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/DenseSet.h"
  49 #include "llvm/ADT/FoldingSet.h"
  50 #include "llvm/ADT/SmallSet.h"
  51 #include "llvm/ADT/Statistic.h"
  52 #include "llvm/ADT/STLExtras.h"
  53 #include "llvm/Constants.h"
  54 #include "llvm/InlineAsm.h"
  55 #include "llvm/Instructions.h"
  56 #include "llvm/LLVMContext.h"
  57 #include "llvm/Module.h"
  58 #include "llvm/Operator.h"
  59 #include "llvm/Pass.h"
  60 #include "llvm/Support/CallSite.h"
  61 #include "llvm/Support/Debug.h"
  62 #include "llvm/Support/ErrorHandling.h"
  63 #include "llvm/Support/IRBuilder.h"
  64 #include "llvm/Support/ValueHandle.h"
  65 #include "llvm/Support/raw_ostream.h"
  66 #include "llvm/Target/TargetData.h"
  67 #include <vector>
  68 using namespace llvm;
  69
  70 STATISTIC(NumFunctionsMerged, "Number of functions merged");
  71 STATISTIC(NumThunksWritten, "Number of thunks generated");
  72 STATISTIC(NumAliasesWritten, "Number of aliases generated");
  73 STATISTIC(NumDoubleWeak, "Number of new functions created");
  74
  75 /// Creates a hash-code for the function which is the same for any two
  76 /// functions that will compare equal, without looking at the instructions
  77 /// inside the function.
  78 static unsigned profileFunction(const Function *F) {
  79   const FunctionType *FTy = F->getFunctionType();
  80
  81   FoldingSetNodeID ID;
  82   ID.AddInteger(F->size());
  83   ID.AddInteger(F->getCallingConv());
  84   ID.AddBoolean(F->hasGC());
  85   ID.AddBoolean(FTy->isVarArg());
  86   ID.AddInteger(FTy->getReturnType()->getTypeID());
  87   for (unsigned i = 0, e = FTy->getNumParams(); i != e; ++i)
  88     ID.AddInteger(FTy->getParamType(i)->getTypeID());
  89   return ID.ComputeHash();
  90 }
  91
  92 namespace {
  93
  94 /// ComparableFunction - A struct that pairs together functions with a
  95 /// TargetData so that we can keep them together as elements in the DenseSet.
  96 class ComparableFunction {
  97 public:
  98   static const ComparableFunction EmptyKey;
  99   static const ComparableFunction TombstoneKey;
 100   static TargetData * const LookupOnly;
 101
 102   ComparableFunction(Function *Func, TargetData *TD)
 103     : Func(Func), Hash(profileFunction(Func)), TD(TD) {}
 104
 105   Function *getFunc() const { return Func; }
 106   unsigned getHash() const { return Hash; }
 107   TargetData *getTD() const { return TD; }
 108
 109   // Drops AssertingVH reference to the function. Outside of debug mode, this
 110   // does nothing.
 111   void release() {
 112     assert(Func &&
 113            "Attempted to release function twice, or release empty/tombstone!");
 114     Func = NULL;
 115   }
 116
 117 private:
 118   explicit ComparableFunction(unsigned Hash)
 119     : Func(NULL), Hash(Hash), TD(NULL) {}
 120
 121   AssertingVH<Function> Func;
 122   unsigned Hash;
 123   TargetData *TD;
 124 };
 125
 126 const ComparableFunction ComparableFunction::EmptyKey = ComparableFunction(0);
 127 const ComparableFunction ComparableFunction::TombstoneKey =
 128     ComparableFunction(1);
 129 TargetData *const ComparableFunction::LookupOnly = (TargetData*)(-1);
 130
 131 }
 132
 133 namespace llvm {
 134   template <>
 135   struct DenseMapInfo<ComparableFunction> {
 136     static ComparableFunction getEmptyKey() {
 137       return ComparableFunction::EmptyKey;
 138     }
 139     static ComparableFunction getTombstoneKey() {
 140       return ComparableFunction::TombstoneKey;
 141     }
 142     static unsigned getHashValue(const ComparableFunction &CF) {
 143       return CF.getHash();
 144     }
 145     static bool isEqual(const ComparableFunction &LHS,
 146                         const ComparableFunction &RHS);
 147   };
 148 }
 149
 150 namespace {
 151
 152 /// FunctionComparator - Compares two functions to determine whether or not
 153 /// they will generate machine code with the same behaviour. TargetData is
 154 /// used if available. The comparator always fails conservatively (erring on the
 155 /// side of claiming that two functions are different).
