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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 "Config.h"
#include "InputFiles.h"
#include "Target.h"
#include "llvm/Object/Archive.h"
#include "llvm/Support/MathExtras.h"
namespace lld {
namespace macho {
class MachHeaderSection;
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 symbolKind; }
StringRef getName() const {
if (nameSize == (uint32_t)-1)
nameSize = strlen(nameData);
return {nameData, nameSize};
bool isLive() const { return used; }
bool isLazy() const {
return symbolKind == LazyArchiveKind || symbolKind == LazyObjectKind;
virtual uint64_t getVA() const { return 0; }
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 { return false; }
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; }
uint64_t getStubVA() const;
uint64_t getLazyPtrVA() const;
uint64_t getGotVA() const;
uint64_t getTlvVA() const;
uint64_t resolveBranchVA() const {
assert(isa<Defined>(this) || isa<DylibSymbol>(this));
return isInStubs() ? getStubVA() : getVA();
uint64_t resolveGotVA() const { return isInGot() ? getGotVA() : getVA(); }
uint64_t resolveTlvVA() const { return isInGot() ? getTlvVA() : getVA(); }
// 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 lazyBindOffset = UINT32_MAX;
uint32_t stubsHelperIndex = UINT32_MAX;
uint32_t stubsIndex = UINT32_MAX;
uint32_t symtabIndex = UINT32_MAX;
InputFile *getFile() const { return file; }
Symbol(Kind k, StringRefZ name, InputFile *file)
: symbolKind(k), nameData(, file(file), nameSize(name.size),
isUsedInRegularObj(!file || isa<ObjFile>(file)),
used(!config->deadStrip) {}
Kind symbolKind;
const char *nameData;
InputFile *file;
mutable uint32_t nameSize;
// True if this symbol was referenced by a regular (non-bitcode) object.
bool isUsedInRegularObj : 1;
// True if this symbol is used from a live section.
bool used : 1;
class Defined : public Symbol {
Defined(StringRefZ name, InputFile *file, InputSection *isec, uint64_t value,
uint64_t size, bool isWeakDef, bool isExternal, bool isPrivateExtern,
bool includeInSymtab, bool isThumb, bool isReferencedDynamically,
bool noDeadStrip, bool canOverrideWeakDef = false,
bool isWeakDefCanBeHidden = false, bool interposable = false);
bool isWeakDef() const override { return weakDef; }
bool isExternalWeakDef() const {
return isWeakDef() && isExternal() && !privateExtern;
bool isTlv() const override;
bool isExternal() const { return external; }
bool isAbsolute() const { return isec == nullptr; }
uint64_t getVA() const override;
// Returns the object file that this symbol was defined in. This value differs
// from `getFile()` if the symbol originated from a bitcode file.
ObjFile *getObjectFile() const;
std::string getSourceLocation();
// Ensure this symbol's pointers to InputSections point to their canonical
// copies.
void canonicalize();
static bool classof(const Symbol *s) { return s->kind() == DefinedKind; }
// Place the bitfields first so that they can get placed in the tail padding
// of the parent class, on platforms which support it.
bool overridesWeakDef : 1;
// Whether this symbol should appear in the output binary's export trie.
bool privateExtern : 1;
// Whether this symbol should appear in the output symbol table.
bool includeInSymtab : 1;
// Whether this symbol was folded into a different symbol during ICF.
bool wasIdenticalCodeFolded : 1;
// Only relevant when compiling for Thumb-supporting arm32 archs.
bool thumb : 1;
// Symbols marked referencedDynamically won't be removed from the output's
// symbol table by tools like strip. In theory, this could be set on arbitrary
// symbols in input object files. In practice, it's used solely for the
// synthetic __mh_execute_header symbol.
// This is information for the static linker, and it's also written to the
// output file's symbol table for tools running later (such as `strip`).
bool referencedDynamically : 1;
// Set on symbols that should not be removed by dead code stripping.
// Set for example on `__attribute__((used))` globals, or on some Objective-C
// metadata. This is information only for the static linker and not written
// to the output.
bool noDeadStrip : 1;
// Whether references to this symbol can be interposed at runtime to point to
// a different symbol definition (with the same name). For example, if both
// dylib A and B define an interposable symbol _foo, and we load A before B at
// runtime, then all references to _foo within dylib B will point to the
// definition in dylib A.
