DXR is a code search and navigation tool aimed at making sense of large projects. It supports full-text and regex searches as well as structural queries.

Implementation

Mercurial (27a812186ff4)

VCS Links

Line Code
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125 126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141 142 143 144 145 146 147 148 149 150 151 152 153 154 155 156 157 158 159 160 161 162 163 164 165 166 167 168 169 170 171 172 173 174 175 176 177 178 179 180 181 182 183 184 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241 242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320 321 322 323 324 325 326 327 328 329 330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352 353 354 355 356 357 358 359 360 361 362 363 364 365 366 367 368 369 370 371 372 373 374 375 376 377 378 379 380 381 382 383 384 385 386 387 388 389 390 391 392 393 394 395 396 397 398 399 400 401 402 403 404 405 406 407 408 409 410 411 412 413 414 415 416 417 418 419 420 421 422 423 424 425 426 427 428 429 430 431 432 433 434 435 436 437 438 439 440 441 442 443 444 445 446 447 448 449 450 451 452 453 454 455 456 457 458 459 460 461 462 463 464 465 466 467 468 469 470 471 472 473 474 475 476 477 478 479 480 481 482 483 484 485 486 487 488 489 490 491 492 493 494 495 496 497 498 499 500 501 502 503 504 505 506 507 508 509 510 511 512 513 514 515 516 517 518 519 520 521 522 523 524 525 526 527 528 529 530 531 532 533 534 535 536 537 538 539 540 541 542 543 544 545 546 547 548 549 550 551 552 553 554 555 556 557 558 559 560 561 562 563 564 565 566 567 568 569 570 571 572 573 574 575 576 577 578 579 580 581 582 583 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 635 636 637 638 639 640 641 642 643 644 645 646 647 648 649 650 651 652 653 654 655 656 657 658 659 660 661 662 663 664 665 666 667 668 669 670 671 672 673 674 675 676 677 678 679 680 681 682 683 684 685 686 687 688 689 690 691 692 693 694 695 696 697 698 699 700 701 702 703 704 705 706 707 708 709 710 711 712 713 714 715 716 717 718 719 720 721 722 723 724 725 726 727 728 729 730 731 732 733 734 735 736 737 738 739 740 741 742 743 744 745 746 747 748 749 750 751 752 753 754 755 756 757 758 759 760 761 762 763 764 765 766 767 768 769 770 771 772 773 774 775 776 777 778 779 780 781 782 783 784 785 786 787 788 789 790 791 792 793 794 795 796 797 798 799 800 801 802 803 804 805 806 807 808 809 810 811 812 813 814 815 816 817 818 819 820 821 822 823 824 825 826 827 828 829 830 831 832 833 834 835 836 837 838 839 840 841 842 843 844 845 846 847 848 849 850 851 852 853 854 855 856 857 858 859 860 861 862 863 864 865 866 867 868 869 870 871 872 873 874 875 876 877 878 879 880 881 882 883 884 885 886 887 888 889 890 891 892 893 894 895 896 897 898 899 900 901 902 903 904 905 906 907 908 909 910 911 912 913 914 915 916 917 918 919 920 921 922 923 924 925 926 927 928 929 930 931 932 933 934 935 936 937 938 939 940 941 942 943 944 945 946 947 948 949 950 951 952 953 954 955 956 957 958 959 960 961 962 963 964 965 966 967 968 969 970 971 972 973 974 975 976 977 978 979 980 981 982 983 984 985 986 987 988 989 990 991 992 993 994 995 996 997 998 999 1000 1001 1002 1003 1004 1005 1006 1007 1008 1009 1010 1011 1012 1013 1014 1015 1016 1017 1018 1019 1020 1021 1022 1023 1024 1025 1026 1027 1028 1029 1030 1031 1032 1033 1034 1035 1036 1037 1038 1039 1040 1041 1042 1043 1044 1045 1046 1047 1048 1049 1050 1051 1052 1053 1054 1055 1056 1057 1058 1059 1060 1061 1062 1063 1064 1065 1066 1067 1068 1069 1070 1071 1072 1073 1074 1075 1076 1077 1078 1079 1080 1081 1082 1083 1084 1085 1086 1087 1088 1089 1090 1091 1092 1093 1094 1095 1096 1097 1098 1099 1100 1101 1102 1103 1104 1105 1106 1107 1108 1109 1110 1111 1112 1113 1114 1115 1116 1117 1118 1119 1120 1121 1122 1123 1124 1125 1126 1127 1128 1129 1130 1131 1132 1133 1134 1135 1136 1137 1138 1139 1140 1141 1142 1143 1144 1145 1146 1147 1148 1149 1150 1151 1152 1153 1154 1155 1156 1157 1158 1159 1160 1161 1162 1163 1164 1165 1166 1167 1168 1169 1170 1171 1172 1173 1174 1175 1176 1177 1178 1179 1180 1181 1182 1183 1184 1185 1186 1187 1188 1189 1190 1191 1192 1193 1194 1195 1196 1197 1198 1199 1200 1201 1202 1203 1204 1205 1206 1207 1208 1209 1210 1211 1212 1213 1214 1215 1216 1217 1218 1219 1220 1221 1222 1223 1224 1225 1226 1227 1228 1229 1230 1231 1232 1233 1234 1235 1236 1237 1238 1239 1240 1241 1242 1243 1244 1245 1246 1247 1248 1249
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * vim: set ts=8 sts=4 et sw=4 tw=99:
 * This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

#ifndef gc_Barrier_h
#define gc_Barrier_h

#include "NamespaceImports.h"

#include "gc/Heap.h"
#ifdef JSGC_GENERATIONAL
# include "gc/StoreBuffer.h"
#endif
#include "js/HashTable.h"
#include "js/Id.h"
#include "js/RootingAPI.h"

