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 minimum cell size ends up as twice the cell alignment because the mark
  71  * bitmap contains one bit per CellBytesPerMarkBit bytes (which is equal to
  72  * CellAlignBytes) and we need two mark bits per cell.
  73  */
  74 const size_t MinCellSize = CellBytesPerMarkBit * MarkBitsPerCell;
  75
  76 /*
  77  * The mark bitmap has one bit per each possible cell start position. This
  78  * wastes some space for larger GC things but allows us to avoid division by the
  79  * cell's size when accessing the bitmap.
  80  */
  81 const size_t ArenaBitmapBits = ArenaSize / CellBytesPerMarkBit;
  82 const size_t ArenaBitmapBytes = HowMany(ArenaBitmapBits, 8);
  83 const size_t ArenaBitmapWords = HowMany(ArenaBitmapBits, JS_BITS_PER_WORD);
  84
  85 // The base class for all GC chunks, either in the nursery or in the tenured
  86 // heap memory. This structure is locatable from any GC pointer by aligning to
  87 // the chunk size.
  88 class alignas(CellAlignBytes) ChunkBase {
  89  protected:
  90   ChunkBase(JSRuntime* rt, StoreBuffer* sb) {
  91     MOZ_ASSERT((uintptr_t(this) & ChunkMask) == 0);
  92     initBase(rt, sb);
  93   }
  94
  95   void initBase(JSRuntime* rt, StoreBuffer* sb) {
  96     runtime = rt;
  97     storeBuffer = sb;
  98   }
  99
 100  public:
 101   // The store buffer for pointers from tenured things to things in this
 102   // chunk. Will be non-null if and only if this is a nursery chunk.
 103   StoreBuffer* storeBuffer;
 104
 105   // Provide quick access to the runtime from absolutely anywhere.
 106   JSRuntime* runtime;
 107 };
 108
 109 // Information about tenured heap chunks.
 110 struct TenuredChunkInfo {
 111  private:
 112   friend class ChunkPool;
 113   TenuredChunk* next = nullptr;
 114   TenuredChunk* prev = nullptr;
 115
 116  public:
 117   /* Number of free arenas, either committed or decommitted. */
 118   uint32_t numArenasFree;
 119
 120   /* Number of free, committed arenas. */
 121   uint32_t numArenasFreeCommitted;
 122 };
 123
 124 /*
 125  * Calculating ArenasPerChunk:
 126  *
 127  * To figure out how many Arenas will fit in a chunk we need to know how much
 128  * extra space is available after we allocate the header data. This is a problem
 129  * because the header size depends on the number of arenas in the chunk.
 130  *
 131  * The dependent fields are markBits, decommittedPages and
 132  * freeCommittedArenas. markBits needs ArenaBitmapBytes bytes per arena,
 133  * decommittedPages needs one bit per page and freeCommittedArenas needs one
 134  * bit per arena.
 135  *
 136  * We can calculate an approximate value by dividing the number of bits of free
 137  * space in the chunk by the number of bits needed per arena. This is an
 138  * approximation because it doesn't take account of the fact that the variable
 139  * sized fields must be rounded up to a whole number of words, or any padding
 140  * the compiler adds between fields.
 141  *
 142  * Fortunately, for the chunk and arena size parameters we use this
 143  * approximation turns out to be correct. If it were not we might need to adjust
 144  * the arena count down by one to allow more space for the padding.
 145  */
 146 const size_t BitsPerPageWithHeaders =
 147     (ArenaSize + ArenaBitmapBytes) * ArenasPerPage * CHAR_BIT + ArenasPerPage +
 148     1;
 149 const size_t ChunkBitsAvailable =
 150     (ChunkSize - sizeof(ChunkBase) - sizeof(TenuredChunkInfo)) * CHAR_BIT;
 151 const size_t PagesPerChunk = ChunkBitsAvailable / BitsPerPageWithHeaders;
 152 const size_t ArenasPerChunk = PagesPerChunk * ArenasPerPage;
 153 const size_t FreeCommittedBits = ArenasPerChunk;
 154 const size_t DecommitBits = PagesPerChunk;
 155 const size_t BitsPerArenaWithHeaders =
 156     (ArenaSize + ArenaBitmapBytes) * CHAR_BIT +
 157     (DecommitBits / ArenasPerChunk) + 1;
 158
 159 const size_t CalculatedChunkSizeRequired =
 160     sizeof(ChunkBase) + sizeof(TenuredChunkInfo) +
 161     RoundUp(ArenasPerChunk * ArenaBitmapBytes, sizeof(uintptr_t)) +
 162     RoundUp(FreeCommittedBits, sizeof(uint32_t) * CHAR_BIT) / CHAR_BIT +
 163     RoundUp(DecommitBits, sizeof(uint32_t) * CHAR_BIT) / CHAR_BIT +
 164     ArenasPerChunk * ArenaSize;
 165 static_assert(CalculatedChunkSizeRequired <= ChunkSize,
 166               "Calculated ArenasPerChunk is too large");
 167
 168 const size_t CalculatedChunkPadSize = ChunkSize - CalculatedChunkSizeRequired;
 169 static_assert(CalculatedChunkPadSize * CHAR_BIT < BitsPerArenaWithHeaders,
 170               "Calculated ArenasPerChunk is too small");
 171
 172 static_assert(ArenasPerChunk == 252,
 173               "Do not accidentally change our heap's density.");
 174
 175 // Mark bitmaps are atomic because they can be written by gray unmarking on the
 176 // main thread while read by sweeping on a background thread. The former does
 177 // not affect the result of the latter.
