Source code

Revision control

Copy as Markdown

Other Tools

/* -*- 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 js_UbiNode_h
#define js_UbiNode_h
#include "mozilla/Alignment.h"
#include "mozilla/Assertions.h"
#include "mozilla/Attributes.h"
#include "mozilla/HashFunctions.h"
#include "mozilla/Maybe.h"
#include "mozilla/MemoryReporting.h"
#include "mozilla/RangedPtr.h"
#include "mozilla/Variant.h"
#include "mozilla/Vector.h"
#include <utility>
#include "jspubtd.h"
#include "js/AllocPolicy.h"
#include "js/ColumnNumber.h" // JS::TaggedColumnNumberOneOrigin
#include "js/HashTable.h"
#include "js/RootingAPI.h"
#include "js/TypeDecls.h"
#include "js/UniquePtr.h"
#include "js/Value.h"
// [SMDOC] ubi::Node (Heap Analysis framework)
// JS::ubi::Node is a pointer-like type designed for internal use by heap
// analysis tools. A ubi::Node can refer to:
// - a JS value, like a string, object, or symbol;
// - an internal SpiderMonkey structure, like a shape or a scope chain object
// - an instance of some embedding-provided type: in Firefox, an XPCOM
// object, or an internal DOM node class instance
// A ubi::Node instance provides metadata about its referent, and can
// enumerate its referent's outgoing edges, so you can implement heap analysis
// algorithms that walk the graph - finding paths between objects, or
// computing heap dominator trees, say - using ubi::Node, while remaining
// ignorant of the details of the types you're operating on.
// Of course, when it comes to presenting the results in a developer-facing
// tool, you'll need to stop being ignorant of those details, because you have
// to discuss the ubi::Nodes' referents with the developer. Here, ubi::Node
// can hand you dynamically checked, properly typed pointers to the original
// objects via the as<T> method, or generate descriptions of the referent
// itself.
// ubi::Node instances are lightweight (two-word) value types. Instances:
// - compare equal if and only if they refer to the same object;
// - have hash values that respect their equality relation; and
// - have serializations that are only equal if the ubi::Nodes are equal.
// A ubi::Node is only valid for as long as its referent is alive; if its
// referent goes away, the ubi::Node becomes a dangling pointer. A ubi::Node
// that refers to a GC-managed object is not automatically a GC root; if the
// GC frees or relocates its referent, the ubi::Node becomes invalid. A
// ubi::Node that refers to a reference-counted object does not bump the
// reference count.
// ubi::Node values require no supporting data structures, making them
// feasible for use in memory-constrained devices --- ideally, the memory
// requirements of the algorithm which uses them will be the limiting factor,
// not the demands of ubi::Node itself.
// One can construct a ubi::Node value given a pointer to a type that ubi::Node
// supports. In the other direction, one can convert a ubi::Node back to a
// pointer; these downcasts are checked dynamically. In particular, one can
// convert a 'JSContext*' to a ubi::Node, yielding a node with an outgoing edge
// for every root registered with the runtime; starting from this, one can walk
// the entire heap. (Of course, one could also start traversal at any other kind
// of type to which one has a pointer.)
// Extending ubi::Node To Handle Your Embedding's Types
// To add support for a new ubi::Node referent type R, you must define a
// specialization of the ubi::Concrete template, ubi::Concrete<R>, which
// inherits from ubi::Base. ubi::Node itself uses the specialization for
// compile-time information (i.e. the checked conversions between R * and
// ubi::Node), and the inheritance for run-time dispatching.
// ubi::Node Exposes Implementation Details
// In many cases, a JavaScript developer's view of their data differs
// substantially from its actual implementation. For example, while the
// ECMAScript specification describes objects as maps from property names to
// sets of attributes (like ECMAScript's [[Value]]), in practice many objects
// have only a pointer to a shape, shared with other similar objects, and
// indexed slots that contain the [[Value]] attributes. As another example, a
// string produced by concatenating two other strings may sometimes be
// represented by a "rope", a structure that points to the two original
// strings.
// We intend to use ubi::Node to write tools that report memory usage, so it's
// important that ubi::Node accurately portray how much memory nodes consume.
// Thus, for example, when data that apparently belongs to multiple nodes is
// in fact shared in a common structure, ubi::Node's graph uses a separate
// node for that shared structure, and presents edges to it from the data's
// apparent owners. For example, ubi::Node exposes SpiderMonkey objects'
// shapes and base shapes, and exposes rope string and substring structure,
// because these optimizations become visible when a tool reports how much
// memory a structure consumes.
// However, fine granularity is not a goal. When a particular object is the
// exclusive owner of a separate block of memory, ubi::Node may present the
// object and its block as a single node, and add their sizes together when
// reporting the node's size, as there is no meaningful loss of data in this
// case. Thus, for example, a ubi::Node referring to a JavaScript object, when
// asked for the object's size in bytes, includes the object's slot and
// element arrays' sizes in the total. There is no separate ubi::Node value
// representing the slot and element arrays, since they are owned exclusively
// by the object.
