lld/ELF/SymbolTable.cpp

//===- SymbolTable.cpp ----------------------------------------------------===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//
//
// Symbol table is a bag of all known symbols. We put all symbols of
// all input files to the symbol table. The symbol table is basically
// a hash table with the logic to resolve symbol name conflicts using
// the symbol types.
//
//===----------------------------------------------------------------------===//

#include "SymbolTable.h"
#include "Config.h"
#include "Error.h"
#include "LinkerScript.h"
#include "Memory.h"
#include "SymbolListFile.h"
#include "Symbols.h"

using namespace llvm;
using namespace llvm::object;
using namespace llvm::ELF;

using namespace lld;
using namespace lld::elf;

// All input object files must be for the same architecture
// (e.g. it does not make sense to link x86 object files with
// MIPS object files.) This function checks for that error.
template <class ELFT> static bool isCompatible(InputFile *F) {
  if (!isa<ELFFileBase<ELFT>>(F) && !isa<BitcodeFile>(F))
    return true;
  if (F->EKind == Config->EKind && F->EMachine == Config->EMachine) {
    if (Config->EMachine != EM_MIPS)
      return true;
    if (isMipsN32Abi(F) == Config->MipsN32Abi)
      return true;
  }
  StringRef A = F->getName();
  StringRef B = Config->Emulation;
  if (B.empty())
    B = Config->FirstElf->getName();
  error(A + " is incompatible with " + B);
  return false;
}

// Add symbols in File to the symbol table.
template <class ELFT> void SymbolTable<ELFT>::addFile(InputFile *File) {
  if (!isCompatible<ELFT>(File))
    return;

  // Binary file
  if (auto *F = dyn_cast<BinaryFile>(File)) {
    BinaryFiles.push_back(F);
    F->parse<ELFT>();
    return;
  }

  // .a file
  if (auto *F = dyn_cast<ArchiveFile>(File)) {
    F->parse<ELFT>();
    return;
  }

  // Lazy object file
  if (auto *F = dyn_cast<LazyObjectFile>(File)) {
    F->parse<ELFT>();
    return;
  }

  if (Config->Trace)
    outs() << getFilename(File) << "\n";

  // .so file
  if (auto *F = dyn_cast<SharedFile<ELFT>>(File)) {
    // DSOs are uniquified not by filename but by soname.
    F->parseSoName();
    if (HasError || !SoNames.insert(F->getSoName()).second)
      return;
    SharedFiles.push_back(F);
    F->parseRest();
    return;
  }

  // LLVM bitcode file
  if (auto *F = dyn_cast<BitcodeFile>(File)) {
    BitcodeFiles.push_back(F);
    F->parse<ELFT>(ComdatGroups);
    return;
  }

  // Regular object file
  auto *F = cast<ObjectFile<ELFT>>(File);
  ObjectFiles.push_back(F);
  F->parse(ComdatGroups);
}

// This function is where all the optimizations of link-time
// optimization happens. When LTO is in use, some input files are
// not in native object file format but in the LLVM bitcode format.
// This function compiles bitcode files into a few big native files
// using LLVM functions and replaces bitcode symbols with the results.
// Because all bitcode files that consist of a program are passed
// to the compiler at once, it can do whole-program optimization.
template <class ELFT> void SymbolTable<ELFT>::addCombinedLtoObject() {
  if (BitcodeFiles.empty())
    return;

  // Compile bitcode files and replace bitcode symbols.
  Lto.reset(new BitcodeCompiler);
  for (BitcodeFile *F : BitcodeFiles)
    Lto->add(*F);

  for (InputFile *File : Lto->compile()) {
    ObjectFile<ELFT> *Obj = cast<ObjectFile<ELFT>>(File);
    DenseSet<CachedHashStringRef> DummyGroups;
    Obj->parse(DummyGroups);
    ObjectFiles.push_back(Obj);
  }
}

template <class ELFT>
DefinedRegular<ELFT> *SymbolTable<ELFT>::addAbsolute(StringRef Name,
                                                     uint8_t Visibility) {
  Symbol *Sym =
      addRegular(Name, Visibility, STT_NOTYPE, 0, 0, STB_GLOBAL, nullptr);
  return cast<DefinedRegular<ELFT>>(Sym->body());
}

