summaryrefslogtreecommitdiffstats
path: root/js/src/vm/String.h
blob: 0937ac24d2b94af806c21b5f78ea2d9440a2b1a9 (plain)
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
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
 * 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 vm_String_h
#define vm_String_h

#include "mozilla/MemoryReporting.h"
#include "mozilla/Range.h"

#include "jsapi.h"
#include "jsfriendapi.h"
#include "jsstr.h"

#include "gc/Barrier.h"
#include "gc/Heap.h"
#include "gc/Marking.h"
#include "gc/Rooting.h"
#include "js/CharacterEncoding.h"
#include "js/GCAPI.h"
#include "js/RootingAPI.h"

class JSDependentString;
class JSExtensibleString;
class JSExternalString;
class JSInlineString;
class JSRope;

namespace js {

class AutoStableStringChars;
class StaticStrings;
class PropertyName;

/* The buffer length required to contain any unsigned 32-bit integer. */
static const size_t UINT32_CHAR_BUFFER_LENGTH = sizeof("4294967295") - 1;

} /* namespace js */

/*
 * JavaScript strings
 *
 * Conceptually, a JS string is just an array of chars and a length. This array
 * of chars may or may not be null-terminated and, if it is, the null character
 * is not included in the length.
 *
 * To improve performance of common operations, the following optimizations are
 * made which affect the engine's representation of strings:
 *
 *  - The plain vanilla representation is a "flat" string which consists of a
 *    string header in the GC heap and a malloc'd null terminated char array.
 *
 *  - To avoid copying a substring of an existing "base" string , a "dependent"
 *    string (JSDependentString) can be created which points into the base
 *    string's char array.
 *
 *  - To avoid O(n^2) char buffer copying, a "rope" node (JSRope) can be created
 *    to represent a delayed string concatenation. Concatenation (called
 *    flattening) is performed if and when a linear char array is requested. In
 *    general, ropes form a binary dag whose internal nodes are JSRope string
 *    headers with no associated char array and whose leaf nodes are either flat
 *    or dependent strings.
 *
 *  - To avoid copying the leftmost string when flattening, we may produce an
 *    "extensible" string, which tracks not only its actual length but also its
 *    buffer's overall size. If such an "extensible" string appears as the
 *    leftmost string in a subsequent flatten, and its buffer has enough unused
 *    space, we can simply flatten the rest of the ropes into its buffer,
 *    leaving its text in place. We then transfer ownership of its buffer to the
 *    flattened rope, and mutate the donor extensible string into a dependent
 *    string referencing its original buffer.
 *
 *    (The term "extensible" does not imply that we ever 'realloc' the buffer.
 *    Extensible strings may have dependent strings pointing into them, and the
 *    JSAPI hands out pointers to flat strings' buffers, so resizing with
 *    'realloc' is generally not possible.)
 *
 *  - To avoid allocating small char arrays, short strings can be stored inline
 *    in the string header (JSInlineString). These come in two flavours:
 *    JSThinInlineString, which is the same size as JSString; and
 *    JSFatInlineString, which has a larger header and so can fit more chars.
 *
 *  - To avoid comparing O(n) string equality comparison, strings can be
 *    canonicalized to "atoms" (JSAtom) such that there is a single atom with a
 *    given (length,chars).
 *
 *  - To avoid copying all strings created through the JSAPI, an "external"
 *    string (JSExternalString) can be created whose chars are managed by the
 *    JSAPI client.
 *
 *  - To avoid using two bytes per character for every string, string characters
 *    are stored as Latin1 instead of TwoByte if all characters are representable
 *    in Latin1.
 *
 * Although all strings share the same basic memory layout, we can conceptually
 * arrange them into a hierarchy of operations/invariants and represent this
 * hierarchy in C++ with classes:
 *
 * C++ type                     operations+fields / invariants+properties
 * ==========================   =========================================
 * JSString (abstract)          get(Latin1|TwoByte)CharsZ, get(Latin1|TwoByte)Chars, length / -
 *  | \
 *  | JSRope                    leftChild, rightChild / -
 *  |
 * JSLinearString (abstract)    latin1Chars, twoByteChars / might be null-terminated
 *  |  |
 *  |  +-- JSDependentString    base / -
 *  |  |
 *  |  +-- JSExternalString     - / char array memory managed by embedding
 *  |
 * JSFlatString                 - / null terminated
 *  |  |
 *  |  +-- JSExtensibleString   tracks total buffer capacity (including current text)
 *  |  |
 *  |  +-- JSUndependedString   original dependent base / -
 *  |  |
 *  |  +-- JSInlineString (abstract)    - / chars stored in header
 *  |      |
 *  |      +-- JSThinInlineString       - / header is normal
 *  |      |
 *  |      +-- JSFatInlineString        - / header is fat
 *  |
 * JSAtom (abstract)            - / string equality === pointer equality
 *  |  |
 *  |  +-- js::NormalAtom       - JSFlatString + atom hash code
 *  |  |
 *  |  +-- js::FatInlineAtom    - JSFatInlineString + atom hash code
 *  |
 * js::PropertyName             - / chars don't contain an index (uint32_t)
 *
 * Classes marked with (abstract) above are not literally C++ Abstract Base
 * Classes (since there are no virtual functions, pure or not, in this
 * hierarchy), but have the same meaning: there are no strings with this type as
 * its most-derived type.
 *
 * Atoms can additionally be permanent, i.e. unable to be collected, and can
 * be combined with other string types to create additional most-derived types
 * that satisfy the invariants of more than one of the abovementioned
 * most-derived types. Furthermore, each atom stores a hash number (based on its
 * chars). This hash number is used as key in the atoms table and when the atom
 * is used as key in a JS Map/Set.
 *
 * Derived string types can be queried from ancestor types via isX() and
 * retrieved with asX() debug-only-checked casts.
 *
 * The ensureX() operations mutate 'this' in place to effectively the type to be
 * at least X (e.g., ensureLinear will change a JSRope to be a JSFlatString).
 */

class JSString : public js::gc::TenuredCell
{
  protected:
    static const size_t NUM_INLINE_CHARS_LATIN1   = 2 * sizeof(void*) / sizeof(JS::Latin1Char);
    static const size_t NUM_INLINE_CHARS_TWO_BYTE = 2 * sizeof(void*) / sizeof(char16_t);