/*
 * A write barrier is a mechanism used by incremental or generation GCs to
 * ensure that every value that needs to be marked is marked. In general, the
 * write barrier should be invoked whenever a write can cause the set of things
 * traced through by the GC to change. This includes:
 *   - writes to object properties
 *   - writes to array slots
 *   - writes to fields like JSObject::shape_ that we trace through
 *   - writes to fields in private data, like JSGenerator::obj
 *   - writes to non-markable fields like JSObject::private that point to
 *     markable data
 * The last category is the trickiest. Even though the private pointers does not
 * point to a GC thing, changing the private pointer may change the set of
 * objects that are traced by the GC. Therefore it needs a write barrier.
 *
 * Every barriered write should have the following form:
 *   <pre-barrier>
 *   obj->field = value; // do the actual write
 *   <post-barrier>
 * The pre-barrier is used for incremental GC and the post-barrier is for
 * generational GC.
 *
 *                               PRE-BARRIER
 *
 * To understand the pre-barrier, let's consider how incremental GC works. The
 * GC itself is divided into "slices". Between each slice, JS code is allowed to
 * run. Each slice should be short so that the user doesn't notice the
 * interruptions. In our GC, the structure of the slices is as follows:
 *
 * 1. ... JS work, which leads to a request to do GC ...
 * 2. [first GC slice, which performs all root marking and possibly more marking]
 * 3. ... more JS work is allowed to run ...
 * 4. [GC mark slice, which runs entirely in drainMarkStack]
 * 5. ... more JS work ...
 * 6. [GC mark slice, which runs entirely in drainMarkStack]
 * 7. ... more JS work ...
 * 8. [GC marking finishes; sweeping done non-incrementally; GC is done]
 * 9. ... JS continues uninterrupted now that GC is finishes ...
 *
 * Of course, there may be a different number of slices depending on how much
 * marking is to be done.
 *
 * The danger inherent in this scheme is that the JS code in steps 3, 5, and 7
 * might change the heap in a way that causes the GC to collect an object that
 * is actually reachable. The write barrier prevents this from happening. We use
 * a variant of incremental GC called "snapshot at the beginning." This approach
 * guarantees the invariant that if an object is reachable in step 2, then we
 * will mark it eventually. The name comes from the idea that we take a
 * theoretical "snapshot" of all reachable objects in step 2; all objects in
 * that snapshot should eventually be marked. (Note that the write barrier
 * verifier code takes an actual snapshot.)
 *
 * The basic correctness invariant of a snapshot-at-the-beginning collector is
 * that any object reachable at the end of the GC (step 9) must either:
 *   (1) have been reachable at the beginning (step 2) and thus in the snapshot
 *   (2) or must have been newly allocated, in steps 3, 5, or 7.
 * To deal with case (2), any objects allocated during an incremental GC are
 * automatically marked black.
 *
 * This strategy is actually somewhat conservative: if an object becomes
 * unreachable between steps 2 and 8, it would be safe to collect it. We won't,
 * mainly for simplicity. (Also, note that the snapshot is entirely
 * theoretical. We don't actually do anything special in step 2 that we wouldn't
 * do in a non-incremental GC.
 *
 * It's the pre-barrier's job to maintain the snapshot invariant. Consider the
 * write "obj->field = value". Let the prior value of obj->field be
 * value0. Since it's possible that value0 may have been what obj->field
 * contained in step 2, when the snapshot was taken, the barrier marks
 * value0. Note that it only does this if we're in the middle of an incremental
 * GC. Since this is rare, the cost of the write barrier is usually just an
 * extra branch.
 *
 * In practice, we implement the pre-barrier differently based on the type of
 * value0. E.g., see JSObject::writeBarrierPre, which is used if obj->field is
 * a JSObject*. It takes value0 as a parameter.
 *
 *                                POST-BARRIER
 *
 * For generational GC, we want to be able to quickly collect the nursery in a
 * minor collection.  Part of the way this is achieved is to only mark the
 * nursery itself; tenured things, which may form the majority of the heap, are
 * not traced through or marked.  This leads to the problem of what to do about
 * tenured objects that have pointers into the nursery: if such things are not
 * marked, they may be discarded while there are still live objects which
 * reference them. The solution is to maintain information about these pointers,
 * and mark their targets when we start a minor collection.
 *
 * The pointers can be thoughs of as edges in object graph, and the set of edges
 * from the tenured generation into the nursery is know as the remembered set.
 * Post barriers are used to track this remembered set.
 *
 * Whenever a slot which could contain such a pointer is written, we use a write
 * barrier to check if the edge created is in the remembered set, and if so we
 * insert it into the store buffer, which is the collector's representation of
 * the remembered set.  This means than when we come to do a minor collection we
 * can examine the contents of the store buffer and mark any edge targets that
 * are in the nursery.
 *
 *                            IMPLEMENTATION DETAILS
 *
 * Since it would be awkward to change every write to memory into a function
 * call, this file contains a bunch of C++ classes and templates that use
 * operator overloading to take care of barriers automatically. In many cases,
 * all that's necessary to make some field be barriered is to replace
 *     Type* field;
 * with
 *     HeapPtr<Type> field;
 * There are also special classes HeapValue and HeapId, which barrier js::Value
 * and jsid, respectively.
 *
 * One additional note: not all object writes need to be barriered. Writes to
 * newly allocated objects do not need a pre-barrier.  In these cases, we use
 * the "obj->field.init(value)" method instead of "obj->field = value". We use
 * the init naming idiom in many places to signify that a field is being
 * assigned for the first time.
 *
 * For each of pointers, Values and jsids this file implements four classes,
 * illustrated here for the pointer (Ptr) classes:
 *
 * BarrieredPtr           abstract base class which provides common operations
 *  |  |  |
 *  |  | EncapsulatedPtr  provides pre-barriers only
 *  |  |
 *  | HeapPtr             provides pre- and post-barriers
 *  |
 * RelocatablePtr         provides pre- and post-barriers and is relocatable
 *
 * These classes are designed to be used by the internals of the JS engine.
 * Barriers designed to be used externally are provided in
 * js/public/RootingAPI.h.
 */

namespace js {

class PropertyName;

