js/src/gc/Cell.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
   2  * vim: set ts=8 sts=2 et sw=2 tw=80:
   3  * This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef gc_Cell_h
   8 #define gc_Cell_h
   9
  10 #include "mozilla/Atomics.h"
  11 #include "mozilla/EndianUtils.h"
  12
  13 #include <type_traits>
  14
  15 #include "gc/GCContext.h"
  16 #include "gc/Heap.h"
  17 #include "gc/TraceKind.h"
  18 #include "js/GCAnnotations.h"
  19 #include "js/shadow/Zone.h"  // JS::shadow::Zone
  20 #include "js/TypeDecls.h"
  21
  22 namespace JS {
  23 enum class TraceKind;
  24 } /* namespace JS */
  25
  26 namespace js {
  27
  28 class JS_PUBLIC_API GenericPrinter;
  29
  30 extern bool RuntimeFromMainThreadIsHeapMajorCollecting(
  31     JS::shadow::Zone* shadowZone);
  32
  33 #ifdef DEBUG
  34 // Barriers can't be triggered during backend Ion compilation, which may run on
  35 // a helper thread.
  36 extern bool CurrentThreadIsIonCompiling();
  37 #endif
  38
  39 extern void TraceManuallyBarrieredGenericPointerEdge(JSTracer* trc,
  40                                                      gc::Cell** thingp,
  41                                                      const char* name);
  42
  43 namespace gc {
  44
  45 enum class AllocKind : uint8_t;
  46 class CellAllocator;  // Declared so subtypes of Cell can friend it easily.
  47 class StoreBuffer;
  48 class TenuredCell;
  49
  50 extern void PerformIncrementalReadBarrier(TenuredCell* cell);
  51 extern void PerformIncrementalPreWriteBarrier(TenuredCell* cell);
  52 extern void PerformIncrementalBarrierDuringFlattening(JSString* str);
  53 extern void UnmarkGrayGCThingRecursively(TenuredCell* cell);
  54
  55 // Like gc::MarkColor but allows the possibility of the cell being unmarked.
  56 enum class CellColor : uint8_t { White = 0, Gray = 1, Black = 2 };
  57 static_assert(uint8_t(CellColor::Gray) == uint8_t(MarkColor::Gray));
  58 static_assert(uint8_t(CellColor::Black) == uint8_t(MarkColor::Black));
  59
  60 inline bool IsMarked(CellColor color) { return color != CellColor::White; }
  61 inline MarkColor AsMarkColor(CellColor color) {
  62   MOZ_ASSERT(IsMarked(color));
  63   return MarkColor(color);
  64 }
  65 inline CellColor AsCellColor(MarkColor color) { return CellColor(color); }
  66 extern const char* CellColorName(CellColor color);
  67
  68 // Cell header word. Stores GC flags and derived class data.
  69 //
  70 // Loads of GC flags + all stores are marked as (relaxed) atomic operations,
  71 // to deal with the following benign data race during compacting GC:
  72 //
  73 // - Thread 1 checks isForwarded (which is always false in this situation).
  74 // - Thread 2 updates the derived class data (without changing the forwarded
  75 //   flag).
  76 //
  77 // To improve performance, we don't use atomic operations for get() because
  78 // atomic operations inhibit certain compiler optimizations: GCC and Clang are
  79 // unable to fold multiple loads even if they're both relaxed atomics. This is
  80 // especially a problem for chained loads such as obj->shape->base->clasp.
  81 class HeaderWord {
  82   // Indicates whether the cell has been forwarded (moved) by generational or
  83   // compacting GC and is now a RelocationOverlay.
  84   static constexpr uintptr_t FORWARD_BIT = Bit(0);
  85   // Bits 1 and 2 are reserved for future use by the GC.
  86
  87   uintptr_t value_;
  88
  89   void setAtomic(uintptr_t value) {
  90     __atomic_store_n(&value_, value, __ATOMIC_RELAXED);
  91   }
  92
  93  public:
  94   static constexpr uintptr_t RESERVED_MASK =
  95       BitMask(gc::CellFlagBitsReservedForGC);
  96   static_assert(gc::CellFlagBitsReservedForGC >= 3,
  97                 "Not enough flag bits reserved for GC");
  98
  99   uintptr_t getAtomic() const {
 100     return __atomic_load_n(&value_, __ATOMIC_RELAXED);
 101   }
 102
 103   // Accessors for derived class data.
 104   uintptr_t get() const {
 105     // Note: non-atomic load. See class comment.
 106     uintptr_t value = value_;
 107     MOZ_ASSERT((value & RESERVED_MASK) == 0);
 108     return value;
 109   }
 110   void set(uintptr_t value) {
 111     MOZ_ASSERT((value & RESERVED_MASK) == 0);
 112     setAtomic(value);
 113   }
 114
 115   // Accessors for GC data.
