summaryrefslogtreecommitdiffstats
path: root/js/src/gc/Memory.cpp
blob: 7d7e22640ddb05116669b44b056b0b6e030e3f0e (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
/* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 * vim: set ts=8 sts=2 et sw=2 tw=80:
 * 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/. */

#include "gc/Memory.h"

#include "mozilla/Atomics.h"
#include "mozilla/MathAlgorithms.h"
#include "mozilla/RandomNum.h"
#include "mozilla/TaggedAnonymousMemory.h"

#include "jit/JitOptions.h"
#include "js/HeapAPI.h"
#include "js/Utility.h"
#include "util/Memory.h"

#ifdef XP_WIN

#  include "util/WindowsWrapper.h"
#  include <psapi.h>

#else

#  include <algorithm>
#  include <errno.h>
#  include <unistd.h>

#  if !defined(__wasi__)
#    include <sys/mman.h>
#    include <sys/resource.h>
#    include <sys/stat.h>
#    include <sys/types.h>
#  endif  // !defined(__wasi__)

#endif  // !XP_WIN

namespace js::gc {

/*
 * System allocation functions generally require the allocation size
 * to be an integer multiple of the page size of the running process.
 */
static size_t pageSize = 0;

/* The OS allocation granularity may not match the page size. */
static size_t allocGranularity = 0;

/* The number of bits used by addresses on this platform. */
static size_t numAddressBits = 0;

/* An estimate of the number of bytes available for virtual memory. */
static size_t virtualMemoryLimit = size_t(-1);

/*
 * System allocation functions may hand out regions of memory in increasing or
 * decreasing order. This ordering is used as a hint during chunk alignment to
 * reduce the number of system calls. On systems with 48-bit addresses, our
 * workarounds to obtain 47-bit pointers cause addresses to be handed out in
 * increasing order.
 *
 * We do not use the growth direction on Windows, as constraints on VirtualAlloc
 * would make its application failure prone and complex. Tests indicate that
 * VirtualAlloc always hands out regions of memory in increasing order.
 */
#if defined(XP_DARWIN)
static mozilla::Atomic<int, mozilla::Relaxed> growthDirection(1);
#elif defined(XP_UNIX)
static mozilla::Atomic<int, mozilla::Relaxed> growthDirection(0);
#endif

/*
 * Data from OOM crashes shows there may be up to 24 chunk-sized but unusable
 * chunks available in low memory situations. These chunks may all need to be
 * used up before we gain access to remaining *alignable* chunk-sized regions,
 * so we use a generous limit of 32 unusable chunks to ensure we reach them.
 */
static const int MaxLastDitchAttempts = 32;

#ifdef JS_64BIT
/*
 * On some 64-bit platforms we can use a random, scattershot allocator that
 * tries addresses from the available range at random. If the address range
 * is large enough this will have a high chance of success and additionally
 * makes the memory layout of our process less predictable.
 *
 * However, not all 64-bit platforms have a very large address range. For
 * example, AArch64 on Linux defaults to using 39-bit addresses to limit the
 * number of translation tables used. On such configurations the scattershot
 * approach to allocation creates a conflict with our desire to reserve large
 * regions of memory for applications like WebAssembly: Small allocations may
 * inadvertently block off all available 4-6GiB regions, and conversely
 * reserving such regions may lower the success rate for smaller allocations to
 * unacceptable levels.
 *
 * So we make a compromise: Instead of using the scattershot on all 64-bit
 * platforms, we only use it on platforms that meet a minimum requirement for
 * the available address range. In addition we split the address range,
 * reserving the upper half for huge allocations and the lower half for smaller
 * allocations. We use a limit of 43 bits so that at least 42 bits are available
 * for huge allocations - this matches the 8TiB per process address space limit
 * that we're already subject to on Windows.
 */
static const size_t MinAddressBitsForRandomAlloc = 43;

/* The lower limit for huge allocations. This is fairly arbitrary. */
static const size_t HugeAllocationSize = 1024 * 1024 * 1024;

/* The minimum and maximum valid addresses that can be allocated into. */
static size_t minValidAddress = 0;
static size_t maxValidAddress = 0;

/* The upper limit for smaller allocations and the lower limit for huge ones. */
static size_t hugeSplit = 0;
#endif

size_t SystemPageSize() { return pageSize; }

size_t SystemAddressBits() { return numAddressBits; }

size_t VirtualMemoryLimit() { return virtualMemoryLimit; }

bool UsingScattershotAllocator() {
#ifdef JS_64BIT
  return numAddressBits >= MinAddressBitsForRandomAlloc;
#else
  return false;
#endif
}

enum class Commit : bool {
  No = false,
  Yes = true,
};

