Add m-esr52 at 52.6.0

1 files changed, 7722 insertions, 0 deletions

diff --git a/js/src/jsgc.cpp b/js/src/jsgc.cpp
new file mode 100644
index 000000000..42d10283c
--- /dev/null
+++ b/js/src/jsgc.cpp
@@ -0,0 +1,7722 @@
+/* -*- 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/. */
+
+/*
+ * This code implements an incremental mark-and-sweep garbage collector, with
+ * most sweeping carried out in the background on a parallel thread.
+ *
+ * Full vs. zone GC
+ * ----------------
+ *
+ * The collector can collect all zones at once, or a subset. These types of
+ * collection are referred to as a full GC and a zone GC respectively.
+ *
+ * The atoms zone is only collected in a full GC since objects in any zone may
+ * have pointers to atoms, and these are not recorded in the cross compartment
+ * pointer map. Also, the atoms zone is not collected if any thread has an
+ * AutoKeepAtoms instance on the stack, or there are any exclusive threads using
+ * the runtime.
+ *
+ * It is possible for an incremental collection that started out as a full GC to
+ * become a zone GC if new zones are created during the course of the
+ * collection.
+ *
+ * Incremental collection
+ * ----------------------
+ *
+ * For a collection to be carried out incrementally the following conditions
+ * must be met:
+ *  - the collection must be run by calling js::GCSlice() rather than js::GC()
+ *  - the GC mode must have been set to JSGC_MODE_INCREMENTAL with
+ *    JS_SetGCParameter()
+ *  - no thread may have an AutoKeepAtoms instance on the stack
+ *
+ * The last condition is an engine-internal mechanism to ensure that incremental
+ * collection is not carried out without the correct barriers being implemented.
+ * For more information see 'Incremental marking' below.
+ *
+ * If the collection is not incremental, all foreground activity happens inside
+ * a single call to GC() or GCSlice(). However the collection is not complete
+ * until the background sweeping activity has finished.
+ *
+ * An incremental collection proceeds as a series of slices, interleaved with
+ * mutator activity, i.e. running JavaScript code. Slices are limited by a time
+ * budget. The slice finishes as soon as possible after the requested time has
+ * passed.
+ *
+ * Collector states
+ * ----------------
+ *
+ * The collector proceeds through the following states, the current state being
+ * held in JSRuntime::gcIncrementalState:
+ *
+ *  - MarkRoots  - marks the stack and other roots
+ *  - Mark       - incrementally marks reachable things
+ *  - Sweep      - sweeps zones in groups and continues marking unswept zones
+ *  - Finalize   - performs background finalization, concurrent with mutator
+ *  - Compact    - incrementally compacts by zone
+ *  - Decommit   - performs background decommit and chunk removal
+ *
+ * The MarkRoots activity always takes place in the first slice. The next two
+ * states can take place over one or more slices.
+ *
+ * In other words an incremental collection proceeds like this:
+ *
+ * Slice 1:   MarkRoots:  Roots pushed onto the mark stack.
+ *            Mark:       The mark stack is processed by popping an element,
+ *                        marking it, and pushing its children.
+ *
+ *          ... JS code runs ...
+ *
+ * Slice 2:   Mark:       More mark stack processing.
+ *
+ *          ... JS code runs ...
+ *
+ * Slice n-1: Mark:       More mark stack processing.
+ *
+ *          ... JS code runs ...
+ *
+ * Slice n:   Mark:       Mark stack is completely drained.
+ *            Sweep:      Select first group of zones to sweep and sweep them.
+ *
+ *          ... JS code runs ...
+ *
+ * Slice n+1: Sweep:      Mark objects in unswept zones that were newly
+ *                        identified as alive (see below). Then sweep more zone
+ *                        groups.
+ *
+ *          ... JS code runs ...
+ *
+ * Slice n+2: Sweep:      Mark objects in unswept zones that were newly
+ *                        identified as alive. Then sweep more zone groups.
+ *
+ *          ... JS code runs ...
+ *
+ * Slice m:   Sweep:      Sweeping is finished, and background sweeping
+ *                        started on the helper thread.
+ *
+ *          ... JS code runs, remaining sweeping done on background thread ...
+ *
+ * When background sweeping finishes the GC is complete.
+ *
+ * Incremental marking
+ * -------------------
+ *
+ * Incremental collection requires close collaboration with the mutator (i.e.,
+ * JS code) to guarantee correctness.
+ *
+ *  - During an incremental GC, if a memory location (except a root) is written
+ *    to, then the value it previously held must be marked. Write barriers
+ *    ensure this.
+ *
+ *  - Any object that is allocated during incremental GC must start out marked.
+ *
+ *  - Roots are marked in the first slice and hence don't need write barriers.
+ *    Roots are things like the C stack and the VM stack.
+ *
+ * The problem that write barriers solve is that between slices the mutator can
+ * change the object graph. We must ensure that it cannot do this in such a way
+ * that makes us fail to mark a reachable object (marking an unreachable object
+ * is tolerable).
+ *
+ * We use a snapshot-at-the-beginning algorithm to do this. This means that we
+ * promise to mark at least everything that is reachable at the beginning of
+ * collection. To implement it we mark the old contents of every non-root memory
+ * location written to by the mutator while the collection is in progress, using
+ * write barriers. This is described in gc/Barrier.h.
+ *
+ * Incremental sweeping
+ * --------------------
+ *
+ * Sweeping is difficult to do incrementally because object finalizers must be
+ * run at the start of sweeping, before any mutator code runs. The reason is
+ * that some objects use their finalizers to remove themselves from caches. If
+ * mutator code was allowed to run after the start of sweeping, it could observe
+ * the state of the cache and create a new reference to an object that was just
+ * about to be destroyed.
+ *
+ * Sweeping all finalizable objects in one go would introduce long pauses, so
+ * instead sweeping broken up into groups of zones. Zones which are not yet
+ * being swept are still marked, so the issue above does not apply.
+ *
+ * The order of sweeping is restricted by cross compartment pointers - for
+ * example say that object |a| from zone A points to object |b| in zone B and
+ * neither object was marked when we transitioned to the Sweep phase. Imagine we
+ * sweep B first and then return to the mutator. It's possible that the mutator
+ * could cause |a| to become alive through a read barrier (perhaps it was a
+ * shape that was accessed via a shape table). Then we would need to mark |b|,
+ * which |a| points to, but |b| has already been swept.
+ *
+ * So if there is such a pointer then marking of zone B must not finish before
+ * marking of zone A.  Pointers which form a cycle between zones therefore
+ * restrict those zones to being swept at the same time, and these are found
+ * using Tarjan's algorithm for finding the strongly connected components of a
+ * graph.
+ *
+ * GC things without finalizers, and things with finalizers that are able to run
+ * in the background, are swept on the background thread. This accounts for most
+ * of the sweeping work.
+ *
+ * Reset
+ * -----
+ *
+ * During incremental collection it is possible, although unlikely, for
+ * conditions to change such that incremental collection is no longer safe. In
+ * this case, the collection is 'reset' by ResetIncrementalGC(). If we are in
+ * the mark state, this just stops marking, but if we have started sweeping
+ * already, we continue until we have swept the current zone group. Following a
+ * reset, a new non-incremental collection is started.
+ *
+ * Compacting GC
+ * -------------
+ *
+ * Compacting GC happens at the end of a major GC as part of the last slice.
+ * There are three parts:
+ *
+ *  - Arenas are selected for compaction.
+ *  - The contents of those arenas are moved to new arenas.
+ *  - All references to moved things are updated.
+ */
+
+#include "jsgcinlines.h"
+
+#include "mozilla/ArrayUtils.h"
+#include "mozilla/DebugOnly.h"
+#include "mozilla/MacroForEach.h"
+#include "mozilla/MemoryReporting.h"
+#include "mozilla/Move.h"
+#include "mozilla/ScopeExit.h"
+
+#include <ctype.h>
+#include <string.h>
+#ifndef XP_WIN
+# include <sys/mman.h>
+# include <unistd.h>
+#endif
+
+#include "jsapi.h"
+#include "jsatom.h"
+#include "jscntxt.h"
+#include "jscompartment.h"
+#include "jsfriendapi.h"
+#include "jsobj.h"
+#include "jsprf.h"
+#include "jsscript.h"
+#include "jstypes.h"
+#include "jsutil.h"
+#include "jswatchpoint.h"
+#include "jsweakmap.h"
+#ifdef XP_WIN
+# include "jswin.h"
+#endif
+
+#include "gc/FindSCCs.h"
+#include "gc/GCInternals.h"
+#include "gc/GCTrace.h"
+#include "gc/Marking.h"
+#include "gc/Memory.h"
+#include "gc/Policy.h"
+#include "jit/BaselineJIT.h"
+#include "jit/IonCode.h"
+#include "jit/JitcodeMap.h"
+#include "js/SliceBudget.h"
+#include "proxy/DeadObjectProxy.h"
+#include "vm/Debugger.h"
+#include "vm/ProxyObject.h"
+#include "vm/Shape.h"
+#include "vm/SPSProfiler.h"
+#include "vm/String.h"
+#include "vm/Symbol.h"
+#include "vm/Time.h"
+#include "vm/TraceLogging.h"
+#include "vm/WrapperObject.h"
+
+#include "jsobjinlines.h"
+#include "jsscriptinlines.h"
+
+#include "vm/Stack-inl.h"
+#include "vm/String-inl.h"
+
+using namespace js;
+using namespace js::gc;
+
+using mozilla::ArrayLength;
+using mozilla::Get;
+using mozilla::HashCodeScrambler;
+using mozilla::Maybe;
+using mozilla::Swap;
+
+using JS::AutoGCRooter;
+
+/* Increase the IGC marking slice time if we are in highFrequencyGC mode. */
+static const int IGC_MARK_SLICE_MULTIPLIER = 2;
+
+const AllocKind gc::slotsToThingKind[] = {
+    /*  0 */ AllocKind::OBJECT0,  AllocKind::OBJECT2,  AllocKind::OBJECT2,  AllocKind::OBJECT4,
+    /*  4 */ AllocKind::OBJECT4,  AllocKind::OBJECT8,  AllocKind::OBJECT8,  AllocKind::OBJECT8,
+    /*  8 */ AllocKind::OBJECT8,  AllocKind::OBJECT12, AllocKind::OBJECT12, AllocKind::OBJECT12,
+    /* 12 */ AllocKind::OBJECT12, AllocKind::OBJECT16, AllocKind::OBJECT16, AllocKind::OBJECT16,
+    /* 16 */ AllocKind::OBJECT16
+};
+
+static_assert(JS_ARRAY_LENGTH(slotsToThingKind) == SLOTS_TO_THING_KIND_LIMIT,
+              "We have defined a slot count for each kind.");
+
+#define CHECK_THING_SIZE(allocKind, traceKind, type, sizedType) \
+    static_assert(sizeof(sizedType) >= SortedArenaList::MinThingSize, \
+                  #sizedType " is smaller than SortedArenaList::MinThingSize!"); \
+    static_assert(sizeof(sizedType) >= sizeof(FreeSpan), \
+                  #sizedType " is smaller than FreeSpan"); \
+    static_assert(sizeof(sizedType) % CellSize == 0, \
+                  "Size of " #sizedType " is not a multiple of CellSize");
+FOR_EACH_ALLOCKIND(CHECK_THING_SIZE);
+#undef CHECK_THING_SIZE
+
+const uint32_t Arena::ThingSizes[] = {
+#define EXPAND_THING_SIZE(allocKind, traceKind, type, sizedType) \
+    sizeof(sizedType),
+FOR_EACH_ALLOCKIND(EXPAND_THING_SIZE)
+#undef EXPAND_THING_SIZE
+};
+
+FreeSpan ArenaLists::placeholder;
+
+#undef CHECK_THING_SIZE_INNER
+#undef CHECK_THING_SIZE
+
+#define OFFSET(type) uint32_t(ArenaHeaderSize + (ArenaSize - ArenaHeaderSize) % sizeof(type))
+
+const uint32_t Arena::FirstThingOffsets[] = {
+#define EXPAND_FIRST_THING_OFFSET(allocKind, traceKind, type, sizedType) \
+    OFFSET(sizedType),
+FOR_EACH_ALLOCKIND(EXPAND_FIRST_THING_OFFSET)
+#undef EXPAND_FIRST_THING_OFFSET
+};
+
+#undef OFFSET
+
+#define COUNT(type) uint32_t((ArenaSize - ArenaHeaderSize) / sizeof(type))
+
+const uint32_t Arena::ThingsPerArena[] = {
+#define EXPAND_THINGS_PER_ARENA(allocKind, traceKind, type, sizedType) \
+    COUNT(sizedType),
+FOR_EACH_ALLOCKIND(EXPAND_THINGS_PER_ARENA)
+#undef EXPAND_THINGS_PER_ARENA
+};
+
+#undef COUNT
+
+struct js::gc::FinalizePhase
+{
+    gcstats::Phase statsPhase;
+    AllocKinds kinds;
+};
+
+/*
+ * Finalization order for GC things swept incrementally on the main thrad.
+ */
+static const FinalizePhase IncrementalFinalizePhases[] = {
+    {
+        gcstats::PHASE_SWEEP_STRING, {
+            AllocKind::EXTERNAL_STRING
+        }
+    },
+    {
+        gcstats::PHASE_SWEEP_SCRIPT, {
+            AllocKind::SCRIPT
+        }
+    },
+    {
+        gcstats::PHASE_SWEEP_JITCODE, {
+            AllocKind::JITCODE
+        }
+    }
+};
+
+/*
+ * Finalization order for GC things swept on the background thread.
+ */
+static const FinalizePhase BackgroundFinalizePhases[] = {
+    {
+        gcstats::PHASE_SWEEP_SCRIPT, {
+            AllocKind::LAZY_SCRIPT
+        }
+    },
+    {
+        gcstats::PHASE_SWEEP_OBJECT, {
+            AllocKind::FUNCTION,
+            AllocKind::FUNCTION_EXTENDED,
+            AllocKind::OBJECT0_BACKGROUND,
+            AllocKind::OBJECT2_BACKGROUND,
+            AllocKind::OBJECT4_BACKGROUND,
+            AllocKind::OBJECT8_BACKGROUND,
+            AllocKind::OBJECT12_BACKGROUND,
+            AllocKind::OBJECT16_BACKGROUND
+        }
+    },
+    {
+        gcstats::PHASE_SWEEP_SCOPE, {
+            AllocKind::SCOPE
+        }
+    },
+    {
+        gcstats::PHASE_SWEEP_STRING, {
+            AllocKind::FAT_INLINE_STRING,
+            AllocKind::STRING,
+            AllocKind::FAT_INLINE_ATOM,
+            AllocKind::ATOM,
+            AllocKind::SYMBOL
+        }
+    },
+    {
+        gcstats::PHASE_SWEEP_SHAPE, {
+            AllocKind::SHAPE,
+            AllocKind::ACCESSOR_SHAPE,
+            AllocKind::BASE_SHAPE,
+            AllocKind::OBJECT_GROUP
+        }
+    }
+};
+
+template<>
+JSObject*
+ArenaCellIterImpl::get<JSObject>() const
+{
+    MOZ_ASSERT(!done());
+    return reinterpret_cast<JSObject*>(getCell());
+}
+
+void
+Arena::unmarkAll()
+{
+    uintptr_t* word = chunk()->bitmap.arenaBits(this);
+    memset(word, 0, ArenaBitmapWords * sizeof(uintptr_t));
+}
+
+/* static */ void
+Arena::staticAsserts()
+{
+    static_assert(size_t(AllocKind::LIMIT) <= 255,
+                  "We must be able to fit the allockind into uint8_t.");
+    static_assert(JS_ARRAY_LENGTH(ThingSizes) == size_t(AllocKind::LIMIT),
+                  "We haven't defined all thing sizes.");
+    static_assert(JS_ARRAY_LENGTH(FirstThingOffsets) == size_t(AllocKind::LIMIT),
+                  "We haven't defined all offsets.");
+    static_assert(JS_ARRAY_LENGTH(ThingsPerArena) == size_t(AllocKind::LIMIT),
+                  "We haven't defined all counts.");
+}
+
+template<typename T>
+inline size_t
+Arena::finalize(FreeOp* fop, AllocKind thingKind, size_t thingSize)
+{
+    /* Enforce requirements on size of T. */
+    MOZ_ASSERT(thingSize % CellSize == 0);
+    MOZ_ASSERT(thingSize <= 255);
+
+    MOZ_ASSERT(allocated());
+    MOZ_ASSERT(thingKind == getAllocKind());
+    MOZ_ASSERT(thingSize == getThingSize());
+    MOZ_ASSERT(!hasDelayedMarking);
+    MOZ_ASSERT(!markOverflow);
+    MOZ_ASSERT(!allocatedDuringIncremental);
+
+    uint_fast16_t firstThing = firstThingOffset(thingKind);
+    uint_fast16_t firstThingOrSuccessorOfLastMarkedThing = firstThing;
+    uint_fast16_t lastThing = ArenaSize - thingSize;
+
+    FreeSpan newListHead;
+    FreeSpan* newListTail = &newListHead;
+    size_t nmarked = 0;
+
+    if (MOZ_UNLIKELY(MemProfiler::enabled())) {
+        for (ArenaCellIterUnderFinalize i(this); !i.done(); i.next()) {
+            T* t = i.get<T>();
+            if (t->asTenured().isMarked())
+                MemProfiler::MarkTenured(reinterpret_cast<void*>(t));
+        }
+    }
+
+    for (ArenaCellIterUnderFinalize i(this); !i.done(); i.next()) {
+        T* t = i.get<T>();
+        if (t->asTenured().isMarked()) {
+            uint_fast16_t thing = uintptr_t(t) & ArenaMask;
+            if (thing != firstThingOrSuccessorOfLastMarkedThing) {
+                // We just finished passing over one or more free things,
+                // so record a new FreeSpan.
+                newListTail->initBounds(firstThingOrSuccessorOfLastMarkedThing,
+                                        thing - thingSize, this);
+                newListTail = newListTail->nextSpanUnchecked(this);
+            }
+            firstThingOrSuccessorOfLastMarkedThing = thing + thingSize;
+            nmarked++;
+        } else {
+            t->finalize(fop);
+            JS_POISON(t, JS_SWEPT_TENURED_PATTERN, thingSize);
+            TraceTenuredFinalize(t);
+        }
+    }
+
+    if (nmarked == 0) {
+        // Do nothing. The caller will update the arena appropriately.
+        MOZ_ASSERT(newListTail == &newListHead);
+        JS_EXTRA_POISON(data, JS_SWEPT_TENURED_PATTERN, sizeof(data));
+        return nmarked;
+    }
+
+    MOZ_ASSERT(firstThingOrSuccessorOfLastMarkedThing != firstThing);
+    uint_fast16_t lastMarkedThing = firstThingOrSuccessorOfLastMarkedThing - thingSize;
+    if (lastThing == lastMarkedThing) {
+        // If the last thing was marked, we will have already set the bounds of
+        // the final span, and we just need to terminate the list.
+        newListTail->initAsEmpty();
+    } else {
+        // Otherwise, end the list with a span that covers the final stretch of free things.
+        newListTail->initFinal(firstThingOrSuccessorOfLastMarkedThing, lastThing, this);
+    }
+
+    firstFreeSpan = newListHead;
+#ifdef DEBUG
+    size_t nfree = numFreeThings(thingSize);
+    MOZ_ASSERT(nfree + nmarked == thingsPerArena(thingKind));
+#endif
+    return nmarked;
+}
+
+// Finalize arenas from src list, releasing empty arenas if keepArenas wasn't
+// specified and inserting the others into the appropriate destination size
+// bins.
+template<typename T>
+static inline bool
+FinalizeTypedArenas(FreeOp* fop,
+                    Arena** src,
+                    SortedArenaList& dest,
+                    AllocKind thingKind,
+                    SliceBudget& budget,
+                    ArenaLists::KeepArenasEnum keepArenas)
+{
+    // When operating in the foreground, take the lock at the top.
+    Maybe<AutoLockGC> maybeLock;
+    if (fop->onMainThread())
+        maybeLock.emplace(fop->runtime());
+
+    // During background sweeping free arenas are released later on in
+    // sweepBackgroundThings().
+    MOZ_ASSERT_IF(!fop->onMainThread(), keepArenas == ArenaLists::KEEP_ARENAS);
+
+    size_t thingSize = Arena::thingSize(thingKind);
+    size_t thingsPerArena = Arena::thingsPerArena(thingKind);
+
+    while (Arena* arena = *src) {
+        *src = arena->next;
+        size_t nmarked = arena->finalize<T>(fop, thingKind, thingSize);
+        size_t nfree = thingsPerArena - nmarked;
+
+        if (nmarked)
+            dest.insertAt(arena, nfree);
+        else if (keepArenas == ArenaLists::KEEP_ARENAS)
+            arena->chunk()->recycleArena(arena, dest, thingsPerArena);
+        else
+            fop->runtime()->gc.releaseArena(arena, maybeLock.ref());
+
+        budget.step(thingsPerArena);
+        if (budget.isOverBudget())
+            return false;
+    }
+
+    return true;
+}
+
+/*
+ * Finalize the list. On return, |al|'s cursor points to the first non-empty
+ * arena in the list (which may be null if all arenas are full).
+ */
+static bool
+FinalizeArenas(FreeOp* fop,
+               Arena** src,
+               SortedArenaList& dest,
+               AllocKind thingKind,
+               SliceBudget& budget,
+               ArenaLists::KeepArenasEnum keepArenas)
+{
+    switch (thingKind) {
+#define EXPAND_CASE(allocKind, traceKind, type, sizedType) \
+      case AllocKind::allocKind: \
+        return FinalizeTypedArenas<type>(fop, src, dest, thingKind, budget, keepArenas);
+FOR_EACH_ALLOCKIND(EXPAND_CASE)
+#undef EXPAND_CASE
+
+      default:
+        MOZ_CRASH("Invalid alloc kind");
+    }
+}
+
+Chunk*
+ChunkPool::pop()
+{
+    MOZ_ASSERT(bool(head_) == bool(count_));
+    if (!count_)
+        return nullptr;
+    return remove(head_);
+}
+
+void
+ChunkPool::push(Chunk* chunk)
+{
+    MOZ_ASSERT(!chunk->info.next);
+    MOZ_ASSERT(!chunk->info.prev);
+
+    chunk->info.next = head_;
+    if (head_)
+        head_->info.prev = chunk;
+    head_ = chunk;
+    ++count_;
+
+    MOZ_ASSERT(verify());
+}
+
+Chunk*
+ChunkPool::remove(Chunk* chunk)
+{
+    MOZ_ASSERT(count_ > 0);
+    MOZ_ASSERT(contains(chunk));
+
+    if (head_ == chunk)
+        head_ = chunk->info.next;
+    if (chunk->info.prev)
+        chunk->info.prev->info.next = chunk->info.next;
+    if (chunk->info.next)
+        chunk->info.next->info.prev = chunk->info.prev;
+    chunk->info.next = chunk->info.prev = nullptr;
+    --count_;
+
+    MOZ_ASSERT(verify());
+    return chunk;
+}
+
+#ifdef DEBUG
+bool
+ChunkPool::contains(Chunk* chunk) const
+{
+    verify();
+    for (Chunk* cursor = head_; cursor; cursor = cursor->info.next) {
+        if (cursor == chunk)
+            return true;
+    }
+    return false;
+}
+
+bool
+ChunkPool::verify() const
+{
+    MOZ_ASSERT(bool(head_) == bool(count_));
+    uint32_t count = 0;
+    for (Chunk* cursor = head_; cursor; cursor = cursor->info.next, ++count) {
+        MOZ_ASSERT_IF(cursor->info.prev, cursor->info.prev->info.next == cursor);
+        MOZ_ASSERT_IF(cursor->info.next, cursor->info.next->info.prev == cursor);
+    }
+    MOZ_ASSERT(count_ == count);
+    return true;
+}
+#endif
+
+void
+ChunkPool::Iter::next()
+{
+    MOZ_ASSERT(!done());
+    current_ = current_->info.next;
+}
+
+ChunkPool
+GCRuntime::expireEmptyChunkPool(const AutoLockGC& lock)
+{
+    MOZ_ASSERT(emptyChunks(lock).verify());
+    MOZ_ASSERT(tunables.minEmptyChunkCount(lock) <= tunables.maxEmptyChunkCount());
+
+    ChunkPool expired;
+    while (emptyChunks(lock).count() > tunables.minEmptyChunkCount(lock)) {
+        Chunk* chunk = emptyChunks(lock).pop();
+        prepareToFreeChunk(chunk->info);
+        expired.push(chunk);
+    }
+
+    MOZ_ASSERT(expired.verify());
+    MOZ_ASSERT(emptyChunks(lock).verify());
+    MOZ_ASSERT(emptyChunks(lock).count() <= tunables.maxEmptyChunkCount());
+    MOZ_ASSERT(emptyChunks(lock).count() <= tunables.minEmptyChunkCount(lock));
+    return expired;
+}
+
+static void
+FreeChunkPool(JSRuntime* rt, ChunkPool& pool)
+{
+    for (ChunkPool::Iter iter(pool); !iter.done();) {
+        Chunk* chunk = iter.get();
+        iter.next();
+        pool.remove(chunk);
+        MOZ_ASSERT(!chunk->info.numArenasFreeCommitted);
+        UnmapPages(static_cast<void*>(chunk), ChunkSize);
+    }
+    MOZ_ASSERT(pool.count() == 0);
+}
+
+void
+GCRuntime::freeEmptyChunks(JSRuntime* rt, const AutoLockGC& lock)
+{
+    FreeChunkPool(rt, emptyChunks(lock));
+}
+
+inline void
+GCRuntime::prepareToFreeChunk(ChunkInfo& info)
+{
+    MOZ_ASSERT(numArenasFreeCommitted >= info.numArenasFreeCommitted);
+    numArenasFreeCommitted -= info.numArenasFreeCommitted;
+    stats.count(gcstats::STAT_DESTROY_CHUNK);
+#ifdef DEBUG
+    /*
+     * Let FreeChunkPool detect a missing prepareToFreeChunk call before it
+     * frees chunk.
+     */
+    info.numArenasFreeCommitted = 0;
+#endif
+}
+
+inline void
+GCRuntime::updateOnArenaFree(const ChunkInfo& info)
+{
+    ++numArenasFreeCommitted;
+}
+
+void
+Chunk::addArenaToFreeList(JSRuntime* rt, Arena* arena)
+{
+    MOZ_ASSERT(!arena->allocated());
+    arena->next = info.freeArenasHead;
+    info.freeArenasHead = arena;
+    ++info.numArenasFreeCommitted;
+    ++info.numArenasFree;
+    rt->gc.updateOnArenaFree(info);
+}
+
+void
+Chunk::addArenaToDecommittedList(JSRuntime* rt, const Arena* arena)
+{
+    ++info.numArenasFree;
+    decommittedArenas.set(Chunk::arenaIndex(arena->address()));
+}
+
+void
+Chunk::recycleArena(Arena* arena, SortedArenaList& dest, size_t thingsPerArena)
+{
+    arena->setAsFullyUnused();
+    dest.insertAt(arena, thingsPerArena);
+}
+
+void
+Chunk::releaseArena(JSRuntime* rt, Arena* arena, const AutoLockGC& lock)
+{
+    MOZ_ASSERT(arena->allocated());
+    MOZ_ASSERT(!arena->hasDelayedMarking);
+
+    arena->setAsNotAllocated();
+    addArenaToFreeList(rt, arena);
+    updateChunkListAfterFree(rt, lock);
+}
+
+bool
+Chunk::decommitOneFreeArena(JSRuntime* rt, AutoLockGC& lock)
+{
+    MOZ_ASSERT(info.numArenasFreeCommitted > 0);
+    Arena* arena = fetchNextFreeArena(rt);
+    updateChunkListAfterAlloc(rt, lock);
+
+    bool ok;
+    {
+        AutoUnlockGC unlock(lock);
+        ok = MarkPagesUnused(arena, ArenaSize);
+    }
+
+    if (ok)
+        addArenaToDecommittedList(rt, arena);
+    else
+        addArenaToFreeList(rt, arena);
+    updateChunkListAfterFree(rt, lock);
+
+    return ok;
+}
+
+void
+Chunk::decommitAllArenasWithoutUnlocking(const AutoLockGC& lock)
+{
+    for (size_t i = 0; i < ArenasPerChunk; ++i) {
+        if (decommittedArenas.get(i) || arenas[i].allocated())
+            continue;
+
+        if (MarkPagesUnused(&arenas[i], ArenaSize)) {
+            info.numArenasFreeCommitted--;
+            decommittedArenas.set(i);
+        }
+    }
+}
+
+void
+Chunk::updateChunkListAfterAlloc(JSRuntime* rt, const AutoLockGC& lock)
+{
+    if (MOZ_UNLIKELY(!hasAvailableArenas())) {
+        rt->gc.availableChunks(lock).remove(this);
+        rt->gc.fullChunks(lock).push(this);
+    }
+}
+
+void
+Chunk::updateChunkListAfterFree(JSRuntime* rt, const AutoLockGC& lock)
+{
+    if (info.numArenasFree == 1) {
+        rt->gc.fullChunks(lock).remove(this);
+        rt->gc.availableChunks(lock).push(this);
+    } else if (!unused()) {
+        MOZ_ASSERT(!rt->gc.fullChunks(lock).contains(this));
+        MOZ_ASSERT(rt->gc.availableChunks(lock).contains(this));
+        MOZ_ASSERT(!rt->gc.emptyChunks(lock).contains(this));
+    } else {
+        MOZ_ASSERT(unused());
+        rt->gc.availableChunks(lock).remove(this);
+        decommitAllArenas(rt);
+        MOZ_ASSERT(info.numArenasFreeCommitted == 0);
+        rt->gc.recycleChunk(this, lock);
+    }
+}
+
+void
+GCRuntime::releaseArena(Arena* arena, const AutoLockGC& lock)
+{
+    arena->zone->usage.removeGCArena();
+    if (isBackgroundSweeping())
+        arena->zone->threshold.updateForRemovedArena(tunables);
+    return arena->chunk()->releaseArena(rt, arena, lock);
+}
+
+GCRuntime::GCRuntime(JSRuntime* rt) :
+    rt(rt),
+    systemZone(nullptr),
+    nursery(rt),
+    storeBuffer(rt, nursery),
+    stats(rt),
+    marker(rt),
+    usage(nullptr),
+    mMemProfiler(rt),
+    maxMallocBytes(0),
+    nextCellUniqueId_(LargestTaggedNullCellPointer + 1), // Ensure disjoint from null tagged pointers.
+    numArenasFreeCommitted(0),
+    verifyPreData(nullptr),
+    chunkAllocationSinceLastGC(false),
+    lastGCTime(PRMJ_Now()),
+    mode(JSGC_MODE_INCREMENTAL),
+    numActiveZoneIters(0),
+    cleanUpEverything(false),
+    grayBufferState(GCRuntime::GrayBufferState::Unused),
+    majorGCTriggerReason(JS::gcreason::NO_REASON),
+    minorGCTriggerReason(JS::gcreason::NO_REASON),
+    fullGCForAtomsRequested_(false),
+    minorGCNumber(0),
+    majorGCNumber(0),
+    jitReleaseNumber(0),
+    number(0),
+    startNumber(0),
+    isFull(false),
+#ifdef DEBUG
+    disableStrictProxyCheckingCount(0),
+#endif
+    incrementalState(gc::State::NotActive),
+    lastMarkSlice(false),
+    sweepOnBackgroundThread(false),
+    blocksToFreeAfterSweeping(JSRuntime::TEMP_LIFO_ALLOC_PRIMARY_CHUNK_SIZE),
+    blocksToFreeAfterMinorGC(JSRuntime::TEMP_LIFO_ALLOC_PRIMARY_CHUNK_SIZE),
+    zoneGroupIndex(0),
+    zoneGroups(nullptr),
+    currentZoneGroup(nullptr),
+    sweepZone(nullptr),
+    sweepKind(AllocKind::FIRST),
+    abortSweepAfterCurrentGroup(false),
+    arenasAllocatedDuringSweep(nullptr),
+    startedCompacting(false),
+    relocatedArenasToRelease(nullptr),
+#ifdef JS_GC_ZEAL
+    markingValidator(nullptr),
+#endif
+    interFrameGC(false),
+    defaultTimeBudget_(SliceBudget::UnlimitedTimeBudget),
+    incrementalAllowed(true),
+    generationalDisabled(0),
+    compactingEnabled(true),
+    compactingDisabledCount(0),
+    manipulatingDeadZones(false),
+    objectsMarkedInDeadZones(0),
+    poked(false),
+#ifdef JS_GC_ZEAL
+    zealModeBits(0),
+    zealFrequency(0),
+    nextScheduled(0),
+    deterministicOnly(false),
+    incrementalLimit(0),
+#endif
+    fullCompartmentChecks(false),
+    mallocBytesUntilGC(0),
+    mallocGCTriggered(false),
+    alwaysPreserveCode(false),
+    inUnsafeRegion(0),
+#ifdef DEBUG
+    noGCOrAllocationCheck(0),
+    noNurseryAllocationCheck(0),
+    arenasEmptyAtShutdown(true),
+#endif
+    lock(mutexid::GCLock),
+    allocTask(rt, emptyChunks_),
+    decommitTask(rt),
+    helperState(rt)
+{
+    setGCMode(JSGC_MODE_GLOBAL);
+}
+
+#ifdef JS_GC_ZEAL
+
+void
+GCRuntime::getZealBits(uint32_t* zealBits, uint32_t* frequency, uint32_t* scheduled)
+{
+    *zealBits = zealModeBits;
+    *frequency = zealFrequency;
+    *scheduled = nextScheduled;
+}
+
+const char* gc::ZealModeHelpText =
+    "  Specifies how zealous the garbage collector should be. Some of these modes can\n"
+    "  be set simultaneously, by passing multiple level options, e.g. \"2;4\" will activate\n"
+    "  both modes 2 and 4. Modes can be specified by name or number.\n"
+    "  \n"
+    "  Values:\n"
+    "    0: (None) Normal amount of collection (resets all modes)\n"
+    "    1: (Poke) Collect when roots are added or removed\n"
+    "    2: (Alloc) Collect when every N allocations (default: 100)\n"
+    "    3: (FrameGC) Collect when the window paints (browser only)\n"
+    "    4: (VerifierPre) Verify pre write barriers between instructions\n"
+    "    5: (FrameVerifierPre) Verify pre write barriers between paints\n"
+    "    6: (StackRooting) Verify stack rooting\n"
+    "    7: (GenerationalGC) Collect the nursery every N nursery allocations\n"
+    "    8: (IncrementalRootsThenFinish) Incremental GC in two slices: 1) mark roots 2) finish collection\n"
+    "    9: (IncrementalMarkAllThenFinish) Incremental GC in two slices: 1) mark all 2) new marking and finish\n"
+    "   10: (IncrementalMultipleSlices) Incremental GC in multiple slices\n"
+    "   11: (IncrementalMarkingValidator) Verify incremental marking\n"
+    "   12: (ElementsBarrier) Always use the individual element post-write barrier, regardless of elements size\n"
+    "   13: (CheckHashTablesOnMinorGC) Check internal hashtables on minor GC\n"
+    "   14: (Compact) Perform a shrinking collection every N allocations\n"
+    "   15: (CheckHeapAfterGC) Walk the heap to check its integrity after every GC\n"
+    "   16: (CheckNursery) Check nursery integrity on minor GC\n";
+
+void
+GCRuntime::setZeal(uint8_t zeal, uint32_t frequency)
+{
+    MOZ_ASSERT(zeal <= unsigned(ZealMode::Limit));
+
+    if (verifyPreData)
+        VerifyBarriers(rt, PreBarrierVerifier);
+
+    if (zeal == 0 && hasZealMode(ZealMode::GenerationalGC)) {
+        evictNursery(JS::gcreason::DEBUG_GC);
+        nursery.leaveZealMode();
+    }
+
+    ZealMode zealMode = ZealMode(zeal);
+    if (zealMode == ZealMode::GenerationalGC)
+        nursery.enterZealMode();
+
+    // Zeal modes 8-10 are mutually exclusive. If we're setting one of those,
+    // we first reset all of them.