 116   uintptr_t flags() const { return getAtomic() & RESERVED_MASK; }
 117   bool isForwarded() const { return flags() & FORWARD_BIT; }
 118   void setForwardingAddress(uintptr_t ptr) {
 119     MOZ_ASSERT((ptr & RESERVED_MASK) == 0);
 120     setAtomic(ptr | FORWARD_BIT);
 121   }
 122   uintptr_t getForwardingAddress() const {
 123     MOZ_ASSERT(isForwarded());
 124     return getAtomic() & ~RESERVED_MASK;
 125   }
 126 };
 127
 128 // [SMDOC] GC Cell
 129 //
 130 // A GC cell is the ultimate base class for all GC things. All types allocated
 131 // on the GC heap extend either gc::Cell or gc::TenuredCell. If a type is always
 132 // tenured, prefer the TenuredCell class as base.
 133 //
 134 // The first word of Cell is a HeaderWord (a uintptr_t) that reserves the low
 135 // three bits for GC purposes. The remaining bits are available to sub-classes
 136 // and can be used store a pointer to another gc::Cell. To make use of the
 137 // remaining space, sub-classes derive from a helper class such as
 138 // TenuredCellWithNonGCPointer.
 139 //
 140 // During moving GC operation a Cell may be marked as forwarded. This indicates
 141 // that a gc::RelocationOverlay is currently stored in the Cell's memory and
 142 // should be used to find the new location of the Cell.
 143 struct Cell {
 144   // Cell header word. Stores GC flags and derived class data.
 145   HeaderWord header_;
 146
 147  public:
 148   Cell() = default;
 149
 150   Cell(const Cell&) = delete;
 151   void operator=(const Cell&) = delete;
 152
 153   bool isForwarded() const { return header_.isForwarded(); }
 154   uintptr_t flags() const { return header_.flags(); }
 155
 156   MOZ_ALWAYS_INLINE bool isTenured() const { return !IsInsideNursery(this); }
 157   MOZ_ALWAYS_INLINE const TenuredCell& asTenured() const;
 158   MOZ_ALWAYS_INLINE TenuredCell& asTenured();
 159
 160   MOZ_ALWAYS_INLINE bool isMarkedAny() const;
 161   MOZ_ALWAYS_INLINE bool isMarkedBlack() const;
 162   MOZ_ALWAYS_INLINE bool isMarkedGray() const;
 163   MOZ_ALWAYS_INLINE bool isMarked(gc::MarkColor color) const;
 164   MOZ_ALWAYS_INLINE bool isMarkedAtLeast(gc::MarkColor color) const;
 165   MOZ_ALWAYS_INLINE CellColor color() const;
 166
 167   inline JSRuntime* runtimeFromMainThread() const;
 168
 169   // Note: Unrestricted access to the runtime of a GC thing from an arbitrary
 170   // thread can easily lead to races. Use this method very carefully.
 171   inline JSRuntime* runtimeFromAnyThread() const;
 172
 173   // May be overridden by GC thing kinds that have a compartment pointer.
 174   inline JS::Compartment* maybeCompartment() const { return nullptr; }
 175
 176   // The StoreBuffer used to record incoming pointers from the tenured heap.
 177   // This will return nullptr for a tenured cell.
 178   inline StoreBuffer* storeBuffer() const;
 179
 180   inline JS::TraceKind getTraceKind() const;
 181
 182   static MOZ_ALWAYS_INLINE bool needPreWriteBarrier(JS::Zone* zone);
 183
 184   template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
 185   inline bool is() const {
 186     return getTraceKind() == JS::MapTypeToTraceKind<T>::kind;
 187   }
 188
 189   template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
 190   inline T* as() {
 191     // |this|-qualify the |is| call below to avoid compile errors with even
 192     // fairly recent versions of gcc, e.g. 7.1.1 according to bz.
 193     MOZ_ASSERT(this->is<T>());
 194     return static_cast<T*>(this);
 195   }
 196
 197   template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
 198   inline const T* as() const {
 199     // |this|-qualify the |is| call below to avoid compile errors with even
 200     // fairly recent versions of gcc, e.g. 7.1.1 according to bz.
 201     MOZ_ASSERT(this->is<T>());
 202     return static_cast<const T*>(this);
 203   }
 204
 205   inline JS::Zone* zone() const;
 206   inline JS::Zone* zoneFromAnyThread() const;
 207
 208   // Get the zone for a cell known to be in the nursery.
 209   inline JS::Zone* nurseryZone() const;
 210   inline JS::Zone* nurseryZoneFromAnyThread() const;
 211
 212   // Default implementation for kinds that cannot be permanent. This may be
 213   // overriden by derived classes.