#ifdef DEBUG
bool
RuntimeFromMainThreadIsHeapMajorCollecting(JS::shadow::Zone* shadowZone);
#endif

namespace gc {

template <typename T>
void
MarkUnbarriered(JSTracer* trc, T** thingp, const char* name);

// Direct value access used by the write barriers and the jits.
void
MarkValueUnbarriered(JSTracer* trc, Value* v, const char* name);

// These two declarations are also present in gc/Marking.h, via the DeclMarker
// macro.  Not great, but hard to avoid.
void
MarkObjectUnbarriered(JSTracer* trc, JSObject** obj, const char* name);
void
MarkStringUnbarriered(JSTracer* trc, JSString** str, const char* name);

// Note that some subclasses (e.g. ObjectImpl) specialize some of these
// methods.
template <typename T>
class BarrieredCell : public gc::Cell
{
  public:
    MOZ_ALWAYS_INLINE JS::Zone* zone() const { return tenuredZone(); }
    MOZ_ALWAYS_INLINE JS::shadow::Zone* shadowZone() const { return JS::shadow::Zone::asShadowZone(zone()); }
    MOZ_ALWAYS_INLINE JS::Zone* zoneFromAnyThread() const { return tenuredZoneFromAnyThread(); }
    MOZ_ALWAYS_INLINE JS::shadow::Zone* shadowZoneFromAnyThread() const {
        return JS::shadow::Zone::asShadowZone(zoneFromAnyThread());
    }

    static MOZ_ALWAYS_INLINE void readBarrier(T* thing) {
#ifdef JSGC_INCREMENTAL
        JS::shadow::Zone* shadowZone = thing->shadowZoneFromAnyThread();
        if (shadowZone->needsBarrier()) {
            MOZ_ASSERT(!RuntimeFromMainThreadIsHeapMajorCollecting(shadowZone));
            T* tmp = thing;
            js::gc::MarkUnbarriered<T>(shadowZone->barrierTracer(), &tmp, "read barrier");
            JS_ASSERT(tmp == thing);
        }
#endif
    }

    static MOZ_ALWAYS_INLINE bool needWriteBarrierPre(JS::Zone* zone) {
#ifdef JSGC_INCREMENTAL
        return JS::shadow::Zone::asShadowZone(zone)->needsBarrier();
#else
        return false;
#endif
    }

    static MOZ_ALWAYS_INLINE bool isNullLike(T* thing) { return !thing; }

    static MOZ_ALWAYS_INLINE void writeBarrierPre(T* thing) {
#ifdef JSGC_INCREMENTAL
        if (isNullLike(thing) || !thing->shadowRuntimeFromAnyThread()->needsBarrier())
            return;

        JS::shadow::Zone* shadowZone = thing->shadowZoneFromAnyThread();
        if (shadowZone->needsBarrier()) {
            MOZ_ASSERT(!RuntimeFromMainThreadIsHeapMajorCollecting(shadowZone));
            T* tmp = thing;
            js::gc::MarkUnbarriered<T>(shadowZone->barrierTracer(), &tmp, "write barrier");
            JS_ASSERT(tmp == thing);
        }
#endif
    }

    static void writeBarrierPost(T* thing, void* addr) {}
    static void writeBarrierPostRelocate(T* thing, void* addr) {}
    static void writeBarrierPostRemove(T* thing, void* addr) {}
};

} // namespace gc

// Note: the following Zone-getting functions must be equivalent to the zone()
// and shadowZone() functions implemented by the subclasses of BarrieredCell.

JS::Zone*
ZoneOfObject(const JSObject& obj);

static inline JS::shadow::Zone*
ShadowZoneOfObject(JSObject* obj)
{
    return JS::shadow::Zone::asShadowZone(ZoneOfObject(*obj));
}

static inline JS::shadow::Zone*
ShadowZoneOfString(JSString* str)
{
    return JS::shadow::Zone::asShadowZone(reinterpret_cast<const js::gc::Cell*>(str)->tenuredZone());
}

MOZ_ALWAYS_INLINE JS::Zone*
ZoneOfValue(const JS::Value& value)
{
    JS_ASSERT(value.isMarkable());
    if (value.isObject())
        return ZoneOfObject(value.toObject());
    return static_cast<js::gc::Cell*>(value.toGCThing())->tenuredZone();
}

JS::Zone*
ZoneOfObjectFromAnyThread(const JSObject& obj);

static inline JS::shadow::Zone*
ShadowZoneOfObjectFromAnyThread(JSObject* obj)
{
    return JS::shadow::Zone::asShadowZone(ZoneOfObjectFromAnyThread(*obj));
}

static inline JS::shadow::Zone*
ShadowZoneOfStringFromAnyThread(JSString* str)
{
    return JS::shadow::Zone::asShadowZone(
        reinterpret_cast<const js::gc::Cell*>(str)->tenuredZoneFromAnyThread());
}

MOZ_ALWAYS_INLINE JS::Zone*
ZoneOfValueFromAnyThread(const JS::Value& value)
{
    JS_ASSERT(value.isMarkable());
    if (value.isObject())
        return ZoneOfObjectFromAnyThread(value.toObject());
    return static_cast<js::gc::Cell*>(value.toGCThing())->tenuredZoneFromAnyThread();
}

/*
 * Base class for barriered pointer types.
 */
template <class T, typename Unioned = uintptr_t>
class BarrieredPtr
{
  protected:
    union {
        T* value;
        Unioned other;
    };

    BarrieredPtr(T* v) : value(v) {}
    ~BarrieredPtr() { pre(); }

  public:
    void init(T* v) {
        JS_ASSERT(!IsPoisonedPtr<T>(v));
        this->value = v;
    }

    /* Use this if the automatic coercion to T* isn't working. */
    T* get() const { return value; }

    /*
     * Use these if you want to change the value without invoking the barrier.
     * Obviously this is dangerous unless you know the barrier is not needed.
     */
    T** unsafeGet() { return &value; }
    void unsafeSet(T* v) { value = v; }

