src/ds/LifoAlloc.h

/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set ts=8 sts=4 et sw=4 tw=99:
 * 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 http://mozilla.org/MPL/2.0/. */

#ifndef ds_LifoAlloc_h
#define ds_LifoAlloc_h

#include "mozilla/Attributes.h"
#include "mozilla/MathAlgorithms.h"
#include "mozilla/MemoryChecking.h"
#include "mozilla/MemoryReporting.h"
#include "mozilla/Move.h"
#include "mozilla/PodOperations.h"
#include "mozilla/TemplateLib.h"
#include "mozilla/TypeTraits.h"

// This data structure supports stacky LIFO allocation (mark/release and
// LifoAllocScope). It does not maintain one contiguous segment; instead, it
// maintains a bunch of linked memory segments. In order to prevent malloc/free
// thrashing, unused segments are deallocated when garbage collection occurs.

#include "jsutil.h"

#include "js/UniquePtr.h"

namespace js {

namespace detail {

template <typename T>
class SingleLinkedList;

template <typename T>
class SingleLinkedListElement {
  friend class SingleLinkedList<T>;
  js::UniquePtr<T> next_;

 public:
  SingleLinkedListElement() : next_(nullptr) {}
  ~SingleLinkedListElement() { MOZ_ASSERT(!next_); }

  T* next() const { return next_.get(); }
};

// Single linked list which is using UniquePtr to hold the next pointers.
// UniquePtr are used to ensure that none of the elements are used
// silmutaneously in 2 different list.
template <typename T>
class SingleLinkedList {
 private:
  // First element of the list which owns the next element, and ensure that
  // that this list is the only owner of the element.
  UniquePtr<T> head_;

  // Weak pointer to the last element of the list.
  T* last_;

  void assertInvariants() {
    MOZ_ASSERT(bool(head_) == bool(last_));
    MOZ_ASSERT_IF(last_, !last_->next_);
  }

 public:
  SingleLinkedList() : head_(nullptr), last_(nullptr) { assertInvariants(); }

  SingleLinkedList(SingleLinkedList&& other)
      : head_(mozilla::Move(other.head_)), last_(other.last_) {
    other.last_ = nullptr;
    assertInvariants();
    other.assertInvariants();
  }

  ~SingleLinkedList() {
    MOZ_ASSERT(!head_);
    MOZ_ASSERT(!last_);
  }

  // Move the elements of the |other| list in the current one, and implicitly
  // remove all the elements of the current list.
  SingleLinkedList& operator=(SingleLinkedList&& other) {
    head_ = mozilla::Move(other.head_);
    last_ = other.last_;
    other.last_ = nullptr;
    assertInvariants();
    other.assertInvariants();
    return *this;
  }

  bool empty() const { return !last_; }

  // Iterates over the elements of the list, this is used to avoid raw
  // manipulation of pointers as much as possible.
  class Iterator {
    T* current_;

   public:
    explicit Iterator(T* current) : current_(current) {}

    T& operator*() const { return *current_; }
    T* operator->() const { return current_; }
    T* get() const { return current_; }

    const Iterator& operator++() {
      current_ = current_->next();
      return *this;
    }

    bool operator!=(Iterator& other) const {
      return current_ != other.current_;
    }
    bool operator==(Iterator& other) const {
      return current_ == other.current_;
    }
  };

  Iterator begin() const { return Iterator(head_.get()); }
  Iterator end() const { return Iterator(nullptr); }
  Iterator last() const { return Iterator(last_); }

