js/public/HeapAPI.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 js_HeapAPI_h
   8 #define js_HeapAPI_h
   9
  10 #include "mozilla/Atomics.h"
  11 #include "mozilla/BitSet.h"
  12
  13 #include <limits.h>
  14 #include <type_traits>
  15
  16 #include "js/AllocPolicy.h"
  17 #include "js/GCAnnotations.h"
  18 #include "js/HashTable.h"
  19 #include "js/shadow/String.h"  // JS::shadow::String
  20 #include "js/shadow/Symbol.h"  // JS::shadow::Symbol
  21 #include "js/shadow/Zone.h"    // JS::shadow::Zone
  22 #include "js/TraceKind.h"
  23 #include "js/TypeDecls.h"
  24
  25 /* These values are private to the JS engine. */
  26 namespace js {
  27
  28 class NurseryDecommitTask;
  29
  30 JS_PUBLIC_API bool CurrentThreadCanAccessZone(JS::Zone* zone);
  31
  32 // To prevent false sharing, some data structures are aligned to a typical cache
  33 // line size.
  34 static constexpr size_t TypicalCacheLineSize = 64;
  35
  36 namespace gc {
  37
  38 class Arena;
  39 struct Cell;
  40 class TenuredChunk;
  41 class StoreBuffer;
  42 class TenuredCell;
  43
  44 const size_t ArenaShift = 12;
  45 const size_t ArenaSize = size_t(1) << ArenaShift;
  46 const size_t ArenaMask = ArenaSize - 1;
  47
  48 #if defined(XP_MACOSX) && defined(__aarch64__)
  49 const size_t PageShift = 14;
  50 #else
  51 const size_t PageShift = 12;
  52 #endif
  53 // Expected page size, so we could initialze ArenasPerPage at compile-time.
  54 // The actual system page size should be queried by SystemPageSize().
  55 const size_t PageSize = size_t(1) << PageShift;
  56 constexpr size_t ArenasPerPage = PageSize / ArenaSize;
  57
  58 const size_t ChunkShift = 20;
  59 const size_t ChunkSize = size_t(1) << ChunkShift;
  60 const size_t ChunkMask = ChunkSize - 1;
  61
  62 const size_t CellAlignShift = 3;
  63 const size_t CellAlignBytes = size_t(1) << CellAlignShift;
  64 const size_t CellAlignMask = CellAlignBytes - 1;
  65
  66 const size_t CellBytesPerMarkBit = CellAlignBytes;
  67 const size_t MarkBitsPerCell = 2;
  68
  69 /*
  70  * The mark bitmap has one bit per each possible cell start position. This
  71  * wastes some space for larger GC things but allows us to avoid division by the
  72  * cell's size when accessing the bitmap.
  73  */
  74 const size_t ArenaBitmapBits = ArenaSize / CellBytesPerMarkBit;
  75 const size_t ArenaBitmapBytes = HowMany(ArenaBitmapBits, 8);
  76 const size_t ArenaBitmapWords = HowMany(ArenaBitmapBits, JS_BITS_PER_WORD);
  77
  78 // The base class for all GC chunks, either in the nursery or in the tenured
  79 // heap memory. This structure is locatable from any GC pointer by aligning to
  80 // the chunk size.
  81 class alignas(CellAlignBytes) ChunkBase {
  82  protected:
  83   ChunkBase(JSRuntime* rt, StoreBuffer* sb) {
  84     MOZ_ASSERT((uintptr_t(this) & ChunkMask) == 0);
  85     initBase(rt, sb);
  86   }
  87
  88   void initBase(JSRuntime* rt, StoreBuffer* sb) {
  89     runtime = rt;
  90     storeBuffer = sb;
  91   }
  92
  93  public:
  94   // The store buffer for pointers from tenured things to things in this
  95   // chunk. Will be non-null if and only if this is a nursery chunk.
  96   StoreBuffer* storeBuffer;
  97
  98   // Provide quick access to the runtime from absolutely anywhere.
  99   JSRuntime* runtime;
 100 };
 101
 102 // Information about tenured heap chunks.
 103 struct TenuredChunkInfo {
 104  private:
 105   friend class ChunkPool;
 106   TenuredChunk* next = nullptr;
 107   TenuredChunk* prev = nullptr;
 108
 109  public:
 110   /* Number of free arenas, either committed or decommitted. */
 111   uint32_t numArenasFree;
 112
 113   /* Number of free, committed arenas. */
 114   uint32_t numArenasFreeCommitted;
 115 };
 116
 117 /*
 118  * Calculating ArenasPerChunk:
 119  *
 120  * To figure out how many Arenas will fit in a chunk we need to know how much
 121  * extra space is available after we allocate the header data. This is a problem
 122  * because the header size depends on the number of arenas in the chunk.
 123  *
 124  * The dependent fields are markBits, decommittedPages and
 125  * freeCommittedArenas. markBits needs ArenaBitmapBytes bytes per arena,
 126  * decommittedPages needs one bit per page and freeCommittedArenas needs one
 127  * bit per arena.