#ifdef XP_WIN
enum class PageAccess : DWORD {
  None = PAGE_NOACCESS,
  Read = PAGE_READONLY,
  ReadWrite = PAGE_READWRITE,
  Execute = PAGE_EXECUTE,
  ReadExecute = PAGE_EXECUTE_READ,
  ReadWriteExecute = PAGE_EXECUTE_READWRITE,
};
#elif defined(__wasi__)
enum class PageAccess : int {
  None = 0,
  Read = 0,
  ReadWrite = 0,
  Execute = 0,
  ReadExecute = 0,
  ReadWriteExecute = 0,
};
#else
enum class PageAccess : int {
  None = PROT_NONE,
  Read = PROT_READ,
  ReadWrite = PROT_READ | PROT_WRITE,
  Execute = PROT_EXEC,
  ReadExecute = PROT_READ | PROT_EXEC,
  ReadWriteExecute = PROT_READ | PROT_WRITE | PROT_EXEC,
};
#endif

template <bool AlwaysGetNew = true>
static bool TryToAlignChunk(void** aRegion, void** aRetainedRegion,
                            size_t length, size_t alignment);

#ifndef __wasi__
static void* MapAlignedPagesSlow(size_t length, size_t alignment);
#endif  // wasi
static void* MapAlignedPagesLastDitch(size_t length, size_t alignment);

#ifdef JS_64BIT
static void* MapAlignedPagesRandom(size_t length, size_t alignment);
#endif

void* TestMapAlignedPagesLastDitch(size_t length, size_t alignment) {
  return MapAlignedPagesLastDitch(length, alignment);
}

bool DecommitEnabled() { return SystemPageSize() == PageSize; }

/* Returns the offset from the nearest aligned address at or below |region|. */
static inline size_t OffsetFromAligned(void* region, size_t alignment) {
  return uintptr_t(region) % alignment;
}

template <Commit commit, PageAccess prot>
static inline void* MapInternal(void* desired, size_t length) {
  void* region = nullptr;
#ifdef XP_WIN
  DWORD flags =
      (commit == Commit::Yes ? MEM_RESERVE | MEM_COMMIT : MEM_RESERVE);
  region = VirtualAlloc(desired, length, flags, DWORD(prot));
#elif defined(__wasi__)
  if (int err = posix_memalign(&region, gc::SystemPageSize(), length)) {
    MOZ_RELEASE_ASSERT(err == ENOMEM);
    return nullptr;
  }
  if (region) {
    memset(region, 0, length);
  }
#else
  int flags = MAP_PRIVATE | MAP_ANON;
  region = MozTaggedAnonymousMmap(desired, length, int(prot), flags, -1, 0,
                                  "js-gc-heap");
  if (region == MAP_FAILED) {
    return nullptr;
  }
#endif
  return region;
}

static inline void UnmapInternal(void* region, size_t length) {
  MOZ_ASSERT(region && OffsetFromAligned(region, allocGranularity) == 0);
  MOZ_ASSERT(length > 0 && length % pageSize == 0);

#ifdef XP_WIN
  MOZ_RELEASE_ASSERT(VirtualFree(region, 0, MEM_RELEASE) != 0);
#elif defined(__wasi__)
  free(region);
#else
  if (munmap(region, length)) {
    MOZ_RELEASE_ASSERT(errno == ENOMEM);
  }
#endif
}

template <Commit commit = Commit::Yes, PageAccess prot = PageAccess::ReadWrite>
static inline void* MapMemory(size_t length) {
  MOZ_ASSERT(length > 0);

  return MapInternal<commit, prot>(nullptr, length);
}

/*
 * Attempts to map memory at the given address, but allows the system
 * to return a different address that may still be suitable.
 */
template <Commit commit = Commit::Yes, PageAccess prot = PageAccess::ReadWrite>
static inline void* MapMemoryAtFuzzy(void* desired, size_t length) {
  MOZ_ASSERT(desired && OffsetFromAligned(desired, allocGranularity) == 0);
  MOZ_ASSERT(length > 0);

  // Note that some platforms treat the requested address as a hint, so the
  // returned address might not match the requested address.
  return MapInternal<commit, prot>(desired, length);
}

/*
 * Attempts to map memory at the given address, returning nullptr if
 * the system returns any address other than the requested one.
 */
template <Commit commit = Commit::Yes, PageAccess prot = PageAccess::ReadWrite>
static inline void* MapMemoryAt(void* desired, size_t length) {
  MOZ_ASSERT(desired && OffsetFromAligned(desired, allocGranularity) == 0);
  MOZ_ASSERT(length > 0);

  void* region = MapInternal<commit, prot>(desired, length);
  if (!region) {
    return nullptr;
  }