 178 using MarkBitmapWord = mozilla::Atomic<uintptr_t, mozilla::Relaxed>;
 179
 180 /*
 181  * Live objects are marked black or gray. Everything reachable from a JS root is
 182  * marked black. Objects marked gray are eligible for cycle collection.
 183  *
 184  *    BlackBit:     GrayOrBlackBit:  Color:
 185  *       0               0           white
 186  *       0               1           gray
 187  *       1               0           black
 188  *       1               1           black
 189  */
 190 enum class ColorBit : uint32_t { BlackBit = 0, GrayOrBlackBit = 1 };
 191
 192 // Mark colors. Order is important here: the greater value the 'more marked' a
 193 // cell is.
 194 enum class MarkColor : uint8_t { Gray = 1, Black = 2 };
 195
 196 // Mark bitmap for a tenured heap chunk.
 197 struct alignas(TypicalCacheLineSize) MarkBitmap {
 198   static constexpr size_t WordCount = ArenaBitmapWords * ArenasPerChunk;
 199   MarkBitmapWord bitmap[WordCount];
 200
 201   inline void getMarkWordAndMask(const TenuredCell* cell, ColorBit colorBit,
 202                                  MarkBitmapWord** wordp, uintptr_t* maskp);
 203
 204   // The following are not exported and are defined in gc/Heap.h:
 205   inline bool markBit(const TenuredCell* cell, ColorBit colorBit);
 206   inline bool isMarkedAny(const TenuredCell* cell);
 207   inline bool isMarkedBlack(const TenuredCell* cell);
 208   inline bool isMarkedGray(const TenuredCell* cell);
 209   inline bool markIfUnmarked(const TenuredCell* cell, MarkColor color);
 210   inline bool markIfUnmarkedAtomic(const TenuredCell* cell, MarkColor color);
 211   inline void markBlack(const TenuredCell* cell);
 212   inline void markBlackAtomic(const TenuredCell* cell);
 213   inline void copyMarkBit(TenuredCell* dst, const TenuredCell* src,
 214                           ColorBit colorBit);
 215   inline void unmark(const TenuredCell* cell);
 216   inline MarkBitmapWord* arenaBits(Arena* arena);
 217 };
 218
 219 static_assert(ArenaBitmapBytes * ArenasPerChunk == sizeof(MarkBitmap),
 220               "Ensure our MarkBitmap actually covers all arenas.");
 221
 222 // Bitmap with one bit per page used for decommitted page set.
 223 using ChunkPageBitmap = mozilla::BitSet<PagesPerChunk, uint32_t>;
 224
 225 // Bitmap with one bit per arena used for free committed arena set.
 226 using ChunkArenaBitmap = mozilla::BitSet<ArenasPerChunk, uint32_t>;
 227
 228 // Base class containing data members for a tenured heap chunk.
 229 class TenuredChunkBase : public ChunkBase {
 230  public:
 231   TenuredChunkInfo info;
 232   MarkBitmap markBits;
 233   ChunkArenaBitmap freeCommittedArenas;
 234   ChunkPageBitmap decommittedPages;
 235
 236  protected:
 237   explicit TenuredChunkBase(JSRuntime* runtime) : ChunkBase(runtime, nullptr) {
 238     info.numArenasFree = ArenasPerChunk;
 239   }
 240
 241   void initAsDecommitted();
 242 };
 243
 244 /*
 245  * We sometimes use an index to refer to a cell in an arena. The index for a
 246  * cell is found by dividing by the cell alignment so not all indices refer to
 247  * valid cells.
