summaryrefslogtreecommitdiffstats
path: root/js/public/HeapAPI.h
blob: 26cca9e1c399886ada40ee72a36a6e562a6285a8 (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
321
322
323
324
325
326
327
328
329
330
331
332
333
334
335
336
337
338
339
340
341
342
343
344
345
346
347
348
349
350
351
352
353
354
355
356
357
358
359
360
361
362
363
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388
389
390
391
392
393
394
395
396
397
398
399
400
401
402
403
404
405
406
407
408
409
410
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
507
508
509
510
511
512
513
514
515
516
517
518
519
520
521
522
523
524
525
526
527
528
529
530
531
532
533
534
535
536
537
538
539
540
541
542
543
544
545
546
547
548
549
550
551
552
553
554
555
556
557
558
559
560
561
562
563
564
565
566
567
568
569
570
571
572
573
574
575
576
577
578
579
580
581
582
583
584
585
586
587
588
589
590
591
592
593
594
595
596
597
598
599
600
601
602
603
604
605
606
607
608
609
610
611
612
613
614
615
616
617
618
619
620
621
622
623
624
625
626
627
628
629
630
631
632
633
634
635
636
637
638
639
640
641
642
643
644
645
646
647
648
649
650
651
652
653
654
655
656
657
658
659
660
661
662
663
664
665
666
667
668
669
670
671
672
673
674
675
676
677
678
679
680
681
682
683
684
685
686
687
688
689
690
691
692
693
694
695
696
697
698
699
700
701
702
703
704
705
706
707
708
709
710
711
712
713
714
715
716
717
718
719
720
721
722
723
724
725
726
727
728
729
730
731
732
733
734
735
736
737
738
739
740
741
742
743
744
745
746
747
748
749
750
751
752
753
754
755
756
757
758
759
760
761
762
763
764
765
766
767
768
769
770
771
772
773
774
775
776
777
778
779
780
781
782
783
784
785
786
787
788
789
790
791
792
793
794
795
796
797
798
799
800
801
802
803
804
805
806
807
808
809
810
811
812
813
814
815
816
817
818
819
820
821
822
823
824
825
826
827
828
829
830
831
832
833
834
835
836
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 * vim: set ts=8 sts=2 et sw=2 tw=80:
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef js_HeapAPI_h
#define js_HeapAPI_h

#include "mozilla/Atomics.h"
#include "mozilla/BitSet.h"

#include <limits.h>
#include <type_traits>

#include "js/AllocPolicy.h"
#include "js/GCAnnotations.h"
#include "js/HashTable.h"
#include "js/shadow/String.h"  // JS::shadow::String
#include "js/shadow/Symbol.h"  // JS::shadow::Symbol
#include "js/shadow/Zone.h"    // JS::shadow::Zone
#include "js/TraceKind.h"
#include "js/TypeDecls.h"

/* These values are private to the JS engine. */
namespace js {

class NurseryDecommitTask;

JS_PUBLIC_API bool CurrentThreadCanAccessZone(JS::Zone* zone);

// To prevent false sharing, some data structures are aligned to a typical cache
// line size.
static constexpr size_t TypicalCacheLineSize = 64;

namespace gc {

class Arena;
struct Cell;
class TenuredChunk;
class StoreBuffer;
class TenuredCell;

const size_t ArenaShift = 12;
const size_t ArenaSize = size_t(1) << ArenaShift;
const size_t ArenaMask = ArenaSize - 1;

#if defined(XP_MACOSX) && defined(__aarch64__)
const size_t PageShift = 14;
#else
const size_t PageShift = 12;
#endif
// Expected page size, so we could initialze ArenasPerPage at compile-time.
// The actual system page size should be queried by SystemPageSize().
const size_t PageSize = size_t(1) << PageShift;
constexpr size_t ArenasPerPage = PageSize / ArenaSize;

const size_t ChunkShift = 20;
const size_t ChunkSize = size_t(1) << ChunkShift;
const size_t ChunkMask = ChunkSize - 1;

const size_t CellAlignShift = 3;
const size_t CellAlignBytes = size_t(1) << CellAlignShift;
const size_t CellAlignMask = CellAlignBytes - 1;

const size_t CellBytesPerMarkBit = CellAlignBytes;
const size_t MarkBitsPerCell = 2;

/*
 * The minimum cell size ends up as twice the cell alignment because the mark
 * bitmap contains one bit per CellBytesPerMarkBit bytes (which is equal to
 * CellAlignBytes) and we need two mark bits per cell.
 */
const size_t MinCellSize = CellBytesPerMarkBit * MarkBitsPerCell;