  // Split the list in 2 single linked lists after the element given as
  // argument.  The front of the list remains in the current list, while the
  // back goes in the newly create linked list.
  //
  // This is used for example to extract one element from a list. (see
  // LifoAlloc::getOrCreateChunk)
  SingleLinkedList splitAfter(T* newLast) {
    MOZ_ASSERT(newLast);
    SingleLinkedList result;
    if (newLast->next_) {
      result.head_ = mozilla::Move(newLast->next_);
      result.last_ = last_;
      last_ = newLast;
    }
    assertInvariants();
    result.assertInvariants();
    return result;
  }

  void pushFront(UniquePtr<T>&& elem) {
    if (!last_) last_ = elem.get();
    elem->next_ = mozilla::Move(head_);
    head_ = mozilla::Move(elem);
    assertInvariants();
  }

  void append(UniquePtr<T>&& elem) {
    if (last_) {
      last_->next_ = mozilla::Move(elem);
      last_ = last_->next_.get();
    } else {
      head_ = mozilla::Move(elem);
      last_ = head_.get();
    }
    assertInvariants();
  }
  void appendAll(SingleLinkedList&& list) {
    if (list.empty()) return;
    if (last_)
      last_->next_ = mozilla::Move(list.head_);
    else
      head_ = mozilla::Move(list.head_);
    last_ = list.last_;
    list.last_ = nullptr;
    assertInvariants();
    list.assertInvariants();
  }
  UniquePtr<T> popFirst() {
    MOZ_ASSERT(head_);
    UniquePtr<T> result = mozilla::Move(head_);
    head_ = mozilla::Move(result->next_);
    if (!head_) last_ = nullptr;
    assertInvariants();
    return result;
  }
};

static const size_t LIFO_ALLOC_ALIGN = 8;

MOZ_ALWAYS_INLINE
uint8_t* AlignPtr(uint8_t* orig) {
  static_assert(mozilla::tl::FloorLog2<LIFO_ALLOC_ALIGN>::value ==
                    mozilla::tl::CeilingLog2<LIFO_ALLOC_ALIGN>::value,
                "LIFO_ALLOC_ALIGN must be a power of two");

  uint8_t* result = (uint8_t*)AlignBytes(uintptr_t(orig), LIFO_ALLOC_ALIGN);
  MOZ_ASSERT(uintptr_t(result) % LIFO_ALLOC_ALIGN == 0);
  return result;
}

// A Chunk represent a single memory allocation made with the system
// allocator. As the owner of the memory, it is responsible for the allocation
// and the deallocation.
//
// This structure is only move-able, but not copyable.
class BumpChunk : public SingleLinkedListElement<BumpChunk> {
 private:
  // Pointer to the last byte allocated in this chunk.
  uint8_t* bump_;
  // Pointer to the last byte available in this chunk.
  const uint8_t* capacity_;

#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
  // Magic number used to check against poisoned values.
  const uintptr_t magic_;
  static constexpr uintptr_t magicNumber = sizeof(uintptr_t) == 4
                                               ? uintptr_t(0x4c69666f)
                                               : uintptr_t(0x4c69666f42756d70);
#endif

  // Poison the memory with memset, in order to catch errors due to
  // use-after-free, with undefinedChunkMemory pattern, or to catch
  // use-before-init with uninitializedChunkMemory.
#if defined(DEBUG)
#define LIFO_HAVE_MEM_CHECKS 1

  // Byte used for poisoning unused memory after releasing memory.
  static constexpr int undefinedChunkMemory = 0xcd;
  // Byte used for poisoning uninitialized memory after reserving memory.
  static constexpr int uninitializedChunkMemory = 0xce;

#define LIFO_MAKE_MEM_NOACCESS(addr, size)  \
  do {                                      \
    uint8_t* base = (addr);                 \
    size_t sz = (size);                     \
    memset(base, undefinedChunkMemory, sz); \
    MOZ_MAKE_MEM_NOACCESS(base, sz);        \
  } while (0)

#define LIFO_MAKE_MEM_UNDEFINED(addr, size)     \
  do {                                          \
    uint8_t* base = (addr);                     \
    size_t sz = (size);                         \
    MOZ_MAKE_MEM_UNDEFINED(base, sz);           \
    memset(base, uninitializedChunkMemory, sz); \
    MOZ_MAKE_MEM_UNDEFINED(base, sz);           \
  } while (0)

