Created attachment 115419 [details]
Wall with light effect triggering the artifact.

On game Shawdow Warrior, with Southern Island chip, drivers from this PPA:
https://launchpad.net/~oibaf/+archive/ubuntu/graphics-drivers

Experienced both on Ubuntu 14.10 and, after upgrade, on Ubuntu 15.04.

Some special effects trigger a visible artifact on triangle primitive
boundaries, probably, see screenshot attached. With fglrx, the issue is not
present.

It seems the problem is on some post-processing shader, because, as can be seen
in the screenshot, the visible triangles does not respect the boundary of scene
objects, and if I move the mouse, they are fixed relative to the screen.

Running with the following env var (otherwise the game is unplayable slow on
current video settings):
R600_DEBUG=sb

Relevant info from glxinfo:
OpenGL vendor string: X.Org
OpenGL renderer string: Gallium 0.4 on AMD TAHITI
OpenGL core profile version string: 3.3 (Core Profile) Mesa 10.6.0-devel
(git-7f5a8ac 2015-04-25 utopic-oibaf-ppa)
OpenGL core profile shading language version string: 3.30
OpenGL core profile context flags: (none)