/*
 * The mark bitmap has one bit per each possible cell start position. This
 * wastes some space for larger GC things but allows us to avoid division by the
 * cell's size when accessing the bitmap.
 */
const size_t ArenaBitmapBits = ArenaSize / CellBytesPerMarkBit;
const size_t ArenaBitmapBytes = HowMany(ArenaBitmapBits, 8);
const size_t ArenaBitmapWords = HowMany(ArenaBitmapBits, JS_BITS_PER_WORD);

// The base class for all GC chunks, either in the nursery or in the tenured
// heap memory. This structure is locatable from any GC pointer by aligning to
// the chunk size.
class alignas(CellAlignBytes) ChunkBase {
 protected:
  ChunkBase(JSRuntime* rt, StoreBuffer* sb) {
    MOZ_ASSERT((uintptr_t(this) & ChunkMask) == 0);
    initBase(rt, sb);
  }

  void initBase(JSRuntime* rt, StoreBuffer* sb) {
    runtime = rt;
    storeBuffer = sb;
  }

 public:
  // The store buffer for pointers from tenured things to things in this
  // chunk. Will be non-null if and only if this is a nursery chunk.
  StoreBuffer* storeBuffer;

  // Provide quick access to the runtime from absolutely anywhere.
  JSRuntime* runtime;
};

// Information about tenured heap chunks.
struct TenuredChunkInfo {
 private:
  friend class ChunkPool;
  TenuredChunk* next = nullptr;
  TenuredChunk* prev = nullptr;

 public:
  /* Number of free arenas, either committed or decommitted. */
  uint32_t numArenasFree;

  /* Number of free, committed arenas. */
  uint32_t numArenasFreeCommitted;
};

/*
 * Calculating ArenasPerChunk:
 *
 * To figure out how many Arenas will fit in a chunk we need to know how much
 * extra space is available after we allocate the header data. This is a problem
 * because the header size depends on the number of arenas in the chunk.
 *
 * The dependent fields are markBits, decommittedPages and
 * freeCommittedArenas. markBits needs ArenaBitmapBytes bytes per arena,
 * decommittedPages needs one bit per page and freeCommittedArenas needs one
 * bit per arena.
 *
 * We can calculate an approximate value by dividing the number of bits of free
 * space in the chunk by the number of bits needed per arena. This is an
 * approximation because it doesn't take account of the fact that the variable
 * sized fields must be rounded up to a whole number of words, or any padding
 * the compiler adds between fields.
 *
 * Fortunately, for the chunk and arena size parameters we use this
 * approximation turns out to be correct. If it were not we might need to adjust
 * the arena count down by one to allow more space for the padding.
 */
const size_t BitsPerPageWithHeaders =
    (ArenaSize + ArenaBitmapBytes) * ArenasPerPage * CHAR_BIT + ArenasPerPage +
    1;
const size_t ChunkBitsAvailable =
    (ChunkSize - sizeof(ChunkBase) - sizeof(TenuredChunkInfo)) * CHAR_BIT;
const size_t PagesPerChunk = ChunkBitsAvailable / BitsPerPageWithHeaders;
const size_t ArenasPerChunk = PagesPerChunk * ArenasPerPage;
const size_t FreeCommittedBits = ArenasPerChunk;
const size_t DecommitBits = PagesPerChunk;
const size_t BitsPerArenaWithHeaders =
    (ArenaSize + ArenaBitmapBytes) * CHAR_BIT +
    (DecommitBits / ArenasPerChunk) + 1;

const size_t CalculatedChunkSizeRequired =
    sizeof(ChunkBase) + sizeof(TenuredChunkInfo) +
    RoundUp(ArenasPerChunk * ArenaBitmapBytes, sizeof(uintptr_t)) +
    RoundUp(FreeCommittedBits, sizeof(uint32_t) * CHAR_BIT) / CHAR_BIT +
    RoundUp(DecommitBits, sizeof(uint32_t) * CHAR_BIT) / CHAR_BIT +
    ArenasPerChunk * ArenaSize;
static_assert(CalculatedChunkSizeRequired <= ChunkSize,
              "Calculated ArenasPerChunk is too large");

const size_t CalculatedChunkPadSize = ChunkSize - CalculatedChunkSizeRequired;
static_assert(CalculatedChunkPadSize * CHAR_BIT < BitsPerArenaWithHeaders,
              "Calculated ArenasPerChunk is too small");

static_assert(ArenasPerChunk == 252,
              "Do not accidentally change our heap's density.");

