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 (a81015259a98)

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 1250 1251 1252 1253 1254 1255 1256 1257 1258 1259 1260 1261 1262 1263 1264 1265 1266 1267 1268 1269 1270 1271 1272 1273 1274 1275 1276 1277 1278 1279 1280 1281 1282 1283 1284 1285 1286 1287 1288 1289 1290 1291 1292 1293 1294 1295 1296 1297 1298 1299 1300 1301 1302 1303 1304 1305 1306 1307 1308 1309 1310 1311 1312 1313 1314 1315 1316 1317 1318 1319 1320 1321 1322 1323 1324 1325 1326 1327 1328 1329 1330 1331 1332 1333 1334 1335 1336 1337 1338 1339 1340 1341 1342 1343 1344 1345 1346 1347 1348 1349 1350 1351 1352 1353 1354 1355 1356 1357 1358 1359 1360 1361 1362 1363 1364 1365 1366 1367 1368 1369 1370 1371 1372 1373 1374 1375 1376 1377 1378 1379 1380 1381 1382 1383 1384 1385 1386 1387 1388 1389 1390 1391 1392 1393 1394 1395 1396 1397 1398 1399 1400 1401 1402 1403 1404 1405 1406 1407 1408 1409 1410 1411 1412 1413 1414 1415 1416 1417 1418 1419 1420 1421 1422 1423 1424 1425 1426 1427 1428 1429 1430 1431 1432 1433 1434 1435 1436 1437 1438 1439 1440 1441 1442 1443 1444 1445 1446 1447 1448 1449 1450 1451 1452 1453 1454 1455 1456 1457 1458 1459 1460 1461 1462 1463 1464 1465 1466 1467 1468 1469 1470 1471 1472 1473 1474 1475 1476 1477 1478 1479 1480 1481 1482 1483 1484 1485 1486 1487 1488 1489 1490 1491 1492 1493 1494 1495 1496 1497 1498 1499 1500 1501 1502 1503 1504 1505 1506 1507 1508 1509 1510 1511 1512 1513 1514 1515 1516 1517 1518 1519 1520 1521 1522 1523 1524 1525 1526 1527 1528 1529 1530 1531 1532 1533 1534 1535 1536 1537 1538 1539 1540 1541 1542 1543 1544 1545 1546 1547 1548 1549 1550 1551 1552 1553 1554 1555 1556 1557 1558 1559 1560 1561 1562 1563 1564 1565 1566 1567 1568 1569 1570 1571 1572 1573 1574 1575 1576 1577 1578 1579 1580 1581 1582 1583 1584 1585 1586 1587 1588 1589 1590 1591 1592 1593 1594 1595 1596 1597 1598 1599 1600 1601 1602 1603 1604 1605 1606 1607 1608 1609 1610 1611 1612 1613 1614 1615 1616 1617 1618 1619 1620 1621 1622 1623 1624 1625 1626 1627 1628 1629 1630 1631 1632 1633 1634 1635 1636 1637 1638 1639 1640 1641 1642 1643 1644 1645 1646 1647 1648 1649 1650 1651 1652 1653 1654 1655 1656 1657 1658 1659 1660 1661 1662 1663 1664 1665 1666 1667 1668 1669 1670 1671 1672 1673 1674 1675 1676 1677 1678 1679 1680 1681 1682 1683 1684 1685 1686 1687 1688 1689 1690 1691 1692 1693 1694 1695 1696 1697 1698 1699 1700 1701 1702 1703 1704 1705 1706 1707 1708 1709 1710 1711 1712 1713 1714 1715 1716 1717 1718 1719 1720 1721 1722 1723 1724 1725 1726 1727 1728 1729 1730 1731 1732 1733 1734 1735 1736 1737 1738 1739 1740 1741 1742 1743 1744 1745 1746 1747 1748 1749 1750 1751 1752 1753 1754 1755 1756 1757 1758 1759 1760 1761 1762 1763 1764 1765 1766 1767 1768 1769 1770 1771 1772 1773 1774 1775 1776 1777 1778 1779 1780 1781 1782 1783 1784
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 *
 * ***** BEGIN LICENSE BLOCK *****
 * Version: MPL 1.1/GPL 2.0/LGPL 2.1
 *
 * The contents of this file are subject to the Mozilla Public License Version
 * 1.1 (the "License"); you may not use this file except in compliance with
 * the License. You may obtain a copy of the License at
 * http://www.mozilla.org/MPL/
 *
 * Software distributed under the License is distributed on an "AS IS" basis,
 * WITHOUT WARRANTY OF ANY KIND, either express or implied. See the License
 * for the specific language governing rights and limitations under the
 * License.
 *
 * The Original Code is Mozilla Communicator client code, released
 * March 31, 1998.
 *
 * The Initial Developer of the Original Code is
 * Netscape Communications Corporation.
 * Portions created by the Initial Developer are Copyright (C) 1998
 * the Initial Developer. All Rights Reserved.
 *
 * Contributor(s):
 *
 * Alternatively, the contents of this file may be used under the terms of
 * either of the GNU General Public License Version 2 or later (the "GPL"),
 * or the GNU Lesser General Public License Version 2.1 or later (the "LGPL"),
 * in which case the provisions of the GPL or the LGPL are applicable instead
 * of those above. If you wish to allow use of your version of this file only
 * under the terms of either the GPL or the LGPL, and not to allow others to
 * use your version of this file under the terms of the MPL, indicate your
 * decision by deleting the provisions above and replace them with the notice
 * and other provisions required by the GPL or the LGPL. If you do not delete
 * the provisions above, a recipient may use your version of this file under
 * the terms of any one of the MPL, the GPL or the LGPL.
 *
 * ***** END LICENSE BLOCK ***** */

#ifndef jsgc_h___
#define jsgc_h___

/*
 * JS Garbage Collector.
 */
#include <setjmp.h>

#include "mozilla/Util.h"

#include "jsalloc.h"
#include "jstypes.h"
#include "jsprvtd.h"
#include "jspubtd.h"
#include "jsdhash.h"
#include "jsgcchunk.h"
#include "jslock.h"
#include "jsutil.h"
#include "jsversion.h"
#include "jsgcstats.h"
#include "jscell.h"

#include "ds/BitArray.h"
#include "gc/Statistics.h"
#include "js/HashTable.h"
#include "js/Vector.h"

struct JSCompartment;

extern "C" void
js_TraceXML(JSTracer *trc, JSXML* thing);

#if JS_STACK_GROWTH_DIRECTION > 0
# define JS_CHECK_STACK_SIZE(limit, lval)  ((jsuword)(lval) < limit)
#else
# define JS_CHECK_STACK_SIZE(limit, lval)  ((jsuword)(lval) > limit)
#endif

namespace js {

class GCHelperThread;
struct Shape;

namespace gc {

struct Arena;

/*
 * This must be an upper bound, but we do not need the least upper bound, so
 * we just exclude non-background objects.
 */
const size_t MAX_BACKGROUND_FINALIZE_KINDS = FINALIZE_LIMIT - (FINALIZE_OBJECT_LAST + 1) / 2;

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

/*
 * This is the maximum number of arenas we allow in the FreeCommitted state
 * before we trigger a GC_SHRINK to release free arenas to the OS.
 */
const static uint32 MaxFreeCommittedArenas = (32 << 20) / ArenaSize;

/*
 * The mark bitmap has one bit per each GC cell. For multi-cell GC things this
 * wastes space but allows to avoid expensive devisions by thing's size when
 * accessing the bitmap. In addition this allows to use some bits for colored
 * marking during the cycle GC.
 */
const size_t ArenaCellCount = size_t(1) << (ArenaShift - Cell::CellShift);
const size_t ArenaBitmapBits = ArenaCellCount;
const size_t ArenaBitmapBytes = ArenaBitmapBits / 8;
const size_t ArenaBitmapWords = ArenaBitmapBits / JS_BITS_PER_WORD;

