Bug#697356:

[Fabio Pedretti]

There are some more informations here:
http://lists.freedesktop.org/archives/mesa-dev/2013-January/032205.html

Note that the llvm backend is optional and experimental for the r600 driver. I 
enabled it some time ago on my Ubuntu PPA ( https://launchpad.
net/~oibaf/+archive/graphics-drivers/ ) and many users experienced problems 
(see the phoronix thread).

About radeonsi, the llvm backend is required to build it, but xserver-xorg-
video-ati must also be compiled with glamor. glamor itself currently only works 
with xorg < 1.13:
http://www.freedesktop.org/wiki/Software/Glamor

So I see no hurry for this, better wait for llvm 3.3 which will include the 
proper and hopefully stabilized AMD backend and also a glamor which works with 
recent xorg.