    /* Fields only apply to string types commented on the right. */
    struct Data
    {
        union {
            struct {
                uint32_t           flags;               /* JSString */
                uint32_t           length;              /* JSString */
            };
            uintptr_t              flattenData;         /* JSRope (temporary while flattening) */
        } u1;
        union {
            union {
                /* JS(Fat)InlineString */
                JS::Latin1Char     inlineStorageLatin1[NUM_INLINE_CHARS_LATIN1];
                char16_t           inlineStorageTwoByte[NUM_INLINE_CHARS_TWO_BYTE];
            };
            struct {
                union {
                    const JS::Latin1Char* nonInlineCharsLatin1; /* JSLinearString, except JS(Fat)InlineString */
                    const char16_t* nonInlineCharsTwoByte;/* JSLinearString, except JS(Fat)InlineString */
                    JSString*      left;               /* JSRope */
                } u2;
                union {
                    JSLinearString* base;               /* JS(Dependent|Undepended)String */
                    JSString*      right;              /* JSRope */
                    size_t         capacity;            /* JSFlatString (extensible) */
                    const JSStringFinalizer* externalFinalizer;/* JSExternalString */
                } u3;
            } s;
        };
    } d;

  public:
    /* Flags exposed only for jits */

    /*
     * The Flags Word
     *
     * The flags word stores both the string's type and its character encoding.
     *
     * If LATIN1_CHARS_BIT is set, the string's characters are stored as Latin1
     * instead of TwoByte. This flag can also be set for ropes, if both the
     * left and right nodes are Latin1. Flattening will result in a Latin1
     * string in this case.
     *
     * The other flags store the string's type. Instead of using a dense index
     * to represent the most-derived type, string types are encoded to allow
     * single-op tests for hot queries (isRope, isDependent, isFlat, isAtom)
     * which, in view of subtyping, would require slower
     * (isX() || isY() || isZ()).
     *
     * The string type encoding can be summarized as follows. The "instance
     * encoding" entry for a type specifies the flag bits used to create a
     * string instance of that type. Abstract types have no instances and thus
     * have no such entry. The "subtype predicate" entry for a type specifies
     * the predicate used to query whether a JSString instance is subtype
     * (reflexively) of that type.
     *
     *   String        Instance     Subtype
     *   type          encoding     predicate
     *   ------------------------------------
     *   Rope          000000       000000
     *   Linear        -           !000000
     *   HasBase       -            xxxx1x
     *   Dependent     000010       000010
     *   External      100000       100000
     *   Flat          -            xxxxx1
     *   Undepended    000011       000011
     *   Extensible    010001       010001
     *   Inline        000101       xxx1xx
     *   FatInline     010101       x1x1xx
     *   Atom          001001       xx1xxx
     *   PermanentAtom 101001       1x1xxx
     *   InlineAtom    -            xx11xx
     *   FatInlineAtom -            x111xx
     *
     * Note that the first 4 flag bits (from right to left in the previous table)
     * have the following meaning and can be used for some hot queries:
     *
     *   Bit 0: IsFlat
     *   Bit 1: HasBase (Dependent, Undepended)
     *   Bit 2: IsInline (Inline, FatInline)
     *   Bit 3: IsAtom (Atom, PermanentAtom)
     *
     *  "HasBase" here refers to the two string types that have a 'base' field:
     *  JSDependentString and JSUndependedString.
     *  A JSUndependedString is a JSDependentString which has been 'fixed' (by ensureFixed)
     *  to be null-terminated.  In such cases, the string must keep marking its base since
     *  there may be any number of *other* JSDependentStrings transitively depending on it.
     *
     */

    static const uint32_t FLAT_BIT               = JS_BIT(0);
    static const uint32_t HAS_BASE_BIT           = JS_BIT(1);
    static const uint32_t INLINE_CHARS_BIT       = JS_BIT(2);
    static const uint32_t ATOM_BIT               = JS_BIT(3);

    static const uint32_t ROPE_FLAGS             = 0;
    static const uint32_t DEPENDENT_FLAGS        = HAS_BASE_BIT;
    static const uint32_t UNDEPENDED_FLAGS       = FLAT_BIT | HAS_BASE_BIT;
    static const uint32_t EXTENSIBLE_FLAGS       = FLAT_BIT | JS_BIT(4);
    static const uint32_t EXTERNAL_FLAGS         = JS_BIT(5);

    static const uint32_t FAT_INLINE_MASK        = INLINE_CHARS_BIT | JS_BIT(4);
    static const uint32_t PERMANENT_ATOM_MASK    = ATOM_BIT | JS_BIT(5);

    /* Initial flags for thin inline and fat inline strings. */
    static const uint32_t INIT_THIN_INLINE_FLAGS = FLAT_BIT | INLINE_CHARS_BIT;
    static const uint32_t INIT_FAT_INLINE_FLAGS  = FLAT_BIT | FAT_INLINE_MASK;

    static const uint32_t TYPE_FLAGS_MASK        = JS_BIT(6) - 1;

    static const uint32_t LATIN1_CHARS_BIT       = JS_BIT(6);

    static const uint32_t MAX_LENGTH             = js::MaxStringLength;

    static const JS::Latin1Char MAX_LATIN1_CHAR = 0xff;

    /*
     * Helper function to validate that a string of a given length is
     * representable by a JSString. An allocation overflow is reported if false
     * is returned.
     */
    static inline bool validateLength(js::ExclusiveContext* maybecx, size_t length);

    static void staticAsserts() {
        static_assert(JSString::MAX_LENGTH < UINT32_MAX, "Length must fit in 32 bits");
        static_assert(sizeof(JSString) ==
                      (offsetof(JSString, d.inlineStorageLatin1) +
                       NUM_INLINE_CHARS_LATIN1 * sizeof(char)),
                      "Inline Latin1 chars must fit in a JSString");
        static_assert(sizeof(JSString) ==
                      (offsetof(JSString, d.inlineStorageTwoByte) +
                       NUM_INLINE_CHARS_TWO_BYTE * sizeof(char16_t)),
                      "Inline char16_t chars must fit in a JSString");

        /* Ensure js::shadow::String has the same layout. */
        using js::shadow::String;
        static_assert(offsetof(JSString, d.u1.length) == offsetof(String, length),
                      "shadow::String length offset must match JSString");
        static_assert(offsetof(JSString, d.u1.flags) == offsetof(String, flags),
                      "shadow::String flags offset must match JSString");
        static_assert(offsetof(JSString, d.s.u2.nonInlineCharsLatin1) == offsetof(String, nonInlineCharsLatin1),
                      "shadow::String nonInlineChars offset must match JSString");
        static_assert(offsetof(JSString, d.s.u2.nonInlineCharsTwoByte) == offsetof(String, nonInlineCharsTwoByte),
                      "shadow::String nonInlineChars offset must match JSString");
        static_assert(offsetof(JSString, d.inlineStorageLatin1) == offsetof(String, inlineStorageLatin1),
                      "shadow::String inlineStorage offset must match JSString");
        static_assert(offsetof(JSString, d.inlineStorageTwoByte) == offsetof(String, inlineStorageTwoByte),
                      "shadow::String inlineStorage offset must match JSString");
        static_assert(INLINE_CHARS_BIT == String::INLINE_CHARS_BIT,
                      "shadow::String::INLINE_CHARS_BIT must match JSString::INLINE_CHARS_BIT");
        static_assert(LATIN1_CHARS_BIT == String::LATIN1_CHARS_BIT,
                      "shadow::String::LATIN1_CHARS_BIT must match JSString::LATIN1_CHARS_BIT");
        static_assert(TYPE_FLAGS_MASK == String::TYPE_FLAGS_MASK,
                      "shadow::String::TYPE_FLAGS_MASK must match JSString::TYPE_FLAGS_MASK");
        static_assert(ROPE_FLAGS == String::ROPE_FLAGS,
                      "shadow::String::ROPE_FLAGS must match JSString::ROPE_FLAGS");
    }

