From cad810f21b803229eb11403f9209855525a25d57 Mon Sep 17 00:00:00 2001
From: Steve Block <steveblock@google.com>
Date: Fri, 6 May 2011 11:45:16 +0100
Subject: Merge WebKit at r75315: Initial merge by git.

Change-Id: I570314b346ce101c935ed22a626b48c2af266b84
---
 Source/JavaScriptCore/runtime/SmallStrings.cpp | 142 +++++++++++++++++++++++++
 1 file changed, 142 insertions(+)
 create mode 100644 Source/JavaScriptCore/runtime/SmallStrings.cpp

(limited to 'Source/JavaScriptCore/runtime/SmallStrings.cpp')

diff --git a/Source/JavaScriptCore/runtime/SmallStrings.cpp b/Source/JavaScriptCore/runtime/SmallStrings.cpp
new file mode 100644
index 0000000..f358727
--- /dev/null
+++ b/Source/JavaScriptCore/runtime/SmallStrings.cpp
@@ -0,0 +1,142 @@
+/*
+ * Copyright (C) 2008, 2010 Apple Inc. All Rights Reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in the
+ *    documentation and/or other materials provided with the distribution.
+ *
+ * THIS SOFTWARE IS PROVIDED BY APPLE INC. ``AS IS'' AND ANY
+ * EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR
+ * PURPOSE ARE DISCLAIMED.  IN NO EVENT SHALL APPLE INC. OR
+ * CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL,
+ * EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO,
+ * PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR
+ * PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY
+ * OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+ * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ */
+
+#include "config.h"
+#include "SmallStrings.h"
+
+#include "JSGlobalObject.h"
+#include "JSString.h"
+#include <wtf/Noncopyable.h>
+#include <wtf/PassOwnPtr.h>
+
+namespace JSC {
+
+static const unsigned numCharactersToStore = 0x100;
+
+static inline bool isMarked(JSString* string)
+{
+    return string && Heap::isCellMarked(string);
+}
+
+class SmallStringsStorage : public Noncopyable {
+public:
+    SmallStringsStorage();
+
+    StringImpl* rep(unsigned char character) { return m_reps[character].get(); }
+
+private:
+    RefPtr<StringImpl> m_reps[numCharactersToStore];
+};
+
+SmallStringsStorage::SmallStringsStorage()
+{
+    UChar* characterBuffer = 0;
+    RefPtr<StringImpl> baseString = StringImpl::createUninitialized(numCharactersToStore, characterBuffer);
+    for (unsigned i = 0; i < numCharactersToStore; ++i) {
+        characterBuffer[i] = i;
+        m_reps[i] = StringImpl::create(baseString, i, 1);
+    }
+}
+
+SmallStrings::SmallStrings()
+{
+    COMPILE_ASSERT(numCharactersToStore == sizeof(m_singleCharacterStrings) / sizeof(m_singleCharacterStrings[0]), IsNumCharactersConstInSyncWithClassUsage);
+    clear();
+}
+
+SmallStrings::~SmallStrings()
+{
+}
+
+void SmallStrings::markChildren(MarkStack& markStack)
+{
+    /*
+       Our hypothesis is that small strings are very common. So, we cache them
+       to avoid GC churn. However, in cases where this hypothesis turns out to
+       be false -- including the degenerate case where all JavaScript execution
+       has terminated -- we don't want to waste memory.
+
+       To test our hypothesis, we check if any small string has been marked. If
+       so, it's probably reasonable to mark the rest. If not, we clear the cache.
+     */
+
+    bool isAnyStringMarked = isMarked(m_emptyString);
+    for (unsigned i = 0; i < numCharactersToStore && !isAnyStringMarked; ++i)
+        isAnyStringMarked = isMarked(m_singleCharacterStrings[i]);
+    
+    if (!isAnyStringMarked) {
+        clear();
+        return;
+    }
+    
+    if (m_emptyString)
+        markStack.append(m_emptyString);
+    for (unsigned i = 0; i < numCharactersToStore; ++i) {
+        if (m_singleCharacterStrings[i])
+            markStack.append(m_singleCharacterStrings[i]);
+    }
+}
+
+void SmallStrings::clear()
+{
+    m_emptyString = 0;
+    for (unsigned i = 0; i < numCharactersToStore; ++i)
+        m_singleCharacterStrings[i] = 0;
+}
+
+unsigned SmallStrings::count() const
+{
+    unsigned count = 0;
+    if (m_emptyString)
+        ++count;
+    for (unsigned i = 0; i < numCharactersToStore; ++i) {
+        if (m_singleCharacterStrings[i])
+            ++count;
+    }
+    return count;
+}
+
+void SmallStrings::createEmptyString(JSGlobalData* globalData)
+{
+    ASSERT(!m_emptyString);
+    m_emptyString = new (globalData) JSString(globalData, "", JSString::HasOtherOwner);
+}
+
+void SmallStrings::createSingleCharacterString(JSGlobalData* globalData, unsigned char character)
+{
+    if (!m_storage)
+        m_storage = adoptPtr(new SmallStringsStorage);
+    ASSERT(!m_singleCharacterStrings[character]);
+    m_singleCharacterStrings[character] = new (globalData) JSString(globalData, PassRefPtr<StringImpl>(m_storage->rep(character)), JSString::HasOtherOwner);
+}
+
+StringImpl* SmallStrings::singleCharacterStringRep(unsigned char character)
+{
+    if (!m_storage)
+        m_storage = adoptPtr(new SmallStringsStorage);
+    return m_storage->rep(character);
+}
+
+} // namespace JSC
-- 
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