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
|
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
* vim: set ts=8 sts=4 et sw=4 tw=99:
* This Source Code Form is subject to the terms of the Mozilla Public
* License, v. 2.0. If a copy of the MPL was not distributed with this
* file, You can obtain one at http://mozilla.org/MPL/2.0/. */
#ifndef js_RootingAPI_h
#define js_RootingAPI_h
#include "mozilla/Attributes.h"
#include "mozilla/GuardObjects.h"
#include "mozilla/LinkedList.h"
#include "mozilla/NullPtr.h"
#include "mozilla/TypeTraits.h"
#include "jspubtd.h"
#include "js/HeapAPI.h"
#include "js/TypeDecls.h"
#include "js/Utility.h"
/*
* Moving GC Stack Rooting
*
* A moving GC may change the physical location of GC allocated things, even
* when they are rooted, updating all pointers to the thing to refer to its new
* location. The GC must therefore know about all live pointers to a thing,
* not just one of them, in order to behave correctly.
*
* The |Rooted| and |Handle| classes below are used to root stack locations
* whose value may be held live across a call that can trigger GC. For a
* code fragment such as:
*
* JSObject* obj = NewObject(cx);
* DoSomething(cx);
* ... = obj->lastProperty();
*
* If |DoSomething()| can trigger a GC, the stack location of |obj| must be
* rooted to ensure that the GC does not move the JSObject referred to by
* |obj| without updating |obj|'s location itself. This rooting must happen
* regardless of whether there are other roots which ensure that the object
* itself will not be collected.
*
* If |DoSomething()| cannot trigger a GC, and the same holds for all other
* calls made between |obj|'s definitions and its last uses, then no rooting
* is required.
*
* SpiderMonkey can trigger a GC at almost any time and in ways that are not
* always clear. For example, the following innocuous-looking actions can
* cause a GC: allocation of any new GC thing; JSObject::hasProperty;
* JS_ReportError and friends; and ToNumber, among many others. The following
* dangerous-looking actions cannot trigger a GC: js_malloc, cx->malloc_,
* rt->malloc_, and friends and JS_ReportOutOfMemory.
*
* The following family of three classes will exactly root a stack location.
* Incorrect usage of these classes will result in a compile error in almost
* all cases. Therefore, it is very hard to be incorrectly rooted if you use
* these classes exclusively. These classes are all templated on the type T of
* the value being rooted.
*
* - Rooted<T> declares a variable of type T, whose value is always rooted.
* Rooted<T> may be automatically coerced to a Handle<T>, below. Rooted<T>
* should be used whenever a local variable's value may be held live across a
* call which can trigger a GC.
*
* - Handle<T> is a const reference to a Rooted<T>. Functions which take GC
* things or values as arguments and need to root those arguments should
* generally use handles for those arguments and avoid any explicit rooting.
* This has two benefits. First, when several such functions call each other
* then redundant rooting of multiple copies of the GC thing can be avoided.
* Second, if the caller does not pass a rooted value a compile error will be
* generated, which is quicker and easier to fix than when relying on a
* separate rooting analysis.
*
* - MutableHandle<T> is a non-const reference to Rooted<T>. It is used in the
* same way as Handle<T> and includes a |set(const T& v)| method to allow
* updating the value of the referenced Rooted<T>. A MutableHandle<T> can be
* created from a Rooted<T> by using |Rooted<T>::operator&()|.
*
* In some cases the small performance overhead of exact rooting (measured to
* be a few nanoseconds on desktop) is too much. In these cases, try the
* following:
*
* - Move all Rooted<T> above inner loops: this allows you to re-use the root
* on each iteration of the loop.
*
* - Pass Handle<T> through your hot call stack to avoid re-rooting costs at
* every invocation.
*
* The following diagram explains the list of supported, implicit type
* conversions between classes of this family:
*
* Rooted<T> ----> Handle<T>
* | ^
* | |
* | |
* +---> MutableHandle<T>
* (via &)
*
* All of these types have an implicit conversion to raw pointers.
*/
namespace js {
class ScriptSourceObject;
template <typename T>
struct GCMethods {};
template <typename T>
class RootedBase {};
template <typename T>
class HandleBase {};
template <typename T>
class MutableHandleBase {};
template <typename T>
class HeapBase {};
/*
* js::NullPtr acts like a nullptr pointer in contexts that require a Handle.
*
* Handle provides an implicit constructor for js::NullPtr so that, given:
* foo(Handle<JSObject*> h);
* callers can simply write:
* foo(js::NullPtr());
* which avoids creating a Rooted<JSObject*> just to pass nullptr.
