[Webkit-unassigned] [Bug 85678] New: Leaf non self-painting layers should bail out early

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=85678

           Summary: Leaf non self-painting layers should bail out early
           Product: WebKit
           Version: 528+ (Nightly build)
          Platform: All
               URL: http://dglazkov.github.com/performance-tests/biggrid.h
                    tml
        OS/Version: All
            Status: NEW
          Severity: Normal
          Priority: P2
         Component: Layout and Rendering
        AssignedTo: webkit-unassigned at lists.webkit.org
        ReportedBy: jchaffraix at webkit.org
                CC: simon.fraser at apple.com, jamesr at chromium.org,
                    enne at google.com


Currently RenderLayer::paintLayer() doesn't check for leaf non self-painting layers (it checks for fully transparent layers though). However in this case, we have no work to do.

For cases like tables with RenderLayers on every cells where the RenderLayer tree is heavily weighted by its leafs, paintLayer() is introducing some overhead.

The benchmark in mind here is http://dglazkov.github.com/performance-tests/biggrid.html. Create a 10,000 rows x 50 columns and tick "overflow: hidden" (which creates an overflow RenderLayer on all the cells): in this case, the paint time when scrolling is around 120 ms on my machine. This is due to walking all RenderLayers and doing some checks to end up not painting as we are not visible and don't need to anyway.

Patch forthcoming.

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