// Mark bitmaps are atomic because they can be written by gray unmarking on the
// main thread while read by sweeping on a background thread. The former does
// not affect the result of the latter.
using MarkBitmapWord = mozilla::Atomic<uintptr_t, mozilla::Relaxed>;

/*
 * Live objects are marked black or gray. Everything reachable from a JS root is
 * marked black. Objects marked gray are eligible for cycle collection.
 *
 *    BlackBit:     GrayOrBlackBit:  Color:
 *       0               0           white
 *       0               1           gray
 *       1               0           black
 *       1               1           black
 */
enum class ColorBit : uint32_t { BlackBit = 0, GrayOrBlackBit = 1 };

// Mark colors. Order is important here: the greater value the 'more marked' a
// cell is.
enum class MarkColor : uint8_t { Gray = 1, Black = 2 };

// Mark bitmap for a tenured heap chunk.
struct alignas(TypicalCacheLineSize) MarkBitmap {
  static constexpr size_t WordCount = ArenaBitmapWords * ArenasPerChunk;
  MarkBitmapWord bitmap[WordCount];

  inline void getMarkWordAndMask(const TenuredCell* cell, ColorBit colorBit,
                                 MarkBitmapWord** wordp, uintptr_t* maskp);

  // The following are not exported and are defined in gc/Heap.h:
  inline bool markBit(const TenuredCell* cell, ColorBit colorBit);
  inline bool isMarkedAny(const TenuredCell* cell);
  inline bool isMarkedBlack(const TenuredCell* cell);
  inline bool isMarkedGray(const TenuredCell* cell);
  inline bool markIfUnmarked(const TenuredCell* cell, MarkColor color);
  inline bool markIfUnmarkedAtomic(const TenuredCell* cell, MarkColor color);
  inline void markBlack(const TenuredCell* cell);
  inline void markBlackAtomic(const TenuredCell* cell);
  inline void copyMarkBit(TenuredCell* dst, const TenuredCell* src,
                          ColorBit colorBit);
  inline void unmark(const TenuredCell* cell);
  inline MarkBitmapWord* arenaBits(Arena* arena);
};

static_assert(ArenaBitmapBytes * ArenasPerChunk == sizeof(MarkBitmap),
              "Ensure our MarkBitmap actually covers all arenas.");

// Bitmap with one bit per page used for decommitted page set.
using ChunkPageBitmap = mozilla::BitSet<PagesPerChunk, uint32_t>;

// Bitmap with one bit per arena used for free committed arena set.
using ChunkArenaBitmap = mozilla::BitSet<ArenasPerChunk, uint32_t>;

// Base class containing data members for a tenured heap chunk.
class TenuredChunkBase : public ChunkBase {
 public:
  TenuredChunkInfo info;
  MarkBitmap markBits;
  ChunkArenaBitmap freeCommittedArenas;
  ChunkPageBitmap decommittedPages;

 protected:
  explicit TenuredChunkBase(JSRuntime* runtime) : ChunkBase(runtime, nullptr) {
    info.numArenasFree = ArenasPerChunk;
  }

  void initAsDecommitted();
};

/*
 * We sometimes use an index to refer to a cell in an arena. The index for a
 * cell is found by dividing by the cell alignment so not all indices refer to
 * valid cells.
 */
const size_t ArenaCellIndexBytes = CellAlignBytes;
const size_t MaxArenaCellIndex = ArenaSize / CellAlignBytes;

const size_t MarkBitmapWordBits = sizeof(MarkBitmapWord) * CHAR_BIT;

constexpr size_t FirstArenaAdjustmentBits =
    RoundUp(sizeof(gc::TenuredChunkBase), ArenaSize) / gc::CellBytesPerMarkBit;

static_assert((FirstArenaAdjustmentBits % MarkBitmapWordBits) == 0);
constexpr size_t FirstArenaAdjustmentWords =
    FirstArenaAdjustmentBits / MarkBitmapWordBits;

const size_t ChunkStoreBufferOffset = offsetof(ChunkBase, storeBuffer);
const size_t ChunkMarkBitmapOffset = offsetof(TenuredChunkBase, markBits);

// Hardcoded offsets into Arena class.
const size_t ArenaZoneOffset = 2 * sizeof(uint32_t);
const size_t ArenaHeaderSize = ArenaZoneOffset + 2 * sizeof(uintptr_t) +
                               sizeof(size_t) + sizeof(uintptr_t);

// The first word of a GC thing has certain requirements from the GC and is used
// to store flags in the low bits.
const size_t CellFlagBitsReservedForGC = 3;

// The first word can be used to store JSClass pointers for some thing kinds, so
// these must be suitably aligned.
const size_t JSClassAlignBytes = size_t(1) << CellFlagBitsReservedForGC;

