Comment by TickleSteve
11 years ago
what you describe is the "-flto" option in GCC and I believe the equivalent in JS is donr by Google Closure compiler.
11 years ago
what you describe is the "-flto" option in GCC and I believe the equivalent in JS is donr by Google Closure compiler.
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.
I saw that discussion years ago about the G++ String implementation. That was a big .o file.
Thanks for the correction. Didn't know that.
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.