/*
 * A FreeSpan represents a contiguous sequence of free cells in an Arena.
 * |first| is the address of the first free cell in the span. |last| is the
 * address of the last free cell in the span. This last cell holds a FreeSpan
 * data structure for the next span unless this is the last span on the list
 * of spans in the arena. For this last span |last| points to the last byte of
 * the last thing in the arena and no linkage is stored there, so
 * |last| == arenaStart + ArenaSize - 1. If the space at the arena end is
 * fully used this last span is empty and |first| == |last + 1|.
 *
 * Thus |first| < |last| implies that we have either the last span with at least
 * one element or that the span is not the last and contains at least 2
 * elements. In both cases to allocate a thing from this span we need simply
 * to increment |first| by the allocation size.
 *
 * |first| == |last| implies that we have a one element span that records the
 * next span. So to allocate from it we need to update the span list head
 * with a copy of the span stored at |last| address so the following
 * allocations will use that span.
 *
 * |first| > |last| implies that we have an empty last span and the arena is
 * fully used.
 *
 * Also only for the last span (|last| & 1)! = 0 as all allocation sizes are
 * multiples of Cell::CellSize.
 */
struct FreeSpan {
    uintptr_t   first;
    uintptr_t   last;

  public:
    FreeSpan() {}

    FreeSpan(uintptr_t first, uintptr_t last)
      : first(first), last(last) {
        checkSpan();
    }

    /*
     * To minimize the size of the arena header the first span is encoded
     * there as offsets from the arena start.
     */
    static size_t encodeOffsets(size_t firstOffset, size_t lastOffset) {
        /* Check that we can pack the offsets into uint16. */
        JS_STATIC_ASSERT(ArenaShift < 16);
        JS_ASSERT(firstOffset <= ArenaSize);
        JS_ASSERT(lastOffset < ArenaSize);
        JS_ASSERT(firstOffset <= ((lastOffset + 1) & ~size_t(1)));
        return firstOffset | (lastOffset << 16);
    }

    /*
     * Encoded offsets for a full arena when its first span is the last one
     * and empty.
     */
    static const size_t FullArenaOffsets = ArenaSize | ((ArenaSize - 1) << 16);

    static FreeSpan decodeOffsets(uintptr_t arenaAddr, size_t offsets) {
        JS_ASSERT(!(arenaAddr & ArenaMask));

        size_t firstOffset = offsets & 0xFFFF;
        size_t lastOffset = offsets >> 16;
        JS_ASSERT(firstOffset <= ArenaSize);
        JS_ASSERT(lastOffset < ArenaSize);

        /*
         * We must not use | when calculating first as firstOffset is
         * ArenaMask + 1 for the empty span.
         */
        return FreeSpan(arenaAddr + firstOffset, arenaAddr | lastOffset);
    }

    void initAsEmpty(uintptr_t arenaAddr = 0) {
        JS_ASSERT(!(arenaAddr & ArenaMask));
        first = arenaAddr + ArenaSize;
        last = arenaAddr | (ArenaSize  - 1);
        JS_ASSERT(isEmpty());
    }

    bool isEmpty() const {
        checkSpan();
        return first > last;
    }

    bool hasNext() const {
        checkSpan();
        return !(last & uintptr_t(1));
    }

    const FreeSpan *nextSpan() const {
        JS_ASSERT(hasNext());
        return reinterpret_cast<FreeSpan *>(last);
    }

    FreeSpan *nextSpanUnchecked(size_t thingSize) const {
#ifdef DEBUG
        uintptr_t lastOffset = last & ArenaMask;
        JS_ASSERT(!(lastOffset & 1));
        JS_ASSERT((ArenaSize - lastOffset) % thingSize == 0);
#endif
        return reinterpret_cast<FreeSpan *>(last);
    }

    uintptr_t arenaAddressUnchecked() const {
        return last & ~ArenaMask;
    }

    uintptr_t arenaAddress() const {
        checkSpan();
        return arenaAddressUnchecked();
    }

    ArenaHeader *arenaHeader() const {
        return reinterpret_cast<ArenaHeader *>(arenaAddress());
    }

    bool isSameNonEmptySpan(const FreeSpan *another) const {
        JS_ASSERT(!isEmpty());
        JS_ASSERT(!another->isEmpty());
        return first == another->first && last == another->last;
    }

    bool isWithinArena(uintptr_t arenaAddr) const {
        JS_ASSERT(!(arenaAddr & ArenaMask));

        /* Return true for the last empty span as well. */
        return arenaAddress() == arenaAddr;
    }

    size_t encodeAsOffsets() const {
        /*
         * We must use first - arenaAddress(), not first & ArenaMask as
         * first == ArenaMask + 1 for an empty span.
         */
        uintptr_t arenaAddr = arenaAddress();
        return encodeOffsets(first - arenaAddr, last & ArenaMask);
    }

    /* See comments before FreeSpan for details. */
    JS_ALWAYS_INLINE void *allocate(size_t thingSize) {
        JS_ASSERT(thingSize % Cell::CellSize == 0);
        checkSpan();
        uintptr_t thing = first;
        if (thing < last) {
            /* Bump-allocate from the current span. */
            first = thing + thingSize;
        } else if (JS_LIKELY(thing == last)) {
            /*
             * Move to the next span. We use JS_LIKELY as without PGO
             * compilers mis-predict == here as unlikely to succeed.
             */
            *this = *reinterpret_cast<FreeSpan *>(thing);
        } else {
            return NULL;
        }
        checkSpan();
        return reinterpret_cast<void *>(thing);
    }

    /* A version of allocate when we know that the span is not empty. */
    JS_ALWAYS_INLINE void *infallibleAllocate(size_t thingSize) {
        JS_ASSERT(thingSize % Cell::CellSize == 0);
        checkSpan();
        uintptr_t thing = first;
        if (thing < last) {
            first = thing + thingSize;
        } else {
            JS_ASSERT(thing == last);
            *this = *reinterpret_cast<FreeSpan *>(thing);
        }
        checkSpan();
        return reinterpret_cast<void *>(thing);
    }

    /*
     * Allocate from a newly allocated arena. We do not move the free list
     * from the arena. Rather we set the arena up as fully used during the
     * initialization so to allocate we simply return the first thing in the
     * arena and set the free list to point to the second.
     */
    JS_ALWAYS_INLINE void *allocateFromNewArena(uintptr_t arenaAddr, size_t firstThingOffset,
                                                size_t thingSize) {
        JS_ASSERT(!(arenaAddr & ArenaMask));
        uintptr_t thing = arenaAddr | firstThingOffset;
        first = thing + thingSize;
        last = arenaAddr | ArenaMask;
        checkSpan();
        return reinterpret_cast<void *>(thing);
    }

    void checkSpan() const {
#ifdef DEBUG
        /* We do not allow spans at the end of the address space. */
        JS_ASSERT(last != uintptr_t(-1));
        JS_ASSERT(first);
        JS_ASSERT(last);
        JS_ASSERT(first - 1 <= last);
        uintptr_t arenaAddr = arenaAddressUnchecked();
        if (last & 1) {
            /* The span is the last. */
            JS_ASSERT((last & ArenaMask) == ArenaMask);

            if (first - 1 == last) {
                /* The span is last and empty. The above start != 0 check
                 * implies that we are not at the end of the address space.
                 */
                return;
            }
            size_t spanLength = last - first + 1;
            JS_ASSERT(spanLength % Cell::CellSize == 0);

            /* Start and end must belong to the same arena. */
            JS_ASSERT((first & ~ArenaMask) == arenaAddr);
            return;
        }

        /* The span is not the last and we have more spans to follow. */
        JS_ASSERT(first <= last);
        size_t spanLengthWithoutOneThing = last - first;
        JS_ASSERT(spanLengthWithoutOneThing % Cell::CellSize == 0);

