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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/Common/LLVM.h"
#include "lld/Common/Memory.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 InputFile;
class ObjFile;
class SymbolTable;
// The base class for real symbol classes.
class Symbol {
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 = DefinedSyntheticKind,
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();
// Indicates that this symbol will be included in the final image. Only valid
// after calling markLive.
bool isLive() const;
friend SymbolTable;
explicit Symbol(Kind K, StringRef N = "")
: SymbolKind(K), IsExternal(true), IsCOMDAT(false),
WrittenToSymtab(false), PendingArchiveLoad(false), IsGCRoot(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 Writer::createSymbolAndStringTable() to prevent
// symbols from being written to the symbol table more than once.
unsigned WrittenToSymtab : 1;
// 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;
/// True if we've already added this symbol to the list of GC roots.
unsigned IsGCRoot : 1;
StringRef Name;
// The base class for any defined symbols, including absolute symbols,
// etc.
class Defined : public Symbol {
Defined(Kind K, StringRef N) : Symbol(K, N) {}
static bool classof(const Symbol *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 chunk containing this symbol. Absolute symbols and __ImageBase
// do not have chunks, so this may return null.
Chunk *getChunk();
// Symbols defined via a COFF object file or bitcode file. For COFF files, this
// stores a coff_symbol_generic*, and names of internal symbols are lazily
// loaded through that. For bitcode files, Sym is nullptr and the name is stored
// as a StringRef.
class DefinedCOFF : public Defined {
friend Symbol;
DefinedCOFF(Kind K, InputFile *F, StringRef N, const coff_symbol_generic *S)
: Defined(K, N), File(F), Sym(S) {}
static bool classof(const Symbol *S) {
return S->kind() <= LastDefinedCOFFKind;
InputFile *getFile() { return File; }
COFFSymbolRef getCOFFSymbol();
InputFile *File;
const coff_symbol_generic *Sym;
// Regular defined symbols read from object file symbol tables.
class DefinedRegular : public DefinedCOFF {
DefinedRegular(InputFile *F, StringRef N, bool IsCOMDAT,
bool IsExternal = false,
const coff_symbol_generic *S = nullptr,
SectionChunk *C = nullptr)
: DefinedCOFF(DefinedRegularKind, F, N, S), Data(C ? &C->Repl : nullptr) {
this->IsExternal = IsExternal;
static bool classof(const Symbol *S) {
return S->kind() == DefinedRegularKind;
uint64_t getRVA() const { return (*Data)->getRVA() + Sym->Value; }
bool isCOMDAT() const { return IsCOMDAT; }
SectionChunk *getChunk() const { return *Data; }
uint32_t getValue() const { return Sym->Value; }
SectionChunk **Data;
class DefinedCommon : public DefinedCOFF {
DefinedCommon(InputFile *F, StringRef N, uint64_t Size,
const coff_symbol_generic *S = nullptr,
CommonChunk *C = nullptr)
: DefinedCOFF(DefinedCommonKind, F, N, S), Data(C), Size(Size) {
this->IsExternal = true;
static bool classof(const Symbol *S) {
return S->kind() == DefinedCommonKind;
uint64_t getRVA() { return Data->getRVA(); }
CommonChunk *getChunk() { return Data; }
friend SymbolTable;
uint64_t getSize() const { return Size; }
CommonChunk *Data;
uint64_t Size;
// 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 Symbol *S) {
return S->kind() == DefinedAbsoluteKind;
uint64_t getRVA() { return VA - Config->ImageBase; }
void setVA(uint64_t V) { VA = V; }
// Section index relocations against absolute symbols resolve to
// this 16 bit number, and it is the largest valid section index
// plus one. This variable keeps it.
static uint16_t NumOutputSections;
uint64_t VA;
// This symbol is used for linker-synthesized symbols like __ImageBase and
// __safe_se_handler_table.
class DefinedSynthetic : public Defined {
explicit DefinedSynthetic(StringRef Name, Chunk *C)
: Defined(DefinedSyntheticKind, Name), C(C) {}
static bool classof(const Symbol *S) {
return S->kind() == DefinedSyntheticKind;
// A null chunk indicates that this is __ImageBase. Otherwise, this is some
// other synthesized chunk, like SEHTableChunk.
uint32_t getRVA() { return C ? C->getRVA() : 0; }
Chunk *getChunk() { return C; }
Chunk *C;
// 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 Symbol {
Lazy(ArchiveFile *F, const Archive::Symbol S)
: Symbol(LazyKind, S.getName()), File(F), Sym(S) {}
static bool classof(const Symbol *S) { return S->kind() == LazyKind; }
ArchiveFile *File;
friend SymbolTable;
const Archive::Symbol Sym;
// Undefined symbols.
class Undefined : public Symbol {
explicit Undefined(StringRef N) : Symbol(UndefinedKind, N) {}
static bool classof(const Symbol *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.