 214   MOZ_ALWAYS_INLINE bool isPermanentAndMayBeShared() const { return false; }
 215
 216 #ifdef DEBUG
 217   static inline void assertThingIsNotGray(Cell* cell);
 218   inline bool isAligned() const;
 219   void dump(GenericPrinter& out) const;
 220   void dump() const;
 221 #endif
 222
 223  protected:
 224   uintptr_t address() const;
 225   inline ChunkBase* chunk() const;
 226
 227  private:
 228   // Cells are destroyed by the GC. Do not delete them directly.
 229   void operator delete(void*) = delete;
 230 } JS_HAZ_GC_THING;
 231
 232 // A GC TenuredCell gets behaviors that are valid for things in the Tenured
 233 // heap, such as access to the arena and mark bits.
 234 class TenuredCell : public Cell {
 235  public:
 236   MOZ_ALWAYS_INLINE bool isTenured() const {
 237     MOZ_ASSERT(!IsInsideNursery(this));
 238     return true;
 239   }
 240
 241   TenuredChunk* chunk() const {
 242     return static_cast<TenuredChunk*>(Cell::chunk());
 243   }
 244
 245   // Mark bit management.
 246   MOZ_ALWAYS_INLINE bool isMarkedAny() const;
 247   MOZ_ALWAYS_INLINE bool isMarkedBlack() const;
 248   MOZ_ALWAYS_INLINE bool isMarkedGray() const;
 249   MOZ_ALWAYS_INLINE CellColor color() const;
 250
 251   // The return value indicates if the cell went from unmarked to marked.
 252   MOZ_ALWAYS_INLINE bool markIfUnmarked(
 253       MarkColor color = MarkColor::Black) const;
 254   MOZ_ALWAYS_INLINE bool markIfUnmarkedAtomic(MarkColor color) const;
 255   MOZ_ALWAYS_INLINE void markBlack() const;
 256   MOZ_ALWAYS_INLINE void markBlackAtomic() const;
 257   MOZ_ALWAYS_INLINE void copyMarkBitsFrom(const TenuredCell* src);
 258   MOZ_ALWAYS_INLINE void unmark();
 259
 260   // Access to the arena.
 261   inline Arena* arena() const;
 262   inline AllocKind getAllocKind() const;
 263   inline JS::TraceKind getTraceKind() const;
 264   inline JS::Zone* zone() const;
 265   inline JS::Zone* zoneFromAnyThread() const;
 266   inline bool isInsideZone(JS::Zone* zone) const;
 267
 268   MOZ_ALWAYS_INLINE JS::shadow::Zone* shadowZone() const {
 269     return JS::shadow::Zone::from(zone());
 270   }
 271   MOZ_ALWAYS_INLINE JS::shadow::Zone* shadowZoneFromAnyThread() const {
 272     return JS::shadow::Zone::from(zoneFromAnyThread());
 273   }
 274
 275   template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
 276   inline bool is() const {
 277     return getTraceKind() == JS::MapTypeToTraceKind<T>::kind;
 278   }
 279
 280   template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
 281   inline T* as() {
 282     // |this|-qualify the |is| call below to avoid compile errors with even
 283     // fairly recent versions of gcc, e.g. 7.1.1 according to bz.
 284     MOZ_ASSERT(this->is<T>());
 285     return static_cast<T*>(this);
 286   }
 287
 288   template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
 289   inline const T* as() const {
 290     // |this|-qualify the |is| call below to avoid compile errors with even
 291     // fairly recent versions of gcc, e.g. 7.1.1 according to bz.
 292     MOZ_ASSERT(this->is<T>());
 293     return static_cast<const T*>(this);
 294   }
 295
 296   // Default implementation for kinds that don't require fixup.