 156 class FunctionComparator {
 157 public:
 158   FunctionComparator(const TargetData *TD, const Function *F1,
 159                      const Function *F2)
 160     : F1(F1), F2(F2), TD(TD) {}
 161
 162   /// Test whether the two functions have equivalent behaviour.
 163   bool compare();
 164
 165 private:
 166   /// Test whether two basic blocks have equivalent behaviour.
 167   bool compare(const BasicBlock *BB1, const BasicBlock *BB2);
 168
 169   /// Assign or look up previously assigned numbers for the two values, and
 170   /// return whether the numbers are equal. Numbers are assigned in the order
 171   /// visited.
 172   bool enumerate(const Value *V1, const Value *V2);
 173
 174   /// Compare two Instructions for equivalence, similar to
 175   /// Instruction::isSameOperationAs but with modifications to the type
 176   /// comparison.
 177   bool isEquivalentOperation(const Instruction *I1,
 178                              const Instruction *I2) const;
 179
 180   /// Compare two GEPs for equivalent pointer arithmetic.
 181   bool isEquivalentGEP(const GEPOperator *GEP1, const GEPOperator *GEP2);
 182   bool isEquivalentGEP(const GetElementPtrInst *GEP1,
 183                        const GetElementPtrInst *GEP2) {
 184     return isEquivalentGEP(cast<GEPOperator>(GEP1), cast<GEPOperator>(GEP2));
 185   }
 186
 187   /// Compare two Types, treating all pointer types as equal.
 188   bool isEquivalentType(const Type *Ty1, const Type *Ty2) const;
 189
 190   // The two functions undergoing comparison.
 191   const Function *F1, *F2;
 192
 193   const TargetData *TD;
 194
 195   DenseMap<const Value *, const Value *> id_map;
 196   DenseSet<const Value *> seen_values;
 197 };
 198
 199 }
 200
 201 // Any two pointers in the same address space are equivalent, intptr_t and
 202 // pointers are equivalent. Otherwise, standard type equivalence rules apply.
 203 bool FunctionComparator::isEquivalentType(const Type *Ty1,
 204                                           const Type *Ty2) const {
 205   if (Ty1 == Ty2)
 206     return true;
 207   if (Ty1->getTypeID() != Ty2->getTypeID()) {
 208     if (TD) {
 209       LLVMContext &Ctx = Ty1->getContext();
 210       if (isa<PointerType>(Ty1) && Ty2 == TD->getIntPtrType(Ctx)) return true;
 211       if (isa<PointerType>(Ty2) && Ty1 == TD->getIntPtrType(Ctx)) return true;
 212     }
 213     return false;
 214   }
 215
 216   switch (Ty1->getTypeID()) {
 217   default:
 218     llvm_unreachable("Unknown type!");
 219     // Fall through in Release mode.
 220   case Type::IntegerTyID:
 221   case Type::OpaqueTyID:
 222   case Type::VectorTyID:
 223     // Ty1 == Ty2 would have returned true earlier.
 224     return false;
 225
 226   case Type::VoidTyID:
 227   case Type::FloatTyID:
 228   case Type::DoubleTyID:
 229   case Type::X86_FP80TyID:
 230   case Type::FP128TyID:
 231   case Type::PPC_FP128TyID:
 232   case Type::LabelTyID:
 233   case Type::MetadataTyID:
 234     return true;
 235
 236   case Type::PointerTyID: {
 237     const PointerType *PTy1 = cast<PointerType>(Ty1);
 238     const PointerType *PTy2 = cast<PointerType>(Ty2);
 239     return PTy1->getAddressSpace() == PTy2->getAddressSpace();
 240   }
 241
 242   case Type::StructTyID: {
 243     const StructType *STy1 = cast<StructType>(Ty1);
 244     const StructType *STy2 = cast<StructType>(Ty2);
 245     if (STy1->getNumElements() != STy2->getNumElements())
 246       return false;
 247
 248     if (STy1->isPacked() != STy2->isPacked())
 249       return false;
 250
 251     for (unsigned i = 0, e = STy1->getNumElements(); i != e; ++i) {
 252       if (!isEquivalentType(STy1->getElementType(i), STy2->getElementType(i)))
 253         return false;
 254     }
 255     return true;
 256   }
 257
 258   case Type::FunctionTyID: {
 259     const FunctionType *FTy1 = cast<FunctionType>(Ty1);
 260     const FunctionType *FTy2 = cast<FunctionType>(Ty2);
 261     if (FTy1->getNumParams() != FTy2->getNumParams() ||
 262         FTy1->isVarArg() != FTy2->isVarArg())
 263       return false;
 264
 265     if (!isEquivalentType(FTy1->getReturnType(), FTy2->getReturnType()))
 266       return false;
 267
 268     for (unsigned i = 0, e = FTy1->getNumParams(); i != e; ++i) {
 269       if (!isEquivalentType(FTy1->getParamType(i), FTy2->getParamType(i)))
 270         return false;
 271     }
 272     return true;
 273   }
 274
 275   case Type::ArrayTyID: {
 276     const ArrayType *ATy1 = cast<ArrayType>(Ty1);
 277     const ArrayType *ATy2 = cast<ArrayType>(Ty2);
 278     return ATy1->getNumElements() == ATy2->getNumElements() &&
 279            isEquivalentType(ATy1->getElementType(), ATy2->getElementType());
 280   }
 281   }
 282 }
 283
 284 // Determine whether the two operations are the same except that pointer-to-A
 285 // and pointer-to-B are equivalent. This should be kept in sync with
 286 // Instruction::isSameOperationAs.