*
* This is the SpiderMonkey internal variant. js::NullPtr should be used in
* preference to JS::NullPtr to avoid the GOT access required for JS_PUBLIC_API
* symbols.
*/
struct NullPtr
{
static void * const constNullValue;
};
namespace gc {
struct Cell;
template<typename T>
struct PersistentRootedMarker;
} /* namespace gc */
} /* namespace js */
namespace JS {
template <typename T> class Rooted;
template <typename T> class PersistentRooted;
/* This is exposing internal state of the GC for inlining purposes. */
JS_FRIEND_API(bool) isGCEnabled();
/*
* JS::NullPtr acts like a nullptr pointer in contexts that require a Handle.
*
* Handle provides an implicit constructor for JS::NullPtr so that, given:
* foo(Handle<JSObject*> h);
* callers can simply write:
* foo(JS::NullPtr());
* which avoids creating a Rooted<JSObject*> just to pass nullptr.
*/
struct JS_PUBLIC_API(NullPtr)
{
static void * const constNullValue;
};
/*
* The Heap<T> class is a heap-stored reference to a JS GC thing. All members of
* heap classes that refer to GC things should use Heap<T> (or possibly
* TenuredHeap<T>, described below).
*
* Heap<T> is an abstraction that hides some of the complexity required to
* maintain GC invariants for the contained reference. It uses operator
* overloading to provide a normal pointer interface, but notifies the GC every
* time the value it contains is updated. This is necessary for generational GC,
* which keeps track of all pointers into the nursery.
*
* Heap<T> instances must be traced when their containing object is traced to
* keep the pointed-to GC thing alive.
*
* Heap<T> objects should only be used on the heap. GC references stored on the
* C/C++ stack must use Rooted/Handle/MutableHandle instead.
*
* Type T must be one of: JS::Value, jsid, JSObject*, JSString*, JSScript*
*/
template <typename T>
class Heap : public js::HeapBase<T>
{
public:
Heap() {
static_assert(sizeof(T) == sizeof(Heap<T>),
"Heap<T> must be binary compatible with T.");
init(js::GCMethods<T>::initial());
}
explicit Heap(T p) { init(p); }
/*
* For Heap, move semantics are equivalent to copy semantics. In C++, a
* copy constructor taking const-ref is the way to get a single function
* that will be used for both lvalue and rvalue copies, so we can simply
* omit the rvalue variant.
*/
explicit Heap(const Heap<T>& p) { init(p.ptr); }
~Heap() {
if (js::GCMethods<T>::needsPostBarrier(ptr))
relocate();
}
bool operator==(const Heap<T>& other) { return ptr == other.ptr; }
bool operator!=(const Heap<T>& other) { return ptr != other.ptr; }
bool operator==(const T& other) const { return ptr == other; }
bool operator!=(const T& other) const { return ptr != other; }
operator T() const { return ptr; }
T operator->() const { return ptr; }
const T* address() const { return &ptr; }
const T& get() const { return ptr; }
T* unsafeGet() { return &ptr; }
Heap<T>& operator=(T p) {
set(p);
return *this;
}
Heap<T>& operator=(const Heap<T>& other) {
set(other.get());
return *this;
}
void set(T newPtr) {
MOZ_ASSERT(!js::GCMethods<T>::poisoned(newPtr));
if (js::GCMethods<T>::needsPostBarrier(newPtr)) {
ptr = newPtr;
post();
} else if (js::GCMethods<T>::needsPostBarrier(ptr)) {
relocate(); /* Called before overwriting ptr. */
ptr = newPtr;
} else {
ptr = newPtr;
}
}
/*
* Set the pointer to a value which will cause a crash if it is
* dereferenced.
*/
void setToCrashOnTouch() {
ptr = reinterpret_cast<T>(crashOnTouchPointer);
}
bool isSetToCrashOnTouch() {
return ptr == crashOnTouchPointer;
}
private:
void init(T newPtr) {
MOZ_ASSERT(!js::GCMethods<T>::poisoned(newPtr));
ptr = newPtr;
if (js::GCMethods<T>::needsPostBarrier(ptr))
post();
}
void post() {
#ifdef JSGC_GENERATIONAL
MOZ_ASSERT(js::GCMethods<T>::needsPostBarrier(ptr));
js::GCMethods<T>::postBarrier(&ptr);
#endif
}
void relocate() {
#ifdef JSGC_GENERATIONAL
js::GCMethods<T>::relocate(&ptr);
#endif
}
enum {
crashOnTouchPointer = 1
};
T ptr;
};
#ifdef JS_DEBUG
/*
* For generational GC, assert that an object is in the tenured generation as
* opposed to being in the nursery.
