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      1 /*
      2  * Copyright (C) 2014 Google 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 are
      6  * met:
      7  *
      8  *     * Redistributions of source code must retain the above copyright
      9  * notice, this list of conditions and the following disclaimer.
     10  *     * Redistributions in binary form must reproduce the above
     11  * copyright notice, this list of conditions and the following disclaimer
     12  * in the documentation and/or other materials provided with the
     13  * distribution.
     14  *     * Neither the name of Google Inc. nor the names of its
     15  * contributors may be used to endorse or promote products derived from
     16  * this software without specific prior written permission.
     17  *
     18  * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
     19  * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
     20  * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
     21  * A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
     22  * OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
     23  * SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
     24  * LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
     25  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
     26  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
     27  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
     28  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     29  */
     30 
     31 #ifndef PurgeableVector_h
     32 #define PurgeableVector_h
     33 
     34 #include "platform/PlatformExport.h"
     35 #include "wtf/Forward.h"
     36 #include "wtf/Noncopyable.h"
     37 #include "wtf/Vector.h"
     38 
     39 namespace blink {
     40 
     41 class WebDiscardableMemory;
     42 
     43 // A simple vector implementation that supports purgeable memory. The vector is
     44 // already locked at construction and locking uses an internal counter which
     45 // means that N calls to lock() must be followed by N+1 calls to unlock() to
     46 // actually make the vector purgeable.
     47 class PLATFORM_EXPORT PurgeableVector {
     48     WTF_MAKE_NONCOPYABLE(PurgeableVector);
     49 public:
     50     enum PurgeableOption {
     51         NotPurgeable,
     52         Purgeable,
     53     };
     54 
     55     // Clients who know in advance that they will call unlock() should construct
     56     // the instance with the Purgeable option so that the instance uses
     57     // discardable memory from the start and unlock() doesn't cause a memcpy().
     58     PurgeableVector(PurgeableOption = Purgeable);
     59 
     60     ~PurgeableVector();
     61 
     62     // WARNING: This causes a memcpy() if the instance was constructed with the
     63     // Purgeable hint or had its internal vector moved to discardable memory
     64     // after a call to unlock().
     65     void adopt(Vector<char>& other);
     66 
     67     void append(const char* data, size_t length);
     68 
     69     void grow(size_t);
     70 
     71     void clear();
     72 
     73     // The instance must be locked before calling this.
     74     char* data();
     75 
     76     size_t size() const;
     77 
     78     // Returns whether the memory is still resident.
     79     bool lock();
     80 
     81     // WARNING: Calling unlock() on an instance that wasn't created with the
     82     // Purgeable option does an extra memcpy().
     83     void unlock();
     84 
     85     bool isLocked() const;
     86 
     87     // Note that this method should be used carefully since it may not use
     88     // exponential growth internally. This means that repeated/invalid uses of
     89     // it can result in O(N^2) append(). If you don't exactly know what you are
     90     // doing then you should probably not call this method.
     91     void reserveCapacity(size_t capacity);
     92 
     93 private:
     94     enum PurgeableAllocationStrategy {
     95         UseExactCapacity,
     96         UseExponentialGrowth,
     97     };
     98 
     99     // Copies data from the discardable buffer to the vector and clears the
    100     // discardable buffer.
    101     void moveDataFromDiscardableToVector();
    102 
    103     void clearDiscardable();
    104 
    105     bool reservePurgeableCapacity(size_t capacity, PurgeableAllocationStrategy);
    106 
    107     size_t adjustPurgeableCapacity(size_t capacity) const;
    108 
    109     // Vector used when the instance is constructed without the purgeability
    110     // hint or when discardable memory allocation fails.
    111     // Note that there can't be data both in |m_vector| and
    112     // |m_discardable|, i.e. only one of them is used at a given time.
    113     Vector<char> m_vector;
    114     OwnPtr<WebDiscardableMemory> m_discardable;
    115     size_t m_discardableCapacity;
    116     size_t m_discardableSize;
    117     bool m_isPurgeable;
    118     int m_locksCount;
    119 };
    120 
    121 } // namespace blink
    122 
    123 #endif // PurgeableVector_h
    124