+    if (zealMode >= ZealMode::IncrementalRootsThenFinish &&
+        zealMode <= ZealMode::IncrementalMultipleSlices)
+    {
+        clearZealMode(ZealMode::IncrementalRootsThenFinish);
+        clearZealMode(ZealMode::IncrementalMarkAllThenFinish);
+        clearZealMode(ZealMode::IncrementalMultipleSlices);
+    }
+
+    bool schedule = zealMode >= ZealMode::Alloc;
+    if (zeal != 0)
+        zealModeBits |= 1 << unsigned(zeal);
+    else
+        zealModeBits = 0;
+    zealFrequency = frequency;
+    nextScheduled = schedule ? frequency : 0;
+}
+
+void
+GCRuntime::setNextScheduled(uint32_t count)
+{
+    nextScheduled = count;
+}
+
+bool
+GCRuntime::parseAndSetZeal(const char* str)
+{
+    int frequency = -1;
+    bool foundFrequency = false;
+    mozilla::Vector<int, 0, SystemAllocPolicy> zeals;
+
+    static const struct {
+        const char* const zealMode;
+        size_t length;
+        uint32_t zeal;
+    } zealModes[] = {
+#define ZEAL_MODE(name, value) {#name, sizeof(#name) - 1, value},
+        JS_FOR_EACH_ZEAL_MODE(ZEAL_MODE)
+#undef ZEAL_MODE
+        {"None", 4, 0}
+    };
+
+    do {
+        int zeal = -1;
+
+        const char* p = nullptr;
+        if (isdigit(str[0])) {
+            zeal = atoi(str);
+
+            size_t offset = strspn(str, "0123456789");
+            p = str + offset;
+        } else {
+            for (auto z : zealModes) {
+                if (!strncmp(str, z.zealMode, z.length)) {
+                    zeal = z.zeal;
+                    p = str + z.length;
+                    break;
+                }
+            }
+        }
+        if (p) {
+            if (!*p || *p == ';') {
+                frequency = JS_DEFAULT_ZEAL_FREQ;
+            } else if (*p == ',') {
+                frequency = atoi(p + 1);
+                foundFrequency = true;
+            }
+        }
+
+        if (zeal < 0 || zeal > int(ZealMode::Limit) || frequency <= 0) {
+            fprintf(stderr, "Format: JS_GC_ZEAL=level(;level)*[,N]\n");
+            fputs(ZealModeHelpText, stderr);
+            return false;
+        }
+
+        if (!zeals.emplaceBack(zeal)) {
+            return false;
+        }
+    } while (!foundFrequency &&
+             (str = strchr(str, ';')) != nullptr &&
+             str++);
+
+    for (auto z : zeals)
+        setZeal(z, frequency);
+    return true;
+}
+
+#endif
+
+/*
+ * Lifetime in number of major GCs for type sets attached to scripts containing
+ * observed types.
+ */
+static const uint64_t JIT_SCRIPT_RELEASE_TYPES_PERIOD = 20;
+
+bool
+GCRuntime::init(uint32_t maxbytes, uint32_t maxNurseryBytes)
+{
+    InitMemorySubsystem();
+
+    if (!rootsHash.init(256))
+        return false;
+
+    {
+        AutoLockGC lock(rt);
+
+        /*
+         * Separate gcMaxMallocBytes from gcMaxBytes but initialize to maxbytes
+         * for default backward API compatibility.
+         */
+        MOZ_ALWAYS_TRUE(tunables.setParameter(JSGC_MAX_BYTES, maxbytes, lock));
+        setMaxMallocBytes(maxbytes);
+
+        const char* size = getenv("JSGC_MARK_STACK_LIMIT");
+        if (size)
+            setMarkStackLimit(atoi(size), lock);
+
+        jitReleaseNumber = majorGCNumber + JIT_SCRIPT_RELEASE_TYPES_PERIOD;
+
+        if (!nursery.init(maxNurseryBytes, lock))
+            return false;
+
+        if (!nursery.isEnabled()) {
+            MOZ_ASSERT(nursery.nurserySize() == 0);
+            ++rt->gc.generationalDisabled;
+        } else {
+            MOZ_ASSERT(nursery.nurserySize() > 0);
+        }
+    }
+
+#ifdef JS_GC_ZEAL
+    const char* zealSpec = getenv("JS_GC_ZEAL");
+    if (zealSpec && zealSpec[0] && !parseAndSetZeal(zealSpec))
+        return false;
+#endif
+
+    if (!InitTrace(*this))
+        return false;
+
+    if (!marker.init(mode))
+        return false;
+
+    return true;
+}
+
+void
+GCRuntime::finish()
+{
+    /* Wait for the nursery sweeping to end. */
+    if (nursery.isEnabled())
+        nursery.waitBackgroundFreeEnd();
+
+    /*
+     * Wait until the background finalization and allocation stops and the
+     * helper thread shuts down before we forcefully release any remaining GC
+     * memory.
+     */
+    helperState.finish();
+    allocTask.cancel(GCParallelTask::CancelAndWait);
+    decommitTask.cancel(GCParallelTask::CancelAndWait);
+
+#ifdef JS_GC_ZEAL
+    /* Free memory associated with GC verification. */
+    finishVerifier();
+#endif
+
+    /* Delete all remaining zones. */
+    if (rt->gcInitialized) {
+        AutoSetThreadIsSweeping threadIsSweeping;
+        for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next()) {
+            for (CompartmentsInZoneIter comp(zone); !comp.done(); comp.next())
+                js_delete(comp.get());
+            js_delete(zone.get());
+        }
+    }
+
+    zones.clear();
+
+    FreeChunkPool(rt, fullChunks_);
+    FreeChunkPool(rt, availableChunks_);
+    FreeChunkPool(rt, emptyChunks_);
+
+    FinishTrace();
+
+    nursery.printTotalProfileTimes();
+    stats.printTotalProfileTimes();
+}
+
+bool
+GCRuntime::setParameter(JSGCParamKey key, uint32_t value, AutoLockGC& lock)
+{
+    switch (key) {
+      case JSGC_MAX_MALLOC_BYTES:
+        setMaxMallocBytes(value);
+        for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next())
+            zone->setGCMaxMallocBytes(maxMallocBytesAllocated() * 0.9);
+        break;
+      case JSGC_SLICE_TIME_BUDGET:
+        defaultTimeBudget_ = value ? value : SliceBudget::UnlimitedTimeBudget;
+        break;
+      case JSGC_MARK_STACK_LIMIT:
+        if (value == 0)
+            return false;
+        setMarkStackLimit(value, lock);
+        break;
+      case JSGC_MODE:
+        if (mode != JSGC_MODE_GLOBAL &&
+            mode != JSGC_MODE_ZONE &&
+            mode != JSGC_MODE_INCREMENTAL)
+        {
+            return false;
+        }
+        mode = JSGCMode(value);
+        break;
+      case JSGC_COMPACTING_ENABLED:
+        compactingEnabled = value != 0;
+        break;
+      default:
+        if (!tunables.setParameter(key, value, lock))
+            return false;
+        for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next()) {
+            zone->threshold.updateAfterGC(zone->usage.gcBytes(), GC_NORMAL, tunables,
+                                          schedulingState, lock);
+        }
+    }
+
+    return true;
+}
+
+bool
+GCSchedulingTunables::setParameter(JSGCParamKey key, uint32_t value, const AutoLockGC& lock)
+{
+    // Limit heap growth factor to one hundred times size of current heap.
+    const double MaxHeapGrowthFactor = 100;
+
+    switch(key) {
+      case JSGC_MAX_BYTES:
+        gcMaxBytes_ = value;
+        break;
+      case JSGC_HIGH_FREQUENCY_TIME_LIMIT:
+        highFrequencyThresholdUsec_ = value * PRMJ_USEC_PER_MSEC;
+        break;
+      case JSGC_HIGH_FREQUENCY_LOW_LIMIT: {
+        uint64_t newLimit = (uint64_t)value * 1024 * 1024;
+        if (newLimit == UINT64_MAX)
+            return false;
+        highFrequencyLowLimitBytes_ = newLimit;
+        if (highFrequencyLowLimitBytes_ >= highFrequencyHighLimitBytes_)
+            highFrequencyHighLimitBytes_ = highFrequencyLowLimitBytes_ + 1;
+        MOZ_ASSERT(highFrequencyHighLimitBytes_ > highFrequencyLowLimitBytes_);
+        break;
+      }
+      case JSGC_HIGH_FREQUENCY_HIGH_LIMIT: {
+        uint64_t newLimit = (uint64_t)value * 1024 * 1024;
+        if (newLimit == 0)
+            return false;
+        highFrequencyHighLimitBytes_ = newLimit;
+        if (highFrequencyHighLimitBytes_ <= highFrequencyLowLimitBytes_)
+            highFrequencyLowLimitBytes_ = highFrequencyHighLimitBytes_ - 1;
+        MOZ_ASSERT(highFrequencyHighLimitBytes_ > highFrequencyLowLimitBytes_);
+        break;
+      }
+      case JSGC_HIGH_FREQUENCY_HEAP_GROWTH_MAX: {
+        double newGrowth = value / 100.0;
+        if (newGrowth <= 0.85 || newGrowth > MaxHeapGrowthFactor)
+            return false;
+        highFrequencyHeapGrowthMax_ = newGrowth;
+        MOZ_ASSERT(highFrequencyHeapGrowthMax_ / 0.85 > 1.0);
+        break;
+      }
+      case JSGC_HIGH_FREQUENCY_HEAP_GROWTH_MIN: {
+        double newGrowth = value / 100.0;
+        if (newGrowth <= 0.85 || newGrowth > MaxHeapGrowthFactor)
+            return false;
+        highFrequencyHeapGrowthMin_ = newGrowth;
+        MOZ_ASSERT(highFrequencyHeapGrowthMin_ / 0.85 > 1.0);
+        break;
+      }
+      case JSGC_LOW_FREQUENCY_HEAP_GROWTH: {
+        double newGrowth = value / 100.0;
+        if (newGrowth <= 0.9 || newGrowth > MaxHeapGrowthFactor)
+            return false;
+        lowFrequencyHeapGrowth_ = newGrowth;
+        MOZ_ASSERT(lowFrequencyHeapGrowth_ / 0.9 > 1.0);
+        break;
+      }
+      case JSGC_DYNAMIC_HEAP_GROWTH:
+        dynamicHeapGrowthEnabled_ = value != 0;
+        break;
+      case JSGC_DYNAMIC_MARK_SLICE:
+        dynamicMarkSliceEnabled_ = value != 0;
+        break;
+      case JSGC_ALLOCATION_THRESHOLD:
+        gcZoneAllocThresholdBase_ = value * 1024 * 1024;
+        break;
+      case JSGC_MIN_EMPTY_CHUNK_COUNT:
+        minEmptyChunkCount_ = value;
+        if (minEmptyChunkCount_ > maxEmptyChunkCount_)
+            maxEmptyChunkCount_ = minEmptyChunkCount_;
+        MOZ_ASSERT(maxEmptyChunkCount_ >= minEmptyChunkCount_);
+        break;
+      case JSGC_MAX_EMPTY_CHUNK_COUNT:
+        maxEmptyChunkCount_ = value;
+        if (minEmptyChunkCount_ > maxEmptyChunkCount_)
+            minEmptyChunkCount_ = maxEmptyChunkCount_;
+        MOZ_ASSERT(maxEmptyChunkCount_ >= minEmptyChunkCount_);
+        break;
+      case JSGC_REFRESH_FRAME_SLICES_ENABLED:
+        refreshFrameSlicesEnabled_ = value != 0;
+        break;
+      default:
+        MOZ_CRASH("Unknown GC parameter.");
+    }
+
+    return true;
+}
+
+uint32_t
+GCRuntime::getParameter(JSGCParamKey key, const AutoLockGC& lock)
+{
+    switch (key) {
+      case JSGC_MAX_BYTES:
+        return uint32_t(tunables.gcMaxBytes());
+      case JSGC_MAX_MALLOC_BYTES:
+        return maxMallocBytes;
+      case JSGC_BYTES:
+        return uint32_t(usage.gcBytes());
+      case JSGC_MODE:
+        return uint32_t(mode);
+      case JSGC_UNUSED_CHUNKS:
+        return uint32_t(emptyChunks(lock).count());
+      case JSGC_TOTAL_CHUNKS:
+        return uint32_t(fullChunks(lock).count() +
+                        availableChunks(lock).count() +
+                        emptyChunks(lock).count());
+      case JSGC_SLICE_TIME_BUDGET:
+        if (defaultTimeBudget_ == SliceBudget::UnlimitedTimeBudget) {
+            return 0;
+        } else {
+            MOZ_RELEASE_ASSERT(defaultTimeBudget_ >= 0);
+            MOZ_RELEASE_ASSERT(defaultTimeBudget_ <= UINT32_MAX);
+            return uint32_t(defaultTimeBudget_);
+        }
+      case JSGC_MARK_STACK_LIMIT:
+        return marker.maxCapacity();
+      case JSGC_HIGH_FREQUENCY_TIME_LIMIT:
+        return tunables.highFrequencyThresholdUsec() / PRMJ_USEC_PER_MSEC;
+      case JSGC_HIGH_FREQUENCY_LOW_LIMIT:
+        return tunables.highFrequencyLowLimitBytes() / 1024 / 1024;
+      case JSGC_HIGH_FREQUENCY_HIGH_LIMIT:
+        return tunables.highFrequencyHighLimitBytes() / 1024 / 1024;
+      case JSGC_HIGH_FREQUENCY_HEAP_GROWTH_MAX:
+        return uint32_t(tunables.highFrequencyHeapGrowthMax() * 100);
+      case JSGC_HIGH_FREQUENCY_HEAP_GROWTH_MIN:
+        return uint32_t(tunables.highFrequencyHeapGrowthMin() * 100);
+      case JSGC_LOW_FREQUENCY_HEAP_GROWTH:
+        return uint32_t(tunables.lowFrequencyHeapGrowth() * 100);
+      case JSGC_DYNAMIC_HEAP_GROWTH:
+        return tunables.isDynamicHeapGrowthEnabled();
+      case JSGC_DYNAMIC_MARK_SLICE:
+        return tunables.isDynamicMarkSliceEnabled();
+      case JSGC_ALLOCATION_THRESHOLD:
+        return tunables.gcZoneAllocThresholdBase() / 1024 / 1024;
+      case JSGC_MIN_EMPTY_CHUNK_COUNT:
+        return tunables.minEmptyChunkCount(lock);
+      case JSGC_MAX_EMPTY_CHUNK_COUNT:
+        return tunables.maxEmptyChunkCount();
+      case JSGC_COMPACTING_ENABLED:
+        return compactingEnabled;
+      case JSGC_REFRESH_FRAME_SLICES_ENABLED:
+        return tunables.areRefreshFrameSlicesEnabled();
+      default:
+        MOZ_ASSERT(key == JSGC_NUMBER);
+        return uint32_t(number);
+    }
+}
+
+void
+GCRuntime::setMarkStackLimit(size_t limit, AutoLockGC& lock)
+{
+    MOZ_ASSERT(!rt->isHeapBusy());
+    AutoUnlockGC unlock(lock);
+    AutoStopVerifyingBarriers pauseVerification(rt, false);
+    marker.setMaxCapacity(limit);
+}
+
+bool
+GCRuntime::addBlackRootsTracer(JSTraceDataOp traceOp, void* data)
+{
+    AssertHeapIsIdle(rt);
+    return !!blackRootTracers.append(Callback<JSTraceDataOp>(traceOp, data));
+}
+
+void
+GCRuntime::removeBlackRootsTracer(JSTraceDataOp traceOp, void* data)
+{
+    // Can be called from finalizers
+    for (size_t i = 0; i < blackRootTracers.length(); i++) {
+        Callback<JSTraceDataOp>* e = &blackRootTracers[i];
+        if (e->op == traceOp && e->data == data) {
+            blackRootTracers.erase(e);
+        }
+    }
+}
+
+void
+GCRuntime::setGrayRootsTracer(JSTraceDataOp traceOp, void* data)
+{
+    AssertHeapIsIdle(rt);
+    grayRootTracer.op = traceOp;
+    grayRootTracer.data = data;
+}
+
+void
+GCRuntime::setGCCallback(JSGCCallback callback, void* data)
+{
+    gcCallback.op = callback;
+    gcCallback.data = data;
+}
+
+void
+GCRuntime::callGCCallback(JSGCStatus status) const
+{
+    if (gcCallback.op)
+        gcCallback.op(rt->contextFromMainThread(), status, gcCallback.data);
+}
+
+void
+GCRuntime::setObjectsTenuredCallback(JSObjectsTenuredCallback callback,
+                                     void* data)
+{
+    tenuredCallback.op = callback;
+    tenuredCallback.data = data;
+}
+
+void
+GCRuntime::callObjectsTenuredCallback()
+{
+    if (tenuredCallback.op)
+        tenuredCallback.op(rt->contextFromMainThread(), tenuredCallback.data);
+}
+
+namespace {
+
+class AutoNotifyGCActivity {
+  public:
+    explicit AutoNotifyGCActivity(GCRuntime& gc) : gc_(gc) {
+        if (!gc_.isIncrementalGCInProgress()) {
+            gcstats::AutoPhase ap(gc_.stats, gcstats::PHASE_GC_BEGIN);
+            gc_.callGCCallback(JSGC_BEGIN);
+        }
+    }
+    ~AutoNotifyGCActivity() {
+        if (!gc_.isIncrementalGCInProgress()) {
+            gcstats::AutoPhase ap(gc_.stats, gcstats::PHASE_GC_END);
+            gc_.callGCCallback(JSGC_END);
+        }
+    }
+
+  private:
+    GCRuntime& gc_;
+};
+
+} // (anon)
+
+bool
+GCRuntime::addFinalizeCallback(JSFinalizeCallback callback, void* data)
+{
+    return finalizeCallbacks.append(Callback<JSFinalizeCallback>(callback, data));
+}
+
+void
+GCRuntime::removeFinalizeCallback(JSFinalizeCallback callback)
+{
+    for (Callback<JSFinalizeCallback>* p = finalizeCallbacks.begin();
+         p < finalizeCallbacks.end(); p++)
+    {
+        if (p->op == callback) {
+            finalizeCallbacks.erase(p);
+            break;
+        }
+    }
+}
+
+void
+GCRuntime::callFinalizeCallbacks(FreeOp* fop, JSFinalizeStatus status) const
+{
+    for (auto& p : finalizeCallbacks)
+        p.op(fop, status, !isFull, p.data);
+}
+
+bool
+GCRuntime::addWeakPointerZoneGroupCallback(JSWeakPointerZoneGroupCallback callback, void* data)
+{
+    return updateWeakPointerZoneGroupCallbacks.append(
+            Callback<JSWeakPointerZoneGroupCallback>(callback, data));
+}
+
+void
+GCRuntime::removeWeakPointerZoneGroupCallback(JSWeakPointerZoneGroupCallback callback)
+{
+    for (auto& p : updateWeakPointerZoneGroupCallbacks) {
+        if (p.op == callback) {
+            updateWeakPointerZoneGroupCallbacks.erase(&p);
+            break;
+        }
+    }
+}
+
+void
+GCRuntime::callWeakPointerZoneGroupCallbacks() const
+{
+    for (auto const& p : updateWeakPointerZoneGroupCallbacks)
+        p.op(rt->contextFromMainThread(), p.data);
+}
+
+bool
+GCRuntime::addWeakPointerCompartmentCallback(JSWeakPointerCompartmentCallback callback, void* data)
+{
+    return updateWeakPointerCompartmentCallbacks.append(
+            Callback<JSWeakPointerCompartmentCallback>(callback, data));
+}
+
+void
+GCRuntime::removeWeakPointerCompartmentCallback(JSWeakPointerCompartmentCallback callback)
+{
+    for (auto& p : updateWeakPointerCompartmentCallbacks) {
+        if (p.op == callback) {
+            updateWeakPointerCompartmentCallbacks.erase(&p);
+            break;
+        }
+    }
+}
+
+void
+GCRuntime::callWeakPointerCompartmentCallbacks(JSCompartment* comp) const
+{
+    for (auto const& p : updateWeakPointerCompartmentCallbacks)
+        p.op(rt->contextFromMainThread(), comp, p.data);
+}
+
+JS::GCSliceCallback
+GCRuntime::setSliceCallback(JS::GCSliceCallback callback) {
+    return stats.setSliceCallback(callback);
+}
+
+JS::GCNurseryCollectionCallback
+GCRuntime::setNurseryCollectionCallback(JS::GCNurseryCollectionCallback callback) {
+    return stats.setNurseryCollectionCallback(callback);
+}
+
+JS::DoCycleCollectionCallback
+GCRuntime::setDoCycleCollectionCallback(JS::DoCycleCollectionCallback callback)
+{
+    auto prior = gcDoCycleCollectionCallback;
+    gcDoCycleCollectionCallback = Callback<JS::DoCycleCollectionCallback>(callback, nullptr);
+    return prior.op;
+}
+
+void
+GCRuntime::callDoCycleCollectionCallback(JSContext* cx)
+{
+    if (gcDoCycleCollectionCallback.op)
+        gcDoCycleCollectionCallback.op(cx);
+}
+
+bool
+GCRuntime::addRoot(Value* vp, const char* name)
+{
+    /*
+     * Sometimes Firefox will hold weak references to objects and then convert
+     * them to strong references by calling AddRoot (e.g., via PreserveWrapper,
+     * or ModifyBusyCount in workers). We need a read barrier to cover these
+     * cases.
+     */
+    if (isIncrementalGCInProgress())
+        GCPtrValue::writeBarrierPre(*vp);
+
+    return rootsHash.put(vp, name);
+}
+
+void
+GCRuntime::removeRoot(Value* vp)
+{
+    rootsHash.remove(vp);
+    poke();
+}
+
+extern JS_FRIEND_API(bool)
+js::AddRawValueRoot(JSContext* cx, Value* vp, const char* name)
+{
+    MOZ_ASSERT(vp);
+    MOZ_ASSERT(name);
+    bool ok = cx->runtime()->gc.addRoot(vp, name);
+    if (!ok)
+        JS_ReportOutOfMemory(cx);
+    return ok;
+}
+
+extern JS_FRIEND_API(void)
+js::RemoveRawValueRoot(JSContext* cx, Value* vp)
+{
+    cx->runtime()->gc.removeRoot(vp);
+}
+
+void
+GCRuntime::setMaxMallocBytes(size_t value)
+{
+    /*
+     * For compatibility treat any value that exceeds PTRDIFF_T_MAX to
+     * mean that value.
+     */
+    maxMallocBytes = (ptrdiff_t(value) >= 0) ? value : size_t(-1) >> 1;
+    resetMallocBytes();
+    for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next())
+        zone->setGCMaxMallocBytes(value);
+}
+
+void
+GCRuntime::resetMallocBytes()
+{
+    mallocBytesUntilGC = ptrdiff_t(maxMallocBytes);
+    mallocGCTriggered = false;
+}
+
+void
+GCRuntime::updateMallocCounter(JS::Zone* zone, size_t nbytes)
+{
+    mallocBytesUntilGC -= ptrdiff_t(nbytes);
+    if (MOZ_UNLIKELY(isTooMuchMalloc()))
+        onTooMuchMalloc();
+    else if (zone)
+        zone->updateMallocCounter(nbytes);
+}
+
+void
+GCRuntime::onTooMuchMalloc()
+{
+    if (!mallocGCTriggered)
+        mallocGCTriggered = triggerGC(JS::gcreason::TOO_MUCH_MALLOC);
+}
+
+double
+ZoneHeapThreshold::allocTrigger(bool highFrequencyGC) const
+{
+    return (highFrequencyGC ? 0.85 : 0.9) * gcTriggerBytes();
+}
+
+/* static */ double
+ZoneHeapThreshold::computeZoneHeapGrowthFactorForHeapSize(size_t lastBytes,
+                                                          const GCSchedulingTunables& tunables,
+                                                          const GCSchedulingState& state)
+{
+    if (!tunables.isDynamicHeapGrowthEnabled())
+        return 3.0;
+
+    // For small zones, our collection heuristics do not matter much: favor
+    // something simple in this case.
+    if (lastBytes < 1 * 1024 * 1024)
+        return tunables.lowFrequencyHeapGrowth();
+
+    // If GC's are not triggering in rapid succession, use a lower threshold so
+    // that we will collect garbage sooner.
+    if (!state.inHighFrequencyGCMode())
+        return tunables.lowFrequencyHeapGrowth();
+
+    // The heap growth factor depends on the heap size after a GC and the GC
+    // frequency. For low frequency GCs (more than 1sec between GCs) we let
+    // the heap grow to 150%. For high frequency GCs we let the heap grow
+    // depending on the heap size:
+    //   lastBytes < highFrequencyLowLimit: 300%
+    //   lastBytes > highFrequencyHighLimit: 150%
+    //   otherwise: linear interpolation between 300% and 150% based on lastBytes
+
+    // Use shorter names to make the operation comprehensible.
+    double minRatio = tunables.highFrequencyHeapGrowthMin();
+    double maxRatio = tunables.highFrequencyHeapGrowthMax();
+    double lowLimit = tunables.highFrequencyLowLimitBytes();
+    double highLimit = tunables.highFrequencyHighLimitBytes();
+
+    if (lastBytes <= lowLimit)
+        return maxRatio;
+
+    if (lastBytes >= highLimit)
+        return minRatio;
+
+    double factor = maxRatio - ((maxRatio - minRatio) * ((lastBytes - lowLimit) /
+                                                         (highLimit - lowLimit)));
+    MOZ_ASSERT(factor >= minRatio);
+    MOZ_ASSERT(factor <= maxRatio);
+    return factor;
+}
+
+/* static */ size_t
+ZoneHeapThreshold::computeZoneTriggerBytes(double growthFactor, size_t lastBytes,
+                                           JSGCInvocationKind gckind,
+                                           const GCSchedulingTunables& tunables,
+                                           const AutoLockGC& lock)
+{
+    size_t base = gckind == GC_SHRINK
+                ? Max(lastBytes, tunables.minEmptyChunkCount(lock) * ChunkSize)
+                : Max(lastBytes, tunables.gcZoneAllocThresholdBase());
+    double trigger = double(base) * growthFactor;
+    return size_t(Min(double(tunables.gcMaxBytes()), trigger));
+}
+
+void
+ZoneHeapThreshold::updateAfterGC(size_t lastBytes, JSGCInvocationKind gckind,
+                                 const GCSchedulingTunables& tunables,
+                                 const GCSchedulingState& state, const AutoLockGC& lock)
+{
+    gcHeapGrowthFactor_ = computeZoneHeapGrowthFactorForHeapSize(lastBytes, tunables, state);
+    gcTriggerBytes_ = computeZoneTriggerBytes(gcHeapGrowthFactor_, lastBytes, gckind, tunables,
+                                              lock);
+}
+
+void
+ZoneHeapThreshold::updateForRemovedArena(const GCSchedulingTunables& tunables)
+{
+    size_t amount = ArenaSize * gcHeapGrowthFactor_;
+    MOZ_ASSERT(amount > 0);
+
+    if ((gcTriggerBytes_ < amount) ||
+        (gcTriggerBytes_ - amount < tunables.gcZoneAllocThresholdBase() * gcHeapGrowthFactor_))
+    {
+        return;
+    }
+
+    gcTriggerBytes_ -= amount;
+}
+
+void
+GCMarker::delayMarkingArena(Arena* arena)
+{
+    if (arena->hasDelayedMarking) {
+        /* Arena already scheduled to be marked later */
+        return;
+    }
+    arena->setNextDelayedMarking(unmarkedArenaStackTop);
+    unmarkedArenaStackTop = arena;
+#ifdef DEBUG
+    markLaterArenas++;
+#endif
+}
+
+void
+GCMarker::delayMarkingChildren(const void* thing)
+{
+    const TenuredCell* cell = TenuredCell::fromPointer(thing);
+    cell->arena()->markOverflow = 1;
+    delayMarkingArena(cell->arena());
+}
+
+inline void
+ArenaLists::prepareForIncrementalGC(JSRuntime* rt)
+{
+    for (auto i : AllAllocKinds()) {
+        FreeSpan* span = freeLists[i];
+        if (span != &placeholder) {
+            if (!span->isEmpty()) {
+                Arena* arena = span->getArena();
+                arena->allocatedDuringIncremental = true;
+                rt->gc.marker.delayMarkingArena(arena);
+            } else {
+                freeLists[i] = &placeholder;
+            }
+        }
+    }
+}
+
+/* Compacting GC */
+
+bool
+GCRuntime::shouldCompact()
+{
+    // Compact on shrinking GC if enabled, but skip compacting in incremental
+    // GCs if we are currently animating.
+    return invocationKind == GC_SHRINK && isCompactingGCEnabled() &&
+        (!isIncremental || rt->lastAnimationTime + PRMJ_USEC_PER_SEC < PRMJ_Now());
+}
+
+void
+GCRuntime::disableCompactingGC()
+{
+    MOZ_ASSERT(CurrentThreadCanAccessRuntime(rt));
+    ++compactingDisabledCount;
+}
+
+void
+GCRuntime::enableCompactingGC()
+{
+    MOZ_ASSERT(CurrentThreadCanAccessRuntime(rt));
+    MOZ_ASSERT(compactingDisabledCount > 0);
+    --compactingDisabledCount;
+}
+
+bool
+GCRuntime::isCompactingGCEnabled() const
+{
+    MOZ_ASSERT(CurrentThreadCanAccessRuntime(rt));
+    return compactingEnabled && compactingDisabledCount == 0;
+}
+
+AutoDisableCompactingGC::AutoDisableCompactingGC(JSContext* cx)
+  : gc(cx->gc)
+{
+    gc.disableCompactingGC();
+    if (gc.isIncrementalGCInProgress() && gc.isCompactingGc())
+        FinishGC(cx);
+}
+
+AutoDisableCompactingGC::~AutoDisableCompactingGC()
+{
+    gc.enableCompactingGC();
+}
+
+static bool
+CanRelocateZone(Zone* zone)
+{
+    return !zone->isAtomsZone() && !zone->isSelfHostingZone();
+}
+
+static const AllocKind AllocKindsToRelocate[] = {
+    AllocKind::FUNCTION,
+    AllocKind::FUNCTION_EXTENDED,
+    AllocKind::OBJECT0,
+    AllocKind::OBJECT0_BACKGROUND,
+    AllocKind::OBJECT2,
+    AllocKind::OBJECT2_BACKGROUND,
+    AllocKind::OBJECT4,
+    AllocKind::OBJECT4_BACKGROUND,
+    AllocKind::OBJECT8,
+    AllocKind::OBJECT8_BACKGROUND,
+    AllocKind::OBJECT12,
+    AllocKind::OBJECT12_BACKGROUND,
+    AllocKind::OBJECT16,
+    AllocKind::OBJECT16_BACKGROUND,
+    AllocKind::SCRIPT,
+    AllocKind::LAZY_SCRIPT,
+    AllocKind::SCOPE,
+    AllocKind::SHAPE,
+    AllocKind::ACCESSOR_SHAPE,
+    AllocKind::BASE_SHAPE,
+    AllocKind::FAT_INLINE_STRING,
+    AllocKind::STRING,
+    AllocKind::EXTERNAL_STRING,
+    AllocKind::FAT_INLINE_ATOM,
+    AllocKind::ATOM
+};
+
+Arena*
+ArenaList::removeRemainingArenas(Arena** arenap)
+{
+    // This is only ever called to remove arenas that are after the cursor, so
+    // we don't need to update it.
+#ifdef DEBUG
+    for (Arena* arena = *arenap; arena; arena = arena->next)
+        MOZ_ASSERT(cursorp_ != &arena->next);
+#endif
+    Arena* remainingArenas = *arenap;
+    *arenap = nullptr;
+    check();
+    return remainingArenas;
+}
+
+static bool
+ShouldRelocateAllArenas(JS::gcreason::Reason reason)
+{
+    return reason == JS::gcreason::DEBUG_GC;
+}
+
+/*
+ * Choose which arenas to relocate all cells from. Return an arena cursor that
+ * can be passed to removeRemainingArenas().
+ */
+Arena**
+ArenaList::pickArenasToRelocate(size_t& arenaTotalOut, size_t& relocTotalOut)
+{
+    // Relocate the greatest number of arenas such that the number of used cells
+    // in relocated arenas is less than or equal to the number of free cells in
+    // unrelocated arenas. In other words we only relocate cells we can move
+    // into existing arenas, and we choose the least full areans to relocate.
+    //
+    // This is made easier by the fact that the arena list has been sorted in
+    // descending order of number of used cells, so we will always relocate a
+    // tail of the arena list. All we need to do is find the point at which to
+    // start relocating.
+
+    check();
+
+    if (isCursorAtEnd())
+        return nullptr;
+
+    Arena** arenap = cursorp_;     // Next arena to consider for relocation.
+    size_t previousFreeCells = 0;  // Count of free cells before arenap.
+    size_t followingUsedCells = 0; // Count of used cells after arenap.
+    size_t fullArenaCount = 0;     // Number of full arenas (not relocated).
+    size_t nonFullArenaCount = 0;  // Number of non-full arenas (considered for relocation).
+    size_t arenaIndex = 0;         // Index of the next arena to consider.