*/
extern JS_FRIEND_API(void)
AssertGCThingMustBeTenured(JSObject* obj);
#else
inline void
AssertGCThingMustBeTenured(JSObject* obj) {}
#endif
/*
* The TenuredHeap<T> class is similar to the Heap<T> class above in that it
* encapsulates the GC concerns of an on-heap reference to a JS object. However,
* it has two important differences:
*
* 1) Pointers which are statically known to only reference "tenured" objects
* can avoid the extra overhead of SpiderMonkey's write barriers.
*
* 2) Objects in the "tenured" heap have stronger alignment restrictions than
* those in the "nursery", so it is possible to store flags in the lower
* bits of pointers known to be tenured. TenuredHeap wraps a normal tagged
* pointer with a nice API for accessing the flag bits and adds various
* assertions to ensure that it is not mis-used.
*
* GC things are said to be "tenured" when they are located in the long-lived
* heap: e.g. they have gained tenure as an object by surviving past at least
* one GC. For performance, SpiderMonkey allocates some things which are known
* to normally be long lived directly into the tenured generation; for example,
* global objects. Additionally, SpiderMonkey does not visit individual objects
* when deleting non-tenured objects, so object with finalizers are also always
* tenured; for instance, this includes most DOM objects.
*
* The considerations to keep in mind when using a TenuredHeap<T> vs a normal
* Heap<T> are:
*
* - It is invalid for a TenuredHeap<T> to refer to a non-tenured thing.
* - It is however valid for a Heap<T> to refer to a tenured thing.
* - It is not possible to store flag bits in a Heap<T>.
*/
template <typename T>
class TenuredHeap : public js::HeapBase<T>
{
public:
TenuredHeap() : bits(0) {
static_assert(sizeof(T) == sizeof(TenuredHeap<T>),
"TenuredHeap<T> must be binary compatible with T.");
}
explicit TenuredHeap(T p) : bits(0) { setPtr(p); }
explicit TenuredHeap(const TenuredHeap<T>& p) : bits(0) { setPtr(p.getPtr()); }
bool operator==(const TenuredHeap<T>& other) { return bits == other.bits; }
bool operator!=(const TenuredHeap<T>& other) { return bits != other.bits; }
void setPtr(T newPtr) {
MOZ_ASSERT((reinterpret_cast<uintptr_t>(newPtr) & flagsMask) == 0);
MOZ_ASSERT(!js::GCMethods<T>::poisoned(newPtr));
if (newPtr)
AssertGCThingMustBeTenured(newPtr);
bits = (bits & flagsMask) | reinterpret_cast<uintptr_t>(newPtr);
}
void setFlags(uintptr_t flagsToSet) {
MOZ_ASSERT((flagsToSet & ~flagsMask) == 0);
bits |= flagsToSet;
}
void unsetFlags(uintptr_t flagsToUnset) {
MOZ_ASSERT((flagsToUnset & ~flagsMask) == 0);
bits &= ~flagsToUnset;
}
bool hasFlag(uintptr_t flag) const {
MOZ_ASSERT((flag & ~flagsMask) == 0);
return (bits & flag) != 0;
}
T getPtr() const { return reinterpret_cast<T>(bits & ~flagsMask); }
uintptr_t getFlags() const { return bits & flagsMask; }
operator T() const { return getPtr(); }
T operator->() const { return getPtr(); }
TenuredHeap<T>& operator=(T p) {
setPtr(p);
return *this;
}
TenuredHeap<T>& operator=(const TenuredHeap<T>& other) {
bits = other.bits;
return *this;
}
private:
enum {
maskBits = 3,
flagsMask = (1 << maskBits) - 1,
};
uintptr_t bits;
};
/*
* Reference to a T that has been rooted elsewhere. This is most useful
* as a parameter type, which guarantees that the T lvalue is properly
* rooted. See "Move GC Stack Rooting" above.
*
* If you want to add additional methods to Handle for a specific
* specialization, define a HandleBase<T> specialization containing them.
