summaryrefslogtreecommitdiffstats
path: root/js/src/vm/Caches.h
blob: dcd0c788227cd7a21ecbba4ab7e6996d9ce7306a (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
/* -*- 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/. */

#ifndef vm_Caches_h
#define vm_Caches_h

#include "mozilla/Array.h"
#include "mozilla/MathAlgorithms.h"
#include "mozilla/Maybe.h"
#include "mozilla/MruCache.h"
#include "mozilla/TemplateLib.h"
#include "mozilla/UniquePtr.h"

#include "frontend/ScopeBindingCache.h"
#include "gc/Tracer.h"
#include "js/RootingAPI.h"
#include "js/TypeDecls.h"
#include "vm/JSScript.h"
#include "vm/Shape.h"
#include "vm/StencilCache.h"  // js::DelazificationCache
#include "vm/StringType.h"

namespace js {

struct EvalCacheEntry {
  JSLinearString* str;
  JSScript* script;
  JSScript* callerScript;
  jsbytecode* pc;

  // We sweep this cache after a nursery collection to update entries with
  // string keys that have been tenured.
  //
  // The entire cache is purged on a major GC, so we don't need to sweep it
  // then.
  bool traceWeak(JSTracer* trc) {
    MOZ_ASSERT(trc->kind() == JS::TracerKind::MinorSweeping);
    return TraceManuallyBarrieredWeakEdge(trc, &str, "EvalCacheEntry::str");
  }
};

struct EvalCacheLookup {
  explicit EvalCacheLookup(JSContext* cx) : str(cx), callerScript(cx) {}
  Rooted<JSLinearString*> str;
  RootedScript callerScript;
  MOZ_INIT_OUTSIDE_CTOR jsbytecode* pc;
};

struct EvalCacheHashPolicy {
  using Lookup = EvalCacheLookup;

  static HashNumber hash(const Lookup& l);
  static bool match(const EvalCacheEntry& entry, const EvalCacheLookup& l);
};

using EvalCache =
    GCHashSet<EvalCacheEntry, EvalCacheHashPolicy, SystemAllocPolicy>;

class MegamorphicCacheEntry {
  // Receiver object's shape.
  Shape* shape_ = nullptr;

  // The atom or symbol property being accessed.
  PropertyKey key_;

  // Slot offset and isFixedSlot flag of the data property.
  TaggedSlotOffset slotOffset_;

  // This entry is valid iff the generation matches the cache's generation.
  uint16_t generation_ = 0;

  // Number of hops on the proto chain to get to the holder object. If this is
  // zero, the property exists on the receiver object. It can also be one of
  // the sentinel values indicating a missing property lookup.
  uint8_t numHops_ = 0;

  friend class MegamorphicCache;

 public:
  static constexpr uint8_t MaxHopsForDataProperty = UINT8_MAX - 2;
  static constexpr uint8_t NumHopsForMissingProperty = UINT8_MAX - 1;
  static constexpr uint8_t NumHopsForMissingOwnProperty = UINT8_MAX;

  void init(Shape* shape, PropertyKey key, uint16_t generation, uint8_t numHops,
            TaggedSlotOffset slotOffset) {
    shape_ = shape;
    key_ = key;
    slotOffset_ = slotOffset;
    generation_ = generation;
    numHops_ = numHops;
    MOZ_ASSERT(numHops_ == numHops, "numHops must fit in numHops_");
  }
  bool isMissingProperty() const {
    return numHops_ == NumHopsForMissingProperty;
  }
  bool isMissingOwnProperty() const {
    return numHops_ == NumHopsForMissingOwnProperty;
  }
  bool isDataProperty() const { return numHops_ <= MaxHopsForDataProperty; }
  uint16_t numHops() const {
    MOZ_ASSERT(isDataProperty());
    return numHops_;
  }
  TaggedSlotOffset slotOffset() const {
    MOZ_ASSERT(isDataProperty());
    return slotOffset_;
  }

  static constexpr size_t offsetOfShape() {
    return offsetof(MegamorphicCacheEntry, shape_);
  }

  static constexpr size_t offsetOfKey() {
    return offsetof(MegamorphicCacheEntry, key_);
  }

  static constexpr size_t offsetOfGeneration() {
    return offsetof(MegamorphicCacheEntry, generation_);
  }

  static constexpr size_t offsetOfSlotOffset() {
    return offsetof(MegamorphicCacheEntry, slotOffset_);
  }

  static constexpr size_t offsetOfNumHops() {
    return offsetof(MegamorphicCacheEntry, numHops_);
  }
};