+
+    for (Arena* arena = head_; arena != *cursorp_; arena = arena->next)
+        fullArenaCount++;
+
+    for (Arena* arena = *cursorp_; arena; arena = arena->next) {
+        followingUsedCells += arena->countUsedCells();
+        nonFullArenaCount++;
+    }
+
+    mozilla::DebugOnly<size_t> lastFreeCells(0);
+    size_t cellsPerArena = Arena::thingsPerArena((*arenap)->getAllocKind());
+
+    while (*arenap) {
+        Arena* arena = *arenap;
+        if (followingUsedCells <= previousFreeCells)
+            break;
+
+        size_t freeCells = arena->countFreeCells();
+        size_t usedCells = cellsPerArena - freeCells;
+        followingUsedCells -= usedCells;
+#ifdef DEBUG
+        MOZ_ASSERT(freeCells >= lastFreeCells);
+        lastFreeCells = freeCells;
+#endif
+        previousFreeCells += freeCells;
+        arenap = &arena->next;
+        arenaIndex++;
+    }
+
+    size_t relocCount = nonFullArenaCount - arenaIndex;
+    MOZ_ASSERT(relocCount < nonFullArenaCount);
+    MOZ_ASSERT((relocCount == 0) == (!*arenap));
+    arenaTotalOut += fullArenaCount + nonFullArenaCount;
+    relocTotalOut += relocCount;
+
+    return arenap;
+}
+
+#ifdef DEBUG
+inline bool
+PtrIsInRange(const void* ptr, const void* start, size_t length)
+{
+    return uintptr_t(ptr) - uintptr_t(start) < length;
+}
+#endif
+
+static TenuredCell*
+AllocRelocatedCell(Zone* zone, AllocKind thingKind, size_t thingSize)
+{
+    AutoEnterOOMUnsafeRegion oomUnsafe;
+    void* dstAlloc = zone->arenas.allocateFromFreeList(thingKind, thingSize);
+    if (!dstAlloc)
+        dstAlloc = GCRuntime::refillFreeListInGC(zone, thingKind);
+    if (!dstAlloc) {
+        // This can only happen in zeal mode or debug builds as we don't
+        // otherwise relocate more cells than we have existing free space
+        // for.
+        oomUnsafe.crash("Could not allocate new arena while compacting");
+    }
+    return TenuredCell::fromPointer(dstAlloc);
+}
+
+static void
+RelocateCell(Zone* zone, TenuredCell* src, AllocKind thingKind, size_t thingSize)
+{
+    JS::AutoSuppressGCAnalysis nogc(zone->contextFromMainThread());
+
+    // Allocate a new cell.
+    MOZ_ASSERT(zone == src->zone());
+    TenuredCell* dst = AllocRelocatedCell(zone, thingKind, thingSize);
+
+    // Copy source cell contents to destination.
+    memcpy(dst, src, thingSize);
+
+    // Move any uid attached to the object.
+    src->zone()->transferUniqueId(dst, src);
+
+    if (IsObjectAllocKind(thingKind)) {
+        JSObject* srcObj = static_cast<JSObject*>(static_cast<Cell*>(src));
+        JSObject* dstObj = static_cast<JSObject*>(static_cast<Cell*>(dst));
+
+        if (srcObj->isNative()) {
+            NativeObject* srcNative = &srcObj->as<NativeObject>();
+            NativeObject* dstNative = &dstObj->as<NativeObject>();
+
+            // Fixup the pointer to inline object elements if necessary.
+            if (srcNative->hasFixedElements())
+                dstNative->setFixedElements();
+
+            // For copy-on-write objects that own their elements, fix up the
+            // owner pointer to point to the relocated object.
+            if (srcNative->denseElementsAreCopyOnWrite()) {
+                GCPtrNativeObject& owner = dstNative->getElementsHeader()->ownerObject();
+                if (owner == srcNative)
+                    owner = dstNative;
+            }
+        }
+
+        // Call object moved hook if present.
+        if (JSObjectMovedOp op = srcObj->getClass()->extObjectMovedOp())
+            op(dstObj, srcObj);
+
+        MOZ_ASSERT_IF(dstObj->isNative(),
+                      !PtrIsInRange((const Value*)dstObj->as<NativeObject>().getDenseElements(),
+                                    src, thingSize));
+    }
+
+    // Copy the mark bits.
+    dst->copyMarkBitsFrom(src);
+
+    // Mark source cell as forwarded and leave a pointer to the destination.
+    RelocationOverlay* overlay = RelocationOverlay::fromCell(src);
+    overlay->forwardTo(dst);
+}
+
+static void
+RelocateArena(Arena* arena, SliceBudget& sliceBudget)
+{
+    MOZ_ASSERT(arena->allocated());
+    MOZ_ASSERT(!arena->hasDelayedMarking);
+    MOZ_ASSERT(!arena->markOverflow);
+    MOZ_ASSERT(!arena->allocatedDuringIncremental);
+    MOZ_ASSERT(arena->bufferedCells->isEmpty());
+
+    Zone* zone = arena->zone;
+
+    AllocKind thingKind = arena->getAllocKind();
+    size_t thingSize = arena->getThingSize();
+
+    for (ArenaCellIterUnderGC i(arena); !i.done(); i.next()) {
+        RelocateCell(zone, i.getCell(), thingKind, thingSize);
+        sliceBudget.step();
+    }
+
+#ifdef DEBUG
+    for (ArenaCellIterUnderGC i(arena); !i.done(); i.next()) {
+        TenuredCell* src = i.getCell();
+        MOZ_ASSERT(RelocationOverlay::isCellForwarded(src));
+        TenuredCell* dest = Forwarded(src);
+        MOZ_ASSERT(src->isMarked(BLACK) == dest->isMarked(BLACK));
+        MOZ_ASSERT(src->isMarked(GRAY) == dest->isMarked(GRAY));
+    }
+#endif
+}
+
+static inline bool
+ShouldProtectRelocatedArenas(JS::gcreason::Reason reason)
+{
+    // For zeal mode collections we don't release the relocated arenas
+    // immediately. Instead we protect them and keep them around until the next
+    // collection so we can catch any stray accesses to them.
+#ifdef DEBUG
+    return reason == JS::gcreason::DEBUG_GC;
+#else
+    return false;
+#endif
+}
+
+/*
+ * Relocate all arenas identified by pickArenasToRelocate: for each arena,
+ * relocate each cell within it, then add it to a list of relocated arenas.
+ */
+Arena*
+ArenaList::relocateArenas(Arena* toRelocate, Arena* relocated, SliceBudget& sliceBudget,
+                          gcstats::Statistics& stats)
+{
+    check();
+
+    while (Arena* arena = toRelocate) {
+        toRelocate = arena->next;
+        RelocateArena(arena, sliceBudget);
+        // Prepend to list of relocated arenas
+        arena->next = relocated;
+        relocated = arena;
+        stats.count(gcstats::STAT_ARENA_RELOCATED);
+    }
+
+    check();
+
+    return relocated;
+}
+
+// Skip compacting zones unless we can free a certain proportion of their GC
+// heap memory.
+static const double MIN_ZONE_RECLAIM_PERCENT = 2.0;
+
+static bool
+ShouldRelocateZone(size_t arenaCount, size_t relocCount, JS::gcreason::Reason reason)
+{
+    if (relocCount == 0)
+        return false;
+
+    if (IsOOMReason(reason))
+        return true;
+
+    return (relocCount * 100.0) / arenaCount >= MIN_ZONE_RECLAIM_PERCENT;
+}
+
+bool
+ArenaLists::relocateArenas(Zone* zone, Arena*& relocatedListOut, JS::gcreason::Reason reason,
+                           SliceBudget& sliceBudget, gcstats::Statistics& stats)
+{
+    // This is only called from the main thread while we are doing a GC, so
+    // there is no need to lock.
+    MOZ_ASSERT(CurrentThreadCanAccessRuntime(runtime_));
+    MOZ_ASSERT(runtime_->gc.isHeapCompacting());
+    MOZ_ASSERT(!runtime_->gc.isBackgroundSweeping());
+
+    // Clear all the free lists.
+    purge();
+
+    if (ShouldRelocateAllArenas(reason)) {
+        zone->prepareForCompacting();
+        for (auto kind : AllocKindsToRelocate) {
+            ArenaList& al = arenaLists[kind];
+            Arena* allArenas = al.head();
+            al.clear();
+            relocatedListOut = al.relocateArenas(allArenas, relocatedListOut, sliceBudget, stats);
+        }
+    } else {
+        size_t arenaCount = 0;
+        size_t relocCount = 0;
+        AllAllocKindArray<Arena**> toRelocate;
+
+        for (auto kind : AllocKindsToRelocate)
+            toRelocate[kind] = arenaLists[kind].pickArenasToRelocate(arenaCount, relocCount);
+
+        if (!ShouldRelocateZone(arenaCount, relocCount, reason))
+            return false;
+
+        zone->prepareForCompacting();
+        for (auto kind : AllocKindsToRelocate) {
+            if (toRelocate[kind]) {
+                ArenaList& al = arenaLists[kind];
+                Arena* arenas = al.removeRemainingArenas(toRelocate[kind]);
+                relocatedListOut = al.relocateArenas(arenas, relocatedListOut, sliceBudget, stats);
+            }
+        }
+    }
+
+    return true;
+}
+
+bool
+GCRuntime::relocateArenas(Zone* zone, JS::gcreason::Reason reason, Arena*& relocatedListOut,
+                          SliceBudget& sliceBudget)
+{
+    gcstats::AutoPhase ap(stats, gcstats::PHASE_COMPACT_MOVE);
+
+    MOZ_ASSERT(!zone->isPreservingCode());
+    MOZ_ASSERT(CanRelocateZone(zone));
+
+    js::CancelOffThreadIonCompile(rt, JS::Zone::Compact);
+
+    if (!zone->arenas.relocateArenas(zone, relocatedListOut, reason, sliceBudget, stats))
+        return false;
+
+#ifdef DEBUG
+    // Check that we did as much compaction as we should have. There
+    // should always be less than one arena's worth of free cells.
+    for (auto i : AllocKindsToRelocate) {
+        ArenaList& al = zone->arenas.arenaLists[i];
+        size_t freeCells = 0;
+        for (Arena* arena = al.arenaAfterCursor(); arena; arena = arena->next)
+            freeCells += arena->countFreeCells();
+        MOZ_ASSERT(freeCells < Arena::thingsPerArena(i));
+    }
+#endif
+
+    return true;
+}
+
+void
+MovingTracer::onObjectEdge(JSObject** objp)
+{
+    JSObject* obj = *objp;
+    if (obj->runtimeFromAnyThread() == runtime() && IsForwarded(obj))
+        *objp = Forwarded(obj);
+}
+
+void
+MovingTracer::onShapeEdge(Shape** shapep)
+{
+    Shape* shape = *shapep;
+    if (shape->runtimeFromAnyThread() == runtime() && IsForwarded(shape))
+        *shapep = Forwarded(shape);
+}
+
+void
+MovingTracer::onStringEdge(JSString** stringp)
+{
+    JSString* string = *stringp;
+    if (string->runtimeFromAnyThread() == runtime() && IsForwarded(string))
+        *stringp = Forwarded(string);
+}
+
+void
+MovingTracer::onScriptEdge(JSScript** scriptp)
+{
+    JSScript* script = *scriptp;
+    if (script->runtimeFromAnyThread() == runtime() && IsForwarded(script))
+        *scriptp = Forwarded(script);
+}
+
+void
+MovingTracer::onLazyScriptEdge(LazyScript** lazyp)
+{
+    LazyScript* lazy = *lazyp;
+    if (lazy->runtimeFromAnyThread() == runtime() && IsForwarded(lazy))
+        *lazyp = Forwarded(lazy);
+}
+
+void
+MovingTracer::onBaseShapeEdge(BaseShape** basep)
+{
+    BaseShape* base = *basep;
+    if (base->runtimeFromAnyThread() == runtime() && IsForwarded(base))
+        *basep = Forwarded(base);
+}
+
+void
+MovingTracer::onScopeEdge(Scope** scopep)
+{
+    Scope* scope = *scopep;
+    if (scope->runtimeFromAnyThread() == runtime() && IsForwarded(scope))
+        *scopep = Forwarded(scope);
+}
+
+void
+Zone::prepareForCompacting()
+{
+    FreeOp* fop = runtimeFromMainThread()->defaultFreeOp();
+    discardJitCode(fop);
+}
+
+void
+GCRuntime::sweepTypesAfterCompacting(Zone* zone)
+{
+    FreeOp* fop = rt->defaultFreeOp();
+    zone->beginSweepTypes(fop, rt->gc.releaseObservedTypes && !zone->isPreservingCode());
+
+    AutoClearTypeInferenceStateOnOOM oom(zone);
+
+    for (auto script = zone->cellIter<JSScript>(); !script.done(); script.next())
+        script->maybeSweepTypes(&oom);
+    for (auto group = zone->cellIter<ObjectGroup>(); !group.done(); group.next())
+        group->maybeSweep(&oom);
+
+    zone->types.endSweep(rt);
+}
+
+void
+GCRuntime::sweepZoneAfterCompacting(Zone* zone)
+{
+    MOZ_ASSERT(zone->isCollecting());
+    FreeOp* fop = rt->defaultFreeOp();
+    sweepTypesAfterCompacting(zone);
+    zone->sweepBreakpoints(fop);
+    zone->sweepWeakMaps();
+    for (auto* cache : zone->weakCaches_)
+        cache->sweep();
+
+    for (CompartmentsInZoneIter c(zone); !c.done(); c.next()) {
+        c->objectGroups.sweep(fop);
+        c->sweepRegExps();
+        c->sweepSavedStacks();
+        c->sweepGlobalObject(fop);
+        c->sweepSelfHostingScriptSource();
+        c->sweepDebugEnvironments();
+        c->sweepJitCompartment(fop);
+        c->sweepNativeIterators();
+        c->sweepTemplateObjects();
+    }
+}
+
+template <typename T>
+static inline void
+UpdateCellPointers(MovingTracer* trc, T* cell)
+{
+    cell->fixupAfterMovingGC();
+    cell->traceChildren(trc);
+}
+
+template <typename T>
+static void
+UpdateArenaPointersTyped(MovingTracer* trc, Arena* arena, JS::TraceKind traceKind)
+{
+    for (ArenaCellIterUnderGC i(arena); !i.done(); i.next())
+        UpdateCellPointers(trc, reinterpret_cast<T*>(i.getCell()));
+}
+
+/*
+ * Update the internal pointers for all cells in an arena.
+ */
+static void
+UpdateArenaPointers(MovingTracer* trc, Arena* arena)
+{
+    AllocKind kind = arena->getAllocKind();
+
+    switch (kind) {
+#define EXPAND_CASE(allocKind, traceKind, type, sizedType) \
+      case AllocKind::allocKind: \
+        UpdateArenaPointersTyped<type>(trc, arena, JS::TraceKind::traceKind); \
+        return;
+FOR_EACH_ALLOCKIND(EXPAND_CASE)
+#undef EXPAND_CASE
+
+      default:
+        MOZ_CRASH("Invalid alloc kind for UpdateArenaPointers");
+    }
+}
+
+namespace js {
+namespace gc {
+
+struct ArenaListSegment
+{
+    Arena* begin;
+    Arena* end;
+};
+
+struct ArenasToUpdate
+{
+    ArenasToUpdate(Zone* zone, AllocKinds kinds);
+    bool done() { return kind == AllocKind::LIMIT; }
+    ArenaListSegment getArenasToUpdate(AutoLockHelperThreadState& lock, unsigned maxLength);
+
+  private:
+    AllocKinds kinds;  // Selects which thing kinds to update
+    Zone* zone;        // Zone to process
+    AllocKind kind;    // Current alloc kind to process
+    Arena* arena;      // Next arena to process
+
+    AllocKind nextAllocKind(AllocKind i) { return AllocKind(uint8_t(i) + 1); }
+    bool shouldProcessKind(AllocKind kind);
+    Arena* next(AutoLockHelperThreadState& lock);
+};
+
+ArenasToUpdate::ArenasToUpdate(Zone* zone, AllocKinds kinds)
+  : kinds(kinds), zone(zone), kind(AllocKind::FIRST), arena(nullptr)
+{
+    MOZ_ASSERT(zone->isGCCompacting());
+}
+
+Arena*
+ArenasToUpdate::next(AutoLockHelperThreadState& lock)
+{
+    // Find the next arena to update.
+    //
+    // This iterates through the GC thing kinds filtered by shouldProcessKind(),
+    // and then through thea arenas of that kind.  All state is held in the
+    // object and we just return when we find an arena.
+
+    for (; kind < AllocKind::LIMIT; kind = nextAllocKind(kind)) {
+        if (kinds.contains(kind)) {
+            if (!arena)
+                arena = zone->arenas.getFirstArena(kind);
+            else
+                arena = arena->next;
+            if (arena)
+                return arena;
+        }
+    }
+
+    MOZ_ASSERT(!arena);
+    MOZ_ASSERT(done());
+    return nullptr;
+}
+
+ArenaListSegment
+ArenasToUpdate::getArenasToUpdate(AutoLockHelperThreadState& lock, unsigned maxLength)
+{
+    Arena* begin = next(lock);
+    if (!begin)
+        return { nullptr, nullptr };
+
+    Arena* last = begin;
+    unsigned count = 1;
+    while (last->next && count < maxLength) {
+        last = last->next;
+        count++;
+    }
+
+    arena = last;
+    return { begin, last->next };
+}
+
+struct UpdatePointersTask : public GCParallelTask
+{
+    // Maximum number of arenas to update in one block.
+#ifdef DEBUG
+    static const unsigned MaxArenasToProcess = 16;
+#else
+    static const unsigned MaxArenasToProcess = 256;
+#endif
+
+    UpdatePointersTask(JSRuntime* rt, ArenasToUpdate* source, AutoLockHelperThreadState& lock)
+      : rt_(rt), source_(source)
+    {
+        arenas_.begin = nullptr;
+        arenas_.end = nullptr;
+    }
+
+    ~UpdatePointersTask() override { join(); }
+
+  private:
+    JSRuntime* rt_;
+    ArenasToUpdate* source_;
+    ArenaListSegment arenas_;
+
+    virtual void run() override;
+    bool getArenasToUpdate();
+    void updateArenas();
+};
+
+bool
+UpdatePointersTask::getArenasToUpdate()
+{
+    AutoLockHelperThreadState lock;
+    arenas_ = source_->getArenasToUpdate(lock, MaxArenasToProcess);
+    return arenas_.begin != nullptr;
+}
+
+void
+UpdatePointersTask::updateArenas()
+{
+    MovingTracer trc(rt_);
+    for (Arena* arena = arenas_.begin; arena != arenas_.end; arena = arena->next)
+        UpdateArenaPointers(&trc, arena);
+}
+
+/* virtual */ void
+UpdatePointersTask::run()
+{
+    while (getArenasToUpdate())
+        updateArenas();
+}
+
+} // namespace gc
+} // namespace js
+
+static const size_t MinCellUpdateBackgroundTasks = 2;
+static const size_t MaxCellUpdateBackgroundTasks = 8;
+
+static size_t
+CellUpdateBackgroundTaskCount()
+{
+    if (!CanUseExtraThreads())
+        return 0;
+
+    size_t targetTaskCount = HelperThreadState().cpuCount / 2;
+    return Min(Max(targetTaskCount, MinCellUpdateBackgroundTasks), MaxCellUpdateBackgroundTasks);
+}
+
+static bool
+CanUpdateKindInBackground(AllocKind kind) {
+    // We try to update as many GC things in parallel as we can, but there are
+    // kinds for which this might not be safe:
+    //  - we assume JSObjects that are foreground finalized are not safe to
+    //    update in parallel
+    //  - updating a shape touches child shapes in fixupShapeTreeAfterMovingGC()
+    if (!js::gc::IsBackgroundFinalized(kind) || IsShapeAllocKind(kind))
+        return false;
+
+    return true;
+}
+
+static AllocKinds
+ForegroundUpdateKinds(AllocKinds kinds)
+{
+    AllocKinds result;
+    for (AllocKind kind : kinds) {
+        if (!CanUpdateKindInBackground(kind))
+            result += kind;
+    }
+    return result;
+}
+
+void
+GCRuntime::updateTypeDescrObjects(MovingTracer* trc, Zone* zone)
+{
+    zone->typeDescrObjects.sweep();
+    for (auto r = zone->typeDescrObjects.all(); !r.empty(); r.popFront())
+        UpdateCellPointers(trc, r.front().get());
+}
+
+void
+GCRuntime::updateCellPointers(MovingTracer* trc, Zone* zone, AllocKinds kinds, size_t bgTaskCount)
+{
+    AllocKinds fgKinds = bgTaskCount == 0 ? kinds : ForegroundUpdateKinds(kinds);
+    AllocKinds bgKinds = kinds - fgKinds;
+
+    ArenasToUpdate fgArenas(zone, fgKinds);
+    ArenasToUpdate bgArenas(zone, bgKinds);
+    Maybe<UpdatePointersTask> fgTask;
+    Maybe<UpdatePointersTask> bgTasks[MaxCellUpdateBackgroundTasks];
+
+    size_t tasksStarted = 0;
+
+    {
+        AutoLockHelperThreadState lock;
+
+        fgTask.emplace(rt, &fgArenas, lock);
+
+        for (size_t i = 0; i < bgTaskCount && !bgArenas.done(); i++) {
+            bgTasks[i].emplace(rt, &bgArenas, lock);
+            startTask(*bgTasks[i], gcstats::PHASE_COMPACT_UPDATE_CELLS, lock);
+            tasksStarted = i;
+        }
+    }
+
+    fgTask->runFromMainThread(rt);
+
+    {
+        AutoLockHelperThreadState lock;
+
+        for (size_t i = 0; i < tasksStarted; i++)
+            joinTask(*bgTasks[i], gcstats::PHASE_COMPACT_UPDATE_CELLS, lock);
+    }
+}
+
+// After cells have been relocated any pointers to a cell's old locations must
+// be updated to point to the new location.  This happens by iterating through
+// all cells in heap and tracing their children (non-recursively) to update
+// them.
+//
+// This is complicated by the fact that updating a GC thing sometimes depends on
+// making use of other GC things.  After a moving GC these things may not be in
+// a valid state since they may contain pointers which have not been updated
+// yet.
+//
+// The main dependencies are:
+//
+//   - Updating a JSObject makes use of its shape
+//   - Updating a typed object makes use of its type descriptor object
+//
+// This means we require at least three phases for update:
+//
+//  1) shapes
+//  2) typed object type descriptor objects
+//  3) all other objects
+//
+// Since we want to minimize the number of phases, we put everything else into
+// the first phase and label it the 'misc' phase.
+
+static const AllocKinds UpdatePhaseMisc {
+    AllocKind::SCRIPT,
+    AllocKind::LAZY_SCRIPT,
+    AllocKind::BASE_SHAPE,
+    AllocKind::SHAPE,
+    AllocKind::ACCESSOR_SHAPE,
+    AllocKind::OBJECT_GROUP,
+    AllocKind::STRING,
+    AllocKind::JITCODE,
+    AllocKind::SCOPE
+};
+
+static const AllocKinds UpdatePhaseObjects {
+    AllocKind::FUNCTION,
+    AllocKind::FUNCTION_EXTENDED,
+    AllocKind::OBJECT0,
+    AllocKind::OBJECT0_BACKGROUND,
+    AllocKind::OBJECT2,
+    AllocKind::OBJECT2_BACKGROUND,
+    AllocKind::OBJECT4,
+    AllocKind::OBJECT4_BACKGROUND,
+    AllocKind::OBJECT8,
+    AllocKind::OBJECT8_BACKGROUND,
+    AllocKind::OBJECT12,
+    AllocKind::OBJECT12_BACKGROUND,
+    AllocKind::OBJECT16,
+    AllocKind::OBJECT16_BACKGROUND
+};
+
+void
+GCRuntime::updateAllCellPointers(MovingTracer* trc, Zone* zone)
+{
+    AutoDisableProxyCheck noProxyCheck(rt); // These checks assert when run in parallel.
+
+    size_t bgTaskCount = CellUpdateBackgroundTaskCount();
+
+    updateCellPointers(trc, zone, UpdatePhaseMisc, bgTaskCount);
+
+    // Update TypeDescrs before all other objects as typed objects access these
+    // objects when we trace them.
+    updateTypeDescrObjects(trc, zone);
+
+    updateCellPointers(trc, zone, UpdatePhaseObjects, bgTaskCount);
+}
+
+/*
+ * Update pointers to relocated cells by doing a full heap traversal and sweep.
+ *
+ * The latter is necessary to update weak references which are not marked as
+ * part of the traversal.
+ */
+void
+GCRuntime::updatePointersToRelocatedCells(Zone* zone, AutoLockForExclusiveAccess& lock)
+{
+    MOZ_ASSERT(!rt->isBeingDestroyed());
+    MOZ_ASSERT(zone->isGCCompacting());
+
+    gcstats::AutoPhase ap(stats, gcstats::PHASE_COMPACT_UPDATE);
+    MovingTracer trc(rt);
+
+    zone->fixupAfterMovingGC();
+
+    // Fixup compartment global pointers as these get accessed during marking.
+    for (CompartmentsInZoneIter comp(zone); !comp.done(); comp.next())
+        comp->fixupAfterMovingGC();
+    JSCompartment::fixupCrossCompartmentWrappersAfterMovingGC(&trc);
+    rt->spsProfiler.fixupStringsMapAfterMovingGC();
+
+    // Iterate through all cells that can contain relocatable pointers to update
+    // them. Since updating each cell is independent we try to parallelize this
+    // as much as possible.
+    updateAllCellPointers(&trc, zone);
+
+    // Mark roots to update them.
+    {
+        traceRuntimeForMajorGC(&trc, lock);
+
+        gcstats::AutoPhase ap(stats, gcstats::PHASE_MARK_ROOTS);
+        Debugger::markAll(&trc);
+        Debugger::markIncomingCrossCompartmentEdges(&trc);
+
+        WeakMapBase::markAll(zone, &trc);
+        for (CompartmentsInZoneIter c(zone); !c.done(); c.next()) {
+            c->trace(&trc);
+            if (c->watchpointMap)
+                c->watchpointMap->markAll(&trc);
+        }
+
+        // Mark all gray roots, making sure we call the trace callback to get the
+        // current set.
+        if (JSTraceDataOp op = grayRootTracer.op)
+            (*op)(&trc, grayRootTracer.data);
+    }
+
+    // Sweep everything to fix up weak pointers
+    WatchpointMap::sweepAll(rt);
+    Debugger::sweepAll(rt->defaultFreeOp());
+    jit::JitRuntime::SweepJitcodeGlobalTable(rt);
+    rt->gc.sweepZoneAfterCompacting(zone);
+
+    // Type inference may put more blocks here to free.
+    blocksToFreeAfterSweeping.freeAll();
+
+    // Call callbacks to get the rest of the system to fixup other untraced pointers.
+    callWeakPointerZoneGroupCallbacks();
+    for (CompartmentsInZoneIter comp(zone); !comp.done(); comp.next())
+        callWeakPointerCompartmentCallbacks(comp);
+    if (rt->sweepZoneCallback)
+        rt->sweepZoneCallback(zone);
+}
+
+void
+GCRuntime::protectAndHoldArenas(Arena* arenaList)
+{
+    for (Arena* arena = arenaList; arena; ) {
+        MOZ_ASSERT(arena->allocated());
+        Arena* next = arena->next;
+        if (!next) {
+            // Prepend to hold list before we protect the memory.
+            arena->next = relocatedArenasToRelease;
+            relocatedArenasToRelease = arenaList;
+        }
+        ProtectPages(arena, ArenaSize);
+        arena = next;
+    }
+}
+
+void
+GCRuntime::unprotectHeldRelocatedArenas()
+{
+    for (Arena* arena = relocatedArenasToRelease; arena; arena = arena->next) {
+        UnprotectPages(arena, ArenaSize);
+        MOZ_ASSERT(arena->allocated());
+    }
+}
+
+void
+GCRuntime::releaseRelocatedArenas(Arena* arenaList)
+{
+    AutoLockGC lock(rt);
+    releaseRelocatedArenasWithoutUnlocking(arenaList, lock);
+}
+
+void
+GCRuntime::releaseRelocatedArenasWithoutUnlocking(Arena* arenaList, const AutoLockGC& lock)
+{
+    // Release the relocated arenas, now containing only forwarding pointers
+    unsigned count = 0;
+    while (arenaList) {
+        Arena* arena = arenaList;
+        arenaList = arenaList->next;
+
+        // Clear the mark bits
+        arena->unmarkAll();
+
+        // Mark arena as empty
+        arena->setAsFullyUnused();
+
+#if defined(JS_CRASH_DIAGNOSTICS) || defined(JS_GC_ZEAL)
+        JS_POISON(reinterpret_cast<void*>(arena->thingsStart()),
+                  JS_MOVED_TENURED_PATTERN, arena->getThingsSpan());
+#endif
+
+        releaseArena(arena, lock);
+        ++count;
+    }
+}
+
+// In debug mode we don't always release relocated arenas straight away.
+// Sometimes protect them instead and hold onto them until the next GC sweep
+// phase to catch any pointers to them that didn't get forwarded.
+
+void
+GCRuntime::releaseHeldRelocatedArenas()
+{
+#ifdef DEBUG
+    unprotectHeldRelocatedArenas();
+    Arena* arenas = relocatedArenasToRelease;
+    relocatedArenasToRelease = nullptr;
+    releaseRelocatedArenas(arenas);
+#endif
+}
+
+void
+GCRuntime::releaseHeldRelocatedArenasWithoutUnlocking(const AutoLockGC& lock)
+{
+#ifdef DEBUG
+    unprotectHeldRelocatedArenas();
+    releaseRelocatedArenasWithoutUnlocking(relocatedArenasToRelease, lock);
+    relocatedArenasToRelease = nullptr;
+#endif
+}
+
+void
+ReleaseArenaList(JSRuntime* rt, Arena* arena, const AutoLockGC& lock)
+{
+    Arena* next;
+    for (; arena; arena = next) {
+        next = arena->next;
+        rt->gc.releaseArena(arena, lock);
+    }
+}
+
+ArenaLists::~ArenaLists()
+{
+    AutoLockGC lock(runtime_);
+
+    for (auto i : AllAllocKinds()) {
+        /*
+         * We can only call this during the shutdown after the last GC when
+         * the background finalization is disabled.
+         */
+        MOZ_ASSERT(backgroundFinalizeState[i] == BFS_DONE);
+        ReleaseArenaList(runtime_, arenaLists[i].head(), lock);
+    }
+    ReleaseArenaList(runtime_, incrementalSweptArenas.head(), lock);
+
+    for (auto i : ObjectAllocKinds())
+        ReleaseArenaList(runtime_, savedObjectArenas[i].head(), lock);
+    ReleaseArenaList(runtime_, savedEmptyObjectArenas, lock);
+}
+
+void
+ArenaLists::finalizeNow(FreeOp* fop, const FinalizePhase& phase)
+{
+    gcstats::AutoPhase ap(fop->runtime()->gc.stats, phase.statsPhase);
+    for (auto kind : phase.kinds)
+        finalizeNow(fop, kind, RELEASE_ARENAS, nullptr);
+}
+
+void
+ArenaLists::finalizeNow(FreeOp* fop, AllocKind thingKind, KeepArenasEnum keepArenas, Arena** empty)
+{
+    MOZ_ASSERT(!IsBackgroundFinalized(thingKind));
+    forceFinalizeNow(fop, thingKind, keepArenas, empty);
+}
+
+void
+ArenaLists::forceFinalizeNow(FreeOp* fop, AllocKind thingKind,
+                             KeepArenasEnum keepArenas, Arena** empty)
+{
+    MOZ_ASSERT(backgroundFinalizeState[thingKind] == BFS_DONE);
+
+    Arena* arenas = arenaLists[thingKind].head();
+    if (!arenas)
+        return;
+    arenaLists[thingKind].clear();
+
+    size_t thingsPerArena = Arena::thingsPerArena(thingKind);
+    SortedArenaList finalizedSorted(thingsPerArena);
+
+    auto unlimited = SliceBudget::unlimited();
+    FinalizeArenas(fop, &arenas, finalizedSorted, thingKind, unlimited, keepArenas);
+    MOZ_ASSERT(!arenas);
+
+    if (empty) {
+        MOZ_ASSERT(keepArenas == KEEP_ARENAS);
+        finalizedSorted.extractEmpty(empty);
+    }
+
+    arenaLists[thingKind] = finalizedSorted.toArenaList();
+}
+
+void
+ArenaLists::queueForForegroundSweep(FreeOp* fop, const FinalizePhase& phase)
+{
+    gcstats::AutoPhase ap(fop->runtime()->gc.stats, phase.statsPhase);
+    for (auto kind : phase.kinds)
+        queueForForegroundSweep(fop, kind);
+}
+
+void
+ArenaLists::queueForForegroundSweep(FreeOp* fop, AllocKind thingKind)
+{
+    MOZ_ASSERT(!IsBackgroundFinalized(thingKind));
+    MOZ_ASSERT(backgroundFinalizeState[thingKind] == BFS_DONE);
+    MOZ_ASSERT(!arenaListsToSweep[thingKind]);
+
+    arenaListsToSweep[thingKind] = arenaLists[thingKind].head();
+    arenaLists[thingKind].clear();
+}
+
+void
+ArenaLists::queueForBackgroundSweep(FreeOp* fop, const FinalizePhase& phase)
+{
+    gcstats::AutoPhase ap(fop->runtime()->gc.stats, phase.statsPhase);
+    for (auto kind : phase.kinds)
+        queueForBackgroundSweep(fop, kind);
+}
+
+inline void
+ArenaLists::queueForBackgroundSweep(FreeOp* fop, AllocKind thingKind)
+{
+    MOZ_ASSERT(IsBackgroundFinalized(thingKind));
+
+    ArenaList* al = &arenaLists[thingKind];
+    if (al->isEmpty()) {
+        MOZ_ASSERT(backgroundFinalizeState[thingKind] == BFS_DONE);
+        return;
+    }
+
+    MOZ_ASSERT(backgroundFinalizeState[thingKind] == BFS_DONE);
+
+    arenaListsToSweep[thingKind] = al->head();
+    al->clear();
+    backgroundFinalizeState[thingKind] = BFS_RUN;
+}
+
+/*static*/ void
+ArenaLists::backgroundFinalize(FreeOp* fop, Arena* listHead, Arena** empty)
+{
+    MOZ_ASSERT(listHead);
+    MOZ_ASSERT(empty);
+
+    AllocKind thingKind = listHead->getAllocKind();
+    Zone* zone = listHead->zone;
+
+    size_t thingsPerArena = Arena::thingsPerArena(thingKind);
+    SortedArenaList finalizedSorted(thingsPerArena);
+
+    auto unlimited = SliceBudget::unlimited();
+    FinalizeArenas(fop, &listHead, finalizedSorted, thingKind, unlimited, KEEP_ARENAS);
+    MOZ_ASSERT(!listHead);
+
+    finalizedSorted.extractEmpty(empty);
+
+    // When arenas are queued for background finalization, all arenas are moved
+    // to arenaListsToSweep[], leaving the arenaLists[] empty. However, new
+    // arenas may be allocated before background finalization finishes; now that
+    // finalization is complete, we want to merge these lists back together.
+    ArenaLists* lists = &zone->arenas;
+    ArenaList* al = &lists->arenaLists[thingKind];
+
+    // Flatten |finalizedSorted| into a regular ArenaList.
+    ArenaList finalized = finalizedSorted.toArenaList();
+
+    // We must take the GC lock to be able to safely modify the ArenaList;
+    // however, this does not by itself make the changes visible to all threads,
+    // as not all threads take the GC lock to read the ArenaLists.
