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      1 /*
      2  * Copyright (C) 2008 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. ``AS IS'' AND ANY
     14  * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
     15  * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
     16  * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
     17  * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
     18  * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
     19  * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
     20  * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
     21  * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
     22  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
     23  * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
     24  */
     25 
     26 #include "config.h"
     27 #include "SmallStrings.h"
     28 
     29 #include "JSGlobalObject.h"
     30 #include "JSString.h"
     31 
     32 #include <wtf/Noncopyable.h>
     33 
     34 namespace JSC {
     35 static const unsigned numCharactersToStore = 0x100;
     36 
     37 static inline bool isMarked(JSString* string)
     38 {
     39     return string && Heap::isCellMarked(string);
     40 }
     41 
     42 class SmallStringsStorage : public Noncopyable {
     43 public:
     44     SmallStringsStorage();
     45 
     46     UString::Rep* rep(unsigned char character) { return &m_reps[character]; }
     47 
     48 private:
     49     UString::Rep m_reps[numCharactersToStore];
     50 };
     51 
     52 SmallStringsStorage::SmallStringsStorage()
     53 {
     54     UChar* characterBuffer = 0;
     55     RefPtr<UStringImpl> baseString = UStringImpl::createUninitialized(numCharactersToStore, characterBuffer);
     56     for (unsigned i = 0; i < numCharactersToStore; ++i) {
     57         characterBuffer[i] = i;
     58         new (&m_reps[i]) UString::Rep(&characterBuffer[i], 1, PassRefPtr<UStringImpl>(baseString));
     59     }
     60 }
     61 
     62 SmallStrings::SmallStrings()
     63 {
     64     COMPILE_ASSERT(numCharactersToStore == sizeof(m_singleCharacterStrings) / sizeof(m_singleCharacterStrings[0]), IsNumCharactersConstInSyncWithClassUsage);
     65     clear();
     66 }
     67 
     68 SmallStrings::~SmallStrings()
     69 {
     70 }
     71 
     72 void SmallStrings::markChildren(MarkStack& markStack)
     73 {
     74     /*
     75        Our hypothesis is that small strings are very common. So, we cache them
     76        to avoid GC churn. However, in cases where this hypothesis turns out to
     77        be false -- including the degenerate case where all JavaScript execution
     78        has terminated -- we don't want to waste memory.
     79 
     80        To test our hypothesis, we check if any small string has been marked. If
     81        so, it's probably reasonable to mark the rest. If not, we clear the cache.
     82      */
     83 
     84     bool isAnyStringMarked = isMarked(m_emptyString);
     85     for (unsigned i = 0; i < numCharactersToStore && !isAnyStringMarked; ++i)
     86         isAnyStringMarked |= isMarked(m_singleCharacterStrings[i]);
     87 
     88     if (!isAnyStringMarked) {
     89         clear();
     90         return;
     91     }
     92 
     93     if (m_emptyString)
     94         markStack.append(m_emptyString);
     95     for (unsigned i = 0; i < numCharactersToStore; ++i) {
     96         if (m_singleCharacterStrings[i])
     97             markStack.append(m_singleCharacterStrings[i]);
     98     }
     99 }
    100 
    101 void SmallStrings::clear()
    102 {
    103     m_emptyString = 0;
    104     for (unsigned i = 0; i < numCharactersToStore; ++i)
    105         m_singleCharacterStrings[i] = 0;
    106 }
    107 
    108 unsigned SmallStrings::count() const
    109 {
    110     unsigned count = 0;
    111     if (m_emptyString)
    112         ++count;
    113     for (unsigned i = 0; i < numCharactersToStore; ++i) {
    114         if (m_singleCharacterStrings[i])
    115             ++count;
    116     }
    117     return count;
    118 }
    119 
    120 void SmallStrings::createEmptyString(JSGlobalData* globalData)
    121 {
    122     ASSERT(!m_emptyString);
    123     m_emptyString = new (globalData) JSString(globalData, "", JSString::HasOtherOwner);
    124 }
    125 
    126 void SmallStrings::createSingleCharacterString(JSGlobalData* globalData, unsigned char character)
    127 {
    128     if (!m_storage)
    129         m_storage.set(new SmallStringsStorage);
    130     ASSERT(!m_singleCharacterStrings[character]);
    131     m_singleCharacterStrings[character] = new (globalData) JSString(globalData, m_storage->rep(character), JSString::HasOtherOwner);
    132 }
    133 
    134 UString::Rep* SmallStrings::singleCharacterStringRep(unsigned char character)
    135 {
    136     if (!m_storage)
    137         m_storage.set(new SmallStringsStorage);
    138     return m_storage->rep(character);
    139 }
    140 
    141 } // namespace JSC
    142