        JS_ASSERT((first & ~ArenaMask) == arenaAddr);

        /*
         * If there is not enough space before the arena end to allocate one
         * more thing, then the span must be marked as the last one to avoid
         * storing useless empty span reference.
         */
        size_t beforeTail = ArenaSize - (last & ArenaMask);
        JS_ASSERT(beforeTail >= sizeof(FreeSpan) + Cell::CellSize);

        FreeSpan *next = reinterpret_cast<FreeSpan *>(last);

        /*
         * The GC things on the list of free spans come from one arena
         * and the spans are linked in ascending address order with
         * at least one non-free thing between spans.
         */
        JS_ASSERT(last < next->first);
        JS_ASSERT(arenaAddr == next->arenaAddressUnchecked());

        if (next->first > next->last) {
            /*
             * The next span is the empty span that terminates the list for
             * arenas that do not have any free things at the end.
             */
            JS_ASSERT(next->first - 1 == next->last);
            JS_ASSERT(arenaAddr + ArenaSize == next->first);
        }
#endif
    }

};

/* Every arena has a header. */
struct ArenaHeader {
    friend struct FreeLists;

    JSCompartment   *compartment;

    /*
     * ArenaHeader::next has two purposes: when unallocated, it points to the
     * next available Arena's header. When allocated, it points to the next
     * arena of the same size class and compartment.
     */
    ArenaHeader     *next;

  private:
    /*
     * The first span of free things in the arena. We encode it as the start
     * and end offsets within the arena, not as FreeSpan structure, to
     * minimize the header size.
     */
    size_t          firstFreeSpanOffsets;

    /*
     * One of AllocKind constants or FINALIZE_LIMIT when the arena does not
     * contain any GC things and is on the list of empty arenas in the GC
     * chunk. The latter allows to quickly check if the arena is allocated
     * during the conservative GC scanning without searching the arena in the
     * list.
     */
    size_t       allocKind          : 8;

    /*
     * When recursive marking uses too much stack the marking is delayed and
     * the corresponding arenas are put into a stack using the following field
     * as a linkage. To distinguish the bottom of the stack from the arenas
     * not present in the stack we use an extra flag to tag arenas on the
     * stack.
     *
     * To minimize the ArenaHeader size we record the next delayed marking
     * linkage as arenaAddress() >> ArenaShift and pack it with the allocKind
     * field and hasDelayedMarking flag. We use 8 bits for the allocKind, not
     * ArenaShift - 1, so the compiler can use byte-level memory instructions
     * to access it.
     */
  public:
    size_t       hasDelayedMarking  : 1;
    size_t       nextDelayedMarking : JS_BITS_PER_WORD - 8 - 1;

    static void staticAsserts() {
        /* We must be able to fit the allockind into uint8. */
        JS_STATIC_ASSERT(FINALIZE_LIMIT <= 255);

        /*
         * nextDelayedMarkingpacking assumes that ArenaShift has enough bits
         * to cover allocKind and hasDelayedMarking.
         */
        JS_STATIC_ASSERT(ArenaShift >= 8 + 1);
    }

    inline uintptr_t address() const;
    inline Chunk *chunk() const;

    bool allocated() const {
        JS_ASSERT(allocKind <= size_t(FINALIZE_LIMIT));
        return allocKind < size_t(FINALIZE_LIMIT);
    }

    void init(JSCompartment *comp, AllocKind kind) {
        JS_ASSERT(!allocated());
        JS_ASSERT(!hasDelayedMarking);
        compartment = comp;

        JS_STATIC_ASSERT(FINALIZE_LIMIT <= 255);
        allocKind = size_t(kind);

        /* See comments in FreeSpan::allocateFromNewArena. */
        firstFreeSpanOffsets = FreeSpan::FullArenaOffsets;
    }

    void setAsNotAllocated() {
        allocKind = size_t(FINALIZE_LIMIT);
        hasDelayedMarking = 0;
        nextDelayedMarking = 0;
    }

    uintptr_t arenaAddress() const {
        return address();
    }

    Arena *getArena() {
        return reinterpret_cast<Arena *>(arenaAddress());
    }

    AllocKind getAllocKind() const {
        JS_ASSERT(allocated());
        return AllocKind(allocKind);
    }

    inline size_t getThingSize() const;

    bool hasFreeThings() const {
        return firstFreeSpanOffsets != FreeSpan::FullArenaOffsets;
    }

    inline bool isEmpty() const;

    void setAsFullyUsed() {
        firstFreeSpanOffsets = FreeSpan::FullArenaOffsets;
    }

    FreeSpan getFirstFreeSpan() const {
#ifdef DEBUG
        checkSynchronizedWithFreeList();
#endif
        return FreeSpan::decodeOffsets(arenaAddress(), firstFreeSpanOffsets);
    }

    void setFirstFreeSpan(const FreeSpan *span) {
        JS_ASSERT(span->isWithinArena(arenaAddress()));
        firstFreeSpanOffsets = span->encodeAsOffsets();
    }

#ifdef DEBUG
    void checkSynchronizedWithFreeList() const;
#endif

    inline Arena *getNextDelayedMarking() const;
    inline void setNextDelayedMarking(Arena *arena);
};

struct Arena {
    /*
     * Layout of an arena:
     * An arena is 4K in size and 4K-aligned. It starts with the ArenaHeader
     * descriptor followed by some pad bytes. The remainder of the arena is
     * filled with the array of T things. The pad bytes ensure that the thing
     * array ends exactly at the end of the arena.
     *
     * +-------------+-----+----+----+-----+----+
     * | ArenaHeader | pad | T0 | T1 | ... | Tn |
     * +-------------+-----+----+----+-----+----+
     *
     * <----------------------------------------> = ArenaSize bytes
     * <-------------------> = first thing offset
     */
    ArenaHeader aheader;
    uint8_t     data[ArenaSize - sizeof(ArenaHeader)];

  private:
    static JS_FRIEND_DATA(const uint32) ThingSizes[];
    static JS_FRIEND_DATA(const uint32) FirstThingOffsets[];

  public:
    static void staticAsserts();

    static size_t thingSize(AllocKind kind) {
        return ThingSizes[kind];
    }

    static size_t firstThingOffset(AllocKind kind) {
        return FirstThingOffsets[kind];
    }

    static size_t thingsPerArena(size_t thingSize) {
        JS_ASSERT(thingSize % Cell::CellSize == 0);

        /* We should be able to fit FreeSpan in any GC thing. */
        JS_ASSERT(thingSize >= sizeof(FreeSpan));

        return (ArenaSize - sizeof(ArenaHeader)) / thingSize;
    }

    static size_t thingsSpan(size_t thingSize) {
        return thingsPerArena(thingSize) * thingSize;
    }

    static bool isAligned(uintptr_t thing, size_t thingSize) {
        /* Things ends at the arena end. */
        uintptr_t tailOffset = (ArenaSize - thing) & ArenaMask;
        return tailOffset % thingSize == 0;
    }

    uintptr_t address() const {
        return aheader.address();
    }

    uintptr_t thingsStart(AllocKind thingKind) {
        return address() | firstThingOffset(thingKind);
    }

    uintptr_t thingsEnd() {
        return address() + ArenaSize;
    }

    template <typename T>
    bool finalize(JSContext *cx, AllocKind thingKind, size_t thingSize);
};

/* The chunk header (located at the end of the chunk to preserve arena alignment). */
struct ChunkInfo {
    Chunk           *next;
    Chunk           **prevp;

    /* Free arenas are linked together with aheader.next. */
    ArenaHeader     *freeArenasHead;

    /*
     * Decommitted arenas are tracked by a bitmap in the chunk header. We use
     * this offset to start our search iteration close to a decommitted arena
     * that we can allocate.
     */
    uint32          lastDecommittedArenaOffset;

