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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 */
#include "gc/AtomMarking.h"
#include "mozilla/Assertions.h"
#include <type_traits>
#include "vm/Realm.h"
#include "vm/SymbolType.h"
#include "gc/Heap-inl.h"
namespace js {
namespace gc {
inline size_t GetAtomBit(TenuredCell* thing) {
Arena* arena = thing->arena();
size_t arenaBit = (reinterpret_cast<uintptr_t>(thing) - arena->address()) /
return arena->atomBitmapStart() * JS_BITS_PER_WORD + arenaBit;
inline bool ThingIsPermanent(JSAtom* atom) {
// This doesn't check for pinned atoms since that might require taking a
// lock. This is not required for correctness.
return atom->isPermanentAtom();
inline bool ThingIsPermanent(JS::Symbol* symbol) {
return symbol->isWellKnownSymbol();
template <typename T, bool Fallible>
MOZ_ALWAYS_INLINE bool AtomMarkingRuntime::inlinedMarkAtomInternal(
JSContext* cx, T* thing) {
static_assert(std::is_same_v<T, JSAtom> || std::is_same_v<T, JS::Symbol>,
"Should only be called with JSAtom* or JS::Symbol* argument");
js::gc::TenuredCell* cell = &thing->asTenured();
// The context's zone will be null during initialization of the runtime.
if (!cx->zone()) {
return true;
if (ThingIsPermanent(thing)) {
return true;
size_t bit = GetAtomBit(cell);
MOZ_ASSERT(bit / JS_BITS_PER_WORD < allocatedWords);
if (Fallible) {
if (!cx->zone()->markedAtoms().setBitFallible(bit)) {
return false;
} else {
if (!cx->isHelperThreadContext()) {
// Trigger a read barrier on the atom, in case there is an incremental
// GC in progress. This is necessary if the atom is being marked
// because a reference to it was obtained from another zone which is
// not being collected by the incremental GC.
// Children of the thing also need to be marked in the context's zone.
// We don't have a JSTracer for this so manually handle the cases in which
// an atom can reference other atoms.
markChildren(cx, thing);
return true;
void AtomMarkingRuntime::markChildren(JSContext* cx, JSAtom*) {}
void AtomMarkingRuntime::markChildren(JSContext* cx, JS::Symbol* symbol) {
if (JSAtom* description = symbol->description()) {
markAtom(cx, description);
template <typename T>
MOZ_ALWAYS_INLINE void AtomMarkingRuntime::inlinedMarkAtom(JSContext* cx,
T* thing) {
MOZ_ALWAYS_TRUE((inlinedMarkAtomInternal<T, false>(cx, thing)));
template <typename T>
MOZ_ALWAYS_INLINE bool AtomMarkingRuntime::inlinedMarkAtomFallible(
JSContext* cx, T* thing) {
return inlinedMarkAtomInternal<T, true>(cx, thing);
} // namespace gc
} // namespace js