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