path: root/js/src/ds/LifoAlloc.h

/* -*- 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 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/PodOperations.h"
#include "mozilla/TemplateLib.h"

#include <algorithm>
#include <new>
#include <stddef.h>  // size_t
#include <type_traits>
#include <utility>

// 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 "js/UniquePtr.h"
#include "util/Memory.h"
#include "util/Poison.h"

namespace js {

namespace detail {

template <typename T, typename D>
class SingleLinkedList;

template <typename T, typename D = JS::DeletePolicy<T>>
class SingleLinkedListElement {
  friend class SingleLinkedList<T, D>;
  js::UniquePtr<T, D> 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, typename D = JS::DeletePolicy<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, D> 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_(std::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_ = std::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!=(const Iterator& other) const {
      return current_ != other.current_;
    }
    bool operator==(const 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_ = std::move(newLast->next_);
      result.last_ = last_;
      last_ = newLast;
    }
    assertInvariants();
    result.assertInvariants();
    return result;
  }

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

  void append(UniquePtr<T, D>&& elem) {
    if (last_) {
      last_->next_ = std::move(elem);
      last_ = last_->next_.get();
    } else {
      head_ = std::move(elem);
      last_ = head_.get();
    }
    assertInvariants();
  }
  void appendAll(SingleLinkedList&& list) {
    if (list.empty()) {
      return;
    }
    if (last_) {
      last_->next_ = std::move(list.head_);
    } else {
      head_ = std::move(list.head_);
    }
    last_ = list.last_;
    list.last_ = nullptr;
    assertInvariants();
    list.assertInvariants();
  }
  void steal(SingleLinkedList&& list) {
    head_ = std::move(list.head_);
    last_ = list.last_;
    list.last_ = nullptr;
    assertInvariants();
    list.assertInvariants();
  }
  void prependAll(SingleLinkedList&& list) {
    list.appendAll(std::move(*this));
    steal(std::move(list));
  }
  UniquePtr<T, D> popFirst() {
    MOZ_ASSERT(head_);
    UniquePtr<T, D> result = std::move(head_);
    head_ = std::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::IsPowerOfTwo(LIFO_ALLOC_ALIGN),
                "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 first byte after this chunk.
  uint8_t* const capacity_;

#ifdef MOZ_DIAGNOSTIC_ASSERT_ENABLED
  // Magic number used to check against poisoned values.
  const uintptr_t magic_ : 24;
  static constexpr uintptr_t magicNumber = uintptr_t(0x4c6966);
#endif

#if defined(DEBUG)
#  define LIFO_CHUNK_PROTECT 1
#endif

  // Poison the memory with memset, in order to catch errors due to
  // use-after-free, with JS_LIFO_UNDEFINED_PATTERN pattern, or to catch
  // use-before-init with JS_LIFO_UNINITIALIZED_PATTERN.
#if defined(DEBUG)
#  define LIFO_HAVE_MEM_CHECKS 1
#  define LIFO_MAKE_MEM_NOACCESS(addr, size)       \
    do {                                           \
      uint8_t* base = (addr);                      \
      size_t sz = (size);                          \
      MOZ_MAKE_MEM_UNDEFINED(base, sz);            \
      memset(base, JS_LIFO_UNDEFINED_PATTERN, 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, JS_LIFO_UNINITIALIZED_PATTERN, 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

#ifdef LIFO_HAVE_MEM_CHECKS
  // Red zone reserved after each allocation.
  static constexpr size_t RedZoneSize = 16;
#else
  static constexpr size_t RedZoneSize = 0;
#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
  {
    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_) {
      MOZ_ASSERT(newBump - RedZoneSize >= bump_);
      LIFO_MAKE_MEM_UNDEFINED(bump_, newBump - RedZoneSize - bump_);
      // The area [newBump - RedZoneSize .. newBump[ is already flagged as
      // no-access either with the previous if-branch or with the
      // BumpChunk constructor. No need to mark it twice.
    }
#endif
    bump_ = newBump;
  }

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

  // 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)
  inline const uint8_t* begin() const;
  inline uint8_t* begin();
  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;
    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_);
  }

  // Given an amount, compute the total size of a chunk for it: reserved
  // space before |begin()|, space for |amount| bytes, and red-zone space
  // after those bytes that will ultimately end at |capacity_|.
  static inline MOZ_MUST_USE bool allocSizeWithRedZone(size_t amount,
                                                       size_t* size);

  // Given a bump chunk pointer, find the next base/end pointers. This is
  // useful for having consistent allocations, and iterating over known size
  // allocations.
  static uint8_t* nextAllocBase(uint8_t* e) { return detail::AlignPtr(e); }
  static uint8_t* nextAllocEnd(uint8_t* b, size_t n) {
    return b + n + RedZoneSize;
  }