 297   void fixupAfterMovingGC() {}
 298
 299   static inline CellColor getColor(MarkBitmap* bitmap, const TenuredCell* cell);
 300
 301 #ifdef DEBUG
 302   inline bool isAligned() const;
 303 #endif
 304 };
 305
 306 MOZ_ALWAYS_INLINE const TenuredCell& Cell::asTenured() const {
 307   MOZ_ASSERT(isTenured());
 308   return *static_cast<const TenuredCell*>(this);
 309 }
 310
 311 MOZ_ALWAYS_INLINE TenuredCell& Cell::asTenured() {
 312   MOZ_ASSERT(isTenured());
 313   return *static_cast<TenuredCell*>(this);
 314 }
 315
 316 MOZ_ALWAYS_INLINE bool Cell::isMarkedAny() const {
 317   return !isTenured() || asTenured().isMarkedAny();
 318 }
 319
 320 MOZ_ALWAYS_INLINE bool Cell::isMarkedBlack() const {
 321   return !isTenured() || asTenured().isMarkedBlack();
 322 }
 323
 324 MOZ_ALWAYS_INLINE bool Cell::isMarkedGray() const {
 325   return isTenured() && asTenured().isMarkedGray();
 326 }
 327
 328 MOZ_ALWAYS_INLINE bool Cell::isMarked(gc::MarkColor color) const {
 329   return color == MarkColor::Gray ? isMarkedGray() : isMarkedBlack();
 330 }
 331
 332 MOZ_ALWAYS_INLINE bool Cell::isMarkedAtLeast(gc::MarkColor color) const {
 333   return color == MarkColor::Gray ? isMarkedAny() : isMarkedBlack();
 334 }
 335
 336 MOZ_ALWAYS_INLINE CellColor Cell::color() const {
 337   return isTenured() ? asTenured().color() : CellColor::Black;
 338 }
 339
 340 inline JSRuntime* Cell::runtimeFromMainThread() const {
 341   JSRuntime* rt = chunk()->runtime;
 342   MOZ_ASSERT(CurrentThreadCanAccessRuntime(rt));
 343   return rt;
 344 }
 345
 346 inline JSRuntime* Cell::runtimeFromAnyThread() const {
 347   return chunk()->runtime;
 348 }
 349
 350 inline uintptr_t Cell::address() const {
 351   uintptr_t addr = uintptr_t(this);
 352   MOZ_ASSERT(addr % CellAlignBytes == 0);
 353   MOZ_ASSERT(TenuredChunk::withinValidRange(addr));
 354   return addr;
 355 }
 356
 357 ChunkBase* Cell::chunk() const {
 358   uintptr_t addr = uintptr_t(this);
 359   MOZ_ASSERT(addr % CellAlignBytes == 0);
 360   addr &= ~ChunkMask;
 361   return reinterpret_cast<ChunkBase*>(addr);
 362 }
 363
 364 inline StoreBuffer* Cell::storeBuffer() const { return chunk()->storeBuffer; }
 365
 366 JS::Zone* Cell::zone() const {
 367   if (isTenured()) {
 368     return asTenured().zone();
 369   }
 370
 371   return nurseryZone();
 372 }
 373
 374 JS::Zone* Cell::zoneFromAnyThread() const {
 375   if (isTenured()) {
 376     return asTenured().zoneFromAnyThread();
 377   }
 378
 379   return nurseryZoneFromAnyThread();
 380 }
 381
 382 JS::Zone* Cell::nurseryZone() const {
 383   JS::Zone* zone = nurseryZoneFromAnyThread();
 384   MOZ_ASSERT(CurrentThreadIsGCMarking() || CurrentThreadCanAccessZone(zone));
 385   return zone;
 386 }
 387
 388 JS::Zone* Cell::nurseryZoneFromAnyThread() const {
 389   return NurseryCellHeader::from(this)->zone();
 390 }
 391
 392 #ifdef DEBUG
 393 extern Cell* UninlinedForwarded(const Cell* cell);
 394 #endif
 395
 396 inline JS::TraceKind Cell::getTraceKind() const {
 397   if (isTenured()) {
 398     MOZ_ASSERT_IF(isForwarded(), UninlinedForwarded(this)->getTraceKind() ==
 399                                      asTenured().getTraceKind());
 400     return asTenured().getTraceKind();
 401   }
 402
 403   return NurseryCellHeader::from(this)->traceKind();
 404 }
 405
 406 /* static */ MOZ_ALWAYS_INLINE bool Cell::needPreWriteBarrier(JS::Zone* zone) {
 407   return JS::shadow::Zone::from(zone)->needsIncrementalBarrier();
 408 }
 409
 410 MOZ_ALWAYS_INLINE bool TenuredCell::isMarkedAny() const {
 411   MOZ_ASSERT(arena()->allocated());
 412   return chunk()->markBits.isMarkedAny(this);
 413 }
 414
 415 MOZ_ALWAYS_INLINE bool TenuredCell::isMarkedBlack() const {
 416   MOZ_ASSERT(arena()->allocated());
 417   return chunk()->markBits.isMarkedBlack(this);
 418 }
 419
 420 MOZ_ALWAYS_INLINE bool TenuredCell::isMarkedGray() const {
 421   MOZ_ASSERT(arena()->allocated());
 422   return chunk()->markBits.isMarkedGray(this);
 423 }
 424
 425 MOZ_ALWAYS_INLINE CellColor TenuredCell::color() const {
 426   return getColor(&chunk()->markBits, this);
 427 }
 428
 429 /* static */
 430 inline CellColor TenuredCell::getColor(MarkBitmap* bitmap,
 431                                        const TenuredCell* cell) {
 432   // Note that this method isn't synchronised so may give surprising results if
 433   // the mark bitmap is being modified concurrently.
