R600 Gallium3D Disables LLVM Back-End By Default
AMD has decided to disable the LLVM compiler back-end by default within the R600 era Gallium3D graphics driver.
The AMD GPU LLVM back-end was never a hard requirement to the R600 Gallium3D driver that supports the Radeon HD 2000 through 6000 series graphics cards, but had become an optional feature on the graphics shader compiler side and then only a requirement for those wishing to take advantage of OpenCL/GPGPU support for this pre-GCN hardware. The AMD GPU LLVM back-end meanwhile is a hard requirement for the RadeonSI Gallium3D driver with AMD's HD 7000 series hardware and newer -- for both graphics and compute support.
Mesa has supported using the --enable-r600-llvm-compiler compile-time switch for months for building R600g with the LLVM back-end on supported versions of the compiler infrastructure, but now not even that will disable the compiler back-end by default for this driver.
AMD's Michel Dänzer wrote in a Git commit a short time ago today, "For graphics, the LLVM compiler backend currently has many shortcomings compared to the non-LLVM one. E.g. it can't handle geometry shaders yet, but that's just the tip of the iceberg."
The R600g LLVM support is now disabled at run-time by default and will require setting the R600_DEBUG=llvm environment variable for enabling the LLVM back-end for graphics on this pre-RadeonSI driver.
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