  // Returns true, if the unused space is large enough for an allocation of
  // |n| bytes.
  bool canAlloc(size_t n) const {
    uint8_t* newBump = nextAllocEnd(nextAllocBase(end()), n);
    // bump_ <= newBump, is necessary to catch overflow.
    return bump_ <= newBump && newBump <= capacity_;
  }

  // Space remaining in the current chunk.
  size_t unused() const {
    uint8_t* aligned = nextAllocBase(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 = nextAllocBase(end());
    uint8_t* newBump = nextAllocEnd(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;
  }

#ifdef LIFO_CHUNK_PROTECT
  void setReadOnly();
  void setReadWrite();
#else
  void setReadOnly() const {}
  void setReadWrite() const {}
#endif
};

// 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 BumpChunkReservedSpace =
    AlignBytes(sizeof(BumpChunk), LIFO_ALLOC_ALIGN);

/* static */ inline MOZ_MUST_USE bool BumpChunk::allocSizeWithRedZone(
    size_t amount, size_t* size) {
  constexpr size_t SpaceBefore = BumpChunkReservedSpace;
  static_assert((SpaceBefore % LIFO_ALLOC_ALIGN) == 0,
                "reserved space presumed already aligned");

  constexpr size_t SpaceAfter = RedZoneSize;  // may be zero

  constexpr size_t SpaceBeforeAndAfter = SpaceBefore + SpaceAfter;
  static_assert(SpaceBeforeAndAfter >= SpaceBefore,
                "intermediate addition must not overflow");

  *size = SpaceBeforeAndAfter + amount;
  return MOZ_LIKELY(*size >= SpaceBeforeAndAfter);
}

inline const uint8_t* BumpChunk::begin() const {
  return base() + BumpChunkReservedSpace;
}

inline uint8_t* BumpChunk::begin() { return base() + BumpChunkReservedSpace; }

}  // 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 UniqueBumpChunk = js::UniquePtr<detail::BumpChunk>;
  using BumpChunkList = detail::SingleLinkedList<detail::BumpChunk>;

  // List of chunks containing allocated data of size smaller than the default
  // chunk size. 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_;

  // List of chunks containing allocated data where each allocation is larger
  // than the oversize threshold. Each chunk contains exactly one allocation.
  // This reduces wasted space in the chunk list.
  //
  // Oversize chunks are allocated on demand and freed as soon as they are
  // released, instead of being pushed to the unused list.
  BumpChunkList oversize_;

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

  size_t markCount;
  size_t defaultChunkSize_;
  size_t oversizeThreshold_;

  // Size of all chunks in chunks_, oversize_, unused_ lists.
  size_t curSize_;
  size_t peakSize_;

  // Size of all chunks containing small bump allocations. This heuristic is
  // used to compute growth rate while ignoring chunks such as oversized,
  // now-unused, or transferred (which followed their own growth patterns).
  size_t smallAllocsSize_;

#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|.
  UniqueBumpChunk newChunkWithCapacity(size_t n, bool oversize);

  // 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_|.
  UniqueBumpChunk getOrCreateChunk(size_t n);

  void reset(size_t defaultChunkSize);

  // 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(std::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(std::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_ASSERT(curSize_ >= smallAllocsSize_);
  }

  void* allocImplColdPath(size_t n);
  void* allocImplOversize(size_t n);

  MOZ_ALWAYS_INLINE
  void* allocImpl(size_t n) {
    void* result;
    // Give oversized allocations their own chunk instead of wasting space
    // due to fragmentation at the end of normal chunk.
    if (MOZ_UNLIKELY(n > oversizeThreshold_)) {
      return allocImplOversize(n);
    }
    if (MOZ_LIKELY(!chunks_.empty() &&
                   (result = chunks_.last()->tryAlloc(n)))) {
      return result;
    }
    return allocImplColdPath(n);
  }

  // Check for space in unused chunks or allocate a new unused chunk.
  MOZ_MUST_USE bool ensureUnusedApproximateColdPath(size_t n, size_t total);

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

  // Set the threshold to allocate data in its own chunk outside the space for
  // small allocations.
  void disableOversize() { oversizeThreshold_ = SIZE_MAX; }
  void setOversizeThreshold(size_t oversizeThreshold) {
    MOZ_ASSERT(oversizeThreshold <= defaultChunkSize_);
    oversizeThreshold_ = oversizeThreshold;
  }

  // Steal allocated chunks from |other|.
  void steal(LifoAlloc* other);

  // 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);
  }

  // Allocates |n| bytes if we can guarantee that we will have
  // |needed| unused bytes remaining. Returns nullptr if we can't.
  // This is useful for maintaining our ballast invariants while
  // attempting fallible allocations.
  MOZ_ALWAYS_INLINE
  void* allocEnsureUnused(size_t n, size_t needed) {
    JS_OOM_POSSIBLY_FAIL();
    MOZ_ASSERT(fallibleScope_);

