/* -*- 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/. */

/* JS Garbage Collector. */

#ifndef jsgc_h
#define jsgc_h

#include "mozilla/Atomics.h"
#include "mozilla/DebugOnly.h"
#include "mozilla/MemoryReporting.h"
#include "mozilla/TypeTraits.h"

#include "jslock.h"

#include "js/GCAPI.h"
#include "js/SliceBudget.h"
#include "js/Vector.h"

#include "vm/NativeObject.h"

namespace js {

class AutoLockGC;

unsigned GetCPUCount();

enum HeapState {
    Idle,             // doing nothing with the GC heap
    Tracing,          // tracing the GC heap without collecting, e.g. IterateCompartments()
    MajorCollecting,  // doing a GC of the major heap
    MinorCollecting   // doing a GC of the minor heap (nursery)
};

enum ThreadType
{
    MainThread,
    BackgroundThread
};

namespace jit {
    class JitCode;
}

namespace gc {

struct FinalizePhase;

enum State {
    NO_INCREMENTAL,
    MARK_ROOTS,
    MARK,
    SWEEP,
    COMPACT
};

/* Map from C++ type to alloc kind. JSObject does not have a 1:1 mapping, so must use Arena::thingSize. */
template <typename T> struct MapTypeToFinalizeKind {};
template <> struct MapTypeToFinalizeKind<JSScript>          { static const AllocKind kind = FINALIZE_SCRIPT; };
template <> struct MapTypeToFinalizeKind<LazyScript>        { static const AllocKind kind = FINALIZE_LAZY_SCRIPT; };
template <> struct MapTypeToFinalizeKind<Shape>             { static const AllocKind kind = FINALIZE_SHAPE; };
template <> struct MapTypeToFinalizeKind<AccessorShape>     { static const AllocKind kind = FINALIZE_ACCESSOR_SHAPE; };
template <> struct MapTypeToFinalizeKind<BaseShape>         { static const AllocKind kind = FINALIZE_BASE_SHAPE; };
template <> struct MapTypeToFinalizeKind<types::TypeObject> { static const AllocKind kind = FINALIZE_TYPE_OBJECT; };
template <> struct MapTypeToFinalizeKind<JSFatInlineString> { static const AllocKind kind = FINALIZE_FAT_INLINE_STRING; };
template <> struct MapTypeToFinalizeKind<JSString>          { static const AllocKind kind = FINALIZE_STRING; };
template <> struct MapTypeToFinalizeKind<JSExternalString>  { static const AllocKind kind = FINALIZE_EXTERNAL_STRING; };
template <> struct MapTypeToFinalizeKind<JS::Symbol>        { static const AllocKind kind = FINALIZE_SYMBOL; };
template <> struct MapTypeToFinalizeKind<jit::JitCode>      { static const AllocKind kind = FINALIZE_JITCODE; };

static inline bool
IsNurseryAllocable(AllocKind kind)
{
    MOZ_ASSERT(kind >= 0 && unsigned(kind) < FINALIZE_LIMIT);
    static const bool map[] = {
        false,     /* FINALIZE_OBJECT0 */
        true,      /* FINALIZE_OBJECT0_BACKGROUND */
        false,     /* FINALIZE_OBJECT2 */
        true,      /* FINALIZE_OBJECT2_BACKGROUND */
        false,     /* FINALIZE_OBJECT4 */
        true,      /* FINALIZE_OBJECT4_BACKGROUND */
        false,     /* FINALIZE_OBJECT8 */
        true,      /* FINALIZE_OBJECT8_BACKGROUND */
        false,     /* FINALIZE_OBJECT12 */
        true,      /* FINALIZE_OBJECT12_BACKGROUND */
        false,     /* FINALIZE_OBJECT16 */
        true,      /* FINALIZE_OBJECT16_BACKGROUND */
        false,     /* FINALIZE_SCRIPT */
        false,     /* FINALIZE_LAZY_SCRIPT */
        false,     /* FINALIZE_SHAPE */
        false,     /* FINALIZE_ACCESSOR_SHAPE */
        false,     /* FINALIZE_BASE_SHAPE */
        false,     /* FINALIZE_TYPE_OBJECT */
        false,     /* FINALIZE_FAT_INLINE_STRING */
        false,     /* FINALIZE_STRING */
        false,     /* FINALIZE_EXTERNAL_STRING */
        false,     /* FINALIZE_SYMBOL */
        false,     /* FINALIZE_JITCODE */
    };
    JS_STATIC_ASSERT(JS_ARRAY_LENGTH(map) == FINALIZE_LIMIT);
    return map[kind];
}

static inline bool
IsBackgroundFinalized(AllocKind kind)
{
    MOZ_ASSERT(kind >= 0 && unsigned(kind) < FINALIZE_LIMIT);
    static const bool map[] = {
        false,     /* FINALIZE_OBJECT0 */
        true,      /* FINALIZE_OBJECT0_BACKGROUND */
        false,     /* FINALIZE_OBJECT2 */
        true,      /* FINALIZE_OBJECT2_BACKGROUND */
        false,     /* FINALIZE_OBJECT4 */
        true,      /* FINALIZE_OBJECT4_BACKGROUND */
        false,     /* FINALIZE_OBJECT8 */
        true,      /* FINALIZE_OBJECT8_BACKGROUND */
        false,     /* FINALIZE_OBJECT12 */
        true,      /* FINALIZE_OBJECT12_BACKGROUND */
        false,     /* FINALIZE_OBJECT16 */
        true,      /* FINALIZE_OBJECT16_BACKGROUND */
        false,     /* FINALIZE_SCRIPT */
        false,     /* FINALIZE_LAZY_SCRIPT */
        true,      /* FINALIZE_SHAPE */
        true,      /* FINALIZE_ACCESSOR_SHAPE */
        true,      /* FINALIZE_BASE_SHAPE */
        true,      /* FINALIZE_TYPE_OBJECT */
        true,      /* FINALIZE_FAT_INLINE_STRING */
        true,      /* FINALIZE_STRING */
        false,     /* FINALIZE_EXTERNAL_STRING */
        true,      /* FINALIZE_SYMBOL */
        false,     /* FINALIZE_JITCODE */
    };
    JS_STATIC_ASSERT(JS_ARRAY_LENGTH(map) == FINALIZE_LIMIT);
    return map[kind];
}