#elif defined(MOZ_HAVE_MEM_CHECKS)
#define LIFO_HAVE_MEM_CHECKS 1
#define LIFO_MAKE_MEM_NOACCESS(addr, size) MOZ_MAKE_MEM_NOACCESS((addr), (size))
#define LIFO_MAKE_MEM_UNDEFINED(addr, size) \
  MOZ_MAKE_MEM_UNDEFINED((addr), (size))
#endif

  void assertInvariants() {
    MOZ_DIAGNOSTIC_ASSERT(magic_ == magicNumber);
    MOZ_ASSERT(begin() <= end());
    MOZ_ASSERT(end() <= capacity_);
  }

  BumpChunk& operator=(const BumpChunk&) = delete;
  BumpChunk(const BumpChunk&) = delete;

  explicit BumpChunk(uintptr_t capacity)
      : bump_(begin()),
        capacity_(base() + capacity)
#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
        ,
        magic_(magicNumber)
#endif
  {
    // We cannot bake this value inside the BumpChunk class, because
    // sizeof(BumpChunk) can only be computed after the closing brace of the
    // BumpChunk class, or within one of its methods. As a work-around, the
    // reservedSpace value is baked in, and we check that it indeed matches
    // with the space taken by the data of the BumpChunk class, and the
    // alignment of a pointer.
    MOZ_ASSERT(BumpChunk::reservedSpace ==
                   AlignBytes(sizeof(BumpChunk), LIFO_ALLOC_ALIGN),
               "Checked that the baked-in value correspond to computed value");

    assertInvariants();
#if defined(LIFO_HAVE_MEM_CHECKS)
    // The memory is freshly allocated and marked as undefined by the
    // allocator of the BumpChunk. Instead, we mark this memory as
    // no-access, as it has not been allocated within the BumpChunk.
    LIFO_MAKE_MEM_NOACCESS(bump_, capacity_ - bump_);
#endif
  }

  // Cast |this| into a uint8_t* pointer.
  //
  // Warning: Are you sure you do not want to use begin() instead?
  const uint8_t* base() const { return reinterpret_cast<const uint8_t*>(this); }
  uint8_t* base() { return reinterpret_cast<uint8_t*>(this); }

  // Update the bump pointer to any value contained in this chunk, which is
  // above the private fields of this chunk.
  //
  // The memory is poisoned and flagged as no-access when the bump pointer is
  // moving backward, such as when memory is released, or when a Mark is used
  // to unwind previous allocations.
  //
  // The memory is flagged as undefined when the bump pointer is moving
  // forward.
  void setBump(uint8_t* newBump) {
    assertInvariants();
    MOZ_ASSERT(begin() <= newBump);
    MOZ_ASSERT(newBump <= capacity_);
#if defined(LIFO_HAVE_MEM_CHECKS)
    // Poison/Unpoison memory that we just free'd/allocated.
    if (bump_ > newBump)
      LIFO_MAKE_MEM_NOACCESS(newBump, bump_ - newBump);
    else if (newBump > bump_)
      LIFO_MAKE_MEM_UNDEFINED(bump_, newBump - bump_);
#endif
    bump_ = newBump;
  }

 public:
  ~BumpChunk() { release(); }

  // Space reserved for the BumpChunk internal data, and the alignment of the
  // first allocation content.  This can be used to ensure there is enough
  // space for the next allocation. (see LifoAlloc::newChunkWithCapacity)
  static constexpr size_t reservedSpace = 4 * sizeof(uintptr_t);

  // Returns true if this chunk contains no allocated content.
  bool empty() const { return end() == begin(); }

  // Returns the size in bytes of the number of allocated space. This includes
  // the size consumed by the alignment of the allocations.
  size_t used() const { return end() - begin(); }

  // These are used for manipulating a chunk as if it was a vector of bytes,
  // and used for iterating over the content of the buffer (see
  // LifoAlloc::Enum)
  const uint8_t* begin() const { return base() + reservedSpace; }
  uint8_t* begin() { return base() + reservedSpace; }
  uint8_t* end() const { return bump_; }

