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
isMarked(JSCell * string)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
rep(unsigned char character)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
SmallStringsStorage()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
SmallStrings()67 SmallStrings::SmallStrings()
68 {
69 COMPILE_ASSERT(singleCharacterStringCount == sizeof(m_singleCharacterStrings) / sizeof(m_singleCharacterStrings[0]), IsNumCharactersConstInSyncWithClassUsage);
70 clear();
71 }
72
~SmallStrings()73 SmallStrings::~SmallStrings()
74 {
75 }
76
markChildren(HeapRootMarker & heapRootMarker)77 void SmallStrings::markChildren(HeapRootMarker& 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
clear()106 void SmallStrings::clear()
107 {
108 m_emptyString = 0;
109 for (unsigned i = 0; i < singleCharacterStringCount; ++i)
110 m_singleCharacterStrings[i] = 0;
111 }
112
count() const113 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
createEmptyString(JSGlobalData * globalData)125 void SmallStrings::createEmptyString(JSGlobalData* globalData)
126 {
127 ASSERT(!m_emptyString);
128 m_emptyString = new (globalData) JSString(globalData, "", JSString::HasOtherOwner);
129 }
130
createSingleCharacterString(JSGlobalData * globalData,unsigned char character)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
singleCharacterStringRep(unsigned char character)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
147