+    // That safety is provided by the ReleaseAcquire memory ordering of the
+    // background finalize state, which we explicitly set as the final step.
+    {
+        AutoLockGC lock(lists->runtime_);
+        MOZ_ASSERT(lists->backgroundFinalizeState[thingKind] == BFS_RUN);
+
+        // Join |al| and |finalized| into a single list.
+        *al = finalized.insertListWithCursorAtEnd(*al);
+
+        lists->arenaListsToSweep[thingKind] = nullptr;
+    }
+
+    lists->backgroundFinalizeState[thingKind] = BFS_DONE;
+}
+
+void
+ArenaLists::queueForegroundObjectsForSweep(FreeOp* fop)
+{
+    gcstats::AutoPhase ap(fop->runtime()->gc.stats, gcstats::PHASE_SWEEP_OBJECT);
+
+#ifdef DEBUG
+    for (auto i : ObjectAllocKinds())
+        MOZ_ASSERT(savedObjectArenas[i].isEmpty());
+    MOZ_ASSERT(savedEmptyObjectArenas == nullptr);
+#endif
+
+    // Foreground finalized objects must be finalized at the beginning of the
+    // sweep phase, before control can return to the mutator. Otherwise,
+    // mutator behavior can resurrect certain objects whose references would
+    // otherwise have been erased by the finalizer.
+    finalizeNow(fop, AllocKind::OBJECT0, KEEP_ARENAS, &savedEmptyObjectArenas);
+    finalizeNow(fop, AllocKind::OBJECT2, KEEP_ARENAS, &savedEmptyObjectArenas);
+    finalizeNow(fop, AllocKind::OBJECT4, KEEP_ARENAS, &savedEmptyObjectArenas);
+    finalizeNow(fop, AllocKind::OBJECT8, KEEP_ARENAS, &savedEmptyObjectArenas);
+    finalizeNow(fop, AllocKind::OBJECT12, KEEP_ARENAS, &savedEmptyObjectArenas);
+    finalizeNow(fop, AllocKind::OBJECT16, KEEP_ARENAS, &savedEmptyObjectArenas);
+
+    // Prevent the arenas from having new objects allocated into them. We need
+    // to know which objects are marked while we incrementally sweep dead
+    // references from type information.
+    savedObjectArenas[AllocKind::OBJECT0] = arenaLists[AllocKind::OBJECT0].copyAndClear();
+    savedObjectArenas[AllocKind::OBJECT2] = arenaLists[AllocKind::OBJECT2].copyAndClear();
+    savedObjectArenas[AllocKind::OBJECT4] = arenaLists[AllocKind::OBJECT4].copyAndClear();
+    savedObjectArenas[AllocKind::OBJECT8] = arenaLists[AllocKind::OBJECT8].copyAndClear();
+    savedObjectArenas[AllocKind::OBJECT12] = arenaLists[AllocKind::OBJECT12].copyAndClear();
+    savedObjectArenas[AllocKind::OBJECT16] = arenaLists[AllocKind::OBJECT16].copyAndClear();
+}
+
+void
+ArenaLists::mergeForegroundSweptObjectArenas()
+{
+    AutoLockGC lock(runtime_);
+    ReleaseArenaList(runtime_, savedEmptyObjectArenas, lock);
+    savedEmptyObjectArenas = nullptr;
+
+    mergeSweptArenas(AllocKind::OBJECT0);
+    mergeSweptArenas(AllocKind::OBJECT2);
+    mergeSweptArenas(AllocKind::OBJECT4);
+    mergeSweptArenas(AllocKind::OBJECT8);
+    mergeSweptArenas(AllocKind::OBJECT12);
+    mergeSweptArenas(AllocKind::OBJECT16);
+}
+
+inline void
+ArenaLists::mergeSweptArenas(AllocKind thingKind)
+{
+    ArenaList* al = &arenaLists[thingKind];
+    ArenaList* saved = &savedObjectArenas[thingKind];
+
+    *al = saved->insertListWithCursorAtEnd(*al);
+    saved->clear();
+}
+
+void
+ArenaLists::queueForegroundThingsForSweep(FreeOp* fop)
+{
+    gcShapeArenasToUpdate = arenaListsToSweep[AllocKind::SHAPE];
+    gcAccessorShapeArenasToUpdate = arenaListsToSweep[AllocKind::ACCESSOR_SHAPE];
+    gcObjectGroupArenasToUpdate = arenaListsToSweep[AllocKind::OBJECT_GROUP];
+    gcScriptArenasToUpdate = arenaListsToSweep[AllocKind::SCRIPT];
+}
+
+SliceBudget::SliceBudget()
+  : timeBudget(UnlimitedTimeBudget), workBudget(UnlimitedWorkBudget)
+{
+    makeUnlimited();
+}
+
+SliceBudget::SliceBudget(TimeBudget time)
+  : timeBudget(time), workBudget(UnlimitedWorkBudget)
+{
+    if (time.budget < 0) {
+        makeUnlimited();
+    } else {
+        // Note: TimeBudget(0) is equivalent to WorkBudget(CounterReset).
+        deadline = PRMJ_Now() + time.budget * PRMJ_USEC_PER_MSEC;
+        counter = CounterReset;
+    }
+}
+
+SliceBudget::SliceBudget(WorkBudget work)
+  : timeBudget(UnlimitedTimeBudget), workBudget(work)
+{
+    if (work.budget < 0) {
+        makeUnlimited();
+    } else {
+        deadline = 0;
+        counter = work.budget;
+    }
+}
+
+int
+SliceBudget::describe(char* buffer, size_t maxlen) const
+{
+    if (isUnlimited())
+        return snprintf(buffer, maxlen, "unlimited");
+    else if (isWorkBudget())
+        return snprintf(buffer, maxlen, "work(%" PRId64 ")", workBudget.budget);
+    else
+        return snprintf(buffer, maxlen, "%" PRId64 "ms", timeBudget.budget);
+}
+
+bool
+SliceBudget::checkOverBudget()
+{
+    bool over = PRMJ_Now() >= deadline;
+    if (!over)
+        counter = CounterReset;
+    return over;
+}
+
+void
+js::MarkCompartmentActive(InterpreterFrame* fp)
+{
+    fp->script()->compartment()->zone()->active = true;
+}
+
+void
+GCRuntime::requestMajorGC(JS::gcreason::Reason reason)
+{
+    MOZ_ASSERT(!CurrentThreadIsPerformingGC());
+
+    if (majorGCRequested())
+        return;
+
+    majorGCTriggerReason = reason;
+
+    // There's no need to use RequestInterruptUrgent here. It's slower because
+    // it has to interrupt (looping) Ion code, but loops in Ion code that
+    // affect GC will have an explicit interrupt check.
+    rt->requestInterrupt(JSRuntime::RequestInterruptCanWait);
+}
+
+void
+GCRuntime::requestMinorGC(JS::gcreason::Reason reason)
+{
+    MOZ_ASSERT(CurrentThreadCanAccessRuntime(rt));
+    MOZ_ASSERT(!CurrentThreadIsPerformingGC());
+
+    if (minorGCRequested())
+        return;
+
+    minorGCTriggerReason = reason;
+
+    // See comment in requestMajorGC.
+    rt->requestInterrupt(JSRuntime::RequestInterruptCanWait);
+}
+
+bool
+GCRuntime::triggerGC(JS::gcreason::Reason reason)
+{
+    /*
+     * Don't trigger GCs if this is being called off the main thread from
+     * onTooMuchMalloc().
+     */
+    if (!CurrentThreadCanAccessRuntime(rt))
+        return false;
+
+    /* GC is already running. */
+    if (rt->isHeapCollecting())
+        return false;
+
+    JS::PrepareForFullGC(rt->contextFromMainThread());
+    requestMajorGC(reason);
+    return true;
+}
+
+void
+GCRuntime::maybeAllocTriggerZoneGC(Zone* zone, const AutoLockGC& lock)
+{
+    size_t usedBytes = zone->usage.gcBytes();
+    size_t thresholdBytes = zone->threshold.gcTriggerBytes();
+    size_t igcThresholdBytes = thresholdBytes * tunables.zoneAllocThresholdFactor();
+
+    if (usedBytes >= thresholdBytes) {
+        // The threshold has been surpassed, immediately trigger a GC,
+        // which will be done non-incrementally.
+        triggerZoneGC(zone, JS::gcreason::ALLOC_TRIGGER);
+    } else if (usedBytes >= igcThresholdBytes) {
+        // Reduce the delay to the start of the next incremental slice.
+        if (zone->gcDelayBytes < ArenaSize)
+            zone->gcDelayBytes = 0;
+        else
+            zone->gcDelayBytes -= ArenaSize;
+
+        if (!zone->gcDelayBytes) {
+            // Start or continue an in progress incremental GC. We do this
+            // to try to avoid performing non-incremental GCs on zones
+            // which allocate a lot of data, even when incremental slices
+            // can't be triggered via scheduling in the event loop.
+            triggerZoneGC(zone, JS::gcreason::ALLOC_TRIGGER);
+
+            // Delay the next slice until a certain amount of allocation
+            // has been performed.
+            zone->gcDelayBytes = tunables.zoneAllocDelayBytes();
+        }
+    }
+}
+
+bool
+GCRuntime::triggerZoneGC(Zone* zone, JS::gcreason::Reason reason)
+{
+    /* Zones in use by a thread with an exclusive context can't be collected. */
+    if (!CurrentThreadCanAccessRuntime(rt)) {
+        MOZ_ASSERT(zone->usedByExclusiveThread || zone->isAtomsZone());
+        return false;
+    }
+
+    /* GC is already running. */
+    if (rt->isHeapCollecting())
+        return false;
+
+#ifdef JS_GC_ZEAL
+    if (hasZealMode(ZealMode::Alloc)) {
+        MOZ_RELEASE_ASSERT(triggerGC(reason));
+        return true;
+    }
+#endif
+
+    if (zone->isAtomsZone()) {
+        /* We can't do a zone GC of the atoms compartment. */
+        if (rt->keepAtoms()) {
+            /* Skip GC and retrigger later, since atoms zone won't be collected
+             * if keepAtoms is true. */
+            fullGCForAtomsRequested_ = true;
+            return false;
+        }
+        MOZ_RELEASE_ASSERT(triggerGC(reason));
+        return true;
+    }
+
+    PrepareZoneForGC(zone);
+    requestMajorGC(reason);
+    return true;
+}
+
+void
+GCRuntime::maybeGC(Zone* zone)
+{
+    MOZ_ASSERT(CurrentThreadCanAccessRuntime(rt));
+
+#ifdef JS_GC_ZEAL
+    if (hasZealMode(ZealMode::Alloc) || hasZealMode(ZealMode::Poke)) {
+        JS::PrepareForFullGC(rt->contextFromMainThread());
+        gc(GC_NORMAL, JS::gcreason::DEBUG_GC);
+        return;
+    }
+#endif
+
+    if (gcIfRequested())
+        return;
+
+    if (zone->usage.gcBytes() > 1024 * 1024 &&
+        zone->usage.gcBytes() >= zone->threshold.allocTrigger(schedulingState.inHighFrequencyGCMode()) &&
+        !isIncrementalGCInProgress() &&
+        !isBackgroundSweeping())
+    {
+        PrepareZoneForGC(zone);
+        startGC(GC_NORMAL, JS::gcreason::EAGER_ALLOC_TRIGGER);
+    }
+}
+
+// Do all possible decommit immediately from the current thread without
+// releasing the GC lock or allocating any memory.
+void
+GCRuntime::decommitAllWithoutUnlocking(const AutoLockGC& lock)
+{
+    MOZ_ASSERT(emptyChunks(lock).count() == 0);
+    for (ChunkPool::Iter chunk(availableChunks(lock)); !chunk.done(); chunk.next())
+        chunk->decommitAllArenasWithoutUnlocking(lock);
+    MOZ_ASSERT(availableChunks(lock).verify());
+}
+
+void
+GCRuntime::startDecommit()
+{
+    MOZ_ASSERT(CurrentThreadCanAccessRuntime(rt));
+    MOZ_ASSERT(!decommitTask.isRunning());
+
+    // If we are allocating heavily enough to trigger "high freqency" GC, then
+    // skip decommit so that we do not compete with the mutator.
+    if (schedulingState.inHighFrequencyGCMode())
+        return;
+
+    BackgroundDecommitTask::ChunkVector toDecommit;
+    {
+        AutoLockGC lock(rt);
+
+        // Verify that all entries in the empty chunks pool are already decommitted.
+        for (ChunkPool::Iter chunk(emptyChunks(lock)); !chunk.done(); chunk.next())
+            MOZ_ASSERT(!chunk->info.numArenasFreeCommitted);
+
+        // Since we release the GC lock while doing the decommit syscall below,
+        // it is dangerous to iterate the available list directly, as the main
+        // thread could modify it concurrently. Instead, we build and pass an
+        // explicit Vector containing the Chunks we want to visit.
+        MOZ_ASSERT(availableChunks(lock).verify());
+        for (ChunkPool::Iter iter(availableChunks(lock)); !iter.done(); iter.next()) {
+            if (!toDecommit.append(iter.get())) {
+                // The OOM handler does a full, immediate decommit.
+                return onOutOfMallocMemory(lock);
+            }
+        }
+    }
+    decommitTask.setChunksToScan(toDecommit);
+
+    if (sweepOnBackgroundThread && decommitTask.start())
+        return;
+
+    decommitTask.runFromMainThread(rt);
+}
+
+void
+js::gc::BackgroundDecommitTask::setChunksToScan(ChunkVector &chunks)
+{
+    MOZ_ASSERT(CurrentThreadCanAccessRuntime(runtime));
+    MOZ_ASSERT(!isRunning());
+    MOZ_ASSERT(toDecommit.empty());
+    Swap(toDecommit, chunks);
+}
+
+/* virtual */ void
+js::gc::BackgroundDecommitTask::run()
+{
+    AutoLockGC lock(runtime);
+
+    for (Chunk* chunk : toDecommit) {
+
+        // The arena list is not doubly-linked, so we have to work in the free
+        // list order and not in the natural order.
+        while (chunk->info.numArenasFreeCommitted) {
+            bool ok = chunk->decommitOneFreeArena(runtime, lock);
+
+            // If we are low enough on memory that we can't update the page
+            // tables, or if we need to return for any other reason, break out
+            // of the loop.
+            if (cancel_ || !ok)
+                break;
+        }
+    }
+    toDecommit.clearAndFree();
+
+    ChunkPool toFree = runtime->gc.expireEmptyChunkPool(lock);
+    if (toFree.count()) {
+        AutoUnlockGC unlock(lock);
+        FreeChunkPool(runtime, toFree);
+    }
+}
+
+void
+GCRuntime::sweepBackgroundThings(ZoneList& zones, LifoAlloc& freeBlocks)
+{
+    freeBlocks.freeAll();
+
+    if (zones.isEmpty())
+        return;
+
+    // We must finalize thing kinds in the order specified by BackgroundFinalizePhases.
+    Arena* emptyArenas = nullptr;
+    FreeOp fop(nullptr);
+    for (unsigned phase = 0 ; phase < ArrayLength(BackgroundFinalizePhases) ; ++phase) {
+        for (Zone* zone = zones.front(); zone; zone = zone->nextZone()) {
+            for (auto kind : BackgroundFinalizePhases[phase].kinds) {
+                Arena* arenas = zone->arenas.arenaListsToSweep[kind];
+                MOZ_RELEASE_ASSERT(uintptr_t(arenas) != uintptr_t(-1));
+                if (arenas)
+                    ArenaLists::backgroundFinalize(&fop, arenas, &emptyArenas);
+            }
+        }
+    }
+
+    AutoLockGC lock(rt);
+
+    // Release swept arenas, dropping and reaquiring the lock every so often to
+    // avoid blocking the main thread from allocating chunks.
+    static const size_t LockReleasePeriod = 32;
+    size_t releaseCount = 0;
+    Arena* next;
+    for (Arena* arena = emptyArenas; arena; arena = next) {
+        next = arena->next;
+        rt->gc.releaseArena(arena, lock);
+        releaseCount++;
+        if (releaseCount % LockReleasePeriod == 0) {
+            lock.unlock();
+            lock.lock();
+        }
+    }
+
+    while (!zones.isEmpty())
+        zones.removeFront();
+}
+
+void
+GCRuntime::assertBackgroundSweepingFinished()
+{
+#ifdef DEBUG
+    MOZ_ASSERT(backgroundSweepZones.isEmpty());
+    for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next()) {
+        for (auto i : AllAllocKinds()) {
+            MOZ_ASSERT(!zone->arenas.arenaListsToSweep[i]);
+            MOZ_ASSERT(zone->arenas.doneBackgroundFinalize(i));
+        }
+    }
+    MOZ_ASSERT(blocksToFreeAfterSweeping.computedSizeOfExcludingThis() == 0);
+#endif
+}
+
+unsigned
+js::GetCPUCount()
+{
+    static unsigned ncpus = 0;
+    if (ncpus == 0) {
+# ifdef XP_WIN
+        SYSTEM_INFO sysinfo;
+        GetSystemInfo(&sysinfo);
+        ncpus = unsigned(sysinfo.dwNumberOfProcessors);
+# else
+        long n = sysconf(_SC_NPROCESSORS_ONLN);
+        ncpus = (n > 0) ? unsigned(n) : 1;
+# endif
+    }
+    return ncpus;
+}
+
+void
+GCHelperState::finish()
+{
+    // Wait for any lingering background sweeping to finish.
+    waitBackgroundSweepEnd();
+}
+
+GCHelperState::State
+GCHelperState::state(const AutoLockGC&)
+{
+    return state_;
+}
+
+void
+GCHelperState::setState(State state, const AutoLockGC&)
+{
+    state_ = state;
+}
+
+void
+GCHelperState::startBackgroundThread(State newState, const AutoLockGC& lock,
+                                     const AutoLockHelperThreadState& helperLock)
+{
+    MOZ_ASSERT(!thread && state(lock) == IDLE && newState != IDLE);
+    setState(newState, lock);
+
+    {
+        AutoEnterOOMUnsafeRegion noOOM;
+        if (!HelperThreadState().gcHelperWorklist(helperLock).append(this))
+            noOOM.crash("Could not add to pending GC helpers list");
+    }
+
+    HelperThreadState().notifyAll(GlobalHelperThreadState::PRODUCER, helperLock);
+}
+
+void
+GCHelperState::waitForBackgroundThread(js::AutoLockGC& lock)
+{
+    done.wait(lock.guard());
+}
+
+void
+GCHelperState::work()
+{
+    MOZ_ASSERT(CanUseExtraThreads());
+
+    AutoLockGC lock(rt);
+
+    MOZ_ASSERT(thread.isNothing());
+    thread = mozilla::Some(ThisThread::GetId());
+
+    TraceLoggerThread* logger = TraceLoggerForCurrentThread();
+
+    switch (state(lock)) {
+
+      case IDLE:
+        MOZ_CRASH("GC helper triggered on idle state");
+        break;
+
+      case SWEEPING: {
+        AutoTraceLog logSweeping(logger, TraceLogger_GCSweeping);
+        doSweep(lock);
+        MOZ_ASSERT(state(lock) == SWEEPING);
+        break;
+      }
+
+    }
+
+    setState(IDLE, lock);
+    thread.reset();
+
+    done.notify_all();
+}
+
+void
+GCRuntime::queueZonesForBackgroundSweep(ZoneList& zones)
+{
+    AutoLockHelperThreadState helperLock;
+    AutoLockGC lock(rt);
+    backgroundSweepZones.transferFrom(zones);
+    helperState.maybeStartBackgroundSweep(lock, helperLock);
+}
+
+void
+GCRuntime::freeUnusedLifoBlocksAfterSweeping(LifoAlloc* lifo)
+{
+    MOZ_ASSERT(rt->isHeapBusy());
+    AutoLockGC lock(rt);
+    blocksToFreeAfterSweeping.transferUnusedFrom(lifo);
+}
+
+void
+GCRuntime::freeAllLifoBlocksAfterSweeping(LifoAlloc* lifo)
+{
+    MOZ_ASSERT(rt->isHeapBusy());
+    AutoLockGC lock(rt);
+    blocksToFreeAfterSweeping.transferFrom(lifo);
+}
+
+void
+GCRuntime::freeAllLifoBlocksAfterMinorGC(LifoAlloc* lifo)
+{
+    MOZ_ASSERT(CurrentThreadCanAccessRuntime(rt));
+    blocksToFreeAfterMinorGC.transferFrom(lifo);
+}
+
+void
+GCHelperState::maybeStartBackgroundSweep(const AutoLockGC& lock,
+                                         const AutoLockHelperThreadState& helperLock)
+{
+    MOZ_ASSERT(CanUseExtraThreads());
+
+    if (state(lock) == IDLE)
+        startBackgroundThread(SWEEPING, lock, helperLock);
+}
+
+void
+GCHelperState::waitBackgroundSweepEnd()
+{
+    AutoLockGC lock(rt);
+    while (state(lock) == SWEEPING)
+        waitForBackgroundThread(lock);
+    if (!rt->gc.isIncrementalGCInProgress())
+        rt->gc.assertBackgroundSweepingFinished();
+}
+
+void
+GCHelperState::doSweep(AutoLockGC& lock)
+{
+    // The main thread may call queueZonesForBackgroundSweep() while this is
+    // running so we must check there is no more work to do before exiting.
+
+    do {
+        while (!rt->gc.backgroundSweepZones.isEmpty()) {
+            AutoSetThreadIsSweeping threadIsSweeping;
+
+            ZoneList zones;
+            zones.transferFrom(rt->gc.backgroundSweepZones);
+            LifoAlloc freeLifoAlloc(JSRuntime::TEMP_LIFO_ALLOC_PRIMARY_CHUNK_SIZE);
+            freeLifoAlloc.transferFrom(&rt->gc.blocksToFreeAfterSweeping);
+
+            AutoUnlockGC unlock(lock);
+            rt->gc.sweepBackgroundThings(zones, freeLifoAlloc);
+        }
+    } while (!rt->gc.backgroundSweepZones.isEmpty());
+}
+
+bool
+GCHelperState::onBackgroundThread()
+{
+    return thread.isSome() && *thread == ThisThread::GetId();
+}
+
+bool
+GCRuntime::shouldReleaseObservedTypes()
+{
+    bool releaseTypes = false;
+
+#ifdef JS_GC_ZEAL
+    if (zealModeBits != 0)
+        releaseTypes = true;
+#endif
+
+    /* We may miss the exact target GC due to resets. */
+    if (majorGCNumber >= jitReleaseNumber)
+        releaseTypes = true;
+
+    if (releaseTypes)
+        jitReleaseNumber = majorGCNumber + JIT_SCRIPT_RELEASE_TYPES_PERIOD;
+
+    return releaseTypes;
+}
+
+struct IsAboutToBeFinalizedFunctor {
+    template <typename T> bool operator()(Cell** t) {
+        mozilla::DebugOnly<const Cell*> prior = *t;
+        bool result = IsAboutToBeFinalizedUnbarriered(reinterpret_cast<T**>(t));
+        // Sweep should not have to deal with moved pointers, since moving GC
+        // handles updating the UID table manually.
+        MOZ_ASSERT(*t == prior);
+        return result;
+    }
+};
+
+/* static */ bool
+UniqueIdGCPolicy::needsSweep(Cell** cell, uint64_t*)
+{
+    return DispatchTraceKindTyped(IsAboutToBeFinalizedFunctor(), (*cell)->getTraceKind(), cell);
+}
+
+void
+JS::Zone::sweepUniqueIds(js::FreeOp* fop)
+{
+    uniqueIds_.sweep();
+}
+
+/*
+ * It's simpler if we preserve the invariant that every zone has at least one
+ * compartment. If we know we're deleting the entire zone, then
+ * SweepCompartments is allowed to delete all compartments. In this case,
+ * |keepAtleastOne| is false. If some objects remain in the zone so that it
+ * cannot be deleted, then we set |keepAtleastOne| to true, which prohibits
+ * SweepCompartments from deleting every compartment. Instead, it preserves an
+ * arbitrary compartment in the zone.
+ */
+void
+Zone::sweepCompartments(FreeOp* fop, bool keepAtleastOne, bool destroyingRuntime)
+{
+    JSRuntime* rt = runtimeFromMainThread();
+    JSDestroyCompartmentCallback callback = rt->destroyCompartmentCallback;
+
+    JSCompartment** read = compartments.begin();
+    JSCompartment** end = compartments.end();
+    JSCompartment** write = read;
+    bool foundOne = false;
+    while (read < end) {
+        JSCompartment* comp = *read++;
+        MOZ_ASSERT(!rt->isAtomsCompartment(comp));
+
+        /*
+         * Don't delete the last compartment if all the ones before it were
+         * deleted and keepAtleastOne is true.
+         */
+        bool dontDelete = read == end && !foundOne && keepAtleastOne;
+        if ((!comp->marked && !dontDelete) || destroyingRuntime) {
+            if (callback)
+                callback(fop, comp);
+            if (comp->principals())
+                JS_DropPrincipals(rt->contextFromMainThread(), comp->principals());
+            js_delete(comp);
+            rt->gc.stats.sweptCompartment();
+        } else {
+            *write++ = comp;
+            foundOne = true;
+        }
+    }
+    compartments.shrinkTo(write - compartments.begin());
+    MOZ_ASSERT_IF(keepAtleastOne, !compartments.empty());
+}
+
+void
+GCRuntime::sweepZones(FreeOp* fop, bool destroyingRuntime)
+{
+    MOZ_ASSERT_IF(destroyingRuntime, numActiveZoneIters == 0);
+    MOZ_ASSERT_IF(destroyingRuntime, arenasEmptyAtShutdown);
+
+    if (rt->gc.numActiveZoneIters)
+        return;
+
+    assertBackgroundSweepingFinished();
+
+    JSZoneCallback callback = rt->destroyZoneCallback;
+
+    /* Skip the atomsCompartment zone. */
+    Zone** read = zones.begin() + 1;
+    Zone** end = zones.end();
+    Zone** write = read;
+    MOZ_ASSERT(zones.length() >= 1);
+    MOZ_ASSERT(zones[0]->isAtomsZone());
+
+    while (read < end) {
+        Zone* zone = *read++;
+
+        if (zone->wasGCStarted()) {
+            MOZ_ASSERT(!zone->isQueuedForBackgroundSweep());
+            const bool zoneIsDead = zone->arenas.arenaListsAreEmpty() &&
+                                    !zone->hasMarkedCompartments();
+            if (zoneIsDead || destroyingRuntime)
+            {
+                // We have just finished sweeping, so we should have freed any
+                // empty arenas back to their Chunk for future allocation.
+                zone->arenas.checkEmptyFreeLists();
+
+                // We are about to delete the Zone; this will leave the Zone*
+                // in the arena header dangling if there are any arenas
+                // remaining at this point.
+#ifdef DEBUG
+                if (!zone->arenas.checkEmptyArenaLists())
+                    arenasEmptyAtShutdown = false;
+#endif
+
+                if (callback)
+                    callback(zone);
+
+                zone->sweepCompartments(fop, false, destroyingRuntime);
+                MOZ_ASSERT(zone->compartments.empty());
+                MOZ_ASSERT_IF(arenasEmptyAtShutdown, zone->typeDescrObjects.empty());
+                fop->delete_(zone);
+                stats.sweptZone();
+                continue;
+            }
+            zone->sweepCompartments(fop, true, destroyingRuntime);
+        }
+        *write++ = zone;
+    }
+    zones.shrinkTo(write - zones.begin());
+}
+
+#ifdef DEBUG
+static const char*
+AllocKindToAscii(AllocKind kind)
+{
+    switch(kind) {
+#define MAKE_CASE(allocKind, traceKind, type, sizedType) \
+      case AllocKind:: allocKind: return #allocKind;
+FOR_EACH_ALLOCKIND(MAKE_CASE)
+#undef MAKE_CASE
+
+      default:
+        MOZ_CRASH("Unknown AllocKind in AllocKindToAscii");
+    }
+}
+#endif // DEBUG
+
+bool
+ArenaLists::checkEmptyArenaList(AllocKind kind)
+{
+    size_t num_live = 0;
+#ifdef DEBUG
+    if (!arenaLists[kind].isEmpty()) {
+        size_t max_cells = 20;
+        char *env = getenv("JS_GC_MAX_LIVE_CELLS");
+        if (env && *env)
+            max_cells = atol(env);
+        for (Arena* current = arenaLists[kind].head(); current; current = current->next) {
+            for (ArenaCellIterUnderGC i(current); !i.done(); i.next()) {
+                TenuredCell* t = i.getCell();
+                MOZ_ASSERT(t->isMarked(), "unmarked cells should have been finalized");
+                if (++num_live <= max_cells) {
+                    fprintf(stderr, "ERROR: GC found live Cell %p of kind %s at shutdown\n",
+                            t, AllocKindToAscii(kind));
+                }
+            }
+        }
+        fprintf(stderr, "ERROR: GC found %" PRIuSIZE " live Cells at shutdown\n", num_live);
+    }
+#endif // DEBUG
+    return num_live == 0;
+}
+
+void
+GCRuntime::purgeRuntime(AutoLockForExclusiveAccess& lock)
+{
+    for (GCCompartmentsIter comp(rt); !comp.done(); comp.next())
+        comp->purge();
+
+    freeUnusedLifoBlocksAfterSweeping(&rt->tempLifoAlloc);
+
+    rt->interpreterStack().purge(rt);
+
+    JSContext* cx = rt->contextFromMainThread();
+    cx->caches.gsnCache.purge();
+    cx->caches.envCoordinateNameCache.purge();
+    cx->caches.newObjectCache.purge();
+    cx->caches.nativeIterCache.purge();
+    cx->caches.uncompressedSourceCache.purge();
+    if (cx->caches.evalCache.initialized())
+        cx->caches.evalCache.clear();
+
+    rt->mainThread.frontendCollectionPool.purge();
+
+    if (auto cache = rt->maybeThisRuntimeSharedImmutableStrings())
+        cache->purge();
+
+    rt->promiseTasksToDestroy.lock()->clear();
+}
+
+bool
+GCRuntime::shouldPreserveJITCode(JSCompartment* comp, int64_t currentTime,
+                                 JS::gcreason::Reason reason, bool canAllocateMoreCode)
+{
+    if (cleanUpEverything)
+        return false;
+    if (!canAllocateMoreCode)
+        return false;
+
+    if (alwaysPreserveCode)
+        return true;
+    if (comp->preserveJitCode())
+        return true;
+    if (comp->lastAnimationTime + PRMJ_USEC_PER_SEC >= currentTime)
+        return true;
+    if (reason == JS::gcreason::DEBUG_GC)
+        return true;
+
+    return false;
+}
+
+#ifdef DEBUG
+class CompartmentCheckTracer : public JS::CallbackTracer
+{
+    void onChild(const JS::GCCellPtr& thing) override;
+
+  public:
+    explicit CompartmentCheckTracer(JSRuntime* rt)
+      : JS::CallbackTracer(rt), src(nullptr), zone(nullptr), compartment(nullptr)
+    {}
+
+    Cell* src;
+    JS::TraceKind srcKind;
+    Zone* zone;
+    JSCompartment* compartment;
+};
+
+namespace {
+struct IsDestComparatorFunctor {
+    JS::GCCellPtr dst_;
+    explicit IsDestComparatorFunctor(JS::GCCellPtr dst) : dst_(dst) {}
+
+    template <typename T> bool operator()(T* t) { return (*t) == dst_.asCell(); }
+};
+} // namespace (anonymous)
+
+static bool
+InCrossCompartmentMap(JSObject* src, JS::GCCellPtr dst)
+{
+    JSCompartment* srccomp = src->compartment();
+
+    if (dst.is<JSObject>()) {
+        Value key = ObjectValue(dst.as<JSObject>());
+        if (WrapperMap::Ptr p = srccomp->lookupWrapper(key)) {
+            if (*p->value().unsafeGet() == ObjectValue(*src))
+                return true;
+        }
+    }
+
+    /*
+     * If the cross-compartment edge is caused by the debugger, then we don't
+     * know the right hashtable key, so we have to iterate.
+     */
+    for (JSCompartment::WrapperEnum e(srccomp); !e.empty(); e.popFront()) {
+        if (e.front().mutableKey().applyToWrapped(IsDestComparatorFunctor(dst)) &&
+            ToMarkable(e.front().value().unbarrieredGet()) == src)
+        {
+            return true;
+        }
+    }
+
+    return false;
+}
+
+struct MaybeCompartmentFunctor {
+    template <typename T> JSCompartment* operator()(T* t) { return t->maybeCompartment(); }
+};
+
+void
+CompartmentCheckTracer::onChild(const JS::GCCellPtr& thing)
+{
+    JSCompartment* comp = DispatchTyped(MaybeCompartmentFunctor(), thing);
+    if (comp && compartment) {
+        MOZ_ASSERT(comp == compartment || runtime()->isAtomsCompartment(comp) ||
+                   (srcKind == JS::TraceKind::Object &&
+                    InCrossCompartmentMap(static_cast<JSObject*>(src), thing)));
+    } else {
+        TenuredCell* tenured = TenuredCell::fromPointer(thing.asCell());
+        Zone* thingZone = tenured->zoneFromAnyThread();
+        MOZ_ASSERT(thingZone == zone || thingZone->isAtomsZone());
+    }
+}
+
+void
+GCRuntime::checkForCompartmentMismatches()
+{
+    if (disableStrictProxyCheckingCount)
+        return;
+
+    CompartmentCheckTracer trc(rt);
+    AutoAssertEmptyNursery empty(rt);
+    for (ZonesIter zone(rt, SkipAtoms); !zone.done(); zone.next()) {
+        trc.zone = zone;
+        for (auto thingKind : AllAllocKinds()) {
+            for (auto i = zone->cellIter<TenuredCell>(thingKind, empty); !i.done(); i.next()) {
+                trc.src = i.getCell();
+                trc.srcKind = MapAllocToTraceKind(thingKind);
+                trc.compartment = DispatchTraceKindTyped(MaybeCompartmentFunctor(),
+                                                         trc.src, trc.srcKind);
+                js::TraceChildren(&trc, trc.src, trc.srcKind);
+            }
+        }
+    }
+}
+#endif
+
+static void
+RelazifyFunctions(Zone* zone, AllocKind kind)
+{
+    MOZ_ASSERT(kind == AllocKind::FUNCTION ||
+               kind == AllocKind::FUNCTION_EXTENDED);
+
+    JSRuntime* rt = zone->runtimeFromMainThread();
+    AutoAssertEmptyNursery empty(rt);
+
+    for (auto i = zone->cellIter<JSObject>(kind, empty); !i.done(); i.next()) {
+        JSFunction* fun = &i->as<JSFunction>();
+        if (fun->hasScript())
+            fun->maybeRelazify(rt);
+    }
+}
+
+bool
+GCRuntime::beginMarkPhase(JS::gcreason::Reason reason, AutoLockForExclusiveAccess& lock)
+{
+    int64_t currentTime = PRMJ_Now();
+
+#ifdef DEBUG
+    if (fullCompartmentChecks)
+        checkForCompartmentMismatches();
+#endif
+
+    isFull = true;
+    bool any = false;
+
+    for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next()) {
+        /* Assert that zone state is as we expect */
+        MOZ_ASSERT(!zone->isCollecting());
+        MOZ_ASSERT(!zone->compartments.empty());
+#ifdef DEBUG
+        for (auto i : AllAllocKinds())
+            MOZ_ASSERT(!zone->arenas.arenaListsToSweep[i]);
+#endif
+
+        /* Set up which zones will be collected. */
+        if (zone->isGCScheduled()) {
+            if (!zone->isAtomsZone()) {
+                any = true;
+                zone->setGCState(Zone::Mark);
+            }
+        } else {
+            isFull = false;
+        }
+
+        zone->setPreservingCode(false);
+    }
+
+    // Discard JIT code more aggressively if the process is approaching its
+    // executable code limit.