static inline bool
CanBeFinalizedInBackground(gc::AllocKind kind, const Class* clasp)
{
    MOZ_ASSERT(kind <= gc::FINALIZE_OBJECT_LAST);
    /* If the class has no finalizer or a finalizer that is safe to call on
     * a different thread, we change the finalize kind. For example,
     * FINALIZE_OBJECT0 calls the finalizer on the main thread,
     * FINALIZE_OBJECT0_BACKGROUND calls the finalizer on the gcHelperThread.
     * IsBackgroundFinalized is called to prevent recursively incrementing
     * the finalize kind; kind may already be a background finalize kind.
     */
    return (!gc::IsBackgroundFinalized(kind) &&
            (!clasp->finalize || (clasp->flags & JSCLASS_BACKGROUND_FINALIZE)));
}

inline JSGCTraceKind
GetGCThingTraceKind(const void* thing);

/* Capacity for slotsToThingKind */
const size_t SLOTS_TO_THING_KIND_LIMIT = 17;

extern const AllocKind slotsToThingKind[];

/* Get the best kind to use when making an object with the given slot count. */
static inline AllocKind
GetGCObjectKind(size_t numSlots)
{
    if (numSlots >= SLOTS_TO_THING_KIND_LIMIT)
        return FINALIZE_OBJECT16;
    return slotsToThingKind[numSlots];
}

/* As for GetGCObjectKind, but for dense array allocation. */
static inline AllocKind
GetGCArrayKind(size_t numSlots)
{
    /*
     * Dense arrays can use their fixed slots to hold their elements array
     * (less two Values worth of ObjectElements header), but if more than the
     * maximum number of fixed slots is needed then the fixed slots will be
     * unused.
     */
    JS_STATIC_ASSERT(ObjectElements::VALUES_PER_HEADER == 2);
    if (numSlots > NativeObject::NELEMENTS_LIMIT || numSlots + 2 >= SLOTS_TO_THING_KIND_LIMIT)
        return FINALIZE_OBJECT2;
    return slotsToThingKind[numSlots + 2];
}

static inline AllocKind
GetGCObjectFixedSlotsKind(size_t numFixedSlots)
{
    MOZ_ASSERT(numFixedSlots < SLOTS_TO_THING_KIND_LIMIT);
    return slotsToThingKind[numFixedSlots];
}

static inline AllocKind
GetBackgroundAllocKind(AllocKind kind)
{
    MOZ_ASSERT(!IsBackgroundFinalized(kind));
    MOZ_ASSERT(kind <= FINALIZE_OBJECT_LAST);
    return (AllocKind) (kind + 1);
}

/* Get the number of fixed slots and initial capacity associated with a kind. */
static inline size_t
GetGCKindSlots(AllocKind thingKind)
{
    /* Using a switch in hopes that thingKind will usually be a compile-time constant. */
    switch (thingKind) {
      case FINALIZE_OBJECT0:
      case FINALIZE_OBJECT0_BACKGROUND:
        return 0;
      case FINALIZE_OBJECT2:
      case FINALIZE_OBJECT2_BACKGROUND:
        return 2;
      case FINALIZE_OBJECT4:
      case FINALIZE_OBJECT4_BACKGROUND:
        return 4;
      case FINALIZE_OBJECT8:
      case FINALIZE_OBJECT8_BACKGROUND:
        return 8;
      case FINALIZE_OBJECT12:
      case FINALIZE_OBJECT12_BACKGROUND:
        return 12;
      case FINALIZE_OBJECT16:
      case FINALIZE_OBJECT16_BACKGROUND:
        return 16;
      default:
        MOZ_CRASH("Bad object finalize kind");
    }
}

static inline size_t
GetGCKindSlots(AllocKind thingKind, const Class* clasp)
{
    size_t nslots = GetGCKindSlots(thingKind);

    /* An object's private data uses the space taken by its last fixed slot. */
    if (clasp->flags & JSCLASS_HAS_PRIVATE) {
        MOZ_ASSERT(nslots > 0);
        nslots--;
    }

    /*
     * Functions have a larger finalize kind than FINALIZE_OBJECT to reserve
     * space for the extra fields in JSFunction, but have no fixed slots.
     */
    if (clasp == FunctionClassPtr)
        nslots = 0;

    return nslots;
}

// Class to assist in triggering background chunk allocation. This cannot be done
// while holding the GC or worker thread state lock due to lock ordering issues.
// As a result, the triggering is delayed using this class until neither of the
// above locks is held.
class AutoMaybeStartBackgroundAllocation;

/*
 * A single segment of a SortedArenaList. Each segment has a head and a tail,
 * which track the start and end of a segment for O(1) append and concatenation.
 */
struct SortedArenaListSegment
{
    ArenaHeader* head;
    ArenaHeader** tailp;

    void clear() {
        head = nullptr;
        tailp = &head;
    }

    bool isEmpty() const {
        return tailp == &head;
    }

    // Appends |aheader| to this segment.
    void append(ArenaHeader* aheader) {
        MOZ_ASSERT(aheader);
        MOZ_ASSERT_IF(head, head->getAllocKind() == aheader->getAllocKind());
        *tailp = aheader;
        tailp = &aheader->next;
    }

    // Points the tail of this segment at |aheader|, which may be null. Note
    // that this does not change the tail itself, but merely which arena
    // follows it. This essentially turns the tail into a cursor (see also the
    // description of ArenaList), but from the perspective of a SortedArenaList
    // this makes no difference.
    void linkTo(ArenaHeader* aheader) {
        *tailp = aheader;
    }
};

