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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 "lld/Common/LLVM.h"
#include "llvm/Object/Archive.h"
#include "llvm/Object/Wasm.h"
namespace lld {
namespace wasm {
// Shared string constants
// The default module name to use for symbol imports.
extern const char *DefaultModule;
// The name under which to import or export the wasm table.
extern const char *FunctionTableName;
using llvm::wasm::WasmSymbolType;
class InputFile;
class InputChunk;
class InputSegment;
class InputFunction;
class InputGlobal;
class InputEvent;
class InputSection;
class OutputSection;
// The base class for real symbol classes.
class Symbol {
enum Kind {
Kind kind() const { return SymbolKind; }
bool isDefined() const { return !isLazy() && !isUndefined(); }
bool isUndefined() const {
return SymbolKind == UndefinedFunctionKind ||
SymbolKind == UndefinedDataKind || SymbolKind == UndefinedGlobalKind;
bool isLazy() const { return SymbolKind == LazyKind; }
bool isLocal() const;
bool isWeak() const;
bool isHidden() const;
// Returns true if this symbol exists in a discarded (due to COMDAT) section
bool isDiscarded() const;
// 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());
// Returns the symbol name.
StringRef getName() const { return Name; }
// Returns the file from which this symbol was created.
InputFile *getFile() const { return File; }
uint32_t getFlags() const { return Flags; }
InputChunk *getChunk() const;
// Indicates that the section or import for this symbol will be included in
// the final image.
bool isLive() const;
// Marks the symbol's InputChunk as Live, so that it will be included in the
// final image.
void markLive();
void setHidden(bool IsHidden);
// Get/set the index in the output symbol table. This is only used for
// relocatable output.
uint32_t getOutputSymbolIndex() const;
void setOutputSymbolIndex(uint32_t Index);
WasmSymbolType getWasmType() const;
bool isExported() const;
// 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.
unsigned IsUsedInRegularObj : 1;
// True if ths symbol is explicity marked for export (i.e. via the -e/--export
// command line flag)
unsigned ForceExport : 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;
// True if this symbol is specified by --trace-symbol option.
unsigned Traced : 1;
const WasmSignature* getSignature() const;
bool isInGOT() const { return GOTIndex != INVALID_INDEX; }
uint32_t getGOTIndex() const {
assert(GOTIndex != INVALID_INDEX);
return GOTIndex;
void setGOTIndex(uint32_t Index);
bool hasGOTIndex() const { return GOTIndex != INVALID_INDEX; }
Symbol(StringRef Name, Kind K, uint32_t Flags, InputFile *F)
: IsUsedInRegularObj(false), ForceExport(false), CanInline(false),
Traced(false), Name(Name), SymbolKind(K), Flags(Flags), File(F),
Referenced(!Config->GcSections) {}
StringRef Name;
Kind SymbolKind;
uint32_t Flags;
InputFile *File;
uint32_t OutputSymbolIndex = INVALID_INDEX;
uint32_t GOTIndex = INVALID_INDEX;
bool Referenced;
class FunctionSymbol : public Symbol {
static bool classof(const Symbol *S) {
return S->kind() == DefinedFunctionKind ||
S->kind() == UndefinedFunctionKind;
// Get/set the table index
void setTableIndex(uint32_t Index);
uint32_t getTableIndex() const;
bool hasTableIndex() const;
// Get/set the function index
uint32_t getFunctionIndex() const;
void setFunctionIndex(uint32_t Index);
bool hasFunctionIndex() const;
const WasmSignature *Signature;
FunctionSymbol(StringRef Name, Kind K, uint32_t Flags, InputFile *F,
const WasmSignature *Sig)
: Symbol(Name, K, Flags, F), Signature(Sig) {}
uint32_t TableIndex = INVALID_INDEX;
uint32_t FunctionIndex = INVALID_INDEX;
class DefinedFunction : public FunctionSymbol {
DefinedFunction(StringRef Name, uint32_t Flags, InputFile *F,
InputFunction *Function);
static bool classof(const Symbol *S) {
return S->kind() == DefinedFunctionKind;
InputFunction *Function;
class UndefinedFunction : public FunctionSymbol {
UndefinedFunction(StringRef Name, StringRef ImportName,
StringRef ImportModule, uint32_t Flags,
InputFile *File = nullptr,
const WasmSignature *Type = nullptr,
bool IsCalledDirectly = true)
: FunctionSymbol(Name, UndefinedFunctionKind, Flags, File, Type),
ImportName(ImportName), ImportModule(ImportModule), IsCalledDirectly(IsCalledDirectly) {}
static bool classof(const Symbol *S) {
return S->kind() == UndefinedFunctionKind;
StringRef ImportName;
StringRef ImportModule;
bool IsCalledDirectly;
// Section symbols for output sections are different from those for input
// section. These are generated by the linker and point the OutputSection
// rather than an InputSection.