// Presenting Analysis Results To JavaScript Developers
// If an analysis provides its results in terms of ubi::Node values, a user
// interface presenting those results will generally need to clean them up
// before they can be understood by JavaScript developers. For example,
// JavaScript developers should not need to understand shapes, only JavaScript
// objects. Similarly, they should not need to understand the distinction
// between DOM nodes and the JavaScript shadow objects that represent them.
// Rooting Restrictions
// At present there is no way to root ubi::Node instances, so instances can't be
// live across any operation that might GC. Analyses using ubi::Node must either
// run to completion and convert their results to some other rootable type, or
// save their intermediate state in some rooted structure if they must GC before
// they complete. (For algorithms like path-finding and dominator tree
// computation, we implement the algorithm avoiding any operation that could
// cause a GC --- and use AutoCheckCannotGC to verify this.)
// If this restriction prevents us from implementing interesting tools, we may
// teach the GC how to root ubi::Nodes, fix up hash tables that use them as
// keys, etc.
// Hostile Graph Structure
// Analyses consuming ubi::Node graphs must be robust when presented with graphs
// that are deliberately constructed to exploit their weaknesses. When operating
// on live graphs, web content has control over the object graph, and less
// direct control over shape and string structure, and analyses should be
// prepared to handle extreme cases gracefully. For example, if an analysis were
// to use the C++ stack in a depth-first traversal, carefully constructed
// content could cause the analysis to overflow the stack.
// When ubi::Nodes refer to nodes deserialized from a heap snapshot, analyses
// must be even more careful: since snapshots often come from potentially
// compromised e10s content processes, even properties normally guaranteed by
// the platform (the proper linking of DOM nodes, for example) might be
// corrupted. While it is the deserializer's responsibility to check the basic
// structure of the snapshot file, the analyses should be prepared for ubi::Node
// graphs constructed from snapshots to be even more bizarre.
namespace js {
class BaseScript;
} // namespace js
namespace JS {
class JS_PUBLIC_API AutoCheckCannotGC;
using ZoneSet =
js::HashSet<Zone*, js::DefaultHasher<Zone*>, js::SystemAllocPolicy>;
using CompartmentSet =
js::HashSet<Compartment*, js::DefaultHasher<Compartment*>,
namespace ubi {
class Edge;
class EdgeRange;
class StackFrame;
using mozilla::Maybe;
using mozilla::RangedPtr;
using mozilla::Variant;
template <typename T>
using Vector = mozilla::Vector<T, 0, js::SystemAllocPolicy>;
/*** ubi::StackFrame **********************************************************/
// Concrete JS::ubi::StackFrame instances backed by a live SavedFrame object
// store their strings as JSAtom*, while deserialized stack frames from offline
// heap snapshots store their strings as const char16_t*. In order to provide
// zero-cost accessors to these strings in a single interface that works with
// both cases, we use this variant type.
class JS_PUBLIC_API AtomOrTwoByteChars
: public Variant<JSAtom*, const char16_t*> {
using Base = Variant<JSAtom*, const char16_t*>;
template <typename T>
MOZ_IMPLICIT AtomOrTwoByteChars(T&& rhs) : Base(std::forward<T>(rhs)) {}
template <typename T>
AtomOrTwoByteChars& operator=(T&& rhs) {
MOZ_ASSERT(this != &rhs, "self-move disallowed");
new (this) AtomOrTwoByteChars(std::forward<T>(rhs));
return *this;
// Return the length of the given AtomOrTwoByteChars string.
size_t length();
// Copy the given AtomOrTwoByteChars string into the destination buffer,
// inflating if necessary. Does NOT null terminate. Returns the number of
// characters written to destination.
size_t copyToBuffer(RangedPtr<char16_t> destination, size_t length);
// The base class implemented by each ConcreteStackFrame<T> type. Subclasses
// must not add data members to this class.
class BaseStackFrame {
friend class StackFrame;
BaseStackFrame(const StackFrame&) = delete;
BaseStackFrame& operator=(const StackFrame&) = delete;
void* ptr;
explicit BaseStackFrame(void* ptr) : ptr(ptr) {}
// This is a value type that should not have a virtual destructor. Don't add
// destructors in subclasses!
// Get a unique identifier for this StackFrame. The identifier is not valid
// across garbage collections.
virtual uint64_t identifier() const { return uint64_t(uintptr_t(ptr)); }
// Get this frame's parent frame.
virtual StackFrame parent() const = 0;
// Get this frame's line number (1-origin).
virtual uint32_t line() const = 0;
// Get this frame's column number in UTF-16 code units.
virtual JS::TaggedColumnNumberOneOrigin column() const = 0;
// Get this frame's source name. Never null.
virtual AtomOrTwoByteChars source() const = 0;
// Get a unique per-process ID for this frame's source. Defaults to zero.
virtual uint32_t sourceId() const = 0;
// Return this frame's function name if named, otherwise the inferred
// display name. Can be null.
virtual AtomOrTwoByteChars functionDisplayName() const = 0;
// Returns true if this frame's function is system JavaScript running with
// trusted principals, false otherwise.
virtual bool isSystem() const = 0;
// Return true if this frame's function is a self-hosted JavaScript builtin,
// false otherwise.
virtual bool isSelfHosted(JSContext* cx) const = 0;
// Construct a SavedFrame stack for the stack starting with this frame and
// containing all of its parents. The SavedFrame objects will be placed into
// cx's current compartment.