/*
 * Arena lists have a head and a cursor. The cursor conceptually lies on arena
 * boundaries, i.e. before the first arena, between two arenas, or after the
 * last arena.
 *
 * Normally the arena following the cursor is the first arena in the list with
 * some free things and all arenas before the cursor are fully allocated. (And
 * if the cursor is at the end of the list, then all the arenas are full.)
 *
 * However, the arena currently being allocated from is considered full while
 * its list of free spans is moved into the freeList. Therefore, during GC or
 * cell enumeration, when an unallocated freeList is moved back to the arena,
 * we can see an arena with some free cells before the cursor.
 *
 * Arenas following the cursor should not be full.
 */
class ArenaList {
    // The cursor is implemented via an indirect pointer, |cursorp_|, to allow
    // for efficient list insertion at the cursor point and other list
    // manipulations.
    //
    // - If the list is empty: |head| is null, |cursorp_| points to |head|, and
    //   therefore |*cursorp_| is null.
    //
    // - If the list is not empty: |head| is non-null, and...
    //
    //   - If the cursor is at the start of the list: |cursorp_| points to
    //     |head|, and therefore |*cursorp_| points to the first arena.
    //
    //   - If cursor is at the end of the list: |cursorp_| points to the |next|
    //     field of the last arena, and therefore |*cursorp_| is null.
    //
    //   - If the cursor is at neither the start nor the end of the list:
    //     |cursorp_| points to the |next| field of the arena preceding the
    //     cursor, and therefore |*cursorp_| points to the arena following the
    //     cursor.
    //
    // |cursorp_| is never null.
    //
    ArenaHeader*    head_;
    ArenaHeader**   cursorp_;

    void copy(const ArenaList& other) {
        other.check();
        head_ = other.head_;
        cursorp_ = other.isCursorAtHead() ? &head_ : other.cursorp_;
        check();
    }

  public:
    ArenaList() {
        clear();
    }

    ArenaList(const ArenaList& other) {
        copy(other);
    }

    ArenaList& operator=(const ArenaList& other) {
        copy(other);
        return *this;
    }

    explicit ArenaList(const SortedArenaListSegment& segment) {
        head_ = segment.head;
        cursorp_ = segment.isEmpty() ? &head_ : segment.tailp;
        check();
    }

    // This does checking just of |head_| and |cursorp_|.
    void check() const {
#ifdef DEBUG
        // If the list is empty, it must have this form.
        MOZ_ASSERT_IF(!head_, cursorp_ == &head_);

        // If there's an arena following the cursor, it must not be full.
        ArenaHeader* cursor = *cursorp_;
        MOZ_ASSERT_IF(cursor, cursor->hasFreeThings());
#endif
    }

    void clear() {
        head_ = nullptr;
        cursorp_ = &head_;
        check();
    }

    ArenaList copyAndClear() {
        ArenaList result = *this;
        clear();
        return result;
    }

    bool isEmpty() const {
        check();
        return !head_;
    }

    // This returns nullptr if the list is empty.
    ArenaHeader* head() const {
        check();
        return head_;
    }

    bool isCursorAtHead() const {
        check();
        return cursorp_ == &head_;
    }

    bool isCursorAtEnd() const {
        check();
        return !*cursorp_;
    }

    // This can return nullptr.
    ArenaHeader* arenaAfterCursor() const {
        check();
        return *cursorp_;
    }

    // This returns the arena after the cursor and moves the cursor past it.
    ArenaHeader* takeNextArena() {
        check();
        ArenaHeader* aheader = *cursorp_;
        if (!aheader)
            return nullptr;
        cursorp_ = &aheader->next;
        check();
        return aheader;
    }

    // This does two things.
    // - Inserts |a| at the cursor.
    // - Leaves the cursor sitting just before |a|, if |a| is not full, or just
    //   after |a|, if |a| is full.
    //
    void insertAtCursor(ArenaHeader* a) {
        check();
        a->next = *cursorp_;
        *cursorp_ = a;
        // At this point, the cursor is sitting before |a|. Move it after |a|
        // if necessary.
        if (!a->hasFreeThings())
            cursorp_ = &a->next;
        check();
    }

    // This inserts |other|, which must be full, at the cursor of |this|.
    ArenaList& insertListWithCursorAtEnd(const ArenaList& other) {
        check();
        other.check();
        MOZ_ASSERT(other.isCursorAtEnd());
        if (other.isCursorAtHead())
            return *this;
        // Insert the full arenas of |other| after those of |this|.
        *other.cursorp_ = *cursorp_;
        *cursorp_ = other.head_;
        cursorp_ = other.cursorp_;
        check();
        return *this;
    }

#ifdef JSGC_COMPACTING
    ArenaHeader* removeRemainingArenas(ArenaHeader** arenap, const AutoLockGC& lock);
    ArenaHeader* pickArenasToRelocate(JSRuntime* runtime);
    ArenaHeader* relocateArenas(ArenaHeader* toRelocate, ArenaHeader* relocated);
#endif
};

/*
 * A class that holds arenas in sorted order by appending arenas to specific
 * segments. Each segment has a head and a tail, which can be linked up to
 * other segments to create a contiguous ArenaList.
 */
class SortedArenaList
{
  public:
    // The minimum size, in bytes, of a GC thing.
    static const size_t MinThingSize = 16;

    static_assert(ArenaSize <= 4096, "When increasing the Arena size, please consider how"\
                                     " this will affect the size of a SortedArenaList.");

    static_assert(MinThingSize >= 16, "When decreasing the minimum thing size, please consider"\
                                      " how this will affect the size of a SortedArenaList.");

  private:
    // The maximum number of GC things that an arena can hold.
    static const size_t MaxThingsPerArena = (ArenaSize - sizeof(ArenaHeader)) / MinThingSize;

    size_t thingsPerArena_;
    SortedArenaListSegment segments[MaxThingsPerArena + 1];

    // Convenience functions to get the nth head and tail.
    ArenaHeader* headAt(size_t n) { return segments[n].head; }
    ArenaHeader** tailAt(size_t n) { return segments[n].tailp; }

  public:
    explicit SortedArenaList(size_t thingsPerArena = MaxThingsPerArena) {
        reset(thingsPerArena);
    }

    void setThingsPerArena(size_t thingsPerArena) {
        MOZ_ASSERT(thingsPerArena && thingsPerArena <= MaxThingsPerArena);
        thingsPerArena_ = thingsPerArena;
    }

