It was anticipated R600 SB shader optimizations might be enabled by default
for this Radeon Gallium3D driver and now they have been for Mesa 9.3/Mesa 10.0.
The R600 SB shader optimization back-end written by Vadim Girlin is very significant for AMD's open-source driver on HD 2000 through HD 6000 series graphics hardware in that it can substantially increase the system's graphics performance
for games/applications making use of GLSL shaders. The performance improvements can be very dramatic and make the open-source Radeon Gallium3D driver much more competitive against Catalyst as many Phoronix benchmarks have shown.
As of this morning, the R600 SB back-end is now enabled by default per this commit
. Those wanting to optionally disable this shader optimization back-end now will have to set the R600_DEBUG=nosb