 434
 435   if (bitmap->isMarkedBlack(cell)) {
 436     return CellColor::Black;
 437   }
 438
 439   if (bitmap->isMarkedGray(cell)) {
 440     return CellColor::Gray;
 441   }
 442
 443   return CellColor::White;
 444 }
 445
 446 bool TenuredCell::markIfUnmarked(MarkColor color /* = Black */) const {
 447   return chunk()->markBits.markIfUnmarked(this, color);
 448 }
 449
 450 bool TenuredCell::markIfUnmarkedAtomic(MarkColor color) const {
 451   return chunk()->markBits.markIfUnmarkedAtomic(this, color);
 452 }
 453
 454 void TenuredCell::markBlack() const { chunk()->markBits.markBlack(this); }
 455 void TenuredCell::markBlackAtomic() const {
 456   chunk()->markBits.markBlackAtomic(this);
 457 }
 458
 459 void TenuredCell::copyMarkBitsFrom(const TenuredCell* src) {
 460   MarkBitmap& markBits = chunk()->markBits;
 461   markBits.copyMarkBit(this, src, ColorBit::BlackBit);
 462   markBits.copyMarkBit(this, src, ColorBit::GrayOrBlackBit);
 463 }
 464
 465 void TenuredCell::unmark() { chunk()->markBits.unmark(this); }
 466
 467 inline Arena* TenuredCell::arena() const {
 468   MOZ_ASSERT(isTenured());
 469   uintptr_t addr = address();
 470   addr &= ~ArenaMask;
 471   return reinterpret_cast<Arena*>(addr);
 472 }
 473
 474 AllocKind TenuredCell::getAllocKind() const { return arena()->getAllocKind(); }
 475
 476 JS::TraceKind TenuredCell::getTraceKind() const {
 477   return MapAllocToTraceKind(getAllocKind());
 478 }
 479
 480 JS::Zone* TenuredCell::zone() const {
 481   JS::Zone* zone = arena()->zone;
 482   MOZ_ASSERT(CurrentThreadIsGCMarking() || CurrentThreadCanAccessZone(zone));
 483   return zone;
 484 }
 485
 486 JS::Zone* TenuredCell::zoneFromAnyThread() const { return arena()->zone; }
 487
 488 bool TenuredCell::isInsideZone(JS::Zone* zone) const {
 489   return zone == arena()->zone;
 490 }
 491
 492 // Read barrier and pre-write barrier implementation for GC cells.
 493
 494 template <typename T>
 495 MOZ_ALWAYS_INLINE void ReadBarrier(T* thing) {
 496   static_assert(std::is_base_of_v<Cell, T>);
 497   static_assert(!std::is_same_v<Cell, T> && !std::is_same_v<TenuredCell, T>);
 498
 499   if (thing) {
 500     ReadBarrierImpl(thing);
 501   }
 502 }
 503
 504 MOZ_ALWAYS_INLINE void ReadBarrierImpl(TenuredCell* thing) {
 505   MOZ_ASSERT(CurrentThreadIsMainThread());
 506   MOZ_ASSERT(!JS::RuntimeHeapIsCollecting());
 507   MOZ_ASSERT(thing);
 508
 509   JS::shadow::Zone* shadowZone = thing->shadowZoneFromAnyThread();
 510   if (shadowZone->needsIncrementalBarrier()) {
 511     PerformIncrementalReadBarrier(thing);
 512     return;
 513   }
 514
 515   if (thing->isMarkedGray()) {
 516     UnmarkGrayGCThingRecursively(thing);
 517   }
 518 }
 519
 520 MOZ_ALWAYS_INLINE void ReadBarrierImpl(Cell* thing) {
 521   MOZ_ASSERT(!CurrentThreadIsGCMarking());
 522   MOZ_ASSERT(thing);
 523
 524   if (thing->isTenured()) {
 525     ReadBarrierImpl(&thing->asTenured());
 526   }
 527 }
 528
 529 MOZ_ALWAYS_INLINE void PreWriteBarrierImpl(TenuredCell* thing) {
 530   MOZ_ASSERT(CurrentThreadIsMainThread() || CurrentThreadIsGCSweeping() ||
 531              CurrentThreadIsGCFinalizing());
 532   MOZ_ASSERT(thing);
 533
 534   // Barriers can be triggered on the main thread while collecting, but are
 535   // disabled. For example, this happens when sweeping HeapPtr wrappers. See
 536   // AutoDisableBarriers.
 537
 538   JS::shadow::Zone* zone = thing->shadowZoneFromAnyThread();
 539   if (zone->needsIncrementalBarrier()) {
 540     PerformIncrementalPreWriteBarrier(thing);
 541   }
 542 }
 543
 544 MOZ_ALWAYS_INLINE void PreWriteBarrierImpl(Cell* thing) {
 545   MOZ_ASSERT(!CurrentThreadIsGCMarking());
 546   MOZ_ASSERT(thing);
 547
 548   if (thing->isTenured()) {
 549     PreWriteBarrierImpl(&thing->asTenured());
 550   }
 551 }
 552
 553 template <typename T>
 554 MOZ_ALWAYS_INLINE void PreWriteBarrier(T* thing) {
 555   static_assert(std::is_base_of_v<Cell, T>);
 556   static_assert(!std::is_same_v<Cell, T> && !std::is_same_v<TenuredCell, T>);
 557
 558   if (thing) {
 559     PreWriteBarrierImpl(thing);
 560   }
 561 }
 562
 563 // Pre-write barrier implementation for structures containing GC cells, taking a
 564 // functor to trace the structure.