    Unioned* unsafeGetUnioned() { return &other; }

    T& operator*() const { return *value; }
    T* operator->() const { return value; }

    operator T*() const { return value; }

  protected:
    void pre() { T::writeBarrierPre(value); }
};

/*
 * EncapsulatedPtr only automatically handles pre-barriers. Post-barriers must
 * be manually implemented when using this class. HeapPtr and RelocatablePtr
 * should be used in all cases that do not require explicit low-level control
 * of moving behavior, e.g. for HashMap keys.
 */
template <class T, typename Unioned = uintptr_t>
class EncapsulatedPtr : public BarrieredPtr<T, Unioned>
{
  public:
    EncapsulatedPtr() : BarrieredPtr<T, Unioned>(nullptr) {}
    EncapsulatedPtr(T* v) : BarrieredPtr<T, Unioned>(v) {}
    explicit EncapsulatedPtr(const EncapsulatedPtr<T, Unioned>& v)
      : BarrieredPtr<T, Unioned>(v.value) {}

    /* Use to set the pointer to nullptr. */
    void clear() {
        this->pre();
        this->value = nullptr;
    }

    EncapsulatedPtr<T, Unioned>& operator=(T* v) {
        this->pre();
        JS_ASSERT(!IsPoisonedPtr<T>(v));
        this->value = v;
        return *this;
    }

    EncapsulatedPtr<T, Unioned>& operator=(const EncapsulatedPtr<T>& v) {
        this->pre();
        JS_ASSERT(!IsPoisonedPtr<T>(v.value));
        this->value = v.value;
        return *this;
    }
};

/*
 * A pre- and post-barriered heap pointer, for use inside the JS engine.
 *
 * Not to be confused with JS::Heap<T>. This is a different class from the
 * external interface and implements substantially different semantics.
 *
 * The post-barriers implemented by this class are faster than those
 * implemented by RelocatablePtr<T> or JS::Heap<T> at the cost of not
 * automatically handling deletion or movement. It should generally only be
 * stored in memory that has GC lifetime. HeapPtr must not be used in contexts
 * where it may be implicitly moved or deleted, e.g. most containers.
 */
template <class T, class Unioned = uintptr_t>
class HeapPtr : public BarrieredPtr<T, Unioned>
{
  public:
    HeapPtr() : BarrieredPtr<T, Unioned>(nullptr) {}
    explicit HeapPtr(T* v) : BarrieredPtr<T, Unioned>(v) { post(); }
    explicit HeapPtr(const HeapPtr<T, Unioned>& v) : BarrieredPtr<T, Unioned>(v) { post(); }

    void init(T* v) {
        JS_ASSERT(!IsPoisonedPtr<T>(v));
        this->value = v;
        post();
    }

    HeapPtr<T, Unioned>& operator=(T* v) {
        this->pre();
        JS_ASSERT(!IsPoisonedPtr<T>(v));
        this->value = v;
        post();
        return *this;
    }

    HeapPtr<T, Unioned>& operator=(const HeapPtr<T, Unioned>& v) {
        this->pre();
        JS_ASSERT(!IsPoisonedPtr<T>(v.value));
        this->value = v.value;
        post();
        return *this;
    }

  protected:
    void post() { T::writeBarrierPost(this->value, (void*)&this->value); }

    /* Make this friend so it can access pre() and post(). */
    template <class T1, class T2>
    friend inline void
    BarrieredSetPair(Zone* zone,
                     HeapPtr<T1>& v1, T1* val1,
                     HeapPtr<T2>& v2, T2* val2);

  private:
    /*
     * Unlike RelocatablePtr<T>, HeapPtr<T> must be managed with GC lifetimes.
     * Specifically, the memory used by the pointer itself must be live until
     * at least the next minor GC. For that reason, move semantics are invalid
     * and are deleted here. Please note that not all containers support move
     * semantics, so this does not completely prevent invalid uses.
     */
    HeapPtr(HeapPtr<T>&&) MOZ_DELETE;
    HeapPtr<T, Unioned>& operator=(HeapPtr<T, Unioned>&&) MOZ_DELETE;
};

/*
 * FixedHeapPtr is designed for one very narrow case: replacing immutable raw
 * pointers to GC-managed things, implicitly converting to a handle type for
 * ease of use.  Pointers encapsulated by this type must:
 *
 *   be immutable (no incremental write barriers),
 *   never point into the nursery (no generational write barriers), and
 *   be traced via MarkRuntime (we use fromMarkedLocation).
 *
 * In short: you *really* need to know what you're doing before you use this
 * class!
 */
template <class T>
class FixedHeapPtr
{
    T* value;

  public:
    operator T*() const { return value; }
    T * operator->() const { return value; }

    operator Handle<T*>() const {
        return Handle<T*>::fromMarkedLocation(&value);
    }

    void init(T* ptr) {
        value = ptr;
    }
};

/*
 * A pre- and post-barriered heap pointer, for use inside the JS engine.
 *
 * Unlike HeapPtr<T>, it can be used in memory that is not managed by the GC,
 * i.e. in C++ containers.  It is, however, somewhat slower, so should only be
 * used in contexts where this ability is necessary.
 */
template <class T>
class RelocatablePtr : public BarrieredPtr<T>
{
  public:
    RelocatablePtr() : BarrieredPtr<T>(nullptr) {}
    explicit RelocatablePtr(T* v) : BarrieredPtr<T>(v) {
        if (v)
            post();
    }

    /*
     * For RelocatablePtr, move semantics are equivalent to copy semantics. In
     * C++, a copy constructor taking const-ref is the way to get a single
     * function that will be used for both lvalue and rvalue copies, so we can
     * simply omit the rvalue variant.
     */
    RelocatablePtr(const RelocatablePtr<T>& v) : BarrieredPtr<T>(v) {
        if (this->value)
            post();
    }