class OutputSectionSymbol : public Symbol {
OutputSectionSymbol(const OutputSection *S)
: Symbol("", OutputSectionKind, llvm::wasm::WASM_SYMBOL_BINDING_LOCAL,
Section(S) {}
static bool classof(const Symbol *S) {
return S->kind() == OutputSectionKind;
const OutputSection *Section;
class SectionSymbol : public Symbol {
SectionSymbol(uint32_t Flags, const InputSection *S, InputFile *F = nullptr)
: Symbol("", SectionKind, Flags, F), Section(S) {}
static bool classof(const Symbol *S) { return S->kind() == SectionKind; }
const OutputSectionSymbol *getOutputSectionSymbol() const;
const InputSection *Section;
class DataSymbol : public Symbol {
static bool classof(const Symbol *S) {
return S->kind() == DefinedDataKind || S->kind() == UndefinedDataKind;
DataSymbol(StringRef Name, Kind K, uint32_t Flags, InputFile *F)
: Symbol(Name, K, Flags, F) {}
class DefinedData : public DataSymbol {
// Constructor for regular data symbols originating from input files.
DefinedData(StringRef Name, uint32_t Flags, InputFile *F,
InputSegment *Segment, uint32_t Offset, uint32_t Size)
: DataSymbol(Name, DefinedDataKind, Flags, F), Segment(Segment),
Offset(Offset), Size(Size) {}
// Constructor for linker synthetic data symbols.
DefinedData(StringRef Name, uint32_t Flags)
: DataSymbol(Name, DefinedDataKind, Flags, nullptr) {}
static bool classof(const Symbol *S) { return S->kind() == DefinedDataKind; }
// Returns the output virtual address of a defined data symbol.
uint32_t getVirtualAddress() const;
void setVirtualAddress(uint32_t VA);
// Returns the offset of a defined data symbol within its OutputSegment.
uint32_t getOutputSegmentOffset() const;
uint32_t getOutputSegmentIndex() const;
uint32_t getSize() const { return Size; }
InputSegment *Segment = nullptr;
uint32_t Offset = 0;
uint32_t Size = 0;
class UndefinedData : public DataSymbol {
UndefinedData(StringRef Name, uint32_t Flags, InputFile *File = nullptr)
: DataSymbol(Name, UndefinedDataKind, Flags, File) {}
static bool classof(const Symbol *S) {
return S->kind() == UndefinedDataKind;
class GlobalSymbol : public Symbol {
static bool classof(const Symbol *S) {
return S->kind() == DefinedGlobalKind || S->kind() == UndefinedGlobalKind;
const WasmGlobalType *getGlobalType() const { return GlobalType; }
// Get/set the global index
uint32_t getGlobalIndex() const;
void setGlobalIndex(uint32_t Index);
bool hasGlobalIndex() const;
GlobalSymbol(StringRef Name, Kind K, uint32_t Flags, InputFile *F,
const WasmGlobalType *GlobalType)
: Symbol(Name, K, Flags, F), GlobalType(GlobalType) {}
const WasmGlobalType *GlobalType;
uint32_t GlobalIndex = INVALID_INDEX;
class DefinedGlobal : public GlobalSymbol {
DefinedGlobal(StringRef Name, uint32_t Flags, InputFile *File,
InputGlobal *Global);
static bool classof(const Symbol *S) {
return S->kind() == DefinedGlobalKind;
InputGlobal *Global;
class UndefinedGlobal : public GlobalSymbol {
UndefinedGlobal(StringRef Name, StringRef ImportName, StringRef ImportModule,
uint32_t Flags, InputFile *File = nullptr,
const WasmGlobalType *Type = nullptr)
: GlobalSymbol(Name, UndefinedGlobalKind, Flags, File, Type),
ImportName(ImportName), ImportModule(ImportModule) {}
static bool classof(const Symbol *S) {
return S->kind() == UndefinedGlobalKind;
StringRef ImportName;
StringRef ImportModule;
// Wasm events are features that suspend the current execution and transfer the
// control flow to a corresponding handler. Currently the only supported event
// kind is exceptions.
// Event tags are values to distinguish different events. For exceptions, they
// can be used to distinguish different language's exceptions, i.e., all C++
// exceptions have the same tag. Wasm can generate code capable of doing
// different handling actions based on the tag of caught exceptions.