 565 template <typename T, typename F>
 566 MOZ_ALWAYS_INLINE void PreWriteBarrier(JS::Zone* zone, T* data,
 567                                        const F& traceFn) {
 568   MOZ_ASSERT(data);
 569   MOZ_ASSERT(!CurrentThreadIsIonCompiling());
 570   MOZ_ASSERT(!CurrentThreadIsGCMarking());
 571
 572   auto* shadowZone = JS::shadow::Zone::from(zone);
 573   if (!shadowZone->needsIncrementalBarrier()) {
 574     return;
 575   }
 576
 577   MOZ_ASSERT(CurrentThreadCanAccessRuntime(shadowZone->runtimeFromAnyThread()));
 578   MOZ_ASSERT(!RuntimeFromMainThreadIsHeapMajorCollecting(shadowZone));
 579
 580   traceFn(shadowZone->barrierTracer(), data);
 581 }
 582
 583 // Pre-write barrier implementation for structures containing GC cells. T must
 584 // support a |trace| method.
 585 template <typename T>
 586 MOZ_ALWAYS_INLINE void PreWriteBarrier(JS::Zone* zone, T* data) {
 587   MOZ_ASSERT(data);
 588   PreWriteBarrier(zone, data, [](JSTracer* trc, T* data) { data->trace(trc); });
 589 }
 590
 591 #ifdef DEBUG
 592
 593 /* static */ void Cell::assertThingIsNotGray(Cell* cell) {
 594   JS::AssertCellIsNotGray(cell);
 595 }
 596
 597 bool Cell::isAligned() const {
 598   if (!isTenured()) {
 599     return true;
 600   }
 601   return asTenured().isAligned();
 602 }
 603
 604 bool TenuredCell::isAligned() const {
 605   return Arena::isAligned(address(), arena()->getThingSize());
 606 }
 607
 608 #endif
 609
 610 // Base class for nusery-allocatable GC things that have 32-bit length and
 611 // 32-bit flags (currently JSString and BigInt).
 612 //
 613 // This tries to store both in Cell::header_, but if that isn't large enough the
 614 // length is stored separately.
 615 //
 616 //          32       0
 617 //  ------------------
 618 //  | Length | Flags |
 619 //  ------------------
 620 //
 621 // The low bits of the flags word (see CellFlagBitsReservedForGC) are reserved
 622 // for GC. Derived classes must ensure they don't use these flags for non-GC
 623 // purposes.
 624 class alignas(gc::CellAlignBytes) CellWithLengthAndFlags : public Cell {
 625 #if JS_BITS_PER_WORD == 32
 626   // Additional storage for length if |header_| is too small to fit both.
 627   uint32_t length_;
 628 #endif
 629
 630  protected:
 631   uint32_t headerLengthField() const {
 632 #if JS_BITS_PER_WORD == 32
 633     return length_;
 634 #else
 635     return uint32_t(header_.get() >> 32);
 636 #endif
 637   }
 638
 639   uint32_t headerFlagsField() const { return uint32_t(header_.get()); }
 640
 641   void setHeaderFlagBit(uint32_t flag) {
 642     header_.set(header_.get() | uintptr_t(flag));
 643   }
 644   void clearHeaderFlagBit(uint32_t flag) {
 645     header_.set(header_.get() & ~uintptr_t(flag));
 646   }
 647   void toggleHeaderFlagBit(uint32_t flag) {
 648     header_.set(header_.get() ^ uintptr_t(flag));
 649   }
 650
 651   void setHeaderLengthAndFlags(uint32_t len, uint32_t flags) {
 652 #if JS_BITS_PER_WORD == 32
 653     header_.set(flags);
 654     length_ = len;
 655 #else
 656     header_.set((uint64_t(len) << 32) | uint64_t(flags));
 657 #endif
 658   }
 659
 660  public:
 661   // Returns the offset of header_. JIT code should use offsetOfFlags
 662   // below.
 663   static constexpr size_t offsetOfRawHeaderFlagsField() {
 664     return offsetof(CellWithLengthAndFlags, header_);
 665   }
 666
 667   // Offsets for direct field from jit code. A number of places directly
 668   // access 32-bit length and flags fields so do endian trickery here.