    // Resets the first |thingsPerArena| segments of this list for further use.
    void reset(size_t thingsPerArena = MaxThingsPerArena) {
        setThingsPerArena(thingsPerArena);
        // Initialize the segments.
        for (size_t i = 0; i <= thingsPerArena; ++i)
            segments[i].clear();
    }

    // Inserts a header, which has room for |nfree| more things, in its segment.
    void insertAt(ArenaHeader* aheader, size_t nfree) {
        MOZ_ASSERT(nfree <= thingsPerArena_);
        segments[nfree].append(aheader);
    }

    // Remove all empty arenas, inserting them as a linked list.
    void extractEmpty(ArenaHeader** empty) {
        SortedArenaListSegment& segment = segments[thingsPerArena_];
        if (segment.head) {
            *segment.tailp = *empty;
            *empty = segment.head;
            segment.clear();
        }
    }

    // Links up the tail of each non-empty segment to the head of the next
    // non-empty segment, creating a contiguous list that is returned as an
    // ArenaList. This is not a destructive operation: neither the head nor tail
    // of any segment is modified. However, note that the ArenaHeaders in the
    // resulting ArenaList should be treated as read-only unless the
    // SortedArenaList is no longer needed: inserting or removing arenas would
    // invalidate the SortedArenaList.
    ArenaList toArenaList() {
        // Link the non-empty segment tails up to the non-empty segment heads.
        size_t tailIndex = 0;
        for (size_t headIndex = 1; headIndex <= thingsPerArena_; ++headIndex) {
            if (headAt(headIndex)) {
                segments[tailIndex].linkTo(headAt(headIndex));
                tailIndex = headIndex;
            }
        }
        // Point the tail of the final non-empty segment at null. Note that if
        // the list is empty, this will just set segments[0].head to null.
        segments[tailIndex].linkTo(nullptr);
        // Create an ArenaList with head and cursor set to the head and tail of
        // the first segment (if that segment is empty, only the head is used).
        return ArenaList(segments[0]);
    }
};

class ArenaLists
{
    JSRuntime* runtime_;

    /*
     * For each arena kind its free list is represented as the first span with
     * free things. Initially all the spans are initialized as empty. After we
     * find a new arena with available things we move its first free span into
     * the list and set the arena as fully allocated. way we do not need to
     * update the arena header after the initial allocation. When starting the
     * GC we only move the head of the of the list of spans back to the arena
     * only for the arena that was not fully allocated.
     */
    FreeList       freeLists[FINALIZE_LIMIT];

    ArenaList      arenaLists[FINALIZE_LIMIT];

    enum BackgroundFinalizeStateEnum { BFS_DONE, BFS_RUN };

    typedef mozilla::Atomic<BackgroundFinalizeStateEnum, mozilla::ReleaseAcquire>
        BackgroundFinalizeState;

    /* The current background finalization state, accessed atomically. */
    BackgroundFinalizeState backgroundFinalizeState[FINALIZE_LIMIT];

  public:
    /* For each arena kind, a list of arenas remaining to be swept. */
    ArenaHeader* arenaListsToSweep[FINALIZE_LIMIT];

    /* During incremental sweeping, a list of the arenas already swept. */
    unsigned incrementalSweptArenaKind;
    ArenaList incrementalSweptArenas;

    // Arena lists which have yet to be swept, but need additional foreground
    // processing before they are swept.
    ArenaHeader* gcShapeArenasToUpdate;
    ArenaHeader* gcAccessorShapeArenasToUpdate;
    ArenaHeader* gcScriptArenasToUpdate;
    ArenaHeader* gcTypeObjectArenasToUpdate;

    // While sweeping type information, these lists save the arenas for the
    // objects which have already been finalized in the foreground (which must
    // happen at the beginning of the GC), so that type sweeping can determine
    // which of the object pointers are marked.
    ArenaList savedObjectArenas[FINALIZE_OBJECT_LIMIT];
    ArenaHeader* savedEmptyObjectArenas;

  public:
    explicit ArenaLists(JSRuntime* rt) : runtime_(rt) {
        for (size_t i = 0; i != FINALIZE_LIMIT; ++i)
            freeLists[i].initAsEmpty();
        for (size_t i = 0; i != FINALIZE_LIMIT; ++i)
            backgroundFinalizeState[i] = BFS_DONE;
        for (size_t i = 0; i != FINALIZE_LIMIT; ++i)
            arenaListsToSweep[i] = nullptr;
        incrementalSweptArenaKind = FINALIZE_LIMIT;
        gcShapeArenasToUpdate = nullptr;
        gcAccessorShapeArenasToUpdate = nullptr;
        gcScriptArenasToUpdate = nullptr;
        gcTypeObjectArenasToUpdate = nullptr;
        savedEmptyObjectArenas = nullptr;
    }

    ~ArenaLists();

    static uintptr_t getFreeListOffset(AllocKind thingKind) {
        uintptr_t offset = offsetof(ArenaLists, freeLists);
        return offset + thingKind * sizeof(FreeList);
    }

    const FreeList* getFreeList(AllocKind thingKind) const {
        return &freeLists[thingKind];
    }

    ArenaHeader* getFirstArena(AllocKind thingKind) const {
        return arenaLists[thingKind].head();
    }

    ArenaHeader* getFirstArenaToSweep(AllocKind thingKind) const {
        return arenaListsToSweep[thingKind];
    }

    ArenaHeader* getFirstSweptArena(AllocKind thingKind) const {
        if (thingKind != incrementalSweptArenaKind)
            return nullptr;
        return incrementalSweptArenas.head();
    }