    Mark m = mark();
    void* result = allocImpl(n);
    if (!ensureUnusedApproximate(needed)) {
      release(m);
      return nullptr;
    }
    cancelMark(m);
    return result;
  }

  template <typename T, typename... Args>
  MOZ_ALWAYS_INLINE T* newWithSize(size_t n, Args&&... args) {
    MOZ_ASSERT(n >= sizeof(T), "must request enough space to store a T");
    static_assert(alignof(T) <= detail::LIFO_ALLOC_ALIGN,
                  "LifoAlloc must provide enough alignment to store T");
    void* ptr = alloc(n);
    if (!ptr) {
      return nullptr;
    }

    return new (ptr) T(std::forward<Args>(args)...);
  }

  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;
      }
    }

    return ensureUnusedApproximateColdPath(n, total);
  }

  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_;

   public:
    explicit AutoFallibleScope(LifoAlloc* lifoAlloc) {
      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(std::is_trivial_v<T>,
                  "T must be trivially constructible so that constructors need "
                  "not be called");
    static_assert(std::is_trivially_destructible_v<T>,
                  "T must be trivially destructible so destructors don't need "
                  "to be called when the LifoAlloc is freed");
    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));
  }

  class Mark {
    friend class LifoAlloc;
    detail::BumpChunk::Mark chunk;
    detail::BumpChunk::Mark oversize;
  };

  // Note: MOZ_NEVER_INLINE is a work around for a Clang 9 (PGO) miscompilation.
  // See bug 1583907.
  MOZ_NEVER_INLINE Mark mark();

  void release(Mark mark);

 private:
  void cancelMark(Mark mark) { markCount--; }

 public:
  void releaseAll() {
    MOZ_ASSERT(!markCount);

    // When releasing all chunks, we can no longer determine which chunks were
    // transferred and which were not, so simply clear the heuristic to zero
    // right away.
    smallAllocsSize_ = 0;

    for (detail::BumpChunk& bc : chunks_) {
      bc.release();
    }
    unused_.appendAll(std::move(chunks_));

    // On release, we free any oversize allocations instead of keeping them
    // in unused chunks.
    while (!oversize_.empty()) {
      UniqueBumpChunk bc = oversize_.popFirst();
      decrementCurSize(bc->computedSizeOfIncludingThis());
    }
  }

  // Protect the content of the LifoAlloc chunks.
#ifdef LIFO_CHUNK_PROTECT
  void setReadOnly();
  void setReadWrite();
#else
  void setReadOnly() const {}
  void setReadWrite() const {}
#endif

  // 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 {
    bool empty = chunks_.empty() ||
                 (chunks_.begin() == chunks_.last() && chunks_.last()->empty());
    MOZ_ASSERT_IF(!oversize_.empty(), !oversize_.last()->empty());
    return empty && oversize_.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 : oversize_) {
      n += chunk.sizeOfIncludingThis(mallocSizeOf);
    }
    for (const detail::BumpChunk& chunk : unused_) {
      n += chunk.sizeOfIncludingThis(mallocSizeOf);
    }
    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 current size of the arena chunks (including unused space and
  // bookkeeping space).
  size_t computedSizeOfExcludingThis() const { return curSize_; }

  // 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;
      }
    }
    for (const detail::BumpChunk& chunk : oversize_) {
      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::BumpChunk::nextAllocBase(head_);
      if (detail::BumpChunk::nextAllocEnd(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_ = detail::BumpChunk::nextAllocEnd(aligned, size);
      MOZ_ASSERT(head_ <= chunkIt_->end());
      return aligned;
    }

   public:
    explicit Enum(LifoAlloc& alloc)
        : chunkIt_(alloc.chunks_.begin()),
          chunkEnd_(alloc.chunks_.end()),
          head_(nullptr) {
      MOZ_RELEASE_ASSERT(alloc.oversize_.empty());
      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;

 public:
  explicit LifoAllocScope(LifoAlloc* lifoAlloc)
      : lifoAlloc(lifoAlloc),
        mark(lifoAlloc->mark()),
        fallibleScope(lifoAlloc) {}

  ~LifoAllocScope() {
    lifoAlloc->release(mark);

    /*
     * The parser can allocate enormous amounts of memory for large functions.
     * Eagerly free the memory now (which otherwise won't be freed until the
     * next GC) to avoid unnecessary OOMs.
     */
    lifoAlloc->freeAllIfHugeAndUnused();
  }

  LifoAlloc& alloc() { return *lifoAlloc; }
};

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, std::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);
  }
  template <typename T>
  void free_(T* p, size_t numElems) {}
  void reportAllocOverflow() const {}
  MOZ_MUST_USE bool checkSimulatedOOM() const {
    return fb == Infallible || !js::oom::ShouldFailWithOOM();
  }
};

}  // namespace js

#endif /* ds_LifoAlloc_h */