*/
template <typename T>
class MOZ_NONHEAP_CLASS Handle : public js::HandleBase<T>
{
friend class JS::MutableHandle<T>;
public:
/* Creates a handle from a handle of a type convertible to T. */
template <typename S>
Handle(Handle<S> handle,
typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy = 0)
{
static_assert(sizeof(Handle<T>) == sizeof(T*),
"Handle must be binary compatible with T*.");
ptr = reinterpret_cast<const T*>(handle.address());
}
/* Create a handle for a nullptr pointer. */
MOZ_IMPLICIT Handle(js::NullPtr) {
static_assert(mozilla::IsPointer<T>::value,
"js::NullPtr overload not valid for non-pointer types");
ptr = reinterpret_cast<const T*>(&js::NullPtr::constNullValue);
}
/* Create a handle for a nullptr pointer. */
MOZ_IMPLICIT Handle(JS::NullPtr) {
static_assert(mozilla::IsPointer<T>::value,
"JS::NullPtr overload not valid for non-pointer types");
ptr = reinterpret_cast<const T*>(&JS::NullPtr::constNullValue);
}
MOZ_IMPLICIT Handle(MutableHandle<T> handle) {
ptr = handle.address();
}
/*
* Take care when calling this method!
*
* This creates a Handle from the raw location of a T.
*
* It should be called only if the following conditions hold:
*
* 1) the location of the T is guaranteed to be marked (for some reason
* other than being a Rooted), e.g., if it is guaranteed to be reachable
* from an implicit root.
*
* 2) the contents of the location are immutable, or at least cannot change
* for the lifetime of the handle, as its users may not expect its value
* to change underneath them.
*/
static MOZ_CONSTEXPR Handle fromMarkedLocation(const T* p) {
return Handle(p, DeliberatelyChoosingThisOverload,
ImUsingThisOnlyInFromFromMarkedLocation);
}
/*
* Construct a handle from an explicitly rooted location. This is the
* normal way to create a handle, and normally happens implicitly.
*/
template <typename S>
inline
Handle(const Rooted<S>& root,
typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy = 0);
template <typename S>
inline
Handle(const PersistentRooted<S>& root,
typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy = 0);
/* Construct a read only handle from a mutable handle. */
template <typename S>
inline
Handle(MutableHandle<S>& root,
typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy = 0);
const T* address() const { return ptr; }
const T& get() const { return *ptr; }
/*
* Return a reference so passing a Handle<T> to something that
* takes a |const T&| is not a GC hazard.
*/
operator const T&() const { return get(); }
T operator->() const { return get(); }
bool operator!=(const T& other) const { return *ptr != other; }
bool operator==(const T& other) const { return *ptr == other; }
private:
Handle() {}
enum Disambiguator { DeliberatelyChoosingThisOverload = 42 };
enum CallerIdentity { ImUsingThisOnlyInFromFromMarkedLocation = 17 };
MOZ_CONSTEXPR Handle(const T* p, Disambiguator, CallerIdentity) : ptr(p) {}
const T* ptr;
template <typename S> void operator=(S) MOZ_DELETE;
void operator=(Handle) MOZ_DELETE;
};
/*
* Similar to a handle, but the underlying storage can be changed. This is
* useful for outparams.
*
* If you want to add additional methods to MutableHandle for a specific
* specialization, define a MutableHandleBase<T> specialization containing
* them.
*/
template <typename T>
class MOZ_STACK_CLASS MutableHandle : public js::MutableHandleBase<T>
{
public:
inline MOZ_IMPLICIT MutableHandle(Rooted<T>* root);
inline MOZ_IMPLICIT MutableHandle(PersistentRooted<T>* root);
private:
// Disallow true nullptr and emulated nullptr (gcc 4.4/4.5, __null, appears
// as int/long [32/64-bit]) for overloading purposes.
template<typename N>
MutableHandle(N,
typename mozilla::EnableIf<mozilla::IsNullPointer<N>::value ||
mozilla::IsSame<N, int>::value ||
mozilla::IsSame<N, long>::value,
int>::Type dummy = 0)
MOZ_DELETE;
public:
void set(T v) {
MOZ_ASSERT(!js::GCMethods<T>::poisoned(v));
*ptr = v;
}
/*
* This may be called only if the location of the T is guaranteed
* to be marked (for some reason other than being a Rooted),
* e.g., if it is guaranteed to be reachable from an implicit root.
*
* Create a MutableHandle from a raw location of a T.
*/
static MutableHandle fromMarkedLocation(T* p) {
MutableHandle h;
h.ptr = p;
return h;
}
T* address() const { return ptr; }
const T& get() const { return *ptr; }
/*
* Return a reference so passing a MutableHandle<T> to something that takes
* a |const T&| is not a GC hazard.