 669 #if JS_BITS_PER_WORD == 32
 670   static constexpr size_t offsetOfHeaderFlags() {
 671     return offsetof(CellWithLengthAndFlags, header_);
 672   }
 673   static constexpr size_t offsetOfHeaderLength() {
 674     return offsetof(CellWithLengthAndFlags, length_);
 675   }
 676 #elif MOZ_LITTLE_ENDIAN()
 677   static constexpr size_t offsetOfHeaderFlags() {
 678     return offsetof(CellWithLengthAndFlags, header_);
 679   }
 680   static constexpr size_t offsetOfHeaderLength() {
 681     return offsetof(CellWithLengthAndFlags, header_) + sizeof(uint32_t);
 682   }
 683 #else
 684   static constexpr size_t offsetOfHeaderFlags() {
 685     return offsetof(CellWithLengthAndFlags, header_) + sizeof(uint32_t);
 686   }
 687   static constexpr size_t offsetOfHeaderLength() {
 688     return offsetof(CellWithLengthAndFlags, header_);
 689   }
 690 #endif
 691 };
 692
 693 // Base class for non-nursery-allocatable GC things that allows storing a non-GC
 694 // thing pointer in the first word.
 695 //
 696 // The low bits of the word (see CellFlagBitsReservedForGC) are reserved for GC.
 697 template <class PtrT>
 698 class alignas(gc::CellAlignBytes) TenuredCellWithNonGCPointer
 699     : public TenuredCell {
 700   static_assert(!std::is_pointer_v<PtrT>,
 701                 "PtrT should be the type of the referent, not of the pointer");
 702   static_assert(
 703       !std::is_base_of_v<Cell, PtrT>,
 704       "Don't use TenuredCellWithNonGCPointer for pointers to GC things");
 705
 706  protected:
 707   TenuredCellWithNonGCPointer() = default;
 708   explicit TenuredCellWithNonGCPointer(PtrT* initial) {
 709     uintptr_t data = uintptr_t(initial);
 710     header_.set(data);
 711   }
 712
 713   PtrT* headerPtr() const {
 714     MOZ_ASSERT(flags() == 0);
 715     return reinterpret_cast<PtrT*>(uintptr_t(header_.get()));
 716   }
 717
 718   void setHeaderPtr(PtrT* newValue) {
 719     // As above, no flags are expected to be set here.
 720     uintptr_t data = uintptr_t(newValue);
 721     MOZ_ASSERT(flags() == 0);
 722     header_.set(data);
 723   }
 724
 725  public:
 726   static constexpr size_t offsetOfHeaderPtr() {
 727     return offsetof(TenuredCellWithNonGCPointer, header_);
 728   }
 729 };
 730
 731 // Base class for non-nursery-allocatable GC things that allows storing flags
 732 // in the first word.
 733 //
 734 // The low bits of the flags word (see CellFlagBitsReservedForGC) are reserved
 735 // for GC.
 736 class alignas(gc::CellAlignBytes) TenuredCellWithFlags : public TenuredCell {
 737  protected:
 738   TenuredCellWithFlags() { header_.set(0); }
 739   explicit TenuredCellWithFlags(uintptr_t initial) { header_.set(initial); }
 740
 741   uintptr_t headerFlagsField() const {
 742     MOZ_ASSERT(flags() == 0);
 743     return header_.get();
 744   }
 745
 746   void setHeaderFlagBits(uintptr_t flags) {
 747     header_.set(header_.get() | flags);
 748   }
 749   void clearHeaderFlagBits(uintptr_t flags) {
 750     header_.set(header_.get() & ~flags);
 751   }
 752 };
 753
 754 // Base class for GC things that have a tenured GC pointer as their first word.
 755 //
 756 // The low bits of the first word (see CellFlagBitsReservedForGC) are reserved
 757 // for GC.
 758 //
 759 // This includes a pre write barrier when the pointer is update. No post barrier
 760 // is necessary as the pointer is always tenured.
 761 template <class BaseCell, class PtrT>
 762 class alignas(gc::CellAlignBytes) CellWithTenuredGCPointer : public BaseCell {
 763   static void staticAsserts() {
 764     // These static asserts are not in class scope because the PtrT may not be
 765     // defined when this class template is instantiated.
 766     static_assert(
 767         std::is_same_v<BaseCell, Cell> || std::is_same_v<BaseCell, TenuredCell>,
 768         "BaseCell must be either Cell or TenuredCell");
 769     static_assert(
 770         !std::is_pointer_v<PtrT>,
 771         "PtrT should be the type of the referent, not of the pointer");
 772     static_assert(
 773         std::is_base_of_v<Cell, PtrT>,
 774         "Only use CellWithTenuredGCPointer for pointers to GC things");
 775   }
 776
 777  protected:
 778   CellWithTenuredGCPointer() = default;
 779   explicit CellWithTenuredGCPointer(PtrT* initial) { initHeaderPtr(initial); }
 780
 781   void initHeaderPtr(PtrT* initial) {
 782     MOZ_ASSERT_IF(initial, !IsInsideNursery(initial));
 783     uintptr_t data = uintptr_t(initial);
 784     this->header_.set(data);
 785   }
 786
 787   void setHeaderPtr(PtrT* newValue) {
 788     // As above, no flags are expected to be set here.
