js/src/jsweakcache.h

   1 /* -*- Mode: C++; tab-width: 8; indent-tabs-mode: nil; c-basic-offset: 4 -*-
   2  * vim: set ts=8 sts=4 et sw=4 tw=99:
   3  * This Source Code Form is subject to the terms of the Mozilla Public
   4  * License, v. 2.0. If a copy of the MPL was not distributed with this
   5  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
   6
   7 #ifndef jsweakcache_h
   8 #define jsweakcache_h
   9
  10 #include "jscntxt.h"
  11 #include "gc/Marking.h"
  12 #include "js/HashTable.h"
  13 #include "vm/Runtime.h"
  14
  15 namespace js {
  16
  17 // A WeakCache is used to map a key to a value similar to an HashMap except
  18 // that its entries are garbage collected.  An entry is kept as long as
  19 // both the key and value are marked.
  20 //
  21 // No mark function is provided with this weak container.  However, this weak
  22 // container should take part in the sweep phase.
  23 template <class Key, class Value,
  24           class HashPolicy = DefaultHasher<Key>,
  25           class AllocPolicy = RuntimeAllocPolicy>
  26 class WeakCache : public HashMap<Key, Value, HashPolicy, AllocPolicy> {
  27   private:
  28     typedef HashMap<Key, Value, HashPolicy, AllocPolicy> Base;
  29     typedef typename Base::Range Range;
  30     typedef typename Base::Enum Enum;
  31
  32   public:
  33     explicit WeakCache(JSRuntime* rt) : Base(rt) { }
  34     explicit WeakCache(JSContext* cx) : Base(cx->runtime()) { }
  35
  36   public:
  37     // Sweep all entries which have unmarked key or value.
  38     void sweep(FreeOp* fop) {
  39         // Remove all entries whose keys/values remain unmarked.
  40         for (Enum e(*this); !e.empty(); e.popFront()) {
  41             // Checking IsMarked() may update the location of the Key (or Value).
  42             // Pass in a stack local, then manually update the backing heap store.
  43             Key k(e.front().key);
  44             bool isKeyDying = gc::IsAboutToBeFinalized(&k);
  45
  46             if (isKeyDying || gc::IsAboutToBeFinalized(e.front().value)) {
  47                 e.removeFront();
  48             } else {
  49                 // Potentially update the location of the Key.
  50                 // The Value had its heap addresses correctly passed to IsMarked(),
  51                 // and therefore has already been updated if necessary.
  52                 // e.rekeyFront(k);
  53             }
  54         }
  55
  56 #if DEBUG
  57         // Once we've swept, all remaining edges should stay within the
  58         // known-live part of the graph.
  59         for (Range r = Base::all(); !r.empty(); r.popFront()) {
  60             Key k(r.front().key);
  61
  62             MOZ_ASSERT(!gc::IsAboutToBeFinalized(&k));
  63             MOZ_ASSERT(!gc::IsAboutToBeFinalized(r.front().value));
  64
  65             // Assert that IsMarked() did not perform relocation.
  66             MOZ_ASSERT(k == r.front().key);
  67         }
  68 #endif
  69     }
  70 };
  71
  72 // A WeakValueCache is similar to a WeakCache, except keys are never marked.
  73 // This is useful for weak maps where the keys are primitive values such as uint32_t.
  74 template <class Key, class Value,
  75           class HashPolicy = DefaultHasher<Key>,
  76           class AllocPolicy = RuntimeAllocPolicy>
  77 class WeakValueCache : public HashMap<Key, Value, HashPolicy, AllocPolicy>
  78 {
  79   public:
  80     typedef HashMap<Key, Value, HashPolicy, AllocPolicy> Base;
  81     typedef typename Base::Range Range;
  82     typedef typename Base::Enum Enum;
  83
  84     explicit WeakValueCache(JSRuntime* rt) : Base(rt) { }
  85     explicit WeakValueCache(JSContext* cx) : Base(cx->runtime()) { }
  86
  87   public:
  88     // Sweep all entries which have unmarked key or value.
  89     void sweep(FreeOp* fop) {
  90         // Remove all entries whose values remain unmarked.
  91         for (Enum e(*this); !e.empty(); e.popFront()) {
  92             if (gc::IsAboutToBeFinalized(e.front().value()))
  93                 e.removeFront();
  94         }
  95
  96 #if DEBUG
  97         // Once we've swept, all remaining edges should stay within the
  98         // known-live part of the graph.
  99         for (Range r = Base::all(); !r.empty(); r.popFront())
 100             MOZ_ASSERT(!gc::IsAboutToBeFinalized(r.front().value()));
 101 #endif
 102     }
 103 };
 104
 105 } // namespace js
 106
 107 #endif /* jsweakcache_h */