I apologize for the chain of comments! I'm also discussing this issue on IRC (#dri-devel on irc.freenode.org) and someone seemingly familiar with the code took a look at the logs I posted. Apparently this is a GPU lockup after all. They suggested I tell the llvm packagers to revert r280589. Someone else also mentioned a regression in llvm 3.9.1. The log of our conversation can also be found online, in case I happened to miss any significant details: https://people.freedesktop.org/~cbrill/dri-log/?channel=dri-devel&date=2017-03-21