 128  *
 129  * We can calculate an approximate value by dividing the number of bits of free
 130  * space in the chunk by the number of bits needed per arena. This is an
 131  * approximation because it doesn't take account of the fact that the variable
 132  * sized fields must be rounded up to a whole number of words, or any padding
 133  * the compiler adds between fields.
 134  *
 135  * Fortunately, for the chunk and arena size parameters we use this
 136  * approximation turns out to be correct. If it were not we might need to adjust
 137  * the arena count down by one to allow more space for the padding.
 138  */
 139 const size_t BitsPerPageWithHeaders =
 140     (ArenaSize + ArenaBitmapBytes) * ArenasPerPage * CHAR_BIT + ArenasPerPage +
 141     1;
 142 const size_t ChunkBitsAvailable =
 143     (ChunkSize - sizeof(ChunkBase) - sizeof(TenuredChunkInfo)) * CHAR_BIT;
 144 const size_t PagesPerChunk = ChunkBitsAvailable / BitsPerPageWithHeaders;
 145 const size_t ArenasPerChunk = PagesPerChunk * ArenasPerPage;
 146 const size_t FreeCommittedBits = ArenasPerChunk;
 147 const size_t DecommitBits = PagesPerChunk;
 148 const size_t BitsPerArenaWithHeaders =
 149     (ArenaSize + ArenaBitmapBytes) * CHAR_BIT +
 150     (DecommitBits / ArenasPerChunk) + 1;
 151
 152 const size_t CalculatedChunkSizeRequired =
 153     sizeof(ChunkBase) + sizeof(TenuredChunkInfo) +
 154     RoundUp(ArenasPerChunk * ArenaBitmapBytes, sizeof(uintptr_t)) +
 155     RoundUp(FreeCommittedBits, sizeof(uint32_t) * CHAR_BIT) / CHAR_BIT +
 156     RoundUp(DecommitBits, sizeof(uint32_t) * CHAR_BIT) / CHAR_BIT +
 157     ArenasPerChunk * ArenaSize;
 158 static_assert(CalculatedChunkSizeRequired <= ChunkSize,
 159               "Calculated ArenasPerChunk is too large");
 160
 161 const size_t CalculatedChunkPadSize = ChunkSize - CalculatedChunkSizeRequired;
 162 static_assert(CalculatedChunkPadSize * CHAR_BIT < BitsPerArenaWithHeaders,
 163               "Calculated ArenasPerChunk is too small");
 164
 165 static_assert(ArenasPerChunk == 252,
 166               "Do not accidentally change our heap's density.");
 167
 168 // Mark bitmaps are atomic because they can be written by gray unmarking on the
 169 // main thread while read by sweeping on a background thread. The former does
 170 // not affect the result of the latter.
 171 using MarkBitmapWord = mozilla::Atomic<uintptr_t, mozilla::Relaxed>;
 172
 173 /*
 174  * Live objects are marked black or gray. Everything reachable from a JS root is
 175  * marked black. Objects marked gray are eligible for cycle collection.
 176  *
 177  *    BlackBit:     GrayOrBlackBit:  Color:
 178  *       0               0           white
 179  *       0               1           gray
 180  *       1               0           black
 181  *       1               1           black
 182  */
 183 enum class ColorBit : uint32_t { BlackBit = 0, GrayOrBlackBit = 1 };
 184
 185 // Mark colors. Order is important here: the greater value the 'more marked' a
 186 // cell is.
 187 enum class MarkColor : uint8_t { Gray = 1, Black = 2 };
 188
 189 // Mark bitmap for a tenured heap chunk.
 190 struct alignas(TypicalCacheLineSize) MarkBitmap {
 191   static constexpr size_t WordCount = ArenaBitmapWords * ArenasPerChunk;
 192   MarkBitmapWord bitmap[WordCount];
 193
 194   inline void getMarkWordAndMask(const TenuredCell* cell, ColorBit colorBit,
 195                                  MarkBitmapWord** wordp, uintptr_t* maskp);
 196
 197   // The following are not exported and are defined in gc/Heap.h:
 198   inline bool markBit(const TenuredCell* cell, ColorBit colorBit);
 199   inline bool isMarkedAny(const TenuredCell* cell);
 200   inline bool isMarkedBlack(const TenuredCell* cell);
 201   inline bool isMarkedGray(const TenuredCell* cell);
 202   inline bool markIfUnmarked(const TenuredCell* cell, MarkColor color);
 203   inline bool markIfUnmarkedAtomic(const TenuredCell* cell, MarkColor color);
 204   inline void markBlack(const TenuredCell* cell);
 205   inline void markBlackAtomic(const TenuredCell* cell);
 206   inline void copyMarkBit(TenuredCell* dst, const TenuredCell* src,
 207                           ColorBit colorBit);
 208   inline void unmark(const TenuredCell* cell);
 209   inline MarkBitmapWord* arenaBits(Arena* arena);
 210 };
 211
 212 static_assert(ArenaBitmapBytes * ArenasPerChunk == sizeof(MarkBitmap),
 213               "Ensure our MarkBitmap actually covers all arenas.");
 214
 215 // Bitmap with one bit per page used for decommitted page set.