    /* Avoid lame compile errors in JSRope::flatten */
    friend class JSRope;

    friend class js::gc::RelocationOverlay;

  protected:
    template <typename CharT>
    MOZ_ALWAYS_INLINE
    void setNonInlineChars(const CharT* chars);

  public:
    /* All strings have length. */

    MOZ_ALWAYS_INLINE
    size_t length() const {
        return d.u1.length;
    }

    MOZ_ALWAYS_INLINE
    bool empty() const {
        return d.u1.length == 0;
    }

    inline bool getChar(js::ExclusiveContext* cx, size_t index, char16_t* code);

    /* Strings have either Latin1 or TwoByte chars. */
    bool hasLatin1Chars() const {
        return d.u1.flags & LATIN1_CHARS_BIT;
    }
    bool hasTwoByteChars() const {
        return !(d.u1.flags & LATIN1_CHARS_BIT);
    }

    /* Fallible conversions to more-derived string types. */

    inline JSLinearString* ensureLinear(js::ExclusiveContext* cx);
    JSFlatString* ensureFlat(JSContext* cx);

    static bool ensureLinear(js::ExclusiveContext* cx, JSString* str) {
        return str->ensureLinear(cx) != nullptr;
    }

    /* Type query and debug-checked casts */

    MOZ_ALWAYS_INLINE
    bool isRope() const {
        return (d.u1.flags & TYPE_FLAGS_MASK) == ROPE_FLAGS;
    }

    MOZ_ALWAYS_INLINE
    JSRope& asRope() const {
        MOZ_ASSERT(isRope());
        return *(JSRope*)this;
    }

    MOZ_ALWAYS_INLINE
    bool isLinear() const {
        return !isRope();
    }

    MOZ_ALWAYS_INLINE
    JSLinearString& asLinear() const {
        MOZ_ASSERT(JSString::isLinear());
        return *(JSLinearString*)this;
    }

    MOZ_ALWAYS_INLINE
    bool isDependent() const {
        return (d.u1.flags & TYPE_FLAGS_MASK) == DEPENDENT_FLAGS;
    }

    MOZ_ALWAYS_INLINE
    JSDependentString& asDependent() const {
        MOZ_ASSERT(isDependent());
        return *(JSDependentString*)this;
    }

    MOZ_ALWAYS_INLINE
    bool isFlat() const {
        return d.u1.flags & FLAT_BIT;
    }

    MOZ_ALWAYS_INLINE
    JSFlatString& asFlat() const {
        MOZ_ASSERT(isFlat());
        return *(JSFlatString*)this;
    }

    MOZ_ALWAYS_INLINE
    bool isExtensible() const {
        return (d.u1.flags & TYPE_FLAGS_MASK) == EXTENSIBLE_FLAGS;
    }

    MOZ_ALWAYS_INLINE
    JSExtensibleString& asExtensible() const {
        MOZ_ASSERT(isExtensible());
        return *(JSExtensibleString*)this;
    }

    MOZ_ALWAYS_INLINE
    bool isInline() const {
        return d.u1.flags & INLINE_CHARS_BIT;
    }

    MOZ_ALWAYS_INLINE
    JSInlineString& asInline() const {
        MOZ_ASSERT(isInline());
        return *(JSInlineString*)this;
    }

    MOZ_ALWAYS_INLINE
    bool isFatInline() const {
        return (d.u1.flags & FAT_INLINE_MASK) == FAT_INLINE_MASK;
    }

    /* For hot code, prefer other type queries. */
    bool isExternal() const {
        return (d.u1.flags & TYPE_FLAGS_MASK) == EXTERNAL_FLAGS;
    }

    MOZ_ALWAYS_INLINE
    JSExternalString& asExternal() const {
        MOZ_ASSERT(isExternal());
        return *(JSExternalString*)this;
    }

    MOZ_ALWAYS_INLINE
    bool isUndepended() const {
        return (d.u1.flags & TYPE_FLAGS_MASK) == UNDEPENDED_FLAGS;
    }

    MOZ_ALWAYS_INLINE
    bool isAtom() const {
        return d.u1.flags & ATOM_BIT;
    }

    MOZ_ALWAYS_INLINE
    bool isPermanentAtom() const {
        return (d.u1.flags & PERMANENT_ATOM_MASK) == PERMANENT_ATOM_MASK;
    }

    MOZ_ALWAYS_INLINE
    JSAtom& asAtom() const {
        MOZ_ASSERT(isAtom());
        return *(JSAtom*)this;
    }

    /* Only called by the GC for dependent or undepended strings. */

    inline bool hasBase() const {
        return d.u1.flags & HAS_BASE_BIT;
    }

    inline JSLinearString* base() const;

    void traceBase(JSTracer* trc);

    /* Only called by the GC for strings with the AllocKind::STRING kind. */

    inline void finalize(js::FreeOp* fop);

    /* Gets the number of bytes that the chars take on the heap. */

    size_t sizeOfExcludingThis(mozilla::MallocSizeOf mallocSizeOf);

    /* Offsets for direct field from jit code. */

    static size_t offsetOfLength() {
        return offsetof(JSString, d.u1.length);
    }
    static size_t offsetOfFlags() {
        return offsetof(JSString, d.u1.flags);
    }

    static size_t offsetOfNonInlineChars() {
        static_assert(offsetof(JSString, d.s.u2.nonInlineCharsTwoByte) ==
                      offsetof(JSString, d.s.u2.nonInlineCharsLatin1),
                      "nonInlineCharsTwoByte and nonInlineCharsLatin1 must have same offset");
        return offsetof(JSString, d.s.u2.nonInlineCharsTwoByte);
    }

    static const JS::TraceKind TraceKind = JS::TraceKind::String;

#ifdef DEBUG
    void dump(FILE* fp);
    void dumpCharsNoNewline(FILE* fp);
    void dump();
    void dumpCharsNoNewline();
    void dumpRepresentation(FILE* fp, int indent) const;
    void dumpRepresentationHeader(FILE* fp, int indent, const char* subclass) const;

    template <typename CharT>
    static void dumpChars(const CharT* s, size_t len, FILE* fp=stderr);

    bool equals(const char* s);
#endif

    void traceChildren(JSTracer* trc);

    static MOZ_ALWAYS_INLINE void readBarrier(JSString* thing) {
        if (thing->isPermanentAtom())
            return;

        TenuredCell::readBarrier(thing);
    }

    static MOZ_ALWAYS_INLINE void writeBarrierPre(JSString* thing) {
        if (isNullLike(thing) || thing->isPermanentAtom())
            return;

