Avoid parsing css text if there are identical inline style blocks.

Source/core/css/StyleSheetContentsCache.cpp

+/*
+ * Copyright (C) 1999 Lars Knoll (knoll@kde.org)
+ * Copyright (C) 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011 Apple Inc. All rights reserved.
+ * Copyright (C) 2013 Google Inc. All rights reserved.
+ *
+ * This library is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU Library General Public
+ * License as published by the Free Software Foundation; either
+ * version 2 of the License, or (at your option) any later version.
+ *
+ * This library is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
+ * Library General Public License for more details.
+ *
+ * You should have received a copy of the GNU Library General Public License
+ * along with this library; see the file COPYING.LIB.  If not, write to
+ * the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor,
+ * Boston, MA 02110-1301, USA.
+ *
+ */
+
+#include "config.h"
+#include "core/css/StyleSheetContentsCache.h"
+
+#include "core/css/StyleSheetContents.h"
+
+namespace WebCore {
+
+PassRefPtr<StyleSheetContentsCacheItem> StyleSheetContentsCacheItem::create(StyleSheetContents* contents)
+{
+    ASSERT(contents);
+    ASSERT(contents->isCacheable());
+    return adoptRef(new StyleSheetContentsCacheItem(contents->copy()));
+}
+
+StyleSheetContentsCache::StyleSheetContentsCache()
+    : m_additionsSinceLastSweep(0)
+    , m_sweepTimer(this, &StyleSheetContentsCache::sweep)
+{
+}
+
+StyleSheetContents* StyleSheetContentsCache::find(const AtomicString& sheetText)
+{
+    if (sheetText.isEmpty())
+        return 0;
+
+    Cache::iterator it = m_cache.find(sheetText);
+    if (it == m_cache.end())
+        return 0;
+    return it->value->contents();
+}
+
+void StyleSheetContentsCache::add(const AtomicString& sheetText, StyleSheetContents* contents)
+{
+    static const unsigned maxAdditionsBetweenSweeps = 100;
+    if (++m_additionsSinceLastSweep >= maxAdditionsBetweenSweeps
+        && !m_sweepTimer.isActive()) {
+        static const unsigned sweepTimeInSeconds = 60;
+        m_sweepTimer.startOneShot(sweepTimeInSeconds);

[esprehn, 2013/10/30 21:33:50]

On second thought I think this timer is too complex and it also doesn't share
between pages in the same process. We can use a static map to make this simpler.
Inside StyleEngine.cpp

static HashMap<AtomicString, StyleSheetContents*>& textToSheetCache()
{
  DEFINE_STATIC_LOCAL(HashMap<AtomicString, StyleSheetContents*>, cache, ());
  return cache;
}

static HashMap<AtomicString, StyleSheetContents*>& sheetToTextCache()
{
  DEFINE_STATIC_LOCAL(HashMap<StyleSheetContents*, AtomicString>, cache, ());
  return cache;
}

then inside ~StyleSheetContents you remove yourself from the cache (since now
you're dead, the cache itself doesn't keep you alive) by looking up the entry in
the sheet -> text map and then removing that entry and the one in the
textToSheetCache.

Inside StyleEngine::createSheet() you check the textToSheetCache to to find a
cached entry, if there isn't one you add one to both maps. The CSSStyleSheet is
the thing keep sheet contents alive though, not the cache. That removes all this
sweeping logic as well.

+    }
+
+    ASSERT(contents);
+    ASSERT(!sheetText.isEmpty());
+
+    Cache::AddResult result = m_cache.add(sheetText, 0);
+    if (result.isNewEntry)
+        result.iterator->value = StyleSheetContentsCacheItem::create(contents);
+    else
+        result.iterator->value->ref();
+}
+
+void StyleSheetContentsCache::remove(const AtomicString& sheetText)
+{
+    Cache::iterator it = m_cache.find(sheetText);
+    if (it == m_cache.end())
+        return;
+
+    if (it->value->hasOneRef()) {
+        m_cache.remove(sheetText);
+        return;
+    }
+    it->value->deref();
+}
+
+void StyleSheetContentsCache::sweep(Timer<StyleSheetContentsCache>*)
+{
+    // Look for cache entries containing a style declaration with a single ref and remove them.
+    Vector<AtomicString*, 16> toRemove;
+    Cache::iterator it = m_cache.begin();
+    Cache::iterator end = m_cache.end();
+    for (; it != end; ++it) {
+        if (it->value->hasOneRef())
+            toRemove.append(&it->key);
+    }
+    for (size_t i = 0; i < toRemove.size(); ++i)
+        m_cache.remove(*toRemove[i]);
+
+    m_additionsSinceLastSweep = 0;
+}
+
+void StyleSheetContentsCache::clear()
+{
+    m_cache.clear();
+}
+
+}