After http://cgit.freedesktop.org/mesa/mesa/commit/?id=c6d79ed289a75f13c65f011be870f7e43a0fedc7 which improve code quality i can enable unsafe-fp-math and grass is now there with trace from Comment 7 Daniel and others, you might want to recheck this. I guess unsafe-fp-math might be reenabled now if is stabilise SI and of course because of better radeonsi performance.