    /* Number of free arenas, either committed or decommitted. */
    uint32          numArenasFree;

    /* Number of free, committed arenas. */
    uint32          numArenasFreeCommitted;

    /* Number of GC cycles this chunk has survived. */
    uint32          age;
};

/*
 * Calculating ArenasPerChunk:
 *
 * In order to figure out how many Arenas will fit in a chunk, we need to know
 * how much extra space is available after we allocate the header data. This
 * is a problem because the header size depends on the number of arenas in the
 * chunk. The two dependent fields are bitmap and decommittedArenas.
 *
 * For the mark bitmap, we know that each arena will use a fixed number of full
 * bytes: ArenaBitmapBytes. The full size of the header data is this number
 * multiplied by the eventual number of arenas we have in the header. We, 
 * conceptually, distribute this header data among the individual arenas and do
 * not include it in the header. This way we do not have to worry about its 
 * variable size: it gets attached to the variable number we are computing.
 *
 * For the decommitted arena bitmap, we only have 1 bit per arena, so this
 * technique will not work. Instead, we observe that we do not have enough
 * header info to fill 8 full arenas: it is currently 4 on 64bit, less on 
 * 32bit. Thus, with current numbers, we need 64 bytes for decommittedArenas.
 * This will not become 63 bytes unless we double the data required in the 
 * header. Therefore, we just compute the number of bytes required to track 
 * every possible arena and do not worry about slop bits, since there are too
 * few to usefully allocate.
 *
 * To actually compute the number of arenas we can allocate in a chunk, we
 * divide the amount of available space less the header info (not including
 * the mark bitmap which is distributed into the arena size) by the size of 
 * the arena (with the mark bitmap bytes it uses).
 */
const size_t BytesPerArenaWithHeader = ArenaSize + ArenaBitmapBytes;
const size_t ChunkDecommitBitmapBytes = ChunkSize / ArenaSize / JS_BITS_PER_BYTE;
const size_t ChunkBytesAvailable = ChunkSize - sizeof(ChunkInfo) - ChunkDecommitBitmapBytes;
const size_t ArenasPerChunk = ChunkBytesAvailable / BytesPerArenaWithHeader;

/* A chunk bitmap contains enough mark bits for all the cells in a chunk. */
struct ChunkBitmap {
    uintptr_t bitmap[ArenaBitmapWords * ArenasPerChunk];

    JS_ALWAYS_INLINE void getMarkWordAndMask(const Cell *cell, uint32 color,
                                             uintptr_t **wordp, uintptr_t *maskp);

    JS_ALWAYS_INLINE bool isMarked(const Cell *cell, uint32 color) {
        uintptr_t *word, mask;
        getMarkWordAndMask(cell, color, &word, &mask);
        return *word & mask;
    }

    JS_ALWAYS_INLINE bool markIfUnmarked(const Cell *cell, uint32 color) {
        uintptr_t *word, mask;
        getMarkWordAndMask(cell, BLACK, &word, &mask);
        if (*word & mask)
            return false;
        *word |= mask;
        if (color != BLACK) {
            /*
             * We use getMarkWordAndMask to recalculate both mask and word as
             * doing just mask << color may overflow the mask.
             */
            getMarkWordAndMask(cell, color, &word, &mask);
            if (*word & mask)
                return false;
            *word |= mask;
        }
        return true;
    }

    JS_ALWAYS_INLINE void unmark(const Cell *cell, uint32 color) {
        uintptr_t *word, mask;
        getMarkWordAndMask(cell, color, &word, &mask);
        *word &= ~mask;
    }

    void clear() {
        PodArrayZero(bitmap);
    }

#ifdef DEBUG
    bool noBitsSet(ArenaHeader *aheader) {
        /*
         * We assume that the part of the bitmap corresponding to the arena
         * has the exact number of words so we do not need to deal with a word
         * that covers bits from two arenas.
         */
        JS_STATIC_ASSERT(ArenaBitmapBits == ArenaBitmapWords * JS_BITS_PER_WORD);

        uintptr_t *word, unused;
        getMarkWordAndMask(reinterpret_cast<Cell *>(aheader->address()), BLACK, &word, &unused);
        for (size_t i = 0; i != ArenaBitmapWords; i++) {
            if (word[i])
                return false;
        }
        return true;
    }
#endif
};

JS_STATIC_ASSERT(ArenaBitmapBytes * ArenasPerChunk == sizeof(ChunkBitmap));

typedef BitArray<ArenasPerChunk> PerArenaBitmap;

const size_t ChunkPadSize = ChunkSize
                            - (sizeof(Arena) * ArenasPerChunk)
                            - sizeof(ChunkBitmap)
                            - sizeof(PerArenaBitmap)
                            - sizeof(ChunkInfo);
JS_STATIC_ASSERT(ChunkPadSize < BytesPerArenaWithHeader);

/*
 * Chunks contain arenas and associated data structures (mark bitmap, delayed
 * marking state).
 */
struct Chunk {
    Arena           arenas[ArenasPerChunk];

    /* Pad to full size to ensure cache alignment of ChunkInfo. */
    uint8           padding[ChunkPadSize];

    ChunkBitmap     bitmap;
    PerArenaBitmap  decommittedArenas;
    ChunkInfo       info;

    static Chunk *fromAddress(uintptr_t addr) {
        addr &= ~ChunkMask;
        return reinterpret_cast<Chunk *>(addr);
    }

    static bool withinArenasRange(uintptr_t addr) {
        uintptr_t offset = addr & ChunkMask;
        return offset < ArenasPerChunk * ArenaSize;
    }

    static size_t arenaIndex(uintptr_t addr) {
        JS_ASSERT(withinArenasRange(addr));
        return (addr & ChunkMask) >> ArenaShift;
    }

    uintptr_t address() const {
        uintptr_t addr = reinterpret_cast<uintptr_t>(this);
        JS_ASSERT(!(addr & ChunkMask));
        return addr;
    }

    bool unused() const {
        return info.numArenasFree == ArenasPerChunk;
    }

    bool noAvailableArenas() const {
        return info.numArenasFree == 0;
    }

    inline void addToAvailableList(JSCompartment *compartment);
    inline void removeFromAvailableList();

    ArenaHeader *allocateArena(JSCompartment *comp, AllocKind kind);

    void releaseArena(ArenaHeader *aheader);

    static Chunk *allocate(JSRuntime *rt);
    static inline void release(JSRuntime *rt, Chunk *chunk);

  private:
    inline void init(JSRuntime *rt);

    /* Search for a decommitted arena to allocate. */
    jsuint findDecommittedArenaOffset();
    ArenaHeader* fetchNextDecommittedArena();

    /* Unlink and return the freeArenasHead. */
    inline ArenaHeader* fetchNextFreeArena(JSRuntime *rt);
};

JS_STATIC_ASSERT(sizeof(Chunk) == ChunkSize);

class ChunkPool {
    Chunk   *emptyChunkListHead;
    size_t  emptyCount;

  public:
    ChunkPool()
      : emptyChunkListHead(NULL),
        emptyCount(0) { }

    size_t getEmptyCount() const {
        return emptyCount;
    }

    inline bool wantBackgroundAllocation(JSRuntime *rt) const;

    /* Must be called with the GC lock taken. */
    inline Chunk *get(JSRuntime *rt);

    /* Must be called either during the GC or with the GC lock taken. */
    inline void put(JSRuntime *rt, Chunk *chunk);

    /* Must be called either during the GC or with the GC lock taken. */
    void expire(JSRuntime *rt, bool releaseAll);