    ~RelocatablePtr() {
        if (this->value)
            relocate();
    }

    RelocatablePtr<T>& operator=(T* v) {
        this->pre();
        JS_ASSERT(!IsPoisonedPtr<T>(v));
        if (v) {
            this->value = v;
            post();
        } else if (this->value) {
            relocate();
            this->value = v;
        }
        return *this;
    }

    RelocatablePtr<T>& operator=(const RelocatablePtr<T>& v) {
        this->pre();
        JS_ASSERT(!IsPoisonedPtr<T>(v.value));
        if (v.value) {
            this->value = v.value;
            post();
        } else if (this->value) {
            relocate();
            this->value = v;
        }
        return *this;
    }

  protected:
    void post() {
#ifdef JSGC_GENERATIONAL
        JS_ASSERT(this->value);
        T::writeBarrierPostRelocate(this->value, &this->value);
#endif
    }

    void relocate() {
#ifdef JSGC_GENERATIONAL
        JS_ASSERT(this->value);
        T::writeBarrierPostRemove(this->value, &this->value);
#endif
    }
};

/*
 * This is a hack for RegExpStatics::updateFromMatch. It allows us to do two
 * barriers with only one branch to check if we're in an incremental GC.
 */
template <class T1, class T2>
static inline void
BarrieredSetPair(Zone* zone,
                 HeapPtr<T1>& v1, T1* val1,
                 HeapPtr<T2>& v2, T2* val2)
{
    if (T1::needWriteBarrierPre(zone)) {
        v1.pre();
        v2.pre();
    }
    v1.unsafeSet(val1);
    v2.unsafeSet(val2);
    v1.post();
    v2.post();
}

class Shape;
class BaseShape;
namespace types { struct TypeObject; }

typedef BarrieredPtr<JSObject> BarrieredPtrObject;
typedef BarrieredPtr<JSScript> BarrieredPtrScript;

typedef EncapsulatedPtr<JSObject> EncapsulatedPtrObject;
typedef EncapsulatedPtr<JSScript> EncapsulatedPtrScript;

typedef RelocatablePtr<JSObject> RelocatablePtrObject;
typedef RelocatablePtr<JSScript> RelocatablePtrScript;

typedef HeapPtr<JSObject> HeapPtrObject;
typedef HeapPtr<JSFunction> HeapPtrFunction;
typedef HeapPtr<JSString> HeapPtrString;
typedef HeapPtr<PropertyName> HeapPtrPropertyName;
typedef HeapPtr<JSScript> HeapPtrScript;
typedef HeapPtr<Shape> HeapPtrShape;
typedef HeapPtr<BaseShape> HeapPtrBaseShape;
typedef HeapPtr<types::TypeObject> HeapPtrTypeObject;

/* Useful for hashtables with a HeapPtr as key. */

template <class T>
struct HeapPtrHasher
{
    typedef HeapPtr<T> Key;
    typedef T* Lookup;

    static HashNumber hash(Lookup obj) { return DefaultHasher<T*>::hash(obj); }
    static bool match(const Key& k, Lookup l) { return k.get() == l; }
    static void rekey(Key& k, const Key& newKey) { k.unsafeSet(newKey); }
};

/* Specialized hashing policy for HeapPtrs. */
template <class T>
struct DefaultHasher< HeapPtr<T> > : HeapPtrHasher<T> { };

template <class T>
struct EncapsulatedPtrHasher
{
    typedef EncapsulatedPtr<T> Key;
    typedef T* Lookup;

    static HashNumber hash(Lookup obj) { return DefaultHasher<T*>::hash(obj); }
    static bool match(const Key& k, Lookup l) { return k.get() == l; }
    static void rekey(Key& k, const Key& newKey) { k.unsafeSet(newKey); }
};

template <class T>
struct DefaultHasher< EncapsulatedPtr<T> > : EncapsulatedPtrHasher<T> { };

bool
StringIsPermanentAtom(JSString* str);

/*
 * Base class for barriered value types.
 */
class BarrieredValue : public ValueOperations<BarrieredValue>
{
  protected:
    Value value;

    /*
     * Ensure that EncapsulatedValue is not constructable, except by our
     * implementations.
     */
    BarrieredValue() MOZ_DELETE;

    BarrieredValue(const Value& v) : value(v) {
        JS_ASSERT(!IsPoisonedValue(v));
    }

    ~BarrieredValue() {
        pre();
    }

  public:
    void init(const Value& v) {
        JS_ASSERT(!IsPoisonedValue(v));
        value = v;
    }
    void init(JSRuntime* rt, const Value& v) {
        JS_ASSERT(!IsPoisonedValue(v));
        value = v;
    }

    bool operator==(const BarrieredValue& v) const { return value == v.value; }
    bool operator!=(const BarrieredValue& v) const { return value != v.value; }

    const Value& get() const { return value; }
    Value* unsafeGet() { return &value; }
    operator const Value&() const { return value; }

    JSGCTraceKind gcKind() const { return value.gcKind(); }

    uint64_t asRawBits() const { return value.asRawBits(); }

    static void writeBarrierPre(const Value& v) {
#ifdef JSGC_INCREMENTAL
        if (v.isMarkable() && shadowRuntimeFromAnyThread(v)->needsBarrier())
            writeBarrierPre(ZoneOfValueFromAnyThread(v), v);
#endif
    }

    static void writeBarrierPre(Zone* zone, const Value& v) {
#ifdef JSGC_INCREMENTAL
        if (v.isString() && StringIsPermanentAtom(v.toString()))
            return;
        JS::shadow::Zone* shadowZone = JS::shadow::Zone::asShadowZone(zone);
        if (shadowZone->needsBarrier()) {
            JS_ASSERT_IF(v.isMarkable(), shadowRuntimeFromMainThread(v)->needsBarrier());
            Value tmp(v);
            js::gc::MarkValueUnbarriered(shadowZone->barrierTracer(), &tmp, "write barrier");
            JS_ASSERT(tmp == v);
        }
#endif
    }

