Is there a way to strip all functions from an object file that I am not using?

Question

I am trying to save space in my executable and I noticed that several functions are being added into my object files, even though I never call them (the code is from a library).

Is there a way to tell gcc to remove these functions automatically or do I need to remove them manually?

Answer 1

Have you looked into calling gcc with -Os (optimize for size.) I'm not sure if it strips unreached code, but it would be simple enough to test. You could also, after getting your executable back, 'strip' it. I'm sure there's a gcc command-line arg to do the same thing - is it --dead_strip?

Answer 2

In addition to -Os to optimize for size, this link may be of help.

Answer 3

If you are compiling into object files (not executables), then a compiler will never remove any non-static functions, since it's always possible you will link the object file against another object file that will call that function. So your first step should be declaring as many functions as possible static.

Secondly, the only way for a compiler to remove any unused functions would be to statically link your executable. In that case, there is at least the possibility that a program might come along and figure out what functions are used and which ones are not used.

The catch is, I don't believe that gcc actually does this type of cross-module optimization. Your best bet is the -Os flag to optimize for code size, but even then, if you have an object file abc.o which has some unused non-static functions and you link statically against some executable def.exe, I don't believe that gcc will go and strip out the code for the unused functions.

If you truly desperately need this to be done, I think you might have to actually #include the files together so that after the preprocessor pass, it results in a single .c file being compiled. With gcc compiling a single monstrous jumbo source file, you stand the best chance of unused functions being eliminated.

Answer 4

IIRC the linker by default does what you want ins some specific cases. The short of it is that library files contain a bunch of object files and only referenced files are linked in. If you can figure out how to get GCC to emit each function into it's own object file and then build this into a library you should get what you are looking.

I only know of one compiler that can actually do this: here (look at the -lib flag)

Answer 5

Since I asked this question, GCC 4.5 was released which includes an option to combine all files so it looks like it is just 1 gigantic source file. Using that option, it is possible to easily strip out the unused functions.

More details here