// Add Name as an "ignored" symbol. An ignored symbol is a regular
// linker-synthesized defined symbol, but is only defined if needed.
template <class ELFT>
DefinedRegular<ELFT> *SymbolTable<ELFT>::addIgnored(StringRef Name,
                                                    uint8_t Visibility) {
  if (!find(Name))
    return nullptr;
  return addAbsolute(Name, Visibility);
}

// Set a flag for --trace-symbol so that we can print out a log message
// if a new symbol with the same name is inserted into the symbol table.
template <class ELFT> void SymbolTable<ELFT>::trace(StringRef Name) {
  Symtab.insert({CachedHashStringRef(Name), {-1, true}});
}

// Rename SYM as __wrap_SYM. The original symbol is preserved as __real_SYM.
// Used to implement --wrap.
template <class ELFT> void SymbolTable<ELFT>::wrap(StringRef Name) {
  SymbolBody *B = find(Name);
  if (!B)
    return;
  Symbol *Sym = B->symbol();
  Symbol *Real = addUndefined(Saver.save("__real_" + Name));
  Symbol *Wrap = addUndefined(Saver.save("__wrap_" + Name));

  // We rename symbols by replacing the old symbol's SymbolBody with the new
  // symbol's SymbolBody. This causes all SymbolBody pointers referring to the
  // old symbol to instead refer to the new symbol.
  memcpy(Real->Body.buffer, Sym->Body.buffer, sizeof(Sym->Body));
  memcpy(Sym->Body.buffer, Wrap->Body.buffer, sizeof(Wrap->Body));
}

static uint8_t getMinVisibility(uint8_t VA, uint8_t VB) {
  if (VA == STV_DEFAULT)
    return VB;
  if (VB == STV_DEFAULT)
    return VA;
  return std::min(VA, VB);
}

// Parses a symbol in the form of <name>@<version> or <name>@@<version>.
static std::pair<StringRef, uint16_t> getSymbolVersion(StringRef S) {
  if (Config->VersionDefinitions.empty())
    return {S, Config->DefaultSymbolVersion};

  size_t Pos = S.find('@');
  if (Pos == 0 || Pos == StringRef::npos)
    return {S, Config->DefaultSymbolVersion};

  StringRef Name = S.substr(0, Pos);
  StringRef Verstr = S.substr(Pos + 1);
  if (Verstr.empty())
    return {S, Config->DefaultSymbolVersion};

  // '@@' in a symbol name means the default version.
  // It is usually the most recent one.
  bool IsDefault = (Verstr[0] == '@');
  if (IsDefault)
    Verstr = Verstr.substr(1);

  for (VersionDefinition &V : Config->VersionDefinitions) {
    if (V.Name == Verstr)
      return {Name, IsDefault ? V.Id : (V.Id | VERSYM_HIDDEN)};
  }

  // It is an error if the specified version was not defined.
  error("symbol " + S + " has undefined version " + Verstr);
  return {S, Config->DefaultSymbolVersion};
}

// Find an existing symbol or create and insert a new one.
template <class ELFT>
std::pair<Symbol *, bool> SymbolTable<ELFT>::insert(StringRef &Name) {
  auto P = Symtab.insert(
      {CachedHashStringRef(Name), SymIndex((int)SymVector.size(), false)});
  SymIndex &V = P.first->second;
  bool IsNew = P.second;

  if (V.Idx == -1) {
    IsNew = true;
    V = SymIndex((int)SymVector.size(), true);
  }

  Symbol *Sym;
  if (IsNew) {
    Sym = new (BAlloc) Symbol;
    Sym->Binding = STB_WEAK;
    Sym->Visibility = STV_DEFAULT;
    Sym->IsUsedInRegularObj = false;
    Sym->ExportDynamic = false;
    Sym->Traced = V.Traced;
    std::tie(Name, Sym->VersionId) = getSymbolVersion(Name);
    SymVector.push_back(Sym);
  } else {
    Sym = SymVector[V.Idx];
  }
  return {Sym, IsNew};
}