 287 bool FunctionComparator::isEquivalentOperation(const Instruction *I1,
 288                                                const Instruction *I2) const {
 289   // Differences from Instruction::isSameOperationAs:
 290   //  * replace type comparison with calls to isEquivalentType.
 291   //  * we test for I->hasSameSubclassOptionalData (nuw/nsw/tail) at the top
 292   //  * because of the above, we don't test for the tail bit on calls later on
 293   if (I1->getOpcode() != I2->getOpcode() ||
 294       I1->getNumOperands() != I2->getNumOperands() ||
 295       !isEquivalentType(I1->getType(), I2->getType()) ||
 296       !I1->hasSameSubclassOptionalData(I2))
 297     return false;
 298
 299   // We have two instructions of identical opcode and #operands.  Check to see
 300   // if all operands are the same type
 301   for (unsigned i = 0, e = I1->getNumOperands(); i != e; ++i)
 302     if (!isEquivalentType(I1->getOperand(i)->getType(),
 303                           I2->getOperand(i)->getType()))
 304       return false;
 305
 306   // Check special state that is a part of some instructions.
 307   if (const LoadInst *LI = dyn_cast<LoadInst>(I1))
 308     return LI->isVolatile() == cast<LoadInst>(I2)->isVolatile() &&
 309            LI->getAlignment() == cast<LoadInst>(I2)->getAlignment();
 310   if (const StoreInst *SI = dyn_cast<StoreInst>(I1))
 311     return SI->isVolatile() == cast<StoreInst>(I2)->isVolatile() &&
 312            SI->getAlignment() == cast<StoreInst>(I2)->getAlignment();
 313   if (const CmpInst *CI = dyn_cast<CmpInst>(I1))
 314     return CI->getPredicate() == cast<CmpInst>(I2)->getPredicate();
 315   if (const CallInst *CI = dyn_cast<CallInst>(I1))
 316     return CI->getCallingConv() == cast<CallInst>(I2)->getCallingConv() &&
 317            CI->getAttributes() == cast<CallInst>(I2)->getAttributes();
 318   if (const InvokeInst *CI = dyn_cast<InvokeInst>(I1))
 319     return CI->getCallingConv() == cast<InvokeInst>(I2)->getCallingConv() &&
 320            CI->getAttributes() == cast<InvokeInst>(I2)->getAttributes();
 321   if (const InsertValueInst *IVI = dyn_cast<InsertValueInst>(I1)) {
 322     if (IVI->getNumIndices() != cast<InsertValueInst>(I2)->getNumIndices())
 323       return false;
 324     for (unsigned i = 0, e = IVI->getNumIndices(); i != e; ++i)
 325       if (IVI->idx_begin()[i] != cast<InsertValueInst>(I2)->idx_begin()[i])
 326         return false;
 327     return true;
 328   }
 329   if (const ExtractValueInst *EVI = dyn_cast<ExtractValueInst>(I1)) {
 330     if (EVI->getNumIndices() != cast<ExtractValueInst>(I2)->getNumIndices())
 331       return false;
 332     for (unsigned i = 0, e = EVI->getNumIndices(); i != e; ++i)
 333       if (EVI->idx_begin()[i] != cast<ExtractValueInst>(I2)->idx_begin()[i])
 334         return false;
 335     return true;
 336   }
 337
 338   return true;
 339 }
 340
 341 // Determine whether two GEP operations perform the same underlying arithmetic.
 342 bool FunctionComparator::isEquivalentGEP(const GEPOperator *GEP1,
 343                                          const GEPOperator *GEP2) {
 344   // When we have target data, we can reduce the GEP down to the value in bytes
 345   // added to the address.
