C++ GlobalVariable::use_empty方法代码示例

/// SplitStaticCtorDtor - A module was recently split into two parts, M1/M2, and
/// M1 has all of the global variables.  If M2 contains any functions that are
/// static ctors/dtors, we need to add an llvm.global_[cd]tors global to M2, and
/// prune appropriate entries out of M1s list.
static void SplitStaticCtorDtor(const char *GlobalName, Module *M1, Module *M2,
                                DenseMap<const Value*, Value*> ValueMap) {
  GlobalVariable *GV = M1->getNamedGlobal(GlobalName);
  if (!GV || GV->isDeclaration() || GV->hasLocalLinkage() ||
      !GV->use_empty()) return;
  
  std::vector<std::pair<Function*, int> > M1Tors, M2Tors;
  ConstantArray *InitList = dyn_cast<ConstantArray>(GV->getInitializer());
  if (!InitList) return;
  
  for (unsigned i = 0, e = InitList->getNumOperands(); i != e; ++i) {
    if (ConstantStruct *CS = dyn_cast<ConstantStruct>(InitList->getOperand(i))){
      if (CS->getNumOperands() != 2) return;  // Not array of 2-element structs.
      
      if (CS->getOperand(1)->isNullValue())
        break;  // Found a null terminator, stop here.
      
      ConstantInt *CI = dyn_cast<ConstantInt>(CS->getOperand(0));
      int Priority = CI ? CI->getSExtValue() : 0;
      
      Constant *FP = CS->getOperand(1);
      if (ConstantExpr *CE = dyn_cast<ConstantExpr>(FP))
        if (CE->isCast())
          FP = CE->getOperand(0);
      if (Function *F = dyn_cast<Function>(FP)) {
        if (!F->isDeclaration())
          M1Tors.push_back(std::make_pair(F, Priority));
        else {
          // Map to M2's version of the function.
          F = cast<Function>(ValueMap[F]);
          M2Tors.push_back(std::make_pair(F, Priority));
        }
      }
    }
  }
  
  GV->eraseFromParent();
  if (!M1Tors.empty()) {
    Constant *M1Init = GetTorInit(M1Tors);
    new GlobalVariable(*M1, M1Init->getType(), false,
                       GlobalValue::AppendingLinkage,
                       M1Init, GlobalName);
  }

  GV = M2->getNamedGlobal(GlobalName);
  assert(GV && "Not a clone of M1?");
  assert(GV->use_empty() && "llvm.ctors shouldn't have uses!");

  GV->eraseFromParent();
  if (!M2Tors.empty()) {
    Constant *M2Init = GetTorInit(M2Tors);
    new GlobalVariable(*M2, M2Init->getType(), false,
                       GlobalValue::AppendingLinkage,
                       M2Init, GlobalName);
  }
}

bool StripDeadPrototypesPass::runOnModule(Module &M) {
  bool MadeChange = false;
  
  // Erase dead function prototypes.
  for (Module::iterator I = M.begin(), E = M.end(); I != E; ) {
    Function *F = I++;
    // Function must be a prototype and unused.
    if (F->isDeclaration() && F->use_empty()) {
      F->eraseFromParent();
      ++NumDeadPrototypes;
      MadeChange = true;
    }
  }

  // Erase dead global var prototypes.
  for (Module::global_iterator I = M.global_begin(), E = M.global_end();
       I != E; ) {
    GlobalVariable *GV = I++;
    // Global must be a prototype and unused.
    if (GV->isDeclaration() && GV->use_empty())
      GV->eraseFromParent();
  }
  