 248  */
 249 const size_t ArenaCellIndexBytes = CellAlignBytes;
 250 const size_t MaxArenaCellIndex = ArenaSize / CellAlignBytes;
 251
 252 const size_t MarkBitmapWordBits = sizeof(MarkBitmapWord) * CHAR_BIT;
 253
 254 constexpr size_t FirstArenaAdjustmentBits =
 255     RoundUp(sizeof(gc::TenuredChunkBase), ArenaSize) / gc::CellBytesPerMarkBit;
 256
 257 static_assert((FirstArenaAdjustmentBits % MarkBitmapWordBits) == 0);
 258 constexpr size_t FirstArenaAdjustmentWords =
 259     FirstArenaAdjustmentBits / MarkBitmapWordBits;
 260
 261 const size_t ChunkStoreBufferOffset = offsetof(ChunkBase, storeBuffer);
 262 const size_t ChunkMarkBitmapOffset = offsetof(TenuredChunkBase, markBits);
 263
 264 // Hardcoded offsets into Arena class.
 265 const size_t ArenaZoneOffset = 2 * sizeof(uint32_t);
 266 const size_t ArenaHeaderSize = ArenaZoneOffset + 2 * sizeof(uintptr_t) +
 267                                sizeof(size_t) + sizeof(uintptr_t);
 268
 269 // The first word of a GC thing has certain requirements from the GC and is used
 270 // to store flags in the low bits.
 271 const size_t CellFlagBitsReservedForGC = 3;
 272
 273 // The first word can be used to store JSClass pointers for some thing kinds, so
 274 // these must be suitably aligned.
 275 const size_t JSClassAlignBytes = size_t(1) << CellFlagBitsReservedForGC;
 276
 277 #ifdef JS_DEBUG
 278 /* When downcasting, ensure we are actually the right type. */
 279 extern JS_PUBLIC_API void AssertGCThingHasType(js::gc::Cell* cell,
 280                                                JS::TraceKind kind);
 281 #else
 282 inline void AssertGCThingHasType(js::gc::Cell* cell, JS::TraceKind kind) {}
 283 #endif
 284
 285 MOZ_ALWAYS_INLINE bool IsInsideNursery(const js::gc::Cell* cell);
 286 MOZ_ALWAYS_INLINE bool IsInsideNursery(const js::gc::TenuredCell* cell);
 287
 288 } /* namespace gc */
 289 } /* namespace js */
 290
 291 namespace JS {
 292
 293 enum class HeapState {
 294   Idle,             // doing nothing with the GC heap
 295   Tracing,          // tracing the GC heap without collecting, e.g.
 296                     // IterateCompartments()
 297   MajorCollecting,  // doing a GC of the major heap
 298   MinorCollecting,  // doing a GC of the minor heap (nursery)
 299   CycleCollecting   // in the "Unlink" phase of cycle collection
 300 };
 301
 302 JS_PUBLIC_API HeapState RuntimeHeapState();
 303
 304 static inline bool RuntimeHeapIsBusy() {
 305   return RuntimeHeapState() != HeapState::Idle;
 306 }
 307
 308 static inline bool RuntimeHeapIsTracing() {
 309   return RuntimeHeapState() == HeapState::Tracing;
 310 }
 311
 312 static inline bool RuntimeHeapIsMajorCollecting() {
 313   return RuntimeHeapState() == HeapState::MajorCollecting;
 314 }
 315
 316 static inline bool RuntimeHeapIsMinorCollecting() {
 317   return RuntimeHeapState() == HeapState::MinorCollecting;
 318 }
 319
 320 static inline bool RuntimeHeapIsCollecting(HeapState state) {
 321   return state == HeapState::MajorCollecting ||
 322          state == HeapState::MinorCollecting;
 323 }
 324
 325 static inline bool RuntimeHeapIsCollecting() {
 326   return RuntimeHeapIsCollecting(RuntimeHeapState());
 327 }
 328
 329 static inline bool RuntimeHeapIsCycleCollecting() {
 330   return RuntimeHeapState() == HeapState::CycleCollecting;
 331 }
 332
 333 /*
 334  * This list enumerates the different types of conceptual stacks we have in
 335  * SpiderMonkey. In reality, they all share the C stack, but we allow different
 336  * stack limits depending on the type of code running.
 337  */
 338 enum StackKind {
 339   StackForSystemCode,       // C++, such as the GC, running on behalf of the VM.
 340   StackForTrustedScript,    // Script running with trusted principals.
 341   StackForUntrustedScript,  // Script running with untrusted principals.
 342   StackKindCount
 343 };
 344
 345 /*
 346  * Default maximum size for the generational nursery in bytes. This is the
 347  * initial value. In the browser this configured by the
 348  * javascript.options.mem.nursery.max_kb pref.