 346   if (TD && GEP1->hasAllConstantIndices() && GEP2->hasAllConstantIndices()) {
 347     SmallVector<Value *, 8> Indices1(GEP1->idx_begin(), GEP1->idx_end());
 348     SmallVector<Value *, 8> Indices2(GEP2->idx_begin(), GEP2->idx_end());
 349     uint64_t Offset1 = TD->getIndexedOffset(GEP1->getPointerOperandType(),
 350                                             Indices1.data(), Indices1.size());
 351     uint64_t Offset2 = TD->getIndexedOffset(GEP2->getPointerOperandType(),
 352                                             Indices2.data(), Indices2.size());
 353     return Offset1 == Offset2;
 354   }
 355
 356   if (GEP1->getPointerOperand()->getType() !=
 357       GEP2->getPointerOperand()->getType())
 358     return false;
 359
 360   if (GEP1->getNumOperands() != GEP2->getNumOperands())
 361     return false;
 362
 363   for (unsigned i = 0, e = GEP1->getNumOperands(); i != e; ++i) {
 364     if (!enumerate(GEP1->getOperand(i), GEP2->getOperand(i)))
 365       return false;
 366   }
 367
 368   return true;
 369 }
 370
 371 // Compare two values used by the two functions under pair-wise comparison. If
 372 // this is the first time the values are seen, they're added to the mapping so
 373 // that we will detect mismatches on next use.
 374 bool FunctionComparator::enumerate(const Value *V1, const Value *V2) {
 375   // Check for function @f1 referring to itself and function @f2 referring to
 376   // itself, or referring to each other, or both referring to either of them.
 377   // They're all equivalent if the two functions are otherwise equivalent.
 378   if (V1 == F1 && V2 == F2)
 379     return true;
 380   if (V1 == F2 && V2 == F1)
 381     return true;
 382
 383   if (const Constant *C1 = dyn_cast<Constant>(V1)) {
 384     if (V1 == V2) return true;
 385     const Constant *C2 = dyn_cast<Constant>(V2);
 386     if (!C2) return false;
 387     // TODO: constant expressions with GEP or references to F1 or F2.
 388     if (C1->isNullValue() && C2->isNullValue() &&
 389         isEquivalentType(C1->getType(), C2->getType()))
 390       return true;
 391     // Try bitcasting C2 to C1's type. If the bitcast is legal and returns C1
 392     // then they must have equal bit patterns.
 393     return C1->getType()->canLosslesslyBitCastTo(C2->getType()) &&
 394       C1 == ConstantExpr::getBitCast(const_cast<Constant*>(C2), C1->getType());
 395   }
 396
 397   if (isa<InlineAsm>(V1) || isa<InlineAsm>(V2))
 398     return V1 == V2;
 399
 400   // Check that V1 maps to V2. If we find a value that V1 maps to then we simply
 401   // check whether it's equal to V2. When there is no mapping then we need to
 402   // ensure that V2 isn't already equivalent to something else. For this
 403   // purpose, we track the V2 values in a set.
 404
 405   const Value *&map_elem = id_map[V1];
 406   if (map_elem)
 407     return map_elem == V2;
 408   if (!seen_values.insert(V2).second)
 409     return false;
 410   map_elem = V2;
 411   return true;
 412 }
 413
 414 // Test whether two basic blocks have equivalent behaviour.
 415 bool FunctionComparator::compare(const BasicBlock *BB1, const BasicBlock *BB2) {
 416   BasicBlock::const_iterator F1I = BB1->begin(), F1E = BB1->end();
 417   BasicBlock::const_iterator F2I = BB2->begin(), F2E = BB2->end();
 418
 419   do {
 420     if (!enumerate(F1I, F2I))
 421       return false;
 422
 423     if (const GetElementPtrInst *GEP1 = dyn_cast<GetElementPtrInst>(F1I)) {
 424       const GetElementPtrInst *GEP2 = dyn_cast<GetElementPtrInst>(F2I);
 425       if (!GEP2)
 426         return false;
 427
 428       if (!enumerate(GEP1->getPointerOperand(), GEP2->getPointerOperand()))
 429         return false;
 430
 431       if (!isEquivalentGEP(GEP1, GEP2))
 432         return false;
 433     } else {
 434       if (!isEquivalentOperation(F1I, F2I))
 435         return false;
 436
 437       assert(F1I->getNumOperands() == F2I->getNumOperands());
 438       for (unsigned i = 0, e = F1I->getNumOperands(); i != e; ++i) {
 439         Value *OpF1 = F1I->getOperand(i);
 440         Value *OpF2 = F2I->getOperand(i);
 441
 442         if (!enumerate(OpF1, OpF2))
 443           return false;
 444
 445         if (OpF1->getValueID() != OpF2->getValueID() ||
 446             !isEquivalentType(OpF1->getType(), OpF2->getType()))
 447           return false;
 448       }
 449     }
 450
 451     ++F1I, ++F2I;
 452   } while (F1I != F1E && F2I != F2E);
 453
 454   return F1I == F1E && F2I == F2E;
 455 }
 456
 457 // Test whether the two functions have equivalent behaviour.
