The R600 LLVM back-end is for targeting LLVM compiler support on ATI/AMD graphics hardware going back to the Radeon HD 2000 series graphics processors (although for GPGPU, only the more recent hardware generations are relevant). This back-end is needed for the LLVM shader compiler and more particularly for the OpenCL support on the open-source Radeon graphics driver stack.
Originally this LLVM back-end was living within the Mesa repository but then months ago is when Tom Stellard of AMD was pushing for it to be included in the upstream LLVM code-base to benefit all users. It took a while of pushing but this week AMD's wish was granted with the R600 LLVM back-end having been merged, as can be seen by this Git copy. The back-end is made up of more than 20,000 lines of code.