llvm.eh.unwind.init : missing documentation/incorrect implementation?

Per subject; I went to look for a description at
http://llvm.org/docs/ExceptionHandling.html , but it wasn't there.

Also, apparently, __builtin_unwind_init is supposed to force
callee-saved registers to be spilled, but llvm.eh.unwind.init doesn't
do that. This matters because Boehm GC expects the intrinsic to do
that.

-Eli