Symbol *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 N, ImportFile *F)
: Defined(DefinedImportDataKind, N), File(F) {
static bool classof(const Symbol *S) {
return S->kind() == DefinedImportDataKind;
uint64_t getRVA() { return File->Location->getRVA(); }
Chunk *getChunk() { return File->Location; }
void setLocation(Chunk *AddressTable) { File->Location = AddressTable; }
StringRef getDLLName() { return File->DLLName; }
StringRef getExternalName() { return File->ExternalName; }
uint16_t getOrdinal() { return File->Hdr->OrdinalHint; }
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 {
DefinedImportThunk(StringRef Name, DefinedImportData *S, uint16_t Machine);
static bool classof(const Symbol *S) {
return S->kind() == DefinedImportThunkKind;
uint64_t getRVA() { return Data->getRVA(); }
Chunk *getChunk() { return Data; }
DefinedImportData *WrappedSym;
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(make<LocalImportChunk>(S)) {}
static bool classof(const Symbol *S) {
return S->kind() == DefinedLocalImportKind;
uint64_t getRVA() { return Data->getRVA(); }
Chunk *getChunk() { return Data; }
LocalImportChunk *Data;
inline uint64_t Defined::getRVA() {
switch (kind()) {
case DefinedAbsoluteKind:
return cast<DefinedAbsolute>(this)->getRVA();
case DefinedSyntheticKind:
return cast<DefinedSynthetic>(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 LazyKind:
case UndefinedKind:
llvm_unreachable("Cannot get the address for an undefined symbol.");
llvm_unreachable("unknown symbol kind");
inline Chunk *Defined::getChunk() {
switch (kind()) {
case DefinedRegularKind:
return cast<DefinedRegular>(this)->getChunk();
case DefinedAbsoluteKind:
return nullptr;
case DefinedSyntheticKind:
return cast<DefinedSynthetic>(this)->getChunk();
case DefinedImportDataKind:
return cast<DefinedImportData>(this)->getChunk();
case DefinedImportThunkKind:
return cast<DefinedImportThunk>(this)->getChunk();
case DefinedLocalImportKind:
return cast<DefinedLocalImport>(this)->getChunk();
case DefinedCommonKind:
return cast<DefinedCommon>(this)->getChunk();
case LazyKind:
case UndefinedKind:
llvm_unreachable("Cannot get the chunk of an undefined symbol.");
llvm_unreachable("unknown symbol kind");
// A buffer class that is large enough to hold any Symbol-derived
// object. We allocate memory using this class and instantiate a symbol
// using the placement new.
union SymbolUnion {
alignas(DefinedRegular) char A[sizeof(DefinedRegular)];
alignas(DefinedCommon) char B[sizeof(DefinedCommon)];
alignas(DefinedAbsolute) char C[sizeof(DefinedAbsolute)];
alignas(DefinedSynthetic) char D[sizeof(DefinedSynthetic)];
alignas(Lazy) char E[sizeof(Lazy)];
alignas(Undefined) char F[sizeof(Undefined)];
alignas(DefinedImportData) char G[sizeof(DefinedImportData)];
alignas(DefinedImportThunk) char H[sizeof(DefinedImportThunk)];
alignas(DefinedLocalImport) char I[sizeof(DefinedLocalImport)];
template <typename T, typename... ArgT>
void replaceSymbol(Symbol *S, ArgT &&... Arg) {
"Symbol types must be trivially destructible");
static_assert(sizeof(T) <= sizeof(SymbolUnion), "Symbol too small");
static_assert(alignof(T) <= alignof(SymbolUnion),
"SymbolUnion not aligned enough");
assert(static_cast<Symbol *>(static_cast<T *>(nullptr)) == nullptr &&
"Not a Symbol");
new (S) T(std::forward<ArgT>(Arg)...);
} // namespace coff
std::string toString(coff::Symbol &B);
} // namespace lld