 349  */
 350 const uint32_t DefaultNurseryMaxBytes = 16 * js::gc::ChunkSize;
 351
 352 /* Default maximum heap size in bytes to pass to JS_NewContext(). */
 353 const uint32_t DefaultHeapMaxBytes = 32 * 1024 * 1024;
 354
 355 /**
 356  * A GC pointer, tagged with the trace kind.
 357  *
 358  * In general, a GC pointer should be stored with an exact type. This class
 359  * is for use when that is not possible because a single pointer must point
 360  * to several kinds of GC thing.
 361  */
 362 class JS_PUBLIC_API GCCellPtr {
 363  public:
 364   GCCellPtr() : GCCellPtr(nullptr) {}
 365
 366   // Construction from a void* and trace kind.
 367   GCCellPtr(void* gcthing, JS::TraceKind traceKind)
 368       : ptr(checkedCast(gcthing, traceKind)) {}
 369
 370   // Automatically construct a null GCCellPtr from nullptr.
 371   MOZ_IMPLICIT GCCellPtr(decltype(nullptr))
 372       : ptr(checkedCast(nullptr, JS::TraceKind::Null)) {}
 373
 374   // Construction from an explicit type.
 375   template <typename T>
 376   explicit GCCellPtr(T* p)
 377       : ptr(checkedCast(p, JS::MapTypeToTraceKind<T>::kind)) {}
 378   explicit GCCellPtr(JSFunction* p)
 379       : ptr(checkedCast(p, JS::TraceKind::Object)) {}
 380   explicit GCCellPtr(JSScript* p)
 381       : ptr(checkedCast(p, JS::TraceKind::Script)) {}
 382   explicit GCCellPtr(const Value& v);
 383
 384   JS::TraceKind kind() const {
 385     uintptr_t kindBits = ptr & OutOfLineTraceKindMask;
 386     if (kindBits != OutOfLineTraceKindMask) {
 387       return JS::TraceKind(kindBits);
 388     }
 389     return outOfLineKind();
 390   }
 391
 392   // Allow GCCellPtr to be used in a boolean context.
 393   explicit operator bool() const {
 394     MOZ_ASSERT(bool(asCell()) == (kind() != JS::TraceKind::Null));
 395     return asCell();
 396   }
 397
 398   // Simplify checks to the kind.
 399   template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
 400   bool is() const {
 401     return kind() == JS::MapTypeToTraceKind<T>::kind;
 402   }
 403
 404   // Conversions to more specific types must match the kind. Access to
 405   // further refined types is not allowed directly from a GCCellPtr.
 406   template <typename T, typename = std::enable_if_t<JS::IsBaseTraceType_v<T>>>
 407   T& as() const {
 408     MOZ_ASSERT(kind() == JS::MapTypeToTraceKind<T>::kind);
 409     // We can't use static_cast here, because the fact that JSObject
 410     // inherits from js::gc::Cell is not part of the public API.
 411     return *reinterpret_cast<T*>(asCell());
 412   }
 413
 414   // Return a pointer to the cell this |GCCellPtr| refers to, or |nullptr|.
 415   // (It would be more symmetrical with |to| for this to return a |Cell&|, but
 416   // the result can be |nullptr|, and null references are undefined behavior.)
 417   js::gc::Cell* asCell() const {
 418     return reinterpret_cast<js::gc::Cell*>(ptr & ~OutOfLineTraceKindMask);
 419   }
 420
 421   // The CC's trace logger needs an identity that is XPIDL serializable.
 422   uint64_t unsafeAsInteger() const {
 423     return static_cast<uint64_t>(unsafeAsUIntPtr());
 424   }
 425   // Inline mark bitmap access requires direct pointer arithmetic.
 426   uintptr_t unsafeAsUIntPtr() const {
 427     MOZ_ASSERT(asCell());
 428     MOZ_ASSERT(!js::gc::IsInsideNursery(asCell()));
 429     return reinterpret_cast<uintptr_t>(asCell());
 430   }
 431
 432   MOZ_ALWAYS_INLINE bool mayBeOwnedByOtherRuntime() const {
 433     if (!is<JSString>() && !is<JS::Symbol>()) {
 434       return false;
 435     }
 436     if (is<JSString>()) {
 437       return JS::shadow::String::isPermanentAtom(asCell());
 438     }
 439     MOZ_ASSERT(is<JS::Symbol>());
 440     return JS::shadow::Symbol::isWellKnownSymbol(asCell());
 441   }
 442
 443  private:
 444   static uintptr_t checkedCast(void* p, JS::TraceKind traceKind) {
 445     auto* cell = static_cast<js::gc::Cell*>(p);
 446     MOZ_ASSERT((uintptr_t(p) & OutOfLineTraceKindMask) == 0);
 447     AssertGCThingHasType(cell, traceKind);
 448     // Store trace in the bottom bits of pointer for common kinds.
