runtime/SmallStrings.cpp

   1 /*
   2  * Copyright (C) 2008, 2010 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 #include <wtf/Noncopyable.h>
  32 #include <wtf/PassOwnPtr.h>
  33
  34 namespace JSC {
  35
  36 static inline bool isMarked(JSCell* string)
  37 {
  38     return string && Heap::isMarked(string);
  39 }
  40
  41 class SmallStringsStorage {
  42     WTF_MAKE_NONCOPYABLE(SmallStringsStorage); WTF_MAKE_FAST_ALLOCATED;
  43 public:
  44     SmallStringsStorage();
  45
  46     StringImpl* rep(unsigned char character)
  47     {
  48         return m_reps[character].get();
  49     }
  50
  51 private:
  52     static const unsigned singleCharacterStringCount = maxSingleCharacterString + 1;
  53
  54     RefPtr<StringImpl> m_reps[singleCharacterStringCount];
  55 };
  56
  57 SmallStringsStorage::SmallStringsStorage()
  58 {
  59     UChar* characterBuffer = 0;
  60     RefPtr<StringImpl> baseString = StringImpl::createUninitialized(singleCharacterStringCount, characterBuffer);
  61     for (unsigned i = 0; i < singleCharacterStringCount; ++i) {
  62         characterBuffer[i] = i;
  63         m_reps[i] = StringImpl::create(baseString, i, 1);
  64     }
  65 }
  66
  67 SmallStrings::SmallStrings()
  68 {
  69     COMPILE_ASSERT(singleCharacterStringCount == sizeof(m_singleCharacterStrings) / sizeof(m_singleCharacterStrings[0]), IsNumCharactersConstInSyncWithClassUsage);
  70     clear();
  71 }
  72
  73 SmallStrings::~SmallStrings()
  74 {
  75 }
  76
  77 void SmallStrings::visitChildren(HeapRootVisitor& heapRootMarker)
  78 {
  79     /*
  80        Our hypothesis is that small strings are very common. So, we cache them
  81        to avoid GC churn. However, in cases where this hypothesis turns out to
  82        be false -- including the degenerate case where all JavaScript execution
  83        has terminated -- we don't want to waste memory.
  84
  85        To test our hypothesis, we check if any small string has been marked. If
  86        so, it's probably reasonable to mark the rest. If not, we clear the cache.
  87      */
  88
  89     bool isAnyStringMarked = isMarked(m_emptyString);
  90     for (unsigned i = 0; i < singleCharacterStringCount && !isAnyStringMarked; ++i)
  91         isAnyStringMarked = isMarked(m_singleCharacterStrings[i]);
  92
  93     if (!isAnyStringMarked) {
  94         clear();
  95         return;
  96     }
  97
  98     if (m_emptyString)
  99         heapRootMarker.mark(&m_emptyString);
 100     for (unsigned i = 0; i < singleCharacterStringCount; ++i) {
 101         if (m_singleCharacterStrings[i])
 102             heapRootMarker.mark(&m_singleCharacterStrings[i]);
 103     }
 104 }
 105
 106 void SmallStrings::clear()
 107 {
 108     m_emptyString = 0;
 109     for (unsigned i = 0; i < singleCharacterStringCount; ++i)
 110         m_singleCharacterStrings[i] = 0;
 111 }
 112
 113 unsigned SmallStrings::count() const
 114 {
 115     unsigned count = 0;
 116     if (m_emptyString)
 117         ++count;
 118     for (unsigned i = 0; i < singleCharacterStringCount; ++i) {
 119         if (m_singleCharacterStrings[i])
 120             ++count;
 121     }
 122     return count;
 123 }
 124
 125 void SmallStrings::createEmptyString(JSGlobalData* globalData)
 126 {
 127     ASSERT(!m_emptyString);
 128     m_emptyString = new (globalData) JSString(globalData, "", JSString::HasOtherOwner);
 129 }
 130
 131 void SmallStrings::createSingleCharacterString(JSGlobalData* globalData, unsigned char character)
 132 {
 133     if (!m_storage)
 134         m_storage = adoptPtr(new SmallStringsStorage);
 135     ASSERT(!m_singleCharacterStrings[character]);
 136     m_singleCharacterStrings[character] = new (globalData) JSString(globalData, PassRefPtr<StringImpl>(m_storage->rep(character)), JSString::HasOtherOwner);
 137 }
 138
 139 StringImpl* SmallStrings::singleCharacterStringRep(unsigned char character)
 140 {
 141     if (!m_storage)
 142         m_storage = adoptPtr(new SmallStringsStorage);
 143     return m_storage->rep(character);
 144 }
 145
 146 } // namespace JSC