// Note that the process of
// SavedFrame
// |
// V
// JS::ubi::StackFrame
// |
// V
// offline heap snapshot
// |
// V
// JS::ubi::StackFrame
// |
// V
// SavedFrame
// is lossy because we cannot serialize and deserialize the SavedFrame's
// principals in the offline heap snapshot, so JS::ubi::StackFrame
// simplifies the principals check into the boolean isSystem() state. This
// is fine because we only expose JS::ubi::Stack to devtools and chrome
// code, and not to the web platform.
[[nodiscard]] virtual bool constructSavedFrameStack(
JSContext* cx, MutableHandleObject outSavedFrameStack) const = 0;
// Trace the concrete implementation of JS::ubi::StackFrame.
virtual void trace(JSTracer* trc) = 0;
// A traits template with a specialization for each backing type that implements
// the ubi::BaseStackFrame interface. Each specialization must be the a subclass
// of ubi::BaseStackFrame.
template <typename T>
class ConcreteStackFrame;
// A JS::ubi::StackFrame represents a frame in a recorded stack. It can be
// backed either by a live SavedFrame object or by a structure deserialized from
// an offline heap snapshot.
// It is a value type that may be memcpy'd hither and thither without worrying
// about constructors or destructors, similar to POD types.
// Its lifetime is the same as the lifetime of the graph that is being analyzed
// by the JS::ubi::Node that the JS::ubi::StackFrame came from. That is, if the
// graph being analyzed is the live heap graph, the JS::ubi::StackFrame is only
// valid within the scope of an AutoCheckCannotGC; if the graph being analyzed
// is an offline heap snapshot, the JS::ubi::StackFrame is valid as long as the
// offline heap snapshot is alive.
class StackFrame {
// Storage in which we allocate BaseStackFrame subclasses.
mozilla::AlignedStorage2<BaseStackFrame> storage;
BaseStackFrame* base() { return storage.addr(); }
const BaseStackFrame* base() const { return storage.addr(); }
template <typename T>
void construct(T* ptr) {
static_assert(std::is_base_of_v<BaseStackFrame, ConcreteStackFrame<T>>,
"ConcreteStackFrame<T> must inherit from BaseStackFrame");
sizeof(ConcreteStackFrame<T>) == sizeof(*base()),
"ubi::ConcreteStackFrame<T> specializations must be the same size as "
ConcreteStackFrame<T>::construct(base(), ptr);
struct ConstructFunctor;
StackFrame() { construct<void>(nullptr); }
template <typename T>
MOZ_IMPLICIT StackFrame(T* ptr) {
template <typename T>
StackFrame& operator=(T* ptr) {
return *this;
// Constructors accepting SpiderMonkey's generic-pointer-ish types.
template <typename T>
explicit StackFrame(const JS::Handle<T*>& handle) {
template <typename T>
StackFrame& operator=(const JS::Handle<T*>& handle) {
return *this;
template <typename T>
explicit StackFrame(const JS::Rooted<T*>& root) {
template <typename T>
StackFrame& operator=(const JS::Rooted<T*>& root) {
return *this;
// Because StackFrame is just a vtable pointer and an instance pointer, we
// can memcpy everything around instead of making concrete classes define
// virtual constructors. See the comment above Node's copy constructor for
// more details; that comment applies here as well.
StackFrame(const StackFrame& rhs) {
memcpy(storage.u.mBytes,, sizeof(storage.u));
StackFrame& operator=(const StackFrame& rhs) {
memcpy(storage.u.mBytes,, sizeof(storage.u));
return *this;
bool operator==(const StackFrame& rhs) const {
return base()->ptr == rhs.base()->ptr;
bool operator!=(const StackFrame& rhs) const { return !(*this == rhs); }
explicit operator bool() const { return base()->ptr != nullptr; }
// Copy this StackFrame's source name into the given |destination|
// buffer. Copy no more than |length| characters. The result is *not* null
// terminated. Returns how many characters were written into the buffer.
size_t source(RangedPtr<char16_t> destination, size_t length) const;
// Copy this StackFrame's function display name into the given |destination|
// buffer. Copy no more than |length| characters. The result is *not* null
// terminated. Returns how many characters were written into the buffer.
size_t functionDisplayName(RangedPtr<char16_t> destination,
size_t length) const;
// Get the size of the respective strings. 0 is returned for null strings.
size_t sourceLength();
size_t functionDisplayNameLength();
// Methods that forward to virtual calls through BaseStackFrame.
void trace(JSTracer* trc) { base()->trace(trc); }
uint64_t identifier() const {
auto id = base()->identifier();
return id;
uint32_t line() const { return base()->line(); }
JS::TaggedColumnNumberOneOrigin column() const { return base()->column(); }
AtomOrTwoByteChars source() const { return base()->source(); }
uint32_t sourceId() const { return base()->sourceId(); }
AtomOrTwoByteChars functionDisplayName() const {
return base()->functionDisplayName();
StackFrame parent() const { return base()->parent(); }
bool isSystem() const { return base()->isSystem(); }
bool isSelfHosted(JSContext* cx) const { return base()->isSelfHosted(cx); }
[[nodiscard]] bool constructSavedFrameStack(
JSContext* cx, MutableHandleObject outSavedFrameStack) const {
return base()->constructSavedFrameStack(cx, outSavedFrameStack);
struct HashPolicy {
using Lookup = JS::ubi::StackFrame;
static js::HashNumber hash(const Lookup& lookup) {
return mozilla::HashGeneric(lookup.identifier());
static bool match(const StackFrame& key, const Lookup& lookup) {
return key == lookup;
static void rekey(StackFrame& k, const StackFrame& newKey) { k = newKey; }
// The ubi::StackFrame null pointer. Any attempt to operate on a null
// ubi::StackFrame crashes.