 216 using ChunkPageBitmap = mozilla::BitSet<PagesPerChunk, uint32_t>;
 217
 218 // Bitmap with one bit per arena used for free committed arena set.
 219 using ChunkArenaBitmap = mozilla::BitSet<ArenasPerChunk, uint32_t>;
 220
 221 // Base class containing data members for a tenured heap chunk.
 222 class TenuredChunkBase : public ChunkBase {
 223  public:
 224   TenuredChunkInfo info;
 225   MarkBitmap markBits;
 226   ChunkArenaBitmap freeCommittedArenas;
 227   ChunkPageBitmap decommittedPages;
 228
 229  protected:
 230   explicit TenuredChunkBase(JSRuntime* runtime) : ChunkBase(runtime, nullptr) {
 231     info.numArenasFree = ArenasPerChunk;
 232   }
 233
 234   void initAsDecommitted();
 235 };
 236
 237 /*
 238  * We sometimes use an index to refer to a cell in an arena. The index for a
 239  * cell is found by dividing by the cell alignment so not all indices refer to
 240  * valid cells.
 241  */
 242 const size_t ArenaCellIndexBytes = CellAlignBytes;
 243 const size_t MaxArenaCellIndex = ArenaSize / CellAlignBytes;
 244
 245 const size_t MarkBitmapWordBits = sizeof(MarkBitmapWord) * CHAR_BIT;
 246
 247 constexpr size_t FirstArenaAdjustmentBits =
 248     RoundUp(sizeof(gc::TenuredChunkBase), ArenaSize) / gc::CellBytesPerMarkBit;
 249
 250 static_assert((FirstArenaAdjustmentBits % MarkBitmapWordBits) == 0);
 251 constexpr size_t FirstArenaAdjustmentWords =
 252     FirstArenaAdjustmentBits / MarkBitmapWordBits;
 253
 254 const size_t ChunkStoreBufferOffset = offsetof(ChunkBase, storeBuffer);
 255 const size_t ChunkMarkBitmapOffset = offsetof(TenuredChunkBase, markBits);
 256
 257 // Hardcoded offsets into Arena class.
 258 const size_t ArenaZoneOffset = 2 * sizeof(uint32_t);
 259 const size_t ArenaHeaderSize = ArenaZoneOffset + 2 * sizeof(uintptr_t) +
 260                                sizeof(size_t) + sizeof(uintptr_t);
 261
 262 // The first word of a GC thing has certain requirements from the GC and is used
 263 // to store flags in the low bits.
 264 const size_t CellFlagBitsReservedForGC = 3;
 265
 266 // The first word can be used to store JSClass pointers for some thing kinds, so
 267 // these must be suitably aligned.
 268 const size_t JSClassAlignBytes = size_t(1) << CellFlagBitsReservedForGC;
 269
 270 #ifdef JS_DEBUG
 271 /* When downcasting, ensure we are actually the right type. */
 272 extern JS_PUBLIC_API void AssertGCThingHasType(js::gc::Cell* cell,
 273                                                JS::TraceKind kind);
 274 #else
 275 inline void AssertGCThingHasType(js::gc::Cell* cell, JS::TraceKind kind) {}
 276 #endif
 277
 278 MOZ_ALWAYS_INLINE bool IsInsideNursery(const js::gc::Cell* cell);
 279 MOZ_ALWAYS_INLINE bool IsInsideNursery(const js::gc::TenuredCell* cell);
 280
 281 } /* namespace gc */
 282 } /* namespace js */
 283
 284 namespace JS {
 285
 286 enum class HeapState {
 287   Idle,             // doing nothing with the GC heap
 288   Tracing,          // tracing the GC heap without collecting, e.g.
 289                     // IterateCompartments()
 290   MajorCollecting,  // doing a GC of the major heap
 291   MinorCollecting,  // doing a GC of the minor heap (nursery)
 292   CycleCollecting   // in the "Unlink" phase of cycle collection
 293 };
 294
 295 JS_PUBLIC_API HeapState RuntimeHeapState();
 296
 297 static inline bool RuntimeHeapIsBusy() {
 298   return RuntimeHeapState() != HeapState::Idle;
 299 }
 300
 301 static inline bool RuntimeHeapIsTracing() {
 302   return RuntimeHeapState() == HeapState::Tracing;
 303 }
 304
 305 static inline bool RuntimeHeapIsMajorCollecting() {
 306   return RuntimeHeapState() == HeapState::MajorCollecting;
 307 }
 308
 309 static inline bool RuntimeHeapIsMinorCollecting() {
 310   return RuntimeHeapState() == HeapState::MinorCollecting;
 311 }
 312
 313 static inline bool RuntimeHeapIsCollecting(HeapState state) {
 314   return state == HeapState::MajorCollecting ||
 315          state == HeapState::MinorCollecting;
 316 }
 317
 318 static inline bool RuntimeHeapIsCollecting() {
 319   return RuntimeHeapIsCollecting(RuntimeHeapState());
 320 }
 321
 322 static inline bool RuntimeHeapIsCycleCollecting() {
 323   return RuntimeHeapState() == HeapState::CycleCollecting;
 324 }
 325
 326 /*
 327  * This list enumerates the different types of conceptual stacks we have in
 328  * SpiderMonkey. In reality, they all share the C stack, but we allow different
 329  * stack limits depending on the type of code running.