  // This function is the only way to allocate and construct a chunk. It
  // returns a UniquePtr to the newly allocated chunk.  The size given as
  // argument includes the space needed for the header of the chunk.
  static UniquePtr<BumpChunk> newWithCapacity(size_t size);

  // Report allocation.
  size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
    return mallocSizeOf(this);
  }

  // Report allocation size.
  size_t computedSizeOfIncludingThis() const { return capacity_ - base(); }

  // Opaque type used to carry a pointer to the current location of the bump_
  // pointer, within a BumpChunk.
  class Mark {
    // Chunk which owns the pointer.
    BumpChunk* chunk_;
    // Recorded of the bump_ pointer of the BumpChunk.
    uint8_t* bump_;

    friend class BumpChunk;
    explicit Mark(BumpChunk* chunk, uint8_t* bump)
        : chunk_(chunk), bump_(bump) {}

   public:
    Mark() : chunk_(nullptr), bump_(nullptr) {}

    BumpChunk* markedChunk() const { return chunk_; }
  };

  // Return a mark to be able to unwind future allocations with the release
  // function. (see LifoAllocScope)
  Mark mark() { return Mark(this, end()); }

  // Check if a pointer is part of the allocated data of this chunk.
  bool contains(void* ptr) const {
    // Note: We cannot check "ptr < end()" because the mark have a 0-size
    // length.
    return begin() <= ptr && ptr <= end();
  }

  // Check if a mark is part of the allocated data of this chunk.
  bool contains(Mark m) const {
    MOZ_ASSERT(m.chunk_ == this);
    return contains(m.bump_);
  }

  // Release the memory allocated in this chunk. This function does not call
  // any of the destructors.
  void release() { setBump(begin()); }

  // Release the memory allocated in this chunk since the corresponding mark
  // got created. This function does not call any of the destructors.
  void release(Mark m) {
    MOZ_RELEASE_ASSERT(contains(m));
    setBump(m.bump_);
  }

  // Returns true, if the unused space is large enough for an allocation of
  // |n| bytes.
  bool canAlloc(size_t n);

  // Space remaining in the current chunk.
  size_t unused() const {
    uint8_t* aligned = AlignPtr(end());
    if (aligned < capacity_) return capacity_ - aligned;
    return 0;
  }

  // Try to perform an allocation of size |n|, returns nullptr if not possible.
  MOZ_ALWAYS_INLINE
  void* tryAlloc(size_t n) {
    uint8_t* aligned = AlignPtr(end());
    uint8_t* newBump = aligned + n;

    if (newBump > capacity_) return nullptr;

    // Check for overflow.
    if (MOZ_UNLIKELY(newBump < bump_)) return nullptr;

    MOZ_ASSERT(canAlloc(n));  // Ensure consistency between "can" and "try".
    setBump(newBump);
    return aligned;
  }
};

}  // namespace detail

// LIFO bump allocator: used for phase-oriented and fast LIFO allocations.
//
// Note: We leave BumpChunks latent in the set of unused chunks after they've
// been released to avoid thrashing before a GC.
class LifoAlloc {
  using BumpChunk = js::UniquePtr<detail::BumpChunk>;
  using BumpChunkList = detail::SingleLinkedList<detail::BumpChunk>;

  // List of chunks containing allocated data. In the common case, the last
  // chunk of this list is always used to perform the allocations. When the
  // allocation cannot be performed, we move a Chunk from the unused set to
  // the list of used chunks.
  BumpChunkList chunks_;

  // Set of unused chunks, which can be reused for future allocations.
  BumpChunkList unused_;

  size_t markCount;
  size_t defaultChunkSize_;
  size_t curSize_;
  size_t peakSize_;
#if defined(DEBUG) || defined(JS_OOM_BREAKPOINT)
  bool fallibleScope_;
#endif

  void operator=(const LifoAlloc&) = delete;
  LifoAlloc(const LifoAlloc&) = delete;