template <>
class ConcreteStackFrame<void> : public BaseStackFrame {
explicit ConcreteStackFrame(void* ptr) : BaseStackFrame(ptr) {}
static void construct(void* storage, void*) {
new (storage) ConcreteStackFrame(nullptr);
uint64_t identifier() const override { return 0; }
void trace(JSTracer* trc) override {}
[[nodiscard]] bool constructSavedFrameStack(
JSContext* cx, MutableHandleObject out) const override {
return true;
uint32_t line() const override { MOZ_CRASH("null JS::ubi::StackFrame"); }
JS::TaggedColumnNumberOneOrigin column() const override {
MOZ_CRASH("null JS::ubi::StackFrame");
AtomOrTwoByteChars source() const override {
MOZ_CRASH("null JS::ubi::StackFrame");
uint32_t sourceId() const override { MOZ_CRASH("null JS::ubi::StackFrame"); }
AtomOrTwoByteChars functionDisplayName() const override {
MOZ_CRASH("null JS::ubi::StackFrame");
StackFrame parent() const override { MOZ_CRASH("null JS::ubi::StackFrame"); }
bool isSystem() const override { MOZ_CRASH("null JS::ubi::StackFrame"); }
bool isSelfHosted(JSContext* cx) const override {
MOZ_CRASH("null JS::ubi::StackFrame");
[[nodiscard]] JS_PUBLIC_API bool ConstructSavedFrameStackSlow(
JSContext* cx, JS::ubi::StackFrame& frame,
MutableHandleObject outSavedFrameStack);
/*** ubi::Node
* ************************************************************************************/
// A concrete node specialization can claim its referent is a member of a
// particular "coarse type" which is less specific than the actual
// implementation type but generally more palatable for web developers. For
// example, JitCode can be considered to have a coarse type of "Script". This is
// used by some analyses for putting nodes into different buckets. The default,
// if a concrete specialization does not provide its own mapping to a CoarseType
// variant, is "Other".
// NB: the values associated with a particular enum variant must not change or
// be reused for new variants. Doing so will cause inspecting ubi::Nodes backed
// by an offline heap snapshot from an older SpiderMonkey/Firefox version to
// break. Consider this enum append only.
enum class CoarseType : uint32_t {
Other = 0,
Object = 1,
Script = 2,
String = 3,
DOMNode = 4,
FIRST = Other,
* Convert a CoarseType enum into a string. The string is statically allocated.
JS_PUBLIC_API const char* CoarseTypeToString(CoarseType type);
inline uint32_t CoarseTypeToUint32(CoarseType type) {
return static_cast<uint32_t>(type);
inline bool Uint32IsValidCoarseType(uint32_t n) {
auto first = static_cast<uint32_t>(CoarseType::FIRST);
auto last = static_cast<uint32_t>(CoarseType::LAST);
MOZ_ASSERT(first < last);
return first <= n && n <= last;
inline CoarseType Uint32ToCoarseType(uint32_t n) {
return static_cast<CoarseType>(n);
// The base class implemented by each ubi::Node referent type. Subclasses must
// not add data members to this class.
class JS_PUBLIC_API Base {
friend class Node;
// For performance's sake, we'd prefer to avoid a virtual destructor; and
// an empty constructor seems consistent with the 'lightweight value type'
// visible behavior we're trying to achieve. But if the destructor isn't
// virtual, and a subclass overrides it, the subclass's destructor will be
// ignored. Is there a way to make the compiler catch that error?
// Space for the actual pointer. Concrete subclasses should define a
// properly typed 'get' member function to access this.
void* ptr;
explicit Base(void* ptr) : ptr(ptr) {}
bool operator==(const Base& rhs) const {
// Some compilers will indeed place objects of different types at
// the same address, so technically, we should include the vtable
// in this comparison. But it seems unlikely to cause problems in
// practice.
return ptr == rhs.ptr;
bool operator!=(const Base& rhs) const { return !(*this == rhs); }
// An identifier for this node, guaranteed to be stable and unique for as
// long as this ubi::Node's referent is alive and at the same address.
// This is probably suitable for use in serializations, as it is an integral
// type. It may also help save memory when constructing HashSets of
// ubi::Nodes: since a uint64_t will always be smaller-or-equal-to the size
// of a ubi::Node, a HashSet<ubi::Node::Id> may use less space per element
// than a HashSet<ubi::Node>.