 458 bool FunctionComparator::compare() {
 459   // We need to recheck everything, but check the things that weren't included
 460   // in the hash first.
 461
 462   if (F1->getAttributes() != F2->getAttributes())
 463     return false;
 464
 465   if (F1->hasGC() != F2->hasGC())
 466     return false;
 467
 468   if (F1->hasGC() && F1->getGC() != F2->getGC())
 469     return false;
 470
 471   if (F1->hasSection() != F2->hasSection())
 472     return false;
 473
 474   if (F1->hasSection() && F1->getSection() != F2->getSection())
 475     return false;
 476
 477   if (F1->isVarArg() != F2->isVarArg())
 478     return false;
 479
 480   // TODO: if it's internal and only used in direct calls, we could handle this
 481   // case too.
 482   if (F1->getCallingConv() != F2->getCallingConv())
 483     return false;
 484
 485   if (!isEquivalentType(F1->getFunctionType(), F2->getFunctionType()))
 486     return false;
 487
 488   assert(F1->arg_size() == F2->arg_size() &&
 489          "Identically typed functions have different numbers of args!");
 490
 491   // Visit the arguments so that they get enumerated in the order they're
 492   // passed in.
 493   for (Function::const_arg_iterator f1i = F1->arg_begin(),
 494          f2i = F2->arg_begin(), f1e = F1->arg_end(); f1i != f1e; ++f1i, ++f2i) {
 495     if (!enumerate(f1i, f2i))
 496       llvm_unreachable("Arguments repeat!");
 497   }
 498
 499   // We do a CFG-ordered walk since the actual ordering of the blocks in the
 500   // linked list is immaterial. Our walk starts at the entry block for both
 501   // functions, then takes each block from each terminator in order. As an
 502   // artifact, this also means that unreachable blocks are ignored.
 503   SmallVector<const BasicBlock *, 8> F1BBs, F2BBs;
 504   SmallSet<const BasicBlock *, 128> VisitedBBs; // in terms of F1.
 505
 506   F1BBs.push_back(&F1->getEntryBlock());
 507   F2BBs.push_back(&F2->getEntryBlock());
 508
 509   VisitedBBs.insert(F1BBs[0]);
 510   while (!F1BBs.empty()) {
 511     const BasicBlock *F1BB = F1BBs.pop_back_val();
 512     const BasicBlock *F2BB = F2BBs.pop_back_val();
 513
 514     if (!enumerate(F1BB, F2BB) || !compare(F1BB, F2BB))
 515       return false;
 516
 517     const TerminatorInst *F1TI = F1BB->getTerminator();
 518     const TerminatorInst *F2TI = F2BB->getTerminator();
 519
 520     assert(F1TI->getNumSuccessors() == F2TI->getNumSuccessors());
 521     for (unsigned i = 0, e = F1TI->getNumSuccessors(); i != e; ++i) {
 522       if (!VisitedBBs.insert(F1TI->getSuccessor(i)))
 523         continue;
 524
 525       F1BBs.push_back(F1TI->getSuccessor(i));
 526       F2BBs.push_back(F2TI->getSuccessor(i));
 527     }
 528   }
 529   return true;
 530 }
 531
 532 namespace {
 533
 534 /// MergeFunctions finds functions which will generate identical machine code,
 535 /// by considering all pointer types to be equivalent. Once identified,
 536 /// MergeFunctions will fold them by replacing a call to one to a call to a
 537 /// bitcast of the other.
 538 ///
 539 class MergeFunctions : public ModulePass {
 540 public:
 541   static char ID;
 542   MergeFunctions()
 543     : ModulePass(ID), HasGlobalAliases(false) {
 544     initializeMergeFunctionsPass(*PassRegistry::getPassRegistry());
 545   }
 546
 547   bool runOnModule(Module &M);
 548
 549 private:
 550   typedef DenseSet<ComparableFunction> FnSetType;
 551
 552   /// A work queue of functions that may have been modified and should be
 553   /// analyzed again.
