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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 nsAtom_h
#define nsAtom_h
#include <type_traits>
#include "mozilla/Atomics.h"
#include "mozilla/Char16.h"
#include "mozilla/MemoryReporting.h"
#include "nsISupports.h"
#include "nsString.h"
namespace mozilla {
struct AtomsSizes;
class nsStaticAtom;
class nsDynamicAtom;
// This class encompasses both static and dynamic atoms.
// - In places where static and dynamic atoms can be used, use RefPtr<nsAtom>.
// This is by far the most common case.
// - In places where only static atoms can appear, use nsStaticAtom* to avoid
// unnecessary refcounting. This is a moderately common case.
// - In places where only dynamic atoms can appear, it doesn't matter much
// whether you use RefPtr<nsAtom> or RefPtr<nsDynamicAtom>. This is an
// extremely rare case.
class nsAtom {
void AddSizeOfIncludingThis(mozilla::MallocSizeOf aMallocSizeOf,
mozilla::AtomsSizes& aSizes) const;
bool Equals(char16ptr_t aString, uint32_t aLength) const {
return mLength == aLength &&
memcmp(GetUTF16String(), aString, mLength * sizeof(char16_t)) == 0;
bool Equals(const nsAString& aString) const {
return Equals(aString.BeginReading(), aString.Length());
bool IsStatic() const { return mIsStatic; }
bool IsDynamic() const { return !IsStatic(); }
inline const nsStaticAtom* AsStatic() const;
inline const nsDynamicAtom* AsDynamic() const;
inline nsDynamicAtom* AsDynamic();
char16ptr_t GetUTF16String() const;
uint32_t GetLength() const { return mLength; }
operator mozilla::Span<const char16_t>() const {
// Explicitly specify template argument here to avoid instantiating
// Span<char16_t> first and then implicitly converting to Span<const
// char16_t>
return mozilla::Span<const char16_t>{GetUTF16String(), GetLength()};
void ToString(nsAString& aString) const;
void ToUTF8String(nsACString& aString) const;
// A hashcode that is better distributed than the actual atom pointer, for
// use in situations that need a well-distributed hashcode. It's called hash()
// rather than Hash() so we can use mozilla::BloomFilter<N, nsAtom>, because
// BloomFilter requires elements to implement a function called hash().
uint32_t hash() const { return mHash; }
// This function returns true if ToLowercaseASCII would return the string
// unchanged.
bool IsAsciiLowercase() const { return mIsAsciiLowercase; }
// This function returns true if this is the empty atom. This is exactly
// equivalent to `this == nsGkAtoms::_empty`, but it's a bit less foot-gunny,
// since we also have `nsGkAtoms::empty`.
// Defined in nsGkAtoms.h
inline bool IsEmpty() const;
// We can't use NS_INLINE_DECL_THREADSAFE_REFCOUNTING because the refcounting
// of this type is special.
inline MozExternalRefCountType AddRef();
inline MozExternalRefCountType Release();
using HasThreadSafeRefCnt = std::true_type;
// Used by nsStaticAtom.
constexpr nsAtom(uint32_t aLength, uint32_t aHash, bool aIsAsciiLowercase)
: mLength(aLength),
mHash(aHash) {}
// Used by nsDynamicAtom.
nsAtom(const nsAString& aString, uint32_t aHash, bool aIsAsciiLowercase)
: mLength(aString.Length()),
mHash(aHash) {}
~nsAtom() = default;
const uint32_t mLength : 30;
const uint32_t mIsStatic : 1;
const uint32_t mIsAsciiLowercase : 1;
const uint32_t mHash;
// This class would be |final| if it wasn't for nsCSSAnonBoxPseudoStaticAtom
// and nsCSSPseudoElementStaticAtom, which are trivial subclasses used to
// ensure only certain static atoms are passed to certain functions.
class nsStaticAtom : public nsAtom {
// These are deleted so it's impossible to RefPtr<nsStaticAtom>. Raw
// nsStaticAtom pointers should be used instead.
MozExternalRefCountType AddRef() = delete;
MozExternalRefCountType Release() = delete;
// The static atom's precomputed hash value is an argument here, but it
// must be the same as would be computed by mozilla::HashString(aStr),
// which is what we use when atomizing strings. We compute this hash in
// and assert in nsAtomTable::RegisterStaticAtoms that the two
// hashes match.
constexpr nsStaticAtom(uint32_t aLength, uint32_t aHash,
uint32_t aStringOffset, bool aIsAsciiLowercase)
: nsAtom(aLength, aHash, aIsAsciiLowercase),
mStringOffset(aStringOffset) {}
const char16_t* String() const {
return reinterpret_cast<const char16_t*>(uintptr_t(this) - mStringOffset);
already_AddRefed<nsAtom> ToAddRefed() {
return already_AddRefed<nsAtom>(static_cast<nsAtom*>(this));
// This is an offset to the string chars, which must be at a lower address in
// memory.
uint32_t mStringOffset;
class nsDynamicAtom : public nsAtom {
// We can't use NS_INLINE_DECL_THREADSAFE_REFCOUNTING because the refcounting
// of this type is special.