    ArenaHeader* getArenaAfterCursor(AllocKind thingKind) const {
        return arenaLists[thingKind].arenaAfterCursor();
    }

    bool arenaListsAreEmpty() const {
        for (size_t i = 0; i != FINALIZE_LIMIT; ++i) {
            /*
             * The arena cannot be empty if the background finalization is not yet
             * done.
             */
            if (backgroundFinalizeState[i] != BFS_DONE)
                return false;
            if (!arenaLists[i].isEmpty())
                return false;
        }
        return true;
    }

    void unmarkAll() {
        for (size_t i = 0; i != FINALIZE_LIMIT; ++i) {
            /* The background finalization must have stopped at this point. */
            MOZ_ASSERT(backgroundFinalizeState[i] == BFS_DONE);
            for (ArenaHeader* aheader = arenaLists[i].head(); aheader; aheader = aheader->next)
                aheader->unmarkAll();
        }
    }

    bool doneBackgroundFinalize(AllocKind kind) const {
        return backgroundFinalizeState[kind] == BFS_DONE;
    }

    bool needBackgroundFinalizeWait(AllocKind kind) const {
        return backgroundFinalizeState[kind] != BFS_DONE;
    }

    /*
     * Return the free list back to the arena so the GC finalization will not
     * run the finalizers over unitialized bytes from free things.
     */
    void purge() {
        for (size_t i = 0; i != FINALIZE_LIMIT; ++i)
            purge(AllocKind(i));
    }

    void purge(AllocKind i) {
        FreeList* freeList = &freeLists[i];
        if (!freeList->isEmpty()) {
            ArenaHeader* aheader = freeList->arenaHeader();
            aheader->setFirstFreeSpan(freeList->getHead());
            freeList->initAsEmpty();
        }
    }

    inline void prepareForIncrementalGC(JSRuntime* rt);

    /*
     * Temporarily copy the free list heads to the arenas so the code can see
     * the proper value in ArenaHeader::freeList when accessing the latter
     * outside the GC.
     */
    void copyFreeListsToArenas() {
        for (size_t i = 0; i != FINALIZE_LIMIT; ++i)
            copyFreeListToArena(AllocKind(i));
    }

    void copyFreeListToArena(AllocKind thingKind) {
        FreeList* freeList = &freeLists[thingKind];
        if (!freeList->isEmpty()) {
            ArenaHeader* aheader = freeList->arenaHeader();
            MOZ_ASSERT(!aheader->hasFreeThings());
            aheader->setFirstFreeSpan(freeList->getHead());
        }
    }

    /*
     * Clear the free lists in arenas that were temporarily set there using
     * copyToArenas.
     */
    void clearFreeListsInArenas() {
        for (size_t i = 0; i != FINALIZE_LIMIT; ++i)
            clearFreeListInArena(AllocKind(i));
    }

    void clearFreeListInArena(AllocKind kind) {
        FreeList* freeList = &freeLists[kind];
        if (!freeList->isEmpty()) {
            ArenaHeader* aheader = freeList->arenaHeader();
            MOZ_ASSERT(freeList->isSameNonEmptySpan(aheader->getFirstFreeSpan()));
            aheader->setAsFullyUsed();
        }
    }

    /*
     * Check that the free list is either empty or were synchronized with the
     * arena using copyToArena().
     */
    bool isSynchronizedFreeList(AllocKind kind) {
        FreeList* freeList = &freeLists[kind];
        if (freeList->isEmpty())
            return true;
        ArenaHeader* aheader = freeList->arenaHeader();
        if (aheader->hasFreeThings()) {
            /*
             * If the arena has a free list, it must be the same as one in
             * lists.
             */
            MOZ_ASSERT(freeList->isSameNonEmptySpan(aheader->getFirstFreeSpan()));
            return true;
        }
        return false;
    }

    /* Check if |aheader|'s arena is in use. */
    bool arenaIsInUse(ArenaHeader* aheader, AllocKind kind) const {
        MOZ_ASSERT(aheader);
        const FreeList& freeList = freeLists[kind];
        if (freeList.isEmpty())
            return false;
        return aheader == freeList.arenaHeader();
    }

    MOZ_ALWAYS_INLINE TenuredCell* allocateFromFreeList(AllocKind thingKind, size_t thingSize) {
        return freeLists[thingKind].allocate(thingSize);
    }

    // Returns false on Out-Of-Memory. This method makes no attempt to
    // synchronize with background finalization, so may miss available memory
    // that is waiting to be finalized.
    TenuredCell* allocateFromArena(JS::Zone* zone, AllocKind thingKind);

    /*
     * Moves all arenas from |fromArenaLists| into |this|.  In
     * parallel blocks, we temporarily create one ArenaLists per
     * parallel thread.  When the parallel block ends, we move
     * whatever allocations may have been performed back into the
     * compartment's main arena list using this function.
     */
    void adoptArenas(JSRuntime* runtime, ArenaLists* fromArenaLists);

    /* True if the ArenaHeader in question is found in this ArenaLists */
    bool containsArena(JSRuntime* runtime, ArenaHeader* arenaHeader);

    void checkEmptyFreeLists() {
#ifdef DEBUG
        for (size_t i = 0; i < mozilla::ArrayLength(freeLists); ++i)
            MOZ_ASSERT(freeLists[i].isEmpty());
#endif
    }

    void checkEmptyFreeList(AllocKind kind) {
        MOZ_ASSERT(freeLists[kind].isEmpty());
    }

#ifdef JSGC_COMPACTING
    ArenaHeader* relocateArenas(ArenaHeader* relocatedList);
#endif

    void queueForegroundObjectsForSweep(FreeOp* fop);
    void queueForegroundThingsForSweep(FreeOp* fop);

    void mergeForegroundSweptObjectArenas();

    bool foregroundFinalize(FreeOp* fop, AllocKind thingKind, SliceBudget& sliceBudget,
                            SortedArenaList& sweepList);
    static void backgroundFinalize(FreeOp* fop, ArenaHeader* listHead, ArenaHeader** empty);

    void wipeDuringParallelExecution(JSRuntime* rt);

    // When finalizing arenas, whether to keep empty arenas on the list or
    // release them immediately.
    enum KeepArenasEnum {
        RELEASE_ARENAS,
        KEEP_ARENAS
    };

  private:
    inline void finalizeNow(FreeOp* fop, const FinalizePhase& phase);
    inline void queueForForegroundSweep(FreeOp* fop, const FinalizePhase& phase);
    inline void queueForBackgroundSweep(FreeOp* fop, const FinalizePhase& phase);

    inline void finalizeNow(FreeOp* fop, AllocKind thingKind,
                            KeepArenasEnum keepArenas, ArenaHeader** empty = nullptr);
    inline void forceFinalizeNow(FreeOp* fop, AllocKind thingKind,
                                 KeepArenasEnum keepArenas, ArenaHeader** empty = nullptr);
    inline void queueForForegroundSweep(FreeOp* fop, AllocKind thingKind);
    inline void queueForBackgroundSweep(FreeOp* fop, AllocKind thingKind);
    inline void mergeSweptArenas(AllocKind thingKind);