  protected:
    void pre() { writeBarrierPre(value); }
    void pre(Zone* zone) { writeBarrierPre(zone, value); }

    static JSRuntime* runtimeFromMainThread(const Value& v) {
        JS_ASSERT(v.isMarkable());
        return static_cast<js::gc::Cell*>(v.toGCThing())->runtimeFromMainThread();
    }
    static JSRuntime* runtimeFromAnyThread(const Value& v) {
        JS_ASSERT(v.isMarkable());
        return static_cast<js::gc::Cell*>(v.toGCThing())->runtimeFromAnyThread();
    }
    static JS::shadow::Runtime* shadowRuntimeFromMainThread(const Value& v) {
        return reinterpret_cast<JS::shadow::Runtime*>(runtimeFromMainThread(v));
    }
    static JS::shadow::Runtime* shadowRuntimeFromAnyThread(const Value& v) {
        return reinterpret_cast<JS::shadow::Runtime*>(runtimeFromAnyThread(v));
    }

  private:
    friend class ValueOperations<BarrieredValue>;
    const Value * extract() const { return &value; }
};

// Like EncapsulatedPtr, but specialized for Value.
// See the comments on that class for details.
class EncapsulatedValue : public BarrieredValue
{
  public:
    EncapsulatedValue(const Value& v) : BarrieredValue(v) {}
    EncapsulatedValue(const EncapsulatedValue& v) : BarrieredValue(v) {}

    EncapsulatedValue& operator=(const Value& v) {
        pre();
        JS_ASSERT(!IsPoisonedValue(v));
        value = v;
        return *this;
    }

    EncapsulatedValue& operator=(const EncapsulatedValue& v) {
        pre();
        JS_ASSERT(!IsPoisonedValue(v));
        value = v.get();
        return *this;
    }
};

// Like HeapPtr, but specialized for Value.
// See the comments on that class for details.
class HeapValue : public BarrieredValue
{
  public:
    explicit HeapValue()
      : BarrieredValue(UndefinedValue())
    {
        post();
    }

    explicit HeapValue(const Value& v)
      : BarrieredValue(v)
    {
        JS_ASSERT(!IsPoisonedValue(v));
        post();
    }

    explicit HeapValue(const HeapValue& v)
      : BarrieredValue(v.value)
    {
        JS_ASSERT(!IsPoisonedValue(v.value));
        post();
    }

    ~HeapValue() {
        pre();
    }

    void init(const Value& v) {
        JS_ASSERT(!IsPoisonedValue(v));
        value = v;
        post();
    }

    void init(JSRuntime* rt, const Value& v) {
        JS_ASSERT(!IsPoisonedValue(v));
        value = v;
        post(rt);
    }

    HeapValue& operator=(const Value& v) {
        pre();
        JS_ASSERT(!IsPoisonedValue(v));
        value = v;
        post();
        return *this;
    }

    HeapValue& operator=(const HeapValue& v) {
        pre();
        JS_ASSERT(!IsPoisonedValue(v.value));
        value = v.value;
        post();
        return *this;
    }

#ifdef DEBUG
    bool preconditionForSet(Zone* zone);
#endif

    /*
     * This is a faster version of operator=. Normally, operator= has to
     * determine the compartment of the value before it can decide whether to do
     * the barrier. If you already know the compartment, it's faster to pass it
     * in.
     */
    void set(Zone* zone, const Value& v) {
        JS::shadow::Zone* shadowZone = JS::shadow::Zone::asShadowZone(zone);
        JS_ASSERT(preconditionForSet(zone));
        pre(zone);
        JS_ASSERT(!IsPoisonedValue(v));
        value = v;
        post(shadowZone->runtimeFromAnyThread());
    }

    static void writeBarrierPost(const Value& value, Value* addr) {
#ifdef JSGC_GENERATIONAL
        if (value.isMarkable())
            shadowRuntimeFromAnyThread(value)->gcStoreBufferPtr()->putValue(addr);
#endif
    }

    static void writeBarrierPost(JSRuntime* rt, const Value& value, Value* addr) {
#ifdef JSGC_GENERATIONAL
        if (value.isMarkable()) {
            JS::shadow::Runtime* shadowRuntime = JS::shadow::Runtime::asShadowRuntime(rt);
            shadowRuntime->gcStoreBufferPtr()->putValue(addr);
        }
#endif
    }

  private:
    void post() {
        writeBarrierPost(value, &value);
    }

    void post(JSRuntime* rt) {
        writeBarrierPost(rt, value, &value);
    }

    HeapValue(HeapValue&&) MOZ_DELETE;
    HeapValue& operator=(HeapValue&&) MOZ_DELETE;
};

// Like RelocatablePtr, but specialized for Value.
// See the comments on that class for details.
class RelocatableValue : public BarrieredValue
{
  public:
    explicit RelocatableValue() : BarrieredValue(UndefinedValue()) {}

    explicit RelocatableValue(const Value& v)
      : BarrieredValue(v)
    {
        if (v.isMarkable())
            post();
    }

    RelocatableValue(const RelocatableValue& v)
      : BarrieredValue(v.value)
    {
        JS_ASSERT(!IsPoisonedValue(v.value));
        if (v.value.isMarkable())
            post();
    }

    ~RelocatableValue()
    {
        if (value.isMarkable())
            relocate(runtimeFromAnyThread(value));
    }

    RelocatableValue& operator=(const Value& v) {
        pre();
        JS_ASSERT(!IsPoisonedValue(v));
        if (v.isMarkable()) {
            value = v;
            post();
        } else if (value.isMarkable()) {
            JSRuntime* rt = runtimeFromAnyThread(value);
            relocate(rt);
            value = v;
        } else {
            value = v;
        }
        return *this;
    }

