lld/COFF/Symbols.h

//===- Symbols.h ------------------------------------------------*- C++ -*-===//
//
//                             The LLVM Linker
//
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
//
//===----------------------------------------------------------------------===//

#ifndef LLD_COFF_SYMBOLS_H
#define LLD_COFF_SYMBOLS_H

#include "Chunks.h"
#include "Config.h"
#include "Memory.h"
#include "lld/Core/LLVM.h"
#include "llvm/ADT/ArrayRef.h"
#include "llvm/Object/Archive.h"
#include "llvm/Object/COFF.h"
#include <atomic>
#include <memory>
#include <vector>

namespace lld {
namespace coff {

using llvm::object::Archive;
using llvm::object::COFFSymbolRef;
using llvm::object::coff_import_header;
using llvm::object::coff_symbol_generic;

class ArchiveFile;
class BitcodeFile;
class InputFile;
class ObjectFile;
struct Symbol;
class SymbolTable;

// The base class for real symbol classes.
class SymbolBody {
public:
  enum Kind {
    // The order of these is significant. We start with the regular defined
    // symbols as those are the most prevelant and the zero tag is the cheapest
    // to set. Among the defined kinds, the lower the kind is preferred over
    // the higher kind when testing wether one symbol should take precedence
    // over another.
    DefinedRegularKind = 0,
    DefinedCommonKind,
    DefinedLocalImportKind,
    DefinedImportThunkKind,
    DefinedImportDataKind,
    DefinedAbsoluteKind,
    DefinedRelativeKind,
    DefinedBitcodeKind,

    UndefinedKind,
    LazyKind,

    LastDefinedCOFFKind = DefinedCommonKind,
    LastDefinedKind = DefinedBitcodeKind,
  };

  Kind kind() const { return static_cast<Kind>(SymbolKind); }

  // Returns true if this is an external symbol.
  bool isExternal() { return IsExternal; }

  // Returns the symbol name.
  StringRef getName();

  // Returns the file from which this symbol was created.
  InputFile *getFile();

  Symbol *symbol();
  const Symbol *symbol() const {
    return const_cast<SymbolBody *>(this)->symbol();
  }

protected:
  friend SymbolTable;
  explicit SymbolBody(Kind K, StringRef N = "")
      : SymbolKind(K), IsExternal(true), IsCOMDAT(false),
        IsReplaceable(false), WrittenToSymtab(false), Name(N) {}

  const unsigned SymbolKind : 8;
  unsigned IsExternal : 1;

  // This bit is used by the \c DefinedRegular subclass.
  unsigned IsCOMDAT : 1;

  // This bit is used by the \c DefinedBitcode subclass.
  unsigned IsReplaceable : 1;

public:
  // This bit is used by Writer::createSymbolAndStringTable().
  unsigned WrittenToSymtab : 1;

protected:
  StringRef Name;
};

// The base class for any defined symbols, including absolute symbols,
// etc.
class Defined : public SymbolBody {
public:
  Defined(Kind K, StringRef N = "") : SymbolBody(K, N) {}

  static bool classof(const SymbolBody *S) {
    return S->kind() <= LastDefinedKind;
  }

  // Returns the RVA (relative virtual address) of this symbol. The
  // writer sets and uses RVAs.
  uint64_t getRVA();

  // Returns the RVA relative to the beginning of the output section.
  // Used to implement SECREL relocation type.
  uint64_t getSecrel();

  // Returns the output section index.
  // Used to implement SECTION relocation type.
  uint64_t getSectionIndex();

  // Returns true if this symbol points to an executable (e.g. .text) section.
  // Used to implement ARM relocations.
  bool isExecutable();
};

// Symbols defined via a COFF object file.
class DefinedCOFF : public Defined {
  friend SymbolBody;
public:
  DefinedCOFF(Kind K, ObjectFile *F, COFFSymbolRef S)
      : Defined(K), File(F), Sym(S.getGeneric()) {}

  static bool classof(const SymbolBody *S) {
    return S->kind() <= LastDefinedCOFFKind;
  }

  ObjectFile *getFile() { return File; }

  COFFSymbolRef getCOFFSymbol();

  ObjectFile *File;

protected:
  const coff_symbol_generic *Sym;
};