    TenuredCell* allocateFromArena(JS::Zone* zone, AllocKind thingKind,
                                   AutoMaybeStartBackgroundAllocation& maybeStartBGAlloc);

    enum ArenaAllocMode { HasFreeThings = true, IsEmpty = false };
    template <ArenaAllocMode hasFreeThings>
    inline TenuredCell* allocateFromArenaInner(JS::Zone* zone, ArenaHeader* aheader,
                                               AllocKind thingKind);

    inline void normalizeBackgroundFinalizeState(AllocKind thingKind);

    friend class GCRuntime;
};

/* The number of GC cycles an empty chunk can survive before been released. */
const size_t MAX_EMPTY_CHUNK_AGE = 4;

} /* namespace gc */

typedef enum JSGCRootType {
    JS_GC_ROOT_VALUE_PTR,
    JS_GC_ROOT_STRING_PTR,
    JS_GC_ROOT_OBJECT_PTR,
    JS_GC_ROOT_SCRIPT_PTR
} JSGCRootType;

struct RootInfo {
    RootInfo() {}
    RootInfo(const char* name, JSGCRootType type) : name(name), type(type) {}
    const char* name;
    JSGCRootType type;
};

typedef js::HashMap<void*,
                    RootInfo,
                    js::DefaultHasher<void*>,
                    js::SystemAllocPolicy> RootedValueMap;

extern bool
AddValueRoot(JSContext* cx, js::Value* vp, const char* name);

extern bool
AddValueRootRT(JSRuntime* rt, js::Value* vp, const char* name);

extern bool
AddStringRoot(JSContext* cx, JSString** rp, const char* name);

extern bool
AddObjectRoot(JSContext* cx, JSObject** rp, const char* name);

extern bool
AddObjectRoot(JSRuntime* rt, JSObject** rp, const char* name);

extern bool
AddScriptRoot(JSContext* cx, JSScript** rp, const char* name);

extern void
RemoveRoot(JSRuntime* rt, void* rp);

} /* namespace js */

extern bool
js_InitGC(JSRuntime* rt, uint32_t maxbytes);

extern void
js_FinishGC(JSRuntime* rt);

namespace js {

class InterpreterFrame;

extern void
MarkCompartmentActive(js::InterpreterFrame* fp);

extern void
TraceRuntime(JSTracer* trc);

extern void
ReleaseAllJITCode(FreeOp* op);

extern void
PrepareForDebugGC(JSRuntime* rt);

/* Functions for managing cross compartment gray pointers. */

extern void
DelayCrossCompartmentGrayMarking(JSObject* src);

extern void
NotifyGCNukeWrapper(JSObject* o);

extern unsigned
NotifyGCPreSwap(JSObject* a, JSObject* b);

extern void
NotifyGCPostSwap(JSObject* a, JSObject* b, unsigned preResult);

/*
 * Helper state for use when JS helper threads sweep and allocate GC thing kinds
 * that can be swept and allocated off the main thread.
 *
 * In non-threadsafe builds, all actual sweeping and allocation is performed
 * on the main thread, but GCHelperState encapsulates this from clients as
 * much as possible.
 */
class GCHelperState
{
    enum State {
        IDLE,
        SWEEPING
    };

    // Associated runtime.
    JSRuntime* const rt;

    // Condvar for notifying the main thread when work has finished. This is
    // associated with the runtime's GC lock --- the worker thread state
    // condvars can't be used here due to lock ordering issues.
    PRCondVar* done;

    // Activity for the helper to do, protected by the GC lock.
    State state_;

    // Thread which work is being performed on, or null.
    PRThread* thread;

    void startBackgroundThread(State newState);
    void waitForBackgroundThread();

    State state();
    void setState(State state);

    bool shrinkFlag;

    friend class js::gc::ArenaLists;

    static void freeElementsAndArray(void** array, void** end) {
        MOZ_ASSERT(array <= end);
        for (void** p = array; p != end; ++p)
            js_free(*p);
        js_free(array);
    }

    void doSweep(AutoLockGC& lock);

  public:
    explicit GCHelperState(JSRuntime* rt)
      : rt(rt),
        done(nullptr),
        state_(IDLE),
        thread(nullptr),
        shrinkFlag(false)
    { }

    bool init();
    void finish();

    void work();

    void maybeStartBackgroundSweep(const AutoLockGC& lock);
    void startBackgroundShrink(const AutoLockGC& lock);

    /* Must be called without the GC lock taken. */
    void waitBackgroundSweepEnd();

    bool onBackgroundThread();

    /*
     * Outside the GC lock may give true answer when in fact the sweeping has
     * been done.
     */
    bool isBackgroundSweeping() const {
        return state_ == SWEEPING;
    }

    bool shouldShrink() const {
        MOZ_ASSERT(isBackgroundSweeping());
        return shrinkFlag;
    }
};

// A generic task used to dispatch work to the helper thread system.
// Users should derive from GCParallelTask add what data they need and
// override |run|.
class GCParallelTask
{
    // The state of the parallel computation.
    enum TaskState {
        NotStarted,
        Dispatched,
        Finished,
    } state;

    // Amount of time this task took to execute.
    uint64_t duration_;

  protected:
    // A flag to signal a request for early completion of the off-thread task.
    mozilla::Atomic<bool> cancel_;

    virtual void run() = 0;

  public:
    GCParallelTask() : state(NotStarted), duration_(0) {}

    // Time spent in the most recent invocation of this task.
    int64_t duration() const { return duration_; }

    // The simple interface to a parallel task works exactly like pthreads.
    bool start();
    void join();

    // If multiple tasks are to be started or joined at once, it is more
    // efficient to take the helper thread lock once and use these methods.
    bool startWithLockHeld();
    void joinWithLockHeld();