*/
operator const T&() const { return get(); }
T operator->() const { return get(); }
private:
MutableHandle() {}
T* ptr;
template <typename S> void operator=(S v) MOZ_DELETE;
void operator=(MutableHandle other) MOZ_DELETE;
};
#ifdef JSGC_GENERATIONAL
JS_FRIEND_API(void) HeapCellPostBarrier(js::gc::Cell** cellp);
JS_FRIEND_API(void) HeapCellRelocate(js::gc::Cell** cellp);
#endif
} /* namespace JS */
namespace js {
/*
* InternalHandle is a handle to an internal pointer into a gcthing. Use
* InternalHandle when you have a pointer to a direct field of a gcthing, or
* when you need a parameter type for something that *may* be a pointer to a
* direct field of a gcthing.
*/
template <typename T>
class InternalHandle {};
template <typename T>
class InternalHandle<T*>
{
void * const* holder;
size_t offset;
public:
/*
* Create an InternalHandle using a Handle to the gcthing containing the
* field in question, and a pointer to the field.
*/
template<typename H>
InternalHandle(const JS::Handle<H>& handle, T* field)
: holder((void**)handle.address()), offset(uintptr_t(field) - uintptr_t(handle.get()))
{}
/*
* Create an InternalHandle to a field within a Rooted<>.
*/
template<typename R>
InternalHandle(const JS::Rooted<R>& root, T* field)
: holder((void**)root.address()), offset(uintptr_t(field) - uintptr_t(root.get()))
{}
InternalHandle(const InternalHandle<T*>& other)
: holder(other.holder), offset(other.offset) {}
T* get() const { return reinterpret_cast<T*>(uintptr_t(*holder) + offset); }
const T& operator*() const { return *get(); }
T* operator->() const { return get(); }
static InternalHandle<T*> fromMarkedLocation(T* fieldPtr) {
return InternalHandle(fieldPtr);
}
private:
/*
* Create an InternalHandle to something that is not a pointer to a
* gcthing, and so does not need to be rooted in the first place. Use these
* InternalHandles to pass pointers into functions that also need to accept
* regular InternalHandles to gcthing fields.
*
* Make this private to prevent accidental misuse; this is only for
* fromMarkedLocation().
*/
explicit InternalHandle(T* field)
: holder(reinterpret_cast<void * const*>(&js::NullPtr::constNullValue)),
offset(uintptr_t(field))
{}
void operator=(InternalHandle<T*> other) MOZ_DELETE;
};
/*
* By default, things should use the inheritance hierarchy to find their
* ThingRootKind. Some pointer types are explicitly set in jspubtd.h so that
* Rooted<T> may be used without the class definition being available.
*/
template <typename T>
struct RootKind
{
static ThingRootKind rootKind() { return T::rootKind(); }
};
template <typename T>
struct RootKind<T*>
{
static ThingRootKind rootKind() { return T::rootKind(); }
};
template <typename T>
struct GCMethods<T*>
{
static T* initial() { return nullptr; }
static bool poisoned(T* v) { return JS::IsPoisonedPtr(v); }
static bool needsPostBarrier(T* v) { return false; }
#ifdef JSGC_GENERATIONAL
static void postBarrier(T** vp) {}
static void relocate(T** vp) {}
#endif
};
template <>
struct GCMethods<JSObject*>
{
static JSObject* initial() { return nullptr; }
static bool poisoned(JSObject* v) { return JS::IsPoisonedPtr(v); }
static bool needsPostBarrier(JSObject* v) {
return v != nullptr && gc::IsInsideNursery(reinterpret_cast<gc::Cell*>(v));
}
#ifdef JSGC_GENERATIONAL
static void postBarrier(JSObject** vp) {
JS::HeapCellPostBarrier(reinterpret_cast<js::gc::Cell**>(vp));
}
static void relocate(JSObject** vp) {
JS::HeapCellRelocate(reinterpret_cast<js::gc::Cell**>(vp));
}
#endif
};
template <>
struct GCMethods<JSFunction*>
{
static JSFunction* initial() { return nullptr; }
static bool poisoned(JSFunction* v) { return JS::IsPoisonedPtr(v); }
static bool needsPostBarrier(JSFunction* v) {
return v != nullptr && gc::IsInsideNursery(reinterpret_cast<gc::Cell*>(v));
}
#ifdef JSGC_GENERATIONAL
static void postBarrier(JSFunction** vp) {
JS::HeapCellPostBarrier(reinterpret_cast<js::gc::Cell**>(vp));
}
static void relocate(JSFunction** vp) {
JS::HeapCellRelocate(reinterpret_cast<js::gc::Cell**>(vp));
}
#endif
};
#ifdef JS_DEBUG
/* This helper allows us to assert that Rooted<T> is scoped within a request. */
extern JS_PUBLIC_API(bool)
IsInRequest(JSContext* cx);
#endif
} /* namespace js */
namespace JS {
/*
* Local variable of type T whose value is always rooted. This is typically
* used for local variables, or for non-rooted values being passed to a
* function that requires a handle, e.g. Foo(Root<T>(cx, x)).