// Regular defined symbols read from object file symbol tables.
class DefinedRegular : public DefinedCOFF {
public:
  DefinedRegular(ObjectFile *F, COFFSymbolRef S, SectionChunk *C)
      : DefinedCOFF(DefinedRegularKind, F, S), Data(&C->Repl) {
    IsExternal = S.isExternal();
    IsCOMDAT = C->isCOMDAT();
  }

  static bool classof(const SymbolBody *S) {
    return S->kind() == DefinedRegularKind;
  }

  uint64_t getRVA() { return (*Data)->getRVA() + Sym->Value; }
  bool isCOMDAT() { return IsCOMDAT; }
  SectionChunk *getChunk() { return *Data; }
  uint32_t getValue() { return Sym->Value; }

private:
  SectionChunk **Data;
};

class DefinedCommon : public DefinedCOFF {
public:
  DefinedCommon(ObjectFile *F, COFFSymbolRef S, CommonChunk *C)
      : DefinedCOFF(DefinedCommonKind, F, S), Data(C) {
    IsExternal = S.isExternal();
  }

  static bool classof(const SymbolBody *S) {
    return S->kind() == DefinedCommonKind;
  }

  uint64_t getRVA() { return Data->getRVA(); }

private:
  friend SymbolTable;
  uint64_t getSize() { return Sym->Value; }
  CommonChunk *Data;
};

// Absolute symbols.
class DefinedAbsolute : public Defined {
public:
  DefinedAbsolute(StringRef N, COFFSymbolRef S)
      : Defined(DefinedAbsoluteKind, N), VA(S.getValue()) {
    IsExternal = S.isExternal();
  }

  DefinedAbsolute(StringRef N, uint64_t V)
      : Defined(DefinedAbsoluteKind, N), VA(V) {}

  static bool classof(const SymbolBody *S) {
    return S->kind() == DefinedAbsoluteKind;
  }

  uint64_t getRVA() { return VA - Config->ImageBase; }
  void setVA(uint64_t V) { VA = V; }

private:
  uint64_t VA;
};

// This is a kind of absolute symbol but relative to the image base.
// Unlike absolute symbols, relocations referring this kind of symbols
// are subject of the base relocation. This type is used rarely --
// mainly for __ImageBase.
class DefinedRelative : public Defined {
public:
  explicit DefinedRelative(StringRef Name, uint64_t V = 0)
      : Defined(DefinedRelativeKind, Name), RVA(V) {}

  static bool classof(const SymbolBody *S) {
    return S->kind() == DefinedRelativeKind;
  }

  uint64_t getRVA() { return RVA; }
  void setRVA(uint64_t V) { RVA = V; }

private:
  uint64_t RVA;
};

// This class represents a symbol defined in an archive file. It is
// created from an archive file header, and it knows how to load an
// object file from an archive to replace itself with a defined
// symbol. If the resolver finds both Undefined and Lazy for
// the same name, it will ask the Lazy to load a file.
class Lazy : public SymbolBody {
public:
  Lazy(ArchiveFile *F, const Archive::Symbol S)
      : SymbolBody(LazyKind, S.getName()), File(F), Sym(S) {}

  static bool classof(const SymbolBody *S) { return S->kind() == LazyKind; }

  ArchiveFile *File;

private:
  friend SymbolTable;

private:
  const Archive::Symbol Sym;
};

// Undefined symbols.
class Undefined : public SymbolBody {
public:
  explicit Undefined(StringRef N) : SymbolBody(UndefinedKind, N) {}

  static bool classof(const SymbolBody *S) {
    return S->kind() == UndefinedKind;
  }

  // An undefined symbol can have a fallback symbol which gives an
  // undefined symbol a second chance if it would remain undefined.
  // If it remains undefined, it'll be replaced with whatever the
  // Alias pointer points to.
  SymbolBody *WeakAlias = nullptr;

  // If this symbol is external weak, try to resolve it to a defined
  // symbol by searching the chain of fallback symbols. Returns the symbol if
  // successful, otherwise returns null.
  Defined *getWeakAlias();
};

// Windows-specific classes.

// This class represents a symbol imported from a DLL. This has two
// names for internal use and external use. The former is used for
// name resolution, and the latter is used for the import descriptor
// table in an output. The former has "__imp_" prefix.
class DefinedImportData : public Defined {
public:
  DefinedImportData(StringRef N, ImportFile *F)
      : Defined(DefinedImportDataKind, N), File(F) {
  }

  static bool classof(const SymbolBody *S) {
    return S->kind() == DefinedImportDataKind;
  }

  uint64_t getRVA() { return File->Location->getRVA(); }
  StringRef getDLLName() { return File->DLLName; }
  StringRef getExternalName() { return File->ExternalName; }
  void setLocation(Chunk *AddressTable) { File->Location = AddressTable; }
  uint16_t getOrdinal() { return File->Hdr->OrdinalHint; }

private:
  ImportFile *File;
};

// This class represents a symbol for a jump table entry which jumps
// to a function in a DLL. Linker are supposed to create such symbols
// without "__imp_" prefix for all function symbols exported from
// DLLs, so that you can call DLL functions as regular functions with
// a regular name. A function pointer is given as a DefinedImportData.
class DefinedImportThunk : public Defined {
public:
  DefinedImportThunk(StringRef Name, DefinedImportData *S, uint16_t Machine);

  static bool classof(const SymbolBody *S) {
    return S->kind() == DefinedImportThunkKind;
  }

  uint64_t getRVA() { return Data->getRVA(); }
  Chunk *getChunk() { return Data; }

private:
  Chunk *Data;
};

