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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_Caches_h
#define vm_Caches_h
#include <iterator>
#include <new>
#include "frontend/SourceNotes.h" // SrcNote
#include "gc/Tracer.h"
#include "js/RootingAPI.h"
#include "js/TypeDecls.h"
#include "js/UniquePtr.h"
#include "util/Memory.h"
#include "vm/ArrayObject.h"
#include "vm/JSAtom.h"
#include "vm/JSObject.h"
#include "vm/JSScript.h"
#include "vm/NativeObject.h"
namespace js {
* GetSrcNote cache to avoid O(n^2) growth in finding a source note for a
* given pc in a script. We use the script->code pointer to tag the cache,
* instead of the script address itself, so that source notes are always found
* by offset from the bytecode with which they were generated.
struct GSNCache {
typedef HashMap<jsbytecode*, const SrcNote*, PointerHasher<jsbytecode*>,
jsbytecode* code;
Map map;
GSNCache() : code(nullptr) {}
void purge();
struct EvalCacheEntry {
JSLinearString* str;
JSScript* script;
JSScript* callerScript;
jsbytecode* pc;
// We sweep this cache before a nursery collection to remove entries with
// string keys in the nursery.
// The entire cache is purged on a major GC, so we don't need to sweep it
// then.
bool needsSweep() { return !str->isTenured(); }
struct EvalCacheLookup {
explicit EvalCacheLookup(JSContext* cx) : str(cx), callerScript(cx) {}
RootedLinearString str;
RootedScript callerScript;
MOZ_INIT_OUTSIDE_CTOR jsbytecode* pc;
struct EvalCacheHashPolicy {
using Lookup = EvalCacheLookup;
static HashNumber hash(const Lookup& l);
static bool match(const EvalCacheEntry& entry, const EvalCacheLookup& l);
typedef GCHashSet<EvalCacheEntry, EvalCacheHashPolicy, SystemAllocPolicy>
* Cache for speeding up repetitive creation of objects in the VM.
* When an object is created which matches the criteria in the 'key' section
* below, an entry is filled with the resulting object.
class NewObjectCache {
static constexpr unsigned MAX_OBJ_SIZE = sizeof(JSObject_Slots16);
static void staticAsserts() {
static_assert(gc::AllocKind::OBJECT_LAST ==
struct Entry {
/* Class of the constructed object. */
const JSClass* clasp;
* Key with one of two possible values:
* - Global for the object. The object must have a standard class and will
* have this global's builtin prototype for this class as proto.
* - Prototype for the object (non-null). Cannot be a global object because
* that would be ambiguous (see previous case).
JSObject* key;
/* Allocation kind for the constructed object. */
gc::AllocKind kind;
/* Number of bytes to copy from the template object. */
uint32_t nbytes;
* Template object to copy from, with the initial values of fields,
* fixed slots (undefined) and private data (nullptr).
char templateObject[MAX_OBJ_SIZE];
using EntryArray = Entry[41]; // TODO: reconsider size;
EntryArray entries;
using EntryIndex = int;
: entries{} // zeroes out the array
void purge() {
new (&entries) EntryArray{}; // zeroes out the array
/* Remove any cached items keyed on moved objects. */
void clearNurseryObjects(JSRuntime* rt);
* Get the entry index for the given lookup, return whether there was a hit
* on an existing entry.
inline bool lookupProto(const JSClass* clasp, JSObject* proto,
gc::AllocKind kind, EntryIndex* pentry);
inline bool lookupGlobal(const JSClass* clasp, js::GlobalObject* global,
gc::AllocKind kind, EntryIndex* pentry);
* Return a new object from a cache hit produced by a lookup method, or
* nullptr if returning the object could possibly trigger GC (does not
* indicate failure).
inline NativeObject* newObjectFromHit(JSContext* cx, EntryIndex entry,
js::gc::InitialHeap heap);
/* Fill an entry after a cache miss. */
void fillProto(EntryIndex entry, const JSClass* clasp, js::TaggedProto proto,
gc::AllocKind kind, NativeObject* obj);
inline void fillGlobal(EntryIndex entry, const JSClass* clasp,
js::GlobalObject* global, gc::AllocKind kind,
NativeObject* obj);
/* Invalidate any entries which might produce an object with shape. */
void invalidateEntriesForShape(Shape* shape);
EntryIndex makeIndex(const JSClass* clasp, gc::Cell* key,
gc::AllocKind kind) {
uintptr_t hash = (uintptr_t(clasp) ^ uintptr_t(key)) + size_t(kind);
return hash % std::size(entries);
bool lookup(const JSClass* clasp, JSObject* key, gc::AllocKind kind,
EntryIndex* pentry) {
*pentry = makeIndex(clasp, key, kind);
Entry* entry = &entries[*pentry];
// N.B. Lookups with the same clasp/key but different kinds map to
// different entries.
return entry->clasp == clasp && entry->key == key;
void fill(EntryIndex entry_, const JSClass* clasp, JSObject* key,
gc::AllocKind kind, NativeObject* obj) {
MOZ_ASSERT(unsigned(entry_) < std::size(entries));
MOZ_ASSERT(entry_ == makeIndex(clasp, key, kind));
Entry* entry = &entries[entry_];
MOZ_ASSERT(obj->hasEmptyElements() || obj->is<ArrayObject>());
entry->clasp = clasp;
entry->key = key;
entry->kind = kind;
entry->nbytes = gc::Arena::thingSize(kind);
js_memcpy(&entry->templateObject, obj, entry->nbytes);
static void copyCachedToObject(NativeObject* dst, NativeObject* src,
gc::AllocKind kind) {
js_memcpy(dst, src, gc::Arena::thingSize(kind));
// Initialize with barriers
// Cache for AtomizeString, mapping JSLinearString* to the corresponding
// JSAtom*. Also used by nursery GC to de-duplicate strings to atoms.
// Purged on minor and major GC.
class StringToAtomCache {
using Map = HashMap<JSLinearString*, JSAtom*, PointerHasher<JSLinearString*>,
Map map_;
// Don't use the cache for short strings. Hashing them is less expensive.
static constexpr size_t MinStringLength = 30;
JSAtom* lookup(JSLinearString* s) {
if (!s->inStringToAtomCache()) {
return nullptr;
MOZ_ASSERT(s->length() >= MinStringLength);
auto p = map_.lookup(s);
JSAtom* atom = p ? p->value() : nullptr;
MOZ_ASSERT_IF(atom, EqualStrings(s, atom));
return atom;
void maybePut(JSLinearString* s, JSAtom* atom) {
if (s->length() < MinStringLength) {
if (!map_.putNew(s, atom)) {
void purge() { map_.clearAndCompact(); }
class RuntimeCaches {
js::GSNCache gsnCache;
js::NewObjectCache newObjectCache;
js::UncompressedSourceCache uncompressedSourceCache;
js::EvalCache evalCache;
js::StringToAtomCache stringToAtomCache;
void purgeForMinorGC(JSRuntime* rt) {
void purgeForCompaction() {
void purge() {
} // namespace js
#endif /* vm_Caches_h */