    RelocatableValue& operator=(const RelocatableValue& v) {
        pre();
        JS_ASSERT(!IsPoisonedValue(v.value));
        if (v.value.isMarkable()) {
            value = v.value;
            post();
        } else if (value.isMarkable()) {
            JSRuntime* rt = runtimeFromAnyThread(value);
            relocate(rt);
            value = v.value;
        } else {
            value = v.value;
        }
        return *this;
    }

  private:
    void post() {
#ifdef JSGC_GENERATIONAL
        JS_ASSERT(value.isMarkable());
        shadowRuntimeFromAnyThread(value)->gcStoreBufferPtr()->putRelocatableValue(&value);
#endif
    }

    void relocate(JSRuntime* rt) {
#ifdef JSGC_GENERATIONAL
        JS::shadow::Runtime* shadowRuntime = JS::shadow::Runtime::asShadowRuntime(rt);
        shadowRuntime->gcStoreBufferPtr()->removeRelocatableValue(&value);
#endif
    }
};

// A pre- and post-barriered Value that is specialized to be aware that it
// resides in a slots or elements vector. This allows it to be relocated in
// memory, but with substantially less overhead than a RelocatablePtr.
class HeapSlot : public BarrieredValue
{
  public:
    enum Kind {
        Slot,
        Element
    };

    explicit HeapSlot() MOZ_DELETE;

    explicit HeapSlot(JSObject* obj, Kind kind, uint32_t slot, const Value& v)
      : BarrieredValue(v)
    {
        JS_ASSERT(!IsPoisonedValue(v));
        post(obj, kind, slot, v);
    }

    explicit HeapSlot(JSObject* obj, Kind kind, uint32_t slot, const HeapSlot& s)
      : BarrieredValue(s.value)
    {
        JS_ASSERT(!IsPoisonedValue(s.value));
        post(obj, kind, slot, s);
    }

    ~HeapSlot() {
        pre();
    }

    void init(JSObject* owner, Kind kind, uint32_t slot, const Value& v) {
        value = v;
        post(owner, kind, slot, v);
    }

    void init(JSRuntime* rt, JSObject* owner, Kind kind, uint32_t slot, const Value& v) {
        value = v;
        post(rt, owner, kind, slot, v);
    }

#ifdef DEBUG
    bool preconditionForSet(JSObject* owner, Kind kind, uint32_t slot);
    bool preconditionForSet(Zone* zone, JSObject* owner, Kind kind, uint32_t slot);
    static void preconditionForWriteBarrierPost(JSObject* obj, Kind kind, uint32_t slot,
                                                Value target);
#endif

    void set(JSObject* owner, Kind kind, uint32_t slot, const Value& v) {
        JS_ASSERT(preconditionForSet(owner, kind, slot));
        pre();
        JS_ASSERT(!IsPoisonedValue(v));
        value = v;
        post(owner, kind, slot, v);
    }

    void set(Zone* zone, JSObject* owner, Kind kind, uint32_t slot, const Value& v) {
        JS_ASSERT(preconditionForSet(zone, owner, kind, slot));
        JS::shadow::Zone* shadowZone = JS::shadow::Zone::asShadowZone(zone);
        pre(zone);
        JS_ASSERT(!IsPoisonedValue(v));
        value = v;
        post(shadowZone->runtimeFromAnyThread(), owner, kind, slot, v);
    }

    static void writeBarrierPost(JSObject* obj, Kind kind, uint32_t slot, Value target)
    {
#ifdef JSGC_GENERATIONAL
        js::gc::Cell* cell = reinterpret_cast<js::gc::Cell*>(obj);
        writeBarrierPost(cell->runtimeFromAnyThread(), obj, kind, slot, target);
#endif
    }

    static void writeBarrierPost(JSRuntime* rt, JSObject* obj, Kind kind, uint32_t slot,
                                 Value target)
    {
#ifdef DEBUG
        preconditionForWriteBarrierPost(obj, kind, slot, target);
#endif
#ifdef JSGC_GENERATIONAL
        if (target.isObject()) {
            JS::shadow::Runtime* shadowRuntime = JS::shadow::Runtime::asShadowRuntime(rt);
            shadowRuntime->gcStoreBufferPtr()->putSlot(obj, kind, slot, &target.toObject());
        }
#endif
    }

  private:
    void post(JSObject* owner, Kind kind, uint32_t slot, Value target) {
        HeapSlot::writeBarrierPost(owner, kind, slot, target);
    }

    void post(JSRuntime* rt, JSObject* owner, Kind kind, uint32_t slot, Value target) {
        HeapSlot::writeBarrierPost(rt, owner, kind, slot, target);
    }
};

static inline const Value*
Valueify(const BarrieredValue* array)
{
    JS_STATIC_ASSERT(sizeof(HeapValue) == sizeof(Value));
    JS_STATIC_ASSERT(sizeof(HeapSlot) == sizeof(Value));
    return (const Value*)array;
}

static inline HeapValue*
HeapValueify(Value* v)
{
    JS_STATIC_ASSERT(sizeof(HeapValue) == sizeof(Value));
    JS_STATIC_ASSERT(sizeof(HeapSlot) == sizeof(Value));
    return (HeapValue*)v;
}

class HeapSlotArray
{
    HeapSlot* array;

  public:
    HeapSlotArray(HeapSlot* array) : array(array) {}

    operator const Value*() const { return Valueify(array); }
    operator HeapSlot*() const { return array; }

    HeapSlotArray operator +(int offset) const { return HeapSlotArray(array + offset); }
    HeapSlotArray operator +(uint32_t offset) const { return HeapSlotArray(array + offset); }
};

