Not quite. When you link to a static library (.a) with lots of .o object files in it, only those object files will be linked that are actually used by your program.

I first learned about this when I looked at the source of dietlibc, and wondered about every function being in a separate file. That enables the aforementioned trivial optimization, even without -flto.

@majewsky: what you're describing is more akin to "-ffunction-sections" and "-gc-sections" which will split ewach function into a separate section and garbage-collect those sections.