[gecko]

Actually, Mach and BSD have bupkis in common. It simply so happens that OS X uses a reasonable amount of BSD user-land code in OS X to make compatibility easier. This probably makes porting FreeBSD to clang easier indirectly--the user-land tools and libraries from the BSDs are written in the same style as the kernel, and so supporting them likely makes supporting the kernel easier that much easier--but saying Mach is similar to BSD is like saying Windows NT is similar to DOS simply because it can run DOS programs.

[ankhmoop]

You're technically correct in regards to Mach, but in the context of Mac OS X a very large percentage of the kernel is composed of BSD-derived code.

Unfortunately, none of this is particularly relevant to clang, as the kernel design (in respect to Mach or BSD) really has little bearing on its compiliability.

[Edit]

I'm very surprised that this was downvoted to 0. The discussion was in context of Mac OS X, and the technical veracity is easily confirmed:

http://en.wikipedia.org/wiki/Xnu http://fxr.watson.org/fxr/source/bsd/?v=xnu-1228