Slow rendering when GC.line_style = LineOnOffDash

[Julian Bradfield]

On 2022-12-03, Farrington, Paul <Paul.Farrington at fisglobal.com> wrote:
> We’ve noticed that XDrawRectangle where the GC.line_style is LineOnOffDash has
> gotten slower in recent X Server releases. We’ve observed this on both Linux
> RH8 and FBSD 11.4 and above. There are two sample programs pasted below, one
> which draws 5 rectangles with LineSolid and one which draws 5 rectangles with
> LineOnOffDash. There is also a makefile for RH8. Running the two programs with
> an X Server version 1.18.4, both programs render the rectangles instantly. But
> in X Server version 1.20.8 or more obviously 1.20.14, rendering the dashed
> line rectangles is much slower. You can watch them being drawn on the screen,
> whereas the solid line version still renders instantly.

Interesting. It's still a bug, but your sample programs use an odd
way to draw lines - setting the GC function to Clear, leaving
foreground and background at default, and using the plane_mask to
specify the color.

If you draw dashed lines the normal way (with GC function Copy and the
color specified in foreground), it's fine - it's the non-trivial
plane_mask that appears to be triggering the problem. Quite how Xorg
can take human-noticeable time to do masked dashed line draw, I can't
imagine!