+    bool canAllocateMoreCode = jit::CanLikelyAllocateMoreExecutableMemory();
+
+    for (CompartmentsIter c(rt, WithAtoms); !c.done(); c.next()) {
+        c->marked = false;
+        c->scheduledForDestruction = false;
+        c->maybeAlive = false;
+        if (shouldPreserveJITCode(c, currentTime, reason, canAllocateMoreCode))
+            c->zone()->setPreservingCode(true);
+    }
+
+    if (!rt->gc.cleanUpEverything && canAllocateMoreCode) {
+        if (JSCompartment* comp = jit::TopmostIonActivationCompartment(rt))
+            comp->zone()->setPreservingCode(true);
+    }
+
+    /*
+     * Atoms are not in the cross-compartment map. So if there are any
+     * zones that are not being collected, we are not allowed to collect
+     * atoms. Otherwise, the non-collected zones could contain pointers
+     * to atoms that we would miss.
+     *
+     * keepAtoms() will only change on the main thread, which we are currently
+     * on. If the value of keepAtoms() changes between GC slices, then we'll
+     * cancel the incremental GC. See IsIncrementalGCSafe.
+     */
+    if (isFull && !rt->keepAtoms()) {
+        Zone* atomsZone = rt->atomsCompartment(lock)->zone();
+        if (atomsZone->isGCScheduled()) {
+            MOZ_ASSERT(!atomsZone->isCollecting());
+            atomsZone->setGCState(Zone::Mark);
+            any = true;
+        }
+    }
+
+    /* Check that at least one zone is scheduled for collection. */
+    if (!any)
+        return false;
+
+    /*
+     * At the end of each incremental slice, we call prepareForIncrementalGC,
+     * which marks objects in all arenas that we're currently allocating
+     * into. This can cause leaks if unreachable objects are in these
+     * arenas. This purge call ensures that we only mark arenas that have had
+     * allocations after the incremental GC started.
+     */
+    if (isIncremental) {
+        for (GCZonesIter zone(rt); !zone.done(); zone.next())
+            zone->arenas.purge();
+    }
+
+    MemProfiler::MarkTenuredStart(rt);
+    marker.start();
+    GCMarker* gcmarker = &marker;
+
+    /* For non-incremental GC the following sweep discards the jit code. */
+    if (isIncremental) {
+        js::CancelOffThreadIonCompile(rt, JS::Zone::Mark);
+        for (GCZonesIter zone(rt); !zone.done(); zone.next()) {
+            gcstats::AutoPhase ap(stats, gcstats::PHASE_MARK_DISCARD_CODE);
+            zone->discardJitCode(rt->defaultFreeOp());
+        }
+    }
+
+    /*
+     * Relazify functions after discarding JIT code (we can't relazify
+     * functions with JIT code) and before the actual mark phase, so that
+     * the current GC can collect the JSScripts we're unlinking here.
+     * We do this only when we're performing a shrinking GC, as too much
+     * relazification can cause performance issues when we have to reparse
+     * the same functions over and over.
+     */
+    if (invocationKind == GC_SHRINK) {
+        {
+            gcstats::AutoPhase ap(stats, gcstats::PHASE_RELAZIFY_FUNCTIONS);
+            for (GCZonesIter zone(rt); !zone.done(); zone.next()) {
+                if (zone->isSelfHostingZone())
+                    continue;
+                RelazifyFunctions(zone, AllocKind::FUNCTION);
+                RelazifyFunctions(zone, AllocKind::FUNCTION_EXTENDED);
+            }
+        }
+
+        /* Purge ShapeTables. */
+        gcstats::AutoPhase ap(stats, gcstats::PHASE_PURGE_SHAPE_TABLES);
+        for (GCZonesIter zone(rt); !zone.done(); zone.next()) {
+            if (zone->keepShapeTables() || zone->isSelfHostingZone())
+                continue;
+            for (auto baseShape = zone->cellIter<BaseShape>(); !baseShape.done(); baseShape.next())
+                baseShape->maybePurgeTable();
+        }
+    }
+
+    startNumber = number;
+
+    /*
+     * We must purge the runtime at the beginning of an incremental GC. The
+     * danger if we purge later is that the snapshot invariant of incremental
+     * GC will be broken, as follows. If some object is reachable only through
+     * some cache (say the dtoaCache) then it will not be part of the snapshot.
+     * If we purge after root marking, then the mutator could obtain a pointer
+     * to the object and start using it. This object might never be marked, so
+     * a GC hazard would exist.
+     */
+    {
+        gcstats::AutoPhase ap(stats, gcstats::PHASE_PURGE);
+        purgeRuntime(lock);
+    }
+
+    /*
+     * Mark phase.
+     */
+    gcstats::AutoPhase ap1(stats, gcstats::PHASE_MARK);
+
+    {
+        gcstats::AutoPhase ap(stats, gcstats::PHASE_UNMARK);
+
+        for (GCZonesIter zone(rt); !zone.done(); zone.next()) {
+            /* Unmark everything in the zones being collected. */
+            zone->arenas.unmarkAll();
+        }
+
+        for (GCZonesIter zone(rt); !zone.done(); zone.next()) {
+            /* Unmark all weak maps in the zones being collected. */
+            WeakMapBase::unmarkZone(zone);
+        }
+    }
+
+    traceRuntimeForMajorGC(gcmarker, lock);
+
+    gcstats::AutoPhase ap2(stats, gcstats::PHASE_MARK_ROOTS);
+
+    if (isIncremental) {
+        gcstats::AutoPhase ap3(stats, gcstats::PHASE_BUFFER_GRAY_ROOTS);
+        bufferGrayRoots();
+    }
+
+    markCompartments();
+
+    return true;
+}
+
+void
+GCRuntime::markCompartments()
+{
+    gcstats::AutoPhase ap(stats, gcstats::PHASE_MARK_COMPARTMENTS);
+
+    /*
+     * This code ensures that if a compartment is "dead", then it will be
+     * collected in this GC. A compartment is considered dead if its maybeAlive
+     * flag is false. The maybeAlive flag is set if:
+     *   (1) the compartment has incoming cross-compartment edges, or
+     *   (2) an object in the compartment was marked during root marking, either
+     *       as a black root or a gray root.
+     * If the maybeAlive is false, then we set the scheduledForDestruction flag.
+     * At the end of the GC, we look for compartments where
+     * scheduledForDestruction is true. These are compartments that were somehow
+     * "revived" during the incremental GC. If any are found, we do a special,
+     * non-incremental GC of those compartments to try to collect them.
+     *
+     * Compartments can be revived for a variety of reasons. On reason is bug
+     * 811587, where a reflector that was dead can be revived by DOM code that
+     * still refers to the underlying DOM node.
+     *
+     * Read barriers and allocations can also cause revival. This might happen
+     * during a function like JS_TransplantObject, which iterates over all
+     * compartments, live or dead, and operates on their objects. See bug 803376
+     * for details on this problem. To avoid the problem, we try to avoid
+     * allocation and read barriers during JS_TransplantObject and the like.
+     */
+
+    /* Set the maybeAlive flag based on cross-compartment edges. */
+    for (CompartmentsIter c(rt, SkipAtoms); !c.done(); c.next()) {
+        for (JSCompartment::WrapperEnum e(c); !e.empty(); e.popFront()) {
+            if (e.front().key().is<JSString*>())
+                continue;
+            JSCompartment* dest = e.front().mutableKey().compartment();
+            if (dest)
+                dest->maybeAlive = true;
+        }
+    }
+
+    /*
+     * For black roots, code in gc/Marking.cpp will already have set maybeAlive
+     * during MarkRuntime.
+     */
+
+    /* Propogate maybeAlive to scheduleForDestruction. */
+    for (GCCompartmentsIter c(rt); !c.done(); c.next()) {
+        if (!c->maybeAlive && !rt->isAtomsCompartment(c))
+            c->scheduledForDestruction = true;
+    }
+}
+
+template <class ZoneIterT>
+void
+GCRuntime::markWeakReferences(gcstats::Phase phase)
+{
+    MOZ_ASSERT(marker.isDrained());
+
+    gcstats::AutoPhase ap1(stats, phase);
+
+    marker.enterWeakMarkingMode();
+
+    // TODO bug 1167452: Make weak marking incremental
+    auto unlimited = SliceBudget::unlimited();
+    MOZ_RELEASE_ASSERT(marker.drainMarkStack(unlimited));
+
+    for (;;) {
+        bool markedAny = false;
+        if (!marker.isWeakMarkingTracer()) {
+            for (ZoneIterT zone(rt); !zone.done(); zone.next())
+                markedAny |= WeakMapBase::markZoneIteratively(zone, &marker);
+        }
+        for (CompartmentsIterT<ZoneIterT> c(rt); !c.done(); c.next()) {
+            if (c->watchpointMap)
+                markedAny |= c->watchpointMap->markIteratively(&marker);
+        }
+        markedAny |= Debugger::markAllIteratively(&marker);
+        markedAny |= jit::JitRuntime::MarkJitcodeGlobalTableIteratively(&marker);
+
+        if (!markedAny)
+            break;
+
+        auto unlimited = SliceBudget::unlimited();
+        MOZ_RELEASE_ASSERT(marker.drainMarkStack(unlimited));
+    }
+    MOZ_ASSERT(marker.isDrained());
+
+    marker.leaveWeakMarkingMode();
+}
+
+void
+GCRuntime::markWeakReferencesInCurrentGroup(gcstats::Phase phase)
+{
+    markWeakReferences<GCZoneGroupIter>(phase);
+}
+
+template <class ZoneIterT, class CompartmentIterT>
+void
+GCRuntime::markGrayReferences(gcstats::Phase phase)
+{
+    gcstats::AutoPhase ap(stats, phase);
+    if (hasBufferedGrayRoots()) {
+        for (ZoneIterT zone(rt); !zone.done(); zone.next())
+            markBufferedGrayRoots(zone);
+    } else {
+        MOZ_ASSERT(!isIncremental);
+        if (JSTraceDataOp op = grayRootTracer.op)
+            (*op)(&marker, grayRootTracer.data);
+    }
+    auto unlimited = SliceBudget::unlimited();
+    MOZ_RELEASE_ASSERT(marker.drainMarkStack(unlimited));
+}
+
+void
+GCRuntime::markGrayReferencesInCurrentGroup(gcstats::Phase phase)
+{
+    markGrayReferences<GCZoneGroupIter, GCCompartmentGroupIter>(phase);
+}
+
+void
+GCRuntime::markAllWeakReferences(gcstats::Phase phase)
+{
+    markWeakReferences<GCZonesIter>(phase);
+}
+
+void
+GCRuntime::markAllGrayReferences(gcstats::Phase phase)
+{
+    markGrayReferences<GCZonesIter, GCCompartmentsIter>(phase);
+}
+
+#ifdef JS_GC_ZEAL
+
+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;
+    }
+};
+
+class js::gc::MarkingValidator
+{
+  public:
+    explicit MarkingValidator(GCRuntime* gc);
+    ~MarkingValidator();
+    void nonIncrementalMark(AutoLockForExclusiveAccess& lock);
+    void validate();
+
+  private:
+    GCRuntime* gc;
+    bool initialized;
+
+    typedef HashMap<Chunk*, ChunkBitmap*, GCChunkHasher, SystemAllocPolicy> BitmapMap;
+    BitmapMap map;
+};
+
+js::gc::MarkingValidator::MarkingValidator(GCRuntime* gc)
+  : gc(gc),
+    initialized(false)
+{}
+
+js::gc::MarkingValidator::~MarkingValidator()
+{
+    if (!map.initialized())
+        return;
+
+    for (BitmapMap::Range r(map.all()); !r.empty(); r.popFront())
+        js_delete(r.front().value());
+}
+
+void
+js::gc::MarkingValidator::nonIncrementalMark(AutoLockForExclusiveAccess& lock)
+{
+    /*
+     * Perform a non-incremental mark for all collecting zones and record
+     * the results for later comparison.
+     *
+     * Currently this does not validate gray marking.
+     */
+
+    if (!map.init())
+        return;
+
+    JSRuntime* runtime = gc->rt;
+    GCMarker* gcmarker = &gc->marker;
+
+    gc->waitBackgroundSweepEnd();
+
+    /* Save existing mark bits. */
+    for (auto chunk = gc->allNonEmptyChunks(); !chunk.done(); chunk.next()) {
+        ChunkBitmap* bitmap = &chunk->bitmap;
+	ChunkBitmap* entry = js_new<ChunkBitmap>();
+        if (!entry)
+            return;
+
+        memcpy((void*)entry->bitmap, (void*)bitmap->bitmap, sizeof(bitmap->bitmap));
+        if (!map.putNew(chunk, entry))
+            return;
+    }
+
+    /*
+     * Temporarily clear the weakmaps' mark flags for the compartments we are
+     * collecting.
+     */
+
+    WeakMapSet markedWeakMaps;
+    if (!markedWeakMaps.init())
+        return;
+
+    /*
+     * For saving, smush all of the keys into one big table and split them back
+     * up into per-zone tables when restoring.
+     */
+    gc::WeakKeyTable savedWeakKeys(SystemAllocPolicy(), runtime->randomHashCodeScrambler());
+    if (!savedWeakKeys.init())
+        return;
+
+    for (GCZonesIter zone(runtime); !zone.done(); zone.next()) {
+        if (!WeakMapBase::saveZoneMarkedWeakMaps(zone, markedWeakMaps))
+            return;
+
+        AutoEnterOOMUnsafeRegion oomUnsafe;
+        for (gc::WeakKeyTable::Range r = zone->gcWeakKeys.all(); !r.empty(); r.popFront()) {
+            if (!savedWeakKeys.put(Move(r.front().key), Move(r.front().value)))
+                oomUnsafe.crash("saving weak keys table for validator");
+        }
+
+        if (!zone->gcWeakKeys.clear())
+            oomUnsafe.crash("clearing weak keys table for validator");
+    }
+
+    /*
+     * After this point, the function should run to completion, so we shouldn't
+     * do anything fallible.
+     */
+    initialized = true;
+
+    /* Re-do all the marking, but non-incrementally. */
+    js::gc::State state = gc->incrementalState;
+    gc->incrementalState = State::MarkRoots;
+
+    {
+        gcstats::AutoPhase ap(gc->stats, gcstats::PHASE_MARK);
+
+        {
+            gcstats::AutoPhase ap(gc->stats, gcstats::PHASE_UNMARK);
+
+            for (GCZonesIter zone(runtime); !zone.done(); zone.next())
+                WeakMapBase::unmarkZone(zone);
+
+            MOZ_ASSERT(gcmarker->isDrained());
+            gcmarker->reset();
+
+            for (auto chunk = gc->allNonEmptyChunks(); !chunk.done(); chunk.next())
+                chunk->bitmap.clear();
+        }
+
+        gc->traceRuntimeForMajorGC(gcmarker, lock);
+
+        gc->incrementalState = State::Mark;
+        auto unlimited = SliceBudget::unlimited();
+        MOZ_RELEASE_ASSERT(gc->marker.drainMarkStack(unlimited));
+    }
+
+    gc->incrementalState = State::Sweep;
+    {
+        gcstats::AutoPhase ap1(gc->stats, gcstats::PHASE_SWEEP);
+        gcstats::AutoPhase ap2(gc->stats, gcstats::PHASE_SWEEP_MARK);
+
+        gc->markAllWeakReferences(gcstats::PHASE_SWEEP_MARK_WEAK);
+
+        /* Update zone state for gray marking. */
+        for (GCZonesIter zone(runtime); !zone.done(); zone.next()) {
+            MOZ_ASSERT(zone->isGCMarkingBlack());
+            zone->setGCState(Zone::MarkGray);
+        }
+        gc->marker.setMarkColorGray();
+
+        gc->markAllGrayReferences(gcstats::PHASE_SWEEP_MARK_GRAY);
+        gc->markAllWeakReferences(gcstats::PHASE_SWEEP_MARK_GRAY_WEAK);
+
+        /* Restore zone state. */
+        for (GCZonesIter zone(runtime); !zone.done(); zone.next()) {
+            MOZ_ASSERT(zone->isGCMarkingGray());
+            zone->setGCState(Zone::Mark);
+        }
+        MOZ_ASSERT(gc->marker.isDrained());
+        gc->marker.setMarkColorBlack();
+    }
+
+    /* Take a copy of the non-incremental mark state and restore the original. */
+    for (auto chunk = gc->allNonEmptyChunks(); !chunk.done(); chunk.next()) {
+        ChunkBitmap* bitmap = &chunk->bitmap;
+        ChunkBitmap* entry = map.lookup(chunk)->value();
+        Swap(*entry, *bitmap);
+    }
+
+    for (GCZonesIter zone(runtime); !zone.done(); zone.next()) {
+        WeakMapBase::unmarkZone(zone);
+        AutoEnterOOMUnsafeRegion oomUnsafe;
+        if (!zone->gcWeakKeys.clear())
+            oomUnsafe.crash("clearing weak keys table for validator");
+    }
+
+    WeakMapBase::restoreMarkedWeakMaps(markedWeakMaps);
+
+    for (gc::WeakKeyTable::Range r = savedWeakKeys.all(); !r.empty(); r.popFront()) {
+        AutoEnterOOMUnsafeRegion oomUnsafe;
+        Zone* zone = gc::TenuredCell::fromPointer(r.front().key.asCell())->zone();
+        if (!zone->gcWeakKeys.put(Move(r.front().key), Move(r.front().value)))
+            oomUnsafe.crash("restoring weak keys table for validator");
+    }
+
+    gc->incrementalState = state;
+}
+
+void
+js::gc::MarkingValidator::validate()
+{
+    /*
+     * Validates the incremental marking for a single compartment by comparing
+     * the mark bits to those previously recorded for a non-incremental mark.
+     */
+
+    if (!initialized)
+        return;
+
+    gc->waitBackgroundSweepEnd();
+
+    for (auto chunk = gc->allNonEmptyChunks(); !chunk.done(); chunk.next()) {
+        BitmapMap::Ptr ptr = map.lookup(chunk);
+        if (!ptr)
+            continue;  /* Allocated after we did the non-incremental mark. */
+
+        ChunkBitmap* bitmap = ptr->value();
+        ChunkBitmap* incBitmap = &chunk->bitmap;
+
+        for (size_t i = 0; i < ArenasPerChunk; i++) {
+            if (chunk->decommittedArenas.get(i))
+                continue;
+            Arena* arena = &chunk->arenas[i];
+            if (!arena->allocated())
+                continue;
+            if (!arena->zone->isGCSweeping())
+                continue;
+            if (arena->allocatedDuringIncremental)
+                continue;
+
+            AllocKind kind = arena->getAllocKind();
+            uintptr_t thing = arena->thingsStart();
+            uintptr_t end = arena->thingsEnd();
+            while (thing < end) {
+                Cell* cell = (Cell*)thing;
+
+                /*
+                 * If a non-incremental GC wouldn't have collected a cell, then
+                 * an incremental GC won't collect it.
+                 */
+                MOZ_ASSERT_IF(bitmap->isMarked(cell, BLACK), incBitmap->isMarked(cell, BLACK));
+
+                /*
+                 * If the cycle collector isn't allowed to collect an object
+                 * after a non-incremental GC has run, then it isn't allowed to
+                 * collected it after an incremental GC.
+                 */
+                MOZ_ASSERT_IF(!bitmap->isMarked(cell, GRAY), !incBitmap->isMarked(cell, GRAY));
+
+                thing += Arena::thingSize(kind);
+            }
+        }
+    }
+}
+
+#endif // JS_GC_ZEAL
+
+void
+GCRuntime::computeNonIncrementalMarkingForValidation(AutoLockForExclusiveAccess& lock)
+{
+#ifdef JS_GC_ZEAL
+    MOZ_ASSERT(!markingValidator);
+    if (isIncremental && hasZealMode(ZealMode::IncrementalMarkingValidator))
+        markingValidator = js_new<MarkingValidator>(this);
+    if (markingValidator)
+        markingValidator->nonIncrementalMark(lock);
+#endif
+}
+
+void
+GCRuntime::validateIncrementalMarking()
+{
+#ifdef JS_GC_ZEAL
+    if (markingValidator)
+        markingValidator->validate();
+#endif
+}
+
+void
+GCRuntime::finishMarkingValidation()
+{
+#ifdef JS_GC_ZEAL
+    js_delete(markingValidator);
+    markingValidator = nullptr;
+#endif
+}
+
+static void
+DropStringWrappers(JSRuntime* rt)
+{
+    /*
+     * String "wrappers" are dropped on GC because their presence would require
+     * us to sweep the wrappers in all compartments every time we sweep a
+     * compartment group.
+     */
+    for (CompartmentsIter c(rt, SkipAtoms); !c.done(); c.next()) {
+        for (JSCompartment::WrapperEnum e(c); !e.empty(); e.popFront()) {
+            if (e.front().key().is<JSString*>())
+                e.removeFront();
+        }
+    }
+}
+
+/*
+ * Group zones that must be swept at the same time.
+ *
+ * If compartment A has an edge to an unmarked object in compartment B, then we
+ * must not sweep A in a later slice than we sweep B. That's because a write
+ * barrier in A could lead to the unmarked object in B becoming marked.
+ * However, if we had already swept that object, we would be in trouble.
+ *
+ * If we consider these dependencies as a graph, then all the compartments in
+ * any strongly-connected component of this graph must be swept in the same
+ * slice.
+ *
+ * Tarjan's algorithm is used to calculate the components.
+ */
+namespace {
+struct AddOutgoingEdgeFunctor {
+    bool needsEdge_;
+    ZoneComponentFinder& finder_;
+
+    AddOutgoingEdgeFunctor(bool needsEdge, ZoneComponentFinder& finder)
+      : needsEdge_(needsEdge), finder_(finder)
+    {}
+
+    template <typename T>
+    void operator()(T tp) {
+        TenuredCell& other = (*tp)->asTenured();
+
+        /*
+         * Add edge to wrapped object compartment if wrapped object is not
+         * marked black to indicate that wrapper compartment not be swept
+         * after wrapped compartment.
+         */
+        if (needsEdge_) {
+            JS::Zone* zone = other.zone();
+            if (zone->isGCMarking())
+                finder_.addEdgeTo(zone);
+        }
+    }
+};
+} // namespace (anonymous)
+
+void
+JSCompartment::findOutgoingEdges(ZoneComponentFinder& finder)
+{
+    for (js::WrapperMap::Enum e(crossCompartmentWrappers); !e.empty(); e.popFront()) {
+        CrossCompartmentKey& key = e.front().mutableKey();
+        MOZ_ASSERT(!key.is<JSString*>());
+        bool needsEdge = true;
+        if (key.is<JSObject*>()) {
+            TenuredCell& other = key.as<JSObject*>()->asTenured();
+            needsEdge = !other.isMarked(BLACK) || other.isMarked(GRAY);
+        }
+        key.applyToWrapped(AddOutgoingEdgeFunctor(needsEdge, finder));
+    }
+}
+
+void
+Zone::findOutgoingEdges(ZoneComponentFinder& finder)
+{
+    /*
+     * Any compartment may have a pointer to an atom in the atoms
+     * compartment, and these aren't in the cross compartment map.
+     */
+    JSRuntime* rt = runtimeFromMainThread();
+    Zone* atomsZone = rt->atomsCompartment(finder.lock)->zone();
+    if (atomsZone->isGCMarking())
+        finder.addEdgeTo(atomsZone);
+
+    for (CompartmentsInZoneIter comp(this); !comp.done(); comp.next())
+        comp->findOutgoingEdges(finder);
+
+    for (ZoneSet::Range r = gcZoneGroupEdges.all(); !r.empty(); r.popFront()) {
+        if (r.front()->isGCMarking())
+            finder.addEdgeTo(r.front());
+    }
+
+    Debugger::findZoneEdges(this, finder);
+}
+
+bool
+GCRuntime::findInterZoneEdges()
+{
+    /*
+     * Weakmaps which have keys with delegates in a different zone introduce the
+     * need for zone edges from the delegate's zone to the weakmap zone.
+     *
+     * Since the edges point into and not away from the zone the weakmap is in
+     * we must find these edges in advance and store them in a set on the Zone.
+     * If we run out of memory, we fall back to sweeping everything in one
+     * group.
+     */
+
+    for (GCZonesIter zone(rt); !zone.done(); zone.next()) {
+        if (!WeakMapBase::findInterZoneEdges(zone))
+            return false;
+    }
+
+    return true;
+}
+
+void
+GCRuntime::findZoneGroups(AutoLockForExclusiveAccess& lock)
+{
+#ifdef DEBUG
+    for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next())
+        MOZ_ASSERT(zone->gcZoneGroupEdges.empty());
+#endif
+
+    JSContext* cx = rt->contextFromMainThread();
+    ZoneComponentFinder finder(cx->nativeStackLimit[StackForSystemCode], lock);
+    if (!isIncremental || !findInterZoneEdges())
+        finder.useOneComponent();
+
+    for (GCZonesIter zone(rt); !zone.done(); zone.next()) {
+        MOZ_ASSERT(zone->isGCMarking());
+        finder.addNode(zone);
+    }
+    zoneGroups = finder.getResultsList();
+    currentZoneGroup = zoneGroups;
+    zoneGroupIndex = 0;
+
+    for (GCZonesIter zone(rt); !zone.done(); zone.next())
+        zone->gcZoneGroupEdges.clear();
+
+#ifdef DEBUG
+    for (Zone* head = currentZoneGroup; head; head = head->nextGroup()) {
+        for (Zone* zone = head; zone; zone = zone->nextNodeInGroup())
+            MOZ_ASSERT(zone->isGCMarking());
+    }
+
+    MOZ_ASSERT_IF(!isIncremental, !currentZoneGroup->nextGroup());
+    for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next())
+        MOZ_ASSERT(zone->gcZoneGroupEdges.empty());
+#endif
+}
+
+static void
+ResetGrayList(JSCompartment* comp);
+
+void
+GCRuntime::getNextZoneGroup()
+{
+    currentZoneGroup = currentZoneGroup->nextGroup();
+    ++zoneGroupIndex;
+    if (!currentZoneGroup) {
+        abortSweepAfterCurrentGroup = false;
+        return;
+    }
+
+    for (Zone* zone = currentZoneGroup; zone; zone = zone->nextNodeInGroup()) {
+        MOZ_ASSERT(zone->isGCMarking());
+        MOZ_ASSERT(!zone->isQueuedForBackgroundSweep());
+    }
+
+    if (!isIncremental)
+        ZoneComponentFinder::mergeGroups(currentZoneGroup);
+
+    if (abortSweepAfterCurrentGroup) {
+        MOZ_ASSERT(!isIncremental);
+        for (GCZoneGroupIter zone(rt); !zone.done(); zone.next()) {
+            MOZ_ASSERT(!zone->gcNextGraphComponent);
+            MOZ_ASSERT(zone->isGCMarking());
+            zone->setNeedsIncrementalBarrier(false, Zone::UpdateJit);
+            zone->setGCState(Zone::NoGC);
+            zone->gcGrayRoots.clearAndFree();
+        }
+
+        for (GCCompartmentGroupIter comp(rt); !comp.done(); comp.next())
+            ResetGrayList(comp);
+
+        abortSweepAfterCurrentGroup = false;
+        currentZoneGroup = nullptr;
+    }
+}
+
+/*
+ * Gray marking:
+ *
+ * At the end of collection, anything reachable from a gray root that has not
+ * otherwise been marked black must be marked gray.
+ *
+ * This means that when marking things gray we must not allow marking to leave
+ * the current compartment group, as that could result in things being marked
+ * grey when they might subsequently be marked black.  To achieve this, when we
+ * find a cross compartment pointer we don't mark the referent but add it to a
+ * singly-linked list of incoming gray pointers that is stored with each
+ * compartment.
+ *
+ * The list head is stored in JSCompartment::gcIncomingGrayPointers and contains
+ * cross compartment wrapper objects. The next pointer is stored in the second
+ * extra slot of the cross compartment wrapper.
+ *
+ * The list is created during gray marking when one of the
+ * MarkCrossCompartmentXXX functions is called for a pointer that leaves the
+ * current compartent group.  This calls DelayCrossCompartmentGrayMarking to
+ * push the referring object onto the list.
+ *
+ * The list is traversed and then unlinked in
+ * MarkIncomingCrossCompartmentPointers.
+ */
+
+static bool
+IsGrayListObject(JSObject* obj)
+{
+    MOZ_ASSERT(obj);
+    return obj->is<CrossCompartmentWrapperObject>() && !IsDeadProxyObject(obj);
+}
+
+/* static */ unsigned
+ProxyObject::grayLinkExtraSlot(JSObject* obj)
+{
+    MOZ_ASSERT(IsGrayListObject(obj));
+    return 1;
+}
+
+#ifdef DEBUG
+static void
+AssertNotOnGrayList(JSObject* obj)
+{
+    MOZ_ASSERT_IF(IsGrayListObject(obj),
+                  GetProxyExtra(obj, ProxyObject::grayLinkExtraSlot(obj)).isUndefined());
+}
+#endif
+
+static void
+AssertNoWrappersInGrayList(JSRuntime* rt)
+{
+#ifdef DEBUG
+    for (CompartmentsIter c(rt, SkipAtoms); !c.done(); c.next()) {
+        MOZ_ASSERT(!c->gcIncomingGrayPointers);
+        for (JSCompartment::WrapperEnum e(c); !e.empty(); e.popFront()) {
+            if (!e.front().key().is<JSString*>())
+                AssertNotOnGrayList(&e.front().value().unbarrieredGet().toObject());
+        }
+    }
+#endif
+}
+
+static JSObject*
+CrossCompartmentPointerReferent(JSObject* obj)
+{
+    MOZ_ASSERT(IsGrayListObject(obj));
+    return &obj->as<ProxyObject>().private_().toObject();
+}
+
+static JSObject*
+NextIncomingCrossCompartmentPointer(JSObject* prev, bool unlink)
+{
+    unsigned slot = ProxyObject::grayLinkExtraSlot(prev);
+    JSObject* next = GetProxyExtra(prev, slot).toObjectOrNull();
+    MOZ_ASSERT_IF(next, IsGrayListObject(next));
+
+    if (unlink)
+        SetProxyExtra(prev, slot, UndefinedValue());
+
+    return next;
+}
+
+void
+js::DelayCrossCompartmentGrayMarking(JSObject* src)
+{
+    MOZ_ASSERT(IsGrayListObject(src));
+
+    /* Called from MarkCrossCompartmentXXX functions. */
+    unsigned slot = ProxyObject::grayLinkExtraSlot(src);
+    JSObject* dest = CrossCompartmentPointerReferent(src);
+    JSCompartment* comp = dest->compartment();
+
+    if (GetProxyExtra(src, slot).isUndefined()) {
+        SetProxyExtra(src, slot, ObjectOrNullValue(comp->gcIncomingGrayPointers));
+        comp->gcIncomingGrayPointers = src;
+    } else {
+        MOZ_ASSERT(GetProxyExtra(src, slot).isObjectOrNull());
+    }
+
+#ifdef DEBUG
+    /*
+     * Assert that the object is in our list, also walking the list to check its
+     * integrity.
+     */
+    JSObject* obj = comp->gcIncomingGrayPointers;
+    bool found = false;
+    while (obj) {
+        if (obj == src)
+            found = true;
+        obj = NextIncomingCrossCompartmentPointer(obj, false);
+    }
+    MOZ_ASSERT(found);
+#endif
+}
+
+static void
+MarkIncomingCrossCompartmentPointers(JSRuntime* rt, const uint32_t color)
+{
+    MOZ_ASSERT(color == BLACK || color == GRAY);
+
+    static const gcstats::Phase statsPhases[] = {
+        gcstats::PHASE_SWEEP_MARK_INCOMING_BLACK,
+        gcstats::PHASE_SWEEP_MARK_INCOMING_GRAY
+    };
+    gcstats::AutoPhase ap1(rt->gc.stats, statsPhases[color]);
+
+    bool unlinkList = color == GRAY;
+
+    for (GCCompartmentGroupIter c(rt); !c.done(); c.next()) {
+        MOZ_ASSERT_IF(color == GRAY, c->zone()->isGCMarkingGray());
+        MOZ_ASSERT_IF(color == BLACK, c->zone()->isGCMarkingBlack());
+        MOZ_ASSERT_IF(c->gcIncomingGrayPointers, IsGrayListObject(c->gcIncomingGrayPointers));
+
+        for (JSObject* src = c->gcIncomingGrayPointers;
+             src;
+             src = NextIncomingCrossCompartmentPointer(src, unlinkList))
+        {
+            JSObject* dst = CrossCompartmentPointerReferent(src);
+            MOZ_ASSERT(dst->compartment() == c);
+
+            if (color == GRAY) {
+                if (IsMarkedUnbarriered(rt, &src) && src->asTenured().isMarked(GRAY))
+                    TraceManuallyBarrieredEdge(&rt->gc.marker, &dst,
+                                               "cross-compartment gray pointer");
+            } else {
+                if (IsMarkedUnbarriered(rt, &src) && !src->asTenured().isMarked(GRAY))
+                    TraceManuallyBarrieredEdge(&rt->gc.marker, &dst,
+                                               "cross-compartment black pointer");
+            }
+        }
+
+        if (unlinkList)
+            c->gcIncomingGrayPointers = nullptr;
+    }
+
+    auto unlimited = SliceBudget::unlimited();
+    MOZ_RELEASE_ASSERT(rt->gc.marker.drainMarkStack(unlimited));
+}
+
+static bool
+RemoveFromGrayList(JSObject* wrapper)
+{
+    if (!IsGrayListObject(wrapper))
+        return false;
+
+    unsigned slot = ProxyObject::grayLinkExtraSlot(wrapper);
+    if (GetProxyExtra(wrapper, slot).isUndefined())
+        return false;  /* Not on our list. */
+
+    JSObject* tail = GetProxyExtra(wrapper, slot).toObjectOrNull();
+    SetProxyExtra(wrapper, slot, UndefinedValue());
+
+    JSCompartment* comp = CrossCompartmentPointerReferent(wrapper)->compartment();
+    JSObject* obj = comp->gcIncomingGrayPointers;
+    if (obj == wrapper) {
+        comp->gcIncomingGrayPointers = tail;
+        return true;
+    }
+
+    while (obj) {
+        unsigned slot = ProxyObject::grayLinkExtraSlot(obj);
+        JSObject* next = GetProxyExtra(obj, slot).toObjectOrNull();
+        if (next == wrapper) {
+            SetProxyExtra(obj, slot, ObjectOrNullValue(tail));
+            return true;
+        }
+        obj = next;
+    }
+
+    MOZ_CRASH("object not found in gray link list");
+}
+
+static void
+ResetGrayList(JSCompartment* comp)
+{
+    JSObject* src = comp->gcIncomingGrayPointers;
+    while (src)
+        src = NextIncomingCrossCompartmentPointer(src, true);
+    comp->gcIncomingGrayPointers = nullptr;
+}
+
+void
+js::NotifyGCNukeWrapper(JSObject* obj)
+{
+    /*
+     * References to target of wrapper are being removed, we no longer have to
+     * remember to mark it.