        TenuredCell::writeBarrierPre(thing);
    }

  private:
    JSString() = delete;
    JSString(const JSString& other) = delete;
    void operator=(const JSString& other) = delete;
};

class JSRope : public JSString
{
    template <typename CharT>
    bool copyCharsInternal(js::ExclusiveContext* cx, js::ScopedJSFreePtr<CharT>& out,
                           bool nullTerminate) const;

    enum UsingBarrier { WithIncrementalBarrier, NoBarrier };

    template<UsingBarrier b, typename CharT>
    JSFlatString* flattenInternal(js::ExclusiveContext* cx);

    template<UsingBarrier b>
    JSFlatString* flattenInternal(js::ExclusiveContext* cx);

    friend class JSString;
    JSFlatString* flatten(js::ExclusiveContext* cx);

    void init(js::ExclusiveContext* cx, JSString* left, JSString* right, size_t length);

  public:
    template <js::AllowGC allowGC>
    static inline JSRope* new_(js::ExclusiveContext* cx,
                               typename js::MaybeRooted<JSString*, allowGC>::HandleType left,
                               typename js::MaybeRooted<JSString*, allowGC>::HandleType right,
                               size_t length);

    bool copyLatin1Chars(js::ExclusiveContext* cx,
                         js::ScopedJSFreePtr<JS::Latin1Char>& out) const;
    bool copyTwoByteChars(js::ExclusiveContext* cx, js::ScopedJSFreePtr<char16_t>& out) const;

    bool copyLatin1CharsZ(js::ExclusiveContext* cx,
                          js::ScopedJSFreePtr<JS::Latin1Char>& out) const;
    bool copyTwoByteCharsZ(js::ExclusiveContext* cx, js::ScopedJSFreePtr<char16_t>& out) const;

    template <typename CharT>
    bool copyChars(js::ExclusiveContext* cx, js::ScopedJSFreePtr<CharT>& out) const;

    JSString* leftChild() const {
        MOZ_ASSERT(isRope());
        return d.s.u2.left;
    }

    JSString* rightChild() const {
        MOZ_ASSERT(isRope());
        return d.s.u3.right;
    }

    void traceChildren(JSTracer* trc);

    static size_t offsetOfLeft() {
        return offsetof(JSRope, d.s.u2.left);
    }
    static size_t offsetOfRight() {
        return offsetof(JSRope, d.s.u3.right);
    }

#ifdef DEBUG
    void dumpRepresentation(FILE* fp, int indent) const;
#endif
};

static_assert(sizeof(JSRope) == sizeof(JSString),
              "string subclasses must be binary-compatible with JSString");

class JSLinearString : public JSString
{
    friend class JSString;
    friend class js::AutoStableStringChars;

    /* Vacuous and therefore unimplemented. */
    JSLinearString* ensureLinear(js::ExclusiveContext* cx) = delete;
    bool isLinear() const = delete;
    JSLinearString& asLinear() const = delete;

  protected:
    /* Returns void pointer to latin1/twoByte chars, for finalizers. */
    MOZ_ALWAYS_INLINE
    void* nonInlineCharsRaw() const {
        MOZ_ASSERT(!isInline());
        static_assert(offsetof(JSLinearString, d.s.u2.nonInlineCharsTwoByte) ==
                      offsetof(JSLinearString, d.s.u2.nonInlineCharsLatin1),
                      "nonInlineCharsTwoByte and nonInlineCharsLatin1 must have same offset");
        return (void*)d.s.u2.nonInlineCharsTwoByte;
    }

    MOZ_ALWAYS_INLINE const JS::Latin1Char* rawLatin1Chars() const;
    MOZ_ALWAYS_INLINE const char16_t* rawTwoByteChars() const;

  public:
    template<typename CharT>
    MOZ_ALWAYS_INLINE
    const CharT* nonInlineChars(const JS::AutoCheckCannotGC& nogc) const;

    MOZ_ALWAYS_INLINE
    const JS::Latin1Char* nonInlineLatin1Chars(const JS::AutoCheckCannotGC& nogc) const {
        MOZ_ASSERT(!isInline());
        MOZ_ASSERT(hasLatin1Chars());
        return d.s.u2.nonInlineCharsLatin1;
    }

    MOZ_ALWAYS_INLINE
    const char16_t* nonInlineTwoByteChars(const JS::AutoCheckCannotGC& nogc) const {
        MOZ_ASSERT(!isInline());
        MOZ_ASSERT(hasTwoByteChars());
        return d.s.u2.nonInlineCharsTwoByte;
    }

    template<typename CharT>
    MOZ_ALWAYS_INLINE
    const CharT* chars(const JS::AutoCheckCannotGC& nogc) const;

    MOZ_ALWAYS_INLINE
    const JS::Latin1Char* latin1Chars(const JS::AutoCheckCannotGC& nogc) const {
        return rawLatin1Chars();
    }

    MOZ_ALWAYS_INLINE
    const char16_t* twoByteChars(const JS::AutoCheckCannotGC& nogc) const {
        return rawTwoByteChars();
    }

    mozilla::Range<const JS::Latin1Char> latin1Range(const JS::AutoCheckCannotGC& nogc) const {
        MOZ_ASSERT(JSString::isLinear());
        return mozilla::Range<const JS::Latin1Char>(latin1Chars(nogc), length());
    }

    mozilla::Range<const char16_t> twoByteRange(const JS::AutoCheckCannotGC& nogc) const {
        MOZ_ASSERT(JSString::isLinear());
        return mozilla::Range<const char16_t>(twoByteChars(nogc), length());
    }

    MOZ_ALWAYS_INLINE
    char16_t latin1OrTwoByteChar(size_t index) const {
        MOZ_ASSERT(JSString::isLinear());
        MOZ_ASSERT(index < length());
        JS::AutoCheckCannotGC nogc;
        return hasLatin1Chars() ? latin1Chars(nogc)[index] : twoByteChars(nogc)[index];
    }

#ifdef DEBUG
    void dumpRepresentationChars(FILE* fp, int indent) const;
#endif
};

static_assert(sizeof(JSLinearString) == sizeof(JSString),
              "string subclasses must be binary-compatible with JSString");

class JSDependentString : public JSLinearString
{
    friend class JSString;
    JSFlatString* undepend(JSContext* cx);

    template <typename CharT>
    JSFlatString* undependInternal(JSContext* cx);

    void init(js::ExclusiveContext* cx, JSLinearString* base, size_t start,
              size_t length);

    /* Vacuous and therefore unimplemented. */
    bool isDependent() const = delete;
    JSDependentString& asDependent() const = delete;