// (Note that 'unique' only means 'up to equality on ubi::Node'; see the
// caveats about multiple objects allocated at the same address for
// 'ubi::Node::operator=='.)
using Id = uint64_t;
virtual Id identifier() const { return Id(uintptr_t(ptr)); }
// Returns true if this node is pointing to something on the live heap, as
// opposed to something from a deserialized core dump. Returns false,
// otherwise.
virtual bool isLive() const { return true; };
// Return the coarse-grained type-of-thing that this node represents.
virtual CoarseType coarseType() const { return CoarseType::Other; }
// Return a human-readable name for the referent's type. The result should
// be statically allocated. (You can use u"strings" for this.)
// This must always return Concrete<T>::concreteTypeName; we use that
// pointer as a tag for this particular referent type.
virtual const char16_t* typeName() const = 0;
// Return the size of this node, in bytes. Include any structures that this
// node owns exclusively that are not exposed as their own ubi::Nodes.
// |mallocSizeOf| should be a malloc block sizing function; see
// |mfbt/MemoryReporting.h|.
// Because we can use |JS::ubi::Node|s backed by a snapshot that was taken
// on a 64-bit platform when we are currently on a 32-bit platform, we
// cannot rely on |size_t| for node sizes. Instead, |Size| is uint64_t on
// all platforms.
using Size = uint64_t;
virtual Size size(mozilla::MallocSizeOf mallocSizeof) const { return 1; }
// Return an EdgeRange that initially contains all the referent's outgoing
// edges. The caller takes ownership of the EdgeRange.
// If wantNames is true, compute names for edges. Doing so can be expensive
// in time and memory.
virtual js::UniquePtr<EdgeRange> edges(JSContext* cx,
bool wantNames) const = 0;
// Return the Zone to which this node's referent belongs, or nullptr if the
// referent is not of a type allocated in SpiderMonkey Zones.
virtual JS::Zone* zone() const { return nullptr; }
// Return the compartment for this node. Some ubi::Node referents are not
// associated with Compartments, such as JSStrings (which are associated
// with Zones). When the referent is not associated with a compartment,
// nullptr is returned.
virtual JS::Compartment* compartment() const { return nullptr; }
// Return the realm for this node. Some ubi::Node referents are not
// associated with Realms, such as JSStrings (which are associated
// with Zones) or cross-compartment wrappers (which are associated with
// compartments). When the referent is not associated with a realm,
// nullptr is returned.
virtual JS::Realm* realm() const { return nullptr; }
// Return whether this node's referent's allocation stack was captured.
virtual bool hasAllocationStack() const { return false; }
// Get the stack recorded at the time this node's referent was
// allocated. This must only be called when hasAllocationStack() is true.
virtual StackFrame allocationStack() const {
"Concrete classes that have an allocation stack must override both "
"hasAllocationStack and allocationStack.");
// In some cases, Concrete<T> can return a more descriptive
// referent type name than simply `T`. This method returns an
// identifier as specific as is efficiently available.
// The string returned is borrowed from the ubi::Node's referent.
// If nothing more specific than typeName() is available, return nullptr.
virtual const char16_t* descriptiveTypeName() const { return nullptr; }
// Methods for JSObject Referents
// These methods are only semantically valid if the referent is either a
// JSObject in the live heap, or represents a previously existing JSObject
// from some deserialized heap snapshot.
// Return the object's [[Class]]'s name.
virtual const char* jsObjectClassName() const { return nullptr; }
// Methods for CoarseType::Script referents
// Return the script's source's filename if available. If unavailable,
// return nullptr.
virtual const char* scriptFilename() const { return nullptr; }
Base(const Base& rhs) = delete;
Base& operator=(const Base& rhs) = delete;
// A traits template with a specialization for each referent type that
// ubi::Node supports. The specialization must be the concrete subclass of Base
// that represents a pointer to the referent type. It must include these
// members:
// // The specific char16_t array returned by Concrete<T>::typeName().
// static const char16_t concreteTypeName[];
// // Construct an instance of this concrete class in |storage| referring
// // to |referent|. Implementations typically use a placement 'new'.
// //
// // In some cases, |referent| will contain dynamic type information that
// // identifies it a some more specific subclass of |Referent|. For
// // example, when |Referent| is |JSObject|, then |referent->getClass()|
// // could tell us that it's actually a JSFunction. Similarly, if
// // |Referent| is |nsISupports|, we would like a ubi::Node that knows its
// // final implementation type.
// //
// // So we delegate the actual construction to this specialization, which
// // knows Referent's details.
// static void construct(void* storage, Referent* referent);
template <typename Referent>
class Concrete;
// A container for a Base instance; all members simply forward to the contained
// instance. This container allows us to pass ubi::Node instances by value.
class Node {
// Storage in which we allocate Base subclasses.
mozilla::AlignedStorage2<Base> storage;
Base* base() { return storage.addr(); }
const Base* base() const { return storage.addr(); }
template <typename T>
void construct(T* ptr) {
sizeof(Concrete<T>) == sizeof(*base()),
"ubi::Base specializations must be the same size as ubi::Base");
static_assert(std::is_base_of_v<Base, Concrete<T>>,
"ubi::Concrete<T> must inherit from ubi::Base");
Concrete<T>::construct(base(), ptr);
struct ConstructFunctor;
Node() { construct<void>(nullptr); }
template <typename T>
MOZ_IMPLICIT Node(T* ptr) {
template <typename T>
Node& operator=(T* ptr) {
return *this;
// We can construct and assign from rooted forms of pointers.
template <typename T>
MOZ_IMPLICIT Node(const Rooted<T*>& root) {
template <typename T>
Node& operator=(const Rooted<T*>& root) {
return *this;
// Constructors accepting SpiderMonkey's other generic-pointer-ish types.
