C++ GlobalValue::isDiscardableIfUnused方法代码示例

/// Make sure GV is visible from both modules. Delete is true if it is
/// being deleted from this module.
/// This also makes sure GV cannot be dropped so that references from
/// the split module remain valid.
static void makeVisible(GlobalValue &GV, bool Delete) {
  bool Local = GV.hasLocalLinkage();
  if (Local)
    GV.setVisibility(GlobalValue::HiddenVisibility);

  if (Local || Delete) {
    GV.setLinkage(GlobalValue::ExternalLinkage);
    return;
  }

  if (!GV.hasLinkOnceLinkage()) {
    assert(!GV.isDiscardableIfUnused());
    return;
  }

  // Map linkonce* to weak* so that llvm doesn't drop this GV.
  switch(GV.getLinkage()) {
  default:
    llvm_unreachable("Unexpected linkage");
  case GlobalValue::LinkOnceAnyLinkage:
    GV.setLinkage(GlobalValue::WeakAnyLinkage);
    return;
  case GlobalValue::LinkOnceODRLinkage:
    GV.setLinkage(GlobalValue::WeakODRLinkage);
    return;
  }
}

/// Make sure GV is visible from both modules. Delete is true if it is
/// being deleted from this module.
/// This also makes sure GV cannot be dropped so that references from
/// the split module remain valid.
static void makeVisible(GlobalValue &GV, bool Delete, bool IsDeletePass) {
  bool Local = GV.hasLocalLinkage();
  if (Local || Delete) {
    // This changes members from private -> hidden -> causes linker errors when using llvm-link
    if (!IsDeletePass)
      GV.setLinkage(GlobalValue::ExternalLinkage);
    if (Local)
      GV.setVisibility(GlobalValue::HiddenVisibility);
    return;
  }

  if (!GV.hasLinkOnceLinkage()) {
    assert(!GV.isDiscardableIfUnused());
    return;
  }

  // Map linkonce* to weak* so that llvm doesn't drop this GV.
  switch(GV.getLinkage()) {
  default:
    llvm_unreachable("Unexpected linkage");
  case GlobalValue::LinkOnceAnyLinkage:
    GV.setLinkage(GlobalValue::WeakAnyLinkage);
    return;
  case GlobalValue::LinkOnceODRLinkage:
    GV.setLinkage(GlobalValue::WeakODRLinkage);
    return;
  }
}

static void keepGlobalValue(GlobalValue &GV,
                            std::vector<GlobalAlias *> &KeptAliases) {
  assert(!GV.hasLocalLinkage());

  if (auto *GA = dyn_cast<GlobalAlias>(&GV))
    KeptAliases.push_back(GA);

  switch (GV.getLinkage()) {
  default:
    break;
  case GlobalValue::LinkOnceAnyLinkage:
    GV.setLinkage(GlobalValue::WeakAnyLinkage);
    break;
  case GlobalValue::LinkOnceODRLinkage:
    GV.setLinkage(GlobalValue::WeakODRLinkage);
    break;
  }

  assert(!GV.isDiscardableIfUnused());
}

static std::unique_ptr<Module>
getModuleForFile(LLVMContext &Context, claimed_file &F,
                 ld_plugin_input_file &Info, raw_fd_ostream *ApiFile,
                 StringSet<> &Internalize, StringSet<> &Maybe) {

  if (get_symbols(F.handle, F.syms.size(), F.syms.data()) != LDPS_OK)
    message(LDPL_FATAL, "Failed to get symbol information");

  const void *View;
  if (get_view(F.handle, &View) != LDPS_OK)
    message(LDPL_FATAL, "Failed to get a view of file");

  MemoryBufferRef BufferRef(StringRef((const char *)View, Info.filesize),
                            Info.name);
  ErrorOr<std::unique_ptr<object::IRObjectFile>> ObjOrErr =
      object::IRObjectFile::create(BufferRef, Context);

  if (std::error_code EC = ObjOrErr.getError())
    message(LDPL_FATAL, "Could not read bitcode from file : %s",
            EC.message().c_str());

  object::IRObjectFile &Obj = **ObjOrErr;

  Module &M = Obj.getModule();

  M.materializeMetadata();
  UpgradeDebugInfo(M);

  SmallPtrSet<GlobalValue *, 8> Used;
  collectUsedGlobalVariables(M, Used, /*CompilerUsed*/ false);

  DenseSet<GlobalValue *> Drop;
  std::vector<GlobalAlias *> KeptAliases;

  unsigned SymNum = 0;
  for (auto &ObjSym : Obj.symbols()) {
    if (shouldSkip(ObjSym.getFlags()))
      continue;
    ld_plugin_symbol &Sym = F.syms[SymNum];
    ++SymNum;

    ld_plugin_symbol_resolution Resolution =
        (ld_plugin_symbol_resolution)Sym.resolution;

    if (options::generate_api_file)
      *ApiFile << Sym.name << ' ' << getResolutionName(Resolution) << '\n';

    GlobalValue *GV = Obj.getSymbolGV(ObjSym.getRawDataRefImpl());
    if (!GV) {
      freeSymName(Sym);
      continue; // Asm symbol.
    }

    if (Resolution != LDPR_PREVAILING_DEF_IRONLY && GV->hasCommonLinkage()) {
      // Common linkage is special. There is no single symbol that wins the
      // resolution. Instead we have to collect the maximum alignment and size.
      // The IR linker does that for us if we just pass it every common GV.
      // We still have to keep track of LDPR_PREVAILING_DEF_IRONLY so we
      // internalize once the IR linker has done its job.
      freeSymName(Sym);
      continue;
    }

    switch (Resolution) {
    case LDPR_UNKNOWN:
      llvm_unreachable("Unexpected resolution");

    case LDPR_RESOLVED_IR:
    case LDPR_RESOLVED_EXEC:
    case LDPR_RESOLVED_DYN:
      assert(GV->isDeclarationForLinker());
      break;

    case LDPR_UNDEF:
      if (!GV->isDeclarationForLinker()) {
        assert(GV->hasComdat());
        Drop.insert(GV);
      }
      break;

    case LDPR_PREVAILING_DEF_IRONLY: {
      keepGlobalValue(*GV, KeptAliases);
      if (!Used.count(GV)) {
        // Since we use the regular lib/Linker, we cannot just internalize GV
        // now or it will not be copied to the merged module. Instead we force
        // it to be copied and then internalize it.
        Internalize.insert(GV->getName());
      }
      break;
    }

    case LDPR_PREVAILING_DEF:
      keepGlobalValue(*GV, KeptAliases);
      break;

    case LDPR_PREEMPTED_IR:
      // Gold might have selected a linkonce_odr and preempted a weak_odr.
      // In that case we have to make sure we don't end up internalizing it.
      if (!GV->isDiscardableIfUnused())
        Maybe.erase(GV->getName());
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