  // Return an indication of whether we changed anything or not.
  return MadeChange;
}

static void rewriteTlsVars(Module &M, std::vector<VarInfo> *TlsVars,
                           PointerType *TemplatePtrType) {
  // Set up the intrinsic that reads the thread pointer.
  Function *ReadTpFunc = Intrinsic::getDeclaration(&M, Intrinsic::nacl_read_tp);

  for (std::vector<VarInfo>::iterator VarInfo = TlsVars->begin();
       VarInfo != TlsVars->end();
       ++VarInfo) {
    GlobalVariable *Var = VarInfo->TlsVar;
    while (!Var->use_empty()) {
      Use *U = &Var->use_begin().getUse();
      Instruction *InsertPt = PhiSafeInsertPt(U);
      Value *RawThreadPtr = CallInst::Create(ReadTpFunc, "tls_raw", InsertPt);
      Value *TypedThreadPtr = new BitCastInst(RawThreadPtr, TemplatePtrType,
                                              "tls_struct", InsertPt);
      SmallVector<Value*, 3> Indexes;
      // We use -1 because we use the x86-style TLS layout in which
      // the TLS data is stored at addresses below the thread pointer.
      // This is largely because a check in nacl_irt_thread_create()
      // in irt/irt_thread.c requires the thread pointer to be a
      // self-pointer on x86-32.
      // TODO(mseaborn): I intend to remove that check because it is
      // non-portable.  In the mean time, we want PNaCl pexes to work
      // in older Chromium releases when translated to nexes.
      Indexes.push_back(ConstantInt::get(
          M.getContext(), APInt(32, -1)));
      Indexes.push_back(ConstantInt::get(
          M.getContext(), APInt(32, VarInfo->IsBss ? 1 : 0)));
      Indexes.push_back(ConstantInt::get(
          M.getContext(), APInt(32, VarInfo->TemplateIndex)));
      Value *TlsField = GetElementPtrInst::Create(TypedThreadPtr, Indexes,
                                                  "field", InsertPt);
      PhiSafeReplaceUses(U, TlsField);
    }
    VarInfo->TlsVar->eraseFromParent();
  }
}

bool ConstantMerge::runOnModule(Module &M) {
  TD = getAnalysisIfAvailable<TargetData>();

  // Find all the globals that are marked "used".  These cannot be merged.
  SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
  FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
  FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
  
  // Map unique <constants, has-unknown-alignment> pairs to globals.  We don't
  // want to merge globals of unknown alignment with those of explicit
  // alignment.  If we have TargetData, we always know the alignment.
  DenseMap<PointerIntPair<Constant*, 1, bool>, GlobalVariable*> CMap;

  // Replacements - This vector contains a list of replacements to perform.
  SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;

  bool MadeChange = false;

  // Iterate constant merging while we are still making progress.  Merging two
  // constants together may allow us to merge other constants together if the
  // second level constants have initializers which point to the globals that
  // were just merged.
  while (1) {

    // First: Find the canonical constants others will be merged with.
    for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
         GVI != E; ) {
      GlobalVariable *GV = GVI++;

      // If this GV is dead, remove it.
      GV->removeDeadConstantUsers();
      if (GV->use_empty() && GV->hasLocalLinkage()) {
        GV->eraseFromParent();
        continue;
      }

      // Only process constants with initializers in the default address space.
      if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
          GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
          // Don't touch values marked with attribute(used).
          UsedGlobals.count(GV))
        continue;

      // This transformation is legal for weak ODR globals in the sense it
      // doesn't change semantics, but we really don't want to perform it
      // anyway; it's likely to pessimize code generation, and some tools
      // (like the Darwin linker in cases involving CFString) don't expect it.
      if (GV->isWeakForLinker())
        continue;

      Constant *Init = GV->getInitializer();

      // Check to see if the initializer is already known.
      PointerIntPair<Constant*, 1, bool> Pair(Init, hasKnownAlignment(GV));
      GlobalVariable *&Slot = CMap[Pair];

      // If this is the first constant we find or if the old one is local,
      // replace with the current one. If the current is externally visible
      // it cannot be replace, but can be the canonical constant we merge with.
      if (Slot == 0 || IsBetterCannonical(*GV, *Slot))
        Slot = GV;
    }

    // Second: identify all globals that can be merged together, filling in
    // the Replacements vector.  We cannot do the replacement in this pass
    // because doing so may cause initializers of other globals to be rewritten,
    // invalidating the Constant* pointers in CMap.
    for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
         GVI != E; ) {
      GlobalVariable *GV = GVI++;