#ifdef JS_DEBUG
/* When downcasting, ensure we are actually the right type. */
extern JS_PUBLIC_API void AssertGCThingHasType(js::gc::Cell* cell,
                                               JS::TraceKind kind);
#else
inline void AssertGCThingHasType(js::gc::Cell* cell, JS::TraceKind kind) {}
#endif

MOZ_ALWAYS_INLINE bool IsInsideNursery(const js::gc::Cell* cell);
MOZ_ALWAYS_INLINE bool IsInsideNursery(const js::gc::TenuredCell* cell);

} /* namespace gc */
} /* namespace js */

namespace JS {

enum class HeapState {
  Idle,             // doing nothing with the GC heap
  Tracing,          // tracing the GC heap without collecting, e.g.
                    // IterateCompartments()
  MajorCollecting,  // doing a GC of the major heap
  MinorCollecting,  // doing a GC of the minor heap (nursery)
  CycleCollecting   // in the "Unlink" phase of cycle collection
};

JS_PUBLIC_API HeapState RuntimeHeapState();

static inline bool RuntimeHeapIsBusy() {
  return RuntimeHeapState() != HeapState::Idle;
}

static inline bool RuntimeHeapIsTracing() {
  return RuntimeHeapState() == HeapState::Tracing;
}

static inline bool RuntimeHeapIsMajorCollecting() {
  return RuntimeHeapState() == HeapState::MajorCollecting;
}

static inline bool RuntimeHeapIsMinorCollecting() {
  return RuntimeHeapState() == HeapState::MinorCollecting;
}

static inline bool RuntimeHeapIsCollecting(HeapState state) {
  return state == HeapState::MajorCollecting ||
         state == HeapState::MinorCollecting;
}

static inline bool RuntimeHeapIsCollecting() {
  return RuntimeHeapIsCollecting(RuntimeHeapState());
}

static inline bool RuntimeHeapIsCycleCollecting() {
  return RuntimeHeapState() == HeapState::CycleCollecting;
}

/*
 * This list enumerates the different types of conceptual stacks we have in
 * SpiderMonkey. In reality, they all share the C stack, but we allow different
 * stack limits depending on the type of code running.
 */
enum StackKind {
  StackForSystemCode,       // C++, such as the GC, running on behalf of the VM.
  StackForTrustedScript,    // Script running with trusted principals.
  StackForUntrustedScript,  // Script running with untrusted principals.
  StackKindCount
};

/*
 * Default maximum size for the generational nursery in bytes. This is the
 * initial value. In the browser this configured by the
 * javascript.options.mem.nursery.max_kb pref.
 */
const uint32_t DefaultNurseryMaxBytes = 16 * js::gc::ChunkSize;

/* Default maximum heap size in bytes to pass to JS_NewContext(). */
const uint32_t DefaultHeapMaxBytes = 32 * 1024 * 1024;

/**
 * A GC pointer, tagged with the trace kind.
 *
 * In general, a GC pointer should be stored with an exact type. This class
 * is for use when that is not possible because a single pointer must point
 * to several kinds of GC thing.
 */
class JS_PUBLIC_API GCCellPtr {
 public:
  GCCellPtr() : GCCellPtr(nullptr) {}

  // Construction from a void* and trace kind.
  GCCellPtr(void* gcthing, JS::TraceKind traceKind)
      : ptr(checkedCast(gcthing, traceKind)) {}

  // Automatically construct a null GCCellPtr from nullptr.
  MOZ_IMPLICIT GCCellPtr(decltype(nullptr))
      : ptr(checkedCast(nullptr, JS::TraceKind::Null)) {}

  // Construction from an explicit type.
  template <typename T>
  explicit GCCellPtr(T* p)
      : ptr(checkedCast(p, JS::MapTypeToTraceKind<T>::kind)) {}
  explicit GCCellPtr(JSFunction* p)
      : ptr(checkedCast(p, JS::TraceKind::Object)) {}
  explicit GCCellPtr(JSScript* p)
      : ptr(checkedCast(p, JS::TraceKind::Script)) {}
  explicit GCCellPtr(const Value& v);

  JS::TraceKind kind() const {
    uintptr_t kindBits = ptr & OutOfLineTraceKindMask;
    if (kindBits != OutOfLineTraceKindMask) {
      return JS::TraceKind(kindBits);
    }
    return outOfLineKind();
  }

  // Allow GCCellPtr to be used in a boolean context.
  explicit operator bool() const {
    MOZ_ASSERT(bool(asCell()) == (kind() != JS::TraceKind::Null));
    return asCell();
  }