// If you have a symbol "__imp_foo" in your object file, a symbol name
// "foo" becomes automatically available as a pointer to "__imp_foo".
// This class is for such automatically-created symbols.
// Yes, this is an odd feature. We didn't intend to implement that.
// This is here just for compatibility with MSVC.
class DefinedLocalImport : public Defined {
public:
  DefinedLocalImport(StringRef N, Defined *S)
      : Defined(DefinedLocalImportKind, N), Data(make<LocalImportChunk>(S)) {}

  static bool classof(const SymbolBody *S) {
    return S->kind() == DefinedLocalImportKind;
  }

  uint64_t getRVA() { return Data->getRVA(); }
  Chunk *getChunk() { return Data; }

private:
  LocalImportChunk *Data;
};

class DefinedBitcode : public Defined {
  friend SymbolBody;
public:
  DefinedBitcode(BitcodeFile *F, StringRef N, bool IsReplaceable)
      : Defined(DefinedBitcodeKind, N), File(F) {
    // IsReplaceable tracks whether the bitcode symbol may be replaced with some
    // other (defined, common or bitcode) symbol. This is the case for common,
    // comdat and weak external symbols. We try to replace bitcode symbols with
    // "real" symbols (see SymbolTable::add{Regular,Bitcode}), and resolve the
    // result against the real symbol from the combined LTO object.
    this->IsReplaceable = IsReplaceable;
  }

  static bool classof(const SymbolBody *S) {
    return S->kind() == DefinedBitcodeKind;
  }

  BitcodeFile *File;
};

inline uint64_t Defined::getRVA() {
  switch (kind()) {
  case DefinedAbsoluteKind:
    return cast<DefinedAbsolute>(this)->getRVA();
  case DefinedRelativeKind:
    return cast<DefinedRelative>(this)->getRVA();
  case DefinedImportDataKind:
    return cast<DefinedImportData>(this)->getRVA();
  case DefinedImportThunkKind:
    return cast<DefinedImportThunk>(this)->getRVA();
  case DefinedLocalImportKind:
    return cast<DefinedLocalImport>(this)->getRVA();
  case DefinedCommonKind:
    return cast<DefinedCommon>(this)->getRVA();
  case DefinedRegularKind:
    return cast<DefinedRegular>(this)->getRVA();
  case DefinedBitcodeKind:
    llvm_unreachable("There is no address for a bitcode symbol.");
  case LazyKind:
  case UndefinedKind:
    llvm_unreachable("Cannot get the address for an undefined symbol.");
  }
  llvm_unreachable("unknown symbol kind");
}

// A real symbol object, SymbolBody, is usually stored within a Symbol. There's
// always one Symbol for each symbol name. The resolver updates the SymbolBody
// stored in the Body field of this object as it resolves symbols. Symbol also
// holds computed properties of symbol names.
struct Symbol {
  // True if this symbol was referenced by a regular (non-bitcode) object.
  unsigned IsUsedInRegularObj : 1;

  // True if we've seen both a lazy and an undefined symbol with this symbol
  // name, which means that we have enqueued an archive member load and should
  // not load any more archive members to resolve the same symbol.
  unsigned PendingArchiveLoad : 1;

  // This field is used to store the Symbol's SymbolBody. This instantiation of
  // AlignedCharArrayUnion gives us a struct with a char array field that is
  // large and aligned enough to store any derived class of SymbolBody.
  llvm::AlignedCharArrayUnion<DefinedRegular, DefinedCommon, DefinedAbsolute,
                              DefinedRelative, Lazy, Undefined,
                              DefinedImportData, DefinedImportThunk,
                              DefinedLocalImport, DefinedBitcode>
      Body;

  SymbolBody *body() {
    return reinterpret_cast<SymbolBody *>(Body.buffer);
  }
  const SymbolBody *body() const { return const_cast<Symbol *>(this)->body(); }
};

template <typename T, typename... ArgT>
void replaceBody(Symbol *S, ArgT &&... Arg) {
  static_assert(sizeof(T) <= sizeof(S->Body), "Body too small");
  static_assert(alignof(T) <= alignof(decltype(S->Body)),
                "Body not aligned enough");
  assert(static_cast<SymbolBody *>(static_cast<T *>(nullptr)) == nullptr &&
         "Not a SymbolBody");
  new (S->Body.buffer) T(std::forward<ArgT>(Arg)...);
}

inline Symbol *SymbolBody::symbol() {
  assert(isExternal());
  return reinterpret_cast<Symbol *>(reinterpret_cast<char *>(this) -
                                    offsetof(Symbol, Body));
}
} // namespace coff

std::string toString(coff::SymbolBody &B);
} // namespace lld

#endif