MozExternalRefCountType AddRef() {
MOZ_ASSERT(int32_t(mRefCnt) >= 0, "illegal refcnt");
nsrefcnt count = ++mRefCnt;
if (count == 1) {
return count;
MozExternalRefCountType Release() {
#ifdef DEBUG
// We set a lower GC threshold for atoms in debug builds so that we exercise
// the GC machinery more often.
static const int32_t kAtomGCThreshold = 20;
static const int32_t kAtomGCThreshold = 10000;
MOZ_ASSERT(int32_t(mRefCnt) > 0, "dup release");
nsrefcnt count = --mRefCnt;
if (count == 0) {
if (++gUnusedAtomCount >= kAtomGCThreshold) {
return count;
const char16_t* String() const {
return reinterpret_cast<const char16_t*>(this + 1);
static nsDynamicAtom* FromChars(char16_t* chars) {
return reinterpret_cast<nsDynamicAtom*>(chars) - 1;
friend class nsAtomTable;
friend class nsAtomSubTable;
friend int32_t NS_GetUnusedAtomCount();
static mozilla::Atomic<int32_t, mozilla::ReleaseAcquire> gUnusedAtomCount;
static void GCAtomTable();
// These shouldn't be used directly, even by friend classes. The
// Create()/Destroy() methods use them.
nsDynamicAtom(const nsAString& aString, uint32_t aHash,
bool aIsAsciiLowercase);
~nsDynamicAtom() = default;
static nsDynamicAtom* Create(const nsAString& aString, uint32_t aHash);
static void Destroy(nsDynamicAtom* aAtom);
mozilla::ThreadSafeAutoRefCnt mRefCnt;
// The atom's chars are stored at the end of the struct.
const nsStaticAtom* nsAtom::AsStatic() const {
return static_cast<const nsStaticAtom*>(this);
const nsDynamicAtom* nsAtom::AsDynamic() const {
return static_cast<const nsDynamicAtom*>(this);
nsDynamicAtom* nsAtom::AsDynamic() {
return static_cast<nsDynamicAtom*>(this);
MozExternalRefCountType nsAtom::AddRef() {
return IsStatic() ? 2 : AsDynamic()->AddRef();
MozExternalRefCountType nsAtom::Release() {
return IsStatic() ? 1 : AsDynamic()->Release();
// The four forms of NS_Atomize (for use with |RefPtr<nsAtom>|) return the
// atom for the string given. At any given time there will always be one atom
// representing a given string. Atoms are intended to make string comparison
// cheaper by simplifying it to pointer equality. A pointer to the atom that
// does not own a reference is not guaranteed to be valid.
// Find an atom that matches the given UTF-8 string. The string is assumed to
// be zero terminated. Never returns null.
already_AddRefed<nsAtom> NS_Atomize(const char* aUTF8String);
// Find an atom that matches the given UTF-8 string. Never returns null.
already_AddRefed<nsAtom> NS_Atomize(const nsACString& aUTF8String);
// Find an atom that matches the given UTF-16 string. The string is assumed to
// be zero terminated. Never returns null.
already_AddRefed<nsAtom> NS_Atomize(const char16_t* aUTF16String);
// Find an atom that matches the given UTF-16 string. Never returns null.
already_AddRefed<nsAtom> NS_Atomize(const nsAString& aUTF16String);
// An optimized version of the method above for the main thread.
already_AddRefed<nsAtom> NS_AtomizeMainThread(const nsAString& aUTF16String);
// Return a count of the total number of atoms currently alive in the system.
// Note that the result is imprecise and racy if other threads are currently
// operating on atoms. It's also slow, since it triggers a GC before counting.
// Currently this function is only used in tests, which should probably remain
// the case.
nsrefcnt NS_GetNumberOfAtoms();
// Return a pointer for a static atom for the string or null if there's no
// static atom for this string.
nsStaticAtom* NS_GetStaticAtom(const nsAString& aUTF16String);
class nsAtomString : public nsString {
explicit nsAtomString(const nsAtom* aAtom) { aAtom->ToString(*this); }
class nsAtomCString : public nsCString {
explicit nsAtomCString(const nsAtom* aAtom) { aAtom->ToUTF8String(*this); }
class nsDependentAtomString : public nsDependentString {
explicit nsDependentAtomString(const nsAtom* aAtom)
: nsDependentString(aAtom->GetUTF16String(), aAtom->GetLength()) {}
// Checks if the ascii chars in a given atom are already lowercase.
// If they are, no-op. Otherwise, converts all the ascii uppercase
// chars to lowercase and atomizes, storing the result in the inout
// param.
void ToLowerCaseASCII(RefPtr<nsAtom>& aAtom);
#endif // nsAtom_h