He described not Link Time Optimization (-flto), but the very basic linker functionality: to only include required functions. C/C++ has a weird compilation model where each source file is translated to machine code separately, with placeholders for unknown function addresses. Thus it is trivial to take only required functions.
-flto, on the other hand, allows to reverse this process to allow interprocedural optimization across different translation units.
@TickleSteve below me: "without this, removal is only performed within the compilation-unit"
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.
He's describing unused-function removal... which "-flto" is the correct option to use (on GCC).
like you say, without this, removal is only performed within the compilation-unit, but my statement was quite correct.
@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.
-flto, on the other hand, allows to reverse this process to allow interprocedural optimization across different translation units.