    /* Must be called either during the GC or with the GC lock taken. */
    JS_FRIEND_API(int64) countDecommittedArenas(JSRuntime *rt);
};

inline uintptr_t
Cell::address() const
{
    uintptr_t addr = uintptr_t(this);
    JS_ASSERT(addr % Cell::CellSize == 0);
    JS_ASSERT(Chunk::withinArenasRange(addr));
    return addr;
}

inline ArenaHeader *
Cell::arenaHeader() const
{
    uintptr_t addr = address();
    addr &= ~ArenaMask;
    return reinterpret_cast<ArenaHeader *>(addr);
}

Chunk *
Cell::chunk() const
{
    uintptr_t addr = uintptr_t(this);
    JS_ASSERT(addr % Cell::CellSize == 0);
    addr &= ~(ChunkSize - 1);
    return reinterpret_cast<Chunk *>(addr);
}

AllocKind
Cell::getAllocKind() const
{
    return arenaHeader()->getAllocKind();
}

#ifdef DEBUG
inline bool
Cell::isAligned() const
{
    return Arena::isAligned(address(), arenaHeader()->getThingSize());
}
#endif

inline uintptr_t
ArenaHeader::address() const
{
    uintptr_t addr = reinterpret_cast<uintptr_t>(this);
    JS_ASSERT(!(addr & ArenaMask));
    JS_ASSERT(Chunk::withinArenasRange(addr));
    return addr;
}

inline Chunk *
ArenaHeader::chunk() const
{
    return Chunk::fromAddress(address());
}

inline bool
ArenaHeader::isEmpty() const
{
    /* Arena is empty if its first span covers the whole arena. */
    JS_ASSERT(allocated());
    size_t firstThingOffset = Arena::firstThingOffset(getAllocKind());
    return firstFreeSpanOffsets == FreeSpan::encodeOffsets(firstThingOffset, ArenaMask);
}

inline size_t
ArenaHeader::getThingSize() const
{
    JS_ASSERT(allocated());
    return Arena::thingSize(getAllocKind());
}

inline Arena *
ArenaHeader::getNextDelayedMarking() const
{
    return reinterpret_cast<Arena *>(nextDelayedMarking << ArenaShift);
}

inline void
ArenaHeader::setNextDelayedMarking(Arena *arena)
{
    JS_ASSERT(!hasDelayedMarking);
    hasDelayedMarking = 1;
    nextDelayedMarking = arena->address() >> ArenaShift;
}

JS_ALWAYS_INLINE void
ChunkBitmap::getMarkWordAndMask(const Cell *cell, uint32 color,
                                uintptr_t **wordp, uintptr_t *maskp)
{
    JS_ASSERT(cell->chunk() == Chunk::fromAddress(reinterpret_cast<uintptr_t>(this)));
    size_t bit = (cell->address() & ChunkMask) / Cell::CellSize + color;
    JS_ASSERT(bit < ArenaBitmapBits * ArenasPerChunk);
    *maskp = uintptr_t(1) << (bit % JS_BITS_PER_WORD);
    *wordp = &bitmap[bit / JS_BITS_PER_WORD];
}

static void
AssertValidColor(const void *thing, uint32 color)
{
#ifdef DEBUG
    ArenaHeader *aheader = reinterpret_cast<const js::gc::Cell *>(thing)->arenaHeader();
    JS_ASSERT_IF(color, color < aheader->getThingSize() / Cell::CellSize);
#endif
}

inline bool
Cell::isMarked(uint32 color) const
{
    AssertValidColor(this, color);
    return chunk()->bitmap.isMarked(this, color);
}

bool
Cell::markIfUnmarked(uint32 color) const
{
    AssertValidColor(this, color);
    return chunk()->bitmap.markIfUnmarked(this, color);
}

void
Cell::unmark(uint32 color) const
{
    JS_ASSERT(color != BLACK);
    AssertValidColor(this, color);
    chunk()->bitmap.unmark(this, color);
}

JSCompartment *
Cell::compartment() const
{
    return arenaHeader()->compartment;
}

/*
 * Lower limit after which we limit the heap growth
 */
const size_t GC_ALLOCATION_THRESHOLD = 30 * 1024 * 1024;

/*
 * A GC is triggered once the number of newly allocated arenas is
 * GC_HEAP_GROWTH_FACTOR times the number of live arenas after the last GC
 * starting after the lower limit of GC_ALLOCATION_THRESHOLD.
 */
const float GC_HEAP_GROWTH_FACTOR = 3.0f;

/* Perform a Full GC every 20 seconds if MaybeGC is called */
static const int64 GC_IDLE_FULL_SPAN = 20 * 1000 * 1000;

static inline JSGCTraceKind
MapAllocToTraceKind(AllocKind thingKind)
{
    static const JSGCTraceKind map[FINALIZE_LIMIT] = {
        JSTRACE_OBJECT,     /* FINALIZE_OBJECT0 */
        JSTRACE_OBJECT,     /* FINALIZE_OBJECT0_BACKGROUND */
        JSTRACE_OBJECT,     /* FINALIZE_OBJECT2 */
        JSTRACE_OBJECT,     /* FINALIZE_OBJECT2_BACKGROUND */
        JSTRACE_OBJECT,     /* FINALIZE_OBJECT4 */
        JSTRACE_OBJECT,     /* FINALIZE_OBJECT4_BACKGROUND */
        JSTRACE_OBJECT,     /* FINALIZE_OBJECT8 */
        JSTRACE_OBJECT,     /* FINALIZE_OBJECT8_BACKGROUND */
        JSTRACE_OBJECT,     /* FINALIZE_OBJECT12 */
        JSTRACE_OBJECT,     /* FINALIZE_OBJECT12_BACKGROUND */
        JSTRACE_OBJECT,     /* FINALIZE_OBJECT16 */
        JSTRACE_OBJECT,     /* FINALIZE_OBJECT16_BACKGROUND */
        JSTRACE_OBJECT,     /* FINALIZE_FUNCTION */
        JSTRACE_SCRIPT,     /* FINALIZE_SCRIPT */
        JSTRACE_SHAPE,      /* FINALIZE_SHAPE */
        JSTRACE_TYPE_OBJECT,/* FINALIZE_TYPE_OBJECT */
#if JS_HAS_XML_SUPPORT      /* FINALIZE_XML */
        JSTRACE_XML,
#endif
        JSTRACE_STRING,     /* FINALIZE_SHORT_STRING */
        JSTRACE_STRING,     /* FINALIZE_STRING */
        JSTRACE_STRING,     /* FINALIZE_EXTERNAL_STRING */
    };
    return map[thingKind];
}

inline JSGCTraceKind
GetGCThingTraceKind(const void *thing);

struct ArenaLists {

    /*
     * ArenaList::head points to the start of the list. Normally cursor points
     * to the first arena in the list with some free things and all arenas
     * before cursor are fully allocated. However, as the arena currently being
     * allocated from is considered full while its list of free spans is moved
     * into the freeList, during the GC or cell enumeration, when an
     * unallocated freeList is moved back to the arena, we can see an arena
     * with some free cells before the cursor. The cursor is an indirect
     * pointer to allow for efficient list insertion at the cursor point and
     * other list manipulations.
     */
    struct ArenaList {
        ArenaHeader     *head;
        ArenaHeader     **cursor;

        ArenaList() {
            clear();
        }

        void clear() {
            head = NULL;
            cursor = &head;
        }
    };

  private:
    /*
     * For each arena kind its free list is represented as the first span with
     * free things. Initially all the spans are initialized as empty. After we
     * find a new arena with available things we move its first free span into
     * the list and set the arena as fully allocated. way we do not need to
     * update the arena header after the initial allocation. When starting the
     * GC we only move the head of the of the list of spans back to the arena
     * only for the arena that was not fully allocated.
     */
    FreeSpan       freeLists[FINALIZE_LIMIT];

    ArenaList      arenaLists[FINALIZE_LIMIT];