 449     uintptr_t kindBits = uintptr_t(traceKind);
 450     if (kindBits >= OutOfLineTraceKindMask) {
 451       kindBits = OutOfLineTraceKindMask;
 452     }
 453     return uintptr_t(p) | kindBits;
 454   }
 455
 456   JS::TraceKind outOfLineKind() const;
 457
 458   uintptr_t ptr;
 459 } JS_HAZ_GC_POINTER;
 460
 461 // Unwraps the given GCCellPtr, calls the functor |f| with a template argument
 462 // of the actual type of the pointer, and returns the result.
 463 template <typename F>
 464 auto MapGCThingTyped(GCCellPtr thing, F&& f) {
 465   switch (thing.kind()) {
 466 #define JS_EXPAND_DEF(name, type, _, _1) \
 467   case JS::TraceKind::name:              \
 468     return f(&thing.as<type>());
 469     JS_FOR_EACH_TRACEKIND(JS_EXPAND_DEF);
 470 #undef JS_EXPAND_DEF
 471     default:
 472       MOZ_CRASH("Invalid trace kind in MapGCThingTyped for GCCellPtr.");
 473   }
 474 }
 475
 476 // Unwraps the given GCCellPtr and calls the functor |f| with a template
 477 // argument of the actual type of the pointer. Doesn't return anything.
 478 template <typename F>
 479 void ApplyGCThingTyped(GCCellPtr thing, F&& f) {
 480   // This function doesn't do anything but is supplied for symmetry with other
 481   // MapGCThingTyped/ApplyGCThingTyped implementations that have to wrap the
 482   // functor to return a dummy value that is ignored.
 483   MapGCThingTyped(thing, f);
 484 }
 485
 486 } /* namespace JS */
 487
 488 // These are defined in the toplevel namespace instead of within JS so that
 489 // they won't shadow other operator== overloads (see bug 1456512.)
 490
 491 inline bool operator==(JS::GCCellPtr ptr1, JS::GCCellPtr ptr2) {
 492   return ptr1.asCell() == ptr2.asCell();
 493 }
 494
 495 inline bool operator!=(JS::GCCellPtr ptr1, JS::GCCellPtr ptr2) {
 496   return !(ptr1 == ptr2);
 497 }
 498
 499 namespace js {
 500 namespace gc {
 501
 502 /* static */
 503 MOZ_ALWAYS_INLINE void MarkBitmap::getMarkWordAndMask(const TenuredCell* cell,
 504                                                       ColorBit colorBit,
 505                                                       MarkBitmapWord** wordp,
 506                                                       uintptr_t* maskp) {
 507   // Note: the JIT pre-barrier trampolines inline this code. Update
 508   // MacroAssembler::emitPreBarrierFastPath code too when making changes here!
 509
 510   MOZ_ASSERT(size_t(colorBit) < MarkBitsPerCell);
 511
 512   size_t offset = uintptr_t(cell) & ChunkMask;
 513   const size_t bit = offset / CellBytesPerMarkBit + size_t(colorBit);
 514   size_t word = bit / MarkBitmapWordBits - FirstArenaAdjustmentWords;
 515   MOZ_ASSERT(word < WordCount);
 516   *wordp = &bitmap[word];
 517   *maskp = uintptr_t(1) << (bit % MarkBitmapWordBits);
 518 }
 519
 520 namespace detail {
 521
 522 static MOZ_ALWAYS_INLINE ChunkBase* GetCellChunkBase(const Cell* cell) {
 523   MOZ_ASSERT(cell);
 524   auto* chunk = reinterpret_cast<ChunkBase*>(uintptr_t(cell) & ~ChunkMask);
 525   MOZ_ASSERT(chunk->runtime);
 526   return chunk;
 527 }
 528
 529 static MOZ_ALWAYS_INLINE TenuredChunkBase* GetCellChunkBase(
 530     const TenuredCell* cell) {
 531   MOZ_ASSERT(cell);
 532   auto* chunk =
 533       reinterpret_cast<TenuredChunkBase*>(uintptr_t(cell) & ~ChunkMask);
 534   MOZ_ASSERT(chunk->runtime);
 535   return chunk;
 536 }
 537
 538 static MOZ_ALWAYS_INLINE JS::Zone* GetTenuredGCThingZone(const void* ptr) {
 539   // This takes a void* because the compiler can't see type relationships in
 540   // this header. |ptr| must be a pointer to a tenured GC thing.