    // Instead of dispatching to a helper, run the task on the main thread.
    void runFromMainThread(JSRuntime* rt);

    // Dispatch a cancelation request.
    enum CancelMode { CancelNoWait, CancelAndWait};
    void cancel(CancelMode mode = CancelNoWait) {
        cancel_ = true;
        if (mode == CancelAndWait)
            join();
    }

    // Check if a task is actively running.
    bool isRunning() const;

    // This should be friended to HelperThread, but cannot be because it
    // would introduce several circular dependencies.
  public:
    virtual void runFromHelperThread();
};

struct GCChunkHasher {
    typedef gc::Chunk* Lookup;

    /*
     * Strip zeros for better distribution after multiplying by the golden
     * ratio.
     */
    static HashNumber hash(gc::Chunk* chunk) {
        MOZ_ASSERT(!(uintptr_t(chunk) & gc::ChunkMask));
        return HashNumber(uintptr_t(chunk) >> gc::ChunkShift);
    }

    static bool match(gc::Chunk* k, gc::Chunk* l) {
        MOZ_ASSERT(!(uintptr_t(k) & gc::ChunkMask));
        MOZ_ASSERT(!(uintptr_t(l) & gc::ChunkMask));
        return k == l;
    }
};

typedef HashSet<js::gc::Chunk*, GCChunkHasher, SystemAllocPolicy> GCChunkSet;

struct GrayRoot {
    void* thing;
    JSGCTraceKind kind;
#ifdef DEBUG
    JSTraceNamePrinter debugPrinter;
    const void* debugPrintArg;
    size_t debugPrintIndex;
#endif

    GrayRoot(void* thing, JSGCTraceKind kind)
        : thing(thing), kind(kind) {}
};

void
MarkStackRangeConservatively(JSTracer* trc, Value* begin, Value* end);

typedef void (*IterateChunkCallback)(JSRuntime* rt, void* data, gc::Chunk* chunk);
typedef void (*IterateZoneCallback)(JSRuntime* rt, void* data, JS::Zone* zone);
typedef void (*IterateArenaCallback)(JSRuntime* rt, void* data, gc::Arena* arena,
                                     JSGCTraceKind traceKind, size_t thingSize);
typedef void (*IterateCellCallback)(JSRuntime* rt, void* data, void* thing,
                                    JSGCTraceKind traceKind, size_t thingSize);

/*
 * This function calls |zoneCallback| on every zone, |compartmentCallback| on
 * every compartment, |arenaCallback| on every in-use arena, and |cellCallback|
 * on every in-use cell in the GC heap.
 */
extern void
IterateZonesCompartmentsArenasCells(JSRuntime* rt, void* data,
                                    IterateZoneCallback zoneCallback,
                                    JSIterateCompartmentCallback compartmentCallback,
                                    IterateArenaCallback arenaCallback,
                                    IterateCellCallback cellCallback);

/*
 * This function is like IterateZonesCompartmentsArenasCells, but does it for a
 * single zone.
 */
extern void
IterateZoneCompartmentsArenasCells(JSRuntime* rt, Zone* zone, void* data,
                                   IterateZoneCallback zoneCallback,
                                   JSIterateCompartmentCallback compartmentCallback,
                                   IterateArenaCallback arenaCallback,
                                   IterateCellCallback cellCallback);

/*
 * Invoke chunkCallback on every in-use chunk.
 */
extern void
IterateChunks(JSRuntime* rt, void* data, IterateChunkCallback chunkCallback);

typedef void (*IterateScriptCallback)(JSRuntime* rt, void* data, JSScript* script);

/*
 * Invoke scriptCallback on every in-use script for
 * the given compartment or for all compartments if it is null.
 */
extern void
IterateScripts(JSRuntime* rt, JSCompartment* compartment,
               void* data, IterateScriptCallback scriptCallback);

} /* namespace js */

extern void
js_FinalizeStringRT(JSRuntime* rt, JSString* str);

namespace js {

JSCompartment*
NewCompartment(JSContext* cx, JS::Zone* zone, JSPrincipals* principals,
               const JS::CompartmentOptions& options);

namespace gc {

/*
 * Merge all contents of source into target. This can only be used if source is
 * the only compartment in its zone.
 */
void
MergeCompartments(JSCompartment* source, JSCompartment* target);

/*
 * This structure overlays a Cell in the Nursery and re-purposes its memory
 * for managing the Nursery collection process.
 */
class RelocationOverlay
{
    friend class MinorCollectionTracer;

    /* The low bit is set so this should never equal a normal pointer. */
    static const uintptr_t Relocated = uintptr_t(0xbad0bad1);

    // Putting the magic value after the forwarding pointer is a terrible hack
    // to make JSObject::zone() work on forwarded objects.

    /* The location |this| was moved to. */
    Cell* newLocation_;

    /* Set to Relocated when moved. */
    uintptr_t magic_;

    /* A list entry to track all relocated things. */
    RelocationOverlay* next_;

  public:
    static RelocationOverlay* fromCell(Cell* cell) {
        return reinterpret_cast<RelocationOverlay*>(cell);
    }

    bool isForwarded() const {
        return magic_ == Relocated;
    }

    Cell* forwardingAddress() const {
        MOZ_ASSERT(isForwarded());
        return newLocation_;
    }

    void forwardTo(Cell* cell) {
        MOZ_ASSERT(!isForwarded());
        MOZ_ASSERT(JSObject::offsetOfShape() == offsetof(RelocationOverlay, newLocation_));
        newLocation_ = cell;
        magic_ = Relocated;
        next_ = nullptr;
    }

    RelocationOverlay* next() const {
        return next_;
    }
};

/* Functions for checking and updating things that might be moved by compacting GC. */

template <typename T>
inline bool
IsForwarded(T* t)
{
    RelocationOverlay* overlay = RelocationOverlay::fromCell(t);
    return overlay->isForwarded();
}

inline bool
IsForwarded(const JS::Value& value)
{
    if (value.isObject())
        return IsForwarded(&value.toObject());

    if (value.isString())
        return IsForwarded(value.toString());

    if (value.isSymbol())
        return IsForwarded(value.toSymbol());

