runtime/WeakGCMap.h

   1 /*
   2  * Copyright (C) 2009 Apple Inc. All rights reserved.
   3  *
   4  * Redistribution and use in source and binary forms, with or without
   5  * modification, are permitted provided that the following conditions
   6  * are met:
   7  * 1. Redistributions of source code must retain the above copyright
   8  *    notice, this list of conditions and the following disclaimer.
   9  * 2. Redistributions in binary form must reproduce the above copyright
  10  *    notice, this list of conditions and the following disclaimer in the
  11  *    documentation and/or other materials provided with the distribution.
  12  *
  13  * THIS SOFTWARE IS PROVIDED BY APPLE INC. AND ITS CONTRIBUTORS ``AS IS''
  14  * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO,
  15  * THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
  16  * PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL APPLE INC. OR ITS CONTRIBUTORS
  17  * BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
  18  * CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF
  19  * SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS
  20  * INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN
  21  * CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  22  * ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF
  23  * THE POSSIBILITY OF SUCH DAMAGE.
  24  */
  25
  26 #ifndef WeakGCMap_h
  27 #define WeakGCMap_h
  28
  29 #include "Collector.h"
  30 #include <wtf/HashMap.h>
  31
  32 namespace JSC {
  33
  34 class JSCell;
  35
  36 // A HashMap whose get() function returns emptyValue() for cells awaiting destruction.
  37 template<typename KeyType, typename MappedType>
  38 class WeakGCMap : public FastAllocBase {
  39     /*
  40     Invariants:
  41         * A value enters the WeakGCMap marked. (Guaranteed by set().)
  42         * A value that becomes unmarked leaves the WeakGCMap before being recycled. (Guaranteed by the value's destructor removing it from the WeakGCMap.)
  43         * A value that becomes unmarked leaves the WeakGCMap before becoming marked again. (Guaranteed by all destructors running before the mark phase begins.)
  44         * During the mark phase, all values in the WeakGCMap are valid. (Guaranteed by all destructors running before the mark phase begins.)
  45     */
  46
  47 public:
  48     typedef typename HashMap<KeyType, MappedType>::iterator iterator;
  49     typedef typename HashMap<KeyType, MappedType>::const_iterator const_iterator;
  50
  51     bool isEmpty() { return m_map.isEmpty(); }
  52
  53     MappedType get(const KeyType& key) const;
  54     pair<iterator, bool> set(const KeyType&, const MappedType&);
  55     MappedType take(const KeyType& key);
  56
  57     // These unchecked functions provide access to a value even if the value's
  58     // mark bit is not set. This is used, among other things, to retrieve values
  59     // during the GC mark phase, which begins by clearing all mark bits.
  60
  61     MappedType uncheckedGet(const KeyType& key) const { return m_map.get(key); }
  62     bool uncheckedRemove(const KeyType&, const MappedType&);
  63
  64     iterator uncheckedBegin() { return m_map.begin(); }
  65     iterator uncheckedEnd() { return m_map.end(); }
  66
  67     const_iterator uncheckedBegin() const { return m_map.begin(); }
  68     const_iterator uncheckedEnd() const { return m_map.end(); }
  69
  70 private:
  71     HashMap<KeyType, MappedType> m_map;
  72 };
  73
  74 template<typename KeyType, typename MappedType>
  75 inline MappedType WeakGCMap<KeyType, MappedType>::get(const KeyType& key) const
  76 {
  77     MappedType result = m_map.get(key);
  78     if (result == HashTraits<MappedType>::emptyValue())
  79         return result;
  80     if (!Heap::isCellMarked(result))
  81         return HashTraits<MappedType>::emptyValue();
  82     return result;
  83 }
  84
  85 template<typename KeyType, typename MappedType>
  86 MappedType WeakGCMap<KeyType, MappedType>::take(const KeyType& key)
  87 {
  88     MappedType result = m_map.take(key);
  89     if (result == HashTraits<MappedType>::emptyValue())
  90         return result;
  91     if (!Heap::isCellMarked(result))
  92         return HashTraits<MappedType>::emptyValue();
  93     return result;
  94 }
  95
  96 template<typename KeyType, typename MappedType>
  97 pair<typename HashMap<KeyType, MappedType>::iterator, bool> WeakGCMap<KeyType, MappedType>::set(const KeyType& key, const MappedType& value)
  98 {
  99     Heap::markCell(value); // If value is newly allocated, it's not marked, so mark it now.
 100     pair<iterator, bool> result = m_map.add(key, value);
 101     if (!result.second) { // pre-existing entry
 102         result.second = !Heap::isCellMarked(result.first->second);
 103         result.first->second = value;
 104     }
 105     return result;
 106 }
 107
 108 template<typename KeyType, typename MappedType>
 109 bool WeakGCMap<KeyType, MappedType>::uncheckedRemove(const KeyType& key, const MappedType& value)
 110 {
 111     iterator it = m_map.find(key);
 112     if (it == m_map.end())
 113         return false;
 114     if (it->second != value)
 115         return false;
 116     m_map.remove(it);
 117     return true;
 118 }
 119
 120 } // namespace JSC
 121
 122 #endif // WeakGCMap_h