    /* The offset of this string's chars in base->chars(). */
    MOZ_ALWAYS_INLINE mozilla::Maybe<size_t> baseOffset() const {
        MOZ_ASSERT(JSString::isDependent());
        JS::AutoCheckCannotGC nogc;
        if (MOZ_UNLIKELY(base()->isUndepended()))
            return mozilla::Nothing();
        size_t offset;
        if (hasTwoByteChars())
            offset = twoByteChars(nogc) - base()->twoByteChars(nogc);
        else
            offset = latin1Chars(nogc) - base()->latin1Chars(nogc);
        MOZ_ASSERT(offset < base()->length());
        return mozilla::Some(offset);
    }

  public:
    static inline JSLinearString* new_(js::ExclusiveContext* cx, JSLinearString* base,
                                       size_t start, size_t length);

    inline static size_t offsetOfBase() {
        return offsetof(JSDependentString, d.s.u3.base);
    }

#ifdef DEBUG
    void dumpRepresentation(FILE* fp, int indent) const;
#endif
};

static_assert(sizeof(JSDependentString) == sizeof(JSString),
              "string subclasses must be binary-compatible with JSString");

class JSFlatString : public JSLinearString
{
    /* Vacuous and therefore unimplemented. */
    JSFlatString* ensureFlat(JSContext* cx) = delete;
    bool isFlat() const = delete;
    JSFlatString& asFlat() const = delete;

    template <typename CharT>
    static bool isIndexSlow(const CharT* s, size_t length, uint32_t* indexp);

    void init(const char16_t* chars, size_t length);
    void init(const JS::Latin1Char* chars, size_t length);

  public:
    template <js::AllowGC allowGC, typename CharT>
    static inline JSFlatString* new_(js::ExclusiveContext* cx,
                                     const CharT* chars, size_t length);

    /*
     * Returns true if this string's characters store an unsigned 32-bit
     * integer value, initializing *indexp to that value if so.  (Thus if
     * calling isIndex returns true, js::IndexToString(cx, *indexp) will be a
     * string equal to this string.)
     */
    inline bool isIndex(uint32_t* indexp) const {
        MOZ_ASSERT(JSString::isFlat());
        JS::AutoCheckCannotGC nogc;
        if (hasLatin1Chars()) {
            const JS::Latin1Char* s = latin1Chars(nogc);
            return JS7_ISDEC(*s) && isIndexSlow(s, length(), indexp);
        }
        const char16_t* s = twoByteChars(nogc);
        return JS7_ISDEC(*s) && isIndexSlow(s, length(), indexp);
    }

    /*
     * Returns a property name represented by this string, or null on failure.
     * You must verify that this is not an index per isIndex before calling
     * this method.
     */
    inline js::PropertyName* toPropertyName(JSContext* cx);

    /*
     * Once a JSFlatString sub-class has been added to the atom state, this
     * operation changes the string to the JSAtom type, in place.
     */
    MOZ_ALWAYS_INLINE JSAtom* morphAtomizedStringIntoAtom(js::HashNumber hash);
    MOZ_ALWAYS_INLINE JSAtom* morphAtomizedStringIntoPermanentAtom(js::HashNumber hash);

    inline void finalize(js::FreeOp* fop);

#ifdef DEBUG
    void dumpRepresentation(FILE* fp, int indent) const;
#endif
};

static_assert(sizeof(JSFlatString) == sizeof(JSString),
              "string subclasses must be binary-compatible with JSString");

class JSExtensibleString : public JSFlatString
{
    /* Vacuous and therefore unimplemented. */
    bool isExtensible() const = delete;
    JSExtensibleString& asExtensible() const = delete;

  public:
    MOZ_ALWAYS_INLINE
    size_t capacity() const {
        MOZ_ASSERT(JSString::isExtensible());
        return d.s.u3.capacity;
    }

#ifdef DEBUG
    void dumpRepresentation(FILE* fp, int indent) const;
#endif
};

static_assert(sizeof(JSExtensibleString) == sizeof(JSString),
              "string subclasses must be binary-compatible with JSString");

class JSInlineString : public JSFlatString
{
  public:
    MOZ_ALWAYS_INLINE
    const JS::Latin1Char* latin1Chars(const JS::AutoCheckCannotGC& nogc) const {
        MOZ_ASSERT(JSString::isInline());
        MOZ_ASSERT(hasLatin1Chars());
        return d.inlineStorageLatin1;
    }

    MOZ_ALWAYS_INLINE
    const char16_t* twoByteChars(const JS::AutoCheckCannotGC& nogc) const {
        MOZ_ASSERT(JSString::isInline());
        MOZ_ASSERT(hasTwoByteChars());
        return d.inlineStorageTwoByte;
    }

    template<typename CharT>
    static bool lengthFits(size_t length);

    static size_t offsetOfInlineStorage() {
        return offsetof(JSInlineString, d.inlineStorageTwoByte);
    }

#ifdef DEBUG
    void dumpRepresentation(FILE* fp, int indent) const;
#endif
};

static_assert(sizeof(JSInlineString) == sizeof(JSString),
              "string subclasses must be binary-compatible with JSString");

/*
 * On 32-bit platforms, JSThinInlineString can store 7 Latin1 characters or 3
 * TwoByte characters (excluding null terminator) inline. On 64-bit platforms,
 * these numbers are 15 and 7, respectively.
 */
class JSThinInlineString : public JSInlineString
{
  public:
    static const size_t MAX_LENGTH_LATIN1 = NUM_INLINE_CHARS_LATIN1 - 1;
    static const size_t MAX_LENGTH_TWO_BYTE = NUM_INLINE_CHARS_TWO_BYTE - 1;

    template <js::AllowGC allowGC>
    static inline JSThinInlineString* new_(js::ExclusiveContext* cx);

    template <typename CharT>
    inline CharT* init(size_t length);

    template<typename CharT>
    static bool lengthFits(size_t length);
};

static_assert(sizeof(JSThinInlineString) == sizeof(JSString),
              "string subclasses must be binary-compatible with JSString");

/*
 * On both 32-bit and 64-bit platforms, MAX_LENGTH_TWO_BYTE is 11 and
 * MAX_LENGTH_LATIN1 is 23 (excluding null terminator). This is deliberate,
 * in order to minimize potential performance differences between 32-bit and
 * 64-bit platforms.
 *
 * There are still some differences due to NUM_INLINE_CHARS_* being different.
 * E.g. TwoByte strings of length 4--7 will be JSFatInlineStrings on 32-bit
 * platforms and JSThinInlineStrings on 64-bit platforms. But the more
 * significant transition from inline strings to non-inline strings occurs at
 * length 11 (for TwoByte strings) and 23 (Latin1 strings) on both 32-bit and
 * 64-bit platforms.
 */
class JSFatInlineString : public JSInlineString
{
    static const size_t INLINE_EXTENSION_CHARS_LATIN1 = 24 - NUM_INLINE_CHARS_LATIN1;
    static const size_t INLINE_EXTENSION_CHARS_TWO_BYTE = 12 - NUM_INLINE_CHARS_TWO_BYTE;

  protected: /* to fool clang into not warning this is unused */
    union {
        char   inlineStorageExtensionLatin1[INLINE_EXTENSION_CHARS_LATIN1];
        char16_t inlineStorageExtensionTwoByte[INLINE_EXTENSION_CHARS_TWO_BYTE];
    };

  public:
    template <js::AllowGC allowGC>
    static inline JSFatInlineString* new_(js::ExclusiveContext* cx);

    static const size_t MAX_LENGTH_LATIN1 = JSString::NUM_INLINE_CHARS_LATIN1 +
                                            INLINE_EXTENSION_CHARS_LATIN1
                                            -1 /* null terminator */;

    static const size_t MAX_LENGTH_TWO_BYTE = JSString::NUM_INLINE_CHARS_TWO_BYTE +
                                              INLINE_EXTENSION_CHARS_TWO_BYTE
                                              -1 /* null terminator */;

    template <typename CharT>
    inline CharT* init(size_t length);

    template<typename CharT>
    static bool lengthFits(size_t length);

