My first thought was "now what is the calling convention for float parameters again? they are passed in registers right? the compiler can probably arrange so they don't have to actually be copied" and then I realized it will probably even inline it.

Anyway, assuming it's not inlined I would guess pass-by-copy, maybe with an occasional exception in code with heavy register pressure.

Edit: Actually since it's a structure, the calling convention is to memory allocate it and pass a pointer, doh. So it should actually compile the same.