// [SMDOC] Megamorphic Property Lookup Cache (MegamorphicCache)
//
// MegamorphicCache is a data structure used to speed up megamorphic property
// lookups from JIT code. The same cache is currently used for both GetProp and
// HasProp (in, hasOwnProperty) operations.
//
// This is implemented as a fixed-size array of entries. Lookups are performed
// based on the receiver object's Shape + PropertyKey. If found in the cache,
// the result of a lookup represents either:
//
// * A data property on the receiver or on its proto chain (stored as number of
//   'hops' up the proto chain + the slot of the data property).
//
// * A missing property on the receiver or its proto chain.
//
// * A missing property on the receiver, but it might exist on the proto chain.
//   This lets us optimize hasOwnProperty better.
//
// Collisions are handled by simply overwriting the previous entry stored in the
// slot. This is sufficient to achieve a high hit rate on typical web workloads
// while ensuring cache lookups are always fast and simple.
//
// Lookups always check the receiver object's shape (ensuring the properties and
// prototype are unchanged). Because the cache also caches lookups on the proto
// chain, Watchtower is used to invalidate the cache when prototype objects are
// mutated. This is done by incrementing the cache's generation counter to
// invalidate all entries.
//
// The cache is also invalidated on each major GC.
class MegamorphicCache {
 public:
  using Entry = MegamorphicCacheEntry;

  static constexpr size_t NumEntries = 1024;
  static constexpr uint8_t ShapeHashShift1 =
      mozilla::tl::FloorLog2<alignof(Shape)>::value;
  static constexpr uint8_t ShapeHashShift2 =
      ShapeHashShift1 + mozilla::tl::FloorLog2<NumEntries>::value;

  static_assert(mozilla::IsPowerOfTwo(alignof(Shape)) &&
                    mozilla::IsPowerOfTwo(NumEntries),
                "FloorLog2 is exact because alignof(Shape) and NumEntries are "
                "both powers of two");

 private:
  mozilla::Array<Entry, NumEntries> entries_;

  // Generation counter used to invalidate all entries.
  uint16_t generation_ = 0;

  // NOTE: this logic is mirrored in MacroAssembler::emitMegamorphicCacheLookup
  Entry& getEntry(Shape* shape, PropertyKey key) {
    static_assert(mozilla::IsPowerOfTwo(NumEntries),
                  "NumEntries must be a power-of-two for fast modulo");
    uintptr_t hash = uintptr_t(shape) >> ShapeHashShift1;
    hash ^= uintptr_t(shape) >> ShapeHashShift2;
    hash += HashAtomOrSymbolPropertyKey(key);
    return entries_[hash % NumEntries];
  }

 public:
  void bumpGeneration() {
    generation_++;
    if (generation_ == 0) {
      // Generation overflowed. Invalidate the whole cache.
      for (size_t i = 0; i < NumEntries; i++) {
        entries_[i].shape_ = nullptr;
      }
    }
  }
  bool lookup(Shape* shape, PropertyKey key, Entry** entryp) {
    Entry& entry = getEntry(shape, key);
    *entryp = &entry;
    return (entry.shape_ == shape && entry.key_ == key &&
            entry.generation_ == generation_);
  }
  void initEntryForMissingProperty(Entry* entry, Shape* shape,
                                   PropertyKey key) {
    entry->init(shape, key, generation_, Entry::NumHopsForMissingProperty,
                TaggedSlotOffset());
  }
  void initEntryForMissingOwnProperty(Entry* entry, Shape* shape,
                                      PropertyKey key) {
    entry->init(shape, key, generation_, Entry::NumHopsForMissingOwnProperty,
                TaggedSlotOffset());
  }
  void initEntryForDataProperty(Entry* entry, Shape* shape, PropertyKey key,
                                size_t numHops, TaggedSlotOffset slotOffset) {
    if (numHops > Entry::MaxHopsForDataProperty) {
      return;
    }
    entry->init(shape, key, generation_, numHops, slotOffset);
  }

  static constexpr size_t offsetOfEntries() {
    return offsetof(MegamorphicCache, entries_);
  }

  static constexpr size_t offsetOfGeneration() {
    return offsetof(MegamorphicCache, generation_);
  }
};

class MegamorphicSetPropCacheEntry {
  Shape* beforeShape_ = nullptr;
  Shape* afterShape_ = nullptr;

  // The atom or symbol property being accessed.
  PropertyKey key_;

  // Slot offset and isFixedSlot flag of the data property.
  TaggedSlotOffset slotOffset_;

  // If slots need to be grown, this is the new capacity we need.
  uint16_t newCapacity_ = 0;