  // Return a BumpChunk that can perform an allocation of at least size |n|.
  BumpChunk newChunkWithCapacity(size_t n);

  // Reuse or allocate a BumpChunk that can perform an allocation of at least
  // size |n|, if successful it is placed at the end the list of |chunks_|.
  MOZ_MUST_USE bool getOrCreateChunk(size_t n);

  void reset(size_t defaultChunkSize) {
    MOZ_ASSERT(mozilla::RoundUpPow2(defaultChunkSize) == defaultChunkSize);
    while (!chunks_.empty()) chunks_.popFirst();
    while (!unused_.empty()) unused_.popFirst();
    defaultChunkSize_ = defaultChunkSize;
    markCount = 0;
    curSize_ = 0;
  }

  // Append unused chunks to the end of this LifoAlloc.
  void appendUnused(BumpChunkList&& otherUnused) {
#ifdef DEBUG
    for (detail::BumpChunk& bc : otherUnused) MOZ_ASSERT(bc.empty());
#endif
    unused_.appendAll(mozilla::Move(otherUnused));
  }

  // Append used chunks to the end of this LifoAlloc. We act as if all the
  // chunks in |this| are used, even if they're not, so memory may be wasted.
  void appendUsed(BumpChunkList&& otherChunks) {
    chunks_.appendAll(mozilla::Move(otherChunks));
  }

  // Track the amount of space allocated in used and unused chunks.
  void incrementCurSize(size_t size) {
    curSize_ += size;
    if (curSize_ > peakSize_) peakSize_ = curSize_;
  }
  void decrementCurSize(size_t size) {
    MOZ_ASSERT(curSize_ >= size);
    curSize_ -= size;
  }

  MOZ_ALWAYS_INLINE
  void* allocImpl(size_t n) {
    void* result;
    if (!chunks_.empty() && (result = chunks_.last()->tryAlloc(n)))
      return result;

    if (!getOrCreateChunk(n)) return nullptr;

    // Since we just created a large enough chunk, this can't fail.
    result = chunks_.last()->tryAlloc(n);
    MOZ_ASSERT(result);
    return result;
  }

 public:
  explicit LifoAlloc(size_t defaultChunkSize)
      : peakSize_(0)
#if defined(DEBUG) || defined(JS_OOM_BREAKPOINT)
        ,
        fallibleScope_(true)
#endif
  {
    reset(defaultChunkSize);
  }

  // Steal allocated chunks from |other|.
  void steal(LifoAlloc* other) {
    MOZ_ASSERT(!other->markCount);
    MOZ_ASSERT(chunks_.empty());

    // Copy everything from |other| to |this| except for |peakSize_|, which
    // requires some care.
    chunks_ = mozilla::Move(other->chunks_);
    unused_ = mozilla::Move(other->unused_);
    markCount = other->markCount;
    defaultChunkSize_ = other->defaultChunkSize_;
    curSize_ = other->curSize_;
    peakSize_ = Max(peakSize_, other->peakSize_);
#if defined(DEBUG) || defined(JS_OOM_BREAKPOINT)
    fallibleScope_ = other->fallibleScope_;
#endif

    other->reset(defaultChunkSize_);
  }

  // Append all chunks from |other|. They are removed from |other|.
  void transferFrom(LifoAlloc* other);

  // Append unused chunks from |other|. They are removed from |other|.
  void transferUnusedFrom(LifoAlloc* other);

  ~LifoAlloc() { freeAll(); }

  size_t defaultChunkSize() const { return defaultChunkSize_; }

  // Frees all held memory.
  void freeAll();

  static const unsigned HUGE_ALLOCATION = 50 * 1024 * 1024;
  void freeAllIfHugeAndUnused() {
    if (markCount == 0 && curSize_ > HUGE_ALLOCATION) freeAll();
  }