 330  */
 331 enum StackKind {
 332   StackForSystemCode,       // C++, such as the GC, running on behalf of the VM.
 333   StackForTrustedScript,    // Script running with trusted principals.
 334   StackForUntrustedScript,  // Script running with untrusted principals.
 335   StackKindCount
 336 };
 337
 338 /*
 339  * Default maximum size for the generational nursery in bytes. This is the
 340  * initial value. In the browser this configured by the
 341  * javascript.options.mem.nursery.max_kb pref.
 342  */
 343 const uint32_t DefaultNurseryMaxBytes = 16 * js::gc::ChunkSize;
 344
 345 /* Default maximum heap size in bytes to pass to JS_NewContext(). */
 346 const uint32_t DefaultHeapMaxBytes = 32 * 1024 * 1024;
 347
 348 /**
 349  * A GC pointer, tagged with the trace kind.
 350  *
 351  * In general, a GC pointer should be stored with an exact type. This class
 352  * is for use when that is not possible because a single pointer must point
 353  * to several kinds of GC thing.
 354  */
 355 class JS_PUBLIC_API GCCellPtr {
 356  public:
 357   GCCellPtr() : GCCellPtr(nullptr) {}
 358
 359   // Construction from a void* and trace kind.
 360   GCCellPtr(void* gcthing, JS::TraceKind traceKind)
 361       : ptr(checkedCast(gcthing, traceKind)) {}
 362
 363   // Automatically construct a null GCCellPtr from nullptr.
 364   MOZ_IMPLICIT GCCellPtr(decltype(nullptr))
 365       : ptr(checkedCast(nullptr, JS::TraceKind::Null)) {}
 366
 367   // Construction from an explicit type.
 368   template <typename T>
 369   explicit GCCellPtr(T* p)
 370       : ptr(checkedCast(p, JS::MapTypeToTraceKind<T>::kind)) {}
 371   explicit GCCellPtr(JSFunction* p)
 372       : ptr(checkedCast(p, JS::TraceKind::Object)) {}
 373   explicit GCCellPtr(JSScript* p)
 374       : ptr(checkedCast(p, JS::TraceKind::Script)) {}
 375   explicit GCCellPtr(const Value& v);
 376
 377   JS::TraceKind kind() const {
 378     uintptr_t kindBits = ptr & OutOfLineTraceKindMask;
 379     if (kindBits != OutOfLineTraceKindMask) {
 380       return JS::TraceKind(kindBits);
 381     }
 382     return outOfLineKind();
 383   }
 384
 385   // Allow GCCellPtr to be used in a boolean context.
 386   explicit operator bool() const {
 387     MOZ_ASSERT(bool(asCell()) == (kind() != JS::TraceKind::Null));
 388     return asCell();
 389   }
 390
 391   // Simplify checks to the kind.
 392   template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
 393   bool is() const {
 394     return kind() == JS::MapTypeToTraceKind<T>::kind;
 395   }
 396
 397   // Conversions to more specific types must match the kind. Access to
 398   // further refined types is not allowed directly from a GCCellPtr.
 399   template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
 400   T& as() const {
 401     MOZ_ASSERT(kind() == JS::MapTypeToTraceKind<T>::kind);
 402     // We can't use static_cast here, because the fact that JSObject
 403     // inherits from js::gc::Cell is not part of the public API.
 404     return *reinterpret_cast<T*>(asCell());
 405   }
 406
 407   // Return a pointer to the cell this |GCCellPtr| refers to, or |nullptr|.
 408   // (It would be more symmetrical with |to| for this to return a |Cell&|, but
 409   // the result can be |nullptr|, and null references are undefined behavior.)
 410   js::gc::Cell* asCell() const {
 411     return reinterpret_cast<js::gc::Cell*>(ptr & ~OutOfLineTraceKindMask);
 412   }
 413
 414   // The CC's trace logger needs an identity that is XPIDL serializable.
 415   uint64_t unsafeAsInteger() const {
 416     return static_cast<uint64_t>(unsafeAsUIntPtr());
 417   }
 418   // Inline mark bitmap access requires direct pointer arithmetic.