/*
 * Base class for barriered jsid types.
 */
class BarrieredId
{
  protected:
    jsid value;

  private:
    BarrieredId(const BarrieredId& v) MOZ_DELETE;

  protected:
    explicit BarrieredId(jsid id) : value(id) {}
    ~BarrieredId() { pre(); }

  public:
    bool operator==(jsid id) const { return value == id; }
    bool operator!=(jsid id) const { return value != id; }

    jsid get() const { return value; }
    jsid* unsafeGet() { return &value; }
    void unsafeSet(jsid newId) { value = newId; }
    operator jsid() const { return value; }

  protected:
    void pre() {
#ifdef JSGC_INCREMENTAL
        if (JSID_IS_OBJECT(value)) {
            JSObject* obj = JSID_TO_OBJECT(value);
            JS::shadow::Zone* shadowZone = ShadowZoneOfObjectFromAnyThread(obj);
            if (shadowZone->needsBarrier()) {
                js::gc::MarkObjectUnbarriered(shadowZone->barrierTracer(), &obj, "write barrier");
                JS_ASSERT(obj == JSID_TO_OBJECT(value));
            }
        } else if (JSID_IS_STRING(value)) {
            if (StringIsPermanentAtom(JSID_TO_STRING(value)))
                return;
            JSString* str = JSID_TO_STRING(value);
            JS::shadow::Zone* shadowZone = ShadowZoneOfStringFromAnyThread(str);
            if (shadowZone->needsBarrier()) {
                js::gc::MarkStringUnbarriered(shadowZone->barrierTracer(), &str, "write barrier");
                JS_ASSERT(str == JSID_TO_STRING(value));
            }
        }
#endif
    }
};

// Like EncapsulatedPtr, but specialized for jsid.
// See the comments on that class for details.
class EncapsulatedId : public BarrieredId
{
  public:
    explicit EncapsulatedId(jsid id) : BarrieredId(id) {}
    explicit EncapsulatedId() : BarrieredId(JSID_VOID) {}

    EncapsulatedId& operator=(const EncapsulatedId& v) {
        if (v.value != value)
            pre();
        JS_ASSERT(!IsPoisonedId(v.value));
        value = v.value;
        return *this;
    }
};

// Like RelocatablePtr, but specialized for jsid.
// See the comments on that class for details.
class RelocatableId : public BarrieredId
{
  public:
    explicit RelocatableId() : BarrieredId(JSID_VOID) {}
    explicit inline RelocatableId(jsid id) : BarrieredId(id) {}
    ~RelocatableId() { pre(); }

    bool operator==(jsid id) const { return value == id; }
    bool operator!=(jsid id) const { return value != id; }

    jsid get() const { return value; }
    operator jsid() const { return value; }

    jsid* unsafeGet() { return &value; }

    RelocatableId& operator=(jsid id) {
        if (id != value)
            pre();
        JS_ASSERT(!IsPoisonedId(id));
        value = id;
        return *this;
    }

    RelocatableId& operator=(const RelocatableId& v) {
        if (v.value != value)
            pre();
        JS_ASSERT(!IsPoisonedId(v.value));
        value = v.value;
        return *this;
    }
};

// Like HeapPtr, but specialized for jsid.
// See the comments on that class for details.
class HeapId : public BarrieredId
{
  public:
    explicit HeapId() : BarrieredId(JSID_VOID) {}

    explicit HeapId(jsid id)
      : BarrieredId(id)
    {
        JS_ASSERT(!IsPoisonedId(id));
        post();
    }

    ~HeapId() { pre(); }

    void init(jsid id) {
        JS_ASSERT(!IsPoisonedId(id));
        value = id;
        post();
    }

    HeapId& operator=(jsid id) {
        if (id != value)
            pre();
        JS_ASSERT(!IsPoisonedId(id));
        value = id;
        post();
        return *this;
    }

    HeapId& operator=(const HeapId& v) {
        if (v.value != value)
            pre();
        JS_ASSERT(!IsPoisonedId(v.value));
        value = v.value;
        post();
        return *this;
    }

  private:
    void post() {};

    HeapId(const HeapId& v) MOZ_DELETE;

    HeapId(HeapId&&) MOZ_DELETE;
    HeapId& operator=(HeapId&&) MOZ_DELETE;
};

/*
 * Incremental GC requires that weak pointers have read barriers. This is mostly
 * an issue for empty shapes stored in JSCompartment. The problem happens when,
 * during an incremental GC, some JS code stores one of the compartment's empty
 * shapes into an object already marked black. Normally, this would not be a
 * problem, because the empty shape would have been part of the initial snapshot
 * when the GC started. However, since this is a weak pointer, it isn't. So we
 * may collect the empty shape even though a live object points to it. To fix
 * this, we mark these empty shapes black whenever they get read out.
 */
template <class T>
class ReadBarriered
{
    T* value;

  public:
    ReadBarriered() : value(nullptr) {}
    ReadBarriered(T* value) : value(value) {}
    ReadBarriered(const Rooted<T*>& rooted) : value(rooted) {}

    T* get() const {
        if (!value)
            return nullptr;
        T::readBarrier(value);
        return value;
    }

    operator T*() const { return get(); }

    T& operator*() const { return *get(); }
    T* operator->() const { return get(); }

    T** unsafeGet() { return &value; }
    T * const * unsafeGet() const { return &value; }

    void set(T* v) { value = v; }

    operator bool() { return !!value; }
};

class ReadBarrieredValue
{
    Value value;

  public:
    ReadBarrieredValue() : value(UndefinedValue()) {}
    ReadBarrieredValue(const Value& value) : value(value) {}

    inline const Value& get() const;
    Value* unsafeGet() { return &value; }
    inline operator const Value&() const;

    inline JSObject& toObject() const;
};

/*
 * Operations on a Heap thing inside the GC need to strip the barriers from
 * pointer operations. This template helps do that in contexts where the type
 * is templatized.
 */
template <typename T> struct Unbarriered {};
template <typename S> struct Unbarriered< EncapsulatedPtr<S> > { typedef S* type; };
template <typename S> struct Unbarriered< RelocatablePtr<S> > { typedef S* type; };
template <> struct Unbarriered<EncapsulatedValue> { typedef Value type; };
template <> struct Unbarriered<RelocatableValue> { typedef Value type; };
template <typename S> struct Unbarriered< DefaultHasher< EncapsulatedPtr<S> > > {
    typedef DefaultHasher<S*> type;
};

} /* namespace js */

#endif /* gc_Barrier_h */