*
* If you want to add additional methods to Rooted for a specific
* specialization, define a RootedBase<T> specialization containing them.
*/
template <typename T>
class MOZ_STACK_CLASS Rooted : public js::RootedBase<T>
{
/* Note: CX is a subclass of either ContextFriendFields or PerThreadDataFriendFields. */
template <typename CX>
void init(CX* cx) {
#ifdef JSGC_TRACK_EXACT_ROOTS
js::ThingRootKind kind = js::RootKind<T>::rootKind();
this->stack = &cx->thingGCRooters[kind];
this->prev = *stack;
*stack = reinterpret_cast<Rooted<void*>*>(this);
MOZ_ASSERT(!js::GCMethods<T>::poisoned(ptr));
#endif
}
public:
explicit Rooted(JSContext* cx
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(js::GCMethods<T>::initial())
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
#ifdef JS_DEBUG
MOZ_ASSERT(js::IsInRequest(cx));
#endif
init(js::ContextFriendFields::get(cx));
}
Rooted(JSContext* cx, T initial
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(initial)
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
#ifdef JS_DEBUG
MOZ_ASSERT(js::IsInRequest(cx));
#endif
init(js::ContextFriendFields::get(cx));
}
explicit Rooted(js::ContextFriendFields* cx
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(js::GCMethods<T>::initial())
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
init(cx);
}
Rooted(js::ContextFriendFields* cx, T initial
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(initial)
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
init(cx);
}
explicit Rooted(js::PerThreadDataFriendFields* pt
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(js::GCMethods<T>::initial())
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
init(pt);
}
Rooted(js::PerThreadDataFriendFields* pt, T initial
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(initial)
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
init(pt);
}
explicit Rooted(JSRuntime* rt
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(js::GCMethods<T>::initial())
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
init(js::PerThreadDataFriendFields::getMainThread(rt));
}
Rooted(JSRuntime* rt, T initial
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(initial)
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
init(js::PerThreadDataFriendFields::getMainThread(rt));
}
// Note that we need to let the compiler generate the default destructor in
// non-exact-rooting builds because of a bug in the instrumented PGO builds
// using MSVC, see bug 915735 for more details.
#ifdef JSGC_TRACK_EXACT_ROOTS
~Rooted() {
MOZ_ASSERT(*stack == reinterpret_cast<Rooted<void*>*>(this));
*stack = prev;
}
#endif
#ifdef JSGC_TRACK_EXACT_ROOTS
Rooted<T>* previous() { return reinterpret_cast<Rooted<T>*>(prev); }
#endif
/*
* Important: Return a reference here so passing a Rooted<T> to
* something that takes a |const T&| is not a GC hazard.
*/
operator const T&() const { return ptr; }
T operator->() const { return ptr; }
T* address() { return &ptr; }
const T* address() const { return &ptr; }
T& get() { return ptr; }
const T& get() const { return ptr; }
T& operator=(T value) {
MOZ_ASSERT(!js::GCMethods<T>::poisoned(value));
ptr = value;
return ptr;
}
T& operator=(const Rooted& value) {
ptr = value;
return ptr;
}
void set(T value) {
MOZ_ASSERT(!js::GCMethods<T>::poisoned(value));
ptr = value;
}
bool operator!=(const T& other) const { return ptr != other; }
bool operator==(const T& other) const { return ptr == other; }
private:
#ifdef JSGC_TRACK_EXACT_ROOTS
/*
* These need to be templated on void* to avoid aliasing issues between, for
* example, Rooted<JSObject> and Rooted<JSFunction>, which use the same
* stack head pointer for different classes.
*/
Rooted<void*>** stack, *prev;
#endif
/*
* |ptr| must be the last field in Rooted because the analysis treats all
* Rooted as Rooted<void*> during the analysis. See bug 829372.
*/
T ptr;
MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
Rooted(const Rooted&) MOZ_DELETE;
};
} /* namespace JS */
namespace js {
/*
* Augment the generic Rooted<T> interface when T = JSObject* with
* class-querying and downcasting operations.
