Region::union on a list of does N^2 copies of Shape vectors

• Component: Layout and Rendering
• Status: NEW ---

[Eric Seidel (no email)]

Region::union on a list of does N^2 copies of Shape vectors

Regions are mutable, but their Shapes are not.  Each Shape has 2 vectors which are both copied into a new Shape object when two Shapes are combined.

To union a list of Regions into a single Region amounts to copying the contents of the first region's Shape vectors N times!  (The second region's shape vectors N-1, times, etc.)

The sum of a list from 1 to N is N(N-1)/2, which is for our purposes is order N^2. :(

Prior to my mitigation in bug 98800, we performed this O(N^2) algorithm every time we did RenderLayerCompositor::computeCompositingRequirements (which is currently on every style recalc!)

This could be solved by making a mutable union which built up a Shape, instead of copying each pair into a new Shape.

I've split this off from bug 98800 so we can land a mitigation for RoboHornet now, and fix the algorithm complexity of Region/Shape combining later.

[Eric Seidel (no email)]

(In reply to comment #0)
> Prior to my mitigation in bug 98800, we performed this O(N^2) algorithm every time we did RenderLayerCompositor::computeCompositingRequirements (which is currently on every style recalc!)

This every-style-recalc behavior is tracked by bug 84393.