  MOZ_ALWAYS_INLINE
  void* alloc(size_t n) {
#if defined(DEBUG) || defined(JS_OOM_BREAKPOINT)
    // Only simulate OOMs when we are not using the LifoAlloc as an
    // infallible allocator.
    if (fallibleScope_) JS_OOM_POSSIBLY_FAIL();
#endif
    return allocImpl(n);
  }

  MOZ_ALWAYS_INLINE
  void* allocInfallible(size_t n) {
    AutoEnterOOMUnsafeRegion oomUnsafe;
    if (void* result = allocImpl(n)) return result;
    oomUnsafe.crash("LifoAlloc::allocInfallible");
    return nullptr;
  }

  // Ensures that enough space exists to satisfy N bytes worth of
  // allocation requests, not necessarily contiguous. Note that this does
  // not guarantee a successful single allocation of N bytes.
  MOZ_ALWAYS_INLINE
  MOZ_MUST_USE bool ensureUnusedApproximate(size_t n) {
    AutoFallibleScope fallibleAllocator(this);
    size_t total = 0;
    if (!chunks_.empty()) {
      total += chunks_.last()->unused();
      if (total >= n) return true;
    }

    for (detail::BumpChunk& bc : unused_) {
      total += bc.unused();
      if (total >= n) return true;
    }

    BumpChunk newChunk = newChunkWithCapacity(n);
    if (!newChunk) return false;
    size_t size = newChunk->computedSizeOfIncludingThis();
    unused_.pushFront(mozilla::Move(newChunk));
    incrementCurSize(size);
    return true;
  }

  MOZ_ALWAYS_INLINE
  void setAsInfallibleByDefault() {
#if defined(DEBUG) || defined(JS_OOM_BREAKPOINT)
    fallibleScope_ = false;
#endif
  }

  class MOZ_NON_TEMPORARY_CLASS AutoFallibleScope {
#if defined(DEBUG) || defined(JS_OOM_BREAKPOINT)
    LifoAlloc* lifoAlloc_;
    bool prevFallibleScope_;
    MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER

   public:
    explicit AutoFallibleScope(
        LifoAlloc* lifoAlloc MOZ_GUARD_OBJECT_NOTIFIER_PARAM) {
      MOZ_GUARD_OBJECT_NOTIFIER_INIT;
      lifoAlloc_ = lifoAlloc;
      prevFallibleScope_ = lifoAlloc->fallibleScope_;
      lifoAlloc->fallibleScope_ = true;
    }

    ~AutoFallibleScope() { lifoAlloc_->fallibleScope_ = prevFallibleScope_; }
#else
   public:
    explicit AutoFallibleScope(LifoAlloc*) {}
#endif
  };

  template <typename T>
  T* newArray(size_t count) {
    static_assert(mozilla::IsPod<T>::value,
                  "T must be POD so that constructors (and destructors, "
                  "when the LifoAlloc is freed) need not be called");
    return newArrayUninitialized<T>(count);
  }

  // Create an array with uninitialized elements of type |T|.
  // The caller is responsible for initialization.
  template <typename T>
  T* newArrayUninitialized(size_t count) {
    size_t bytes;
    if (MOZ_UNLIKELY(!CalculateAllocSize<T>(count, &bytes))) return nullptr;
    return static_cast<T*>(alloc(bytes));
  }

  using Mark = detail::BumpChunk::Mark;

  Mark mark() {
    markCount++;
    if (chunks_.empty()) return Mark();
    return chunks_.last()->mark();
  }

  void release(Mark mark) {
    markCount--;

    // Move the blocks which are after the mark to the set of unused chunks.
    BumpChunkList released;
    if (!mark.markedChunk())
      released = mozilla::Move(chunks_);
    else
      released = mozilla::Move(chunks_.splitAfter(mark.markedChunk()));

    // Release the content of all the blocks which are after the marks.
    for (detail::BumpChunk& bc : released) bc.release();
    unused_.appendAll(mozilla::Move(released));