*
* Given a Rooted<JSObject*> obj, one can view
* Handle<StringObject*> h = obj.as<StringObject*>();
* as an optimization of
* Rooted<StringObject*> rooted(cx, &obj->as<StringObject*>());
* Handle<StringObject*> h = rooted;
*/
template <>
class RootedBase<JSObject*>
{
public:
template <class U>
JS::Handle<U*> as() const;
};
/* Interface substitute for Rooted<T> which does not root the variable's memory. */
template <typename T>
class FakeRooted : public RootedBase<T>
{
public:
template <typename CX>
FakeRooted(CX* cx
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(GCMethods<T>::initial())
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
}
template <typename CX>
FakeRooted(CX* cx, T initial
MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
: ptr(initial)
{
MOZ_GUARD_OBJECT_NOTIFIER_INIT;
}
operator T() const { return ptr; }
T operator->() const { return ptr; }
T* address() { return &ptr; }
const T* address() const { return &ptr; }
T& get() { return ptr; }
const T& get() const { return ptr; }
FakeRooted<T>& operator=(T value) {
MOZ_ASSERT(!GCMethods<T>::poisoned(value));
ptr = value;
return *this;
}
FakeRooted<T>& operator=(const FakeRooted<T>& other) {
MOZ_ASSERT(!GCMethods<T>::poisoned(other.ptr));
ptr = other.ptr;
return *this;
}
bool operator!=(const T& other) const { return ptr != other; }
bool operator==(const T& other) const { return ptr == other; }
private:
T ptr;
MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
FakeRooted(const FakeRooted&) MOZ_DELETE;
};
/* Interface substitute for MutableHandle<T> which is not required to point to rooted memory. */
template <typename T>
class FakeMutableHandle : public js::MutableHandleBase<T>
{
public:
MOZ_IMPLICIT FakeMutableHandle(T* t) {
ptr = t;
}
MOZ_IMPLICIT FakeMutableHandle(FakeRooted<T>* root) {
ptr = root->address();
}
void set(T v) {
MOZ_ASSERT(!js::GCMethods<T>::poisoned(v));
*ptr = v;
}
T* address() const { return ptr; }
T get() const { return *ptr; }
operator T() const { return get(); }
T operator->() const { return get(); }
private:
FakeMutableHandle() {}
T* ptr;
template <typename S>
void operator=(S v) MOZ_DELETE;
void operator=(const FakeMutableHandle<T>& other) MOZ_DELETE;
};
/*
* Types for a variable that either should or shouldn't be rooted, depending on
* the template parameter allowGC. Used for implementing functions that can
* operate on either rooted or unrooted data.
*
* The toHandle() and toMutableHandle() functions are for calling functions
* which require handle types and are only called in the CanGC case. These
* allow the calling code to type check.
*/
enum AllowGC {
NoGC = 0,
CanGC = 1
};
template <typename T, AllowGC allowGC>
class MaybeRooted
{
};
template <typename T> class MaybeRooted<T, CanGC>
{
public:
typedef JS::Handle<T> HandleType;
typedef JS::Rooted<T> RootType;
typedef JS::MutableHandle<T> MutableHandleType;
static inline JS::Handle<T> toHandle(HandleType v) {
return v;
}
static inline JS::MutableHandle<T> toMutableHandle(MutableHandleType v) {
return v;
}
};
template <typename T> class MaybeRooted<T, NoGC>
{
public:
typedef T HandleType;
typedef FakeRooted<T> RootType;
typedef FakeMutableHandle<T> MutableHandleType;
static inline JS::Handle<T> toHandle(HandleType v) {
MOZ_ASSUME_UNREACHABLE("Bad conversion");
}
static inline JS::MutableHandle<T> toMutableHandle(MutableHandleType v) {
MOZ_ASSUME_UNREACHABLE("Bad conversion");
}
};
} /* namespace js */
namespace JS {
template <typename T> template <typename S>
inline
Handle<T>::Handle(const Rooted<S>& root,
typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy)
{
ptr = reinterpret_cast<const T*>(root.address());
}
template <typename T> template <typename S>
inline
Handle<T>::Handle(const PersistentRooted<S>& root,
typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy)
{
ptr = reinterpret_cast<const T*>(root.address());
}
template <typename T> template <typename S>
inline
Handle<T>::Handle(MutableHandle<S>& root,
typename mozilla::EnableIf<mozilla::IsConvertible<S, T>::value, int>::Type dummy)
{
ptr = reinterpret_cast<const T*>(root.address());
}
template <typename T>
inline
MutableHandle<T>::MutableHandle(Rooted<T>* root)
{
static_assert(sizeof(MutableHandle<T>) == sizeof(T*),
"MutableHandle must be binary compatible with T*.");
ptr = root->address();
}
template <typename T>
inline
MutableHandle<T>::MutableHandle(PersistentRooted<T>* root)
{
static_assert(sizeof(MutableHandle<T>) == sizeof(T*),
"MutableHandle must be binary compatible with T*.");
ptr = root->address();
}
/*
* A copyable, assignable global GC root type with arbitrary lifetime, an
* infallible constructor, and automatic unrooting on destruction.