// Construct a string in the form of "Sym in File1 and File2".
// Used to construct an error message.
static std::string conflictMsg(SymbolBody *Existing, InputFile *NewFile) {
  return "'" + maybeDemangle(Existing->getName()) + "' in " +
         getFilename(Existing->File) + " and " + getFilename(NewFile);
}

// Find an existing symbol or create and insert a new one, then apply the given
// attributes.
template <class ELFT>
std::pair<Symbol *, bool>
SymbolTable<ELFT>::insert(StringRef &Name, uint8_t Type, uint8_t Visibility,
                          bool CanOmitFromDynSym, InputFile *File) {
  bool IsUsedInRegularObj = !File || File->kind() == InputFile::ObjectKind;
  Symbol *S;
  bool WasInserted;
  std::tie(S, WasInserted) = insert(Name);

  // Merge in the new symbol's visibility.
  S->Visibility = getMinVisibility(S->Visibility, Visibility);
  if (!CanOmitFromDynSym && (Config->Shared || Config->ExportDynamic))
    S->ExportDynamic = true;
  if (IsUsedInRegularObj)
    S->IsUsedInRegularObj = true;
  if (!WasInserted && S->body()->Type != SymbolBody::UnknownType &&
      ((Type == STT_TLS) != S->body()->isTls()))
    error("TLS attribute mismatch for symbol " + conflictMsg(S->body(), File));

  return {S, WasInserted};
}

template <class ELFT> Symbol *SymbolTable<ELFT>::addUndefined(StringRef Name) {
  return addUndefined(Name, STB_GLOBAL, STV_DEFAULT, /*Type*/ 0,
                      /*CanOmitFromDynSym*/ false, /*File*/ nullptr);
}

template <class ELFT>
Symbol *SymbolTable<ELFT>::addUndefined(StringRef Name, uint8_t Binding,
                                        uint8_t StOther, uint8_t Type,
                                        bool CanOmitFromDynSym,
                                        InputFile *File) {
  Symbol *S;
  bool WasInserted;
  std::tie(S, WasInserted) =
      insert(Name, Type, StOther & 3, CanOmitFromDynSym, File);
  if (WasInserted) {
    S->Binding = Binding;
    replaceBody<Undefined>(S, Name, StOther, Type, File);
    return S;
  }
  if (Binding != STB_WEAK) {
    if (S->body()->isShared() || S->body()->isLazy())
      S->Binding = Binding;
    if (auto *SS = dyn_cast<SharedSymbol<ELFT>>(S->body()))
      SS->file()->IsUsed = true;
  }
  if (auto *L = dyn_cast<Lazy>(S->body())) {
    // An undefined weak will not fetch archive members, but we have to remember
    // its type. See also comment in addLazyArchive.
    if (S->isWeak())
      L->Type = Type;
    else if (InputFile *F = L->fetch())
      addFile(F);
  }
  return S;
}

// We have a new defined symbol with the specified binding. Return 1 if the new
// symbol should win, -1 if the new symbol should lose, or 0 if both symbols are
// strong defined symbols.
static int compareDefined(Symbol *S, bool WasInserted, uint8_t Binding) {
  if (WasInserted)
    return 1;
  SymbolBody *Body = S->body();
  if (Body->isLazy() || Body->isUndefined() || Body->isShared())
    return 1;
  if (Binding == STB_WEAK)
    return -1;
  if (S->isWeak())
    return 1;
  return 0;
}

// We have a new non-common defined symbol with the specified binding. Return 1
// if the new symbol should win, -1 if the new symbol should lose, or 0 if there
// is a conflict. If the new symbol wins, also update the binding.
static int compareDefinedNonCommon(Symbol *S, bool WasInserted,
                                   uint8_t Binding) {
  if (int Cmp = compareDefined(S, WasInserted, Binding)) {
    if (Cmp > 0)
      S->Binding = Binding;
    return Cmp;
  }
  if (isa<DefinedCommon>(S->body())) {
    // Non-common symbols take precedence over common symbols.
    if (Config->WarnCommon)
      warn("common " + S->body()->getName() + " is overridden");
    return 1;
  }
  return 0;
}