    // Release everything which follows the mark in the last chunk.
    if (!chunks_.empty()) chunks_.last()->release(mark);
  }

  void releaseAll() {
    MOZ_ASSERT(!markCount);
    for (detail::BumpChunk& bc : chunks_) bc.release();
    unused_.appendAll(mozilla::Move(chunks_));
  }

  // Get the total "used" (occupied bytes) count for the arena chunks.
  size_t used() const {
    size_t accum = 0;
    for (const detail::BumpChunk& chunk : chunks_) accum += chunk.used();
    return accum;
  }

  // Return true if the LifoAlloc does not currently contain any allocations.
  bool isEmpty() const { return chunks_.empty() || !chunks_.last()->empty(); }

  // Return the number of bytes remaining to allocate in the current chunk.
  // e.g. How many bytes we can allocate before needing a new block.
  size_t availableInCurrentChunk() const {
    if (!chunks_.empty()) return 0;
    return chunks_.last()->unused();
  }

  // Get the total size of the arena chunks (including unused space).
  size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
    size_t n = 0;
    for (const detail::BumpChunk& chunk : chunks_)
      n += chunk.sizeOfIncludingThis(mallocSizeOf);
    for (const detail::BumpChunk& chunk : unused_)
      n += chunk.sizeOfIncludingThis(mallocSizeOf);
    return n;
  }

  // Get the total size of the arena chunks (including unused space).
  size_t computedSizeOfExcludingThis() const {
    size_t n = 0;
    for (const detail::BumpChunk& chunk : chunks_)
      n += chunk.computedSizeOfIncludingThis();
    for (const detail::BumpChunk& chunk : unused_)
      n += chunk.computedSizeOfIncludingThis();
    return n;
  }

  // Like sizeOfExcludingThis(), but includes the size of the LifoAlloc itself.
  size_t sizeOfIncludingThis(mozilla::MallocSizeOf mallocSizeOf) const {
    return mallocSizeOf(this) + sizeOfExcludingThis(mallocSizeOf);
  }

  // Get the peak size of the arena chunks (including unused space and
  // bookkeeping space).
  size_t peakSizeOfExcludingThis() const { return peakSize_; }

  // Doesn't perform construction; useful for lazily-initialized POD types.
  template <typename T>
  MOZ_ALWAYS_INLINE T* pod_malloc() {
    return static_cast<T*>(alloc(sizeof(T)));
  }

  JS_DECLARE_NEW_METHODS(new_, alloc, MOZ_ALWAYS_INLINE)
  JS_DECLARE_NEW_METHODS(newInfallible, allocInfallible, MOZ_ALWAYS_INLINE)

#ifdef DEBUG
  bool contains(void* ptr) const {
    for (const detail::BumpChunk& chunk : chunks_) {
      if (chunk.contains(ptr)) return true;
    }
    return false;
  }
#endif

  // Iterate over the data allocated in a LifoAlloc, and interpret it.
  class Enum {
    friend class LifoAlloc;
    friend class detail::BumpChunk;

    // Iterator over the list of bump chunks.
    BumpChunkList::Iterator chunkIt_;
    BumpChunkList::Iterator chunkEnd_;
    // Read head (must be within chunk_).
    uint8_t* head_;

    // If there is not enough room in the remaining block for |size|,
    // advance to the next block and update the position.
    uint8_t* seekBaseAndAdvanceBy(size_t size) {
      MOZ_ASSERT(!empty());

      uint8_t* aligned = detail::AlignPtr(head_);
      if (aligned + size > chunkIt_->end()) {
        ++chunkIt_;
        aligned = chunkIt_->begin();
        // The current code assumes that if we have a chunk, then we
        // have allocated something it in.
        MOZ_ASSERT(!chunkIt_->empty());
      }
      head_ = aligned + size;
      MOZ_ASSERT(head_ <= chunkIt_->end());
      return aligned;
    }

   public:
    explicit Enum(LifoAlloc& alloc)
        : chunkIt_(alloc.chunks_.begin()),
          chunkEnd_(alloc.chunks_.end()),
          head_(nullptr) {
      if (chunkIt_ != chunkEnd_) head_ = chunkIt_->begin();
    }

