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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
// This file defines various types of Symbols.
#include "InputFiles.h"
#include "InputSection.h"
#include "lld/Common/LLVM.h"
#include "lld/Common/Strings.h"
#include "llvm/ADT/DenseMap.h"
#include "llvm/Object/Archive.h"
#include "llvm/Object/ELF.h"
namespace lld {
// Returns a string representation for a symbol for diagnostics.
std::string toString(const elf::Symbol &);
// There are two different ways to convert an Archive::Symbol to a string:
// One for Microsoft name mangling and one for Itanium name mangling.
// Call the functions toCOFFString and toELFString, not just toString.
std::string toELFString(const llvm::object::Archive::Symbol &);
namespace elf {
class CommonSymbol;
class Defined;
class InputFile;
class LazyArchive;
class LazyObject;
class SharedSymbol;
class Symbol;
class Undefined;
// This is a StringRef-like container that doesn't run strlen().
// ELF string tables contain a lot of null-terminated strings. Most of them
// are not necessary for the linker because they are names of local symbols,
// and the linker doesn't use local symbol names for name resolution. So, we
// use this class to represents strings read from string tables.
struct StringRefZ {
StringRefZ(const char *s) : data(s), size(-1) {}
StringRefZ(StringRef s) : data(, size(s.size()) {}
const char *data;
const uint32_t size;
// The base class for real symbol classes.
class Symbol {
enum Kind {
Kind kind() const { return static_cast<Kind>(symbolKind); }
// The file from which this symbol was created.
InputFile *file;
const char *nameData;
mutable uint32_t nameSize;
uint32_t dynsymIndex = 0;
uint32_t gotIndex = -1;
uint32_t pltIndex = -1;
uint32_t globalDynIndex = -1;
// This field is a index to the symbol's version definition.
uint32_t verdefIndex = -1;
// Version definition index.
uint16_t versionId;
// Symbol binding. This is not overwritten by replace() to track
// changes during resolution. In particular:
// - An undefined weak is still weak when it resolves to a shared library.
// - An undefined weak will not fetch archive members, but we have to
// remember it is weak.
uint8_t binding;
// The following fields have the same meaning as the ELF symbol attributes.
uint8_t type; // symbol type
uint8_t stOther; // st_other field value
uint8_t symbolKind;
// Symbol visibility. This is the computed minimum visibility of all
// observed non-DSO symbols.
uint8_t visibility : 2;
// True if the symbol was used for linking and thus need to be added to the
// output file's symbol table. This is true for all symbols except for
// unreferenced DSO symbols, lazy (archive) symbols, and bitcode symbols that
// are unreferenced except by other bitcode objects.
uint8_t isUsedInRegularObj : 1;
// Used by a Defined symbol with protected or default visibility, to record
// whether it is required to be exported into .dynsym. This is set when any of
// the following conditions hold:
// - If there is an interposable symbol from a DSO.
// - If -shared or --export-dynamic is specified, any symbol in an object
// file/bitcode sets this property, unless suppressed by LTO
// canBeOmittedFromSymbolTable().
uint8_t exportDynamic : 1;
// True if the symbol is in the --dynamic-list file. A Defined symbol with
// protected or default visibility with this property is required to be
// exported into .dynsym.
uint8_t inDynamicList : 1;
// False if LTO shouldn't inline whatever this symbol points to. If a symbol
// is overwritten after LTO, LTO shouldn't inline the symbol because it
// doesn't know the final contents of the symbol.
uint8_t canInline : 1;
// Used to track if there has been at least one undefined reference to the
// symbol. For Undefined and SharedSymbol, the binding may change to STB_WEAK
// if the first undefined reference from a non-shared object is weak.
// This is also used to retain __wrap_foo when foo is referenced.
uint8_t referenced : 1;
// True if this symbol is specified by --trace-symbol option.
uint8_t traced : 1;
inline void replace(const Symbol &newSym);
bool includeInDynsym() const;
uint8_t computeBinding() const;
bool isWeak() const { return binding == llvm::ELF::STB_WEAK; }
bool isUndefined() const { return symbolKind == UndefinedKind; }
bool isCommon() const { return symbolKind == CommonKind; }
bool isDefined() const { return symbolKind == DefinedKind; }
bool isShared() const { return symbolKind == SharedKind; }
bool isPlaceholder() const { return symbolKind == PlaceholderKind; }
bool isLocal() const { return binding == llvm::ELF::STB_LOCAL; }
bool isLazy() const {
return symbolKind == LazyArchiveKind || symbolKind == LazyObjectKind;
// True if this is an undefined weak symbol. This only works once
// all input files have been added.
bool isUndefWeak() const {
// See comment on lazy symbols for details.
return isWeak() && (isUndefined() || isLazy());
StringRef getName() const {
if (nameSize == (uint32_t)-1)
nameSize = strlen(nameData);
return {nameData, nameSize};
void setName(StringRef s) {
nameData =;
nameSize = s.size();
void parseSymbolVersion();
// Get the NUL-terminated version suffix ("", "@...", or "@@...").
