RADV ACO Adds A Version Hack/Workaround For An Extra 2~5% Boost In SotTR

While many hoped that with the adoption of Vulkan it would lead to avoiding driver-specific checks/workarounds, that's still not been the case to workaround various driver bugs and other issues. The RADV ACO compiler back-end has resorted to altering its version string in order to obtain 2~5% higher performance in Feral's latest Linux game release.

While RADV+ACO doesn't make use of LLVM, they are now appending "LLVM 9.0.1" to the version string when enabled via a new DRI option (radv_report_llvm9_version_string). Reporting LLVM 9.0.1 even though LLVM isn't being used was added as a workaround for Shadow of the Tomb Raider to achieve 2~5% higher performance with ACO.

The Linux game port is applying shader workarounds for Radeon graphics when LLVM 8.0 or older is detected or if LLVM isn't present. Those workarounds hurt the out-of-the-box ACO performance so now there is this ACO workaround to expose LLVM 9.0.1 in turn to avoid the game's shader workarounds.

This workaround was merged today in Mesa 20.2-devel.