    // Return true if there are no more bytes to enumerate.
    bool empty() {
      return chunkIt_ == chunkEnd_ ||
             (chunkIt_->next() == chunkEnd_.get() && head_ >= chunkIt_->end());
    }

    // Move the read position forward by the size of one T.
    template <typename T>
    T* read(size_t size = sizeof(T)) {
      return reinterpret_cast<T*>(read(size));
    }

    // Return a pointer to the item at the current position. This returns a
    // pointer to the inline storage, not a copy, and moves the read-head by
    // the requested |size|.
    void* read(size_t size) { return seekBaseAndAdvanceBy(size); }
  };
};

class MOZ_NON_TEMPORARY_CLASS LifoAllocScope {
  LifoAlloc* lifoAlloc;
  LifoAlloc::Mark mark;
  LifoAlloc::AutoFallibleScope fallibleScope;
  bool shouldRelease;
  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER

 public:
  explicit LifoAllocScope(LifoAlloc* lifoAlloc MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
      : lifoAlloc(lifoAlloc),
        mark(lifoAlloc->mark()),
        fallibleScope(lifoAlloc),
        shouldRelease(true) {
    MOZ_GUARD_OBJECT_NOTIFIER_INIT;
  }

  ~LifoAllocScope() {
    if (shouldRelease) lifoAlloc->release(mark);
  }

  LifoAlloc& alloc() { return *lifoAlloc; }

  void releaseEarly() {
    MOZ_ASSERT(shouldRelease);
    lifoAlloc->release(mark);
    shouldRelease = false;
  }
};

enum Fallibility { Fallible, Infallible };

template <Fallibility fb>
class LifoAllocPolicy {
  LifoAlloc& alloc_;

 public:
  MOZ_IMPLICIT LifoAllocPolicy(LifoAlloc& alloc) : alloc_(alloc) {}
  template <typename T>
  T* maybe_pod_malloc(size_t numElems) {
    size_t bytes;
    if (MOZ_UNLIKELY(!CalculateAllocSize<T>(numElems, &bytes))) return nullptr;
    void* p =
        fb == Fallible ? alloc_.alloc(bytes) : alloc_.allocInfallible(bytes);
    return static_cast<T*>(p);
  }
  template <typename T>
  T* maybe_pod_calloc(size_t numElems) {
    T* p = maybe_pod_malloc<T>(numElems);
    if (MOZ_UNLIKELY(!p)) return nullptr;
    memset(p, 0, numElems * sizeof(T));
    return p;
  }
  template <typename T>
  T* maybe_pod_realloc(T* p, size_t oldSize, size_t newSize) {
    T* n = maybe_pod_malloc<T>(newSize);
    if (MOZ_UNLIKELY(!n)) return nullptr;
    MOZ_ASSERT(!(oldSize & mozilla::tl::MulOverflowMask<sizeof(T)>::value));
    memcpy(n, p, Min(oldSize * sizeof(T), newSize * sizeof(T)));
    return n;
  }
  template <typename T>
  T* pod_malloc(size_t numElems) {
    return maybe_pod_malloc<T>(numElems);
  }
  template <typename T>
  T* pod_calloc(size_t numElems) {
    return maybe_pod_calloc<T>(numElems);
  }
  template <typename T>
  T* pod_realloc(T* p, size_t oldSize, size_t newSize) {
    return maybe_pod_realloc<T>(p, oldSize, newSize);
  }
  void free_(void* p) {}
  void reportAllocOverflow() const {}
  MOZ_MUST_USE bool checkSimulatedOOM() const {
    return fb == Infallible || !js::oom::ShouldFailWithOOM();
  }
};

}  // namespace js

#endif /* ds_LifoAlloc_h */