Usually how it works is it does that a couple of times, while gathering some kind of stats.

If those stats prove that it is always an array of floats, then machine code gets generated for an array of floats, with a trap handler for when the code fails.

The trap handler, if called, will throw away the code and restart the analysis.

So if you are nice for the compiler and keep your types regular, the machine code will not be thrown away and there are no speed bumps.