Swift Performance

[Joe_Groff]

@scanon

A cursory glance at instruments shows that the majority of the time in the Swift implementations (both "naive" and "unmanaged") is spent in retain/release traffic, which suggests that there's an easy 2-3x performance improvement to be had with very minor changes (and also that further improvements to how retain/release are handled at the language level would net big wins here--cc @Michael_Gottesman).

The tests were done with Swift 5.0 as well, and there are already substantial improvements to RC optimization in 5.1 (and even more in top-of-tree). Using Unmanaged is "cheating" in a sense because it's unsafe, and it should not be necessary in normal circumstances to get adequate performance from idiomatic code—if it's safe to use unmanaged references, then the compiler should know that and avoid reference counting in the first place. The current Swift implementation is still nowhere near representative of the performance that should be possible, since we've been primarily in the "make it work" phase of development, and are only now starting to get into the "make it fast" work.