  // This entry is valid iff the generation matches the cache's generation.
  uint16_t generation_ = 0;

  friend class MegamorphicSetPropCache;

 public:
  void init(Shape* beforeShape, Shape* afterShape, PropertyKey key,
            uint16_t generation, TaggedSlotOffset slotOffset,
            uint16_t newCapacity) {
    beforeShape_ = beforeShape;
    afterShape_ = afterShape;
    key_ = key;
    slotOffset_ = slotOffset;
    newCapacity_ = newCapacity;
    generation_ = generation;
  }
  TaggedSlotOffset slotOffset() const { return slotOffset_; }
  Shape* afterShape() const { return afterShape_; }

  static constexpr size_t offsetOfShape() {
    return offsetof(MegamorphicSetPropCacheEntry, beforeShape_);
  }
  static constexpr size_t offsetOfAfterShape() {
    return offsetof(MegamorphicSetPropCacheEntry, afterShape_);
  }

  static constexpr size_t offsetOfKey() {
    return offsetof(MegamorphicSetPropCacheEntry, key_);
  }

  static constexpr size_t offsetOfNewCapacity() {
    return offsetof(MegamorphicSetPropCacheEntry, newCapacity_);
  }

  static constexpr size_t offsetOfGeneration() {
    return offsetof(MegamorphicSetPropCacheEntry, generation_);
  }

  static constexpr size_t offsetOfSlotOffset() {
    return offsetof(MegamorphicSetPropCacheEntry, slotOffset_);
  }
};

class MegamorphicSetPropCache {
 public:
  using Entry = MegamorphicSetPropCacheEntry;
  // We can get more hits if we increase this, but this seems to be around
  // the sweet spot where we are getting most of the hits we would get with
  // an infinitely sized cache
  static constexpr size_t NumEntries = 1024;
  static constexpr uint8_t ShapeHashShift1 =
      mozilla::tl::FloorLog2<alignof(Shape)>::value;
  static constexpr uint8_t ShapeHashShift2 =
      ShapeHashShift1 + mozilla::tl::FloorLog2<NumEntries>::value;

  static_assert(mozilla::IsPowerOfTwo(alignof(Shape)) &&
                    mozilla::IsPowerOfTwo(NumEntries),
                "FloorLog2 is exact because alignof(Shape) and NumEntries are "
                "both powers of two");

 private:
  mozilla::Array<Entry, NumEntries> entries_;

  // Generation counter used to invalidate all entries.
  uint16_t generation_ = 0;

  Entry& getEntry(Shape* beforeShape, PropertyKey key) {
    static_assert(mozilla::IsPowerOfTwo(NumEntries),
                  "NumEntries must be a power-of-two for fast modulo");
    uintptr_t hash = uintptr_t(beforeShape) >> ShapeHashShift1;
    hash ^= uintptr_t(beforeShape) >> ShapeHashShift2;
    hash += HashAtomOrSymbolPropertyKey(key);
    return entries_[hash % NumEntries];
  }

 public:
  void bumpGeneration() {
    generation_++;
    if (generation_ == 0) {
      // Generation overflowed. Invalidate the whole cache.
      for (size_t i = 0; i < NumEntries; i++) {
        entries_[i].beforeShape_ = nullptr;
      }
    }
  }
  void set(Shape* beforeShape, Shape* afterShape, PropertyKey key,
           TaggedSlotOffset slotOffset, uint32_t newCapacity) {
    uint16_t newSlots = (uint16_t)newCapacity;
    if (newSlots != newCapacity) {
      return;
    }
    Entry& entry = getEntry(beforeShape, key);
    entry.init(beforeShape, afterShape, key, generation_, slotOffset, newSlots);
  }

#ifdef DEBUG
  bool lookup(Shape* beforeShape, PropertyKey key, Entry** entryp) {
    Entry& entry = getEntry(beforeShape, key);
    *entryp = &entry;
    return (entry.beforeShape_ == beforeShape && entry.key_ == key &&
            entry.generation_ == generation_);
  }
#endif

  static constexpr size_t offsetOfEntries() {
    return offsetof(MegamorphicSetPropCache, entries_);
  }

  static constexpr size_t offsetOfGeneration() {
    return offsetof(MegamorphicSetPropCache, generation_);
  }
};

// Cache for AtomizeString, mapping JSString* or JS::Latin1Char* to the
// corresponding JSAtom*. The cache has three different optimizations:
//
// * The two most recent lookups are cached. This has a hit rate of 30-65% on
//   typical web workloads.
//
// * MruCache is used for short JS::Latin1Char strings.
//
// * For longer strings, there's also a JSLinearString* => JSAtom* HashMap,
//   because hashing the string characters repeatedly can be slow.
//   This map is also used by nursery GC to de-duplicate strings to atoms.
//
// This cache is purged on minor and major GC.
class StringToAtomCache {
 public:
  struct LastLookup {
    JSString* string = nullptr;
    JSAtom* atom = nullptr;

    static constexpr size_t offsetOfString() {
      return offsetof(LastLookup, string);
    }

    static constexpr size_t offsetOfAtom() {
      return offsetof(LastLookup, atom);
    }
  };
  static constexpr size_t NumLastLookups = 2;

