Find cached display items directly during painting

third_party/WebKit/Source/platform/graphics/ContiguousContainer.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "platform/graphics/ContiguousContainer.h"
 
 #include "wtf/Allocator.h"
 #include "wtf/ContainerAnnotations.h"
 #include "wtf/PtrUtil.h"
 #include "wtf/allocator/PartitionAlloc.h"
@@ skipping 153 matching lines @@
 }
 
 void ContiguousContainerBase::swap(ContiguousContainerBase& other)
 {
     m_elements.swap(other.m_elements);
     m_buffers.swap(other.m_buffers);
     std::swap(m_endIndex, other.m_endIndex);
     std::swap(m_maxObjectSize, other.m_maxObjectSize);
 }
 
+void ContiguousContainerBase::removeEmptyBuffers()
+{
+    while (m_endIndex < m_buffers.size() - 1)

[jbroman, 2016/07/06 17:34:03]

How are we getting into a state where this is needed?
ContiguousContainerBase::removeLast should already be removing all but one empty
buffer.

[Xianzhu, 2016/07/06 18:05:27]

The method is to remove the last empty buffer. For a committed display item
list, because we will no more change the list, the empty buffer (which is big)
will be never used.

This was not needed because we copied the items in m_newDisplayItemList into
updatedList which doesn't have removeLast() calls so doesn't have the trailing
empty buffer, and then move updatedList into m_currentPaintArtifact. Now we move
m_newDisplayItemList into m_currentPaintArtifact directly. If there is any
trailing empty buffer it will be alive until the next paint.

[jbroman, 2016/07/06 19:09:37]

Ah, okay, it's a "shrink to fit"-ish thing. Makes sense.

+        m_buffers.removeLast();
+}
+
 ContiguousContainerBase::Buffer*
 ContiguousContainerBase::allocateNewBufferForNextAllocation(size_t bufferSize, const char* typeName)
 {
     ASSERT(m_buffers.isEmpty() || m_endIndex == m_buffers.size() - 1);
     std::unique_ptr<Buffer> newBuffer = wrapUnique(new Buffer(bufferSize, typeName));
     Buffer* bufferToReturn = newBuffer.get();
     m_buffers.append(std::move(newBuffer));
     m_endIndex = m_buffers.size() - 1;
     return bufferToReturn;
 }
 
 } // namespace blink