  // On some platforms mmap treats the desired address as a hint, so
  // check that the address we got is the address we requested.
  if (region != desired) {
    UnmapInternal(region, length);
    return nullptr;
  }
  return region;
}

#ifdef JS_64BIT

/* Returns a random number in the given range. */
static inline uint64_t GetNumberInRange(uint64_t minNum, uint64_t maxNum) {
  const uint64_t MaxRand = UINT64_C(0xffffffffffffffff);
  maxNum -= minNum;
  uint64_t binSize = 1 + (MaxRand - maxNum) / (maxNum + 1);

  uint64_t rndNum;
  do {
    mozilla::Maybe<uint64_t> result;
    do {
      result = mozilla::RandomUint64();
    } while (!result);
    rndNum = result.value() / binSize;
  } while (rndNum > maxNum);

  return minNum + rndNum;
}

#  ifndef XP_WIN
static inline uint64_t FindAddressLimitInner(size_t highBit, size_t tries);

/*
 * The address range available to applications depends on both hardware and
 * kernel configuration. For example, AArch64 on Linux uses addresses with
 * 39 significant bits by default, but can be configured to use addresses with
 * 48 significant bits by enabling a 4th translation table. Unfortunately,
 * there appears to be no standard way to query the limit at runtime
 * (Windows exposes this via GetSystemInfo()).
 *
 * This function tries to find the address limit by performing a binary search
 * on the index of the most significant set bit in the addresses it attempts to
 * allocate. As the requested address is often treated as a hint by the
 * operating system, we use the actual returned addresses to narrow the range.
 * We return the number of bits of an address that may be set.
 */
static size_t FindAddressLimit() {
  // Use 32 bits as a lower bound in case we keep getting nullptr.
  uint64_t low = 31;
  uint64_t highestSeen = (UINT64_C(1) << 32) - allocGranularity - 1;

  // Exclude 48-bit and 47-bit addresses first.
  uint64_t high = 47;
  for (; high >= std::max(low, UINT64_C(46)); --high) {
    highestSeen = std::max(FindAddressLimitInner(high, 4), highestSeen);
    low = mozilla::FloorLog2(highestSeen);
  }
  // If those didn't work, perform a modified binary search.
  while (high - 1 > low) {
    uint64_t middle = low + (high - low) / 2;
    highestSeen = std::max(FindAddressLimitInner(middle, 4), highestSeen);
    low = mozilla::FloorLog2(highestSeen);
    if (highestSeen < (UINT64_C(1) << middle)) {
      high = middle;
    }
  }
  // We can be sure of the lower bound, but check the upper bound again.
  do {
    high = low + 1;
    highestSeen = std::max(FindAddressLimitInner(high, 8), highestSeen);
    low = mozilla::FloorLog2(highestSeen);
  } while (low >= high);

  // `low` is the highest set bit, so `low + 1` is the number of bits.
  return low + 1;
}

static inline uint64_t FindAddressLimitInner(size_t highBit, size_t tries) {
  const size_t length = allocGranularity;  // Used as both length and alignment.

  uint64_t highestSeen = 0;
  uint64_t startRaw = UINT64_C(1) << highBit;
  uint64_t endRaw = 2 * startRaw - length - 1;
  uint64_t start = (startRaw + length - 1) / length;
  uint64_t end = (endRaw - (length - 1)) / length;
  for (size_t i = 0; i < tries; ++i) {
    uint64_t desired = length * GetNumberInRange(start, end);
    void* address = MapMemoryAtFuzzy(reinterpret_cast<void*>(desired), length);
    uint64_t actual = uint64_t(address);
    if (address) {
      UnmapInternal(address, length);
    }
    if (actual > highestSeen) {
      highestSeen = actual;
      if (actual >= startRaw) {
        break;
      }
    }
  }
  return highestSeen;
}
#  endif  // !defined(XP_WIN)

#endif  // defined(JS_64BIT)

