In Road To OpenCL, R600g LLVM Back-End Arrives
Written by Michael Larabel in AMD on 9 December 2011 at 04:24 PM EST. 30 Comments
Before calling it a week, Tom Stellard at AMD published a Git branch that offers up an LLVM shader back-end for the AMD R600 Gallium3D driver. This is one of the steps in bringing Compute/OpenCL support to the open-source AMD Radeon Linux graphics drivers.

Tom Stellard announced on Friday afternoon this R600g LLVM shader back-end branch to the Mesa developer's mailing list. Tom's r600g-llvm-shader branch of his Mesa repository offers a TGSI (the current Gallium3D IR) to LLVM IR converter, changes to Gallivm to make it portable with the TGSI-to-LLVM converter, and the LLVM back-end for R600g itself. The TGSI-to-LLVM converter will likely be of use to more Gallium3D drivers besides the R600g driver itself, since TGSI is universal to the drivers and not Radeon-specific.

This LLVM back-end is based upon AMD's AMDIL LLVM back-end for OpenCL with various modifications. Tom says this new shader back-end currently passes 99% of the Piglit regression tests that work with the current R600g shader back-end. The failures are coming from unimplemented texture instructions and on a current lack of indirect addressing support. The LLVM back-end also isn't optimized for VLIW at this point plus other optimizations are lacking. "The optimizations in r600_asm.c are able to do some instruction packing, but the resulting code is not yet as good as the current backend."

And here's the key part of his message:
The main motivation for this LLVM backend is to help bring compute/OpenCL support to r600g by making it easier to support different compiler frontends. I don't have a concrete plan for integrating this into mainline Mesa yet, but I don't expect it to be in the next release. I would really like to make it compatible with LLVM 3.0 before it gets merged (it only works with LLVM 2.9 now), but if compute support evolves quickly, I might be tempted to push the 2.9 version into the master branch.

More information coming soon.

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Michael Larabel is the principal author of and founded the site in 2004 with a focus on enriching the Linux hardware experience. Michael has written more than 10,000 articles covering the state of Linux hardware support, Linux performance, graphics drivers, and other topics. Michael is also the lead developer of the Phoronix Test Suite, Phoromatic, and automated benchmarking software. He can be followed via Twitter or contacted via

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