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 const unsigned numCharactersToStore = 0x100;

    37 

    38 static inline bool isMarked(JSString* string)

    39 {

    40     return string && Heap::isCellMarked(string);

    41 }

    42 

    43 class SmallStringsStorage : public Noncopyable {

    44 public:

    45     SmallStringsStorage();

    46 

    47     UString::Rep* rep(unsigned char character) { return m_reps[character].get(); }

    48 

    49 private:

    50     RefPtr<UString::Rep> m_reps[numCharactersToStore];

    51 };

    52 

    53 SmallStringsStorage::SmallStringsStorage()

    54 {

    55     UChar* characterBuffer = 0;

    56     RefPtr<UStringImpl> baseString = UStringImpl::createUninitialized(numCharactersToStore, characterBuffer);

    57     for (unsigned i = 0; i < numCharactersToStore; ++i) {

    58         characterBuffer[i] = i;

    59         m_reps[i] = UStringImpl::create(baseString, i, 1);

    60     }

    61 }

    62 

    63 SmallStrings::SmallStrings()

    64 {

    65     COMPILE_ASSERT(numCharactersToStore == sizeof(m_singleCharacterStrings) / sizeof(m_singleCharacterStrings[0]), IsNumCharactersConstInSyncWithClassUsage);

    66     clear();

    67 }

    68 

    69 SmallStrings::~SmallStrings()

    70 {

    71 }

    72 

    73 void SmallStrings::markChildren(MarkStack& markStack)

    74 {

    75     /*

    76        Our hypothesis is that small strings are very common. So, we cache them

    77        to avoid GC churn. However, in cases where this hypothesis turns out to

    78        be false -- including the degenerate case where all JavaScript execution

    79        has terminated -- we don't want to waste memory.

    80 

    81        To test our hypothesis, we check if any small string has been marked. If

    82        so, it's probably reasonable to mark the rest. If not, we clear the cache.

    83      */

    84 

    85     bool isAnyStringMarked = isMarked(m_emptyString);

    86     for (unsigned i = 0; i < numCharactersToStore && !isAnyStringMarked; ++i)

    87         isAnyStringMarked = isMarked(m_singleCharacterStrings[i]);

    88     

    89     if (!isAnyStringMarked) {

    90         clear();

    91         return;

    92     }

    93     

    94     if (m_emptyString)

    95         markStack.append(m_emptyString);

    96     for (unsigned i = 0; i < numCharactersToStore; ++i) {

    97         if (m_singleCharacterStrings[i])

    98             markStack.append(m_singleCharacterStrings[i]);

    99     }

   100 }

   101 

   102 void SmallStrings::clear()

   103 {

   104     m_emptyString = 0;

   105     for (unsigned i = 0; i < numCharactersToStore; ++i)

   106         m_singleCharacterStrings[i] = 0;

   107 }

   108 

   109 unsigned SmallStrings::count() const

   110 {

   111     unsigned count = 0;

   112     if (m_emptyString)

   113         ++count;

   114     for (unsigned i = 0; i < numCharactersToStore; ++i) {

   115         if (m_singleCharacterStrings[i])

   116             ++count;

   117     }

   118     return count;

   119 }

   120 

   121 void SmallStrings::createEmptyString(JSGlobalData* globalData)

   122 {

   123     ASSERT(!m_emptyString);

   124     m_emptyString = new (globalData) JSString(globalData, "", JSString::HasOtherOwner);

   125 }

   126 

   127 void SmallStrings::createSingleCharacterString(JSGlobalData* globalData, unsigned char character)

   128 {

   129     if (!m_storage)

   130         m_storage = adoptPtr(new SmallStringsStorage);

   131     ASSERT(!m_singleCharacterStrings[character]);

   132     m_singleCharacterStrings[character] = new (globalData) JSString(globalData, m_storage->rep(character), JSString::HasOtherOwner);

   133 }

   134 

   135 UString::Rep* SmallStrings::singleCharacterStringRep(unsigned char character)

   136 {

   137     if (!m_storage)

   138         m_storage = adoptPtr(new SmallStringsStorage);

   139     return m_storage->rep(character);

   140 }

   141 

   142 } // namespace JSC