https://bugs.freedesktop.org/show_bug.cgi?id=66847
--- Comment #1 from Andreas Boll andreas.boll.dev@gmail.com --- The llvm backends of r600g and the radeonsi on mesa 9.1 are only compatible with tstellar's llvm-3.2 branch.
The llvm backends of r600g and the radeonsi on mesa 9.2 should be compatible with the official llvm-3.3 and llvm-3.4. It's not compatible with tstellar's llvm-3.2 branch.
So I think we should abort in configure if someone wants to build mesa 9.1 with enabled llvm backend and llvm != 3.2.