[Webkit-unassigned] [Bug 72452] [Chromium] Avoid clear and color mask operations for root layers

[bugzilla-daemon at webkit.org]

https://bugs.webkit.org/show_bug.cgi?id=72452





--- Comment #11 from Daniel Sievers <sievers at chromium.org>  2011-11-16 13:19:24 PST ---

> On a tiled architecture, the GPU has a piece of embedded memory / cache that is smaller than the frame buffer. It collects all screen-space draw operations (clear, triangles post transform etc.), map them to a set of tiles that fit into the cache, then goes for all the tiles:
> 1- read frame buffer from memory to cache
> 2- do all draw operations scissored to the tile
> 3- write frame buffer from cache to memory
> 
> If you clear first, it can skip (1) because it's easy to know it will erase all the data. Otherwise, it would have to go through all the triangles and check whether or not they fully cover the tile which would be expensive, so it generally doesn't do that.


Although I do believe you :) I feel like we should rather make the more naive assumption that clearing before rendering over the same area is potentially slower (and remove the clear as part of this patch). Ideally, this should be left to the driver or hardware to try to optimize these cases, as I find it counter-intuitive at the OpenGL API level.

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