// Note that we *do* want an implicit constructor here: JS::Value and
// JS::ubi::Node are both essentially tagged references to other sorts of
// objects, so letting conversions happen automatically is appropriate.
MOZ_IMPLICIT Node(JS::HandleValue value);
explicit Node(JS::GCCellPtr thing);
// copy construction and copy assignment just use memcpy, since we know
// instances contain nothing but a vtable pointer and a data pointer.
// To be completely correct, concrete classes could provide a virtual
// 'construct' member function, which we could invoke on rhs to construct an
// instance in our storage. But this is good enough; there's no need to jump
// through vtables for copying and assignment that are just going to move
// two words around. The compiler knows how to optimize memcpy.
Node(const Node& rhs) {
memcpy(storage.u.mBytes,, sizeof(storage.u));
Node& operator=(const Node& rhs) {
memcpy(storage.u.mBytes,, sizeof(storage.u));
return *this;
bool operator==(const Node& rhs) const { return *base() == *rhs.base(); }
bool operator!=(const Node& rhs) const { return *base() != *rhs.base(); }
explicit operator bool() const { return base()->ptr != nullptr; }
bool isLive() const { return base()->isLive(); }
// Get the canonical type name for the given type T.
template <typename T>
static const char16_t* canonicalTypeName() {
return Concrete<T>::concreteTypeName;
template <typename T>
bool is() const {
return base()->typeName() == canonicalTypeName<T>();
template <typename T>
T* as() const {
return static_cast<T*>(base()->ptr);
template <typename T>
T* asOrNull() const {
return this->is<T>() ? static_cast<T*>(base()->ptr) : nullptr;
// If this node refers to something that can be represented as a JavaScript
// value that is safe to expose to JavaScript code, return that value.
// Otherwise return UndefinedValue(). JSStrings, JS::Symbols, and some (but
// not all!) JSObjects can be exposed.
JS::Value exposeToJS() const;
CoarseType coarseType() const { return base()->coarseType(); }
const char16_t* typeName() const { return base()->typeName(); }
JS::Zone* zone() const { return base()->zone(); }
JS::Compartment* compartment() const { return base()->compartment(); }
JS::Realm* realm() const { return base()->realm(); }
const char* jsObjectClassName() const { return base()->jsObjectClassName(); }
const char16_t* descriptiveTypeName() const {
return base()->descriptiveTypeName();
const char* scriptFilename() const { return base()->scriptFilename(); }
using Size = Base::Size;
Size size(mozilla::MallocSizeOf mallocSizeof) const {
auto size = base()->size(mallocSizeof);
size > 0,
"C++ does not have zero-sized types! Choose 1 if you just need a "
"conservative default.");
return size;
js::UniquePtr<EdgeRange> edges(JSContext* cx, bool wantNames = true) const {
return base()->edges(cx, wantNames);
bool hasAllocationStack() const { return base()->hasAllocationStack(); }
StackFrame allocationStack() const { return base()->allocationStack(); }
using Id = Base::Id;
Id identifier() const {
auto id = base()->identifier();
return id;
// A hash policy for ubi::Nodes.
// This simply uses the stock PointerHasher on the ubi::Node's pointer.
// We specialize DefaultHasher below to make this the default.
class HashPolicy {
typedef js::PointerHasher<void*> PtrHash;
typedef Node Lookup;
static js::HashNumber hash(const Lookup& l) {
return PtrHash::hash(l.base()->ptr);
static bool match(const Node& k, const Lookup& l) { return k == l; }
static void rekey(Node& k, const Node& newKey) { k = newKey; }
using NodeSet =
js::HashSet<Node, js::DefaultHasher<Node>, js::SystemAllocPolicy>;
using NodeSetPtr = mozilla::UniquePtr<NodeSet, JS::DeletePolicy<NodeSet>>;
/*** Edge and EdgeRange *******************************************************/
using EdgeName = UniqueTwoByteChars;
// An outgoing edge to a referent node.
class Edge {
Edge() = default;
// Construct an initialized Edge, taking ownership of |name|.
Edge(char16_t* name, const Node& referent) : name(name), referent(referent) {}
// Move construction and assignment.
Edge(Edge&& rhs) : name(std::move(, referent(rhs.referent) {}
Edge& operator=(Edge&& rhs) {
MOZ_ASSERT(&rhs != this);
new (this) Edge(std::move(rhs));
return *this;
Edge(const Edge&) = delete;
Edge& operator=(const Edge&) = delete;
// This edge's name. This may be nullptr, if Node::edges was called with
// false as the wantNames parameter.
// The storage is owned by this Edge, and will be freed when this Edge is
// destructed. You may take ownership of the name by `std::move`ing it
// out of the edge; it is just a UniquePtr.
