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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 nsGkAtoms_h___
#define nsGkAtoms_h___
#include "nsAtom.h"
// Static atoms are structured carefully to satisfy a lot of constraints.
// - We have ~2300 static atoms.
// - We want them to be constexpr so they end up in .rodata, and thus shared
// between processes, minimizing memory usage.
// - We need them to be in an array, so we can iterate over them (for
// registration and lookups).
// - Each static atom has a string literal associated with it. We can't use a
// pointer to the string literal because then the atoms won't end up in
// .rodata. Therefore the string literals and the atoms must be arranged in a
// way such that a numeric index can be used instead. This numeric index
// (nsStaticAtom::mStringOffset) must be computable at compile-time to keep
// the static atom constexpr. It should also not be too large (a uint32_t is
// reasonable).
// - Each static atom stores the hash value of its associated string literal;
// it's used in various ways. The hash value must be specified at
// compile-time, to keep the static atom constexpr.
// - As well as accessing each static atom via array indexing, we need an
// individual pointer, e.g. nsGkAtoms::foo. We want this to be constexpr so
// it doesn't take up any space in memory.
// - The array of static atoms can't be in a .h file, because it's a huge
// constexpr expression, which would blow out compile times. But the
// individual pointers for the static atoms must be in a .h file so they are
// public.
// nsGkAtoms below defines static atoms in a way that satisfies these
// constraints. It uses nsGkAtomList.h, which defines the names and values of
// the atoms. nsGkAtomList.h is generated by and has entries
// that look like this:
// GK_ATOM(a, "a", 0x01234567, nsStaticAtom, Atom)
// GK_ATOM(bb, "bb", 0x12345678, nsCSSPseudoElementStaticAtom,
// PseudoElementAtom)
// GK_ATOM(Ccc, "Ccc", 0x23456789, nsCSSAnonBoxPseudoStaticAtom,
// InheritingAnonBoxAtom)
// Comments throughout this file and nsGkAtoms.cpp show how these entries get
// expanded by macros.
// Trivial subclasses of nsStaticAtom so that function signatures can require
// an atom from a specific atom list.
class name_ : public nsStaticAtom { \
public: \
constexpr name_(uint32_t aLength, uint32_t aHash, uint32_t aOffset, \
bool aIsAsciiLowercase) \
: nsStaticAtom(aLength, aHash, aOffset, aIsAsciiLowercase) {} \
namespace mozilla {
namespace detail {
// This `detail` class contains the atom strings and the atom objects. Because
// they are together in a class, the `mStringOffset` field of the atoms will be
// small and can be initialized at compile time.
// A `detail` namespace is used because the things within it aren't directly
// referenced by external users of these static atoms.
struct GkAtoms {
// The declaration of each atom's string.
// Expansion of the example GK_ATOM entries from above:
// const char16_t a_string[sizeof("a")];
// const char16_t bb_string[sizeof("bb")];
// const char16_t Ccc_string[sizeof("Ccc")];
#define GK_ATOM(name_, value_, hash_, is_ascii_lower_, type_, atom_type_) \
const char16_t name_##_string[sizeof(value_)];
#include "nsGkAtomList.h"
#undef GK_ATOM
// The enum value for each atom.
enum class Atoms {
// Expansion of the example GK_ATOM entries above:
// a,
// bb,
// Ccc,
#define GK_ATOM(name_, value_, hash_, is_ascii_lower_, type_, atom_type_) name_,
#include "nsGkAtomList.h"
#undef GK_ATOM
const nsStaticAtom mAtoms[static_cast<size_t>(Atoms::AtomsCount)];
// The offset from the start of the GkAtoms object to the start of the
// nsStaticAtom array inside it. This is used in Rust to avoid problems
// with lld-link.exe on Windows when rust-bindgen generates a non-opaque
// version of GkAtoms.
const ptrdiff_t kGkAtomsArrayOffset = offsetof(GkAtoms, mAtoms);
// The GkAtoms instance is `extern const` so it can be defined in a .cpp file.
// XXX: The NS_EXTERNAL_VIS is necessary to work around an apparent GCC bug:
#if defined(__GNUC__) && !defined(__clang__)
extern NS_EXTERNAL_VIS const GkAtoms gGkAtoms;
extern const GkAtoms gGkAtoms;
} // namespace detail
} // namespace mozilla
// This class holds the pointers to the individual atoms.
class nsGkAtoms {
friend void NS_InitAtomTable();
// This is a useful handle to the array of atoms, used below and also
// possibly by Rust code.
static const nsStaticAtom* const sAtoms;
// The number of atoms, used below.
static constexpr size_t sAtomsLen =
static nsStaticAtom* GetAtomByIndex(size_t aIndex) {
MOZ_ASSERT(aIndex < sAtomsLen);
return const_cast<nsStaticAtom*>(&sAtoms[aIndex]);
static size_t IndexOf(const nsStaticAtom* atom) {
nsStaticAtom* firstAtom = GetAtomByIndex(0);
size_t ret = atom - firstAtom;
MOZ_ASSERT(ret < sAtomsLen);
return ret;
// The definition of the pointer to each static atom.
// These types are not `static constexpr <type>* const` -- even though these
// atoms are immutable -- because they are often passed to functions with
// `nsAtom*` parameters that can be passed both dynamic and static atoms.
// Expansion of the example GK_ATOM entries above:
// static constexpr nsStaticAtom* a =
// const_cast<nsStaticAtom*>(
// &mozilla::detail::gGkAtoms.mAtoms[
// static_cast<size_t>(mozilla::detail::GkAtoms::Atoms::a)]);
// static constexpr nsStaticAtom* bb =
// const_cast<nsStaticAtom*>(
// &mozilla::detail::gGkAtoms.mAtoms[
// static_cast<size_t>(mozilla::detail::GkAtoms::Atoms::bb)]);
// static constexpr nsStaticAtom* Ccc =
// const_cast<nsStaticAtom*>(
// &mozilla::detail::gGkAtoms.mAtoms[
// static_cast<size_t>(mozilla::detail::GkAtoms::Atoms::Ccc)]);
#define GK_ATOM(name_, value_, hash_, is_ascii_lower_, type_, atom_type_) \
static constexpr nsStaticAtom* name_ = const_cast<nsStaticAtom*>( \
&mozilla::detail::gGkAtoms.mAtoms[static_cast<size_t>( \
#include "nsGkAtomList.h"
#undef GK_ATOM
inline bool nsAtom::IsEmpty() const { return this == nsGkAtoms::_empty; }
#endif /* nsGkAtoms_h___ */