void InitMemorySubsystem() {
  if (pageSize == 0) {
#ifdef XP_WIN
    SYSTEM_INFO sysinfo;
    GetSystemInfo(&sysinfo);
    pageSize = sysinfo.dwPageSize;
    allocGranularity = sysinfo.dwAllocationGranularity;
#else
    pageSize = size_t(sysconf(_SC_PAGESIZE));
    allocGranularity = pageSize;
#endif
#ifdef JS_64BIT
#  ifdef XP_WIN
    minValidAddress = size_t(sysinfo.lpMinimumApplicationAddress);
    maxValidAddress = size_t(sysinfo.lpMaximumApplicationAddress);
    numAddressBits = mozilla::FloorLog2(maxValidAddress) + 1;
#  else
    // No standard way to determine these, so fall back to FindAddressLimit().
    numAddressBits = FindAddressLimit();
    minValidAddress = allocGranularity;
    maxValidAddress = (UINT64_C(1) << numAddressBits) - 1 - allocGranularity;
#  endif
    // Sanity check the address to ensure we don't use more than 47 bits.
    uint64_t maxJSAddress = UINT64_C(0x00007fffffffffff) - allocGranularity;
    if (maxValidAddress > maxJSAddress) {
      maxValidAddress = maxJSAddress;
      hugeSplit = UINT64_C(0x00003fffffffffff) - allocGranularity;
    } else {
      hugeSplit = (UINT64_C(1) << (numAddressBits - 1)) - 1 - allocGranularity;
    }
#else  // !defined(JS_64BIT)
    numAddressBits = 32;
#endif
#ifdef RLIMIT_AS
    if (jit::HasJitBackend()) {
      rlimit as_limit;
      if (getrlimit(RLIMIT_AS, &as_limit) == 0 &&
          as_limit.rlim_max != RLIM_INFINITY) {
        virtualMemoryLimit = as_limit.rlim_max;
      }
    }
#endif
  }
}

#ifdef JS_64BIT
/* The JS engine uses 47-bit pointers; all higher bits must be clear. */
static inline bool IsInvalidRegion(void* region, size_t length) {
  const uint64_t invalidPointerMask = UINT64_C(0xffff800000000000);
  return (uintptr_t(region) + length - 1) & invalidPointerMask;
}
#endif

void* MapAlignedPages(size_t length, size_t alignment) {
  MOZ_RELEASE_ASSERT(length > 0 && alignment > 0);
  MOZ_RELEASE_ASSERT(length % pageSize == 0);
  MOZ_RELEASE_ASSERT(std::max(alignment, allocGranularity) %
                         std::min(alignment, allocGranularity) ==
                     0);

  // Smaller alignments aren't supported by the allocation functions.
  if (alignment < allocGranularity) {
    alignment = allocGranularity;
  }

#ifdef __wasi__
  void* region = nullptr;
  if (int err = posix_memalign(&region, alignment, length)) {
    MOZ_ASSERT(err == ENOMEM);
    return nullptr;
  }
  MOZ_ASSERT(region != nullptr);
  memset(region, 0, length);
  return region;
#else

#  ifdef JS_64BIT
  // Use the scattershot allocator if the address range is large enough.
  if (UsingScattershotAllocator()) {
    void* region = MapAlignedPagesRandom(length, alignment);

    MOZ_RELEASE_ASSERT(!IsInvalidRegion(region, length));
    MOZ_ASSERT(OffsetFromAligned(region, alignment) == 0);

    return region;
  }
#  endif

  // Try to allocate the region. If the returned address is aligned,
  // either we OOMed (region is nullptr) or we're done.
  void* region = MapMemory(length);
  if (OffsetFromAligned(region, alignment) == 0) {
    return region;
  }

  // Try to align the region. On success, TryToAlignChunk() returns
  // true and we can return the aligned region immediately.
  void* retainedRegion;
  if (TryToAlignChunk(&region, &retainedRegion, length, alignment)) {
    MOZ_ASSERT(region && OffsetFromAligned(region, alignment) == 0);
    MOZ_ASSERT(!retainedRegion);
    return region;
  }

  // On failure, the unaligned region is retained unless we OOMed. We don't
  // use the retained region on this path (see the last ditch allocator).
  if (retainedRegion) {
    UnmapInternal(retainedRegion, length);
  }

  // If it fails to align the given region, TryToAlignChunk() returns the
  // next valid region that we might be able to align (unless we OOMed).
  if (region) {
    MOZ_ASSERT(OffsetFromAligned(region, alignment) != 0);
    UnmapInternal(region, length);
  }

  // Since we couldn't align the first region, fall back to allocating a
  // region large enough that we can definitely align it.
  region = MapAlignedPagesSlow(length, alignment);
  if (!region) {
    // If there wasn't enough contiguous address space left for that,
    // try to find an alignable region using the last ditch allocator.
    region = MapAlignedPagesLastDitch(length, alignment);
  }

  // At this point we should either have an aligned region or nullptr.
  MOZ_ASSERT(OffsetFromAligned(region, alignment) == 0);
  return region;
#endif  // !__wasi__
}