    /* Only called by the GC for strings with the AllocKind::FAT_INLINE_STRING kind. */

    MOZ_ALWAYS_INLINE void finalize(js::FreeOp* fop);
};

static_assert(sizeof(JSFatInlineString) % js::gc::CellSize == 0,
              "fat inline strings shouldn't waste space up to the next cell "
              "boundary");

class JSExternalString : public JSLinearString
{
    void init(const char16_t* chars, size_t length, const JSStringFinalizer* fin);

    /* Vacuous and therefore unimplemented. */
    bool isExternal() const = delete;
    JSExternalString& asExternal() const = delete;

  public:
    static inline JSExternalString* new_(JSContext* cx, const char16_t* chars, size_t length,
                                         const JSStringFinalizer* fin);

    const JSStringFinalizer* externalFinalizer() const {
        MOZ_ASSERT(JSString::isExternal());
        return d.s.u3.externalFinalizer;
    }

    /*
     * External chars are never allocated inline or in the nursery, so we can
     * safely expose this without requiring an AutoCheckCannotGC argument.
     */
    const char16_t* twoByteChars() const {
        return rawTwoByteChars();
    }

    /* Only called by the GC for strings with the AllocKind::EXTERNAL_STRING kind. */

    inline void finalize(js::FreeOp* fop);

    JSFlatString* ensureFlat(JSContext* cx);

#ifdef DEBUG
    void dumpRepresentation(FILE* fp, int indent) const;
#endif
};

static_assert(sizeof(JSExternalString) == sizeof(JSString),
              "string subclasses must be binary-compatible with JSString");

class JSUndependedString : public JSFlatString
{
    /*
     * JSUndependedString is not explicitly used and is only present for
     * consistency. See JSDependentString::undepend for how a JSDependentString
     * gets morphed into a JSUndependedString.
     */
};

static_assert(sizeof(JSUndependedString) == sizeof(JSString),
              "string subclasses must be binary-compatible with JSString");

class JSAtom : public JSFlatString
{
    /* Vacuous and therefore unimplemented. */
    bool isAtom() const = delete;
    JSAtom& asAtom() const = delete;

  public:
    /* Returns the PropertyName for this.  isIndex() must be false. */
    inline js::PropertyName* asPropertyName();

    inline void finalize(js::FreeOp* fop);

    MOZ_ALWAYS_INLINE
    bool isPermanent() const {
        return JSString::isPermanentAtom();
    }

    // Transform this atom into a permanent atom. This is only done during
    // initialization of the runtime.
    MOZ_ALWAYS_INLINE void morphIntoPermanentAtom() {
        d.u1.flags |= PERMANENT_ATOM_MASK;
    }

    inline js::HashNumber hash() const;
    inline void initHash(js::HashNumber hash);

#ifdef DEBUG
    void dump(FILE* fp);
    void dump();
#endif
};

static_assert(sizeof(JSAtom) == sizeof(JSString),
              "string subclasses must be binary-compatible with JSString");

namespace js {

class NormalAtom : public JSAtom
{
  protected: // Silence Clang unused-field warning.
    HashNumber hash_;
    uint32_t padding_; // Ensure the size is a multiple of gc::CellSize.

  public:
    HashNumber hash() const {
        return hash_;
    }
    void initHash(HashNumber hash) {
        hash_ = hash;
    }
};

static_assert(sizeof(NormalAtom) == sizeof(JSString) + sizeof(uint64_t),
              "NormalAtom must have size of a string + HashNumber, "
              "aligned to gc::CellSize");

class FatInlineAtom : public JSAtom
{
  protected: // Silence Clang unused-field warning.
    char inlineStorage_[sizeof(JSFatInlineString) - sizeof(JSString)];
    HashNumber hash_;
    uint32_t padding_; // Ensure the size is a multiple of gc::CellSize.

  public:
    HashNumber hash() const {
        return hash_;
    }
    void initHash(HashNumber hash) {
        hash_ = hash;
    }
};

static_assert(sizeof(FatInlineAtom) == sizeof(JSFatInlineString) + sizeof(uint64_t),
              "FatInlineAtom must have size of a fat inline string + HashNumber, "
              "aligned to gc::CellSize");

} // namespace js

inline js::HashNumber
JSAtom::hash() const
{
    if (isFatInline())
        return static_cast<const js::FatInlineAtom*>(this)->hash();
    return static_cast<const js::NormalAtom*>(this)->hash();
}

inline void
JSAtom::initHash(js::HashNumber hash)
{
    if (isFatInline())
        return static_cast<js::FatInlineAtom*>(this)->initHash(hash);
    return static_cast<js::NormalAtom*>(this)->initHash(hash);
}

MOZ_ALWAYS_INLINE JSAtom*
JSFlatString::morphAtomizedStringIntoAtom(js::HashNumber hash)
{
    d.u1.flags |= ATOM_BIT;
    JSAtom* atom = &asAtom();
    atom->initHash(hash);
    return atom;
}

MOZ_ALWAYS_INLINE JSAtom*
JSFlatString::morphAtomizedStringIntoPermanentAtom(js::HashNumber hash)
{
    d.u1.flags |= PERMANENT_ATOM_MASK;
    JSAtom* atom = &asAtom();
    atom->initHash(hash);
    return atom;
}

namespace js {

class StaticStrings
{
  private:
    /* Bigger chars cannot be in a length-2 string. */
    static const size_t SMALL_CHAR_LIMIT    = 128U;
    static const size_t NUM_SMALL_CHARS     = 64U;

    JSAtom* length2StaticTable[NUM_SMALL_CHARS * NUM_SMALL_CHARS] = {}; // zeroes

  public:
    /* We keep these public for the JITs. */
    static const size_t UNIT_STATIC_LIMIT   = 256U;
    JSAtom* unitStaticTable[UNIT_STATIC_LIMIT] = {}; // zeroes

    static const size_t INT_STATIC_LIMIT    = 256U;
    JSAtom* intStaticTable[INT_STATIC_LIMIT] = {}; // zeroes

