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//===- Symbols.h ------------------------------------------------*- C++ -*-===//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
#include "InputSection.h"
#include "Target.h"
#include "lld/Common/ErrorHandler.h"
#include "lld/Common/Strings.h"
#include "llvm/Object/Archive.h"
#include "llvm/Support/MathExtras.h"
namespace lld {
namespace macho {
class InputSection;
class MachHeaderSection;
class DylibFile;
class ArchiveFile;
struct StringRefZ {
StringRefZ(const char *s) : data(s), size(-1) {}
StringRefZ(StringRef s) : data(, size(s.size()) {}
const char *data;
const uint32_t size;
class Symbol {
enum Kind {
virtual ~Symbol() {}
Kind kind() const { return static_cast<Kind>(symbolKind); }
StringRef getName() const {
if (nameSize == (uint32_t)-1)
nameSize = strlen(nameData);
return {nameData, nameSize};
virtual uint64_t getVA() const { return 0; }
virtual uint64_t getFileOffset() const {
llvm_unreachable("attempt to get an offset from a non-defined symbol");
virtual bool isWeakDef() const { llvm_unreachable("cannot be weak def"); }
// Only undefined or dylib symbols can be weak references. A weak reference
// need not be satisfied at runtime, e.g. due to the symbol not being
// available on a given target platform.
virtual bool isWeakRef() const { llvm_unreachable("cannot be a weak ref"); }
virtual bool isTlv() const { llvm_unreachable("cannot be TLV"); }
// Whether this symbol is in the GOT or TLVPointer sections.
bool isInGot() const { return gotIndex != UINT32_MAX; }
// Whether this symbol is in the StubsSection.
bool isInStubs() const { return stubsIndex != UINT32_MAX; }
// The index of this symbol in the GOT or the TLVPointer section, depending
// on whether it is a thread-local. A given symbol cannot be referenced by
// both these sections at once.
uint32_t gotIndex = UINT32_MAX;
uint32_t stubsIndex = UINT32_MAX;
uint32_t symtabIndex = UINT32_MAX;
Symbol(Kind k, StringRefZ name)
: symbolKind(k), nameData(, nameSize(name.size) {}
Kind symbolKind;
const char *nameData;
mutable uint32_t nameSize;
class Defined : public Symbol {
Defined(StringRefZ name, InputSection *isec, uint32_t value, bool isWeakDef,
bool isExternal, bool isPrivateExtern)
: Symbol(DefinedKind, name), isec(isec), value(value),
overridesWeakDef(false), privateExtern(isPrivateExtern),
weakDef(isWeakDef), external(isExternal) {}
bool isWeakDef() const override { return weakDef; }
bool isExternalWeakDef() const {
return isWeakDef() && isExternal() && !privateExtern;
bool isTlv() const override {
return !isAbsolute() && isThreadLocalVariables(isec->flags);
bool isExternal() const { return external; }
bool isAbsolute() const { return isec == nullptr; }
uint64_t getVA() const override;
uint64_t getFileOffset() const override;
static bool classof(const Symbol *s) { return s->kind() == DefinedKind; }
InputSection *isec;
uint32_t value;
bool overridesWeakDef : 1;
bool privateExtern : 1;
const bool weakDef : 1;
const bool external : 1;
// This enum does double-duty: as a symbol property, it indicates whether & how
// a dylib symbol is referenced. As a DylibFile property, it indicates the kind
// of referenced symbols contained within the file. If there are both weak
// and strong references to the same file, we will count the file as
// strongly-referenced.
enum class RefState : uint8_t { Unreferenced = 0, Weak = 1, Strong = 2 };
class Undefined : public Symbol {
Undefined(StringRefZ name, RefState refState)
: Symbol(UndefinedKind, name), refState(refState) {
assert(refState != RefState::Unreferenced);
bool isWeakRef() const override { return refState == RefState::Weak; }
static bool classof(const Symbol *s) { return s->kind() == UndefinedKind; }
RefState refState : 2;
// On Unix, it is traditionally allowed to write variable definitions without
// initialization expressions (such as "int foo;") to header files. These are
// called tentative definitions.
// Using tentative definitions is usually considered a bad practice; you should
// write only declarations (such as "extern int foo;") to header files.
// Nevertheless, the linker and the compiler have to do something to support
// bad code by allowing duplicate definitions for this particular case.
