547591 - Rendering of -moz-border-*-colors is much slower than of border-color

Status: RESOLVED INVALID

(Core :: Web Painting, defect)

Description

[Markus Stange [:mstange]]

Attachment: testcase

Load the attached testcase and click the button several times in quick succession. It will alternate between setting border-color and -moz-border-*-colors on the circles.

Observe how the measured times are different. On my machine the it's roughly
100ms vs 1000ms.

Comment 1

[Boris Zbarsky [:bzbarsky]]

See nsCSSBorderRenderer::DrawBorders (in nsCSSRenderingBorders.cpp) and specifically the forceSeparateCorners boolean.  Your border-color testcase ends up with forceSeparateCorners false, which triggers a fast path.  Your border-*-colors testcase ends up with forceSeparateCorners true (see code comments for why; we could probably detect the special case of only one color and fast-path it if that would be useful) and takes a slower path involving clipping, etc.

About 30% of the slower-path time is under pushing groups and painting with alpha from nsCSSBorderRenderer::DrawBorders itself.  The rest is under the FillSolidBorder calls that DrawBorderSidesCompositeColors makes; I'm not sure why these are so much slower than the FillSolidBorder calls DrawBorderSides makes in the non-moz-border-colors case....

Comment 2

[Joe Drew (not getting mail)]

It's not clear to me who should be working on this, but it feels sort of like this can be optimized from within the style system.

Comment 3

[David Baron :dbaron: (⌚️UTC-4, no longer working on Mozilla)]

This is code in nsCSSBorderRenderer.

Comment 4

[Sebastian Zartner [:sebo]]

-moz-border-*-colors were removed in bug 1429723.