 789     MOZ_ASSERT_IF(newValue, !IsInsideNursery(newValue));
 790     PreWriteBarrier(headerPtr());
 791     unbarrieredSetHeaderPtr(newValue);
 792   }
 793
 794  public:
 795   PtrT* headerPtr() const {
 796     staticAsserts();
 797     MOZ_ASSERT(this->flags() == 0);
 798     return reinterpret_cast<PtrT*>(uintptr_t(this->header_.get()));
 799   }
 800   PtrT* headerPtrAtomic() const {
 801     staticAsserts();
 802     MOZ_ASSERT(this->flags() == 0);
 803     return reinterpret_cast<PtrT*>(uintptr_t(this->header_.getAtomic()));
 804   }
 805
 806   void unbarrieredSetHeaderPtr(PtrT* newValue) {
 807     uintptr_t data = uintptr_t(newValue);
 808     MOZ_ASSERT(this->flags() == 0);
 809     this->header_.set(data);
 810   }
 811
 812   static constexpr size_t offsetOfHeaderPtr() {
 813     return offsetof(CellWithTenuredGCPointer, header_);
 814   }
 815 };
 816
 817 void CellHeaderPostWriteBarrier(JSObject** ptr, JSObject* prev, JSObject* next);
 818
 819 template <typename T>
 820 constexpr inline bool GCTypeIsTenured() {
 821   static_assert(std::is_base_of_v<Cell, T>);
 822   static_assert(!std::is_same_v<Cell, T> && !std::is_same_v<TenuredCell, T>);
 823
 824   return std::is_base_of_v<TenuredCell, T> || std::is_base_of_v<JSAtom, T>;
 825 }
 826
 827 template <class PtrT>
 828 class alignas(gc::CellAlignBytes) TenuredCellWithGCPointer
 829     : public TenuredCell {
 830   static void staticAsserts() {
 831     // These static asserts are not in class scope because the PtrT may not be
 832     // defined when this class template is instantiated.
 833     static_assert(
 834         !std::is_pointer_v<PtrT>,
 835         "PtrT should be the type of the referent, not of the pointer");
 836     static_assert(
 837         std::is_base_of_v<Cell, PtrT>,
 838         "Only use TenuredCellWithGCPointer for pointers to GC things");
 839     static_assert(
 840         !GCTypeIsTenured<PtrT>,
 841         "Don't use TenuredCellWithGCPointer for always-tenured GC things");
 842   }
 843
 844  protected:
 845   TenuredCellWithGCPointer() = default;
 846   explicit TenuredCellWithGCPointer(PtrT* initial) { initHeaderPtr(initial); }
 847
 848   void initHeaderPtr(PtrT* initial) {
 849     uintptr_t data = uintptr_t(initial);
 850     this->header_.set(data);
 851     if (initial && IsInsideNursery(initial)) {
 852       CellHeaderPostWriteBarrier(headerPtrAddress(), nullptr, initial);
 853     }
 854   }
 855
 856   PtrT** headerPtrAddress() {
 857     MOZ_ASSERT(this->flags() == 0);
 858     return reinterpret_cast<PtrT**>(&this->header_);
 859   }
 860
 861  public:
 862   PtrT* headerPtr() const {
 863     MOZ_ASSERT(this->flags() == 0);
 864     return reinterpret_cast<PtrT*>(uintptr_t(this->header_.get()));
 865   }
 866
 867   void unbarrieredSetHeaderPtr(PtrT* newValue) {
 868     uintptr_t data = uintptr_t(newValue);
 869     MOZ_ASSERT(this->flags() == 0);
 870     this->header_.set(data);
 871   }
 872
 873   static constexpr size_t offsetOfHeaderPtr() {
 874     return offsetof(TenuredCellWithGCPointer, header_);
 875   }
 876 };
 877
 878 // Check whether a typed GC thing is marked at all. Doesn't check gray bits for
 879 // kinds that can't be marked gray.
 880 template <typename T>
 881 static inline bool TenuredThingIsMarkedAny(T* thing) {
 882   using BaseT = typename BaseGCType<T>::type;
 883   TenuredCell* cell = &thing->asTenured();
 884   if constexpr (TraceKindCanBeGray<BaseT>::value) {
 885     return cell->isMarkedAny();
 886   } else {
 887     MOZ_ASSERT(!cell->isMarkedGray());
 888     return cell->isMarkedBlack();
 889   }
 890 }
 891
 892 template <>
 893 inline bool TenuredThingIsMarkedAny<Cell>(Cell* thing) {
 894   return thing->asTenured().isMarkedAny();
 895 }
 896
 897 } /* namespace gc */
 898 } /* namespace js */
 899
 900 #endif /* gc_Cell_h */