Xcode only lets you do WMO & optimizations on in the normal UX. Really bad that they don't expose this :/
Also to other people thinking about swift: don't. 200-100kloc and your build times become 5-20+ minutes. The conditional compiler is not very good either. And once your at that level of code, xcode spins it's indexer for a long time, autocomplete fails a lot and the debugger just doesn't work that well. Printing an object just spins for over a minute, and often it fails to even print the variable. Also configuring your project into many modules become painful with all the targets you have to manage.
Maybe in 4+ years when the language has matured enough and these issues start going away? And you have a build system that doesn't depend on large multimegabyte xml files that can't be written by a human. But for now, no.
It's good enough for a couple of developers doing contract apps although!
This makes XCode warn you about functions with long compile times.