#ifdef JS_64BIT

/*
 * This allocator takes advantage of the large address range on some 64-bit
 * platforms to allocate in a scattershot manner, choosing addresses at random
 * from the range. By controlling the range we can avoid returning addresses
 * that have more than 47 significant bits (as required by SpiderMonkey).
 * This approach also has some other advantages over the methods employed by
 * the other allocation functions in this file:
 * 1) Allocations are extremely likely to succeed on the first try.
 * 2) The randomness makes our memory layout becomes harder to predict.
 * 3) The low probability of reusing regions guards against use-after-free.
 *
 * The main downside is that detecting physical OOM situations becomes more
 * difficult; to guard against this, we occasionally try a regular allocation.
 * In addition, sprinkling small allocations throughout the full address range
 * might get in the way of large address space reservations such as those
 * employed by WebAssembly. To avoid this (or the opposite problem of such
 * reservations reducing the chance of success for smaller allocations) we
 * split the address range in half, with one half reserved for huge allocations
 * and the other for regular (usually chunk sized) allocations.
 */
static void* MapAlignedPagesRandom(size_t length, size_t alignment) {
  uint64_t minNum, maxNum;
  if (length < HugeAllocationSize) {
    // Use the lower half of the range.
    minNum = (minValidAddress + alignment - 1) / alignment;
    maxNum = (hugeSplit - (length - 1)) / alignment;
  } else {
    // Use the upper half of the range.
    minNum = (hugeSplit + 1 + alignment - 1) / alignment;
    maxNum = (maxValidAddress - (length - 1)) / alignment;
  }

  // Try to allocate in random aligned locations.
  void* region = nullptr;
  for (size_t i = 1; i <= 1024; ++i) {
    if (i & 0xf) {
      uint64_t desired = alignment * GetNumberInRange(minNum, maxNum);
      region = MapMemoryAtFuzzy(reinterpret_cast<void*>(desired), length);
      if (!region) {
        continue;
      }
    } else {
      // Check for OOM.
      region = MapMemory(length);
      if (!region) {
        return nullptr;
      }
    }
    if (IsInvalidRegion(region, length)) {
      UnmapInternal(region, length);
      continue;
    }
    if (OffsetFromAligned(region, alignment) == 0) {
      return region;
    }
    void* retainedRegion = nullptr;
    if (TryToAlignChunk<false>(&region, &retainedRegion, length, alignment)) {
      MOZ_ASSERT(region && OffsetFromAligned(region, alignment) == 0);
      MOZ_ASSERT(!retainedRegion);
      return region;
    }
    MOZ_ASSERT(region && !retainedRegion);
    UnmapInternal(region, length);
  }

  if (numAddressBits < 48) {
    // Try the reliable fallback of overallocating.
    // Note: This will not respect the address space split.
    region = MapAlignedPagesSlow(length, alignment);
    if (region) {
      return region;
    }
  }
  if (length < HugeAllocationSize) {
    MOZ_CRASH("Couldn't allocate even after 1000 tries!");
  }

  return nullptr;
}

#endif  // defined(JS_64BIT)

#ifndef __wasi__
static void* MapAlignedPagesSlow(size_t length, size_t alignment) {
  void* alignedRegion = nullptr;
  do {
    size_t reserveLength = length + alignment - pageSize;
#  ifdef XP_WIN
    // Don't commit the requested pages as we won't use the region directly.
    void* region = MapMemory<Commit::No>(reserveLength);
#  else
    void* region = MapMemory(reserveLength);
#  endif
    if (!region) {
      return nullptr;
    }
    alignedRegion =
        reinterpret_cast<void*>(AlignBytes(uintptr_t(region), alignment));
#  ifdef XP_WIN
    // Windows requires that map and unmap calls be matched, so deallocate
    // and immediately reallocate at the desired (aligned) address.
    UnmapInternal(region, reserveLength);
    alignedRegion = MapMemoryAt(alignedRegion, length);
#  else
    // munmap allows us to simply unmap the pages that don't interest us.
    if (alignedRegion != region) {
      UnmapInternal(region, uintptr_t(alignedRegion) - uintptr_t(region));
    }
    void* regionEnd =
        reinterpret_cast<void*>(uintptr_t(region) + reserveLength);
    void* alignedEnd =
        reinterpret_cast<void*>(uintptr_t(alignedRegion) + length);
    if (alignedEnd != regionEnd) {
      UnmapInternal(alignedEnd, uintptr_t(regionEnd) - uintptr_t(alignedEnd));
    }
#  endif
    // On Windows we may have raced with another thread; if so, try again.
  } while (!alignedRegion);