    StaticStrings() = default;

    bool init(JSContext* cx);
    void trace(JSTracer* trc);

    static bool hasUint(uint32_t u) { return u < INT_STATIC_LIMIT; }

    JSAtom* getUint(uint32_t u) {
        MOZ_ASSERT(hasUint(u));
        return intStaticTable[u];
    }

    static bool hasInt(int32_t i) {
        return uint32_t(i) < INT_STATIC_LIMIT;
    }

    JSAtom* getInt(int32_t i) {
        MOZ_ASSERT(hasInt(i));
        return getUint(uint32_t(i));
    }

    static bool hasUnit(char16_t c) { return c < UNIT_STATIC_LIMIT; }

    JSAtom* getUnit(char16_t c) {
        MOZ_ASSERT(hasUnit(c));
        return unitStaticTable[c];
    }

    /* May not return atom, returns null on (reported) failure. */
    inline JSLinearString* getUnitStringForElement(JSContext* cx, JSString* str, size_t index);

    template <typename CharT>
    static bool isStatic(const CharT* chars, size_t len);
    static bool isStatic(JSAtom* atom);

    /* Return null if no static atom exists for the given (chars, length). */
    template <typename CharT>
    JSAtom* lookup(const CharT* chars, size_t length) {
        switch (length) {
          case 1: {
            char16_t c = chars[0];
            if (c < UNIT_STATIC_LIMIT)
                return getUnit(c);
            return nullptr;
          }
          case 2:
            if (fitsInSmallChar(chars[0]) && fitsInSmallChar(chars[1]))
                return getLength2(chars[0], chars[1]);
            return nullptr;
          case 3:
            /*
             * Here we know that JSString::intStringTable covers only 256 (or at least
             * not 1000 or more) chars. We rely on order here to resolve the unit vs.
             * int string/length-2 string atom identity issue by giving priority to unit
             * strings for "0" through "9" and length-2 strings for "10" through "99".
             */
            static_assert(INT_STATIC_LIMIT <= 999,
                          "static int strings assumed below to be at most "
                          "three digits");
            if ('1' <= chars[0] && chars[0] <= '9' &&
                '0' <= chars[1] && chars[1] <= '9' &&
                '0' <= chars[2] && chars[2] <= '9') {
                int i = (chars[0] - '0') * 100 +
                          (chars[1] - '0') * 10 +
                          (chars[2] - '0');

                if (unsigned(i) < INT_STATIC_LIMIT)
                    return getInt(i);
            }
            return nullptr;
        }

        return nullptr;
    }

  private:
    typedef uint8_t SmallChar;
    static const SmallChar INVALID_SMALL_CHAR = -1;

    static bool fitsInSmallChar(char16_t c) {
        return c < SMALL_CHAR_LIMIT && toSmallChar[c] != INVALID_SMALL_CHAR;
    }

    static const SmallChar toSmallChar[];

    JSAtom* getLength2(char16_t c1, char16_t c2);
    JSAtom* getLength2(uint32_t u) {
        MOZ_ASSERT(u < 100);
        return getLength2('0' + u / 10, '0' + u % 10);
    }
};

/*
 * Represents an atomized string which does not contain an index (that is, an
 * unsigned 32-bit value).  Thus for any PropertyName propname,
 * ToString(ToUint32(propname)) never equals propname.
 *
 * To more concretely illustrate the utility of PropertyName, consider that it
 * is used to partition, in a type-safe manner, the ways to refer to a
 * property, as follows:
 *
 *   - uint32_t indexes,
 *   - PropertyName strings which don't encode uint32_t indexes, and
 *   - jsspecial special properties (non-ES5 properties like object-valued
 *     jsids, JSID_EMPTY, JSID_VOID, and maybe in the future Harmony-proposed
 *     private names).
 */
class PropertyName : public JSAtom
{
  private:
    /* Vacuous and therefore unimplemented. */
    PropertyName* asPropertyName() = delete;
};

static_assert(sizeof(PropertyName) == sizeof(JSString),
              "string subclasses must be binary-compatible with JSString");

static MOZ_ALWAYS_INLINE jsid
NameToId(PropertyName* name)
{
    return NON_INTEGER_ATOM_TO_JSID(name);
}

using PropertyNameVector = JS::GCVector<PropertyName*>;

template <typename CharT>
void
CopyChars(CharT* dest, const JSLinearString& str);

static inline UniqueChars
StringToNewUTF8CharsZ(ExclusiveContext* maybecx, JSString& str)
{
    JS::AutoCheckCannotGC nogc;

    JSLinearString* linear = str.ensureLinear(maybecx);
    if (!linear)
        return nullptr;

    return UniqueChars(linear->hasLatin1Chars()
                       ? JS::CharsToNewUTF8CharsZ(maybecx, linear->latin1Range(nogc)).c_str()
                       : JS::CharsToNewUTF8CharsZ(maybecx, linear->twoByteRange(nogc)).c_str());
}

/* GC-allocate a string descriptor for the given malloc-allocated chars. */
template <js::AllowGC allowGC, typename CharT>
extern JSFlatString*
NewString(js::ExclusiveContext* cx, CharT* chars, size_t length);

/* Like NewString, but doesn't try to deflate to Latin1. */
template <js::AllowGC allowGC, typename CharT>
extern JSFlatString*
NewStringDontDeflate(js::ExclusiveContext* cx, CharT* chars, size_t length);

extern JSLinearString*
NewDependentString(JSContext* cx, JSString* base, size_t start, size_t length);

/* Take ownership of an array of Latin1Chars. */
extern JSFlatString*
NewLatin1StringZ(js::ExclusiveContext* cx, UniqueChars chars);

/* Copy a counted string and GC-allocate a descriptor for it. */
template <js::AllowGC allowGC, typename CharT>
extern JSFlatString*
NewStringCopyN(js::ExclusiveContext* cx, const CharT* s, size_t n);

template <js::AllowGC allowGC>
inline JSFlatString*
NewStringCopyN(ExclusiveContext* cx, const char* s, size_t n)
{
    return NewStringCopyN<allowGC>(cx, reinterpret_cast<const Latin1Char*>(s), n);
}

/* Like NewStringCopyN, but doesn't try to deflate to Latin1. */
template <js::AllowGC allowGC, typename CharT>
extern JSFlatString*
NewStringCopyNDontDeflate(js::ExclusiveContext* cx, const CharT* s, size_t n);

/* Copy a C string and GC-allocate a descriptor for it. */
template <js::AllowGC allowGC>
inline JSFlatString*
NewStringCopyZ(js::ExclusiveContext* cx, const char16_t* s)
{
    return NewStringCopyN<allowGC>(cx, s, js_strlen(s));
}

template <js::AllowGC allowGC>
inline JSFlatString*
NewStringCopyZ(js::ExclusiveContext* cx, const char* s)
{
    return NewStringCopyN<allowGC>(cx, s, strlen(s));
}

template <js::AllowGC allowGC>
extern JSFlatString*
NewStringCopyUTF8N(JSContext* cx, const JS::UTF8Chars utf8);

template <js::AllowGC allowGC>
inline JSFlatString*
NewStringCopyUTF8Z(JSContext* cx, const JS::ConstUTF8CharsZ utf8)
{
    return NewStringCopyUTF8N<allowGC>(cx, JS::UTF8Chars(utf8.c_str(), strlen(utf8.c_str())));
}