template <class ELFT>
Symbol *SymbolTable<ELFT>::addCommon(StringRef N, uint64_t Size,
                                     uint64_t Alignment, uint8_t Binding,
                                     uint8_t StOther, uint8_t Type,
                                     InputFile *File) {
  Symbol *S;
  bool WasInserted;
  std::tie(S, WasInserted) =
      insert(N, Type, StOther & 3, /*CanOmitFromDynSym*/ false, File);
  int Cmp = compareDefined(S, WasInserted, Binding);
  if (Cmp > 0) {
    S->Binding = Binding;
    replaceBody<DefinedCommon>(S, N, Size, Alignment, StOther, Type, File);
  } else if (Cmp == 0) {
    auto *C = dyn_cast<DefinedCommon>(S->body());
    if (!C) {
      // Non-common symbols take precedence over common symbols.
      if (Config->WarnCommon)
        warn("common " + S->body()->getName() + " is overridden");
      return S;
    }

    if (Config->WarnCommon)
      warn("multiple common of " + S->body()->getName());

    Alignment = C->Alignment = std::max(C->Alignment, Alignment);
    if (Size > C->Size)
      replaceBody<DefinedCommon>(S, N, Size, Alignment, StOther, Type, File);
  }
  return S;
}

static void print(const Twine &Msg) {
  if (Config->AllowMultipleDefinition)
    warn(Msg);
  else
    error(Msg);
}

static void reportDuplicate(SymbolBody *Existing, InputFile *NewFile) {
  print("duplicate symbol " + conflictMsg(Existing, NewFile));
}

template <class ELFT>
static void reportDuplicate(SymbolBody *Existing,
                            InputSectionBase<ELFT> *ErrSec,
                            typename ELFT::uint ErrOffset) {
  DefinedRegular<ELFT> *D = dyn_cast<DefinedRegular<ELFT>>(Existing);
  if (!D || !D->Section || !ErrSec) {
    reportDuplicate(Existing, ErrSec ? ErrSec->getFile() : nullptr);
    return;
  }

  std::string OldLoc = getLocation(*D->Section, D->Value);
  std::string NewLoc = getLocation(*ErrSec, ErrOffset);

  print(NewLoc + ": duplicate symbol '" + maybeDemangle(Existing->getName()) +
        "'");
  print(OldLoc + ": previous definition was here");
}

template <typename ELFT>
Symbol *SymbolTable<ELFT>::addRegular(StringRef Name, const Elf_Sym &Sym,
                                      InputSectionBase<ELFT> *Section) {
  return addRegular(Name, Sym.st_other, Sym.getType(), Sym.st_value,
                    Sym.st_size, Sym.getBinding(), Section);
}

template <typename ELFT>
Symbol *SymbolTable<ELFT>::addRegular(StringRef Name, uint8_t StOther,
                                      uint8_t Type, uintX_t Value, uintX_t Size,
                                      uint8_t Binding,
                                      InputSectionBase<ELFT> *Section) {
  Symbol *S;
  bool WasInserted;
  std::tie(S, WasInserted) = insert(Name, Type, StOther & 3,
                                    /*CanOmitFromDynSym*/ false,
                                    Section ? Section->getFile() : nullptr);
  int Cmp = compareDefinedNonCommon(S, WasInserted, Binding);
  if (Cmp > 0)
    replaceBody<DefinedRegular<ELFT>>(S, Name, StOther, Type, Value, Size,
                                      Section);
  else if (Cmp == 0)
    reportDuplicate(S->body(), Section, Value);
  return S;
}

template <typename ELFT>
Symbol *SymbolTable<ELFT>::addSynthetic(StringRef N, OutputSectionBase *Section,
                                        uintX_t Value, uint8_t StOther) {
  Symbol *S;
  bool WasInserted;
  std::tie(S, WasInserted) = insert(N, STT_NOTYPE, /*Visibility*/ StOther & 0x3,
                                    /*CanOmitFromDynSym*/ false, nullptr);
  int Cmp = compareDefinedNonCommon(S, WasInserted, STB_GLOBAL);
  if (Cmp > 0)
    replaceBody<DefinedSynthetic<ELFT>>(S, N, Value, Section);
  else if (Cmp == 0)
    reportDuplicate(S->body(), nullptr);
  return S;
}