// For @@, the name has been truncated by insert(). For @, the name has been
// truncated by Symbol::parseSymbolVersion().
const char *getVersionSuffix() const {
return nameData + nameSize;
bool isInGot() const { return gotIndex != -1U; }
bool isInPlt() const { return pltIndex != -1U; }
uint64_t getVA(int64_t addend = 0) const;
uint64_t getGotOffset() const;
uint64_t getGotVA() const;
uint64_t getGotPltOffset() const;
uint64_t getGotPltVA() const;
uint64_t getPltVA() const;
uint64_t getSize() const;
OutputSection *getOutputSection() const;
// The following two functions are used for symbol resolution.
// You are expected to call mergeProperties for all symbols in input
// files so that attributes that are attached to names rather than
// indivisual symbol (such as visibility) are merged together.
// Every time you read a new symbol from an input, you are supposed
// to call resolve() with the new symbol. That function replaces
// "this" object as a result of name resolution if the new symbol is
// more appropriate to be included in the output.
// For example, if "this" is an undefined symbol and a new symbol is
// a defined symbol, "this" is replaced with the new symbol.
void mergeProperties(const Symbol &other);
void resolve(const Symbol &other);
// If this is a lazy symbol, fetch an input file and add the symbol
// in the file to the symbol table. Calling this function on
// non-lazy object causes a runtime error.
void fetch() const;
static bool isExportDynamic(Kind k, uint8_t visibility) {
if (k == SharedKind)
return visibility == llvm::ELF::STV_DEFAULT;
return config->shared || config->exportDynamic;
void resolveUndefined(const Undefined &other);
void resolveCommon(const CommonSymbol &other);
void resolveDefined(const Defined &other);
template <class LazyT> void resolveLazy(const LazyT &other);
void resolveShared(const SharedSymbol &other);
int compare(const Symbol *other) const;
inline size_t getSymbolSize() const;
Symbol(Kind k, InputFile *file, StringRefZ name, uint8_t binding,
uint8_t stOther, uint8_t type)
: file(file), nameData(, nameSize(name.size), binding(binding),
type(type), stOther(stOther), symbolKind(k), visibility(stOther & 3),
isUsedInRegularObj(!file || file->kind() == InputFile::ObjKind),
exportDynamic(isExportDynamic(k, visibility)), inDynamicList(false),
canInline(false), referenced(false), traced(false), needsPltAddr(false),
isInIplt(false), gotInIgot(false), isPreemptible(false),
used(!config->gcSections), needsTocRestore(false),
scriptDefined(false) {}
// True the symbol should point to its PLT entry.
// For SharedSymbol only.
uint8_t needsPltAddr : 1;
// True if this symbol is in the Iplt sub-section of the Plt and the Igot
// sub-section of the .got.plt or .got.
uint8_t isInIplt : 1;
// True if this symbol needs a GOT entry and its GOT entry is actually in
// Igot. This will be true only for certain non-preemptible ifuncs.
uint8_t gotInIgot : 1;
// True if this symbol is preemptible at load time.
uint8_t isPreemptible : 1;
// True if an undefined or shared symbol is used from a live section.
// NOTE: In Writer.cpp the field is used to mark local defined symbols
// which are referenced by relocations when -r or --emit-relocs is given.
uint8_t used : 1;
// True if a call to this symbol needs to be followed by a restore of the
// PPC64 toc pointer.
uint8_t needsTocRestore : 1;
// True if this symbol is defined by a linker script.
uint8_t scriptDefined : 1;
// The partition whose dynamic symbol table contains this symbol's definition.
uint8_t partition = 1;
bool isSection() const { return type == llvm::ELF::STT_SECTION; }
bool isTls() const { return type == llvm::ELF::STT_TLS; }
bool isFunc() const { return type == llvm::ELF::STT_FUNC; }
bool isGnuIFunc() const { return type == llvm::ELF::STT_GNU_IFUNC; }
bool isObject() const { return type == llvm::ELF::STT_OBJECT; }
bool isFile() const { return type == llvm::ELF::STT_FILE; }
// Represents a symbol that is defined in the current output file.
class Defined : public Symbol {
Defined(InputFile *file, StringRefZ name, uint8_t binding, uint8_t stOther,
uint8_t type, uint64_t value, uint64_t size, SectionBase *section)
: Symbol(DefinedKind, file, name, binding, stOther, type), value(value),
size(size), section(section) {}
static bool classof(const Symbol *s) { return s->isDefined(); }
uint64_t value;
uint64_t size;
SectionBase *section;
// Represents a common symbol.