    MOZ_ASSERT(!value.isGCThing());
    return false;
}

template <typename T>
inline T*
Forwarded(T* t)
{
    RelocationOverlay* overlay = RelocationOverlay::fromCell(t);
    MOZ_ASSERT(overlay->isForwarded());
    return reinterpret_cast<T*>(overlay->forwardingAddress());
}

inline Value
Forwarded(const JS::Value& value)
{
    if (value.isObject())
        return ObjectValue(*Forwarded(&value.toObject()));
    else if (value.isString())
        return StringValue(Forwarded(value.toString()));
    else if (value.isSymbol())
        return SymbolValue(Forwarded(value.toSymbol()));

    MOZ_ASSERT(!value.isGCThing());
    return value;
}

template <typename T>
inline T
MaybeForwarded(T t)
{
    return IsForwarded(t) ? Forwarded(t) : t;
}

#ifdef JSGC_HASH_TABLE_CHECKS

template <typename T>
inline void
CheckGCThingAfterMovingGC(T* t)
{
    MOZ_ASSERT_IF(t, !IsInsideNursery(t));
#ifdef JSGC_COMPACTING
    MOZ_ASSERT_IF(t, !IsForwarded(t));
#endif
}

inline void
CheckValueAfterMovingGC(const JS::Value& value)
{
    if (value.isObject())
        return CheckGCThingAfterMovingGC(&value.toObject());
    else if (value.isString())
        return CheckGCThingAfterMovingGC(value.toString());
    else if (value.isSymbol())
        return CheckGCThingAfterMovingGC(value.toSymbol());
}

#endif // JSGC_HASH_TABLE_CHECKS

const int ZealPokeValue = 1;
const int ZealAllocValue = 2;
const int ZealFrameGCValue = 3;
const int ZealVerifierPreValue = 4;
const int ZealFrameVerifierPreValue = 5;
const int ZealStackRootingValue = 6;
const int ZealGenerationalGCValue = 7;
const int ZealIncrementalRootsThenFinish = 8;
const int ZealIncrementalMarkAllThenFinish = 9;
const int ZealIncrementalMultipleSlices = 10;
const int ZealVerifierPostValue = 11;
const int ZealFrameVerifierPostValue = 12;
const int ZealCheckHashTablesOnMinorGC = 13;
const int ZealCompactValue = 14;
const int ZealLimit = 14;

extern const char* ZealModeHelpText;

enum VerifierType {
    PreBarrierVerifier,
    PostBarrierVerifier
};

#ifdef JS_GC_ZEAL

/* Check that write barriers have been used correctly. See jsgc.cpp. */
void
VerifyBarriers(JSRuntime* rt, VerifierType type);

void
MaybeVerifyBarriers(JSContext* cx, bool always = false);

#else

static inline void
VerifyBarriers(JSRuntime* rt, VerifierType type)
{
}

static inline void
MaybeVerifyBarriers(JSContext* cx, bool always = false)
{
}

#endif

/*
 * Instances of this class set the |JSRuntime::suppressGC| flag for the duration
 * that they are live. Use of this class is highly discouraged. Please carefully
 * read the comment in vm/Runtime.h above |suppressGC| and take all appropriate
 * precautions before instantiating this class.
 */
class AutoSuppressGC
{
    int32_t& suppressGC_;

  public:
    explicit AutoSuppressGC(ExclusiveContext* cx);
    explicit AutoSuppressGC(JSCompartment* comp);
    explicit AutoSuppressGC(JSRuntime* rt);

    ~AutoSuppressGC()
    {
        suppressGC_--;
    }
};

#ifdef DEBUG
/* Disable OOM testing in sections which are not OOM safe. */
class AutoEnterOOMUnsafeRegion
{
    uint32_t saved_;

  public:
    AutoEnterOOMUnsafeRegion() : saved_(OOM_maxAllocations) {
        OOM_maxAllocations = UINT32_MAX;
    }
    ~AutoEnterOOMUnsafeRegion() {
        OOM_maxAllocations = saved_;
    }
};
#else
class AutoEnterOOMUnsafeRegion {};
#endif /* DEBUG */

// This tests whether something is inside the GGC's nursery only;
// use sparingly, mostly testing for any nursery, using IsInsideNursery,
// is appropriate.
bool
IsInsideGGCNursery(const gc::Cell* cell);

// A singly linked list of zones.
class ZoneList
{
    static Zone * const End;

    Zone* head;
    Zone* tail;

  public:
    ZoneList();
    ~ZoneList();

    bool isEmpty() const;
    Zone* front() const;

    void append(Zone* zone);
    void transferFrom(ZoneList& other);
    void removeFront();

  private:
    explicit ZoneList(Zone* singleZone);
    void check() const;

    ZoneList(const ZoneList& other) = delete;
    ZoneList& operator=(const ZoneList& other) = delete;
};

} /* namespace gc */

#ifdef DEBUG
/* Use this to avoid assertions when manipulating the wrapper map. */
class AutoDisableProxyCheck
{
    MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER;
    gc::GCRuntime& gc;

  public:
    explicit AutoDisableProxyCheck(JSRuntime* rt
                                   MOZ_GUARD_OBJECT_NOTIFIER_PARAM);
    ~AutoDisableProxyCheck();
};
#else
struct AutoDisableProxyCheck
{
    explicit AutoDisableProxyCheck(JSRuntime* rt) {}
};
#endif

struct AutoDisableCompactingGC
{
#ifdef JSGC_COMPACTING
    explicit AutoDisableCompactingGC(JSRuntime* rt);
    ~AutoDisableCompactingGC();

  private:
    gc::GCRuntime& gc;
#else
    explicit AutoDisableCompactingGC(JSRuntime* rt) {}
    ~AutoDisableCompactingGC() {}
#endif
};

void
PurgeJITCaches(JS::Zone* zone);

// This is the same as IsInsideNursery, but not inlined.
bool
UninlinedIsInsideNursery(const gc::Cell* cell);

} /* namespace js */

#endif /* jsgc_h */