  // Simplify checks to the kind.
  template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
  bool is() const {
    return kind() == JS::MapTypeToTraceKind<T>::kind;
  }

  // Conversions to more specific types must match the kind. Access to
  // further refined types is not allowed directly from a GCCellPtr.
  template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
  T& as() const {
    MOZ_ASSERT(kind() == JS::MapTypeToTraceKind<T>::kind);
    // We can't use static_cast here, because the fact that JSObject
    // inherits from js::gc::Cell is not part of the public API.
    return *reinterpret_cast<T*>(asCell());
  }

  // Return a pointer to the cell this |GCCellPtr| refers to, or |nullptr|.
  // (It would be more symmetrical with |to| for this to return a |Cell&|, but
  // the result can be |nullptr|, and null references are undefined behavior.)
  js::gc::Cell* asCell() const {
    return reinterpret_cast<js::gc::Cell*>(ptr & ~OutOfLineTraceKindMask);
  }

  // The CC's trace logger needs an identity that is XPIDL serializable.
  uint64_t unsafeAsInteger() const {
    return static_cast<uint64_t>(unsafeAsUIntPtr());
  }
  // Inline mark bitmap access requires direct pointer arithmetic.
  uintptr_t unsafeAsUIntPtr() const {
    MOZ_ASSERT(asCell());
    MOZ_ASSERT(!js::gc::IsInsideNursery(asCell()));
    return reinterpret_cast<uintptr_t>(asCell());
  }

  MOZ_ALWAYS_INLINE bool mayBeOwnedByOtherRuntime() const {
    if (!is<JSString>() && !is<JS::Symbol>()) {
      return false;
    }
    if (is<JSString>()) {
      return JS::shadow::String::isPermanentAtom(asCell());
    }
    MOZ_ASSERT(is<JS::Symbol>());
    return JS::shadow::Symbol::isWellKnownSymbol(asCell());
  }

 private:
  static uintptr_t checkedCast(void* p, JS::TraceKind traceKind) {
    auto* cell = static_cast<js::gc::Cell*>(p);
    MOZ_ASSERT((uintptr_t(p) & OutOfLineTraceKindMask) == 0);
    AssertGCThingHasType(cell, traceKind);
    // Store trace in the bottom bits of pointer for common kinds.
    uintptr_t kindBits = uintptr_t(traceKind);
    if (kindBits >= OutOfLineTraceKindMask) {
      kindBits = OutOfLineTraceKindMask;
    }
    return uintptr_t(p) | kindBits;
  }

  JS::TraceKind outOfLineKind() const;

  uintptr_t ptr;
} JS_HAZ_GC_POINTER;

// Unwraps the given GCCellPtr, calls the functor |f| with a template argument
// of the actual type of the pointer, and returns the result.
template <typename F>
auto MapGCThingTyped(GCCellPtr thing, F&& f) {
  switch (thing.kind()) {
#define JS_EXPAND_DEF(name, type, _, _1) \
  case JS::TraceKind::name:              \
    return f(&thing.as<type>());
    JS_FOR_EACH_TRACEKIND(JS_EXPAND_DEF);
#undef JS_EXPAND_DEF
    default:
      MOZ_CRASH("Invalid trace kind in MapGCThingTyped for GCCellPtr.");
  }
}

// Unwraps the given GCCellPtr and calls the functor |f| with a template
// argument of the actual type of the pointer. Doesn't return anything.
template <typename F>
void ApplyGCThingTyped(GCCellPtr thing, F&& f) {
  // This function doesn't do anything but is supplied for symmetry with other
  // MapGCThingTyped/ApplyGCThingTyped implementations that have to wrap the
  // functor to return a dummy value that is ignored.
  MapGCThingTyped(thing, f);
}

} /* namespace JS */

// These are defined in the toplevel namespace instead of within JS so that
// they won't shadow other operator== overloads (see bug 1456512.)

inline bool operator==(JS::GCCellPtr ptr1, JS::GCCellPtr ptr2) {
  return ptr1.asCell() == ptr2.asCell();
}

inline bool operator!=(JS::GCCellPtr ptr1, JS::GCCellPtr ptr2) {
  return !(ptr1 == ptr2);
}

namespace js {
namespace gc {

/* static */
MOZ_ALWAYS_INLINE void MarkBitmap::getMarkWordAndMask(const TenuredCell* cell,
                                                      ColorBit colorBit,
                                                      MarkBitmapWord** wordp,
                                                      uintptr_t* maskp) {
  // Note: the JIT pre-barrier trampolines inline this code. Update
  // MacroAssembler::emitPreBarrierFastPath code too when making changes here!