+     */
+    RemoveFromGrayList(obj);
+}
+
+enum {
+    JS_GC_SWAP_OBJECT_A_REMOVED = 1 << 0,
+    JS_GC_SWAP_OBJECT_B_REMOVED = 1 << 1
+};
+
+unsigned
+js::NotifyGCPreSwap(JSObject* a, JSObject* b)
+{
+    /*
+     * Two objects in the same compartment are about to have had their contents
+     * swapped.  If either of them are in our gray pointer list, then we remove
+     * them from the lists, returning a bitset indicating what happened.
+     */
+    return (RemoveFromGrayList(a) ? JS_GC_SWAP_OBJECT_A_REMOVED : 0) |
+           (RemoveFromGrayList(b) ? JS_GC_SWAP_OBJECT_B_REMOVED : 0);
+}
+
+void
+js::NotifyGCPostSwap(JSObject* a, JSObject* b, unsigned removedFlags)
+{
+    /*
+     * Two objects in the same compartment have had their contents swapped.  If
+     * either of them were in our gray pointer list, we re-add them again.
+     */
+    if (removedFlags & JS_GC_SWAP_OBJECT_A_REMOVED)
+        DelayCrossCompartmentGrayMarking(b);
+    if (removedFlags & JS_GC_SWAP_OBJECT_B_REMOVED)
+        DelayCrossCompartmentGrayMarking(a);
+}
+
+void
+GCRuntime::endMarkingZoneGroup()
+{
+    gcstats::AutoPhase ap(stats, gcstats::PHASE_SWEEP_MARK);
+
+    /*
+     * Mark any incoming black pointers from previously swept compartments
+     * whose referents are not marked. This can occur when gray cells become
+     * black by the action of UnmarkGray.
+     */
+    MarkIncomingCrossCompartmentPointers(rt, BLACK);
+    markWeakReferencesInCurrentGroup(gcstats::PHASE_SWEEP_MARK_WEAK);
+
+    /*
+     * Change state of current group to MarkGray to restrict marking to this
+     * group.  Note that there may be pointers to the atoms compartment, and
+     * these will be marked through, as they are not marked with
+     * MarkCrossCompartmentXXX.
+     */
+    for (GCZoneGroupIter zone(rt); !zone.done(); zone.next()) {
+        MOZ_ASSERT(zone->isGCMarkingBlack());
+        zone->setGCState(Zone::MarkGray);
+    }
+    marker.setMarkColorGray();
+
+    /* Mark incoming gray pointers from previously swept compartments. */
+    MarkIncomingCrossCompartmentPointers(rt, GRAY);
+
+    /* Mark gray roots and mark transitively inside the current compartment group. */
+    markGrayReferencesInCurrentGroup(gcstats::PHASE_SWEEP_MARK_GRAY);
+    markWeakReferencesInCurrentGroup(gcstats::PHASE_SWEEP_MARK_GRAY_WEAK);
+
+    /* Restore marking state. */
+    for (GCZoneGroupIter zone(rt); !zone.done(); zone.next()) {
+        MOZ_ASSERT(zone->isGCMarkingGray());
+        zone->setGCState(Zone::Mark);
+    }
+    MOZ_ASSERT(marker.isDrained());
+    marker.setMarkColorBlack();
+}
+
+class GCSweepTask : public GCParallelTask
+{
+    GCSweepTask(const GCSweepTask&) = delete;
+
+  protected:
+    JSRuntime* runtime;
+
+  public:
+    explicit GCSweepTask(JSRuntime* rt) : runtime(rt) {}
+    GCSweepTask(GCSweepTask&& other)
+      : GCParallelTask(mozilla::Move(other)),
+        runtime(other.runtime)
+    {}
+};
+
+// Causes the given WeakCache to be swept when run.
+class SweepWeakCacheTask : public GCSweepTask
+{
+    JS::WeakCache<void*>& cache;
+
+    SweepWeakCacheTask(const SweepWeakCacheTask&) = delete;
+
+  public:
+    SweepWeakCacheTask(JSRuntime* rt, JS::WeakCache<void*>& wc) : GCSweepTask(rt), cache(wc) {}
+    SweepWeakCacheTask(SweepWeakCacheTask&& other)
+      : GCSweepTask(mozilla::Move(other)), cache(other.cache)
+    {}
+
+    void run() override {
+        cache.sweep();
+    }
+};
+
+#define MAKE_GC_SWEEP_TASK(name)                                              \
+    class name : public GCSweepTask {                                         \
+        void run() override;                                                  \
+      public:                                                                 \
+        explicit name (JSRuntime* rt) : GCSweepTask(rt) {}                    \
+    }
+MAKE_GC_SWEEP_TASK(SweepAtomsTask);
+MAKE_GC_SWEEP_TASK(SweepCCWrappersTask);
+MAKE_GC_SWEEP_TASK(SweepBaseShapesTask);
+MAKE_GC_SWEEP_TASK(SweepInitialShapesTask);
+MAKE_GC_SWEEP_TASK(SweepObjectGroupsTask);
+MAKE_GC_SWEEP_TASK(SweepRegExpsTask);
+MAKE_GC_SWEEP_TASK(SweepMiscTask);
+#undef MAKE_GC_SWEEP_TASK
+
+/* virtual */ void
+SweepAtomsTask::run()
+{
+    runtime->sweepAtoms();
+    for (CompartmentsIter comp(runtime, SkipAtoms); !comp.done(); comp.next())
+        comp->sweepVarNames();
+}
+
+/* virtual */ void
+SweepCCWrappersTask::run()
+{
+    for (GCCompartmentGroupIter c(runtime); !c.done(); c.next())
+        c->sweepCrossCompartmentWrappers();
+}
+
+/* virtual */ void
+SweepObjectGroupsTask::run()
+{
+    for (GCCompartmentGroupIter c(runtime); !c.done(); c.next())
+        c->objectGroups.sweep(runtime->defaultFreeOp());
+}
+
+/* virtual */ void
+SweepRegExpsTask::run()
+{
+    for (GCCompartmentGroupIter c(runtime); !c.done(); c.next())
+        c->sweepRegExps();
+}
+
+/* virtual */ void
+SweepMiscTask::run()
+{
+    for (GCCompartmentGroupIter c(runtime); !c.done(); c.next()) {
+        c->sweepSavedStacks();
+        c->sweepSelfHostingScriptSource();
+        c->sweepNativeIterators();
+    }
+}
+
+void
+GCRuntime::startTask(GCParallelTask& task, gcstats::Phase phase, AutoLockHelperThreadState& locked)
+{
+    if (!task.startWithLockHeld(locked)) {
+        AutoUnlockHelperThreadState unlock(locked);
+        gcstats::AutoPhase ap(stats, phase);
+        task.runFromMainThread(rt);
+    }
+}
+
+void
+GCRuntime::joinTask(GCParallelTask& task, gcstats::Phase phase, AutoLockHelperThreadState& locked)
+{
+    gcstats::AutoPhase ap(stats, task, phase);
+    task.joinWithLockHeld(locked);
+}
+
+using WeakCacheTaskVector = mozilla::Vector<SweepWeakCacheTask, 0, SystemAllocPolicy>;
+
+static void
+SweepWeakCachesFromMainThread(JSRuntime* rt)
+{
+    for (GCZoneGroupIter zone(rt); !zone.done(); zone.next()) {
+        for (JS::WeakCache<void*>* cache : zone->weakCaches_) {
+            SweepWeakCacheTask task(rt, *cache);
+            task.runFromMainThread(rt);
+        }
+    }
+}
+
+static WeakCacheTaskVector
+PrepareWeakCacheTasks(JSRuntime* rt)
+{
+    WeakCacheTaskVector out;
+    for (GCZoneGroupIter zone(rt); !zone.done(); zone.next()) {
+        for (JS::WeakCache<void*>* cache : zone->weakCaches_) {
+            if (!out.append(SweepWeakCacheTask(rt, *cache))) {
+                SweepWeakCachesFromMainThread(rt);
+                return WeakCacheTaskVector();
+            }
+        }
+    }
+    return out;
+}
+
+void
+GCRuntime::beginSweepingZoneGroup(AutoLockForExclusiveAccess& lock)
+{
+    /*
+     * Begin sweeping the group of zones in gcCurrentZoneGroup,
+     * performing actions that must be done before yielding to caller.
+     */
+
+    bool sweepingAtoms = false;
+    for (GCZoneGroupIter zone(rt); !zone.done(); zone.next()) {
+        /* Set the GC state to sweeping. */
+        MOZ_ASSERT(zone->isGCMarking());
+        zone->setGCState(Zone::Sweep);
+
+        /* Purge the ArenaLists before sweeping. */
+        zone->arenas.purge();
+
+        if (zone->isAtomsZone())
+            sweepingAtoms = true;
+
+        if (rt->sweepZoneCallback)
+            rt->sweepZoneCallback(zone);
+
+#ifdef DEBUG
+        zone->gcLastZoneGroupIndex = zoneGroupIndex;
+#endif
+    }
+
+    validateIncrementalMarking();
+
+    FreeOp fop(rt);
+    SweepAtomsTask sweepAtomsTask(rt);
+    SweepCCWrappersTask sweepCCWrappersTask(rt);
+    SweepObjectGroupsTask sweepObjectGroupsTask(rt);
+    SweepRegExpsTask sweepRegExpsTask(rt);
+    SweepMiscTask sweepMiscTask(rt);
+    WeakCacheTaskVector sweepCacheTasks = PrepareWeakCacheTasks(rt);
+
+    for (GCZoneGroupIter zone(rt); !zone.done(); zone.next()) {
+        /* Clear all weakrefs that point to unmarked things. */
+        for (auto edge : zone->gcWeakRefs) {
+            /* Edges may be present multiple times, so may already be nulled. */
+            if (*edge && IsAboutToBeFinalizedDuringSweep(**edge))
+                *edge = nullptr;
+        }
+        zone->gcWeakRefs.clear();
+
+        /* No need to look up any more weakmap keys from this zone group. */
+        AutoEnterOOMUnsafeRegion oomUnsafe;
+        if (!zone->gcWeakKeys.clear())
+            oomUnsafe.crash("clearing weak keys in beginSweepingZoneGroup()");
+    }
+
+    {
+        gcstats::AutoPhase ap(stats, gcstats::PHASE_FINALIZE_START);
+        callFinalizeCallbacks(&fop, JSFINALIZE_GROUP_START);
+        {
+            gcstats::AutoPhase ap2(stats, gcstats::PHASE_WEAK_ZONEGROUP_CALLBACK);
+            callWeakPointerZoneGroupCallbacks();
+        }
+        {
+            gcstats::AutoPhase ap2(stats, gcstats::PHASE_WEAK_COMPARTMENT_CALLBACK);
+            for (GCZoneGroupIter zone(rt); !zone.done(); zone.next()) {
+                for (CompartmentsInZoneIter comp(zone); !comp.done(); comp.next())
+                    callWeakPointerCompartmentCallbacks(comp);
+            }
+        }
+    }
+
+    if (sweepingAtoms) {
+        AutoLockHelperThreadState helperLock;
+        startTask(sweepAtomsTask, gcstats::PHASE_SWEEP_ATOMS, helperLock);
+    }
+
+    {
+        gcstats::AutoPhase ap(stats, gcstats::PHASE_SWEEP_COMPARTMENTS);
+        gcstats::AutoSCC scc(stats, zoneGroupIndex);
+
+        {
+            AutoLockHelperThreadState helperLock;
+            startTask(sweepCCWrappersTask, gcstats::PHASE_SWEEP_CC_WRAPPER, helperLock);
+            startTask(sweepObjectGroupsTask, gcstats::PHASE_SWEEP_TYPE_OBJECT, helperLock);
+            startTask(sweepRegExpsTask, gcstats::PHASE_SWEEP_REGEXP, helperLock);
+            startTask(sweepMiscTask, gcstats::PHASE_SWEEP_MISC, helperLock);
+            for (auto& task : sweepCacheTasks)
+                startTask(task, gcstats::PHASE_SWEEP_MISC, helperLock);
+        }
+
+        // The remainder of the of the tasks run in parallel on the main
+        // thread until we join, below.
+        {
+            gcstats::AutoPhase ap(stats, gcstats::PHASE_SWEEP_MISC);
+
+            // Cancel any active or pending off thread compilations.
+            js::CancelOffThreadIonCompile(rt, JS::Zone::Sweep);
+
+            for (GCCompartmentGroupIter c(rt); !c.done(); c.next()) {
+                c->sweepGlobalObject(&fop);
+                c->sweepDebugEnvironments();
+                c->sweepJitCompartment(&fop);
+                c->sweepTemplateObjects();
+            }
+
+            for (GCZoneGroupIter zone(rt); !zone.done(); zone.next())
+                zone->sweepWeakMaps();
+
+            // Bug 1071218: the following two methods have not yet been
+            // refactored to work on a single zone-group at once.
+
+            // Collect watch points associated with unreachable objects.
+            WatchpointMap::sweepAll(rt);
+
+            // Detach unreachable debuggers and global objects from each other.
+            Debugger::sweepAll(&fop);
+
+            // Sweep entries containing about-to-be-finalized JitCode and
+            // update relocated TypeSet::Types inside the JitcodeGlobalTable.
+            jit::JitRuntime::SweepJitcodeGlobalTable(rt);
+        }
+
+        {
+            gcstats::AutoPhase apdc(stats, gcstats::PHASE_SWEEP_DISCARD_CODE);
+            for (GCZoneGroupIter zone(rt); !zone.done(); zone.next())
+                zone->discardJitCode(&fop);
+        }
+
+        {
+            gcstats::AutoPhase ap1(stats, gcstats::PHASE_SWEEP_TYPES);
+            gcstats::AutoPhase ap2(stats, gcstats::PHASE_SWEEP_TYPES_BEGIN);
+            for (GCZoneGroupIter zone(rt); !zone.done(); zone.next())
+                zone->beginSweepTypes(&fop, releaseObservedTypes && !zone->isPreservingCode());
+        }
+
+        {
+            gcstats::AutoPhase ap(stats, gcstats::PHASE_SWEEP_BREAKPOINT);
+            for (GCZoneGroupIter zone(rt); !zone.done(); zone.next())
+                zone->sweepBreakpoints(&fop);
+        }
+
+        {
+            gcstats::AutoPhase ap(stats, gcstats::PHASE_SWEEP_BREAKPOINT);
+            for (GCZoneGroupIter zone(rt); !zone.done(); zone.next())
+                zone->sweepUniqueIds(&fop);
+        }
+    }
+
+    if (sweepingAtoms) {
+        gcstats::AutoPhase ap(stats, gcstats::PHASE_SWEEP_SYMBOL_REGISTRY);
+        rt->symbolRegistry(lock).sweep();
+    }
+
+    // Rejoin our off-main-thread tasks.
+    if (sweepingAtoms) {
+        AutoLockHelperThreadState helperLock;
+        joinTask(sweepAtomsTask, gcstats::PHASE_SWEEP_ATOMS, helperLock);
+    }
+
+    {
+        gcstats::AutoPhase ap(stats, gcstats::PHASE_SWEEP_COMPARTMENTS);
+        gcstats::AutoSCC scc(stats, zoneGroupIndex);
+
+        AutoLockHelperThreadState helperLock;
+        joinTask(sweepCCWrappersTask, gcstats::PHASE_SWEEP_CC_WRAPPER, helperLock);
+        joinTask(sweepObjectGroupsTask, gcstats::PHASE_SWEEP_TYPE_OBJECT, helperLock);
+        joinTask(sweepRegExpsTask, gcstats::PHASE_SWEEP_REGEXP, helperLock);
+        joinTask(sweepMiscTask, gcstats::PHASE_SWEEP_MISC, helperLock);
+        for (auto& task : sweepCacheTasks)
+            joinTask(task, gcstats::PHASE_SWEEP_MISC, helperLock);
+    }
+
+    /*
+     * Queue all GC things in all zones for sweeping, either in the
+     * foreground or on the background thread.
+     *
+     * Note that order is important here for the background case.
+     *
+     * Objects are finalized immediately but this may change in the future.
+     */
+
+    for (GCZoneGroupIter zone(rt); !zone.done(); zone.next()) {
+        gcstats::AutoSCC scc(stats, zoneGroupIndex);
+        zone->arenas.queueForegroundObjectsForSweep(&fop);
+    }
+    for (GCZoneGroupIter zone(rt); !zone.done(); zone.next()) {
+        gcstats::AutoSCC scc(stats, zoneGroupIndex);
+        for (unsigned i = 0; i < ArrayLength(IncrementalFinalizePhases); ++i)
+            zone->arenas.queueForForegroundSweep(&fop, IncrementalFinalizePhases[i]);
+    }
+    for (GCZoneGroupIter zone(rt); !zone.done(); zone.next()) {
+        gcstats::AutoSCC scc(stats, zoneGroupIndex);
+        for (unsigned i = 0; i < ArrayLength(BackgroundFinalizePhases); ++i)
+            zone->arenas.queueForBackgroundSweep(&fop, BackgroundFinalizePhases[i]);
+    }
+    for (GCZoneGroupIter zone(rt); !zone.done(); zone.next()) {
+        gcstats::AutoSCC scc(stats, zoneGroupIndex);
+        zone->arenas.queueForegroundThingsForSweep(&fop);
+    }
+
+    sweepingTypes = true;
+
+    finalizePhase = 0;
+    sweepZone = currentZoneGroup;
+    sweepKind = AllocKind::FIRST;
+
+    {
+        gcstats::AutoPhase ap(stats, gcstats::PHASE_FINALIZE_END);
+        callFinalizeCallbacks(&fop, JSFINALIZE_GROUP_END);
+    }
+}
+
+void
+GCRuntime::endSweepingZoneGroup()
+{
+    /* Update the GC state for zones we have swept. */
+    for (GCZoneGroupIter zone(rt); !zone.done(); zone.next()) {
+        MOZ_ASSERT(zone->isGCSweeping());
+        AutoLockGC lock(rt);
+        zone->setGCState(Zone::Finished);
+        zone->threshold.updateAfterGC(zone->usage.gcBytes(), invocationKind, tunables,
+                                      schedulingState, lock);
+    }
+
+    /* Start background thread to sweep zones if required. */
+    ZoneList zones;
+    for (GCZoneGroupIter zone(rt); !zone.done(); zone.next())
+        zones.append(zone);
+    if (sweepOnBackgroundThread)
+        queueZonesForBackgroundSweep(zones);
+    else
+        sweepBackgroundThings(zones, blocksToFreeAfterSweeping);
+
+    /* Reset the list of arenas marked as being allocated during sweep phase. */
+    while (Arena* arena = arenasAllocatedDuringSweep) {
+        arenasAllocatedDuringSweep = arena->getNextAllocDuringSweep();
+        arena->unsetAllocDuringSweep();
+    }
+}
+
+void
+GCRuntime::beginSweepPhase(bool destroyingRuntime, AutoLockForExclusiveAccess& lock)
+{
+    /*
+     * Sweep phase.
+     *
+     * Finalize as we sweep, outside of lock but with rt->isHeapBusy()
+     * true so that any attempt to allocate a GC-thing from a finalizer will
+     * fail, rather than nest badly and leave the unmarked newborn to be swept.
+     */
+
+    MOZ_ASSERT(!abortSweepAfterCurrentGroup);
+
+    AutoSetThreadIsSweeping threadIsSweeping;
+
+    releaseHeldRelocatedArenas();
+
+    computeNonIncrementalMarkingForValidation(lock);
+
+    gcstats::AutoPhase ap(stats, gcstats::PHASE_SWEEP);
+
+    sweepOnBackgroundThread =
+        !destroyingRuntime && !TraceEnabled() && CanUseExtraThreads();
+
+    releaseObservedTypes = shouldReleaseObservedTypes();
+
+    AssertNoWrappersInGrayList(rt);
+    DropStringWrappers(rt);
+
+    findZoneGroups(lock);
+    endMarkingZoneGroup();
+    beginSweepingZoneGroup(lock);
+}
+
+bool
+ArenaLists::foregroundFinalize(FreeOp* fop, AllocKind thingKind, SliceBudget& sliceBudget,
+                               SortedArenaList& sweepList)
+{
+    if (!arenaListsToSweep[thingKind] && incrementalSweptArenas.isEmpty())
+        return true;
+
+    if (!FinalizeArenas(fop, &arenaListsToSweep[thingKind], sweepList,
+                        thingKind, sliceBudget, RELEASE_ARENAS))
+    {
+        incrementalSweptArenaKind = thingKind;
+        incrementalSweptArenas = sweepList.toArenaList();
+        return false;
+    }
+
+    // Clear any previous incremental sweep state we may have saved.
+    incrementalSweptArenas.clear();
+
+    // Join |arenaLists[thingKind]| and |sweepList| into a single list.
+    ArenaList finalized = sweepList.toArenaList();
+    arenaLists[thingKind] =
+        finalized.insertListWithCursorAtEnd(arenaLists[thingKind]);
+
+    return true;
+}
+
+GCRuntime::IncrementalProgress
+GCRuntime::drainMarkStack(SliceBudget& sliceBudget, gcstats::Phase phase)
+{
+    /* Run a marking slice and return whether the stack is now empty. */
+    gcstats::AutoPhase ap(stats, phase);
+    return marker.drainMarkStack(sliceBudget) ? Finished : NotFinished;
+}
+
+static void
+SweepThing(Shape* shape)
+{
+    if (!shape->isMarked())
+        shape->sweep();
+}
+
+static void
+SweepThing(JSScript* script, AutoClearTypeInferenceStateOnOOM* oom)
+{
+    script->maybeSweepTypes(oom);
+}
+
+static void
+SweepThing(ObjectGroup* group, AutoClearTypeInferenceStateOnOOM* oom)
+{
+    group->maybeSweep(oom);
+}
+
+template <typename T, typename... Args>
+static bool
+SweepArenaList(Arena** arenasToSweep, SliceBudget& sliceBudget, Args... args)
+{
+    while (Arena* arena = *arenasToSweep) {
+        for (ArenaCellIterUnderGC i(arena); !i.done(); i.next())
+            SweepThing(i.get<T>(), args...);
+
+        *arenasToSweep = (*arenasToSweep)->next;
+        AllocKind kind = MapTypeToFinalizeKind<T>::kind;
+        sliceBudget.step(Arena::thingsPerArena(kind));
+        if (sliceBudget.isOverBudget())
+            return false;
+    }
+
+    return true;
+}
+
+GCRuntime::IncrementalProgress
+GCRuntime::sweepPhase(SliceBudget& sliceBudget, AutoLockForExclusiveAccess& lock)
+{
+    AutoSetThreadIsSweeping threadIsSweeping;
+
+    gcstats::AutoPhase ap(stats, gcstats::PHASE_SWEEP);
+    FreeOp fop(rt);
+
+    if (drainMarkStack(sliceBudget, gcstats::PHASE_SWEEP_MARK) == NotFinished)
+        return NotFinished;
+
+
+    for (;;) {
+        // Sweep dead type information stored in scripts and object groups, but
+        // don't finalize them yet. We have to sweep dead information from both
+        // live and dead scripts and object groups, so that no dead references
+        // remain in them. Type inference can end up crawling these zones
+        // again, such as for TypeCompartment::markSetsUnknown, and if this
+        // happens after sweeping for the zone group finishes we won't be able
+        // to determine which things in the zone are live.
+        if (sweepingTypes) {
+            gcstats::AutoPhase ap1(stats, gcstats::PHASE_SWEEP_COMPARTMENTS);
+            gcstats::AutoPhase ap2(stats, gcstats::PHASE_SWEEP_TYPES);
+
+            for (; sweepZone; sweepZone = sweepZone->nextNodeInGroup()) {
+                ArenaLists& al = sweepZone->arenas;
+
+                AutoClearTypeInferenceStateOnOOM oom(sweepZone);
+
+                if (!SweepArenaList<JSScript>(&al.gcScriptArenasToUpdate, sliceBudget, &oom))
+                    return NotFinished;
+
+                if (!SweepArenaList<ObjectGroup>(
+                        &al.gcObjectGroupArenasToUpdate, sliceBudget, &oom))
+                {
+                    return NotFinished;
+                }
+
+                // Finish sweeping type information in the zone.
+                {
+                    gcstats::AutoPhase ap(stats, gcstats::PHASE_SWEEP_TYPES_END);
+                    sweepZone->types.endSweep(rt);
+                }
+
+                // Foreground finalized objects have already been finalized,
+                // and now their arenas can be reclaimed by freeing empty ones
+                // and making non-empty ones available for allocation.
+                al.mergeForegroundSweptObjectArenas();
+            }
+
+            sweepZone = currentZoneGroup;
+            sweepingTypes = false;
+        }
+
+        /* Finalize foreground finalized things. */
+        for (; finalizePhase < ArrayLength(IncrementalFinalizePhases) ; ++finalizePhase) {
+            gcstats::AutoPhase ap(stats, IncrementalFinalizePhases[finalizePhase].statsPhase);
+
+            for (; sweepZone; sweepZone = sweepZone->nextNodeInGroup()) {
+                Zone* zone = sweepZone;
+
+                for (auto kind : SomeAllocKinds(sweepKind, AllocKind::LIMIT)) {
+                    if (!IncrementalFinalizePhases[finalizePhase].kinds.contains(kind))
+                        continue;
+
+                    /* Set the number of things per arena for this AllocKind. */
+                    size_t thingsPerArena = Arena::thingsPerArena(kind);
+                    incrementalSweepList.setThingsPerArena(thingsPerArena);
+
+                    if (!zone->arenas.foregroundFinalize(&fop, kind, sliceBudget,
+                                                         incrementalSweepList))
+                    {
+                        sweepKind = kind;
+                        return NotFinished;
+                    }
+
+                    /* Reset the slots of the sweep list that we used. */
+                    incrementalSweepList.reset(thingsPerArena);
+                }
+                sweepKind = AllocKind::FIRST;
+            }
+            sweepZone = currentZoneGroup;
+        }
+
+        /* Remove dead shapes from the shape tree, but don't finalize them yet. */
+        {
+            gcstats::AutoPhase ap(stats, gcstats::PHASE_SWEEP_SHAPE);
+
+            for (; sweepZone; sweepZone = sweepZone->nextNodeInGroup()) {
+                ArenaLists& al = sweepZone->arenas;
+
+                if (!SweepArenaList<Shape>(&al.gcShapeArenasToUpdate, sliceBudget))
+                    return NotFinished;
+
+                if (!SweepArenaList<AccessorShape>(&al.gcAccessorShapeArenasToUpdate, sliceBudget))
+                    return NotFinished;
+            }
+        }
+
+        endSweepingZoneGroup();
+        getNextZoneGroup();
+        if (!currentZoneGroup)
+            return Finished;
+
+        endMarkingZoneGroup();
+        beginSweepingZoneGroup(lock);
+    }
+}
+
+void
+GCRuntime::endSweepPhase(bool destroyingRuntime, AutoLockForExclusiveAccess& lock)
+{
+    AutoSetThreadIsSweeping threadIsSweeping;
+
+    gcstats::AutoPhase ap(stats, gcstats::PHASE_SWEEP);
+    FreeOp fop(rt);
+
+    MOZ_ASSERT_IF(destroyingRuntime, !sweepOnBackgroundThread);
+
+    /*
+     * Recalculate whether GC was full or not as this may have changed due to
+     * newly created zones.  Can only change from full to not full.
+     */
+    if (isFull) {
+        for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next()) {
+            if (!zone->isCollecting()) {
+                isFull = false;
+                break;
+            }
+        }
+    }
+
+    {
+        gcstats::AutoPhase ap(stats, gcstats::PHASE_DESTROY);
+
+        /*
+         * Sweep script filenames after sweeping functions in the generic loop
+         * above. In this way when a scripted function's finalizer destroys the
+         * script and calls rt->destroyScriptHook, the hook can still access the
+         * script's filename. See bug 323267.
+         */
+        SweepScriptData(rt, lock);
+
+        /* Clear out any small pools that we're hanging on to. */
+        if (jit::JitRuntime* jitRuntime = rt->jitRuntime()) {
+            jitRuntime->execAlloc().purge();
+            jitRuntime->backedgeExecAlloc().purge();
+        }
+    }
+
+    {
+        gcstats::AutoPhase ap(stats, gcstats::PHASE_FINALIZE_END);
+        callFinalizeCallbacks(&fop, JSFINALIZE_COLLECTION_END);
+
+        /* If we finished a full GC, then the gray bits are correct. */
+        if (isFull)
+            rt->setGCGrayBitsValid(true);
+    }
+
+    finishMarkingValidation();
+
+#ifdef DEBUG
+    for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next()) {
+        for (auto i : AllAllocKinds()) {
+            MOZ_ASSERT_IF(!IsBackgroundFinalized(i) ||
+                          !sweepOnBackgroundThread,
+                          !zone->arenas.arenaListsToSweep[i]);
+        }
+    }
+#endif
+
+    AssertNoWrappersInGrayList(rt);
+}
+
+void
+GCRuntime::beginCompactPhase()
+{
+    MOZ_ASSERT(!isBackgroundSweeping());
+
+    gcstats::AutoPhase ap(stats, gcstats::PHASE_COMPACT);
+
+    MOZ_ASSERT(zonesToMaybeCompact.isEmpty());
+    for (GCZonesIter zone(rt); !zone.done(); zone.next()) {
+        if (CanRelocateZone(zone))
+            zonesToMaybeCompact.append(zone);
+    }
+
+    MOZ_ASSERT(!relocatedArenasToRelease);
+    startedCompacting = true;
+}
+
+GCRuntime::IncrementalProgress
+GCRuntime::compactPhase(JS::gcreason::Reason reason, SliceBudget& sliceBudget,
+                        AutoLockForExclusiveAccess& lock)
+{
+    MOZ_ASSERT(rt->gc.nursery.isEmpty());
+    assertBackgroundSweepingFinished();
+    MOZ_ASSERT(startedCompacting);
+
+    gcstats::AutoPhase ap(stats, gcstats::PHASE_COMPACT);
+
+    Arena* relocatedArenas = nullptr;
+    while (!zonesToMaybeCompact.isEmpty()) {
+        // TODO: JSScripts can move. If the sampler interrupts the GC in the
+        // middle of relocating an arena, invalid JSScript pointers may be
+        // accessed. Suppress all sampling until a finer-grained solution can be
+        // found. See bug 1295775.
+        AutoSuppressProfilerSampling suppressSampling(rt);
+
+        Zone* zone = zonesToMaybeCompact.front();
+        MOZ_ASSERT(zone->isGCFinished());
+        zone->setGCState(Zone::Compact);
+        if (relocateArenas(zone, reason, relocatedArenas, sliceBudget))
+            updatePointersToRelocatedCells(zone, lock);
+        zone->setGCState(Zone::Finished);
+        zonesToMaybeCompact.removeFront();
+        if (sliceBudget.isOverBudget())
+            break;
+    }
+
+    if (ShouldProtectRelocatedArenas(reason))
+        protectAndHoldArenas(relocatedArenas);
+    else
+        releaseRelocatedArenas(relocatedArenas);
+
+    // Clear caches that can contain cell pointers.