// (In real life we'll want a better representation for names, to avoid
// creating tons of strings when the names follow a pattern; and we'll need
// to think about lifetimes carefully to ensure traversal stays cheap.)
EdgeName name = nullptr;
// This edge's referent.
Node referent;
// EdgeRange is an abstract base class for iterating over a node's outgoing
// edges. (This is modeled after js::HashTable<K,V>::Range.)
// Concrete instances of this class need not be as lightweight as Node itself,
// since they're usually only instantiated while iterating over a particular
// object's edges. For example, a dumb implementation for JS Cells might use
// JS::TraceChildren to to get the outgoing edges, and then store them in an
// array internal to the EdgeRange.
class EdgeRange {
// The current front edge of this range, or nullptr if this range is empty.
Edge* front_;
EdgeRange() : front_(nullptr) {}
virtual ~EdgeRange() = default;
// True if there are no more edges in this range.
bool empty() const { return !front_; }
// The front edge of this range. This is owned by the EdgeRange, and is
// only guaranteed to live until the next call to popFront, or until
// the EdgeRange is destructed.
const Edge& front() const { return *front_; }
Edge& front() { return *front_; }
// Remove the front edge from this range. This should only be called if
// !empty().
virtual void popFront() = 0;
EdgeRange(const EdgeRange&) = delete;
EdgeRange& operator=(const EdgeRange&) = delete;
typedef mozilla::Vector<Edge, 8, js::SystemAllocPolicy> EdgeVector;
// An EdgeRange concrete class that holds a pre-existing vector of
// Edges. A PreComputedEdgeRange does not take ownership of its
// EdgeVector; it is up to the PreComputedEdgeRange's consumer to manage
// that lifetime.
class PreComputedEdgeRange : public EdgeRange {
EdgeVector& edges;
size_t i;
void settle() { front_ = i < edges.length() ? &edges[i] : nullptr; }
explicit PreComputedEdgeRange(EdgeVector& edges) : edges(edges), i(0) {
void popFront() override {
/*** RootList *****************************************************************/
// RootList is a class that can be pointed to by a |ubi::Node|, creating a
// fictional root-of-roots which has edges to every GC root in the JS
// runtime. Having a single root |ubi::Node| is useful for algorithms written
// with the assumption that there aren't multiple roots (such as computing
// dominator trees) and you want a single point of entry. It also ensures that
// the roots themselves get visited by |ubi::BreadthFirst| (they would otherwise
// only be used as starting points).
// RootList::init itself causes a minor collection, but once the list of roots
// has been created, GC must not occur, as the referent ubi::Nodes are not
// stable across GC. It returns a [[nodiscard]] AutoCheckCannotGC token in order
// to enforce this. The token's lifetime must extend at least as long as the
// RootList itself. Note that the RootList does not itself contain a nogc field,
// which means that it is possible to store it somewhere that it can escape
// the init()'s nogc scope. Don't do that. (Or you could call some function
// and pass in the RootList and GC, but that would be caught.)
// Example usage:
// {
// JS::ubi::RootList rootList(cx);
// auto [ok, nogc] = rootList.init();
// if (!ok()) {
// return false;
// }
// JS::ubi::Node root(&rootList);
// ...
// }
JSContext* cx;
EdgeVector edges;
bool wantNames;
bool inited;
explicit RootList(JSContext* cx, bool wantNames = false);
// Find all GC roots.
[[nodiscard]] std::pair<bool, JS::AutoCheckCannotGC> init();
// Find only GC roots in the provided set of |JS::Compartment|s. Note: it's
// important to take a CompartmentSet and not a RealmSet: objects in
// same-compartment realms can reference each other directly, without going
// through CCWs, so if we used a RealmSet here we would miss edges.
[[nodiscard]] std::pair<bool, JS::AutoCheckCannotGC> init(
CompartmentSet& debuggees);
// Find only GC roots in the given Debugger object's set of debuggee
// compartments.
[[nodiscard]] std::pair<bool, JS::AutoCheckCannotGC> init(
HandleObject debuggees);
// Returns true if the RootList has been initialized successfully, false
// otherwise.
bool initialized() { return inited; }
// Explicitly add the given Node as a root in this RootList. If wantNames is
// true, you must pass an edgeName. The RootList does not take ownership of
// edgeName.
[[nodiscard]] bool addRoot(Node node, const char16_t* edgeName = nullptr);
/*** Concrete classes for ubi::Node referent types ****************************/
template <>
class JS_PUBLIC_API Concrete<RootList> : public Base {
explicit Concrete(RootList* ptr) : Base(ptr) {}
RootList& get() const { return *static_cast<RootList*>(ptr); }
static void construct(void* storage, RootList* ptr) {
new (storage) Concrete(ptr);
js::UniquePtr<EdgeRange> edges(JSContext* cx, bool wantNames) const override;
const char16_t* typeName() const override { return concreteTypeName; }
static const char16_t concreteTypeName[];
// A reusable ubi::Concrete specialization base class for types supported by
// JS::TraceChildren.
template <typename Referent>
class JS_PUBLIC_API TracerConcrete : public Base {
JS::Zone* zone() const override;
js::UniquePtr<EdgeRange> edges(JSContext* cx, bool wantNames) const override;
explicit TracerConcrete(Referent* ptr) : Base(ptr) {}
Referent& get() const { return *static_cast<Referent*>(ptr); }
// For JS::TraceChildren-based types that have 'realm' and 'compartment'
// methods.
template <typename Referent>
class JS_PUBLIC_API TracerConcreteWithRealm : public TracerConcrete<Referent> {
typedef TracerConcrete<Referent> TracerBase;
JS::Compartment* compartment() const override;
JS::Realm* realm() const override;
explicit TracerConcreteWithRealm(Referent* ptr) : TracerBase(ptr) {}
// Define specializations for some commonly-used public JSAPI types.