#ifdef JS_THREADSAFE
    /*
     * The background finalization adds the finalized arenas to the list at
     * the *cursor position. backgroundFinalizeState controls the interaction
     * between the GC lock and the access to the list from the allocation
     * thread.
     *
     * BFS_DONE indicates that the finalizations is not running or cannot
     * affect this arena list. The allocation thread can access the list
     * outside the GC lock.
     *
     * In BFS_RUN and BFS_JUST_FINISHED the allocation thread must take the
     * lock. The former indicates that the finalization still runs. The latter
     * signals that finalization just added to the list finalized arenas. In
     * that case the lock effectively serves as a read barrier to ensure that
     * the allocation thread see all the writes done during finalization.
     */
    enum BackgroundFinalizeState {
        BFS_DONE,
        BFS_RUN,
        BFS_JUST_FINISHED
    };

    volatile uintptr_t backgroundFinalizeState[FINALIZE_LIMIT];
#endif

  public:
    ArenaLists() {
        for (size_t i = 0; i != FINALIZE_LIMIT; ++i)
            freeLists[i].initAsEmpty();
#ifdef JS_THREADSAFE
        for (size_t i = 0; i != FINALIZE_LIMIT; ++i)
            backgroundFinalizeState[i] = BFS_DONE;
#endif
    }

    ~ArenaLists() {
        for (size_t i = 0; i != FINALIZE_LIMIT; ++i) {
#ifdef JS_THREADSAFE
            /*
             * We can only call this during the shutdown after the last GC when
             * the background finalization is disabled.
             */
            JS_ASSERT(backgroundFinalizeState[i] == BFS_DONE);
#endif
            ArenaHeader **headp = &arenaLists[i].head;
            while (ArenaHeader *aheader = *headp) {
                *headp = aheader->next;
                aheader->chunk()->releaseArena(aheader);
            }
        }
    }

    const FreeSpan *getFreeList(AllocKind thingKind) const {
        return &freeLists[thingKind];
    }

    ArenaHeader *getFirstArena(AllocKind thingKind) const {
        return arenaLists[thingKind].head;
    }

    bool arenaListsAreEmpty() const {
        for (size_t i = 0; i != FINALIZE_LIMIT; ++i) {
#ifdef JS_THREADSAFE
            /*
             * The arena cannot be empty if the background finalization is not yet
             * done.
             */
            if (backgroundFinalizeState[i] != BFS_DONE)
                return false;
#endif
            if (arenaLists[i].head)
                return false;
        }
        return true;
    }

#ifdef DEBUG
    bool checkArenaListAllUnmarked() const {
        for (size_t i = 0; i != FINALIZE_LIMIT; ++i) {
# ifdef JS_THREADSAFE
            /* The background finalization must have stopped at this point. */
            JS_ASSERT(backgroundFinalizeState[i] == BFS_DONE ||
                      backgroundFinalizeState[i] == BFS_JUST_FINISHED);
# endif
            for (ArenaHeader *aheader = arenaLists[i].head; aheader; aheader = aheader->next) {
                if (!aheader->chunk()->bitmap.noBitsSet(aheader))
                    return false;
            }
        }
        return true;
    }
#endif

#ifdef JS_THREADSAFE
    bool doneBackgroundFinalize(AllocKind kind) const {
        return backgroundFinalizeState[kind] == BFS_DONE;
    }
#endif

    /*
     * Return the free list back to the arena so the GC finalization will not
     * run the finalizers over unitialized bytes from free things.
     */
    void purge() {
        for (size_t i = 0; i != FINALIZE_LIMIT; ++i) {
            FreeSpan *headSpan = &freeLists[i];
            if (!headSpan->isEmpty()) {
                ArenaHeader *aheader = headSpan->arenaHeader();
                JS_ASSERT(!aheader->hasFreeThings());
                aheader->setFirstFreeSpan(headSpan);
                headSpan->initAsEmpty();
            }
        }
    }

    /*
     * Temporarily copy the free list heads to the arenas so the code can see
     * the proper value in ArenaHeader::freeList when accessing the latter
     * outside the GC.
     */
    void copyFreeListsToArenas() {
        for (size_t i = 0; i != FINALIZE_LIMIT; ++i)
            copyFreeListToArena(AllocKind(i));
    }

    void copyFreeListToArena(AllocKind thingKind) {
        FreeSpan *headSpan = &freeLists[thingKind];
        if (!headSpan->isEmpty()) {
            ArenaHeader *aheader = headSpan->arenaHeader();
            JS_ASSERT(!aheader->hasFreeThings());
            aheader->setFirstFreeSpan(headSpan);
        }
    }

    /*
     * Clear the free lists in arenas that were temporarily set there using
     * copyToArenas.
     */
    void clearFreeListsInArenas() {
        for (size_t i = 0; i != FINALIZE_LIMIT; ++i)
            clearFreeListInArena(AllocKind(i));
    }


    void clearFreeListInArena(AllocKind kind) {
        FreeSpan *headSpan = &freeLists[kind];
        if (!headSpan->isEmpty()) {
            ArenaHeader *aheader = headSpan->arenaHeader();
            JS_ASSERT(aheader->getFirstFreeSpan().isSameNonEmptySpan(headSpan));
            aheader->setAsFullyUsed();
        }
    }

    /*
     * Check that the free list is either empty or were synchronized with the
     * arena using copyToArena().
     */
    bool isSynchronizedFreeList(AllocKind kind) {
        FreeSpan *headSpan = &freeLists[kind];
        if (headSpan->isEmpty())
            return true;
        ArenaHeader *aheader = headSpan->arenaHeader();
        if (aheader->hasFreeThings()) {
            /*
             * If the arena has a free list, it must be the same as one in
             * lists.
             */
            JS_ASSERT(aheader->getFirstFreeSpan().isSameNonEmptySpan(headSpan));
            return true;
        }
        return false;
    }

    JS_ALWAYS_INLINE void *allocateFromFreeList(AllocKind thingKind, size_t thingSize) {
        return freeLists[thingKind].allocate(thingSize);
    }

    static void *refillFreeList(JSContext *cx, AllocKind thingKind);

    void checkEmptyFreeLists() {
#ifdef DEBUG
        for (size_t i = 0; i < mozilla::ArrayLength(freeLists); ++i)
            JS_ASSERT(freeLists[i].isEmpty());
#endif
    }

    void checkEmptyFreeList(AllocKind kind) {
        JS_ASSERT(freeLists[kind].isEmpty());
    }

    void finalizeObjects(JSContext *cx);
    void finalizeStrings(JSContext *cx);
    void finalizeShapes(JSContext *cx);
    void finalizeScripts(JSContext *cx);

#ifdef JS_THREADSAFE
    static void backgroundFinalize(JSContext *cx, ArenaHeader *listHead);
#endif

  private:
    inline void finalizeNow(JSContext *cx, AllocKind thingKind);
    inline void finalizeLater(JSContext *cx, AllocKind thingKind);

    inline void *allocateFromArena(JSCompartment *comp, AllocKind thingKind);
};

/*
 * Initial allocation size for data structures holding chunks is set to hold
 * chunks with total capacity of 16MB to avoid buffer resizes during browser
 * startup.
 */
const size_t INITIAL_CHUNK_CAPACITY = 16 * 1024 * 1024 / ChunkSize;

/* The number of GC cycles an empty chunk can survive before been released. */
const size_t MAX_EMPTY_CHUNK_AGE = 4;

} /* namespace gc */

struct GCPtrHasher
{
    typedef void *Lookup;

    static HashNumber hash(void *key) {
        return HashNumber(uintptr_t(key) >> JS_GCTHING_ZEROBITS);
    }

    static bool match(void *l, void *k) { return l == k; }
};

typedef HashMap<void *, uint32, GCPtrHasher, SystemAllocPolicy> GCLocks;

struct RootInfo {
    RootInfo() {}
    RootInfo(const char *name, JSGCRootType type) : name(name), type(type) {}
    const char *name;
    JSGCRootType type;
};

typedef js::HashMap<void *,
                    RootInfo,
                    js::DefaultHasher<void *>,
                    js::SystemAllocPolicy> RootedValueMap;