 419   uintptr_t unsafeAsUIntPtr() const {
 420     MOZ_ASSERT(asCell());
 421     MOZ_ASSERT(!js::gc::IsInsideNursery(asCell()));
 422     return reinterpret_cast<uintptr_t>(asCell());
 423   }
 424
 425   MOZ_ALWAYS_INLINE bool mayBeOwnedByOtherRuntime() const {
 426     if (!is<JSString>() && !is<JS::Symbol>()) {
 427       return false;
 428     }
 429     if (is<JSString>()) {
 430       return JS::shadow::String::isPermanentAtom(asCell());
 431     }
 432     MOZ_ASSERT(is<JS::Symbol>());
 433     return JS::shadow::Symbol::isWellKnownSymbol(asCell());
 434   }
 435
 436  private:
 437   static uintptr_t checkedCast(void* p, JS::TraceKind traceKind) {
 438     auto* cell = static_cast<js::gc::Cell*>(p);
 439     MOZ_ASSERT((uintptr_t(p) & OutOfLineTraceKindMask) == 0);
 440     AssertGCThingHasType(cell, traceKind);
 441     // Store trace in the bottom bits of pointer for common kinds.
 442     uintptr_t kindBits = uintptr_t(traceKind);
 443     if (kindBits >= OutOfLineTraceKindMask) {
 444       kindBits = OutOfLineTraceKindMask;
 445     }
 446     return uintptr_t(p) | kindBits;
 447   }
 448
 449   JS::TraceKind outOfLineKind() const;
 450
 451   uintptr_t ptr;
 452 } JS_HAZ_GC_POINTER;
 453
 454 // Unwraps the given GCCellPtr, calls the functor |f| with a template argument
 455 // of the actual type of the pointer, and returns the result.
 456 template <typename F>
 457 auto MapGCThingTyped(GCCellPtr thing, F&& f) {
 458   switch (thing.kind()) {
 459 #define JS_EXPAND_DEF(name, type, _, _1) \
 460   case JS::TraceKind::name:              \
 461     return f(&thing.as<type>());
 462     JS_FOR_EACH_TRACEKIND(JS_EXPAND_DEF);
 463 #undef JS_EXPAND_DEF
 464     default:
 465       MOZ_CRASH("Invalid trace kind in MapGCThingTyped for GCCellPtr.");
 466   }
 467 }
 468
 469 // Unwraps the given GCCellPtr and calls the functor |f| with a template
 470 // argument of the actual type of the pointer. Doesn't return anything.
 471 template <typename F>
 472 void ApplyGCThingTyped(GCCellPtr thing, F&& f) {
 473   // This function doesn't do anything but is supplied for symmetry with other
 474   // MapGCThingTyped/ApplyGCThingTyped implementations that have to wrap the
 475   // functor to return a dummy value that is ignored.
 476   MapGCThingTyped(thing, f);
 477 }
 478
 479 } /* namespace JS */
 480
 481 // These are defined in the toplevel namespace instead of within JS so that
 482 // they won't shadow other operator== overloads (see bug 1456512.)
 483
 484 inline bool operator==(JS::GCCellPtr ptr1, JS::GCCellPtr ptr2) {
 485   return ptr1.asCell() == ptr2.asCell();
 486 }
 487
 488 inline bool operator!=(JS::GCCellPtr ptr1, JS::GCCellPtr ptr2) {
 489   return !(ptr1 == ptr2);
 490 }
 491
 492 namespace js {
 493 namespace gc {
 494
 495 /* static */
 496 MOZ_ALWAYS_INLINE void MarkBitmap::getMarkWordAndMask(const TenuredCell* cell,
 497                                                       ColorBit colorBit,
 498                                                       MarkBitmapWord** wordp,
 499                                                       uintptr_t* maskp) {
 500   // Note: the JIT pre-barrier trampolines inline this code. Update
 501   // MacroAssembler::emitPreBarrierFastPath code too when making changes here!
 502
 503   MOZ_ASSERT(size_t(colorBit) < MarkBitsPerCell);
 504
 505   size_t offset = uintptr_t(cell) & ChunkMask;
 506   const size_t bit = offset / CellBytesPerMarkBit + size_t(colorBit);
 507   size_t word = bit / MarkBitmapWordBits - FirstArenaAdjustmentWords;
 508   MOZ_ASSERT(word < WordCount);
 509   *wordp = &bitmap[word];
 510   *maskp = uintptr_t(1) << (bit % MarkBitmapWordBits);
 511 }
 512
 513 namespace detail {
 514
 515 static MOZ_ALWAYS_INLINE ChunkBase* GetCellChunkBase(const Cell* cell) {
 516   MOZ_ASSERT(cell);
 517   auto* chunk = reinterpret_cast<ChunkBase*>(uintptr_t(cell) & ~ChunkMask);
 518   MOZ_ASSERT(chunk->runtime);
 519   return chunk;
 520 }
 521
 522 static MOZ_ALWAYS_INLINE TenuredChunkBase* GetCellChunkBase(
 523     const TenuredCell* cell) {
 524   MOZ_ASSERT(cell);
 525   auto* chunk =
 526       reinterpret_cast<TenuredChunkBase*>(uintptr_t(cell) & ~ChunkMask);
 527   MOZ_ASSERT(chunk->runtime);
 528   return chunk;
 529 }
 530
 531 static MOZ_ALWAYS_INLINE JS::Zone* GetTenuredGCThingZone(const uintptr_t addr) {
 532   MOZ_ASSERT(addr);
 533   const uintptr_t zone_addr = (addr & ~ArenaMask) | ArenaZoneOffset;
 534   return *reinterpret_cast<JS::Zone**>(zone_addr);
 535 }
 536
 537 static MOZ_ALWAYS_INLINE bool TenuredCellIsMarkedBlack(
 538     const TenuredCell* cell) {
 539   // Return true if BlackBit is set.
