Note: Article has been updated to make it clear this is with later LLVM versions.
DiRT Rally [Steam, Feral Store] seemed to have an issue with rendering properly in certain cases with newer versions of LLVM with RadeonSI Mesa (AMD), but it seems to have been tracked down as an issue in LLVM.
https://t.co/P1nITBK8oG Samuel tracked down the last rendering issue with DiRT Rally on RadeonSI; renders properly with these patches.— Pierre-Loup Griffais (@Plagman2) March 8, 2017
I did notice an issue during Feral's livestream before release, which other people also noticed where the car dashboard wasn't rendering. Hopefully this is the fix for that, as I haven't heard about any other issues.
Looks like Feral helped directly, as the Linux Dev Lead from Feral also tweeted about helping to get it found.
I've been told this was actually a regression with LLVM 4.0, so it's good to see it get cleaned up. It's not a bug in the port itself, if that wasn't clear. So hopefully with later Mesa releases DiRT will continue to work nicely on Mesa.