Re: Symbols which were not used, still in binary

[Michael Eager <eager at eagercon dot com>]

Steve Kreyer wrote:
> Hi,
>
> I have a question about an optimization issue.
>
> I have compiled the follwing code with the command ''gcc file.c''
>
> ---------------------------------------------------------------
> int add(int a, int b){
>    return a + b;
> }
>
> int main(){
> }
> --------------------------------------------------------------
>
> My question is:
> Why does the symbol "add" appear in the outcoming binary file?
> nm gives me:
> --------------------------------------------------------------
> redwing@euklid:~ $ nm a.out | grep add
> 08048324 T add
> --------------------------------------------------------------
>
> Neither is it used in this piece of code, nor is it compiled into an 
> object file,
> so in my opinion it should be optimized away by gcc.
> So perhaps anybody can tell me whats wrong with my thought...

add is a global symbol.  GCC has no way of knowing whether or not
this object file will be linked with another file which contains
a reference to the function.  Arguably, the linker should know
that add is not referenced and could remove it, but linkers are not
usually able to slice and dice object files.

[If you declare add to be static, then GCC should be able
to eliminate it.  But it doesn't.]

Programs often contain functions which are designed
to be called by someone using a debugger but are otherwise
not referenced.

--
Michael Eager	 eager@eagercon.com
1960 Park Blvd., Palo Alto, CA 94306  650-325-8077