 540
 541   MOZ_ASSERT(cell);
 542   MOZ_ASSERT(!js::gc::IsInsideNursery(cell));
 543
 544   MarkBitmapWord* blackWord;
 545   uintptr_t blackMask;
 546   TenuredChunkBase* chunk = GetCellChunkBase(cell);
 547   chunk->markBits.getMarkWordAndMask(cell, js::gc::ColorBit::BlackBit,
 548                                      &blackWord, &blackMask);
 549   return *blackWord & blackMask;
 550 }
 551
 552 static MOZ_ALWAYS_INLINE bool NonBlackCellIsMarkedGray(
 553     const TenuredCell* cell) {
 554   // Return true if GrayOrBlackBit is set. Callers should check BlackBit first.
 555
 556   MOZ_ASSERT(cell);
 557   MOZ_ASSERT(!js::gc::IsInsideNursery(cell));
 558   MOZ_ASSERT(!TenuredCellIsMarkedBlack(cell));
 559
 560   MarkBitmapWord* grayWord;
 561   uintptr_t grayMask;
 562   TenuredChunkBase* chunk = GetCellChunkBase(cell);
 563   chunk->markBits.getMarkWordAndMask(cell, js::gc::ColorBit::GrayOrBlackBit,
 564                                      &grayWord, &grayMask);
 565   return *grayWord & grayMask;
 566 }
 567
 568 static MOZ_ALWAYS_INLINE bool TenuredCellIsMarkedGray(const TenuredCell* cell) {
 569   return !TenuredCellIsMarkedBlack(cell) && NonBlackCellIsMarkedGray(cell);
 570 }
 571
 572 static MOZ_ALWAYS_INLINE bool CellIsMarkedGray(const Cell* cell) {
 573   MOZ_ASSERT(cell);
 574   if (js::gc::IsInsideNursery(cell)) {
 575     return false;
 576   }
 577   return TenuredCellIsMarkedGray(reinterpret_cast<const TenuredCell*>(cell));
 578 }
 579
 580 extern JS_PUBLIC_API bool CanCheckGrayBits(const TenuredCell* cell);
 581
 582 extern JS_PUBLIC_API bool CellIsMarkedGrayIfKnown(const TenuredCell* cell);
 583
 584 #ifdef DEBUG
 585 extern JS_PUBLIC_API void AssertCellIsNotGray(const Cell* cell);
 586
 587 extern JS_PUBLIC_API bool ObjectIsMarkedBlack(const JSObject* obj);
 588 #endif
 589
 590 MOZ_ALWAYS_INLINE bool CellHasStoreBuffer(const Cell* cell) {
 591   return GetCellChunkBase(cell)->storeBuffer;
 592 }
 593
 594 } /* namespace detail */
 595
 596 MOZ_ALWAYS_INLINE bool IsInsideNursery(const Cell* cell) {
 597   MOZ_ASSERT(cell);
 598   return detail::CellHasStoreBuffer(cell);
 599 }
 600
 601 MOZ_ALWAYS_INLINE bool IsInsideNursery(const TenuredCell* cell) {
 602   MOZ_ASSERT(cell);
 603   MOZ_ASSERT(!IsInsideNursery(reinterpret_cast<const Cell*>(cell)));
 604   return false;
 605 }
 606
 607 // Allow use before the compiler knows the derivation of JSObject, JSString, and
 608 // JS::BigInt.