 541   MOZ_ASSERT(ptr);
 542   const uintptr_t zone_addr = (uintptr_t(ptr) & ~ArenaMask) | ArenaZoneOffset;
 543   return *reinterpret_cast<JS::Zone**>(zone_addr);
 544 }
 545
 546 static MOZ_ALWAYS_INLINE bool TenuredCellIsMarkedBlack(
 547     const TenuredCell* cell) {
 548   // Return true if BlackBit is set.
 549
 550   MOZ_ASSERT(cell);
 551   MOZ_ASSERT(!js::gc::IsInsideNursery(cell));
 552
 553   MarkBitmapWord* blackWord;
 554   uintptr_t blackMask;
 555   TenuredChunkBase* chunk = GetCellChunkBase(cell);
 556   chunk->markBits.getMarkWordAndMask(cell, js::gc::ColorBit::BlackBit,
 557                                      &blackWord, &blackMask);
 558   return *blackWord & blackMask;
 559 }
 560
 561 static MOZ_ALWAYS_INLINE bool NonBlackCellIsMarkedGray(
 562     const TenuredCell* cell) {
 563   // Return true if GrayOrBlackBit is set. Callers should check BlackBit first.
 564
 565   MOZ_ASSERT(cell);
 566   MOZ_ASSERT(!js::gc::IsInsideNursery(cell));
 567   MOZ_ASSERT(!TenuredCellIsMarkedBlack(cell));
 568
 569   MarkBitmapWord* grayWord;
 570   uintptr_t grayMask;
 571   TenuredChunkBase* chunk = GetCellChunkBase(cell);
 572   chunk->markBits.getMarkWordAndMask(cell, js::gc::ColorBit::GrayOrBlackBit,
 573                                      &grayWord, &grayMask);
 574   return *grayWord & grayMask;
 575 }
 576
 577 static MOZ_ALWAYS_INLINE bool TenuredCellIsMarkedGray(const TenuredCell* cell) {
 578   return !TenuredCellIsMarkedBlack(cell) && NonBlackCellIsMarkedGray(cell);
 579 }
 580
 581 static MOZ_ALWAYS_INLINE bool CellIsMarkedGray(const Cell* cell) {
 582   MOZ_ASSERT(cell);
 583   if (js::gc::IsInsideNursery(cell)) {
 584     return false;
 585   }
 586   return TenuredCellIsMarkedGray(reinterpret_cast<const TenuredCell*>(cell));
 587 }
 588
 589 extern JS_PUBLIC_API bool CanCheckGrayBits(const TenuredCell* cell);
 590
 591 extern JS_PUBLIC_API bool CellIsMarkedGrayIfKnown(const TenuredCell* cell);
 592
 593 #ifdef DEBUG
 594 extern JS_PUBLIC_API void AssertCellIsNotGray(const Cell* cell);
 595
 596 extern JS_PUBLIC_API bool ObjectIsMarkedBlack(const JSObject* obj);
 597 #endif
 598
 599 MOZ_ALWAYS_INLINE bool CellHasStoreBuffer(const Cell* cell) {
 600   return GetCellChunkBase(cell)->storeBuffer;
 601 }
 602
 603 } /* namespace detail */
 604
 605 MOZ_ALWAYS_INLINE bool IsInsideNursery(const Cell* cell) {
 606   MOZ_ASSERT(cell);
 607   return detail::CellHasStoreBuffer(cell);
 608 }
 609
 610 MOZ_ALWAYS_INLINE bool IsInsideNursery(const TenuredCell* cell) {
 611   MOZ_ASSERT(cell);
 612   MOZ_ASSERT(!IsInsideNursery(reinterpret_cast<const Cell*>(cell)));
 613   return false;
 614 }
 615
 616 // Allow use before the compiler knows the derivation of JSObject, JSString, and
 617 // JS::BigInt.
