Comment by oautholaf

Here's one thing I don't understand: In addition to enabling dynamic linking, this mechanism allows Swift to compile less code (generic functions only need to be compiled once) and therefore reduce instruction cache misses.

But certainly the tradeoff for this type-info-vtable "witness table" and much more heavy boxing must impact the data cache miss rate. What's the tradeoff end up being in practical terms? Is it worth it?

Also, although it seems there's features that let you "freeze" a type, is there a practical way that a non-language expert could ever profile and discover that they may want to use such a feature to improve performance?

Especially given that Swift programs on iOS probably only dynamically link to the system libraries, this seems like a powerful language feature to enable something that could have been achieved by writing the Swift system libraries as typed facades around a typeless core, which you so often see in C++ STL implementations.