 609 MOZ_ALWAYS_INLINE bool IsInsideNursery(const JSObject* obj) {
 610   return IsInsideNursery(reinterpret_cast<const Cell*>(obj));
 611 }
 612 MOZ_ALWAYS_INLINE bool IsInsideNursery(const JSString* str) {
 613   return IsInsideNursery(reinterpret_cast<const Cell*>(str));
 614 }
 615 MOZ_ALWAYS_INLINE bool IsInsideNursery(const JS::BigInt* bi) {
 616   return IsInsideNursery(reinterpret_cast<const Cell*>(bi));
 617 }
 618
 619 MOZ_ALWAYS_INLINE bool IsCellPointerValid(const void* ptr) {
 620   auto addr = uintptr_t(ptr);
 621   if (addr < ChunkSize || addr % CellAlignBytes != 0) {
 622     return false;
 623   }
 624
 625   auto* cell = reinterpret_cast<const Cell*>(ptr);
 626   if (!IsInsideNursery(cell)) {
 627     return detail::GetTenuredGCThingZone(addr) != nullptr;
 628   }
 629
 630   return true;
 631 }
 632
 633 MOZ_ALWAYS_INLINE bool IsCellPointerValidOrNull(const void* cell) {
 634   if (!cell) {
 635     return true;
 636   }
 637   return IsCellPointerValid(cell);
 638 }
 639
 640 } /* namespace gc */
 641 } /* namespace js */
 642
 643 namespace JS {
 644
 645 static MOZ_ALWAYS_INLINE Zone* GetTenuredGCThingZone(GCCellPtr thing) {
 646   MOZ_ASSERT(!js::gc::IsInsideNursery(thing.asCell()));
 647   return js::gc::detail::GetTenuredGCThingZone(thing.unsafeAsUIntPtr());
 648 }
 649
 650 extern JS_PUBLIC_API Zone* GetNurseryCellZone(js::gc::Cell* cell);
 651
 652 static MOZ_ALWAYS_INLINE Zone* GetGCThingZone(GCCellPtr thing) {
 653   if (!js::gc::IsInsideNursery(thing.asCell())) {
 654     return js::gc::detail::GetTenuredGCThingZone(thing.unsafeAsUIntPtr());
 655   }
 656
 657   return GetNurseryCellZone(thing.asCell());
 658 }
 659
 660 static MOZ_ALWAYS_INLINE Zone* GetStringZone(JSString* str) {
 661   if (!js::gc::IsInsideNursery(str)) {
 662     return js::gc::detail::GetTenuredGCThingZone(
 663         reinterpret_cast<uintptr_t>(str));
 664   }
 665   return GetNurseryCellZone(reinterpret_cast<js::gc::Cell*>(str));
 666 }
 667
 668 extern JS_PUBLIC_API Zone* GetObjectZone(JSObject* obj);
 669
 670 static MOZ_ALWAYS_INLINE bool GCThingIsMarkedGray(GCCellPtr thing) {
 671   js::gc::Cell* cell = thing.asCell();
 672   if (IsInsideNursery(cell)) {
 673     return false;
 674   }
 675
 676   auto* tenuredCell = reinterpret_cast<js::gc::TenuredCell*>(cell);
 677   return js::gc::detail::CellIsMarkedGrayIfKnown(tenuredCell);
 678 }
 679
 680 // Specialised gray marking check for use by the cycle collector. This is not
 681 // called during incremental GC or when the gray bits are invalid.
 682 static MOZ_ALWAYS_INLINE bool GCThingIsMarkedGrayInCC(GCCellPtr thing) {
 683   js::gc::Cell* cell = thing.asCell();
 684   if (IsInsideNursery(cell)) {
 685     return false;
 686   }
 687
 688   auto* tenuredCell = reinterpret_cast<js::gc::TenuredCell*>(cell);
 689   if (!js::gc::detail::TenuredCellIsMarkedGray(tenuredCell)) {
 690     return false;
 691   }
 692
 693   MOZ_ASSERT(js::gc::detail::CanCheckGrayBits(tenuredCell));
 694
 695   return true;
 696 }
 697
 698 extern JS_PUBLIC_API JS::TraceKind GCThingTraceKind(void* thing);
 699
 700 extern JS_PUBLIC_API void EnableNurseryStrings(JSContext* cx);
 701
 702 extern JS_PUBLIC_API void DisableNurseryStrings(JSContext* cx);
 703
 704 extern JS_PUBLIC_API void EnableNurseryBigInts(JSContext* cx);
 705
 706 extern JS_PUBLIC_API void DisableNurseryBigInts(JSContext* cx);
 707
 708 /*
 709  * Returns true when writes to GC thing pointers (and reads from weak pointers)
 710  * must call an incremental barrier. This is generally only true when running
 711  * mutator code in-between GC slices. At other times, the barrier may be elided
 712  * for performance.
 713  */
 714 extern JS_PUBLIC_API bool IsIncrementalBarrierNeeded(JSContext* cx);
 715
 716 /*
 717  * Notify the GC that a reference to a JSObject is about to be overwritten.
 718  * This method must be called if IsIncrementalBarrierNeeded.
 719  */
 720 extern JS_PUBLIC_API void IncrementalPreWriteBarrier(JSObject* obj);
 721
 722 /*
 723  * Notify the GC that a reference to a tenured GC cell is about to be
 724  * overwritten. This method must be called if IsIncrementalBarrierNeeded.