+    JSContext* cx = rt->contextFromMainThread();
+    cx->caches.newObjectCache.purge();
+    cx->caches.nativeIterCache.purge();
+    if (cx->caches.evalCache.initialized())
+        cx->caches.evalCache.clear();
+
+#ifdef DEBUG
+    CheckHashTablesAfterMovingGC(rt);
+#endif
+
+    return zonesToMaybeCompact.isEmpty() ? Finished : NotFinished;
+}
+
+void
+GCRuntime::endCompactPhase(JS::gcreason::Reason reason)
+{
+    startedCompacting = false;
+}
+
+void
+GCRuntime::finishCollection(JS::gcreason::Reason reason)
+{
+    assertBackgroundSweepingFinished();
+    MOZ_ASSERT(marker.isDrained());
+    marker.stop();
+    clearBufferedGrayRoots();
+    MemProfiler::SweepTenured(rt);
+
+    uint64_t currentTime = PRMJ_Now();
+    schedulingState.updateHighFrequencyMode(lastGCTime, currentTime, tunables);
+
+    for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next()) {
+        if (zone->isCollecting()) {
+            MOZ_ASSERT(zone->isGCFinished());
+            zone->setGCState(Zone::NoGC);
+            zone->active = false;
+        }
+
+        MOZ_ASSERT(!zone->isCollecting());
+        MOZ_ASSERT(!zone->wasGCStarted());
+    }
+
+    MOZ_ASSERT(zonesToMaybeCompact.isEmpty());
+
+    lastGCTime = currentTime;
+}
+
+static const char*
+HeapStateToLabel(JS::HeapState heapState)
+{
+    switch (heapState) {
+      case JS::HeapState::MinorCollecting:
+        return "js::Nursery::collect";
+      case JS::HeapState::MajorCollecting:
+        return "js::GCRuntime::collect";
+      case JS::HeapState::Tracing:
+        return "JS_IterateCompartments";
+      case JS::HeapState::Idle:
+      case JS::HeapState::CycleCollecting:
+        MOZ_CRASH("Should never have an Idle or CC heap state when pushing GC pseudo frames!");
+    }
+    MOZ_ASSERT_UNREACHABLE("Should have exhausted every JS::HeapState variant!");
+    return nullptr;
+}
+
+/* Start a new heap session. */
+AutoTraceSession::AutoTraceSession(JSRuntime* rt, JS::HeapState heapState)
+  : lock(rt),
+    runtime(rt),
+    prevState(rt->heapState()),
+    pseudoFrame(rt, HeapStateToLabel(heapState), ProfileEntry::Category::GC)
+{
+    MOZ_ASSERT(prevState == JS::HeapState::Idle);
+    MOZ_ASSERT(heapState != JS::HeapState::Idle);
+    MOZ_ASSERT_IF(heapState == JS::HeapState::MajorCollecting, rt->gc.nursery.isEmpty());
+    rt->setHeapState(heapState);
+}
+
+AutoTraceSession::~AutoTraceSession()
+{
+    MOZ_ASSERT(runtime->isHeapBusy());
+    runtime->setHeapState(prevState);
+}
+
+void
+GCRuntime::resetIncrementalGC(gc::AbortReason reason, AutoLockForExclusiveAccess& lock)
+{
+    MOZ_ASSERT(reason != gc::AbortReason::None);
+
+    switch (incrementalState) {
+      case State::NotActive:
+        return;
+
+      case State::MarkRoots:
+        MOZ_CRASH("resetIncrementalGC did not expect MarkRoots state");
+        break;
+
+      case State::Mark: {
+        /* Cancel any ongoing marking. */
+        marker.reset();
+        marker.stop();
+        clearBufferedGrayRoots();
+
+        for (GCCompartmentsIter c(rt); !c.done(); c.next())
+            ResetGrayList(c);
+
+        for (GCZonesIter zone(rt); !zone.done(); zone.next()) {
+            MOZ_ASSERT(zone->isGCMarking());
+            zone->setNeedsIncrementalBarrier(false, Zone::UpdateJit);
+            zone->setGCState(Zone::NoGC);
+        }
+
+        blocksToFreeAfterSweeping.freeAll();
+
+        incrementalState = State::NotActive;
+
+        MOZ_ASSERT(!marker.shouldCheckCompartments());
+
+        break;
+      }
+
+      case State::Sweep: {
+        marker.reset();
+
+        for (CompartmentsIter c(rt, SkipAtoms); !c.done(); c.next())
+            c->scheduledForDestruction = false;
+
+        /* Finish sweeping the current zone group, then abort. */
+        abortSweepAfterCurrentGroup = true;
+
+        /* Don't perform any compaction after sweeping. */
+        bool wasCompacting = isCompacting;
+        isCompacting = false;
+
+        auto unlimited = SliceBudget::unlimited();
+        incrementalCollectSlice(unlimited, JS::gcreason::RESET, lock);
+
+        isCompacting = wasCompacting;
+
+        {
+            gcstats::AutoPhase ap(stats, gcstats::PHASE_WAIT_BACKGROUND_THREAD);
+            rt->gc.waitBackgroundSweepOrAllocEnd();
+        }
+        break;
+      }
+
+      case State::Finalize: {
+        {
+            gcstats::AutoPhase ap(stats, gcstats::PHASE_WAIT_BACKGROUND_THREAD);
+            rt->gc.waitBackgroundSweepOrAllocEnd();
+        }
+
+        bool wasCompacting = isCompacting;
+        isCompacting = false;
+
+        auto unlimited = SliceBudget::unlimited();
+        incrementalCollectSlice(unlimited, JS::gcreason::RESET, lock);
+
+        isCompacting = wasCompacting;
+
+        break;
+      }
+
+      case State::Compact: {
+        bool wasCompacting = isCompacting;
+
+        isCompacting = true;
+        startedCompacting = true;
+        zonesToMaybeCompact.clear();
+
+        auto unlimited = SliceBudget::unlimited();
+        incrementalCollectSlice(unlimited, JS::gcreason::RESET, lock);
+
+        isCompacting = wasCompacting;
+        break;
+      }
+
+      case State::Decommit: {
+        auto unlimited = SliceBudget::unlimited();
+        incrementalCollectSlice(unlimited, JS::gcreason::RESET, lock);
+        break;
+      }
+    }
+
+    stats.reset(reason);
+
+#ifdef DEBUG
+    assertBackgroundSweepingFinished();
+    for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next()) {
+        MOZ_ASSERT(!zone->isCollecting());
+        MOZ_ASSERT(!zone->needsIncrementalBarrier());
+        MOZ_ASSERT(!zone->isOnList());
+    }
+    MOZ_ASSERT(zonesToMaybeCompact.isEmpty());
+    MOZ_ASSERT(incrementalState == State::NotActive);
+#endif
+}
+
+namespace {
+
+class AutoGCSlice {
+  public:
+    explicit AutoGCSlice(JSRuntime* rt);
+    ~AutoGCSlice();
+
+  private:
+    JSRuntime* runtime;
+    AutoSetThreadIsPerformingGC performingGC;
+};
+
+} /* anonymous namespace */
+
+AutoGCSlice::AutoGCSlice(JSRuntime* rt)
+  : runtime(rt)
+{
+    /*
+     * During incremental GC, the compartment's active flag determines whether
+     * there are stack frames active for any of its scripts. Normally this flag
+     * is set at the beginning of the mark phase. During incremental GC, we also
+     * set it at the start of every phase.
+     */
+    for (ActivationIterator iter(rt); !iter.done(); ++iter)
+        iter->compartment()->zone()->active = true;
+
+    for (GCZonesIter zone(rt); !zone.done(); zone.next()) {
+        /*
+         * Clear needsIncrementalBarrier early so we don't do any write
+         * barriers during GC. We don't need to update the Ion barriers (which
+         * is expensive) because Ion code doesn't run during GC. If need be,
+         * we'll update the Ion barriers in ~AutoGCSlice.
+         */
+        if (zone->isGCMarking()) {
+            MOZ_ASSERT(zone->needsIncrementalBarrier());
+            zone->setNeedsIncrementalBarrier(false, Zone::DontUpdateJit);
+        } else {
+            MOZ_ASSERT(!zone->needsIncrementalBarrier());
+        }
+    }
+}
+
+AutoGCSlice::~AutoGCSlice()
+{
+    /* We can't use GCZonesIter if this is the end of the last slice. */
+    for (ZonesIter zone(runtime, WithAtoms); !zone.done(); zone.next()) {
+        if (zone->isGCMarking()) {
+            zone->setNeedsIncrementalBarrier(true, Zone::UpdateJit);
+            zone->arenas.prepareForIncrementalGC(runtime);
+        } else {
+            zone->setNeedsIncrementalBarrier(false, Zone::UpdateJit);
+        }
+    }
+}
+
+void
+GCRuntime::pushZealSelectedObjects()
+{
+#ifdef JS_GC_ZEAL
+    /* Push selected objects onto the mark stack and clear the list. */
+    for (JSObject** obj = selectedForMarking.begin(); obj != selectedForMarking.end(); obj++)
+        TraceManuallyBarrieredEdge(&marker, obj, "selected obj");
+#endif
+}
+
+static bool
+IsShutdownGC(JS::gcreason::Reason reason)
+{
+    return reason == JS::gcreason::SHUTDOWN_CC || reason == JS::gcreason::DESTROY_RUNTIME;
+}
+
+static bool
+ShouldCleanUpEverything(JS::gcreason::Reason reason, JSGCInvocationKind gckind)
+{
+    // During shutdown, we must clean everything up, for the sake of leak
+    // detection. When a runtime has no contexts, or we're doing a GC before a
+    // shutdown CC, those are strong indications that we're shutting down.
+    return IsShutdownGC(reason) || gckind == GC_SHRINK;
+}
+
+void
+GCRuntime::incrementalCollectSlice(SliceBudget& budget, JS::gcreason::Reason reason,
+                                   AutoLockForExclusiveAccess& lock)
+{
+    AutoGCSlice slice(rt);
+
+    bool destroyingRuntime = (reason == JS::gcreason::DESTROY_RUNTIME);
+
+    gc::State initialState = incrementalState;
+
+    bool useZeal = false;
+#ifdef JS_GC_ZEAL
+    if (reason == JS::gcreason::DEBUG_GC && !budget.isUnlimited()) {
+        /*
+         * Do the incremental collection type specified by zeal mode if the
+         * collection was triggered by runDebugGC() and incremental GC has not
+         * been cancelled by resetIncrementalGC().
+         */
+        useZeal = true;
+    }
+#endif
+
+    MOZ_ASSERT_IF(isIncrementalGCInProgress(), isIncremental);
+    isIncremental = !budget.isUnlimited();
+
+    if (useZeal && (hasZealMode(ZealMode::IncrementalRootsThenFinish) ||
+                    hasZealMode(ZealMode::IncrementalMarkAllThenFinish)))
+    {
+        /*
+         * Yields between slices occurs at predetermined points in these modes;
+         * the budget is not used.
+         */
+        budget.makeUnlimited();
+    }
+
+    switch (incrementalState) {
+      case State::NotActive:
+        initialReason = reason;
+        cleanUpEverything = ShouldCleanUpEverything(reason, invocationKind);
+        isCompacting = shouldCompact();
+        lastMarkSlice = false;
+
+        incrementalState = State::MarkRoots;
+
+        MOZ_FALLTHROUGH;
+
+      case State::MarkRoots:
+        if (!beginMarkPhase(reason, lock)) {
+            incrementalState = State::NotActive;
+            return;
+        }
+
+        if (!destroyingRuntime)
+            pushZealSelectedObjects();
+
+        incrementalState = State::Mark;
+
+        if (isIncremental && useZeal && hasZealMode(ZealMode::IncrementalRootsThenFinish))
+            break;
+
+        MOZ_FALLTHROUGH;
+
+      case State::Mark:
+        AutoGCRooter::traceAllWrappers(&marker);
+
+        /* If we needed delayed marking for gray roots, then collect until done. */
+        if (!hasBufferedGrayRoots()) {
+            budget.makeUnlimited();
+            isIncremental = false;
+        }
+
+        if (drainMarkStack(budget, gcstats::PHASE_MARK) == NotFinished)
+            break;
+
+        MOZ_ASSERT(marker.isDrained());
+
+        /*
+         * In incremental GCs where we have already performed more than once
+         * slice we yield after marking with the aim of starting the sweep in
+         * the next slice, since the first slice of sweeping can be expensive.
+         *
+         * This is modified by the various zeal modes.  We don't yield in
+         * IncrementalRootsThenFinish mode and we always yield in
+         * IncrementalMarkAllThenFinish mode.
+         *
+         * We will need to mark anything new on the stack when we resume, so
+         * we stay in Mark state.
+         */
+        if (!lastMarkSlice && isIncremental &&
+            ((initialState == State::Mark &&
+              !(useZeal && hasZealMode(ZealMode::IncrementalRootsThenFinish))) ||
+             (useZeal && hasZealMode(ZealMode::IncrementalMarkAllThenFinish))))
+        {
+            lastMarkSlice = true;
+            break;
+        }
+
+        incrementalState = State::Sweep;
+
+        /*
+         * This runs to completion, but we don't continue if the budget is
+         * now exhasted.
+         */
+        beginSweepPhase(destroyingRuntime, lock);
+        if (budget.isOverBudget())
+            break;
+
+        /*
+         * Always yield here when running in incremental multi-slice zeal
+         * mode, so RunDebugGC can reset the slice buget.
+         */
+        if (isIncremental && useZeal && hasZealMode(ZealMode::IncrementalMultipleSlices))
+            break;
+
+        MOZ_FALLTHROUGH;
+
+      case State::Sweep:
+        if (sweepPhase(budget, lock) == NotFinished)
+            break;
+
+        endSweepPhase(destroyingRuntime, lock);
+
+        incrementalState = State::Finalize;
+
+        MOZ_FALLTHROUGH;
+
+      case State::Finalize:
+        {
+            gcstats::AutoPhase ap(stats, gcstats::PHASE_WAIT_BACKGROUND_THREAD);
+
+            // Yield until background finalization is done.
+            if (isIncremental) {
+                // Poll for end of background sweeping
+                AutoLockGC lock(rt);
+                if (isBackgroundSweeping())
+                    break;
+            } else {
+                waitBackgroundSweepEnd();
+            }
+        }
+
+        {
+            // Re-sweep the zones list, now that background finalization is
+            // finished to actually remove and free dead zones.
+            gcstats::AutoPhase ap1(stats, gcstats::PHASE_SWEEP);
+            gcstats::AutoPhase ap2(stats, gcstats::PHASE_DESTROY);
+            AutoSetThreadIsSweeping threadIsSweeping;
+            FreeOp fop(rt);
+            sweepZones(&fop, destroyingRuntime);
+        }
+
+        MOZ_ASSERT(!startedCompacting);
+        incrementalState = State::Compact;
+
+        // Always yield before compacting since it is not incremental.
+        if (isCompacting && isIncremental)
+            break;
+
+        MOZ_FALLTHROUGH;
+
+      case State::Compact:
+        if (isCompacting) {
+            if (!startedCompacting)
+                beginCompactPhase();
+
+            if (compactPhase(reason, budget, lock) == NotFinished)
+                break;
+
+            endCompactPhase(reason);
+        }
+
+        startDecommit();
+        incrementalState = State::Decommit;
+
+        MOZ_FALLTHROUGH;
+
+      case State::Decommit:
+        {
+            gcstats::AutoPhase ap(stats, gcstats::PHASE_WAIT_BACKGROUND_THREAD);
+
+            // Yield until background decommit is done.
+            if (isIncremental && decommitTask.isRunning())
+                break;
+
+            decommitTask.join();
+        }
+
+        finishCollection(reason);
+        incrementalState = State::NotActive;
+        break;
+    }
+}
+
+gc::AbortReason
+gc::IsIncrementalGCUnsafe(JSRuntime* rt)
+{
+    MOZ_ASSERT(!rt->mainThread.suppressGC);
+
+    if (rt->keepAtoms())
+        return gc::AbortReason::KeepAtomsSet;
+
+    if (!rt->gc.isIncrementalGCAllowed())
+        return gc::AbortReason::IncrementalDisabled;
+
+    return gc::AbortReason::None;
+}
+
+void
+GCRuntime::budgetIncrementalGC(SliceBudget& budget, AutoLockForExclusiveAccess& lock)
+{
+    AbortReason unsafeReason = IsIncrementalGCUnsafe(rt);
+    if (unsafeReason != AbortReason::None) {
+        resetIncrementalGC(unsafeReason, lock);
+        budget.makeUnlimited();
+        stats.nonincremental(unsafeReason);
+        return;
+    }
+
+    if (mode != JSGC_MODE_INCREMENTAL) {
+        resetIncrementalGC(AbortReason::ModeChange, lock);
+        budget.makeUnlimited();
+        stats.nonincremental(AbortReason::ModeChange);
+        return;
+    }
+
+    if (isTooMuchMalloc()) {
+        budget.makeUnlimited();
+        stats.nonincremental(AbortReason::MallocBytesTrigger);
+    }
+
+    bool reset = false;
+    for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next()) {
+        if (zone->usage.gcBytes() >= zone->threshold.gcTriggerBytes()) {
+            budget.makeUnlimited();
+            stats.nonincremental(AbortReason::GCBytesTrigger);
+        }
+
+        if (isIncrementalGCInProgress() && zone->isGCScheduled() != zone->wasGCStarted())
+            reset = true;
+
+        if (zone->isTooMuchMalloc()) {
+            budget.makeUnlimited();
+            stats.nonincremental(AbortReason::MallocBytesTrigger);
+        }
+    }
+
+    if (reset)
+        resetIncrementalGC(AbortReason::ZoneChange, lock);
+}
+
+namespace {
+
+class AutoScheduleZonesForGC
+{
+    JSRuntime* rt_;
+
+  public:
+    explicit AutoScheduleZonesForGC(JSRuntime* rt) : rt_(rt) {
+        for (ZonesIter zone(rt_, WithAtoms); !zone.done(); zone.next()) {
+            if (rt->gc.gcMode() == JSGC_MODE_GLOBAL)
+                zone->scheduleGC();
+
+            /* This is a heuristic to avoid resets. */
+            if (rt->gc.isIncrementalGCInProgress() && zone->needsIncrementalBarrier())
+                zone->scheduleGC();
+
+            /* This is a heuristic to reduce the total number of collections. */
+            if (zone->usage.gcBytes() >=
+                zone->threshold.allocTrigger(rt->gc.schedulingState.inHighFrequencyGCMode()))
+            {
+                zone->scheduleGC();
+            }
+        }
+    }
+
+    ~AutoScheduleZonesForGC() {
+        for (ZonesIter zone(rt_, WithAtoms); !zone.done(); zone.next())
+            zone->unscheduleGC();
+    }
+};
+
+/*
+ * An invariant of our GC/CC interaction is that there must not ever be any
+ * black to gray edges in the system. It is possible to violate this with
+ * simple compartmental GC. For example, in GC[n], we collect in both
+ * compartmentA and compartmentB, and mark both sides of the cross-compartment
+ * edge gray. Later in GC[n+1], we only collect compartmentA, but this time
+ * mark it black. Now we are violating the invariants and must fix it somehow.
+ *
+ * To prevent this situation, we explicitly detect the black->gray state when
+ * marking cross-compartment edges -- see ShouldMarkCrossCompartment -- adding
+ * each violating edges to foundBlackGrayEdges. After we leave the trace
+ * session for each GC slice, we "ExposeToActiveJS" on each of these edges
+ * (which we cannot do safely from the guts of the GC).
+ */
+class AutoExposeLiveCrossZoneEdges
+{
+    BlackGrayEdgeVector* edges;
+
+  public:
+    explicit AutoExposeLiveCrossZoneEdges(BlackGrayEdgeVector* edgesPtr) : edges(edgesPtr) {
+        MOZ_ASSERT(edges->empty());
+    }
+    ~AutoExposeLiveCrossZoneEdges() {
+        for (auto& target : *edges) {
+            MOZ_ASSERT(target);
+            MOZ_ASSERT(!target->zone()->isCollecting());
+            UnmarkGrayCellRecursively(target, target->getTraceKind());
+        }
+        edges->clear();
+    }
+};
+
+} /* anonymous namespace */
+
+/*
+ * Run one GC "cycle" (either a slice of incremental GC or an entire
+ * non-incremental GC. We disable inlining to ensure that the bottom of the
+ * stack with possible GC roots recorded in MarkRuntime excludes any pointers we
+ * use during the marking implementation.
+ *
+ * Returns true if we "reset" an existing incremental GC, which would force us
+ * to run another cycle.
+ */
+MOZ_NEVER_INLINE bool
+GCRuntime::gcCycle(bool nonincrementalByAPI, SliceBudget& budget, JS::gcreason::Reason reason)
+{
+    // Note that the following is allowed to re-enter GC in the finalizer.
+    AutoNotifyGCActivity notify(*this);
+
+    gcstats::AutoGCSlice agc(stats, scanZonesBeforeGC(), invocationKind, budget, reason);
+
+    AutoExposeLiveCrossZoneEdges aelcze(&foundBlackGrayEdges);
+
+    evictNursery(reason);
+
+    AutoTraceSession session(rt, JS::HeapState::MajorCollecting);
+
+    majorGCTriggerReason = JS::gcreason::NO_REASON;
+    interFrameGC = true;
+
+    number++;
+    if (!isIncrementalGCInProgress())
+        incMajorGcNumber();
+
+    // It's ok if threads other than the main thread have suppressGC set, as
+    // they are operating on zones which will not be collected from here.
+    MOZ_ASSERT(!rt->mainThread.suppressGC);
+
+    // Assert if this is a GC unsafe region.
+    verifyIsSafeToGC();
+
+    {
+        gcstats::AutoPhase ap(stats, gcstats::PHASE_WAIT_BACKGROUND_THREAD);
+
+        // Background finalization and decommit are finished by defininition
+        // before we can start a new GC session.
+        if (!isIncrementalGCInProgress()) {
+            assertBackgroundSweepingFinished();
+            MOZ_ASSERT(!decommitTask.isRunning());
+        }
+
+        // We must also wait for background allocation to finish so we can
+        // avoid taking the GC lock when manipulating the chunks during the GC.
+        // The background alloc task can run between slices, so we must wait
+        // for it at the start of every slice.
+        allocTask.cancel(GCParallelTask::CancelAndWait);
+    }
+
+    State prevState = incrementalState;
+
+    if (nonincrementalByAPI) {
+        // Reset any in progress incremental GC if this was triggered via the
+        // API. This isn't required for correctness, but sometimes during tests
+        // the caller expects this GC to collect certain objects, and we need
+        // to make sure to collect everything possible.
+        if (reason != JS::gcreason::ALLOC_TRIGGER)
+            resetIncrementalGC(gc::AbortReason::NonIncrementalRequested, session.lock);
+
+        stats.nonincremental(gc::AbortReason::NonIncrementalRequested);
+        budget.makeUnlimited();
+    } else {
+        budgetIncrementalGC(budget, session.lock);
+    }
+
+    /* The GC was reset, so we need a do-over. */
+    if (prevState != State::NotActive && !isIncrementalGCInProgress())
+        return true;
+
+    TraceMajorGCStart();
+
+    incrementalCollectSlice(budget, reason, session.lock);
+
+    chunkAllocationSinceLastGC = false;
+
+#ifdef JS_GC_ZEAL
+    /* Keeping these around after a GC is dangerous. */
+    clearSelectedForMarking();
+#endif
+
+    /* Clear gcMallocBytes for all zones. */
+    for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next())
+        zone->resetGCMallocBytes();
+
+    resetMallocBytes();
+
+    TraceMajorGCEnd();
+
+    return false;
+}
+
+#ifdef JS_GC_ZEAL
+static bool
+IsDeterministicGCReason(JS::gcreason::Reason reason)
+{
+    if (reason > JS::gcreason::DEBUG_GC &&
+        reason != JS::gcreason::CC_FORCED && reason != JS::gcreason::SHUTDOWN_CC)
+    {
+        return false;
+    }
+
+    if (reason == JS::gcreason::EAGER_ALLOC_TRIGGER)
+        return false;
+
+    return true;
+}
+#endif
+
+gcstats::ZoneGCStats
+GCRuntime::scanZonesBeforeGC()
+{
+    gcstats::ZoneGCStats zoneStats;
+    for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next()) {
+        zoneStats.zoneCount++;
+        if (zone->isGCScheduled()) {
+            zoneStats.collectedZoneCount++;
+            zoneStats.collectedCompartmentCount += zone->compartments.length();
+        }
+    }
+
+    for (CompartmentsIter c(rt, WithAtoms); !c.done(); c.next())
+        zoneStats.compartmentCount++;
+
+    return zoneStats;
+}
+
+// The GC can only clean up scheduledForDestruction compartments that were
+// marked live by a barrier (e.g. by RemapWrappers from a navigation event).
+// It is also common to have compartments held live because they are part of a
+// cycle in gecko, e.g. involving the HTMLDocument wrapper. In this case, we
+// need to run the CycleCollector in order to remove these edges before the
+// compartment can be freed.
+void
+GCRuntime::maybeDoCycleCollection()
+{
+    const static double ExcessiveGrayCompartments = 0.8;
+    const static size_t LimitGrayCompartments = 200;
+
+    size_t compartmentsTotal = 0;
+    size_t compartmentsGray = 0;
+    for (CompartmentsIter c(rt, SkipAtoms); !c.done(); c.next()) {
+        ++compartmentsTotal;
+        GlobalObject* global = c->unsafeUnbarrieredMaybeGlobal();
+        if (global && global->asTenured().isMarked(GRAY))
+            ++compartmentsGray;
+    }
+    double grayFraction = double(compartmentsGray) / double(compartmentsTotal);
+    if (grayFraction > ExcessiveGrayCompartments || compartmentsGray > LimitGrayCompartments)
+        callDoCycleCollectionCallback(rt->contextFromMainThread());
+}
+
+void
+GCRuntime::checkCanCallAPI()
+{
+    MOZ_RELEASE_ASSERT(CurrentThreadCanAccessRuntime(rt));
+
+    /* If we attempt to invoke the GC while we are running in the GC, assert. */
+    MOZ_RELEASE_ASSERT(!rt->isHeapBusy());
+
+    MOZ_ASSERT(isAllocAllowed());
+}
+
+bool
+GCRuntime::checkIfGCAllowedInCurrentState(JS::gcreason::Reason reason)
+{
+    if (rt->mainThread.suppressGC)
+        return false;
+
+    // Only allow shutdown GCs when we're destroying the runtime. This keeps
+    // the GC callback from triggering a nested GC and resetting global state.
+    if (rt->isBeingDestroyed() && !IsShutdownGC(reason))
+        return false;
+
+#ifdef JS_GC_ZEAL
+    if (deterministicOnly && !IsDeterministicGCReason(reason))
+        return false;
+#endif
+
+    return true;
+}
+
+void
+GCRuntime::collect(bool nonincrementalByAPI, SliceBudget budget, JS::gcreason::Reason reason)
+{
+    // Checks run for each request, even if we do not actually GC.
+    checkCanCallAPI();
+
+    // Check if we are allowed to GC at this time before proceeding.
+    if (!checkIfGCAllowedInCurrentState(reason))
+        return;
+
+    AutoTraceLog logGC(TraceLoggerForMainThread(rt), TraceLogger_GC);
+    AutoStopVerifyingBarriers av(rt, IsShutdownGC(reason));
+    AutoEnqueuePendingParseTasksAfterGC aept(*this);
+    AutoScheduleZonesForGC asz(rt);
+
+    bool repeat = false;
+    do {
+        poked = false;
+        bool wasReset = gcCycle(nonincrementalByAPI, budget, reason);
+
+        /* Need to re-schedule all zones for GC. */
+        if (poked && cleanUpEverything)
+            JS::PrepareForFullGC(rt->contextFromMainThread());
+
+        /*
+         * This code makes an extra effort to collect compartments that we
+         * thought were dead at the start of the GC. See the large comment in
+         * beginMarkPhase.
+         */
+        bool repeatForDeadZone = false;
+        if (!nonincrementalByAPI && !isIncrementalGCInProgress()) {
+            for (CompartmentsIter c(rt, SkipAtoms); !c.done(); c.next()) {
+                if (c->scheduledForDestruction) {
+                    nonincrementalByAPI = true;
+                    repeatForDeadZone = true;
+                    reason = JS::gcreason::COMPARTMENT_REVIVED;
+                    c->zone()->scheduleGC();
+                }
+            }
+        }
+
+        /*
+         * If we reset an existing GC, we need to start a new one. Also, we
+         * repeat GCs that happen during shutdown (the gcShouldCleanUpEverything
+         * case) until we can be sure that no additional garbage is created
+         * (which typically happens if roots are dropped during finalizers).
+         */
+        repeat = (poked && cleanUpEverything) || wasReset || repeatForDeadZone;
+    } while (repeat);
+
+    if (reason == JS::gcreason::COMPARTMENT_REVIVED)
+        maybeDoCycleCollection();
+
+#ifdef JS_GC_ZEAL
+    if (rt->hasZealMode(ZealMode::CheckHeapAfterGC)) {
+        gcstats::AutoPhase ap(rt->gc.stats, gcstats::PHASE_TRACE_HEAP);
+        CheckHeapAfterGC(rt);
+    }
+#endif
+}
+
+js::AutoEnqueuePendingParseTasksAfterGC::~AutoEnqueuePendingParseTasksAfterGC()
+{
+    if (!OffThreadParsingMustWaitForGC(gc_.rt))
+        EnqueuePendingParseTasksAfterGC(gc_.rt);
+}
+
+SliceBudget
+GCRuntime::defaultBudget(JS::gcreason::Reason reason, int64_t millis)
+{
+    if (millis == 0) {
+        if (reason == JS::gcreason::ALLOC_TRIGGER)
+            millis = defaultSliceBudget();
+        else if (schedulingState.inHighFrequencyGCMode() && tunables.isDynamicMarkSliceEnabled())
+            millis = defaultSliceBudget() * IGC_MARK_SLICE_MULTIPLIER;
+        else
+            millis = defaultSliceBudget();
+    }
+
+    return SliceBudget(TimeBudget(millis));
+}
+
+void
+GCRuntime::gc(JSGCInvocationKind gckind, JS::gcreason::Reason reason)
+{
+    invocationKind = gckind;
+    collect(true, SliceBudget::unlimited(), reason);
+}
+
+void
+GCRuntime::startGC(JSGCInvocationKind gckind, JS::gcreason::Reason reason, int64_t millis)
+{
+    MOZ_ASSERT(!isIncrementalGCInProgress());
+    if (!JS::IsIncrementalGCEnabled(rt->contextFromMainThread())) {
+        gc(gckind, reason);
+        return;
+    }
+    invocationKind = gckind;
+    collect(false, defaultBudget(reason, millis), reason);
+}
+
+void
+GCRuntime::gcSlice(JS::gcreason::Reason reason, int64_t millis)
+{
+    MOZ_ASSERT(isIncrementalGCInProgress());
+    collect(false, defaultBudget(reason, millis), reason);
+}
+
+void
+GCRuntime::finishGC(JS::gcreason::Reason reason)
+{
+    MOZ_ASSERT(isIncrementalGCInProgress());
+
+    // If we're not collecting because we're out of memory then skip the
+    // compacting phase if we need to finish an ongoing incremental GC
+    // non-incrementally to avoid janking the browser.
+    if (!IsOOMReason(initialReason)) {
+        if (incrementalState == State::Compact) {
+            abortGC();
+            return;
+        }
+
+        isCompacting = false;
+    }
+
+    collect(false, SliceBudget::unlimited(), reason);
+}
+
+void
+GCRuntime::abortGC()
+{
+    checkCanCallAPI();
+    MOZ_ASSERT(!rt->mainThread.suppressGC);
+
+    AutoStopVerifyingBarriers av(rt, false);
+    AutoEnqueuePendingParseTasksAfterGC aept(*this);
+
+    gcstats::AutoGCSlice agc(stats, scanZonesBeforeGC(), invocationKind,
+                             SliceBudget::unlimited(), JS::gcreason::ABORT_GC);
+
+    evictNursery(JS::gcreason::ABORT_GC);
+    AutoTraceSession session(rt, JS::HeapState::MajorCollecting);
+
+    number++;
+    resetIncrementalGC(gc::AbortReason::AbortRequested, session.lock);
+}
+
+void
+GCRuntime::notifyDidPaint()
+{
+    MOZ_ASSERT(CurrentThreadCanAccessRuntime(rt));
+
+#ifdef JS_GC_ZEAL
+    if (hasZealMode(ZealMode::FrameVerifierPre))
+        verifyPreBarriers();
+
+    if (hasZealMode(ZealMode::FrameGC)) {
+        JS::PrepareForFullGC(rt->contextFromMainThread());
+        gc(GC_NORMAL, JS::gcreason::REFRESH_FRAME);
+        return;
+    }
+#endif
+
+    if (isIncrementalGCInProgress() && !interFrameGC && tunables.areRefreshFrameSlicesEnabled()) {
+        JS::PrepareForIncrementalGC(rt->contextFromMainThread());
+        gcSlice(JS::gcreason::REFRESH_FRAME);
+    }
+
+    interFrameGC = false;
+}
+
+static bool
+ZonesSelected(JSRuntime* rt)
+{
+    for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next()) {
+        if (zone->isGCScheduled())
+            return true;
+    }
+    return false;
+}
+
+void
+GCRuntime::startDebugGC(JSGCInvocationKind gckind, SliceBudget& budget)
+{
+    MOZ_ASSERT(!isIncrementalGCInProgress());
+    if (!ZonesSelected(rt))
+        JS::PrepareForFullGC(rt->contextFromMainThread());
+    invocationKind = gckind;
+    collect(false, budget, JS::gcreason::DEBUG_GC);
+}
+
+void
+GCRuntime::debugGCSlice(SliceBudget& budget)
+{
+    MOZ_ASSERT(isIncrementalGCInProgress());
+    if (!ZonesSelected(rt))
+        JS::PrepareForIncrementalGC(rt->contextFromMainThread());
+    collect(false, budget, JS::gcreason::DEBUG_GC);
+}
+
+/* Schedule a full GC unless a zone will already be collected. */
+void
+js::PrepareForDebugGC(JSRuntime* rt)
+{
+    if (!ZonesSelected(rt))
+        JS::PrepareForFullGC(rt->contextFromMainThread());
+}
+
+void
+GCRuntime::onOutOfMallocMemory()
+{
+    // Stop allocating new chunks.
+    allocTask.cancel(GCParallelTask::CancelAndWait);
+
+    // Make sure we release anything queued for release.
+    decommitTask.join();
+
+    // Wait for background free of nursery huge slots to finish.
+    nursery.waitBackgroundFreeEnd();
+
+    AutoLockGC lock(rt);
+    onOutOfMallocMemory(lock);
+}
+
+void
+GCRuntime::onOutOfMallocMemory(const AutoLockGC& lock)
+{
+    // Release any relocated arenas we may be holding on to, without releasing
+    // the GC lock.
+    releaseHeldRelocatedArenasWithoutUnlocking(lock);
+
+    // Throw away any excess chunks we have lying around.
+    freeEmptyChunks(rt, lock);
+
+    // Immediately decommit as many arenas as possible in the hopes that this
+    // might let the OS scrape together enough pages to satisfy the failing
+    // malloc request.
+    decommitAllWithoutUnlocking(lock);
+}
+
+void
+GCRuntime::minorGC(JS::gcreason::Reason reason, gcstats::Phase phase)
+{
+    MOZ_ASSERT(!rt->isHeapBusy());
+
+    if (rt->mainThread.suppressGC)
+        return;
+
+    gcstats::AutoPhase ap(stats, phase);
+
+    minorGCTriggerReason = JS::gcreason::NO_REASON;
+    TraceLoggerThread* logger = TraceLoggerForMainThread(rt);
+    AutoTraceLog logMinorGC(logger, TraceLogger_MinorGC);
+    nursery.collect(rt, reason);
+    MOZ_ASSERT(nursery.isEmpty());
+
+    blocksToFreeAfterMinorGC.freeAll();
+
+#ifdef JS_GC_ZEAL
+    if (rt->hasZealMode(ZealMode::CheckHeapAfterGC))
+        CheckHeapAfterGC(rt);
+#endif
+
+    {
+        AutoLockGC lock(rt);
+        for (ZonesIter zone(rt, WithAtoms); !zone.done(); zone.next())
+            maybeAllocTriggerZoneGC(zone, lock);
+    }
+}
+
+void
+GCRuntime::disableGenerationalGC()
+{
+    if (isGenerationalGCEnabled()) {
+        evictNursery(JS::gcreason::API);
+        nursery.disable();
+    }
+    ++rt->gc.generationalDisabled;
+}
+
+void
+GCRuntime::enableGenerationalGC()
+{
+    MOZ_ASSERT(generationalDisabled > 0);
+    --generationalDisabled;
+    if (generationalDisabled == 0)
+        nursery.enable();
+}
+
+bool
+GCRuntime::gcIfRequested()
+{
+    // This method returns whether a major GC was performed.
+
+    if (minorGCRequested())
+        minorGC(minorGCTriggerReason);
+
+    if (majorGCRequested()) {
+        if (!isIncrementalGCInProgress())
+            startGC(GC_NORMAL, majorGCTriggerReason);
+        else
+            gcSlice(majorGCTriggerReason);
+        return true;
+    }
+
+    return false;
+}
+
+void
+js::gc::FinishGC(JSContext* cx)
+{
+    if (JS::IsIncrementalGCInProgress(cx)) {
+        JS::PrepareForIncrementalGC(cx);
+        JS::FinishIncrementalGC(cx, JS::gcreason::API);
+    }
+
+    cx->gc.nursery.waitBackgroundFreeEnd();
+}
+
+AutoPrepareForTracing::AutoPrepareForTracing(JSContext* cx, ZoneSelector selector)
+{
+    js::gc::FinishGC(cx);
+    session_.emplace(cx);
+}
+
+JSCompartment*
+js::NewCompartment(JSContext* cx, Zone* zone, JSPrincipals* principals,
+                   const JS::CompartmentOptions& options)
+{
+    JSRuntime* rt = cx->runtime();
+    JS_AbortIfWrongThread(cx);
+
+    ScopedJSDeletePtr<Zone> zoneHolder;
+    if (!zone) {
+        zone = cx->new_<Zone>(rt);
+        if (!zone)
+            return nullptr;
+
+        zoneHolder.reset(zone);
+
+        const JSPrincipals* trusted = rt->trustedPrincipals();
+        bool isSystem = principals && principals == trusted;
+        if (!zone->init(isSystem)) {
+            ReportOutOfMemory(cx);
+            return nullptr;
+        }
+    }
+
+    ScopedJSDeletePtr<JSCompartment> compartment(cx->new_<JSCompartment>(zone, options));
+    if (!compartment || !compartment->init(cx))
+        return nullptr;
+
+    // Set up the principals.