 554   std::vector<WeakVH> Deferred;
 555
 556   /// Insert a ComparableFunction into the FnSet, or merge it away if it's
 557   /// equal to one that's already present.
 558   bool insert(ComparableFunction &NewF);
 559
 560   /// Remove a Function from the FnSet and queue it up for a second sweep of
 561   /// analysis.
 562   void remove(Function *F);
 563
 564   /// Find the functions that use this Value and remove them from FnSet and
 565   /// queue the functions.
 566   void removeUsers(Value *V);
 567
 568   /// Replace all direct calls of Old with calls of New. Will bitcast New if
 569   /// necessary to make types match.
 570   void replaceDirectCallers(Function *Old, Function *New);
 571
 572   /// Merge two equivalent functions. Upon completion, G may be deleted, or may
 573   /// be converted into a thunk. In either case, it should never be visited
 574   /// again.
 575   void mergeTwoFunctions(Function *F, Function *G);
 576
 577   /// Replace G with a thunk or an alias to F. Deletes G.
 578   void writeThunkOrAlias(Function *F, Function *G);
 579
 580   /// Replace G with a simple tail call to bitcast(F). Also replace direct uses
 581   /// of G with bitcast(F). Deletes G.
 582   void writeThunk(Function *F, Function *G);
 583
 584   /// Replace G with an alias to F. Deletes G.
 585   void writeAlias(Function *F, Function *G);
 586
 587   /// The set of all distinct functions. Use the insert() and remove() methods
 588   /// to modify it.
 589   FnSetType FnSet;
 590
 591   /// TargetData for more accurate GEP comparisons. May be NULL.
 592   TargetData *TD;
 593
 594   /// Whether or not the target supports global aliases.
 595   bool HasGlobalAliases;
 596 };
 597
 598 }  // end anonymous namespace
 599
 600 char MergeFunctions::ID = 0;
 601 INITIALIZE_PASS(MergeFunctions, "mergefunc", "Merge Functions", false, false)
 602
 603 ModulePass *llvm::createMergeFunctionsPass() {
 604   return new MergeFunctions();
 605 }
 606
 607 bool MergeFunctions::runOnModule(Module &M) {
 608   bool Changed = false;
 609   TD = getAnalysisIfAvailable<TargetData>();
 610
 611   for (Module::iterator I = M.begin(), E = M.end(); I != E; ++I) {
 612     if (!I->isDeclaration() && !I->hasAvailableExternallyLinkage())
 613       Deferred.push_back(WeakVH(I));
 614   }
 615   FnSet.resize(Deferred.size());
 616
 617   do {
 618     std::vector<WeakVH> Worklist;
 619     Deferred.swap(Worklist);
 620
 621     DEBUG(dbgs() << "size of module: " << M.size() << '\n');
 622     DEBUG(dbgs() << "size of worklist: " << Worklist.size() << '\n');
 623
 624     // Insert only strong functions and merge them. Strong function merging
 625     // always deletes one of them.
 626     for (std::vector<WeakVH>::iterator I = Worklist.begin(),
 627            E = Worklist.end(); I != E; ++I) {
 628       if (!*I) continue;
 629       Function *F = cast<Function>(*I);
 630       if (!F->isDeclaration() && !F->hasAvailableExternallyLinkage() &&
 631           !F->mayBeOverridden()) {
 632         ComparableFunction CF = ComparableFunction(F, TD);
 633         Changed |= insert(CF);
 634       }
 635     }
 636
 637     // Insert only weak functions and merge them. By doing these second we
 638     // create thunks to the strong function when possible. When two weak
 639     // functions are identical, we create a new strong function with two weak
 640     // weak thunks to it which are identical but not mergable.
 641     for (std::vector<WeakVH>::iterator I = Worklist.begin(),
 642            E = Worklist.end(); I != E; ++I) {
 643       if (!*I) continue;
 644       Function *F = cast<Function>(*I);
 645       if (!F->isDeclaration() && !F->hasAvailableExternallyLinkage() &&
 646           F->mayBeOverridden()) {
 647         ComparableFunction CF = ComparableFunction(F, TD);
 648         Changed |= insert(CF);
 649       }
 650     }
 651     DEBUG(dbgs() << "size of FnSet: " << FnSet.size() << '\n');
 652   } while (!Deferred.empty());
 653
 654   FnSet.clear();
 655
 656   return Changed;
 657 }
 658
 659 bool DenseMapInfo<ComparableFunction>::isEqual(const ComparableFunction &LHS,
 660                                                const ComparableFunction &RHS) {
 661   if (LHS.getFunc() == RHS.getFunc() &&
 662       LHS.getHash() == RHS.getHash())
 663     return true;
 664   if (!LHS.getFunc() || !RHS.getFunc())
 665     return false;
 666
 667   // One of these is a special "underlying pointer comparison only" object.