 725  */
 726 extern JS_PUBLIC_API void IncrementalPreWriteBarrier(GCCellPtr thing);
 727
 728 /**
 729  * Unsets the gray bit for anything reachable from |thing|. |kind| should not be
 730  * JS::TraceKind::Shape. |thing| should be non-null. The return value indicates
 731  * if anything was unmarked.
 732  */
 733 extern JS_PUBLIC_API bool UnmarkGrayGCThingRecursively(GCCellPtr thing);
 734
 735 }  // namespace JS
 736
 737 namespace js {
 738 namespace gc {
 739
 740 extern JS_PUBLIC_API void PerformIncrementalReadBarrier(JS::GCCellPtr thing);
 741
 742 static MOZ_ALWAYS_INLINE void ExposeGCThingToActiveJS(JS::GCCellPtr thing) {
 743   // TODO: I'd like to assert !RuntimeHeapIsBusy() here but this gets
 744   // called while we are tracing the heap, e.g. during memory reporting
 745   // (see bug 1313318).
 746   MOZ_ASSERT(!JS::RuntimeHeapIsCollecting());
 747
 748   // GC things residing in the nursery cannot be gray: they have no mark bits.
 749   // All live objects in the nursery are moved to tenured at the beginning of
 750   // each GC slice, so the gray marker never sees nursery things.
 751   if (IsInsideNursery(thing.asCell())) {
 752     return;
 753   }
 754
 755   auto* cell = reinterpret_cast<TenuredCell*>(thing.asCell());
 756   if (detail::TenuredCellIsMarkedBlack(cell)) {
 757     return;
 758   }
 759
 760   // GC things owned by other runtimes are always black.
 761   MOZ_ASSERT(!thing.mayBeOwnedByOtherRuntime());
 762
 763   auto* zone = JS::shadow::Zone::from(JS::GetTenuredGCThingZone(thing));
 764   if (zone->needsIncrementalBarrier()) {
 765     PerformIncrementalReadBarrier(thing);
 766   } else if (!zone->isGCPreparing() && detail::NonBlackCellIsMarkedGray(cell)) {
 767     MOZ_ALWAYS_TRUE(JS::UnmarkGrayGCThingRecursively(thing));
 768   }
 769
 770   MOZ_ASSERT_IF(!zone->isGCPreparing(), !detail::TenuredCellIsMarkedGray(cell));
 771 }
 772
 773 static MOZ_ALWAYS_INLINE void IncrementalReadBarrier(JS::GCCellPtr thing) {
 774   // This is a lighter version of ExposeGCThingToActiveJS that doesn't do gray
 775   // unmarking.
 776
 777   if (IsInsideNursery(thing.asCell())) {
 778     return;
 779   }
 780
 781   auto* zone = JS::shadow::Zone::from(JS::GetTenuredGCThingZone(thing));
 782   auto* cell = reinterpret_cast<TenuredCell*>(thing.asCell());
 783   if (zone->needsIncrementalBarrier() &&
 784       !detail::TenuredCellIsMarkedBlack(cell)) {
 785     // GC things owned by other runtimes are always black.
 786     MOZ_ASSERT(!thing.mayBeOwnedByOtherRuntime());
 787     PerformIncrementalReadBarrier(thing);
 788   }
 789 }
 790
 791 template <typename T>
 792 extern JS_PUBLIC_API bool EdgeNeedsSweepUnbarrieredSlow(T* thingp);
 793
 794 static MOZ_ALWAYS_INLINE bool EdgeNeedsSweepUnbarriered(JSObject** objp) {
 795   // This function does not handle updating nursery pointers. Raw JSObject
 796   // pointers should be updated separately or replaced with
 797   // JS::Heap<JSObject*> which handles this automatically.
 798   MOZ_ASSERT(!JS::RuntimeHeapIsMinorCollecting());
 799   if (IsInsideNursery(*objp)) {
 800     return false;
 801   }
 802
 803   auto zone =
 804       JS::shadow::Zone::from(detail::GetTenuredGCThingZone(uintptr_t(*objp)));
 805   if (!zone->isGCSweepingOrCompacting()) {
 806     return false;
 807   }
 808
 809   return EdgeNeedsSweepUnbarrieredSlow(objp);
 810 }
 811
 812 }  // namespace gc
 813 }  // namespace js
 814
 815 namespace JS {
 816
 817 /*
 818  * This should be called when an object that is marked gray is exposed to the JS
 819  * engine (by handing it to running JS code or writing it into live JS
 820  * data). During incremental GC, since the gray bits haven't been computed yet,
 821  * we conservatively mark the object black.
 822  */
 823 static MOZ_ALWAYS_INLINE void ExposeObjectToActiveJS(JSObject* obj) {
 824   MOZ_ASSERT(obj);
 825   MOZ_ASSERT(!js::gc::EdgeNeedsSweepUnbarrieredSlow(&obj));
 826   js::gc::ExposeGCThingToActiveJS(GCCellPtr(obj));
 827 }
 828
 829 } /* namespace JS */
 830
 831 #endif /* js_HeapAPI_h */