  MOZ_ASSERT(size_t(colorBit) < MarkBitsPerCell);

  size_t offset = uintptr_t(cell) & ChunkMask;
  const size_t bit = offset / CellBytesPerMarkBit + size_t(colorBit);
  size_t word = bit / MarkBitmapWordBits - FirstArenaAdjustmentWords;
  MOZ_ASSERT(word < WordCount);
  *wordp = &bitmap[word];
  *maskp = uintptr_t(1) << (bit % MarkBitmapWordBits);
}

namespace detail {

static MOZ_ALWAYS_INLINE ChunkBase* GetCellChunkBase(const Cell* cell) {
  MOZ_ASSERT(cell);
  auto* chunk = reinterpret_cast<ChunkBase*>(uintptr_t(cell) & ~ChunkMask);
  MOZ_ASSERT(chunk->runtime);
  return chunk;
}

static MOZ_ALWAYS_INLINE TenuredChunkBase* GetCellChunkBase(
    const TenuredCell* cell) {
  MOZ_ASSERT(cell);
  auto* chunk =
      reinterpret_cast<TenuredChunkBase*>(uintptr_t(cell) & ~ChunkMask);
  MOZ_ASSERT(chunk->runtime);
  return chunk;
}

static MOZ_ALWAYS_INLINE JS::Zone* GetTenuredGCThingZone(const void* ptr) {
  // This takes a void* because the compiler can't see type relationships in
  // this header. |ptr| must be a pointer to a tenured GC thing.
  MOZ_ASSERT(ptr);
  const uintptr_t zone_addr = (uintptr_t(ptr) & ~ArenaMask) | ArenaZoneOffset;
  return *reinterpret_cast<JS::Zone**>(zone_addr);
}

static MOZ_ALWAYS_INLINE bool TenuredCellIsMarkedBlack(
    const TenuredCell* cell) {
  // Return true if BlackBit is set.

  MOZ_ASSERT(cell);
  MOZ_ASSERT(!js::gc::IsInsideNursery(cell));

  MarkBitmapWord* blackWord;
  uintptr_t blackMask;
  TenuredChunkBase* chunk = GetCellChunkBase(cell);
  chunk->markBits.getMarkWordAndMask(cell, js::gc::ColorBit::BlackBit,
                                     &blackWord, &blackMask);
  return *blackWord & blackMask;
}

static MOZ_ALWAYS_INLINE bool NonBlackCellIsMarkedGray(
    const TenuredCell* cell) {
  // Return true if GrayOrBlackBit is set. Callers should check BlackBit first.

  MOZ_ASSERT(cell);
  MOZ_ASSERT(!js::gc::IsInsideNursery(cell));
  MOZ_ASSERT(!TenuredCellIsMarkedBlack(cell));

  MarkBitmapWord* grayWord;
  uintptr_t grayMask;
  TenuredChunkBase* chunk = GetCellChunkBase(cell);
  chunk->markBits.getMarkWordAndMask(cell, js::gc::ColorBit::GrayOrBlackBit,
                                     &grayWord, &grayMask);
  return *grayWord & grayMask;
}

static MOZ_ALWAYS_INLINE bool TenuredCellIsMarkedGray(const TenuredCell* cell) {
  return !TenuredCellIsMarkedBlack(cell) && NonBlackCellIsMarkedGray(cell);
}

static MOZ_ALWAYS_INLINE bool CellIsMarkedGray(const Cell* cell) {
  MOZ_ASSERT(cell);
  if (js::gc::IsInsideNursery(cell)) {
    return false;
  }
  return TenuredCellIsMarkedGray(reinterpret_cast<const TenuredCell*>(cell));
}

extern JS_PUBLIC_API bool CanCheckGrayBits(const TenuredCell* cell);

extern JS_PUBLIC_API bool CellIsMarkedGrayIfKnown(const TenuredCell* cell);

#ifdef DEBUG
extern JS_PUBLIC_API void AssertCellIsNotGray(const Cell* cell);

extern JS_PUBLIC_API bool ObjectIsMarkedBlack(const JSObject* obj);
#endif

MOZ_ALWAYS_INLINE bool CellHasStoreBuffer(const Cell* cell) {
  return GetCellChunkBase(cell)->storeBuffer;
}

} /* namespace detail */

MOZ_ALWAYS_INLINE bool IsInsideNursery(const Cell* cell) {
  MOZ_ASSERT(cell);
  return detail::CellHasStoreBuffer(cell);
}

