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      1 // Copyright 2012 the V8 project authors. All rights reserved.
      2 // Use of this source code is governed by a BSD-style license that can be
      3 // found in the LICENSE file.
      4 
      5 #ifndef V8_STUB_CACHE_H_
      6 #define V8_STUB_CACHE_H_
      7 
      8 #include "src/macro-assembler.h"
      9 
     10 namespace v8 {
     11 namespace internal {
     12 
     13 
     14 // The stub cache is used for megamorphic property accesses.
     15 // It maps (map, name, type) to property access handlers. The cache does not
     16 // need explicit invalidation when a prototype chain is modified, since the
     17 // handlers verify the chain.
     18 
     19 
     20 class SCTableReference {
     21  public:
     22   Address address() const { return address_; }
     23 
     24  private:
     25   explicit SCTableReference(Address address) : address_(address) {}
     26 
     27   Address address_;
     28 
     29   friend class StubCache;
     30 };
     31 
     32 
     33 class StubCache {
     34  public:
     35   struct Entry {
     36     Name* key;
     37     Code* value;
     38     Map* map;
     39   };
     40 
     41   void Initialize();
     42   // Access cache for entry hash(name, map).
     43   Code* Set(Name* name, Map* map, Code* code);
     44   Code* Get(Name* name, Map* map, Code::Flags flags);
     45   // Clear the lookup table (@ mark compact collection).
     46   void Clear();
     47   // Collect all maps that match the name and flags.
     48   void CollectMatchingMaps(SmallMapList* types, Handle<Name> name,
     49                            Code::Flags flags, Handle<Context> native_context,
     50                            Zone* zone);
     51   // Generate code for probing the stub cache table.
     52   // Arguments extra, extra2 and extra3 may be used to pass additional scratch
     53   // registers. Set to no_reg if not needed.
     54   // If leave_frame is true, then exit a frame before the tail call.
     55   void GenerateProbe(MacroAssembler* masm, Code::Flags flags, bool leave_frame,
     56                      Register receiver, Register name, Register scratch,
     57                      Register extra, Register extra2 = no_reg,
     58                      Register extra3 = no_reg);
     59 
     60   enum Table { kPrimary, kSecondary };
     61 
     62   SCTableReference key_reference(StubCache::Table table) {
     63     return SCTableReference(
     64         reinterpret_cast<Address>(&first_entry(table)->key));
     65   }
     66 
     67   SCTableReference map_reference(StubCache::Table table) {
     68     return SCTableReference(
     69         reinterpret_cast<Address>(&first_entry(table)->map));
     70   }
     71 
     72   SCTableReference value_reference(StubCache::Table table) {
     73     return SCTableReference(
     74         reinterpret_cast<Address>(&first_entry(table)->value));
     75   }
     76 
     77   StubCache::Entry* first_entry(StubCache::Table table) {
     78     switch (table) {
     79       case StubCache::kPrimary:
     80         return StubCache::primary_;
     81       case StubCache::kSecondary:
     82         return StubCache::secondary_;
     83     }
     84     UNREACHABLE();
     85     return NULL;
     86   }
     87 
     88   Isolate* isolate() { return isolate_; }
     89 
     90   // Setting the entry size such that the index is shifted by Name::kHashShift
     91   // is convenient; shifting down the length field (to extract the hash code)
     92   // automatically discards the hash bit field.
     93   static const int kCacheIndexShift = Name::kHashShift;
     94 
     95  private:
     96   explicit StubCache(Isolate* isolate);
     97 
     98   // The stub cache has a primary and secondary level.  The two levels have
     99   // different hashing algorithms in order to avoid simultaneous collisions
    100   // in both caches.  Unlike a probing strategy (quadratic or otherwise) the
    101   // update strategy on updates is fairly clear and simple:  Any existing entry
    102   // in the primary cache is moved to the secondary cache, and secondary cache
    103   // entries are overwritten.
    104 
    105   // Hash algorithm for the primary table.  This algorithm is replicated in
    106   // assembler for every architecture.  Returns an index into the table that
    107   // is scaled by 1 << kCacheIndexShift.
    108   static int PrimaryOffset(Name* name, Code::Flags flags, Map* map) {
    109     STATIC_ASSERT(kCacheIndexShift == Name::kHashShift);
    110     // Compute the hash of the name (use entire hash field).
    111     DCHECK(name->HasHashCode());
    112     uint32_t field = name->hash_field();
    113     // Using only the low bits in 64-bit mode is unlikely to increase the
    114     // risk of collision even if the heap is spread over an area larger than
    115     // 4Gb (and not at all if it isn't).
    116     uint32_t map_low32bits =
    117         static_cast<uint32_t>(reinterpret_cast<uintptr_t>(map));
    118     // We always set the in_loop bit to zero when generating the lookup code
    119     // so do it here too so the hash codes match.
    120     uint32_t iflags =
    121         (static_cast<uint32_t>(flags) & ~Code::kFlagsNotUsedInLookup);
    122     // Base the offset on a simple combination of name, flags, and map.
    123     uint32_t key = (map_low32bits + field) ^ iflags;
    124     return key & ((kPrimaryTableSize - 1) << kCacheIndexShift);
    125   }
    126 
    127   // Hash algorithm for the secondary table.  This algorithm is replicated in
    128   // assembler for every architecture.  Returns an index into the table that
    129   // is scaled by 1 << kCacheIndexShift.
    130   static int SecondaryOffset(Name* name, Code::Flags flags, int seed) {
    131     // Use the seed from the primary cache in the secondary cache.
    132     uint32_t name_low32bits =
    133         static_cast<uint32_t>(reinterpret_cast<uintptr_t>(name));
    134     // We always set the in_loop bit to zero when generating the lookup code
    135     // so do it here too so the hash codes match.
    136     uint32_t iflags =
    137         (static_cast<uint32_t>(flags) & ~Code::kFlagsNotUsedInLookup);
    138     uint32_t key = (seed - name_low32bits) + iflags;
    139     return key & ((kSecondaryTableSize - 1) << kCacheIndexShift);
    140   }
    141 
    142   // Compute the entry for a given offset in exactly the same way as
    143   // we do in generated code.  We generate an hash code that already
    144   // ends in Name::kHashShift 0s.  Then we multiply it so it is a multiple
    145   // of sizeof(Entry).  This makes it easier to avoid making mistakes
    146   // in the hashed offset computations.
    147   static Entry* entry(Entry* table, int offset) {
    148     const int multiplier = sizeof(*table) >> Name::kHashShift;
    149     return reinterpret_cast<Entry*>(reinterpret_cast<Address>(table) +
    150                                     offset * multiplier);
    151   }
    152 
    153   static const int kPrimaryTableBits = 11;
    154   static const int kPrimaryTableSize = (1 << kPrimaryTableBits);
    155   static const int kSecondaryTableBits = 9;
    156   static const int kSecondaryTableSize = (1 << kSecondaryTableBits);
    157 
    158  private:
    159   Entry primary_[kPrimaryTableSize];
    160   Entry secondary_[kSecondaryTableSize];
    161   Isolate* isolate_;
    162 
    163   friend class Isolate;
    164   friend class SCTableReference;
    165 
    166   DISALLOW_COPY_AND_ASSIGN(StubCache);
    167 };
    168 }
    169 }  // namespace v8::internal
    170 
    171 #endif  // V8_STUB_CACHE_H_
    172