// On Unix, it is traditionally allowed to write variable definitions
// without initialization expressions (such as "int foo;") to header
// files. Such definition is called "tentative definition".
// Using tentative definition is usually considered a bad practice
// because 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.
// Common symbols represent variable definitions without initializations.
// The compiler creates common symbols when it sees variable definitions
// without initialization (you can suppress this behavior and let the
// compiler create a regular defined symbol by -fno-common).
// The linker allows common symbols to be replaced by regular defined
// symbols. If there are remaining common symbols after name resolution is
// complete, they are converted to regular defined symbols in a .bss
// section. (Therefore, the later passes don't see any CommonSymbols.)
class CommonSymbol : public Symbol {
CommonSymbol(InputFile *file, StringRefZ name, uint8_t binding,
uint8_t stOther, uint8_t type, uint64_t alignment, uint64_t size)
: Symbol(CommonKind, file, name, binding, stOther, type),
alignment(alignment), size(size) {}
static bool classof(const Symbol *s) { return s->isCommon(); }
uint32_t alignment;
uint64_t size;
class Undefined : public Symbol {
Undefined(InputFile *file, StringRefZ name, uint8_t binding, uint8_t stOther,
uint8_t type, uint32_t discardedSecIdx = 0)
: Symbol(UndefinedKind, file, name, binding, stOther, type),
discardedSecIdx(discardedSecIdx) {}
static bool classof(const Symbol *s) { return s->kind() == UndefinedKind; }
// The section index if in a discarded section, 0 otherwise.
uint32_t discardedSecIdx;
class SharedSymbol : public Symbol {
static bool classof(const Symbol *s) { return s->kind() == SharedKind; }
SharedSymbol(InputFile &file, StringRef name, uint8_t binding,
uint8_t stOther, uint8_t type, uint64_t value, uint64_t size,
uint32_t alignment, uint32_t verdefIndex)
: Symbol(SharedKind, &file, name, binding, stOther, type), value(value),
size(size), alignment(alignment) {
this->verdefIndex = verdefIndex;
// GNU ifunc is a mechanism to allow user-supplied functions to
// resolve PLT slot values at load-time. This is contrary to the
// regular symbol resolution scheme in which symbols are resolved just
// by name. Using this hook, you can program how symbols are solved
// for you program. For example, you can make "memcpy" to be resolved
// to a SSE-enabled version of memcpy only when a machine running the
// program supports the SSE instruction set.
// Naturally, such symbols should always be called through their PLT
// slots. What GNU ifunc symbols point to are resolver functions, and
// calling them directly doesn't make sense (unless you are writing a
// loader).
// For DSO symbols, we always call them through PLT slots anyway.
// So there's no difference between GNU ifunc and regular function
// symbols if they are in DSOs. So we can handle GNU_IFUNC as FUNC.
if (this->type == llvm::ELF::STT_GNU_IFUNC)
this->type = llvm::ELF::STT_FUNC;
SharedFile &getFile() const { return *cast<SharedFile>(file); }
uint64_t value; // st_value
uint64_t size; // st_size
uint32_t alignment;
// LazyArchive and LazyObject represent a symbols that is not yet in the link,
// but we know where to find it if needed. If the resolver finds both Undefined
// and Lazy for the same name, it will ask the Lazy to load a file.
// A special complication is the handling of weak undefined symbols. They should
// not load a file, but we have to remember we have seen both the weak undefined
// and the lazy. We represent that with a lazy symbol with a weak binding. This
// means that code looking for undefined symbols normally also has to take lazy
// symbols into consideration.
// 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.
class LazyArchive : public Symbol {
LazyArchive(InputFile &file, const llvm::object::Archive::Symbol s)
: Symbol(LazyArchiveKind, &file, s.getName(), llvm::ELF::STB_GLOBAL,
sym(s) {}
static bool classof(const Symbol *s) { return s->kind() == LazyArchiveKind; }
MemoryBufferRef getMemberBuffer();
const llvm::object::Archive::Symbol sym;
// LazyObject symbols represents symbols in object files between
// --start-lib and --end-lib options.
class LazyObject : public Symbol {
LazyObject(InputFile &file, StringRef name)
: Symbol(LazyObjectKind, &file, name, llvm::ELF::STB_GLOBAL,
static bool classof(const Symbol *s) { return s->kind() == LazyObjectKind; }
// Some linker-generated symbols need to be created as
// Defined symbols.