template <typename ELFT>
void SymbolTable<ELFT>::addShared(SharedFile<ELFT> *F, StringRef Name,
                                  const Elf_Sym &Sym,
                                  const typename ELFT::Verdef *Verdef) {
  // DSO symbols do not affect visibility in the output, so we pass STV_DEFAULT
  // as the visibility, which will leave the visibility in the symbol table
  // unchanged.
  Symbol *S;
  bool WasInserted;
  std::tie(S, WasInserted) =
      insert(Name, Sym.getType(), STV_DEFAULT, /*CanOmitFromDynSym*/ true, F);
  // Make sure we preempt DSO symbols with default visibility.
  if (Sym.getVisibility() == STV_DEFAULT)
    S->ExportDynamic = true;
  if (WasInserted || isa<Undefined>(S->body())) {
    replaceBody<SharedSymbol<ELFT>>(S, F, Name, Sym, Verdef);
    if (!S->isWeak())
      F->IsUsed = true;
  }
}

template <class ELFT>
Symbol *SymbolTable<ELFT>::addBitcode(StringRef Name, uint8_t Binding,
                                      uint8_t StOther, uint8_t Type,
                                      bool CanOmitFromDynSym, BitcodeFile *F) {
  Symbol *S;
  bool WasInserted;
  std::tie(S, WasInserted) =
      insert(Name, Type, StOther & 3, CanOmitFromDynSym, F);
  int Cmp = compareDefinedNonCommon(S, WasInserted, Binding);
  if (Cmp > 0)
    replaceBody<DefinedRegular<ELFT>>(S, Name, StOther, Type, F);
  else if (Cmp == 0)
    reportDuplicate(S->body(), F);
  return S;
}

template <class ELFT> SymbolBody *SymbolTable<ELFT>::find(StringRef Name) {
  auto It = Symtab.find(CachedHashStringRef(Name));
  if (It == Symtab.end())
    return nullptr;
  SymIndex V = It->second;
  if (V.Idx == -1)
    return nullptr;
  return SymVector[V.Idx]->body();
}

// Returns a list of defined symbols that match with a given regex.
template <class ELFT>
std::vector<SymbolBody *> SymbolTable<ELFT>::findAll(const StringMatcher &M) {
  std::vector<SymbolBody *> Res;
  for (Symbol *Sym : SymVector) {
    SymbolBody *B = Sym->body();
    StringRef Name = B->getName();
    if (!B->isUndefined() && M.match(Name))
      Res.push_back(B);
  }
  return Res;
}

template <class ELFT>
void SymbolTable<ELFT>::addLazyArchive(ArchiveFile *F,
                                       const object::Archive::Symbol Sym) {
  Symbol *S;
  bool WasInserted;
  StringRef Name = Sym.getName();
  std::tie(S, WasInserted) = insert(Name);
  if (WasInserted) {
    replaceBody<LazyArchive>(S, *F, Sym, SymbolBody::UnknownType);
    return;
  }
  if (!S->body()->isUndefined())
    return;

  // Weak undefined symbols should not fetch members from archives. If we were
  // to keep old symbol we would not know that an archive member was available
  // if a strong undefined symbol shows up afterwards in the link. If a strong
  // undefined symbol never shows up, this lazy symbol will get to the end of
  // the link and must be treated as the weak undefined one. We already marked
  // this symbol as used when we added it to the symbol table, but we also need
  // to preserve its type. FIXME: Move the Type field to Symbol.
  if (S->isWeak()) {
    replaceBody<LazyArchive>(S, *F, Sym, S->body()->Type);
    return;
  }
  std::pair<MemoryBufferRef, uint64_t> MBInfo = F->getMember(&Sym);
  if (!MBInfo.first.getBuffer().empty())
    addFile(createObjectFile(MBInfo.first, F->getName(), MBInfo.second));
}

template <class ELFT>
void SymbolTable<ELFT>::addLazyObject(StringRef Name, LazyObjectFile &Obj) {
  Symbol *S;
  bool WasInserted;
  std::tie(S, WasInserted) = insert(Name);
  if (WasInserted) {
    replaceBody<LazyObject>(S, Name, Obj, SymbolBody::UnknownType);
    return;
  }
  if (!S->body()->isUndefined())
    return;

