https://bugs.freedesktop.org/show_bug.cgi?id=100697
--- Comment #1 from network723@rkmail.ru --- While I'm not a dev, but this is very similar to what I see in Xonotic, due to llvm regression, see this bug: https://bugs.freedesktop.org/show_bug.cgi?id=100681
If you can, try building mesa with earlier llvm revision, I myself use 299800 for now.