 668   if (LHS.getTD() == ComparableFunction::LookupOnly ||
 669       RHS.getTD() == ComparableFunction::LookupOnly)
 670     return false;
 671
 672   assert(LHS.getTD() == RHS.getTD() &&
 673          "Comparing functions for different targets");
 674
 675   return FunctionComparator(LHS.getTD(), LHS.getFunc(),
 676                             RHS.getFunc()).compare();
 677 }
 678
 679 // Replace direct callers of Old with New.
 680 void MergeFunctions::replaceDirectCallers(Function *Old, Function *New) {
 681   Constant *BitcastNew = ConstantExpr::getBitCast(New, Old->getType());
 682   for (Value::use_iterator UI = Old->use_begin(), UE = Old->use_end();
 683        UI != UE;) {
 684     Value::use_iterator TheIter = UI;
 685     ++UI;
 686     CallSite CS(*TheIter);
 687     if (CS && CS.isCallee(TheIter)) {
 688       remove(CS.getInstruction()->getParent()->getParent());
 689       TheIter.getUse().set(BitcastNew);
 690     }
 691   }
 692 }
 693
 694 // Replace G with an alias to F if possible, or else a thunk to F. Deletes G.
 695 void MergeFunctions::writeThunkOrAlias(Function *F, Function *G) {
 696   if (HasGlobalAliases && G->hasUnnamedAddr()) {
 697     if (G->hasExternalLinkage() || G->hasLocalLinkage() ||
 698         G->hasWeakLinkage()) {
 699       writeAlias(F, G);
 700       return;
 701     }
 702   }
 703
 704   writeThunk(F, G);
 705 }
 706
 707 // Replace G with a simple tail call to bitcast(F). Also replace direct uses
 708 // of G with bitcast(F). Deletes G.
 709 void MergeFunctions::writeThunk(Function *F, Function *G) {
 710   if (!G->mayBeOverridden()) {
 711     // Redirect direct callers of G to F.
 712     replaceDirectCallers(G, F);
 713   }
 714
 715   // If G was internal then we may have replaced all uses of G with F. If so,
 716   // stop here and delete G. There's no need for a thunk.
 717   if (G->hasLocalLinkage() && G->use_empty()) {
 718     G->eraseFromParent();
 719     return;
 720   }
 721
 722   Function *NewG = Function::Create(G->getFunctionType(), G->getLinkage(), "",
 723                                     G->getParent());
 724   BasicBlock *BB = BasicBlock::Create(F->getContext(), "", NewG);
 725   IRBuilder<false> Builder(BB);
 726
 727   SmallVector<Value *, 16> Args;
 728   unsigned i = 0;
 729   const FunctionType *FFTy = F->getFunctionType();
 730   for (Function::arg_iterator AI = NewG->arg_begin(), AE = NewG->arg_end();
 731        AI != AE; ++AI) {
 732     Args.push_back(Builder.CreateBitCast(AI, FFTy->getParamType(i)));
 733     ++i;
 734   }
 735
 736   CallInst *CI = Builder.CreateCall(F, Args.begin(), Args.end());
 737   CI->setTailCall();
 738   CI->setCallingConv(F->getCallingConv());
 739   if (NewG->getReturnType()->isVoidTy()) {
 740     Builder.CreateRetVoid();
 741   } else {
 742     Builder.CreateRet(Builder.CreateBitCast(CI, NewG->getReturnType()));
 743   }
 744
 745   NewG->copyAttributesFrom(G);
 746   NewG->takeName(G);
 747   removeUsers(G);
 748   G->replaceAllUsesWith(NewG);
 749   G->eraseFromParent();
 750
 751   DEBUG(dbgs() << "writeThunk: " << NewG->getName() << '\n');
 752   ++NumThunksWritten;
 753 }
 754
 755 // Replace G with an alias to F and delete G.
 756 void MergeFunctions::writeAlias(Function *F, Function *G) {
 757   Constant *BitcastF = ConstantExpr::getBitCast(F, G->getType());
 758   GlobalAlias *GA = new GlobalAlias(G->getType(), G->getLinkage(), "",
 759                                     BitcastF, G->getParent());
 760   F->setAlignment(std::max(F->getAlignment(), G->getAlignment()));
 761   GA->takeName(G);
 762   GA->setVisibility(G->getVisibility());
 763   removeUsers(G);
 764   G->replaceAllUsesWith(GA);
 765   G->eraseFromParent();
 766
 767   DEBUG(dbgs() << "writeAlias: " << GA->getName() << '\n');
 768   ++NumAliasesWritten;
 769 }
 770
 771 // Merge two equivalent functions. Upon completion, Function G is deleted.
