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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 util_TrailingArray_h
#define util_TrailingArray_h
#include "mozilla/Assertions.h" // MOZ_ASSERT
#include <stddef.h> // size_t
#include <stdint.h> // uint32_t, uintptr_t
namespace js {
// This is a mixin class to use for types that have trailing arrays and use
// offsets to delimit them. It provides helper methods to do casting and
// initialization while avoiding C++ undefined behaviour.
class TrailingArray {
// Offsets are measured in bytes relative to 'this'.
using Offset = uint32_t;
// Test if offset is correctly aligned for type.
template <typename T>
static constexpr bool isAlignedOffset(Offset offset) {
return offset % alignof(T) == 0;
template <size_t N>
static constexpr bool isAlignedOffset(Offset offset) {
return offset % N == 0;
// Translate an offset into a concrete pointer.
template <typename T>
T* offsetToPointer(Offset offset) {
uintptr_t base = reinterpret_cast<uintptr_t>(this);
return reinterpret_cast<T*>(base + offset);
template <typename T>
const T* offsetToPointer(Offset offset) const {
uintptr_t base = reinterpret_cast<uintptr_t>(this);
return reinterpret_cast<const T*>(base + offset);
// Placement-new the elements of an array. This optimizes away for types with
// trivial default initialization and plays nicely with compiler vectorization
// passes.
template <typename T>
void initElements(Offset offset, size_t nelem) {
// Address of first array element.
uintptr_t elem = reinterpret_cast<uintptr_t>(this) + offset;
for (size_t i = 0; i < nelem; ++i) {
void* raw = reinterpret_cast<void*>(elem);
new (raw) T;
elem += sizeof(T);
// Compute the length of an array from its start and end offset.
template <typename T>
size_t numElements(Offset start, Offset end) const {
constexpr size_t ElemSize = sizeof(T);
return numElements<ElemSize>(start, end);
template <size_t ElemSize>
size_t numElements(Offset start, Offset end) const {
MOZ_ASSERT(start <= end);
MOZ_ASSERT((end - start) % ElemSize == 0);
return (end - start) / ElemSize;
// Constructor is protected so a derived type is required.
TrailingArray() = default;
// Type has trailing data so isn't copyable or movable.
TrailingArray(const TrailingArray&) = delete;
TrailingArray& operator=(const TrailingArray&) = delete;
} // namespace js
#endif // util_TrailingArray_h