// The compiler creates common symbols when it sees tentative definitions.
// (You can suppress this behavior and let the compiler create a regular
// defined symbol by passing -fno-common. -fno-common is the default in clang
// as of LLVM 11.0.) When linking the final binary, if there are remaining
// common symbols after name resolution is complete, the linker converts them
// to regular defined symbols in a __common section.
class CommonSymbol : public Symbol {
CommonSymbol(StringRefZ name, InputFile *file, uint64_t size, uint32_t align,
bool isPrivateExtern)
: Symbol(CommonKind, name), file(file), size(size),
align(align != 1 ? align : llvm::PowerOf2Ceil(size)),
privateExtern(isPrivateExtern) {
// TODO: cap maximum alignment
static bool classof(const Symbol *s) { return s->kind() == CommonKind; }
InputFile *const file;
const uint64_t size;
const uint32_t align;
const bool privateExtern;
class DylibSymbol : public Symbol {
DylibSymbol(DylibFile *file, StringRefZ name, bool isWeakDef,
RefState refState, bool isTlv)
: Symbol(DylibKind, name), file(file), refState(refState),
weakDef(isWeakDef), tlv(isTlv) {}
bool isWeakDef() const override { return weakDef; }
bool isWeakRef() const override { return refState == RefState::Weak; }
bool isReferenced() const { return refState != RefState::Unreferenced; }
bool isTlv() const override { return tlv; }
bool hasStubsHelper() const { return stubsHelperIndex != UINT32_MAX; }
static bool classof(const Symbol *s) { return s->kind() == DylibKind; }
DylibFile *file;
uint32_t stubsHelperIndex = UINT32_MAX;
uint32_t lazyBindOffset = UINT32_MAX;
RefState refState : 2;
const bool weakDef : 1;
const bool tlv : 1;
class LazySymbol : public Symbol {
LazySymbol(ArchiveFile *file, const llvm::object::Archive::Symbol &sym)
: Symbol(LazyKind, sym.getName()), file(file), sym(sym) {}
static bool classof(const Symbol *s) { return s->kind() == LazyKind; }
void fetchArchiveMember();
ArchiveFile *file;
const llvm::object::Archive::Symbol sym;
// The Itanium C++ ABI requires dylibs to pass a pointer to __cxa_atexit which
// does e.g. cleanup of static global variables. The ABI document says that the
// pointer can point to any address in one of the dylib's segments, but in
// practice ld64 seems to set it to point to the header, so that's what's
// implemented here.
// The ARM C++ ABI uses __dso_handle similarly, but I (int3) have not yet
// tested this on an ARM platform.
// DSOHandle effectively functions like a Defined symbol, but it doesn't belong
// to an InputSection.
class DSOHandle : public Symbol {
DSOHandle(const MachHeaderSection *header)
: Symbol(DSOHandleKind, name), header(header) {}
const MachHeaderSection *header;
uint64_t getVA() const override;
uint64_t getFileOffset() const override;
bool isWeakDef() const override { return false; }
bool isTlv() const override { return false; }
static constexpr StringRef name = "___dso_handle";
static bool classof(const Symbol *s) { return s->kind() == DSOHandleKind; }
union SymbolUnion {
alignas(Defined) char a[sizeof(Defined)];
alignas(Undefined) char b[sizeof(Undefined)];
alignas(CommonSymbol) char c[sizeof(CommonSymbol)];
alignas(DylibSymbol) char d[sizeof(DylibSymbol)];
alignas(LazySymbol) char e[sizeof(LazySymbol)];
alignas(DSOHandle) char f[sizeof(DSOHandle)];
template <typename T, typename... ArgT>
T *replaceSymbol(Symbol *s, ArgT &&... arg) {
static_assert(sizeof(T) <= sizeof(SymbolUnion), "SymbolUnion too small");
static_assert(alignof(T) <= alignof(SymbolUnion),
"SymbolUnion not aligned enough");
assert(static_cast<Symbol *>(static_cast<T *>(nullptr)) == nullptr &&
"Not a Symbol");
return new (s) T(std::forward<ArgT>(arg)...);
} // namespace macho
std::string toString(const macho::Symbol &);
std::string toMachOString(const llvm::object::Archive::Symbol &);
} // namespace lld