Oilpan: promptly free per-frame document heap vector.

Source/core/page/PageAnimator.cpp

 // Copyright 2014 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 "config.h"
 #include "core/page/PageAnimator.h"
 
 #include "core/animation/DocumentAnimations.h"
 #include "core/frame/FrameView.h"
 #include "core/frame/LocalFrame.h"
@@ skipping 50 matching lines @@
         }
     }
 
     for (size_t i = 0; i < documents.size(); ++i) {
         DocumentAnimations::updateAnimationTimingForAnimationFrame(*documents[i], monotonicAnimationStartTime);
         SVGDocumentExtensions::serviceOnAnimationFrame(*documents[i], monotonicAnimationStartTime);
     }
 
     for (size_t i = 0; i < documents.size(); ++i)
         documents[i]->serviceScriptedAnimations(monotonicAnimationStartTime);
+
+#if ENABLE(OILPAN)
+    documents.clear();

[dstockwell, 2015/03/10 22:38:31]

This might be a reasonable optimiziation, but to a non-oilpan expert it's not
clear what the purpose is. Once the oilpan if-defs are removed there's nothing
to prevent someone from cleaning this up. Is there something we can do to
improve the readability here? Maybe a different named HeapVector?

[haraken, 2015/03/10 23:26:08]

Yeah, that's something we're considering.

Option 1: Introduce StackVector.

Option 2: Add a destructor for an on-stack HeapVector so that the HeapVector is
promptly cleared. (But I'm failing at implementing the destructor in a way in
which it doesn't regress performance.)

Option 3: Explicitly add .clear() with a comment.

+#endif
 }
 
 void PageAnimator::scheduleVisualUpdate(LocalFrame* frame)
 {
     // FIXME: also include m_animationFramePending here. It is currently not there due to crbug.com/353756.
     if (m_servicingAnimations || m_updatingLayoutAndStyleForPainting)
         return;
     // FIXME: The frame-specific version of scheduleAnimation() is for
     // out-of-process iframes. Passing 0 or the top-level frame to this method
     // causes scheduleAnimation() to be called for the page, which still uses
@@ skipping 18 matching lines @@
     // setFrameRect(). This will be a quick operation for most frames, but the
     // NativeWindowWidgets will update a proper clipping region.
     view->setFrameRect(view->frameRect());
 
     // setFrameRect may have the side-effect of causing existing page layout to
     // be invalidated, so layout needs to be called last.
     view->updateLayoutAndStyleForPainting();
 }
 
 }