https://bugs.freedesktop.org/show_bug.cgi?id=32296
--- Comment #4 from Marek Olšák maraeo@gmail.com 2011-01-08 15:39:14 PST --- None. Fast Z Clear is the only feature that's left and is required to initialize ZMASK RAM so that zbuffer compression can be used.
I know that the compression doesn't always work and I can reproduce it. But it's one of the features that are hard to fix.