/* If HashNumber grows, need to change WrapperHasher. */
JS_STATIC_ASSERT(sizeof(HashNumber) == 4);

struct WrapperHasher
{
    typedef Value Lookup;

    static HashNumber hash(Value key) {
        uint64 bits = key.asRawBits();
        return (uint32)bits ^ (uint32)(bits >> 32);
    }

    static bool match(const Value &l, const Value &k) { return l == k; }
};

typedef HashMap<Value, Value, WrapperHasher, SystemAllocPolicy> WrapperMap;

class AutoValueVector;
class AutoIdVector;

} /* namespace js */

#ifdef DEBUG
extern bool
CheckAllocation(JSContext *cx);
#endif

extern JS_FRIEND_API(JSGCTraceKind)
js_GetGCThingTraceKind(void *thing);

extern JSBool
js_InitGC(JSRuntime *rt, uint32 maxbytes);

extern void
js_FinishGC(JSRuntime *rt);

extern JSBool
js_AddRoot(JSContext *cx, js::Value *vp, const char *name);

extern JSBool
js_AddGCThingRoot(JSContext *cx, void **rp, const char *name);

#ifdef DEBUG
extern void
js_DumpNamedRoots(JSRuntime *rt,
                  void (*dump)(const char *name, void *rp, JSGCRootType type, void *data),
                  void *data);
#endif

extern uint32
js_MapGCRoots(JSRuntime *rt, JSGCRootMapFun map, void *data);

/* Table of pointers with count valid members. */
typedef struct JSPtrTable {
    size_t      count;
    void        **array;
} JSPtrTable;

extern JSBool
js_RegisterCloseableIterator(JSContext *cx, JSObject *obj);

#ifdef JS_TRACER
extern JSBool
js_ReserveObjects(JSContext *cx, size_t nobjects);
#endif

extern JSBool
js_LockGCThingRT(JSRuntime *rt, void *thing);

extern void
js_UnlockGCThingRT(JSRuntime *rt, void *thing);

extern JS_FRIEND_API(bool)
IsAboutToBeFinalized(JSContext *cx, const void *thing);

extern JS_FRIEND_API(bool)
js_GCThingIsMarked(void *thing, uintN color);

extern void
js_TraceStackFrame(JSTracer *trc, js::StackFrame *fp);

namespace js {

extern JS_REQUIRES_STACK void
MarkRuntime(JSTracer *trc);

extern void
TraceRuntime(JSTracer *trc);

extern JS_REQUIRES_STACK JS_FRIEND_API(void)
MarkContext(JSTracer *trc, JSContext *acx);

/* Must be called with GC lock taken. */
extern void
TriggerGC(JSRuntime *rt, js::gcstats::Reason reason);

/* Must be called with GC lock taken. */
extern void
TriggerCompartmentGC(JSCompartment *comp, js::gcstats::Reason reason);

extern void
MaybeGC(JSContext *cx);

} /* namespace js */

/*
 * Kinds of js_GC invocation.
 */
typedef enum JSGCInvocationKind {
    /* Normal invocation. */
    GC_NORMAL           = 0,

    /*
     * Called from js_DestroyContext for last JSContext in a JSRuntime, when
     * it is imperative that rt->gcPoke gets cleared early in js_GC.
     */
    GC_LAST_CONTEXT     = 1,

    /* Minimize GC triggers and release empty GC chunks right away. */
    GC_SHRINK             = 2
} JSGCInvocationKind;

/* Pass NULL for |comp| to get a full GC. */
extern void
js_GC(JSContext *cx, JSCompartment *comp, JSGCInvocationKind gckind, js::gcstats::Reason r);

#ifdef JS_THREADSAFE
/*
 * This is a helper for code at can potentially run outside JS request to
 * ensure that the GC is not running when the function returns.
 *
 * This function must be called with the GC lock held.
 */
extern void
js_WaitForGC(JSRuntime *rt);

#else /* !JS_THREADSAFE */

# define js_WaitForGC(rt)    ((void) 0)

#endif

namespace js {

#ifdef JS_THREADSAFE

class GCHelperThread {
    enum State {
        IDLE,
        SWEEPING,
        ALLOCATING,
        CANCEL_ALLOCATION,
        SHUTDOWN
    };

    /*
     * During the finalization we do not free immediately. Rather we add the
     * corresponding pointers to a buffer which we later release on a
     * separated thread.
     *
     * The buffer is implemented as a vector of 64K arrays of pointers, not as
     * a simple vector, to avoid realloc calls during the vector growth and to
     * not bloat the binary size of the inlined freeLater method. Any OOM
     * during buffer growth results in the pointer being freed immediately.
     */
    static const size_t FREE_ARRAY_SIZE = size_t(1) << 16;
    static const size_t FREE_ARRAY_LENGTH = FREE_ARRAY_SIZE / sizeof(void *);

    JSRuntime         *const rt;
    PRThread          *thread;
    PRCondVar         *wakeup;
    PRCondVar         *done;
    volatile State    state;

    JSContext         *context;
    bool              shrinkFlag;

    Vector<void **, 16, js::SystemAllocPolicy> freeVector;
    void            **freeCursor;
    void            **freeCursorEnd;

    Vector<js::gc::ArenaHeader *, 64, js::SystemAllocPolicy> finalizeVector;

    bool    backgroundAllocation;

    friend struct js::gc::ArenaLists;

    JS_FRIEND_API(void)
    replenishAndFreeLater(void *ptr);

    static void freeElementsAndArray(void **array, void **end) {
        JS_ASSERT(array <= end);
        for (void **p = array; p != end; ++p)
            js::Foreground::free_(*p);
        js::Foreground::free_(array);
    }

    static void threadMain(void* arg);
    void threadLoop();

    /* Must be called with the GC lock taken. */
    void doSweep();

  public:
    GCHelperThread(JSRuntime *rt)
      : rt(rt),
        thread(NULL),
        wakeup(NULL),
        done(NULL),
        state(IDLE),
        freeCursor(NULL),
        freeCursorEnd(NULL),
        backgroundAllocation(true)
    { }

    bool init();
    void finish();

    /* Must be called with the GC lock taken. */
    inline void startBackgroundSweep(bool shouldShrink);

    /* Must be called with the GC lock taken. */
    void waitBackgroundSweepEnd();

    /* Must be called with the GC lock taken. */
    void waitBackgroundSweepOrAllocEnd();

    /* Must be called with the GC lock taken. */
    inline void startBackgroundAllocationIfIdle();

    bool canBackgroundAllocate() const {
        return backgroundAllocation;
    }

    void disableBackgroundAllocation() {
        backgroundAllocation = false;
    }

    /*
     * Outside the GC lock may give true answer when in fact the sweeping has
     * been done.
     */
    bool sweeping() const {
        return state == SWEEPING;
    }

    bool shouldShrink() const {
        JS_ASSERT(sweeping());
        return shrinkFlag;
    }

    void freeLater(void *ptr) {
        JS_ASSERT(!sweeping());
        if (freeCursor != freeCursorEnd)
            *freeCursor++ = ptr;
        else
            replenishAndFreeLater(ptr);
    }

    /* Must be called with the GC lock taken. */
    bool prepareForBackgroundSweep(JSContext *cx);
};

#endif /* JS_THREADSAFE */

struct GCChunkHasher {
    typedef gc::Chunk *Lookup;

