is immaterial. Our walk starts at the entry block for both
 478   // functions, then takes each block from each terminator in order. As an
 479   // artifact, this also means that unreachable blocks are ignored.
 480   SmallVector<const BasicBlock *, 8> F1BBs, F2BBs;
 481   SmallSet<const BasicBlock *, 128> VisitedBBs; // in terms of F1.
 482
 483   F1BBs.push_back(&F1->getEntryBlock());
 484   F2BBs.push_back(&F2->getEntryBlock());
 485
 486   VisitedBBs.insert(F1BBs[0]);
 487   while (!F1BBs.empty()) {
 488     const BasicBlock *F1BB = F1BBs.pop_back_val();
 489     const BasicBlock *F2BB = F2BBs.pop_back_val();
 490
 491     if (!Enumerate(F1BB, F2BB) || !Compare(F1BB, F2BB))
 492       return false;
 493
 494     const TerminatorInst *F1TI = F1BB->getTerminator();
 495     const TerminatorInst *F2TI = F2BB->getTerminator();
 496
 497     assert(F1TI->getNumSuccessors() == F2TI->getNumSuccessors());
 498     for (unsigned i = 0, e = F1TI->getNumSuccessors(); i != e; ++i) {
 499       if (!VisitedBBs.insert(F1TI->getSuccessor(i)))
 500         continue;
 501
 502       F1BBs.push_back(F1TI->getSuccessor(i));
 503       F2BBs.push_back(F2TI->getSuccessor(i));
 504     }
 505   }
 506   return true;
 507 }
 508
 509 /// WriteThunk - Replace G with a simple tail call to bitcast(F). Also replace
 510 /// direct uses of G with bitcast(F).
 511 void MergeFunctions::WriteThunk(Function *F, Function *G) const {
 512   if (!G->mayBeOverridden()) {
 513     // Redirect direct callers of G to F.
 514     Constant *BitcastF = ConstantExpr::getBitCast(F, G->getType());
 515     for (Value::use_iterator UI = G->use_begin(), UE = G->use_end();
 516          UI != UE;) {
 517       Value::use_iterator TheIter = UI;
 518       ++UI;
 519       CallSite CS(*TheIter);
 520       if (CS && CS.isCallee(TheIter))
 521         TheIter.getUse().set(BitcastF);
 522     }
 523   }
 524
 525   // If G was internal then we may have replaced all uses if G with F. If so,
 526   // stop here and delete G. There's no need for a thunk.
 527   if (G->hasLocalLinkage() && G->use_empty()) {
 528     G->eraseFromParent();
 529     return;
 530   }
 531
 532   Function *NewG = Function::Create(G->getFunctionType(), G->getLinkage(), "",
 533                                     G->getParent());
 534   BasicBlock *BB = BasicBlock::Create(F->getContext(), "", NewG);
 535   IRBuilder<false> Builder(BB);
 536
 537   SmallVector<Value *, 16> Args;
 538   unsigned i = 0;
 539   const FunctionType *FFTy = F->getFunctionType();
 540   for (Function::arg_iterator AI = NewG->arg_begin(), AE = NewG->arg_end();
 541        AI != AE; ++AI) {
 542     Args.push_back(Builder.CreateBitCast(AI, FFTy->getParamType(i)));
 543     ++i;
 544   }
 545
 546   CallInst *CI = Builder.CreateCall(F, Args.begin(), Args.end());
 547   CI->setTailCall();
 548   CI->setCallingConv(F->getCallingConv());
 549   if (NewG->getReturnType()->isVoidTy()) {
 550     Builder.CreateRetVoid();
 551   } else {
 552     Builder.CreateRet(Builder.CreateBitCast(CI, NewG->getReturnType()));
 553   }
 554
 555   NewG->copyAttributesFrom(G);
 556   NewG->takeName(G);
 557   G->replaceAllUsesWith(NewG);
 558   G->eraseFromParent();
 559 }
 560
 561 /// MergeTwoFunctions - Merge two equivalent functions. Upon completion,
 562 /// FnVec[j] should never be visited again.
 563 void MergeFunctions::MergeTwoFunctions(std::vector<Function *> &FnVec,
 564                                        unsigned i, unsigned j) const {
 565   Function *F = FnVec[i];
 566   Function *G = FnVec[j];
 567
 568   if (F->isWeakForLinker() && !G->isWeakForLinker()) {
 569     std::swap(FnVec[i], FnVec[j]);
 570     std::swap(F, G);
 571   }
 572
 573   if (F->isWeakForLinker()) {
 574     assert(G->isWeakForLinker());
 575
 576     // Make them both thunks to the same internal function.
 577     Function *H = Function::Create(F->getFunctionType(), F->getLinkage(), "",
 578                                    F->getParent());
 579     H->copyAttributesFrom(F);
 580     H->takeName(F);
 581     F->replaceAllUsesWith(H);
 582
 583     WriteThunk(F, G);
 584     WriteThunk(F, H);
 585
 586     F->setAlignment(std::max(G->getAlignment(), H->getAlignment()));
 587     F->setLinkage(GlobalValue::InternalLinkage);
 588   } else {
 589     WriteThunk(F, G);
 590   }
 591
 592   ++NumFunctionsMerged;
 593 }
 594
 595 /// PairwiseCompareAndMerge - Given a list of functions, compare each pair and
 596 /// merge the pairs of equivalent functions.
 597 bool MergeFunctions::PairwiseCompareAndMerge(std::vector<Function *> &FnVec) {
 598   bool Changed = false;
 599   for (int i = 0, e = FnVec.size(); i != e; ++i) {
 600     for (int j = i + 1; j != e; ++j) {
 601       bool isEqual = FunctionComparator(TD, FnVec[i], FnVec[j]).Compare();
 602
 603       DEBUG(dbgs() << "  " << FnVec[i]->getName()
 604             << (isEqual ? " == " : " != ") << FnVec[j]->getName() << "\n");
 605
 606       if (isEqual) {
 607         MergeTwoFunctions(FnVec, i, j);
 608         Changed = true;
 609         FnVec.erase(FnVec.begin() + j);
 610         --j, --e;
 611       }
 612     }
 613   }
 614   return Changed;
 615 }
 616
 617 bool MergeFunctions::runOnModule(Module &M) {
 618   bool Changed = false;
 619
 620   std::map<unsigned long, std::vector<Function *> > FnMap;
 621
 622   for (Module::iterator F = M.begin(), E = M.end(); F != E; ++F) {
 623     if (F->isDeclaration() || F->hasAvailableExternallyLinkage())
 624       continue;
 625
 626     FnMap[ProfileFunction(F)].push_back(F);
 627   }
 628
 629   TD = getAnalysisIfAvailable<TargetData>();
 630
 631   bool LocalChanged;
 632   do {
 633     LocalChanged = false;
 634     DEBUG(dbgs() << "size: " << FnMap.size() << "\n");
 635     for (std::map<unsigned long, std::vector<Function *> >::iterator
 636            I = FnMap.begin(), E = FnMap.end(); I != E; ++I) {
 637       std::vector<Function *> &FnVec = I->second;
 638       DEBUG(dbgs() << "hash (" << I->first << "): " << FnVec.size() << "\n");
 639       LocalChanged |= PairwiseCompareAndMerge(FnVec);
 640     }
 641     Changed |= LocalChanged;
 642   } while (LocalChanged);
 643
 644   return Changed;
 645 }