// A single EventSymbol object represents a single tag. C++ exception event
// symbol is a weak symbol generated in every object file in which exceptions
// are used, and has name '__cpp_exception' for linking.
class EventSymbol : public Symbol {
static bool classof(const Symbol *S) { return S->kind() == DefinedEventKind; }
const WasmEventType *getEventType() const { return EventType; }
// Get/set the event index
uint32_t getEventIndex() const;
void setEventIndex(uint32_t Index);
bool hasEventIndex() const;
const WasmSignature *Signature;
EventSymbol(StringRef Name, Kind K, uint32_t Flags, InputFile *F,
const WasmEventType *EventType, const WasmSignature *Sig)
: Symbol(Name, K, Flags, F), Signature(Sig), EventType(EventType) {}
const WasmEventType *EventType;
uint32_t EventIndex = INVALID_INDEX;
class DefinedEvent : public EventSymbol {
DefinedEvent(StringRef Name, uint32_t Flags, InputFile *File,
InputEvent *Event);
static bool classof(const Symbol *S) { return S->kind() == DefinedEventKind; }
InputEvent *Event;
// LazySymbol represents a symbol 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.
class LazySymbol : public Symbol {
LazySymbol(StringRef Name, uint32_t Flags, InputFile *File,
const llvm::object::Archive::Symbol &Sym)
: Symbol(Name, LazyKind, Flags, File), ArchiveSymbol(Sym) {}
static bool classof(const Symbol *S) { return S->kind() == LazyKind; }
void fetch();
// Lazy symbols can have a signature because they can replace an
// UndefinedFunction which which case we need to be able to preserve the
// signture.
// TODO(sbc): This repetition of the signature field is inelegant. Revisit
// the use of class hierarchy to represent symbol taxonomy.
const WasmSignature *Signature = nullptr;
llvm::object::Archive::Symbol ArchiveSymbol;
// linker-generated symbols
struct WasmSym {
// __global_base
// Symbol marking the start of the global section.
static DefinedData *GlobalBase;
// __stack_pointer
// Global that holds the address of the top of the explicit value stack in
// linear memory.
static GlobalSymbol *StackPointer;
// __data_end
// Symbol marking the end of the data and bss.
static DefinedData *DataEnd;
// __heap_base
// Symbol marking the end of the data, bss and explicit stack. Any linear
// memory following this address is not used by the linked code and can
// therefore be used as a backing store for brk()/malloc() implementations.
static DefinedData *HeapBase;
// __wasm_call_ctors
// Function that directly calls all ctors in priority order.
static DefinedFunction *CallCtors;
// __wasm_apply_relocs
// Function that applies relocations to data segment post-instantiation.
static DefinedFunction *ApplyRelocs;
// __dso_handle
// Symbol used in calls to __cxa_atexit to determine current DLL
static DefinedData *DsoHandle;
// __table_base
// Used in PIC code for offset of indirect function table
static UndefinedGlobal *TableBase;
// __memory_base
// Used in PIC code for offset of global data
static UndefinedGlobal *MemoryBase;
// 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(DefinedFunction) char A[sizeof(DefinedFunction)];
alignas(DefinedData) char B[sizeof(DefinedData)];
alignas(DefinedGlobal) char C[sizeof(DefinedGlobal)];
alignas(DefinedEvent) char D[sizeof(DefinedEvent)];
alignas(LazySymbol) char E[sizeof(LazySymbol)];
alignas(UndefinedFunction) char F[sizeof(UndefinedFunction)];
alignas(UndefinedData) char G[sizeof(UndefinedData)];
alignas(UndefinedGlobal) char H[sizeof(UndefinedGlobal)];
alignas(SectionSymbol) char I[sizeof(SectionSymbol)];
void printTraceSymbol(Symbol *Sym);
void printTraceSymbolUndefined(StringRef Name, const InputFile* File);
template <typename T, typename... ArgT>
T *replaceSymbol(Symbol *S, ArgT &&... Arg) {
"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");
assert(static_cast<Symbol *>(static_cast<T *>(nullptr)) == nullptr &&
"Not a Symbol");
Symbol SymCopy = *S;
T *S2 = new (S) T(std::forward<ArgT>(Arg)...);
S2->IsUsedInRegularObj = SymCopy.IsUsedInRegularObj;
S2->ForceExport = SymCopy.ForceExport;
S2->CanInline = SymCopy.CanInline;
S2->Traced = SymCopy.Traced;
// Print out a log message if --trace-symbol was specified.
// This is for debugging.
if (S2->Traced)
return S2;
} // namespace wasm
// Returns a symbol name for an error message.
std::string toString(const wasm::Symbol &Sym);
std::string toString(wasm::Symbol::Kind Kind);
std::string maybeDemangleSymbol(StringRef Name);
} // namespace lld