  return alignedRegion;
}
#endif  // wasi

/*
 * In a low memory or high fragmentation situation, alignable chunks of the
 * desired length may still be available, even if there are no more contiguous
 * free chunks that meet the |length + alignment - pageSize| requirement of
 * MapAlignedPagesSlow. In this case, try harder to find an alignable chunk
 * by temporarily holding onto the unaligned parts of each chunk until the
 * allocator gives us a chunk that either is, or can be aligned.
 */
static void* MapAlignedPagesLastDitch(size_t length, size_t alignment) {
  void* tempMaps[MaxLastDitchAttempts];
  int attempt = 0;
  void* region = MapMemory(length);
  if (OffsetFromAligned(region, alignment) == 0) {
    return region;
  }
  for (; attempt < MaxLastDitchAttempts; ++attempt) {
    if (TryToAlignChunk(&region, tempMaps + attempt, length, alignment)) {
      MOZ_ASSERT(region && OffsetFromAligned(region, alignment) == 0);
      MOZ_ASSERT(!tempMaps[attempt]);
      break;  // Success!
    }
    if (!region || !tempMaps[attempt]) {
      break;  // We ran out of memory, so give up.
    }
  }
  if (OffsetFromAligned(region, alignment)) {
    UnmapInternal(region, length);
    region = nullptr;
  }
  while (--attempt >= 0) {
    UnmapInternal(tempMaps[attempt], length);
  }
  return region;
}

#ifdef XP_WIN

/*
 * On Windows, map and unmap calls must be matched, so we deallocate the
 * unaligned chunk, then reallocate the unaligned part to block off the
 * old address and force the allocator to give us a new one.
 */
template <bool>
static bool TryToAlignChunk(void** aRegion, void** aRetainedRegion,
                            size_t length, size_t alignment) {
  void* region = *aRegion;
  MOZ_ASSERT(region && OffsetFromAligned(region, alignment) != 0);

  size_t retainedLength = 0;
  void* retainedRegion = nullptr;
  do {
    size_t offset = OffsetFromAligned(region, alignment);
    if (offset == 0) {
      // If the address is aligned, either we hit OOM or we're done.
      break;
    }
    UnmapInternal(region, length);
    retainedLength = alignment - offset;
    retainedRegion = MapMemoryAt<Commit::No>(region, retainedLength);
    region = MapMemory(length);

    // If retainedRegion is null here, we raced with another thread.
  } while (!retainedRegion);

  bool result = OffsetFromAligned(region, alignment) == 0;
  if (result && retainedRegion) {
    UnmapInternal(retainedRegion, retainedLength);
    retainedRegion = nullptr;
  }

  *aRegion = region;
  *aRetainedRegion = retainedRegion;
  return region && result;
}

#else  // !defined(XP_WIN)

/*
 * mmap calls don't have to be matched with calls to munmap, so we can unmap
 * just the pages we don't need. However, as we don't know a priori if addresses
 * are handed out in increasing or decreasing order, we have to try both
 * directions (depending on the environment, one will always fail).
 */
template <bool AlwaysGetNew>
static bool TryToAlignChunk(void** aRegion, void** aRetainedRegion,
                            size_t length, size_t alignment) {
  void* regionStart = *aRegion;
  MOZ_ASSERT(regionStart && OffsetFromAligned(regionStart, alignment) != 0);

  bool addressesGrowUpward = growthDirection > 0;
  bool directionUncertain = -8 < growthDirection && growthDirection <= 8;
  size_t offsetLower = OffsetFromAligned(regionStart, alignment);
  size_t offsetUpper = alignment - offsetLower;
  for (size_t i = 0; i < 2; ++i) {
    if (addressesGrowUpward) {
      void* upperStart =
          reinterpret_cast<void*>(uintptr_t(regionStart) + offsetUpper);
      void* regionEnd =
          reinterpret_cast<void*>(uintptr_t(regionStart) + length);
      if (MapMemoryAt(regionEnd, offsetUpper)) {
        UnmapInternal(regionStart, offsetUpper);
        if (directionUncertain) {
          ++growthDirection;
        }
        regionStart = upperStart;
        break;
      }
    } else {
      auto* lowerStart =
          reinterpret_cast<void*>(uintptr_t(regionStart) - offsetLower);
      auto* lowerEnd = reinterpret_cast<void*>(uintptr_t(lowerStart) + length);
      if (MapMemoryAt(lowerStart, offsetLower)) {
        UnmapInternal(lowerEnd, offsetLower);
        if (directionUncertain) {
          --growthDirection;
        }
        regionStart = lowerStart;
        break;
      }
    }
    // If we're confident in the growth direction, don't try the other.
    if (!directionUncertain) {
      break;
    }
    addressesGrowUpward = !addressesGrowUpward;
  }

  void* retainedRegion = nullptr;
  bool result = OffsetFromAligned(regionStart, alignment) == 0;
  if (AlwaysGetNew && !result) {
    // If our current chunk cannot be aligned, just get a new one.
    retainedRegion = regionStart;
    regionStart = MapMemory(length);
    // Our new region might happen to already be aligned.
    result = OffsetFromAligned(regionStart, alignment) == 0;
    if (result) {
      UnmapInternal(retainedRegion, length);
      retainedRegion = nullptr;
    }
  }

  *aRegion = regionStart;
  *aRetainedRegion = retainedRegion;
  return regionStart && result;
}