 772 void MergeFunctions::mergeTwoFunctions(Function *F, Function *G) {
 773   if (F->mayBeOverridden()) {
 774     assert(G->mayBeOverridden());
 775
 776     if (HasGlobalAliases) {
 777       // Make them both thunks to the same internal function.
 778       Function *H = Function::Create(F->getFunctionType(), F->getLinkage(), "",
 779                                      F->getParent());
 780       H->copyAttributesFrom(F);
 781       H->takeName(F);
 782       removeUsers(F);
 783       F->replaceAllUsesWith(H);
 784
 785       unsigned MaxAlignment = std::max(G->getAlignment(), H->getAlignment());
 786
 787       writeAlias(F, G);
 788       writeAlias(F, H);
 789
 790       F->setAlignment(MaxAlignment);
 791       F->setLinkage(GlobalValue::PrivateLinkage);
 792     } else {
 793       // We can't merge them. Instead, pick one and update all direct callers
 794       // to call it and hope that we improve the instruction cache hit rate.
 795       replaceDirectCallers(G, F);
 796     }
 797
 798     ++NumDoubleWeak;
 799   } else {
 800     writeThunkOrAlias(F, G);
 801   }
 802
 803   ++NumFunctionsMerged;
 804 }
 805
 806 // Insert a ComparableFunction into the FnSet, or merge it away if equal to one
 807 // that was already inserted.
 808 bool MergeFunctions::insert(ComparableFunction &NewF) {
 809   std::pair<FnSetType::iterator, bool> Result = FnSet.insert(NewF);
 810   if (Result.second) {
 811     DEBUG(dbgs() << "Inserting as unique: " << NewF.getFunc()->getName() << '\n');
 812     return false;
 813   }
 814
 815   const ComparableFunction &OldF = *Result.first;
 816
 817   // Never thunk a strong function to a weak function.
 818   assert(!OldF.getFunc()->mayBeOverridden() ||
 819          NewF.getFunc()->mayBeOverridden());
 820
 821   DEBUG(dbgs() << "  " << OldF.getFunc()->getName() << " == "
 822                << NewF.getFunc()->getName() << '\n');
 823
 824   Function *DeleteF = NewF.getFunc();
 825   NewF.release();
 826   mergeTwoFunctions(OldF.getFunc(), DeleteF);
 827   return true;
 828 }
 829
 830 // Remove a function from FnSet. If it was already in FnSet, add it to Deferred
 831 // so that we'll look at it in the next round.
 832 void MergeFunctions::remove(Function *F) {
 833   // We need to make sure we remove F, not a function "equal" to F per the
 834   // function equality comparator.
 835   //
 836   // The special "lookup only" ComparableFunction bypasses the expensive
 837   // function comparison in favour of a pointer comparison on the underlying
 838   // Function*'s.
 839   ComparableFunction CF = ComparableFunction(F, ComparableFunction::LookupOnly);
 840   if (FnSet.erase(CF)) {
 841     DEBUG(dbgs() << "Removed " << F->getName() << " from set and deferred it.\n");
 842     Deferred.push_back(F);
 843   }
 844 }
 845
 846 // For each instruction used by the value, remove() the function that contains
 847 // the instruction. This should happen right before a call to RAUW.
 848 void MergeFunctions::removeUsers(Value *V) {
 849   std::vector<Value *> Worklist;
 850   Worklist.push_back(V);
 851   while (!Worklist.empty()) {
 852     Value *V = Worklist.back();
 853     Worklist.pop_back();
 854
 855     for (Value::use_iterator UI = V->use_begin(), UE = V->use_end();
 856          UI != UE; ++UI) {
 857       Use &U = UI.getUse();
 858       if (Instruction *I = dyn_cast<Instruction>(U.getUser())) {
 859         remove(I->getParent()->getParent());
 860       } else if (isa<GlobalValue>(U.getUser())) {
 861         // do nothing
 862       } else if (Constant *C = dyn_cast<Constant>(U.getUser())) {
 863         for (Value::use_iterator CUI = C->use_begin(), CUE = C->use_end();
 864              CUI != CUE; ++CUI)
 865           Worklist.push_back(*CUI);
 866       }
 867     }
 868   }
 869 }