(In reply to comment #3)
> I've rebuilt mesa with this re-enabled and I saw a lot of rendering
> glitches. I suspect that DISCARD_RANGE being re-enabled is the problem,
> although I've also built and installed an updated xf86-video-ati
> (cdf522f→20bfc65).
> 
> My current mesa build has 1eedebc reverted; I'll re-check with that undone
> later.

Do you still have the rendering glitches with enabled DISCARD_RANGE?
If so please reopen this bug.