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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 "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 {
std::string toString(coff::Symbol &b);
// 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 toCOFFString(const coff::Archive::Symbol &b);
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 prevalent 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 whether one symbol should take precedence
// over another.
DefinedRegularKind = 0,
LastDefinedCOFFKind = DefinedCommonKind,
LastDefinedKind = DefinedSyntheticKind,
Kind kind() const { return static_cast<Kind>(symbolKind); }
// Returns the symbol name.
StringRef getName() {
// COFF symbol names are read lazily for a performance reason.
// Non-external symbol names are never used by the linker except for logging
// or debugging. Their internal references are resolved not by name but by
// symbol index. And because they are not external, no one can refer them by
// name. Object files contain lots of non-external symbols, and creating
// StringRefs for them (which involves lots of strlen() on the string table)
// is a waste of time.
if (nameData == nullptr)
return StringRef(nameData, nameSize);
void replaceKeepingName(Symbol *other, size_t size);
// 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;
bool isLazy() const {
return symbolKind == LazyArchiveKind || symbolKind == LazyObjectKind ||
symbolKind == LazyDLLSymbolKind;
void computeName();
friend SymbolTable;
explicit Symbol(Kind k, StringRef n = "")
: symbolKind(k), isExternal(true), isCOMDAT(false),
writtenToSymtab(false), pendingArchiveLoad(false), isGCRoot(false),
isRuntimePseudoReloc(false), deferUndefined(false), canInline(true),
nameSize(n.size()), nameData(n.empty() ? nullptr : {}
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;
unsigned isRuntimePseudoReloc : 1;
// True if we want to allow this symbol to be undefined in the early
// undefined check pass in SymbolTable::reportUnresolvable(), as it
// might be fixed up later.
unsigned deferUndefined : 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.
unsigned canInline : 1;
// Symbol name length. Assume symbol lengths fit in a 32-bit integer.
uint32_t nameSize;
const char *nameData;
// 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 decomposed 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;
this->isCOMDAT = isCOMDAT;
static bool classof(const Symbol *s) {
return s->kind() == DefinedRegularKind;
uint64_t getRVA() const { return (*data)->getRVA() + sym->Value; }
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; }
uint64_t getVA() const { return va; }
// 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 LazyArchive for
// the same name, it will ask the LazyArchive to load a file.
class LazyArchive : public Symbol {
LazyArchive(ArchiveFile *f, const Archive::Symbol s)
: Symbol(LazyArchiveKind, s.getName()), file(f), sym(s) {}
static bool classof(const Symbol *s) { return s->kind() == LazyArchiveKind; }
MemoryBufferRef getMemberBuffer();
ArchiveFile *file;
const Archive::Symbol sym;
class LazyObject : public Symbol {
LazyObject(LazyObjFile *f, StringRef n)
: Symbol(LazyObjectKind, n), file(f) {}
static bool classof(const Symbol *s) { return s->kind() == LazyObjectKind; }
LazyObjFile *file;
// MinGW only.
class LazyDLLSymbol : public Symbol {
LazyDLLSymbol(DLLFile *f, DLLFile::Symbol *s, StringRef n)
: Symbol(LazyDLLSymbolKind, n), file(f), sym(s) {}
static bool classof(const Symbol *s) {
return s->kind() == LazyDLLSymbolKind;
DLLFile *file;
DLLFile::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 is a pointer to the synthetic symbol associated with the load thunk
// for this symbol that will be called if the DLL is delay-loaded. This is
// needed for Control Flow Guard because if this DefinedImportData symbol is a
// valid call target, the corresponding load thunk must also be marked as a
// valid call target.
DefinedSynthetic *loadThunkSym = 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 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 "foo" in your object file, a symbol name
// "__imp_foo" becomes automatically available as a pointer to "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 LazyArchiveKind:
case LazyObjectKind:
case LazyDLLSymbolKind:
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 LazyArchiveKind:
case LazyObjectKind:
case LazyDLLSymbolKind:
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(LazyArchive) char e[sizeof(LazyArchive)];
alignas(Undefined) char f[sizeof(Undefined)];
alignas(DefinedImportData) char g[sizeof(DefinedImportData)];
alignas(DefinedImportThunk) char h[sizeof(DefinedImportThunk)];
alignas(DefinedLocalImport) char i[sizeof(DefinedLocalImport)];
alignas(LazyObject) char j[sizeof(LazyObject)];
alignas(LazyDLLSymbol) char k[sizeof(LazyDLLSymbol)];
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");
bool canInline = s->canInline;
new (s) T(std::forward<ArgT>(arg)...);
s->canInline = canInline;
} // namespace coff
} // namespace lld