*
* These roots can be used in heap-allocated data structures, so they are not
* associated with any particular JSContext or stack. They are registered with
* the JSRuntime itself, without locking, so they require a full JSContext to be
* constructed, not one of its more restricted superclasses.
*
* Note that you must not use an PersistentRooted in an object owned by a JS
* object:
*
* Whenever one object whose lifetime is decided by the GC refers to another
* such object, that edge must be traced only if the owning JS object is traced.
* This applies not only to JS objects (which obviously are managed by the GC)
* but also to C++ objects owned by JS objects.
*
* If you put a PersistentRooted in such a C++ object, that is almost certainly
* a leak. When a GC begins, the referent of the PersistentRooted is treated as
* live, unconditionally (because a PersistentRooted is a *root*), even if the
* JS object that owns it is unreachable. If there is any path from that
* referent back to the JS object, then the C++ object containing the
* PersistentRooted will not be destructed, and the whole blob of objects will
* not be freed, even if there are no references to them from the outside.
*
* In the context of Firefox, this is a severe restriction: almost everything in
* Firefox is owned by some JS object or another, so using PersistentRooted in
* such objects would introduce leaks. For these kinds of edges, Heap<T> or
* TenuredHeap<T> would be better types. It's up to the implementor of the type
* containing Heap<T> or TenuredHeap<T> members to make sure their referents get
* marked when the object itself is marked.
*/
template<typename T>
class PersistentRooted : private mozilla::LinkedListElement<PersistentRooted<T> > {
friend class mozilla::LinkedList<PersistentRooted>;
friend class mozilla::LinkedListElement<PersistentRooted>;
friend class js::gc::PersistentRootedMarker<T>;
void registerWithRuntime(JSRuntime* rt) {
JS::shadow::Runtime* srt = JS::shadow::Runtime::asShadowRuntime(rt);
srt->getPersistentRootedList<T>().insertBack(this);
}
public:
explicit PersistentRooted(JSContext* cx) : ptr(js::GCMethods<T>::initial())
{
registerWithRuntime(js::GetRuntime(cx));
}
PersistentRooted(JSContext* cx, T initial) : ptr(initial)
{
registerWithRuntime(js::GetRuntime(cx));
}
explicit PersistentRooted(JSRuntime* rt) : ptr(js::GCMethods<T>::initial())
{
registerWithRuntime(rt);
}
PersistentRooted(JSRuntime* rt, T initial) : ptr(initial)
{
registerWithRuntime(rt);
}
PersistentRooted(PersistentRooted& rhs)
: mozilla::LinkedListElement<PersistentRooted<T> >(),
ptr(rhs.ptr)
{
/*
* Copy construction takes advantage of the fact that the original
* is already inserted, and simply adds itself to whatever list the
* original was on - no JSRuntime pointer needed.
*/
rhs.setNext(this);
}
/*
* Important: Return a reference here so passing a Rooted<T> to
* something that takes a |const T&| is not a GC hazard.
*/
operator const T&() const { return ptr; }
T operator->() const { return ptr; }
T* address() { return &ptr; }
const T* address() const { return &ptr; }
T& get() { return ptr; }
const T& get() const { return ptr; }
T& operator=(T value) {
MOZ_ASSERT(!js::GCMethods<T>::poisoned(value));
ptr = value;
return ptr;
}
T& operator=(const PersistentRooted& value) {
ptr = value;
return ptr;
}
void set(T value) {
MOZ_ASSERT(!js::GCMethods<T>::poisoned(value));
ptr = value;
}
bool operator!=(const T& other) const { return ptr != other; }
bool operator==(const T& other) const { return ptr == other; }
private:
T ptr;
};
} /* namespace JS */
namespace js {
/* Base class for automatic read-only object rooting during compilation. */
class CompilerRootNode
{
protected:
explicit CompilerRootNode(js::gc::Cell* ptr) : next(nullptr), ptr_(ptr) {}
public:
void** address() { return (void**)&ptr_; }
public:
CompilerRootNode* next;
protected:
js::gc::Cell* ptr_;
};
} /* namespace js */
#endif /* js_RootingAPI_h */
|