+    JS_SetCompartmentPrincipals(compartment, principals);
+
+    AutoLockGC lock(rt);
+
+    if (!zone->compartments.append(compartment.get())) {
+        ReportOutOfMemory(cx);
+        return nullptr;
+    }
+
+    if (zoneHolder && !rt->gc.zones.append(zone)) {
+        ReportOutOfMemory(cx);
+        return nullptr;
+    }
+
+    zoneHolder.forget();
+    return compartment.forget();
+}
+
+void
+gc::MergeCompartments(JSCompartment* source, JSCompartment* target)
+{
+    // The source compartment must be specifically flagged as mergable.  This
+    // also implies that the compartment is not visible to the debugger.
+    MOZ_ASSERT(source->creationOptions_.mergeable());
+    MOZ_ASSERT(source->creationOptions_.invisibleToDebugger());
+
+    MOZ_ASSERT(source->creationOptions().addonIdOrNull() ==
+               target->creationOptions().addonIdOrNull());
+
+    JSContext* cx = source->contextFromMainThread();
+
+    AutoPrepareForTracing prepare(cx, SkipAtoms);
+
+    // Cleanup tables and other state in the source compartment that will be
+    // meaningless after merging into the target compartment.
+
+    source->clearTables();
+    source->zone()->clearTables();
+    source->unsetIsDebuggee();
+
+    // The delazification flag indicates the presence of LazyScripts in a
+    // compartment for the Debugger API, so if the source compartment created
+    // LazyScripts, the flag must be propagated to the target compartment.
+    if (source->needsDelazificationForDebugger())
+        target->scheduleDelazificationForDebugger();
+
+    // Release any relocated arenas which we may be holding on to as they might
+    // be in the source zone
+    cx->gc.releaseHeldRelocatedArenas();
+
+    // Fixup compartment pointers in source to refer to target, and make sure
+    // type information generations are in sync.
+
+    for (auto script = source->zone()->cellIter<JSScript>(); !script.done(); script.next()) {
+        MOZ_ASSERT(script->compartment() == source);
+        script->compartment_ = target;
+        script->setTypesGeneration(target->zone()->types.generation);
+    }
+
+    for (auto group = source->zone()->cellIter<ObjectGroup>(); !group.done(); group.next()) {
+        group->setGeneration(target->zone()->types.generation);
+        group->compartment_ = target;
+
+        // Remove any unboxed layouts from the list in the off thread
+        // compartment. These do not need to be reinserted in the target
+        // compartment's list, as the list is not required to be complete.
+        if (UnboxedLayout* layout = group->maybeUnboxedLayoutDontCheckGeneration())
+            layout->detachFromCompartment();
+    }
+
+    // Fixup zone pointers in source's zone to refer to target's zone.
+
+    for (auto thingKind : AllAllocKinds()) {
+        for (ArenaIter aiter(source->zone(), thingKind); !aiter.done(); aiter.next()) {
+            Arena* arena = aiter.get();
+            arena->zone = target->zone();
+        }
+    }
+
+    // The source should be the only compartment in its zone.
+    for (CompartmentsInZoneIter c(source->zone()); !c.done(); c.next())
+        MOZ_ASSERT(c.get() == source);
+
+    // Merge the allocator, stats and UIDs in source's zone into target's zone.
+    target->zone()->arenas.adoptArenas(cx, &source->zone()->arenas);
+    target->zone()->usage.adopt(source->zone()->usage);
+    target->zone()->adoptUniqueIds(source->zone());
+
+    // Merge other info in source's zone into target's zone.
+    target->zone()->types.typeLifoAlloc.transferFrom(&source->zone()->types.typeLifoAlloc);
+}
+
+void
+GCRuntime::runDebugGC()
+{
+#ifdef JS_GC_ZEAL
+    if (rt->mainThread.suppressGC)
+        return;
+
+    if (hasZealMode(ZealMode::GenerationalGC))
+        return minorGC(JS::gcreason::DEBUG_GC);
+
+    PrepareForDebugGC(rt);
+
+    auto budget = SliceBudget::unlimited();
+    if (hasZealMode(ZealMode::IncrementalRootsThenFinish) ||
+        hasZealMode(ZealMode::IncrementalMarkAllThenFinish) ||
+        hasZealMode(ZealMode::IncrementalMultipleSlices))
+    {
+        js::gc::State initialState = incrementalState;
+        if (hasZealMode(ZealMode::IncrementalMultipleSlices)) {
+            /*
+             * Start with a small slice limit and double it every slice. This
+             * ensure that we get multiple slices, and collection runs to
+             * completion.
+             */
+            if (!isIncrementalGCInProgress())
+                incrementalLimit = zealFrequency / 2;
+            else
+                incrementalLimit *= 2;
+            budget = SliceBudget(WorkBudget(incrementalLimit));
+        } else {
+            // This triggers incremental GC but is actually ignored by IncrementalMarkSlice.
+            budget = SliceBudget(WorkBudget(1));
+        }
+
+        if (!isIncrementalGCInProgress())
+            invocationKind = GC_SHRINK;
+        collect(false, budget, JS::gcreason::DEBUG_GC);
+
+        /*
+         * For multi-slice zeal, reset the slice size when we get to the sweep
+         * or compact phases.
+         */
+        if (hasZealMode(ZealMode::IncrementalMultipleSlices)) {
+            if ((initialState == State::Mark && incrementalState == State::Sweep) ||
+                (initialState == State::Sweep && incrementalState == State::Compact))
+            {
+                incrementalLimit = zealFrequency / 2;
+            }
+        }
+    } else if (hasZealMode(ZealMode::Compact)) {
+        gc(GC_SHRINK, JS::gcreason::DEBUG_GC);
+    } else {
+        gc(GC_NORMAL, JS::gcreason::DEBUG_GC);
+    }
+
+#endif
+}
+
+void
+GCRuntime::setFullCompartmentChecks(bool enabled)
+{
+    MOZ_ASSERT(!rt->isHeapMajorCollecting());
+    fullCompartmentChecks = enabled;
+}
+
+#ifdef JS_GC_ZEAL
+bool
+GCRuntime::selectForMarking(JSObject* object)
+{
+    MOZ_ASSERT(!rt->isHeapMajorCollecting());
+    return selectedForMarking.append(object);
+}
+
+void
+GCRuntime::clearSelectedForMarking()
+{
+    selectedForMarking.clearAndFree();
+}
+
+void
+GCRuntime::setDeterministic(bool enabled)
+{
+    MOZ_ASSERT(!rt->isHeapMajorCollecting());
+    deterministicOnly = enabled;
+}
+#endif
+
+#ifdef DEBUG
+
+/* Should only be called manually under gdb */
+void PreventGCDuringInteractiveDebug()
+{
+    TlsPerThreadData.get()->suppressGC++;
+}
+
+#endif
+
+void
+js::ReleaseAllJITCode(FreeOp* fop)
+{
+    js::CancelOffThreadIonCompile(fop->runtime());
+    for (ZonesIter zone(fop->runtime(), SkipAtoms); !zone.done(); zone.next()) {
+        zone->setPreservingCode(false);
+        zone->discardJitCode(fop);
+    }
+}
+
+void
+js::PurgeJITCaches(Zone* zone)
+{
+    /* Discard Ion caches. */
+    for (auto script = zone->cellIter<JSScript>(); !script.done(); script.next())
+        jit::PurgeCaches(script);
+}
+
+void
+ArenaLists::normalizeBackgroundFinalizeState(AllocKind thingKind)
+{
+    ArenaLists::BackgroundFinalizeState* bfs = &backgroundFinalizeState[thingKind];
+    switch (*bfs) {
+      case BFS_DONE:
+        break;
+      default:
+        MOZ_ASSERT_UNREACHABLE("Background finalization in progress, but it should not be.");
+        break;
+    }
+}
+
+void
+ArenaLists::adoptArenas(JSRuntime* rt, ArenaLists* fromArenaLists)
+{
+    // GC should be inactive, but still take the lock as a kind of read fence.
+    AutoLockGC lock(rt);
+
+    fromArenaLists->purge();
+
+    for (auto thingKind : AllAllocKinds()) {
+        // When we enter a parallel section, we join the background
+        // thread, and we do not run GC while in the parallel section,
+        // so no finalizer should be active!
+        normalizeBackgroundFinalizeState(thingKind);
+        fromArenaLists->normalizeBackgroundFinalizeState(thingKind);
+
+        ArenaList* fromList = &fromArenaLists->arenaLists[thingKind];
+        ArenaList* toList = &arenaLists[thingKind];
+        fromList->check();
+        toList->check();
+        Arena* next;
+        for (Arena* fromArena = fromList->head(); fromArena; fromArena = next) {
+            // Copy fromArena->next before releasing/reinserting.
+            next = fromArena->next;
+
+            MOZ_ASSERT(!fromArena->isEmpty());
+            toList->insertAtCursor(fromArena);
+        }
+        fromList->clear();
+        toList->check();
+    }
+}
+
+bool
+ArenaLists::containsArena(JSRuntime* rt, Arena* needle)
+{
+    AutoLockGC lock(rt);
+    ArenaList& list = arenaLists[needle->getAllocKind()];
+    for (Arena* arena = list.head(); arena; arena = arena->next) {
+        if (arena == needle)
+            return true;
+    }
+    return false;
+}
+
+
+AutoSuppressGC::AutoSuppressGC(ExclusiveContext* cx)
+  : suppressGC_(cx->perThreadData->suppressGC)
+{
+    suppressGC_++;
+}
+
+AutoSuppressGC::AutoSuppressGC(JSCompartment* comp)
+  : suppressGC_(comp->runtimeFromMainThread()->mainThread.suppressGC)
+{
+    suppressGC_++;
+}
+
+AutoSuppressGC::AutoSuppressGC(JSContext* cx)
+  : suppressGC_(cx->mainThread().suppressGC)
+{
+    suppressGC_++;
+}
+
+bool
+js::UninlinedIsInsideNursery(const gc::Cell* cell)
+{
+    return IsInsideNursery(cell);
+}
+
+#ifdef DEBUG
+AutoDisableProxyCheck::AutoDisableProxyCheck(JSRuntime* rt)
+  : gc(rt->gc)
+{
+    gc.disableStrictProxyChecking();
+}
+
+AutoDisableProxyCheck::~AutoDisableProxyCheck()
+{
+    gc.enableStrictProxyChecking();
+}
+
+JS_FRIEND_API(void)
+JS::AssertGCThingMustBeTenured(JSObject* obj)
+{
+    MOZ_ASSERT(obj->isTenured() &&
+               (!IsNurseryAllocable(obj->asTenured().getAllocKind()) ||
+                obj->getClass()->hasFinalize()));
+}
+
+JS_FRIEND_API(void)
+JS::AssertGCThingIsNotAnObjectSubclass(Cell* cell)
+{
+    MOZ_ASSERT(cell);
+    MOZ_ASSERT(cell->getTraceKind() != JS::TraceKind::Object);
+}
+
+JS_FRIEND_API(void)
+js::gc::AssertGCThingHasType(js::gc::Cell* cell, JS::TraceKind kind)
+{
+    if (!cell)
+        MOZ_ASSERT(kind == JS::TraceKind::Null);
+    else if (IsInsideNursery(cell))
+        MOZ_ASSERT(kind == JS::TraceKind::Object);
+    else
+        MOZ_ASSERT(MapAllocToTraceKind(cell->asTenured().getAllocKind()) == kind);
+}
+
+JS_PUBLIC_API(size_t)
+JS::GetGCNumber()
+{
+    JSRuntime* rt = js::TlsPerThreadData.get()->runtimeFromMainThread();
+    if (!rt)
+        return 0;
+    return rt->gc.gcNumber();
+}
+#endif
+
+JS::AutoAssertNoGC::AutoAssertNoGC()
+  : gc(nullptr), gcNumber(0)
+{
+    js::PerThreadData* data = js::TlsPerThreadData.get();
+    if (data) {
+        /*
+         * GC's from off-thread will always assert, so off-thread is implicitly
+         * AutoAssertNoGC. We still need to allow AutoAssertNoGC to be used in
+         * code that works from both threads, however. We also use this to
+         * annotate the off thread run loops.
+         */
+        JSRuntime* runtime = data->runtimeIfOnOwnerThread();
+        if (runtime) {
+            gc = &runtime->gc;
+            gcNumber = gc->gcNumber();
+            gc->enterUnsafeRegion();
+        }
+    }
+}
+
+JS::AutoAssertNoGC::AutoAssertNoGC(JSRuntime* rt)
+  : gc(&rt->gc), gcNumber(rt->gc.gcNumber())
+{
+    gc->enterUnsafeRegion();
+}
+
+JS::AutoAssertNoGC::AutoAssertNoGC(JSContext* cx)
+  : gc(&cx->gc), gcNumber(cx->gc.gcNumber())
+{
+    gc->enterUnsafeRegion();
+}
+
+JS::AutoAssertNoGC::~AutoAssertNoGC()
+{
+    if (gc) {
+        gc->leaveUnsafeRegion();
+
+        /*
+         * The following backstop assertion should never fire: if we bumped the
+         * gcNumber, we should have asserted because inUnsafeRegion was true.
+         */
+        MOZ_ASSERT(gcNumber == gc->gcNumber(), "GC ran inside an AutoAssertNoGC scope.");
+    }
+}
+
+JS::AutoAssertOnBarrier::AutoAssertOnBarrier(JSContext* cx)
+  : context(cx),
+    prev(cx->runtime()->allowGCBarriers())
+{
+    context->runtime()->allowGCBarriers_ = false;
+}
+
+JS::AutoAssertOnBarrier::~AutoAssertOnBarrier()
+{
+    MOZ_ASSERT(!context->runtime()->allowGCBarriers_);
+    context->runtime()->allowGCBarriers_ = prev;
+}
+
+#ifdef DEBUG
+JS::AutoAssertNoAlloc::AutoAssertNoAlloc(JSContext* cx)
+  : gc(nullptr)
+{
+    disallowAlloc(cx);
+}
+
+void JS::AutoAssertNoAlloc::disallowAlloc(JSRuntime* rt)
+{
+    MOZ_ASSERT(!gc);
+    gc = &rt->gc;
+    gc->disallowAlloc();
+}
+
+JS::AutoAssertNoAlloc::~AutoAssertNoAlloc()
+{
+    if (gc)
+        gc->allowAlloc();
+}
+
+AutoAssertNoNurseryAlloc::AutoAssertNoNurseryAlloc(JSRuntime* rt)
+  : gc(rt->gc)
+{
+    gc.disallowNurseryAlloc();
+}
+
+AutoAssertNoNurseryAlloc::~AutoAssertNoNurseryAlloc()
+{
+    gc.allowNurseryAlloc();
+}
+
+JS::AutoEnterCycleCollection::AutoEnterCycleCollection(JSContext* cx)
+  : runtime(cx->runtime())
+{
+    MOZ_ASSERT(!runtime->isHeapBusy());
+    runtime->setHeapState(HeapState::CycleCollecting);
+}
+
+JS::AutoEnterCycleCollection::~AutoEnterCycleCollection()
+{
+    MOZ_ASSERT(runtime->isCycleCollecting());
+    runtime->setHeapState(HeapState::Idle);
+}
+#endif
+
+JS::AutoAssertGCCallback::AutoAssertGCCallback(JSObject* obj)
+  : AutoSuppressGCAnalysis()
+{
+    MOZ_ASSERT(obj->runtimeFromMainThread()->isHeapCollecting());
+}
+
+JS_FRIEND_API(const char*)
+JS::GCTraceKindToAscii(JS::TraceKind kind)
+{
+    switch(kind) {
+#define MAP_NAME(name, _0, _1) case JS::TraceKind::name: return #name;
+JS_FOR_EACH_TRACEKIND(MAP_NAME);
+#undef MAP_NAME
+      default: return "Invalid";
+    }
+}
+
+JS::GCCellPtr::GCCellPtr(const Value& v)
+  : ptr(0)
+{
+    if (v.isString())
+        ptr = checkedCast(v.toString(), JS::TraceKind::String);
+    else if (v.isObject())
+        ptr = checkedCast(&v.toObject(), JS::TraceKind::Object);
+    else if (v.isSymbol())
+        ptr = checkedCast(v.toSymbol(), JS::TraceKind::Symbol);
+    else if (v.isPrivateGCThing())
+        ptr = checkedCast(v.toGCThing(), v.toGCThing()->getTraceKind());
+    else
+        ptr = checkedCast(nullptr, JS::TraceKind::Null);
+}
+
+JS::TraceKind
+JS::GCCellPtr::outOfLineKind() const
+{
+    MOZ_ASSERT((ptr & OutOfLineTraceKindMask) == OutOfLineTraceKindMask);
+    MOZ_ASSERT(asCell()->isTenured());
+    return MapAllocToTraceKind(asCell()->asTenured().getAllocKind());
+}
+
+bool
+JS::GCCellPtr::mayBeOwnedByOtherRuntime() const
+{
+    return (is<JSString>() && as<JSString>().isPermanentAtom()) ||
+           (is<Symbol>() && as<Symbol>().isWellKnownSymbol());
+}
+
+#ifdef JSGC_HASH_TABLE_CHECKS
+void
+js::gc::CheckHashTablesAfterMovingGC(JSRuntime* rt)
+{
+    /*
+     * Check that internal hash tables no longer have any pointers to things
+     * that have been moved.
+     */
+    rt->spsProfiler.checkStringsMapAfterMovingGC();
+    for (ZonesIter zone(rt, SkipAtoms); !zone.done(); zone.next()) {
+        zone->checkUniqueIdTableAfterMovingGC();
+        zone->checkInitialShapesTableAfterMovingGC();
+        zone->checkBaseShapeTableAfterMovingGC();
+
+        JS::AutoCheckCannotGC nogc;
+        for (auto baseShape = zone->cellIter<BaseShape>(); !baseShape.done(); baseShape.next()) {
+            if (ShapeTable* table = baseShape->maybeTable(nogc))
+                table->checkAfterMovingGC();
+        }
+    }
+    for (CompartmentsIter c(rt, SkipAtoms); !c.done(); c.next()) {
+        c->objectGroups.checkTablesAfterMovingGC();
+        c->dtoaCache.checkCacheAfterMovingGC();
+        c->checkWrapperMapAfterMovingGC();
+        c->checkScriptMapsAfterMovingGC();
+        if (c->debugEnvs)
+            c->debugEnvs->checkHashTablesAfterMovingGC(rt);
+    }
+}
+#endif
+
+JS_PUBLIC_API(void)
+JS::PrepareZoneForGC(Zone* zone)
+{
+    zone->scheduleGC();
+}
+
+JS_PUBLIC_API(void)
+JS::PrepareForFullGC(JSContext* cx)
+{
+    for (ZonesIter zone(cx, WithAtoms); !zone.done(); zone.next())
+        zone->scheduleGC();
+}
+
+JS_PUBLIC_API(void)
+JS::PrepareForIncrementalGC(JSContext* cx)
+{
+    if (!JS::IsIncrementalGCInProgress(cx))
+        return;
+
+    for (ZonesIter zone(cx, WithAtoms); !zone.done(); zone.next()) {
+        if (zone->wasGCStarted())
+            PrepareZoneForGC(zone);
+    }
+}
+
+JS_PUBLIC_API(bool)
+JS::IsGCScheduled(JSContext* cx)
+{
+    for (ZonesIter zone(cx, WithAtoms); !zone.done(); zone.next()) {
+        if (zone->isGCScheduled())
+            return true;
+    }
+
+    return false;
+}
+
+JS_PUBLIC_API(void)
+JS::SkipZoneForGC(Zone* zone)
+{
+    zone->unscheduleGC();
+}
+
+JS_PUBLIC_API(void)
+JS::GCForReason(JSContext* cx, JSGCInvocationKind gckind, gcreason::Reason reason)
+{
+    MOZ_ASSERT(gckind == GC_NORMAL || gckind == GC_SHRINK);
+    cx->gc.gc(gckind, reason);
+}
+
+JS_PUBLIC_API(void)
+JS::StartIncrementalGC(JSContext* cx, JSGCInvocationKind gckind, gcreason::Reason reason, int64_t millis)
+{
+    MOZ_ASSERT(gckind == GC_NORMAL || gckind == GC_SHRINK);
+    cx->gc.startGC(gckind, reason, millis);
+}
+
+JS_PUBLIC_API(void)
+JS::IncrementalGCSlice(JSContext* cx, gcreason::Reason reason, int64_t millis)
+{
+    cx->gc.gcSlice(reason, millis);
+}
+
+JS_PUBLIC_API(void)
+JS::FinishIncrementalGC(JSContext* cx, gcreason::Reason reason)
+{
+    cx->gc.finishGC(reason);
+}
+
+JS_PUBLIC_API(void)
+JS::AbortIncrementalGC(JSContext* cx)
+{
+    cx->gc.abortGC();
+}
+
+char16_t*
+JS::GCDescription::formatSliceMessage(JSContext* cx) const
+{
+    UniqueChars cstr = cx->gc.stats.formatCompactSliceMessage();
+
+    size_t nchars = strlen(cstr.get());
+    UniqueTwoByteChars out(js_pod_malloc<char16_t>(nchars + 1));
+    if (!out)
+        return nullptr;
+    out.get()[nchars] = 0;
+
+    CopyAndInflateChars(out.get(), cstr.get(), nchars);
+    return out.release();
+}
+
+char16_t*
+JS::GCDescription::formatSummaryMessage(JSContext* cx) const
+{
+    UniqueChars cstr = cx->gc.stats.formatCompactSummaryMessage();
+
+    size_t nchars = strlen(cstr.get());
+    UniqueTwoByteChars out(js_pod_malloc<char16_t>(nchars + 1));
+    if (!out)
+        return nullptr;
+    out.get()[nchars] = 0;
+
+    CopyAndInflateChars(out.get(), cstr.get(), nchars);
+    return out.release();
+}
+
+JS::dbg::GarbageCollectionEvent::Ptr
+JS::GCDescription::toGCEvent(JSContext* cx) const
+{
+    return JS::dbg::GarbageCollectionEvent::Create(cx, cx->gc.stats, cx->gc.majorGCCount());
+}
+
+char16_t*
+JS::GCDescription::formatJSON(JSContext* cx, uint64_t timestamp) const
+{
+    UniqueChars cstr = cx->gc.stats.formatJsonMessage(timestamp);
+
+    size_t nchars = strlen(cstr.get());
+    UniqueTwoByteChars out(js_pod_malloc<char16_t>(nchars + 1));
+    if (!out)
+        return nullptr;
+    out.get()[nchars] = 0;
+
+    CopyAndInflateChars(out.get(), cstr.get(), nchars);
+    return out.release();
+}
+
+JS_PUBLIC_API(JS::GCSliceCallback)
+JS::SetGCSliceCallback(JSContext* cx, GCSliceCallback callback)
+{
+    return cx->gc.setSliceCallback(callback);
+}
+
+JS_PUBLIC_API(JS::DoCycleCollectionCallback)
+JS::SetDoCycleCollectionCallback(JSContext* cx, JS::DoCycleCollectionCallback callback)
+{
+    return cx->gc.setDoCycleCollectionCallback(callback);
+}
+
+JS_PUBLIC_API(JS::GCNurseryCollectionCallback)
+JS::SetGCNurseryCollectionCallback(JSContext* cx, GCNurseryCollectionCallback callback)
+{
+    return cx->gc.setNurseryCollectionCallback(callback);
+}
+
+JS_PUBLIC_API(void)
+JS::DisableIncrementalGC(JSContext* cx)
+{
+    cx->gc.disallowIncrementalGC();
+}
+
+JS_PUBLIC_API(bool)
+JS::IsIncrementalGCEnabled(JSContext* cx)
+{
+    return cx->gc.isIncrementalGCEnabled();
+}
+
+JS_PUBLIC_API(bool)
+JS::IsIncrementalGCInProgress(JSContext* cx)
+{
+    return cx->gc.isIncrementalGCInProgress() && !cx->gc.isVerifyPreBarriersEnabled();
+}
+
+JS_PUBLIC_API(bool)
+JS::IsIncrementalBarrierNeeded(JSContext* cx)
+{
+    if (cx->isHeapBusy())
+        return false;
+
+    auto state = cx->gc.state();
+    return state != gc::State::NotActive && state <= gc::State::Sweep;
+}
+
+struct IncrementalReferenceBarrierFunctor {
+    template <typename T> void operator()(T* t) { T::writeBarrierPre(t); }
+};
+
+JS_PUBLIC_API(void)
+JS::IncrementalReferenceBarrier(GCCellPtr thing)
+{
+    if (!thing)
+        return;
+
+    DispatchTyped(IncrementalReferenceBarrierFunctor(), thing);
+}
+
+JS_PUBLIC_API(void)
+JS::IncrementalValueBarrier(const Value& v)
+{
+    js::GCPtrValue::writeBarrierPre(v);
+}
+
+JS_PUBLIC_API(void)
+JS::IncrementalObjectBarrier(JSObject* obj)
+{
+    if (!obj)
+        return;
+
+    MOZ_ASSERT(!obj->zone()->runtimeFromMainThread()->isHeapMajorCollecting());
+
+    JSObject::writeBarrierPre(obj);
+}
+
+JS_PUBLIC_API(bool)
+JS::WasIncrementalGC(JSContext* cx)
+{
+    return cx->gc.isIncrementalGc();
+}
+
+JS::AutoDisableGenerationalGC::AutoDisableGenerationalGC(JSRuntime* rt)
+  : gc(&rt->gc)
+{
+    gc->disableGenerationalGC();
+}
+
+JS::AutoDisableGenerationalGC::~AutoDisableGenerationalGC()
+{
+    gc->enableGenerationalGC();
+}
+
+JS_PUBLIC_API(bool)
+JS::IsGenerationalGCEnabled(JSRuntime* rt)
+{
+    return rt->gc.isGenerationalGCEnabled();
+}
+
+uint64_t
+js::gc::NextCellUniqueId(JSRuntime* rt)
+{
+    return rt->gc.nextCellUniqueId();
+}
+
+namespace js {
+namespace gc {
+namespace MemInfo {
+
+static bool
+GCBytesGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setNumber(double(cx->runtime()->gc.usage.gcBytes()));
+    return true;
+}
+
+static bool
+GCMaxBytesGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setNumber(double(cx->runtime()->gc.tunables.gcMaxBytes()));
+    return true;
+}
+
+static bool
+MallocBytesGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setNumber(double(cx->runtime()->gc.getMallocBytes()));
+    return true;
+}
+
+static bool
+MaxMallocGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setNumber(double(cx->runtime()->gc.maxMallocBytesAllocated()));
+    return true;
+}
+
+static bool
+GCHighFreqGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setBoolean(cx->runtime()->gc.schedulingState.inHighFrequencyGCMode());
+    return true;
+}
+
+static bool
+GCNumberGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setNumber(double(cx->runtime()->gc.gcNumber()));
+    return true;
+}
+
+static bool
+MajorGCCountGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setNumber(double(cx->runtime()->gc.majorGCCount()));
+    return true;
+}
+
+static bool
+MinorGCCountGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setNumber(double(cx->runtime()->gc.minorGCCount()));
+    return true;
+}
+
+static bool
+ZoneGCBytesGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setNumber(double(cx->zone()->usage.gcBytes()));
+    return true;
+}
+
+static bool
+ZoneGCTriggerBytesGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setNumber(double(cx->zone()->threshold.gcTriggerBytes()));
+    return true;
+}
+
+static bool
+ZoneGCAllocTriggerGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setNumber(double(cx->zone()->threshold.allocTrigger(cx->runtime()->gc.schedulingState.inHighFrequencyGCMode())));
+    return true;
+}
+
+static bool
+ZoneMallocBytesGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setNumber(double(cx->zone()->gcMallocBytes));
+    return true;
+}
+
+static bool
+ZoneMaxMallocGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setNumber(double(cx->zone()->gcMaxMallocBytes));
+    return true;
+}
+
+static bool
+ZoneGCDelayBytesGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setNumber(double(cx->zone()->gcDelayBytes));
+    return true;
+}
+
+static bool
+ZoneGCHeapGrowthFactorGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setNumber(cx->zone()->threshold.gcHeapGrowthFactor());
+    return true;
+}
+
+static bool
+ZoneGCNumberGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setNumber(double(cx->zone()->gcNumber()));
+    return true;
+}
+
+#ifdef JS_MORE_DETERMINISTIC
+static bool
+DummyGetter(JSContext* cx, unsigned argc, Value* vp)
+{
+    CallArgs args = CallArgsFromVp(argc, vp);
+    args.rval().setUndefined();
+    return true;
+}
+#endif
+
+} /* namespace MemInfo */
+
+JSObject*
+NewMemoryInfoObject(JSContext* cx)
+{
+    RootedObject obj(cx, JS_NewObject(cx, nullptr));
+    if (!obj)
+        return nullptr;
+
+    using namespace MemInfo;
+    struct NamedGetter {
+        const char* name;
+        JSNative getter;
+    } getters[] = {
+        { "gcBytes", GCBytesGetter },
+        { "gcMaxBytes", GCMaxBytesGetter },
+        { "mallocBytesRemaining", MallocBytesGetter },
+        { "maxMalloc", MaxMallocGetter },
+        { "gcIsHighFrequencyMode", GCHighFreqGetter },
+        { "gcNumber", GCNumberGetter },
+        { "majorGCCount", MajorGCCountGetter },
+        { "minorGCCount", MinorGCCountGetter }
+    };
+
+    for (auto pair : getters) {
+#ifdef JS_MORE_DETERMINISTIC
+        JSNative getter = DummyGetter;
+#else
+        JSNative getter = pair.getter;
+#endif
+        if (!JS_DefineProperty(cx, obj, pair.name, UndefinedHandleValue,
+                               JSPROP_ENUMERATE | JSPROP_SHARED,
+                               getter, nullptr))
+        {
+            return nullptr;
+        }
+    }
+
+    RootedObject zoneObj(cx, JS_NewObject(cx, nullptr));
+    if (!zoneObj)
+        return nullptr;
+
+    if (!JS_DefineProperty(cx, obj, "zone", zoneObj, JSPROP_ENUMERATE))
+        return nullptr;
+
+    struct NamedZoneGetter {
+        const char* name;
+        JSNative getter;
+    } zoneGetters[] = {
+        { "gcBytes", ZoneGCBytesGetter },
+        { "gcTriggerBytes", ZoneGCTriggerBytesGetter },
+        { "gcAllocTrigger", ZoneGCAllocTriggerGetter },
+        { "mallocBytesRemaining", ZoneMallocBytesGetter },
+        { "maxMalloc", ZoneMaxMallocGetter },
+        { "delayBytes", ZoneGCDelayBytesGetter },
+        { "heapGrowthFactor", ZoneGCHeapGrowthFactorGetter },
+        { "gcNumber", ZoneGCNumberGetter }
+    };
+
+    for (auto pair : zoneGetters) {
+ #ifdef JS_MORE_DETERMINISTIC
+        JSNative getter = DummyGetter;
+#else
+        JSNative getter = pair.getter;
+#endif
+        if (!JS_DefineProperty(cx, zoneObj, pair.name, UndefinedHandleValue,
+                               JSPROP_ENUMERATE | JSPROP_SHARED,
+                               getter, nullptr))
+        {
+            return nullptr;
+        }
+    }
+
+    return obj;
+}
+
+const char*
+StateName(State state)
+{
+    switch(state) {
+#define MAKE_CASE(name) case State::name: return #name;
+      GCSTATES(MAKE_CASE)
+#undef MAKE_CASE
+    }
+    MOZ_MAKE_COMPILER_ASSUME_IS_UNREACHABLE("invalide gc::State enum value");
+}
+
+void
+AutoAssertHeapBusy::checkCondition(JSRuntime *rt)
+{
+    this->rt = rt;
+    MOZ_ASSERT(rt->isHeapBusy());
+}
+
+void
+AutoAssertEmptyNursery::checkCondition(JSRuntime *rt) {
+    if (!noAlloc)
+        noAlloc.emplace(rt);
+    this->rt = rt;
+    MOZ_ASSERT(rt->gc.nursery.isEmpty());
+}
+
+AutoEmptyNursery::AutoEmptyNursery(JSRuntime *rt)
+  : AutoAssertEmptyNursery()
+{
+    MOZ_ASSERT(!rt->mainThread.suppressGC);
+    rt->gc.stats.suspendPhases();
+    rt->gc.evictNursery();
+    rt->gc.stats.resumePhases();
+    checkCondition(rt);
+}
+
+} /* namespace gc */
+} /* namespace js */
+
+#ifdef DEBUG
+void
+js::gc::Cell::dump(FILE* fp) const
+{
+    switch (getTraceKind()) {
+      case JS::TraceKind::Object:
+        reinterpret_cast<const JSObject*>(this)->dump(fp);
+        break;
+
+      case JS::TraceKind::String:
+          js::DumpString(reinterpret_cast<JSString*>(const_cast<Cell*>(this)), fp);
+        break;
+
+      case JS::TraceKind::Shape:
+        reinterpret_cast<const Shape*>(this)->dump(fp);
+        break;
+
+      default:
+        fprintf(fp, "%s(%p)\n", JS::GCTraceKindToAscii(getTraceKind()), (void*) this);
+    }
+}
+
+// For use in a debugger.
+void
+js::gc::Cell::dump() const
+{
+    dump(stderr);
+}
+#endif
+
+JS_PUBLIC_API(bool)
+js::gc::detail::CellIsMarkedGrayIfKnown(const Cell* cell)
+{
+    MOZ_ASSERT(cell);
+    if (!cell->isTenured())
+        return false;
+
+    // We ignore the gray marking state of cells and return false in two cases:
+    //
+    // 1) When OOM has caused us to clear the gcGrayBitsValid_ flag.
+    //
+    // 2) When we are in an incremental GC and examine a cell that is in a zone
+    // that is not being collected. Gray targets of CCWs that are marked black
+    // by a barrier will eventually be marked black in the next GC slice.
+    auto tc = &cell->asTenured();
+    auto rt = tc->runtimeFromMainThread();
+    if (!rt->areGCGrayBitsValid() ||
+        (rt->gc.isIncrementalGCInProgress() && !tc->zone()->wasGCStarted()))
+    {
+        return false;
+    }
+
+    return detail::CellIsMarkedGray(tc);
+}