#endif

void UnmapPages(void* region, size_t length) {
  MOZ_RELEASE_ASSERT(region &&
                     OffsetFromAligned(region, allocGranularity) == 0);
  MOZ_RELEASE_ASSERT(length > 0 && length % pageSize == 0);

  // ASan does not automatically unpoison memory, so we have to do this here.
  MOZ_MAKE_MEM_UNDEFINED(region, length);

  UnmapInternal(region, length);
}

static void CheckDecommit(void* region, size_t length) {
  MOZ_RELEASE_ASSERT(region);
  MOZ_RELEASE_ASSERT(length > 0);

  // pageSize == ArenaSize doesn't necessarily hold, but this function is
  // used by the GC to decommit unused Arenas, so we don't want to assert
  // if pageSize > ArenaSize.
  MOZ_ASSERT(OffsetFromAligned(region, ArenaSize) == 0);
  MOZ_ASSERT(length % ArenaSize == 0);

  MOZ_RELEASE_ASSERT(OffsetFromAligned(region, pageSize) == 0);
  MOZ_RELEASE_ASSERT(length % pageSize == 0);
}

bool MarkPagesUnusedSoft(void* region, size_t length) {
  MOZ_ASSERT(DecommitEnabled());
  CheckDecommit(region, length);

  MOZ_MAKE_MEM_NOACCESS(region, length);

#if defined(XP_WIN)
  return VirtualAlloc(region, length, MEM_RESET,
                      DWORD(PageAccess::ReadWrite)) == region;
#elif defined(__wasi__)
  return 0;
#else
  int status;
  do {
#  if defined(XP_DARWIN)
    status = madvise(region, length, MADV_FREE_REUSABLE);
#  elif defined(XP_SOLARIS)
    status = posix_madvise(region, length, POSIX_MADV_DONTNEED);
#  else
    status = madvise(region, length, MADV_DONTNEED);
#  endif
  } while (status == -1 && errno == EAGAIN);
  return status == 0;
#endif
}

bool MarkPagesUnusedHard(void* region, size_t length) {
  CheckDecommit(region, length);

  MOZ_MAKE_MEM_NOACCESS(region, length);

  if (!DecommitEnabled()) {
    return true;
  }

#if defined(XP_WIN)
  return VirtualFree(region, length, MEM_DECOMMIT);
#else
  return MarkPagesUnusedSoft(region, length);
#endif
}

void MarkPagesInUseSoft(void* region, size_t length) {
  MOZ_ASSERT(DecommitEnabled());
  CheckDecommit(region, length);

#if defined(XP_DARWIN)
  while (madvise(region, length, MADV_FREE_REUSE) == -1 && errno == EAGAIN) {
  }
#endif

  MOZ_MAKE_MEM_UNDEFINED(region, length);
}

bool MarkPagesInUseHard(void* region, size_t length) {
  if (js::oom::ShouldFailWithOOM()) {
    return false;
  }

  CheckDecommit(region, length);

  MOZ_MAKE_MEM_UNDEFINED(region, length);

  if (!DecommitEnabled()) {
    return true;
  }

#if defined(XP_WIN)
  return VirtualAlloc(region, length, MEM_COMMIT,
                      DWORD(PageAccess::ReadWrite)) == region;
#else
  return true;
#endif
}

size_t GetPageFaultCount() {
#ifdef XP_WIN
  PROCESS_MEMORY_COUNTERS pmc;
  if (GetProcessMemoryInfo(GetCurrentProcess(), &pmc, sizeof(pmc)) == 0) {
    return 0;
  }
  return pmc.PageFaultCount;
#elif defined(__wasi__)
  return 0;
#else
  struct rusage usage;
  int err = getrusage(RUSAGE_SELF, &usage);
  if (err) {
    return 0;
  }
  return usage.ru_majflt;
#endif
}

void* AllocateMappedContent(int fd, size_t offset, size_t length,
                            size_t alignment) {
#ifdef __wasi__
  MOZ_CRASH("Not yet supported for WASI");
#else
  if (length == 0 || alignment == 0 || offset % alignment != 0 ||
      std::max(alignment, allocGranularity) %
              std::min(alignment, allocGranularity) !=
          0) {
    return nullptr;
  }

  size_t alignedOffset = offset - (offset % allocGranularity);
  size_t alignedLength = length + (offset % allocGranularity);

  // We preallocate the mapping using MapAlignedPages, which expects
  // the length parameter to be an integer multiple of the page size.
  size_t mappedLength = alignedLength;
  if (alignedLength % pageSize != 0) {
    mappedLength += pageSize - alignedLength % pageSize;
  }