MOZ_ALWAYS_INLINE bool IsInsideNursery(const TenuredCell* cell) {
  MOZ_ASSERT(cell);
  MOZ_ASSERT(!IsInsideNursery(reinterpret_cast<const Cell*>(cell)));
  return false;
}

// Allow use before the compiler knows the derivation of JSObject, JSString, and
// JS::BigInt.
MOZ_ALWAYS_INLINE bool IsInsideNursery(const JSObject* obj) {
  return IsInsideNursery(reinterpret_cast<const Cell*>(obj));
}
MOZ_ALWAYS_INLINE bool IsInsideNursery(const JSString* str) {
  return IsInsideNursery(reinterpret_cast<const Cell*>(str));
}
MOZ_ALWAYS_INLINE bool IsInsideNursery(const JS::BigInt* bi) {
  return IsInsideNursery(reinterpret_cast<const Cell*>(bi));
}

MOZ_ALWAYS_INLINE bool IsCellPointerValid(const void* ptr) {
  auto addr = uintptr_t(ptr);
  if (addr < ChunkSize || addr % CellAlignBytes != 0) {
    return false;
  }

  auto* cell = reinterpret_cast<const Cell*>(ptr);
  if (!IsInsideNursery(cell)) {
    return detail::GetTenuredGCThingZone(cell) != nullptr;
  }

  return true;
}

MOZ_ALWAYS_INLINE bool IsCellPointerValidOrNull(const void* cell) {
  if (!cell) {
    return true;
  }
  return IsCellPointerValid(cell);
}

} /* namespace gc */
} /* namespace js */

namespace JS {

extern JS_PUBLIC_API Zone* GetTenuredGCThingZone(GCCellPtr thing);

extern JS_PUBLIC_API Zone* GetNurseryCellZone(js::gc::Cell* cell);

static MOZ_ALWAYS_INLINE Zone* GetGCThingZone(GCCellPtr thing) {
  if (!js::gc::IsInsideNursery(thing.asCell())) {
    return js::gc::detail::GetTenuredGCThingZone(thing.asCell());
  }

  return GetNurseryCellZone(thing.asCell());
}

static MOZ_ALWAYS_INLINE Zone* GetStringZone(JSString* str) {
  if (!js::gc::IsInsideNursery(str)) {
    return js::gc::detail::GetTenuredGCThingZone(str);
  }

  return GetNurseryCellZone(reinterpret_cast<js::gc::Cell*>(str));
}

extern JS_PUBLIC_API Zone* GetObjectZone(JSObject* obj);

static MOZ_ALWAYS_INLINE bool GCThingIsMarkedGray(GCCellPtr thing) {
  js::gc::Cell* cell = thing.asCell();
  if (IsInsideNursery(cell)) {
    return false;
  }

  auto* tenuredCell = reinterpret_cast<js::gc::TenuredCell*>(cell);
  return js::gc::detail::CellIsMarkedGrayIfKnown(tenuredCell);
}

// Specialised gray marking check for use by the cycle collector. This is not
// called during incremental GC or when the gray bits are invalid.
static MOZ_ALWAYS_INLINE bool GCThingIsMarkedGrayInCC(GCCellPtr thing) {
  js::gc::Cell* cell = thing.asCell();
  if (IsInsideNursery(cell)) {
    return false;
  }

  auto* tenuredCell = reinterpret_cast<js::gc::TenuredCell*>(cell);
  if (!js::gc::detail::TenuredCellIsMarkedGray(tenuredCell)) {
    return false;
  }

  MOZ_ASSERT(js::gc::detail::CanCheckGrayBits(tenuredCell));

  return true;
}

extern JS_PUBLIC_API JS::TraceKind GCThingTraceKind(void* thing);

extern JS_PUBLIC_API void EnableNurseryStrings(JSContext* cx);

extern JS_PUBLIC_API void DisableNurseryStrings(JSContext* cx);

extern JS_PUBLIC_API void EnableNurseryBigInts(JSContext* cx);

extern JS_PUBLIC_API void DisableNurseryBigInts(JSContext* cx);

/*
 * Returns true when writes to GC thing pointers (and reads from weak pointers)
 * must call an incremental barrier. This is generally only true when running
 * mutator code in-between GC slices. At other times, the barrier may be elided
 * for performance.
 */
extern JS_PUBLIC_API bool IsIncrementalBarrierNeeded(JSContext* cx);

/*
 * Notify the GC that a reference to a JSObject is about to be overwritten.
 * This method must be called if IsIncrementalBarrierNeeded.
 */
extern JS_PUBLIC_API void IncrementalPreWriteBarrier(JSObject* obj);