// Only extern symbols may be interposable.
bool interposable : 1;
bool weakDefCanBeHidden : 1;
const bool weakDef : 1;
const bool external : 1;
InputSection *isec;
// Contains the offset from the containing subsection. Note that this is
// different from nlist::n_value, which is the absolute address of the symbol.
uint64_t value;
// size is only calculated for regular (non-bitcode) symbols.
uint64_t size;
// This can be a subsection of either __compact_unwind or __eh_frame.
ConcatInputSection *unwindEntry = nullptr;
// 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, InputFile *file, RefState refState,
bool wasBitcodeSymbol)
: Symbol(UndefinedKind, name, file), refState(refState),
wasBitcodeSymbol(wasBitcodeSymbol) {
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;
bool wasBitcodeSymbol;
// 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), 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; }
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), refState(refState), weakDef(isWeakDef),
tlv(isTlv) {
if (file && refState > RefState::Unreferenced)
uint64_t getVA() const override;
bool isWeakDef() const override { return weakDef; }
// Symbols from weak libraries/frameworks are also weakly-referenced.
bool isWeakRef() const override {
return refState == RefState::Weak ||
(file && getFile()->umbrella->forceWeakImport);
bool isReferenced() const { return refState != RefState::Unreferenced; }
bool isTlv() const override { return tlv; }
bool isDynamicLookup() const { return file == nullptr; }
bool hasStubsHelper() const { return stubsHelperIndex != UINT32_MAX; }
DylibFile *getFile() const {
return cast<DylibFile>(file);
static bool classof(const Symbol *s) { return s->kind() == DylibKind; }
RefState getRefState() const { return refState; }
void reference(RefState newState) {
assert(newState > RefState::Unreferenced);
if (refState == RefState::Unreferenced && file)
refState = std::max(refState, newState);
void unreference() {
// dynamic_lookup symbols have no file.
if (refState > RefState::Unreferenced && file) {
assert(getFile()->numReferencedSymbols > 0);
RefState refState : 2;
const bool weakDef : 1;
const bool tlv : 1;
class LazyArchive : public Symbol {
LazyArchive(ArchiveFile *file, const llvm::object::Archive::Symbol &sym)
: Symbol(LazyArchiveKind, sym.getName(), file), sym(sym) {}
ArchiveFile *getFile() const { return cast<ArchiveFile>(file); }
void fetchArchiveMember();
static bool classof(const Symbol *s) { return s->kind() == LazyArchiveKind; }
const llvm::object::Archive::Symbol sym;
// A defined symbol in an ObjFile/BitcodeFile surrounded by --start-lib and
// --end-lib.
class LazyObject : public Symbol {
LazyObject(InputFile &file, StringRef name)
: Symbol(LazyObjectKind, name, &file) {
isUsedInRegularObj = false;
static bool classof(const Symbol *s) { return s->kind() == LazyObjectKind; }
// Represents N_INDR symbols. Note that if we are given valid, linkable inputs,
// then all AliasSymbol instances will be converted into one of the other Symbol
// types after `createAliases()` runs.
class AliasSymbol final : public Symbol {
AliasSymbol(InputFile *file, StringRef name, StringRef aliasedName,
bool isPrivateExtern)
: Symbol(AliasKind, name, file), privateExtern(isPrivateExtern),
aliasedName(aliasedName) {}
StringRef getAliasedName() const { return aliasedName; }
static bool classof(const Symbol *s) { return s->kind() == AliasKind; }
const bool privateExtern;
StringRef aliasedName;
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(LazyArchive) char e[sizeof(LazyArchive)];
alignas(LazyObject) char f[sizeof(LazyObject)];
alignas(AliasSymbol) char g[sizeof(AliasSymbol)];
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");
bool isUsedInRegularObj = s->isUsedInRegularObj;
bool used = s->used;
T *sym = new (s) T(std::forward<ArgT>(arg)...);
sym->isUsedInRegularObj |= isUsedInRegularObj;
sym->used |= used;
return sym;
// Can a symbol's address only be resolved at runtime?
inline bool needsBinding(const Symbol *sym) {
if (isa<DylibSymbol>(sym))
return true;
if (const auto *defined = dyn_cast<Defined>(sym))
return defined->isExternalWeakDef() || defined->interposable;
return false;
} // namespace macho
std::string toString(const macho::Symbol &);
std::string toMachOString(const llvm::object::Archive::Symbol &);
} // namespace lld