    /*
     * Strip zeros for better distribution after multiplying by the golden
     * ratio.
     */
    static HashNumber hash(gc::Chunk *chunk) {
        JS_ASSERT(!(jsuword(chunk) & gc::ChunkMask));
        return HashNumber(jsuword(chunk) >> gc::ChunkShift);
    }

    static bool match(gc::Chunk *k, gc::Chunk *l) {
        JS_ASSERT(!(jsuword(k) & gc::ChunkMask));
        JS_ASSERT(!(jsuword(l) & gc::ChunkMask));
        return k == l;
    }
};

typedef HashSet<js::gc::Chunk *, GCChunkHasher, SystemAllocPolicy> GCChunkSet;

struct ConservativeGCThreadData {

    /*
     * The GC scans conservatively between ThreadData::nativeStackBase and
     * nativeStackTop unless the latter is NULL.
     */
    jsuword             *nativeStackTop;

    union {
        jmp_buf         jmpbuf;
        jsuword         words[JS_HOWMANY(sizeof(jmp_buf), sizeof(jsuword))];
    } registerSnapshot;

    /*
     * Cycle collector uses this to communicate that the native stack of the
     * GC thread should be scanned only if the thread have more than the given
     * threshold of requests.
     */
    unsigned requestThreshold;

    ConservativeGCThreadData()
      : nativeStackTop(NULL), requestThreshold(0)
    {
    }

    ~ConservativeGCThreadData() {
#ifdef JS_THREADSAFE
        /*
         * The conservative GC scanner should be disabled when the thread leaves
         * the last request.
         */
        JS_ASSERT(!hasStackToScan());
#endif
    }

    JS_NEVER_INLINE void recordStackTop();

#ifdef JS_THREADSAFE
    void updateForRequestEnd(unsigned suspendCount) {
        if (suspendCount)
            recordStackTop();
        else
            nativeStackTop = NULL;
    }
#endif

    bool hasStackToScan() const {
        return !!nativeStackTop;
    }
};

template<class T>
struct MarkStack {
    T *stack;
    uintN tos, limit;

    bool push(T item) {
        if (tos == limit)
            return false;
        stack[tos++] = item;
        return true;
    }

    bool isEmpty() { return tos == 0; }

    T pop() {
        JS_ASSERT(!isEmpty());
        return stack[--tos];
    }

    T &peek() {
        JS_ASSERT(!isEmpty());
        return stack[tos-1];
    }

    MarkStack(void **buffer, size_t size)
    {
        tos = 0;
        limit = size / sizeof(T) - 1;
        stack = (T *)buffer;
    }
};

struct LargeMarkItem
{
    JSObject *obj;
    uintN markpos;

    LargeMarkItem(JSObject *obj) : obj(obj), markpos(0) {}
};

static const size_t OBJECT_MARK_STACK_SIZE = 32768 * sizeof(JSObject *);
static const size_t ROPES_MARK_STACK_SIZE = 1024 * sizeof(JSString *);
static const size_t XML_MARK_STACK_SIZE = 1024 * sizeof(JSXML *);
static const size_t TYPE_MARK_STACK_SIZE = 1024 * sizeof(types::TypeObject *);
static const size_t LARGE_MARK_STACK_SIZE = 64 * sizeof(LargeMarkItem);

struct GCMarker : public JSTracer {
  private:
    /* The color is only applied to objects, functions and xml. */
    uint32 color;

  public:
    /* Pointer to the top of the stack of arenas we are delaying marking on. */
    js::gc::Arena *unmarkedArenaStackTop;
    /* Count of arenas that are currently in the stack. */
    DebugOnly<size_t> markLaterArenas;

#ifdef JS_DUMP_CONSERVATIVE_GC_ROOTS
    js::gc::ConservativeGCStats conservativeStats;
    Vector<void *, 0, SystemAllocPolicy> conservativeRoots;
    const char *conservativeDumpFileName;
    void dumpConservativeRoots();
#endif

    MarkStack<JSObject *> objStack;
    MarkStack<JSRope *> ropeStack;
    MarkStack<types::TypeObject *> typeStack;
    MarkStack<JSXML *> xmlStack;
    MarkStack<LargeMarkItem> largeStack;

  public:
    explicit GCMarker(JSContext *cx);
    ~GCMarker();

    uint32 getMarkColor() const {
        return color;
    }

    /*
     * The only valid color transition during a GC is from black to gray. It is
     * wrong to switch the mark color from gray to black. The reason is that the
     * cycle collector depends on the invariant that there are no black to gray
     * edges in the GC heap. This invariant lets the CC not trace through black
     * objects. If this invariant is violated, the cycle collector may free
     * objects that are still reachable.
     *
     * We don't assert this yet, but we should.
     */
    void setMarkColor(uint32 newColor) {
        //JS_ASSERT(color == BLACK && newColor == GRAY);
        color = newColor;
    }

    void delayMarkingChildren(const void *thing);

    void markDelayedChildren();

    bool isMarkStackEmpty() {
        return objStack.isEmpty() &&
               ropeStack.isEmpty() &&
               typeStack.isEmpty() &&
               xmlStack.isEmpty() &&
               largeStack.isEmpty();
    }

    void drainMarkStack();

    void pushObject(JSObject *obj) {
        if (!objStack.push(obj))
            delayMarkingChildren(obj);
    }

    void pushRope(JSRope *rope) {
        if (!ropeStack.push(rope))
            delayMarkingChildren(rope);
    }

    void pushType(types::TypeObject *type) {
        if (!typeStack.push(type))
            delayMarkingChildren(type);
    }

    void pushXML(JSXML *xml) {
        if (!xmlStack.push(xml))
            delayMarkingChildren(xml);
    }
};

void
MarkStackRangeConservatively(JSTracer *trc, Value *begin, Value *end);

typedef void (*IterateCompartmentCallback)(JSContext *cx, void *data, JSCompartment *compartment);
typedef void (*IterateChunkCallback)(JSContext *cx, void *data, gc::Chunk *chunk);
typedef void (*IterateArenaCallback)(JSContext *cx, void *data, gc::Arena *arena,
                                     JSGCTraceKind traceKind, size_t thingSize);
typedef void (*IterateCellCallback)(JSContext *cx, void *data, void *thing,
                                    JSGCTraceKind traceKind, size_t thingSize);

/*
 * This function calls |compartmentCallback| on every compartment,
 * |arenaCallback| on every in-use arena, and |cellCallback| on every in-use
 * cell in the GC heap.
 */
extern JS_FRIEND_API(void)
IterateCompartmentsArenasCells(JSContext *cx, void *data,
                               IterateCompartmentCallback compartmentCallback,
                               IterateArenaCallback arenaCallback,
                               IterateCellCallback cellCallback);

/*
 * Invoke chunkCallback on every in-use chunk.
 */
extern JS_FRIEND_API(void)
IterateChunks(JSContext *cx, void *data, IterateChunkCallback chunkCallback);

/*
 * Invoke cellCallback on every in-use object of the specified thing kind for
 * the given compartment or for all compartments if it is null.
 */
extern JS_FRIEND_API(void)
IterateCells(JSContext *cx, JSCompartment *compartment, gc::AllocKind thingKind,
             void *data, IterateCellCallback cellCallback);

} /* namespace js */

extern void
js_FinalizeStringRT(JSRuntime *rt, JSString *str);

/*
 * Macro to test if a traversal is the marking phase of the GC.
 */
#define IS_GC_MARKING_TRACER(trc) ((trc)->callback == NULL)

namespace js {
namespace gc {

JSCompartment *
NewCompartment(JSContext *cx, JSPrincipals *principals);

/* Tries to run a GC no matter what (used for GC zeal). */
void
RunDebugGC(JSContext *cx);

} /* namespace gc */

static inline JSCompartment *
GetObjectCompartment(JSObject *obj) { return reinterpret_cast<js::gc::Cell *>(obj)->compartment(); }

} /* namespace js */

#endif /* jsgc_h___ */