/*
 * Notify the GC that a reference to a tenured GC cell is about to be
 * overwritten. This method must be called if IsIncrementalBarrierNeeded.
 */
extern JS_PUBLIC_API void IncrementalPreWriteBarrier(GCCellPtr thing);

/**
 * Unsets the gray bit for anything reachable from |thing|. |kind| should not be
 * JS::TraceKind::Shape. |thing| should be non-null. The return value indicates
 * if anything was unmarked.
 */
extern JS_PUBLIC_API bool UnmarkGrayGCThingRecursively(GCCellPtr thing);

}  // namespace JS

namespace js {
namespace gc {

extern JS_PUBLIC_API void PerformIncrementalReadBarrier(JS::GCCellPtr thing);

static MOZ_ALWAYS_INLINE void ExposeGCThingToActiveJS(JS::GCCellPtr thing) {
  // TODO: I'd like to assert !RuntimeHeapIsBusy() here but this gets
  // called while we are tracing the heap, e.g. during memory reporting
  // (see bug 1313318).
  MOZ_ASSERT(!JS::RuntimeHeapIsCollecting());

  // GC things residing in the nursery cannot be gray: they have no mark bits.
  // All live objects in the nursery are moved to tenured at the beginning of
  // each GC slice, so the gray marker never sees nursery things.
  if (IsInsideNursery(thing.asCell())) {
    return;
  }

  auto* cell = reinterpret_cast<TenuredCell*>(thing.asCell());
  if (detail::TenuredCellIsMarkedBlack(cell)) {
    return;
  }

  // GC things owned by other runtimes are always black.
  MOZ_ASSERT(!thing.mayBeOwnedByOtherRuntime());

  auto* zone = JS::shadow::Zone::from(detail::GetTenuredGCThingZone(cell));
  if (zone->needsIncrementalBarrier()) {
    PerformIncrementalReadBarrier(thing);
  } else if (!zone->isGCPreparing() && detail::NonBlackCellIsMarkedGray(cell)) {
    MOZ_ALWAYS_TRUE(JS::UnmarkGrayGCThingRecursively(thing));
  }

  MOZ_ASSERT_IF(!zone->isGCPreparing(), !detail::TenuredCellIsMarkedGray(cell));
}

static MOZ_ALWAYS_INLINE void IncrementalReadBarrier(JS::GCCellPtr thing) {
  // This is a lighter version of ExposeGCThingToActiveJS that doesn't do gray
  // unmarking.

  if (IsInsideNursery(thing.asCell())) {
    return;
  }

  auto* cell = reinterpret_cast<TenuredCell*>(thing.asCell());
  auto* zone = JS::shadow::Zone::from(detail::GetTenuredGCThingZone(cell));
  if (zone->needsIncrementalBarrier() &&
      !detail::TenuredCellIsMarkedBlack(cell)) {
    // GC things owned by other runtimes are always black.
    MOZ_ASSERT(!thing.mayBeOwnedByOtherRuntime());
    PerformIncrementalReadBarrier(thing);
  }
}

template <typename T>
extern JS_PUBLIC_API bool EdgeNeedsSweepUnbarrieredSlow(T* thingp);

static MOZ_ALWAYS_INLINE bool EdgeNeedsSweepUnbarriered(JSObject** objp) {
  // This function does not handle updating nursery pointers. Raw JSObject
  // pointers should be updated separately or replaced with
  // JS::Heap<JSObject*> which handles this automatically.
  MOZ_ASSERT(!JS::RuntimeHeapIsMinorCollecting());
  if (IsInsideNursery(*objp)) {
    return false;
  }

  auto zone = JS::shadow::Zone::from(detail::GetTenuredGCThingZone(*objp));
  if (!zone->isGCSweepingOrCompacting()) {
    return false;
  }

  return EdgeNeedsSweepUnbarrieredSlow(objp);
}

}  // namespace gc
}  // namespace js

namespace JS {

/*
 * This should be called when an object that is marked gray is exposed to the JS
 * engine (by handing it to running JS code or writing it into live JS
 * data). During incremental GC, since the gray bits haven't been computed yet,
 * we conservatively mark the object black.
 */
static MOZ_ALWAYS_INLINE void ExposeObjectToActiveJS(JSObject* obj) {
  MOZ_ASSERT(obj);
  MOZ_ASSERT(!js::gc::EdgeNeedsSweepUnbarrieredSlow(&obj));
  js::gc::ExposeGCThingToActiveJS(GCCellPtr(obj));
}

} /* namespace JS */

#endif /* js_HeapAPI_h */