struct ElfSym {
// __bss_start
static Defined *bss;
// etext and _etext
static Defined *etext1;
static Defined *etext2;
// edata and _edata
static Defined *edata1;
static Defined *edata2;
// end and _end
static Defined *end1;
static Defined *end2;
// The _GLOBAL_OFFSET_TABLE_ symbol is defined by target convention to
// be at some offset from the base of the .got section, usually 0 or
// the end of the .got.
static Defined *globalOffsetTable;
// _gp, _gp_disp and __gnu_local_gp symbols. Only for MIPS.
static Defined *mipsGp;
static Defined *mipsGpDisp;
static Defined *mipsLocalGp;
// __rel{,a}_iplt_{start,end} symbols.
static Defined *relaIpltStart;
static Defined *relaIpltEnd;
// __global_pointer$ for RISC-V.
static Defined *riscvGlobalPointer;
// _TLS_MODULE_BASE_ on targets that support TLSDESC.
static Defined *tlsModuleBase;
// 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(Defined) char a[sizeof(Defined)];
alignas(CommonSymbol) char b[sizeof(CommonSymbol)];
alignas(Undefined) char c[sizeof(Undefined)];
alignas(SharedSymbol) char d[sizeof(SharedSymbol)];
alignas(LazyArchive) char e[sizeof(LazyArchive)];
alignas(LazyObject) char f[sizeof(LazyObject)];
// It is important to keep the size of SymbolUnion small for performance and
// memory usage reasons. 80 bytes is a soft limit based on the size of Defined
// on a 64-bit system.
static_assert(sizeof(SymbolUnion) <= 80, "SymbolUnion too large");
template <typename T> struct AssertSymbol {
"Symbol types must be trivially destructible");
static_assert(sizeof(T) <= sizeof(SymbolUnion), "SymbolUnion too small");
static_assert(alignof(T) <= alignof(SymbolUnion),
"SymbolUnion not aligned enough");
static inline void assertSymbols() {
void printTraceSymbol(const Symbol *sym);
size_t Symbol::getSymbolSize() const {
switch (kind()) {
case CommonKind:
return sizeof(CommonSymbol);
case DefinedKind:
return sizeof(Defined);
case LazyArchiveKind:
return sizeof(LazyArchive);
case LazyObjectKind:
return sizeof(LazyObject);
case SharedKind:
return sizeof(SharedSymbol);
case UndefinedKind:
return sizeof(Undefined);
case PlaceholderKind:
return sizeof(Symbol);
llvm_unreachable("unknown symbol kind");
// replace() replaces "this" object with a given symbol by memcpy'ing
// it over to "this". This function is called as a result of name
// resolution, e.g. to replace an undefind symbol with a defined symbol.
void Symbol::replace(const Symbol &newSym) {
using llvm::ELF::STT_TLS;
// st_value of STT_TLS represents the assigned offset, not the actual address
// which is used by STT_FUNC and STT_OBJECT. STT_TLS symbols can only be
// referenced by special TLS relocations. It is usually an error if a STT_TLS
// symbol is replaced by a non-STT_TLS symbol, vice versa. There are two
// exceptions: (a) a STT_NOTYPE lazy/undefined symbol can be replaced by a
// STT_TLS symbol, (b) a STT_TLS undefined symbol can be replaced by a
// STT_NOTYPE lazy symbol.
if (symbolKind != PlaceholderKind && !newSym.isLazy() &&
(type == STT_TLS) != (newSym.type == STT_TLS) &&
type != llvm::ELF::STT_NOTYPE)
error("TLS attribute mismatch: " + toString(*this) + "\n>>> defined in " +
toString(newSym.file) + "\n>>> defined in " + toString(file));
Symbol old = *this;
memcpy(this, &newSym, newSym.getSymbolSize());
// old may be a placeholder. The referenced fields must be initialized in
// SymbolTable::insert.
versionId = old.versionId;
visibility = old.visibility;
isUsedInRegularObj = old.isUsedInRegularObj;
exportDynamic = old.exportDynamic;
inDynamicList = old.inDynamicList;
canInline = old.canInline;
referenced = old.referenced;
traced = old.traced;
isPreemptible = old.isPreemptible;
scriptDefined = old.scriptDefined;
partition = old.partition;
// Symbol length is computed lazily. If we already know a symbol length,
// propagate it.
if (nameData == old.nameData && nameSize == 0 && old.nameSize != 0)
nameSize = old.nameSize;
// Print out a log message if --trace-symbol was specified.
// This is for debugging.
if (traced)
void maybeWarnUnorderableSymbol(const Symbol *sym);
bool computeIsPreemptible(const Symbol &sym);
void reportBackrefs();
// A mapping from a symbol to an InputFile referencing it backward. Used by
// --warn-backrefs.
extern llvm::DenseMap<const Symbol *,
std::pair<const InputFile *, const InputFile *>>
} // namespace elf
} // namespace lld