 618 MOZ_ALWAYS_INLINE bool IsInsideNursery(const JSObject* obj) {
 619   return IsInsideNursery(reinterpret_cast<const Cell*>(obj));
 620 }
 621 MOZ_ALWAYS_INLINE bool IsInsideNursery(const JSString* str) {
 622   return IsInsideNursery(reinterpret_cast<const Cell*>(str));
 623 }
 624 MOZ_ALWAYS_INLINE bool IsInsideNursery(const JS::BigInt* bi) {
 625   return IsInsideNursery(reinterpret_cast<const Cell*>(bi));
 626 }
 627
 628 MOZ_ALWAYS_INLINE bool IsCellPointerValid(const void* ptr) {
 629   auto addr = uintptr_t(ptr);
 630   if (addr < ChunkSize || addr % CellAlignBytes != 0) {
 631     return false;
 632   }
 633
 634   auto* cell = reinterpret_cast<const Cell*>(ptr);
 635   if (!IsInsideNursery(cell)) {
 636     return detail::GetTenuredGCThingZone(cell) != nullptr;
 637   }
 638
 639   return true;
 640 }
 641
 642 MOZ_ALWAYS_INLINE bool IsCellPointerValidOrNull(const void* cell) {
 643   if (!cell) {
 644     return true;
 645   }
 646   return IsCellPointerValid(cell);
 647 }
 648
 649 } /* namespace gc */
 650 } /* namespace js */
 651
 652 namespace JS {
 653
 654 extern JS_PUBLIC_API Zone* GetTenuredGCThingZone(GCCellPtr thing);
 655
 656 extern JS_PUBLIC_API Zone* GetNurseryCellZone(js::gc::Cell* cell);
 657
 658 static MOZ_ALWAYS_INLINE Zone* GetGCThingZone(GCCellPtr thing) {
 659   if (!js::gc::IsInsideNursery(thing.asCell())) {
 660     return js::gc::detail::GetTenuredGCThingZone(thing.asCell());
 661   }
 662
 663   return GetNurseryCellZone(thing.asCell());
 664 }
 665
 666 static MOZ_ALWAYS_INLINE Zone* GetStringZone(JSString* str) {
 667   if (!js::gc::IsInsideNursery(str)) {
 668     return js::gc::detail::GetTenuredGCThingZone(str);
 669   }
 670
 671   return GetNurseryCellZone(reinterpret_cast<js::gc::Cell*>(str));
 672 }
 673
 674 extern JS_PUBLIC_API Zone* GetObjectZone(JSObject* obj);
 675
 676 static MOZ_ALWAYS_INLINE bool GCThingIsMarkedGray(GCCellPtr thing) {
 677   js::gc::Cell* cell = thing.asCell();
 678   if (IsInsideNursery(cell)) {
 679     return false;
 680   }
 681
 682   auto* tenuredCell = reinterpret_cast<js::gc::TenuredCell*>(cell);
 683   return js::gc::detail::CellIsMarkedGrayIfKnown(tenuredCell);
 684 }
 685
 686 // Specialised gray marking check for use by the cycle collector. This is not
 687 // called during incremental GC or when the gray bits are invalid.
 688 static MOZ_ALWAYS_INLINE bool GCThingIsMarkedGrayInCC(GCCellPtr thing) {
 689   js::gc::Cell* cell = thing.asCell();
 690   if (IsInsideNursery(cell)) {
 691     return false;
 692   }
 693
 694   auto* tenuredCell = reinterpret_cast<js::gc::TenuredCell*>(cell);
 695   if (!js::gc::detail::TenuredCellIsMarkedGray(tenuredCell)) {
 696     return false;
 697   }
 698
 699   MOZ_ASSERT(js::gc::detail::CanCheckGrayBits(tenuredCell));
 700
 701   return true;
 702 }
 703
 704 extern JS_PUBLIC_API JS::TraceKind GCThingTraceKind(void* thing);
 705
 706 extern JS_PUBLIC_API void EnableNurseryStrings(JSContext* cx);
 707
 708 extern JS_PUBLIC_API void DisableNurseryStrings(JSContext* cx);
 709
 710 extern JS_PUBLIC_API void EnableNurseryBigInts(JSContext* cx);
 711
 712 extern JS_PUBLIC_API void DisableNurseryBigInts(JSContext* cx);
 713
 714 /*
 715  * Returns true when writes to GC thing pointers (and reads from weak pointers)
 716  * must call an incremental barrier. This is generally only true when running
 717  * mutator code in-between GC slices. At other times, the barrier may be elided
 718  * for performance.
 719  */
 720 extern JS_PUBLIC_API bool IsIncrementalBarrierNeeded(JSContext* cx);
 721
 722 /*
 723  * Notify the GC that a reference to a JSObject is about to be overwritten.
 724  * This method must be called if IsIncrementalBarrierNeeded.
 725  */
 726 extern JS_PUBLIC_API void IncrementalPreWriteBarrier(JSObject* obj);
 727
 728 /*
 729  * Notify the GC that a reference to a tenured GC cell is about to be
 730  * overwritten. This method must be called if IsIncrementalBarrierNeeded.
 731  */
 732 extern JS_PUBLIC_API void IncrementalPreWriteBarrier(GCCellPtr thing);
 733
 734 /**
 735  * Unsets the gray bit for anything reachable from |thing|. |kind| should not be
 736  * JS::TraceKind::Shape. |thing| should be non-null. The return value indicates
 737  * if anything was unmarked.