  // See comment for addLazyArchive above.
  if (S->isWeak()) {
    replaceBody<LazyObject>(S, Name, Obj, S->body()->Type);
  } else {
    MemoryBufferRef MBRef = Obj.getBuffer();
    if (!MBRef.getBuffer().empty())
      addFile(createObjectFile(MBRef));
  }
}

// Process undefined (-u) flags by loading lazy symbols named by those flags.
template <class ELFT> void SymbolTable<ELFT>::scanUndefinedFlags() {
  for (StringRef S : Config->Undefined)
    if (auto *L = dyn_cast_or_null<Lazy>(find(S)))
      if (InputFile *File = L->fetch())
        addFile(File);
}

// This function takes care of the case in which shared libraries depend on
// the user program (not the other way, which is usual). Shared libraries
// may have undefined symbols, expecting that the user program provides
// the definitions for them. An example is BSD's __progname symbol.
// We need to put such symbols to the main program's .dynsym so that
// shared libraries can find them.
// Except this, we ignore undefined symbols in DSOs.
template <class ELFT> void SymbolTable<ELFT>::scanShlibUndefined() {
  for (SharedFile<ELFT> *File : SharedFiles)
    for (StringRef U : File->getUndefinedSymbols())
      if (SymbolBody *Sym = find(U))
        if (Sym->isDefined())
          Sym->symbol()->ExportDynamic = true;
}

// This function processes --export-dynamic-symbol and --dynamic-list.
template <class ELFT> void SymbolTable<ELFT>::scanDynamicList() {
  for (StringRef S : Config->DynamicList)
    if (SymbolBody *B = find(S))
      B->symbol()->ExportDynamic = true;
}

static void setVersionId(SymbolBody *Body, StringRef VersionName,
                         StringRef Name, uint16_t Version) {
  if (!Body || Body->isUndefined()) {
    if (Config->NoUndefinedVersion)
      error("version script assignment of " + VersionName + " to symbol " +
            Name + " failed: symbol not defined");
    return;
  }

  Symbol *Sym = Body->symbol();
  if (Sym->VersionId != Config->DefaultSymbolVersion)
    warn("duplicate symbol " + Name + " in version script");
  Sym->VersionId = Version;
}

// Returns a map from demangled symbols to symbol objects.
// The relationship is 1:N instead of 1:1 because with the symbol
// versioning, more than one symbol may have the same name.
template <class ELFT>
std::map<std::string, std::vector<SymbolBody *>>
SymbolTable<ELFT>::getDemangledSyms() {
  std::map<std::string, std::vector<SymbolBody *>> Result;
  for (Symbol *Sym : SymVector) {
    SymbolBody *B = Sym->body();
    Result[demangle(B->getName())].push_back(B);
  }
  return Result;
}

static bool hasExternCpp() {
  for (VersionDefinition &V : Config->VersionDefinitions)
    for (SymbolVersion Sym : V.Globals)
      if (Sym.IsExternCpp)
        return true;
  return false;
}

static ArrayRef<SymbolBody *>
findDemangled(std::map<std::string, std::vector<SymbolBody *>> &D,
              StringRef Name) {
  auto I = D.find(Name);
  if (I != D.end())
    return I->second;
  return {};
}

static std::vector<SymbolBody *>
findAllDemangled(const std::map<std::string, std::vector<SymbolBody *>> &D,
                 StringMatcher &M) {
  std::vector<SymbolBody *> Res;
  for (auto &P : D) {
    if (M.match(P.first))
      for (SymbolBody *Body : P.second)
        if (!Body->isUndefined())
          Res.push_back(Body);
  }
  return Res;
}