#  ifdef XP_WIN
  HANDLE hFile = reinterpret_cast<HANDLE>(intptr_t(fd));

  // This call will fail if the file does not exist.
  HANDLE hMap =
      CreateFileMappingW(hFile, nullptr, PAGE_READONLY, 0, 0, nullptr);
  if (!hMap) {
    return nullptr;
  }

  DWORD offsetH = uint32_t(uint64_t(alignedOffset) >> 32);
  DWORD offsetL = uint32_t(alignedOffset);

  uint8_t* map = nullptr;
  for (;;) {
    // The value of a pointer is technically only defined while the region
    // it points to is allocated, so explicitly treat this one as a number.
    uintptr_t region = uintptr_t(MapAlignedPages(mappedLength, alignment));
    if (region == 0) {
      break;
    }
    UnmapInternal(reinterpret_cast<void*>(region), mappedLength);
    // If the offset or length are out of bounds, this call will fail.
    map = static_cast<uint8_t*>(
        MapViewOfFileEx(hMap, FILE_MAP_COPY, offsetH, offsetL, alignedLength,
                        reinterpret_cast<void*>(region)));

    // Retry if another thread mapped the address we were trying to use.
    if (map || GetLastError() != ERROR_INVALID_ADDRESS) {
      break;
    }
  }

  // This just decreases the file mapping object's internal reference count;
  // it won't actually be destroyed until we unmap the associated view.
  CloseHandle(hMap);

  if (!map) {
    return nullptr;
  }
#  else  // !defined(XP_WIN)
  // Sanity check the offset and length, as mmap does not do this for us.
  struct stat st;
  if (fstat(fd, &st) || offset >= uint64_t(st.st_size) ||
      length > uint64_t(st.st_size) - offset) {
    return nullptr;
  }

  void* region = MapAlignedPages(mappedLength, alignment);
  if (!region) {
    return nullptr;
  }

  // Calling mmap with MAP_FIXED will replace the previous mapping, allowing
  // us to reuse the region we obtained without racing with other threads.
  uint8_t* map =
      static_cast<uint8_t*>(mmap(region, alignedLength, PROT_READ | PROT_WRITE,
                                 MAP_PRIVATE | MAP_FIXED, fd, alignedOffset));
  if (map == MAP_FAILED) {
    UnmapInternal(region, mappedLength);
    return nullptr;
  }
#  endif

#  ifdef DEBUG
  // Zero out data before and after the desired mapping to catch errors early.
  if (offset != alignedOffset) {
    memset(map, 0, offset - alignedOffset);
  }
  if (alignedLength % pageSize) {
    memset(map + alignedLength, 0, pageSize - (alignedLength % pageSize));
  }
#  endif

  return map + (offset - alignedOffset);
#endif  // __wasi__
}

void DeallocateMappedContent(void* region, size_t length) {
#ifdef __wasi__
  MOZ_CRASH("Not yet supported for WASI");
#else
  if (!region) {
    return;
  }

  // Due to bug 1502562, the following assertion does not currently hold.
  // MOZ_RELEASE_ASSERT(length > 0);

  // Calculate the address originally returned by the system call.
  // This is needed because AllocateMappedContent returns a pointer
  // that might be offset from the mapping, as the beginning of a
  // mapping must be aligned with the allocation granularity.
  uintptr_t map = uintptr_t(region) - (uintptr_t(region) % allocGranularity);
#  ifdef XP_WIN
  MOZ_RELEASE_ASSERT(UnmapViewOfFile(reinterpret_cast<void*>(map)) != 0);
#  else
  size_t alignedLength = length + (uintptr_t(region) % allocGranularity);
  if (munmap(reinterpret_cast<void*>(map), alignedLength)) {
    MOZ_RELEASE_ASSERT(errno == ENOMEM);
  }
#  endif
#endif  // __wasi__
}

static inline void ProtectMemory(void* region, size_t length, PageAccess prot) {
  MOZ_RELEASE_ASSERT(region && OffsetFromAligned(region, pageSize) == 0);
  MOZ_RELEASE_ASSERT(length > 0 && length % pageSize == 0);
#ifdef XP_WIN
  DWORD oldProtect;
  MOZ_RELEASE_ASSERT(VirtualProtect(region, length, DWORD(prot), &oldProtect) !=
                     0);
#elif defined(__wasi__)
  /* nothing */
#else
  MOZ_RELEASE_ASSERT(mprotect(region, length, int(prot)) == 0);
#endif
}

void ProtectPages(void* region, size_t length) {
  ProtectMemory(region, length, PageAccess::None);
}

void MakePagesReadOnly(void* region, size_t length) {
  ProtectMemory(region, length, PageAccess::Read);
}

void UnprotectPages(void* region, size_t length) {
  ProtectMemory(region, length, PageAccess::ReadWrite);
}

}  // namespace js::gc