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//===- Symbols.h ------------------------------------------------*- C++ -*-===//
// The LLVM Linker
// This file is distributed under the University of Illinois Open Source
// License. See LICENSE.TXT for details.
#include "Chunks.h"
#include "Config.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;
class SymbolBody;
// A real symbol object, SymbolBody, is usually accessed indirectly
// through a Symbol. There's always one Symbol for each symbol name.
// The resolver updates SymbolBody pointers as it resolves symbols.
struct Symbol {
explicit Symbol(SymbolBody *P) : Body(P) {}
SymbolBody *Body;
// The base class for real symbol classes.
class SymbolBody {
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,
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();
// A SymbolBody has a backreference to a Symbol. Originally they are
// doubly-linked. A backreference will never change. But the pointer
// in the Symbol may be mutated by the resolver. If you have a
// pointer P to a SymbolBody and are not sure whether the resolver
// has chosen the object among other objects having the same name,
// you can access P->Backref->Body to get the resolver's result.
void setBackref(Symbol *P) { Backref = P; }
SymbolBody *repl() { return Backref ? Backref->Body : this; }
// Decides which symbol should "win" in the symbol table, this or
// the Other. Returns 1 if this wins, -1 if the Other wins, or 0 if
// they are duplicate (conflicting) symbols.
int compare(SymbolBody *Other);
// Returns a name of this symbol including source file name.
// Used only for debugging and logging.
std::string getDebugName();
explicit SymbolBody(Kind K, StringRef N = "")
: SymbolKind(K), IsExternal(true), IsCOMDAT(false),
IsReplaceable(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;
StringRef Name;
Symbol *Backref = nullptr;
// The base class for any defined symbols, including absolute symbols,
// etc.
class Defined : public SymbolBody {
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;
DefinedCOFF(Kind K, ObjectFile *F, COFFSymbolRef S)
: Defined(K), File(F), Sym(S.getGeneric()) {}
static bool classof(const SymbolBody *S) {
return S->kind() <= LastDefinedCOFFKind;
int getFileIndex() { return File->Index; }
COFFSymbolRef getCOFFSymbol();
ObjectFile *File;
const coff_symbol_generic *Sym;
// Regular defined symbols read from object file symbol tables.
class DefinedRegular : public DefinedCOFF {
DefinedRegular(ObjectFile *F, COFFSymbolRef S, SectionChunk *C)
: DefinedCOFF(DefinedRegularKind, F, S), Data(&C->Repl) {
IsExternal = S.isExternal();
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; }
SectionChunk **Data;
class DefinedCommon : public DefinedCOFF {
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(); }
friend SymbolBody;
uint64_t getSize() { return Sym->Value; }
CommonChunk *Data;
// Absolute symbols.
class DefinedAbsolute : public Defined {
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; }
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 {
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; }
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 {
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; }
// Returns an object file for this symbol, or a nullptr if the file
// was already returned.
std::unique_ptr<InputFile> getMember();
int getFileIndex() { return File->Index; }
ArchiveFile *File;
const Archive::Symbol Sym;
// Undefined symbols.
class Undefined : public SymbolBody {
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 {
DefinedImportData(StringRef D, StringRef N, StringRef E,
const coff_import_header *H)
: Defined(DefinedImportDataKind, N), DLLName(D), ExternalName(E), Hdr(H) {
static bool classof(const SymbolBody *S) {
return S->kind() == DefinedImportDataKind;
uint64_t getRVA() { return Location->getRVA(); }
StringRef getDLLName() { return DLLName; }
StringRef getExternalName() { return ExternalName; }
void setLocation(Chunk *AddressTable) { Location = AddressTable; }
uint16_t getOrdinal() { return Hdr->OrdinalHint; }
StringRef DLLName;
StringRef ExternalName;
const coff_import_header *Hdr;
Chunk *Location = nullptr;
// 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 {
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.get(); }
std::unique_ptr<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 {
DefinedLocalImport(StringRef N, Defined *S)
: Defined(DefinedLocalImportKind, N), Data(S) {}
static bool classof(const SymbolBody *S) {
return S->kind() == DefinedLocalImportKind;
uint64_t getRVA() { return Data.getRVA(); }
Chunk *getChunk() { return &Data; }
LocalImportChunk Data;
class DefinedBitcode : public Defined {
friend SymbolBody;
DefinedBitcode(BitcodeFile *F, StringRef N, bool IsReplaceable)
: Defined(DefinedBitcodeKind, N), File(F) {
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");
} // namespace coff
} // namespace lld