 738  */
 739 extern JS_PUBLIC_API bool UnmarkGrayGCThingRecursively(GCCellPtr thing);
 740
 741 }  // namespace JS
 742
 743 namespace js {
 744 namespace gc {
 745
 746 extern JS_PUBLIC_API void PerformIncrementalReadBarrier(JS::GCCellPtr thing);
 747
 748 static MOZ_ALWAYS_INLINE void ExposeGCThingToActiveJS(JS::GCCellPtr thing) {
 749   // TODO: I'd like to assert !RuntimeHeapIsBusy() here but this gets
 750   // called while we are tracing the heap, e.g. during memory reporting
 751   // (see bug 1313318).
 752   MOZ_ASSERT(!JS::RuntimeHeapIsCollecting());
 753
 754   // GC things residing in the nursery cannot be gray: they have no mark bits.
 755   // All live objects in the nursery are moved to tenured at the beginning of
 756   // each GC slice, so the gray marker never sees nursery things.
 757   if (IsInsideNursery(thing.asCell())) {
 758     return;
 759   }
 760
 761   auto* cell = reinterpret_cast<TenuredCell*>(thing.asCell());
 762   if (detail::TenuredCellIsMarkedBlack(cell)) {
 763     return;
 764   }
 765
 766   // GC things owned by other runtimes are always black.
 767   MOZ_ASSERT(!thing.mayBeOwnedByOtherRuntime());
 768
 769   auto* zone = JS::shadow::Zone::from(detail::GetTenuredGCThingZone(cell));
 770   if (zone->needsIncrementalBarrier()) {
 771     PerformIncrementalReadBarrier(thing);
 772   } else if (!zone->isGCPreparing() && detail::NonBlackCellIsMarkedGray(cell)) {
 773     MOZ_ALWAYS_TRUE(JS::UnmarkGrayGCThingRecursively(thing));
 774   }
 775
 776   MOZ_ASSERT_IF(!zone->isGCPreparing(), !detail::TenuredCellIsMarkedGray(cell));
 777 }
 778
 779 static MOZ_ALWAYS_INLINE void IncrementalReadBarrier(JS::GCCellPtr thing) {
 780   // This is a lighter version of ExposeGCThingToActiveJS that doesn't do gray
 781   // unmarking.
 782
 783   if (IsInsideNursery(thing.asCell())) {
 784     return;
 785   }
 786
 787   auto* cell = reinterpret_cast<TenuredCell*>(thing.asCell());
 788   auto* zone = JS::shadow::Zone::from(detail::GetTenuredGCThingZone(cell));
 789   if (zone->needsIncrementalBarrier() &&
 790       !detail::TenuredCellIsMarkedBlack(cell)) {
 791     // GC things owned by other runtimes are always black.
 792     MOZ_ASSERT(!thing.mayBeOwnedByOtherRuntime());
 793     PerformIncrementalReadBarrier(thing);
 794   }
 795 }
 796
 797 template <typename T>
 798 extern JS_PUBLIC_API bool EdgeNeedsSweepUnbarrieredSlow(T* thingp);
 799
 800 static MOZ_ALWAYS_INLINE bool EdgeNeedsSweepUnbarriered(JSObject** objp) {
 801   // This function does not handle updating nursery pointers. Raw JSObject
 802   // pointers should be updated separately or replaced with
 803   // JS::Heap<JSObject*> which handles this automatically.
 804   MOZ_ASSERT(!JS::RuntimeHeapIsMinorCollecting());
 805   if (IsInsideNursery(*objp)) {
 806     return false;
 807   }
 808
 809   auto zone = JS::shadow::Zone::from(detail::GetTenuredGCThingZone(*objp));
 810   if (!zone->isGCSweepingOrCompacting()) {
 811     return false;
 812   }
 813
 814   return EdgeNeedsSweepUnbarrieredSlow(objp);
 815 }
 816
 817 }  // namespace gc
 818 }  // namespace js
 819
 820 namespace JS {
 821
 822 /*
 823  * This should be called when an object that is marked gray is exposed to the JS
 824  * engine (by handing it to running JS code or writing it into live JS
 825  * data). During incremental GC, since the gray bits haven't been computed yet,
 826  * we conservatively mark the object black.
 827  */
 828 static MOZ_ALWAYS_INLINE void ExposeObjectToActiveJS(JSObject* obj) {
 829   MOZ_ASSERT(obj);
 830   MOZ_ASSERT(!js::gc::EdgeNeedsSweepUnbarrieredSlow(&obj));
 831   js::gc::ExposeGCThingToActiveJS(GCCellPtr(obj));
 832 }
 833
 834 } /* namespace JS */
 835
 836 #endif /* js_HeapAPI_h */