// These can use the generic templates above.
template <>
class JS_PUBLIC_API Concrete<JS::Symbol> : TracerConcrete<JS::Symbol> {
explicit Concrete(JS::Symbol* ptr) : TracerConcrete(ptr) {}
static void construct(void* storage, JS::Symbol* ptr) {
new (storage) Concrete(ptr);
Size size(mozilla::MallocSizeOf mallocSizeOf) const override;
const char16_t* typeName() const override { return concreteTypeName; }
static const char16_t concreteTypeName[];
template <>
class JS_PUBLIC_API Concrete<JS::BigInt> : TracerConcrete<JS::BigInt> {
explicit Concrete(JS::BigInt* ptr) : TracerConcrete(ptr) {}
static void construct(void* storage, JS::BigInt* ptr) {
new (storage) Concrete(ptr);
Size size(mozilla::MallocSizeOf mallocSizeOf) const override;
const char16_t* typeName() const override { return concreteTypeName; }
static const char16_t concreteTypeName[];
template <>
class JS_PUBLIC_API Concrete<js::BaseScript>
: TracerConcreteWithRealm<js::BaseScript> {
explicit Concrete(js::BaseScript* ptr)
: TracerConcreteWithRealm<js::BaseScript>(ptr) {}
static void construct(void* storage, js::BaseScript* ptr) {
new (storage) Concrete(ptr);
CoarseType coarseType() const final { return CoarseType::Script; }
Size size(mozilla::MallocSizeOf mallocSizeOf) const override;
const char* scriptFilename() const final;
const char16_t* typeName() const override { return concreteTypeName; }
static const char16_t concreteTypeName[];
// The JSObject specialization.
template <>
class JS_PUBLIC_API Concrete<JSObject> : public TracerConcrete<JSObject> {
explicit Concrete(JSObject* ptr) : TracerConcrete<JSObject>(ptr) {}
static void construct(void* storage, JSObject* ptr);
JS::Compartment* compartment() const override;
JS::Realm* realm() const override;
const char* jsObjectClassName() const override;
Size size(mozilla::MallocSizeOf mallocSizeOf) const override;
bool hasAllocationStack() const override;
StackFrame allocationStack() const override;
CoarseType coarseType() const final { return CoarseType::Object; }
const char16_t* typeName() const override { return concreteTypeName; }
static const char16_t concreteTypeName[];
// For JSString, we extend the generic template with a 'size' implementation.
template <>
class JS_PUBLIC_API Concrete<JSString> : TracerConcrete<JSString> {
explicit Concrete(JSString* ptr) : TracerConcrete<JSString>(ptr) {}
static void construct(void* storage, JSString* ptr) {
new (storage) Concrete(ptr);
Size size(mozilla::MallocSizeOf mallocSizeOf) const override;
CoarseType coarseType() const final { return CoarseType::String; }
const char16_t* typeName() const override { return concreteTypeName; }
static const char16_t concreteTypeName[];
// The ubi::Node null pointer. Any attempt to operate on a null ubi::Node
// asserts.
template <>
class JS_PUBLIC_API Concrete<void> : public Base {
const char16_t* typeName() const override;
Size size(mozilla::MallocSizeOf mallocSizeOf) const override;
js::UniquePtr<EdgeRange> edges(JSContext* cx, bool wantNames) const override;
JS::Zone* zone() const override;
JS::Compartment* compartment() const override;
JS::Realm* realm() const override;
CoarseType coarseType() const final;
explicit Concrete(void* ptr) : Base(ptr) {}
static void construct(void* storage, void* ptr) {
new (storage) Concrete(ptr);
// The |callback| callback is much like the |Concrete<T>::construct| method: a
// call to |callback| should construct an instance of the most appropriate
// JS::ubi::Base subclass for |obj| in |storage|. The callback may assume that
// |obj->getClass()->isDOMClass()|, and that |storage| refers to the
// sizeof(JS::ubi::Base) bytes of space that all ubi::Base implementations
// should require.
// Set |cx|'s runtime hook for constructing ubi::Nodes for DOM classes to
// |callback|.
void SetConstructUbiNodeForDOMObjectCallback(JSContext* cx,
void (*callback)(void*,
} // namespace ubi
} // namespace JS
namespace mozilla {
// Make ubi::Node::HashPolicy the default hash policy for ubi::Node.
template <>
struct DefaultHasher<JS::ubi::Node> : JS::ubi::Node::HashPolicy {};
template <>
struct DefaultHasher<JS::ubi::StackFrame> : JS::ubi::StackFrame::HashPolicy {};
} // namespace mozilla
#endif // js_UbiNode_h