      // Only process constants with initializers in the default address space.
      if (!GV->isConstant() || !GV->hasDefinitiveInitializer() ||
          GV->getType()->getAddressSpace() != 0 || GV->hasSection() ||
          // Don't touch values marked with attribute(used).
          UsedGlobals.count(GV))
        continue;

      // We can only replace constant with local linkage.
      if (!GV->hasLocalLinkage())
        continue;

      Constant *Init = GV->getInitializer();

      // Check to see if the initializer is already known.
      PointerIntPair<Constant*, 1, bool> Pair(Init, hasKnownAlignment(GV));
      GlobalVariable *Slot = CMap[Pair];

      if (!Slot || Slot == GV)
        continue;

      if (!Slot->hasUnnamedAddr() && !GV->hasUnnamedAddr())
        continue;

      if (!GV->hasUnnamedAddr())
        Slot->setUnnamedAddr(false);

      // Make all uses of the duplicate constant use the canonical version.
      Replacements.push_back(std::make_pair(GV, Slot));
    }
//.........这里部分代码省略.........

bool ConstantMerge::runOnModule(Module &M) {
  // Find all the globals that are marked "used".  These cannot be merged.
  SmallPtrSet<const GlobalValue*, 8> UsedGlobals;
  FindUsedValues(M.getGlobalVariable("llvm.used"), UsedGlobals);
  FindUsedValues(M.getGlobalVariable("llvm.compiler.used"), UsedGlobals);
  
  // Map unique constant/section pairs to globals.  We don't want to merge
  // globals in different sections.
  DenseMap<Constant*, GlobalVariable*> CMap;

  // Replacements - This vector contains a list of replacements to perform.
  SmallVector<std::pair<GlobalVariable*, GlobalVariable*>, 32> Replacements;

  bool MadeChange = false;

  // Iterate constant merging while we are still making progress.  Merging two
  // constants together may allow us to merge other constants together if the
  // second level constants have initializers which point to the globals that
  // were just merged.
  while (1) {
    // First pass: identify all globals that can be merged together, filling in
    // the Replacements vector.  We cannot do the replacement in this pass
    // because doing so may cause initializers of other globals to be rewritten,
    // invalidating the Constant* pointers in CMap.
    //
    for (Module::global_iterator GVI = M.global_begin(), E = M.global_end();
         GVI != E; ) {
      GlobalVariable *GV = GVI++;
      
      // If this GV is dead, remove it.
      GV->removeDeadConstantUsers();
      if (GV->use_empty() && GV->hasLocalLinkage()) {
        GV->eraseFromParent();
        continue;
      }
      
      // Only process constants with initializers in the default addres space.
      if (!GV->isConstant() ||!GV->hasDefinitiveInitializer() ||
          GV->getType()->getAddressSpace() != 0 || !GV->getSection().empty() ||
          // Don't touch values marked with attribute(used).
          UsedGlobals.count(GV))
        continue;
      
      
      
      Constant *Init = GV->getInitializer();

      // Check to see if the initializer is already known.
      GlobalVariable *&Slot = CMap[Init];

      if (Slot == 0) {    // Nope, add it to the map.
        Slot = GV;
      } else if (GV->hasLocalLinkage()) {    // Yup, this is a duplicate!
        // Make all uses of the duplicate constant use the canonical version.
        Replacements.push_back(std::make_pair(GV, Slot));
      }
    }

    if (Replacements.empty())
      return MadeChange;
    CMap.clear();

    // Now that we have figured out which replacements must be made, do them all
    // now.  This avoid invalidating the pointers in CMap, which are unneeded
    // now.
    for (unsigned i = 0, e = Replacements.size(); i != e; ++i) {
      // Eliminate any uses of the dead global.
      Replacements[i].first->replaceAllUsesWith(Replacements[i].second);

      // Delete the global value from the module.
      Replacements[i].first->eraseFromParent();
    }

    NumMerged += Replacements.size();
    Replacements.clear();
  }
}