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/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
* vim: set ts=8 sts=2 et sw=2 tw=80:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at */
#ifndef vm_SymbolType_h
#define vm_SymbolType_h
#include <stdio.h>
#include "jsapi.h"
#include "gc/Barrier.h"
#include "gc/Tracer.h"
#include "js/AllocPolicy.h"
#include "js/GCHashTable.h"
#include "js/HeapAPI.h"
#include "js/RootingAPI.h"
#include "js/shadow/Symbol.h" // JS::shadow::Symbol
#include "js/Symbol.h"
#include "js/TypeDecls.h"
#include "js/Utility.h"
#include "vm/Printer.h"
#include "vm/StringType.h"
namespace js {
class AutoAccessAtomsZone;
} // namespace js
namespace JS {
class Symbol
: public js::gc::CellWithTenuredGCPointer<js::gc::TenuredCell, JSAtom> {
// User description of symbol, stored in the cell header.
JSAtom* description() const { return headerPtr(); }
SymbolCode code_;
// Each Symbol gets its own hash code so that we don't have to use
// addresses as hash codes (a security hazard).
js::HashNumber hash_;
Symbol(SymbolCode code, js::HashNumber hash, JSAtom* desc)
: CellWithTenuredGCPointer(desc), code_(code), hash_(hash) {}
Symbol(const Symbol&) = delete;
void operator=(const Symbol&) = delete;
static Symbol* newInternal(JSContext* cx, SymbolCode code,
js::HashNumber hash, js::HandleAtom description);
static void staticAsserts() {
static_assert(uint32_t(SymbolCode::WellKnownAPILimit) ==
"JS::shadow::Symbol::WellKnownAPILimit must match "
offsetof(Symbol, code_) == offsetof(JS::shadow::Symbol, code_),
"JS::shadow::Symbol::code_ offset must match SymbolCode::code_");
static Symbol* new_(JSContext* cx, SymbolCode code,
js::HandleString description);
static Symbol* for_(JSContext* cx, js::HandleString description);
SymbolCode code() const { return code_; }
js::HashNumber hash() const { return hash_; }
bool isWellKnownSymbol() const {
return uint32_t(code_) < WellKnownSymbolLimit;
// An "interesting symbol" is a well-known symbol, like @@toStringTag,
// that's often looked up on random objects but is usually not present. We
// optimize this by setting a flag on the object's BaseShape when such
// symbol properties are added, so we can optimize lookups on objects that
// don't have the BaseShape flag.
bool isInterestingSymbol() const {
return code_ == SymbolCode::toStringTag || code_ == SymbolCode::toPrimitive;
// Symbol created for the #PrivateName syntax.
bool isPrivateName() const { return code_ == SymbolCode::PrivateNameSymbol; }
static const JS::TraceKind TraceKind = JS::TraceKind::Symbol;
inline void traceChildren(JSTracer* trc) {
js::TraceNullableCellHeaderEdge(trc, this, "symbol description");
inline void finalize(JSFreeOp*) {}
// Override base class implementation to tell GC about well-known symbols.
bool isPermanentAndMayBeShared() const { return isWellKnownSymbol(); }
size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
return mallocSizeOf(this);
#if defined(DEBUG) || defined(JS_JITSPEW)
void dump(); // Debugger-friendly stderr dump.
void dump(js::GenericPrinter& out);
} /* namespace JS */
namespace js {
/* Hash policy used by the SymbolRegistry. */
struct HashSymbolsByDescription {
using Key = JS::Symbol*;
using Lookup = JSAtom*;
static HashNumber hash(Lookup l) { return HashNumber(l->hash()); }
static bool match(Key sym, Lookup l) { return sym->description() == l; }
* [SMDOC] Symbol.for() registry (ES6 GlobalSymbolRegistry)
* The runtime-wide symbol registry, used to implement Symbol.for().
* ES6 draft rev 25 (2014 May 22) calls this the GlobalSymbolRegistry List. In
* our implementation, it is not global. There is one per JSRuntime. The
* symbols in the symbol registry, like all symbols, are allocated in the atoms
* compartment and can be directly referenced from any compartment. They are
* never shared across runtimes.
* The memory management strategy here is modeled after js::AtomSet. It's like
* a WeakSet. The registry itself does not keep any symbols alive; when a
* symbol in the registry is collected, the registry entry is removed. No GC
* nondeterminism is exposed to scripts, because there is no API for
* enumerating the symbol registry, querying its size, etc.
class SymbolRegistry
: public GCHashSet<WeakHeapPtrSymbol, HashSymbolsByDescription,
SystemAllocPolicy> {
SymbolRegistry() = default;
// ES6 rev 27 (2014 Aug 24)
bool SymbolDescriptiveString(JSContext* cx, JS::Symbol* sym,
JS::MutableHandleValue result);
} /* namespace js */
#endif /* vm_SymbolType_h */