  struct AtomTableKey {
    explicit AtomTableKey(const JS::Latin1Char* str, size_t len)
        : string_(str), length_(len) {
      hash_ = mozilla::HashString(string_, length_);
    }

    const JS::Latin1Char* string_;
    size_t length_;
    uint32_t hash_;
  };

 private:
  struct RopeAtomCache
      : public mozilla::MruCache<AtomTableKey, JSAtom*, RopeAtomCache> {
    static HashNumber Hash(const AtomTableKey& key) { return key.hash_; }
    static bool Match(const AtomTableKey& key, const JSAtom* val) {
      JS::AutoCheckCannotGC nogc;
      return val->length() == key.length_ &&
             EqualChars(key.string_, val->latin1Chars(nogc), key.length_);
    }
  };
  using Map =
      HashMap<JSString*, JSAtom*, PointerHasher<JSString*>, SystemAllocPolicy>;
  Map map_;
  mozilla::Array<LastLookup, NumLastLookups> lastLookups_;
  RopeAtomCache ropeCharCache_;

 public:
  // Don't use the HashMap for short strings. Hashing them is less expensive.
  // But the length needs to long enough to cover common identifiers in React.
  static constexpr size_t MinStringLength = 39;

  JSAtom* lookupInMap(JSString* s) const {
    MOZ_ASSERT(s->inStringToAtomCache());
    MOZ_ASSERT(s->length() >= MinStringLength);

    auto p = map_.lookup(s);
    JSAtom* atom = p ? p->value() : nullptr;
    return atom;
  }

  MOZ_ALWAYS_INLINE JSAtom* lookup(JSString* s) const {
    MOZ_ASSERT(!s->isAtom());
    for (const LastLookup& entry : lastLookups_) {
      if (entry.string == s) {
        return entry.atom;
      }
    }

    if (!s->inStringToAtomCache()) {
      MOZ_ASSERT(!map_.lookup(s));
      return nullptr;
    }

    return lookupInMap(s);
  }

  MOZ_ALWAYS_INLINE JSAtom* lookupWithRopeChars(
      const JS::Latin1Char* str, size_t len,
      mozilla::Maybe<AtomTableKey>& key) {
    MOZ_ASSERT(len < MinStringLength);
    key.emplace(str, len);
    if (auto p = ropeCharCache_.Lookup(key.value())) {
      return p.Data();
    }
    return nullptr;
  }

  static constexpr size_t offsetOfLastLookups() {
    return offsetof(StringToAtomCache, lastLookups_);
  }

  void maybePut(JSString* s, JSAtom* atom, mozilla::Maybe<AtomTableKey>& key) {
    if (key.isSome()) {
      ropeCharCache_.Put(key.value(), atom);
    }

    for (size_t i = NumLastLookups - 1; i > 0; i--) {
      lastLookups_[i] = lastLookups_[i - 1];
    }
    lastLookups_[0].string = s;
    lastLookups_[0].atom = atom;

    if (s->length() < MinStringLength) {
      return;
    }
    if (!map_.putNew(s, atom)) {
      return;
    }
    s->setInStringToAtomCache();
  }

  void purge() {
    map_.clearAndCompact();
    for (LastLookup& entry : lastLookups_) {
      entry.string = nullptr;
      entry.atom = nullptr;
    }

    ropeCharCache_.Clear();
  }
};

class RuntimeCaches {
 public:
  MegamorphicCache megamorphicCache;
  UniquePtr<MegamorphicSetPropCache> megamorphicSetPropCache;
  UncompressedSourceCache uncompressedSourceCache;
  EvalCache evalCache;
  StringToAtomCache stringToAtomCache;

  // Delazification: Cache binding for runtime objects which are used during
  // delazification to quickly resolve NameLocation of bindings without linearly
  // iterating over the list of bindings.
  frontend::RuntimeScopeBindingCache scopeCache;

  void sweepAfterMinorGC(JSTracer* trc) { evalCache.traceWeak(trc); }
#ifdef JSGC_HASH_TABLE_CHECKS
  void checkEvalCacheAfterMinorGC();
#endif

  void purgeForCompaction() {
    evalCache.clear();
    stringToAtomCache.purge();
    megamorphicCache.bumpGeneration();
    if (megamorphicSetPropCache) {
      // MegamorphicSetPropCache can be null if we failed out of
      // JSRuntime::init. We will then try to destroy the runtime which will
      // do a GC and land us here.
      megamorphicSetPropCache->bumpGeneration();
    }
    scopeCache.purge();
  }

  void purgeStencils() {
    DelazificationCache& cache = DelazificationCache::getSingleton();
    cache.clearAndDisable();
  }

  void purge() {
    purgeForCompaction();
    uncompressedSourceCache.purge();
    purgeStencils();
  }
};

}  // namespace js

#endif /* vm_Caches_h */