JS_STATIC_ASSERT(sizeof(HashNumber) == 4);

} /* namespace js */

// Addon IDs are interned atoms which are never destroyed. This detail is
// not exposed outside the API.
class JSAddonId : public JSAtom
{};

MOZ_ALWAYS_INLINE bool
JSString::getChar(js::ExclusiveContext* cx, size_t index, char16_t* code)
{
    MOZ_ASSERT(index < length());

    /*
     * Optimization for one level deep ropes.
     * This is common for the following pattern:
     *
     * while() {
     *   text = text.substr(0, x) + "bla" + text.substr(x)
     *   test.charCodeAt(x + 1)
     * }
     */
    JSString* str;
    if (isRope()) {
        JSRope* rope = &asRope();
        if (uint32_t(index) < rope->leftChild()->length()) {
            str = rope->leftChild();
        } else {
            str = rope->rightChild();
            index -= rope->leftChild()->length();
        }
    } else {
        str = this;
    }

    if (!str->ensureLinear(cx))
        return false;

    *code = str->asLinear().latin1OrTwoByteChar(index);
    return true;
}

MOZ_ALWAYS_INLINE JSLinearString*
JSString::ensureLinear(js::ExclusiveContext* cx)
{
    return isLinear()
           ? &asLinear()
           : asRope().flatten(cx);
}

inline JSLinearString*
JSString::base() const
{
    MOZ_ASSERT(hasBase());
    MOZ_ASSERT(!d.s.u3.base->isInline());
    return d.s.u3.base;
}

template<>
MOZ_ALWAYS_INLINE const char16_t*
JSLinearString::nonInlineChars(const JS::AutoCheckCannotGC& nogc) const
{
    return nonInlineTwoByteChars(nogc);
}

template<>
MOZ_ALWAYS_INLINE const JS::Latin1Char*
JSLinearString::nonInlineChars(const JS::AutoCheckCannotGC& nogc) const
{
    return nonInlineLatin1Chars(nogc);
}

template<>
MOZ_ALWAYS_INLINE const char16_t*
JSLinearString::chars(const JS::AutoCheckCannotGC& nogc) const
{
    return rawTwoByteChars();
}

template<>
MOZ_ALWAYS_INLINE const JS::Latin1Char*
JSLinearString::chars(const JS::AutoCheckCannotGC& nogc) const
{
    return rawLatin1Chars();
}

template <>
MOZ_ALWAYS_INLINE bool
JSRope::copyChars<JS::Latin1Char>(js::ExclusiveContext* cx,
                                  js::ScopedJSFreePtr<JS::Latin1Char>& out) const
{
    return copyLatin1Chars(cx, out);
}

template <>
MOZ_ALWAYS_INLINE bool
JSRope::copyChars<char16_t>(js::ExclusiveContext* cx, js::ScopedJSFreePtr<char16_t>& out) const
{
    return copyTwoByteChars(cx, out);
}

template<>
MOZ_ALWAYS_INLINE bool
JSThinInlineString::lengthFits<JS::Latin1Char>(size_t length)
{
    return length <= MAX_LENGTH_LATIN1;
}

template<>
MOZ_ALWAYS_INLINE bool
JSThinInlineString::lengthFits<char16_t>(size_t length)
{
    return length <= MAX_LENGTH_TWO_BYTE;
}

template<>
MOZ_ALWAYS_INLINE bool
JSFatInlineString::lengthFits<JS::Latin1Char>(size_t length)
{
    static_assert((INLINE_EXTENSION_CHARS_LATIN1 * sizeof(char)) % js::gc::CellSize == 0,
                  "fat inline strings' Latin1 characters don't exactly "
                  "fill subsequent cells and thus are wasteful");
    static_assert(MAX_LENGTH_LATIN1 + 1 ==
                  (sizeof(JSFatInlineString) -
                   offsetof(JSFatInlineString, d.inlineStorageLatin1)) / sizeof(char),
                  "MAX_LENGTH_LATIN1 must be one less than inline Latin1 "
                  "storage count");

    return length <= MAX_LENGTH_LATIN1;
}

template<>
MOZ_ALWAYS_INLINE bool
JSFatInlineString::lengthFits<char16_t>(size_t length)
{
    static_assert((INLINE_EXTENSION_CHARS_TWO_BYTE * sizeof(char16_t)) % js::gc::CellSize == 0,
                  "fat inline strings' char16_t characters don't exactly "
                  "fill subsequent cells and thus are wasteful");
    static_assert(MAX_LENGTH_TWO_BYTE + 1 ==
                  (sizeof(JSFatInlineString) -
                   offsetof(JSFatInlineString, d.inlineStorageTwoByte)) / sizeof(char16_t),
                  "MAX_LENGTH_TWO_BYTE must be one less than inline "
                  "char16_t storage count");

    return length <= MAX_LENGTH_TWO_BYTE;
}

template<>
MOZ_ALWAYS_INLINE bool
JSInlineString::lengthFits<JS::Latin1Char>(size_t length)
{
    // If it fits in a fat inline string, it fits in any inline string.
    return JSFatInlineString::lengthFits<JS::Latin1Char>(length);
}

template<>
MOZ_ALWAYS_INLINE bool
JSInlineString::lengthFits<char16_t>(size_t length)
{
    // If it fits in a fat inline string, it fits in any inline string.
    return JSFatInlineString::lengthFits<char16_t>(length);
}

template<>
MOZ_ALWAYS_INLINE void
JSString::setNonInlineChars(const char16_t* chars)
{
    d.s.u2.nonInlineCharsTwoByte = chars;
}

template<>
MOZ_ALWAYS_INLINE void
JSString::setNonInlineChars(const JS::Latin1Char* chars)
{
    d.s.u2.nonInlineCharsLatin1 = chars;
}

MOZ_ALWAYS_INLINE const JS::Latin1Char*
JSLinearString::rawLatin1Chars() const
{
    MOZ_ASSERT(JSString::isLinear());
    MOZ_ASSERT(hasLatin1Chars());
    return isInline() ? d.inlineStorageLatin1 : d.s.u2.nonInlineCharsLatin1;
}

MOZ_ALWAYS_INLINE const char16_t*
JSLinearString::rawTwoByteChars() const
{
    MOZ_ASSERT(JSString::isLinear());
    MOZ_ASSERT(hasTwoByteChars());
    return isInline() ? d.inlineStorageTwoByte : d.s.u2.nonInlineCharsTwoByte;
}

inline js::PropertyName*
JSAtom::asPropertyName()
{
#ifdef DEBUG
    uint32_t dummy;
    MOZ_ASSERT(!isIndex(&dummy));
#endif
    return static_cast<js::PropertyName*>(this);
}

#endif /* vm_String_h */