[Webkit-unassigned] [Bug 100413] [cg] RenderBlock::selectionGaps() is extremely slow when there are many floats

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=100413





--- Comment #2 from Darin Adler <darin at apple.com>  2012-10-25 14:30:03 PST ---
(From update of attachment 170730)
View in context: https://bugs.webkit.org/attachment.cgi?id=170730&action=review

I’m OK with this, but I suspect there may be two other variations that work equally well or perhaps even better.

> Source/WebCore/ChangeLog:10
> +        RenderBlock::selectionGaps() calls GraphicsContext::clipOut(const IntRect&) for each float.
> +        With Core Graphics, A function that takes a vector of rectangles and clips them all out at
> +        once is faster than multiple calls to the function that clips out a single rectangle.

Is the issue here the performance of repeatedly calling the CGContextGetClipBoundingBox function?

If so, could we fix this performance issue another way by simply using a wide open rectangle instead of calling CGContextGetClipBoundingBox?

> Source/WebCore/platform/graphics/cg/GraphicsContextCG.cpp:1080
> +        CGContextBeginPath(context);
> +        CGContextAddRects(context, cgRects, 2);
> +        CGContextEOClip(context);

Can the calls to CGContextBeginPath and CGContextEOClip be moved outside the loop? I don’t entirely understand the semantics here, but if all the rectangles are exclusive or'ed with each other, it seems that it could all be done with a single path.

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