// If there's only one anonymous version definition in a version
// script file, the script does not actually define any symbol version,
// but just specifies symbols visibilities. We assume that the script was
// in the form of { global: foo; bar; local *; }. So, local is default.
// In this function, we make specified symbols global.
template <class ELFT> void SymbolTable<ELFT>::handleAnonymousVersion() {
  std::vector<StringRef> Patterns;
  for (SymbolVersion &Sym : Config->VersionScriptGlobals) {
    if (hasWildcard(Sym.Name)) {
      Patterns.push_back(Sym.Name);
      continue;
    }
    if (SymbolBody *B = find(Sym.Name))
      B->symbol()->VersionId = VER_NDX_GLOBAL;
  }
  if (Patterns.empty())
    return;
  StringMatcher M(Patterns);
  std::vector<SymbolBody *> Syms = findAll(M);
  for (SymbolBody *B : Syms)
    B->symbol()->VersionId = VER_NDX_GLOBAL;
}

// This function processes version scripts by updating VersionId
// member of symbols.
template <class ELFT> void SymbolTable<ELFT>::scanVersionScript() {
  // Handle edge cases first.
  if (!Config->VersionScriptGlobals.empty()) {
    handleAnonymousVersion();
    return;
  }

  if (Config->VersionDefinitions.empty())
    return;

  // Now we have version definitions, so we need to set version ids to symbols.
  // Each version definition has a glob pattern, and all symbols that match
  // with the pattern get that version.

  // Users can use "extern C++ {}" directive to match against demangled
  // C++ symbols. For example, you can write a pattern such as
  // "llvm::*::foo(int, ?)". Obviously, there's no way to handle this
  // other than trying to match a regexp against all demangled symbols.
  // So, if "extern C++" feature is used, we demangle all known symbols.
  std::map<std::string, std::vector<SymbolBody *>> Demangled;
  if (hasExternCpp())
    Demangled = getDemangledSyms();

  // First, we assign versions to exact matching symbols,
  // i.e. version definitions not containing any glob meta-characters.
  for (VersionDefinition &V : Config->VersionDefinitions) {
    for (SymbolVersion Sym : V.Globals) {
      if (Sym.HasWildcards)
        continue;

      StringRef N = Sym.Name;
      if (Sym.IsExternCpp) {
        for (SymbolBody *B : findDemangled(Demangled, N))
          setVersionId(B, V.Name, N, V.Id);
        continue;
      }
      setVersionId(find(N), V.Name, N, V.Id);
    }
    for (SymbolVersion Sym : V.Locals) {
      if (Sym.HasWildcards)
        continue;
      setVersionId(find(Sym.Name), V.Name, Sym.Name, VER_NDX_LOCAL);
    }
  }

  // Next, we assign versions to fuzzy matching symbols,
  // i.e. version definitions containing glob meta-characters.
  // Note that because the last match takes precedence over previous matches,
  // we iterate over the definitions in the reverse order.
  auto assignFuzzyVersion = [&](SymbolVersion &Sym, size_t Version) {
    if (!Sym.HasWildcards)
      return;
    StringMatcher M({Sym.Name});
    std::vector<SymbolBody *> Syms =
        Sym.IsExternCpp ? findAllDemangled(Demangled, M) : findAll(M);
    // Exact matching takes precendence over fuzzy matching,
    // so we set a version to a symbol only if no version has been assigned
    // to the symbol. This behavior is compatible with GNU.
    for (SymbolBody *B : Syms)
      if (B->symbol()->VersionId == Config->DefaultSymbolVersion)
        B->symbol()->VersionId = Version;
  };

  for (size_t I = Config->VersionDefinitions.size() - 1; I != (size_t)-1; --I) {
    VersionDefinition &V = Config->VersionDefinitions[I];
    for (SymbolVersion &Sym : V.Locals)
      assignFuzzyVersion(Sym, VER_NDX_LOCAL);
    for (SymbolVersion &Sym : V.Globals)
      assignFuzzyVersion(Sym, V.Id);
  }
}

template class elf::SymbolTable<ELF32LE>;
template class elf::SymbolTable<ELF32BE>;
template class elf::SymbolTable<ELF64LE>;
template class elf::SymbolTable<ELF64BE>;