Tell gcc/g++ compiler remove orphaned functions

Question

There are some functions which are never called. Not because they are not called under some logic but because they are never called from tree of main function. assuming:

int A()
{
   if(...)
      call F();
}

int B()
{
}

int C()
{
    call B();
}

int D()
{
    call A();
}

int F()
{
    call A();
}

int main()
{
    call D();
    call F();
}

so in this example:

main  ---> D
       |
       --> F

D  ------> A
F -------> A
A -------> F
C -------> B

So in no way by running this application B and C have chance of being called. They are orphan. But it seems gcc/g++ do not remove orphan functions as I have checked:

program 1:

int main()
{
    int a=4;
    int b=3;
    int c=a+b;
    b=c-a;
    a=c-b;
    return 0;
}

running in command line:

g++ -std=c++11 test.cpp
md5sum a.out

I get:

546da269abddb8dcb3883527a362f769  a.out

Now by adding an orpan function (test), I get different executive file:

program 2:

int test()
{

}

int main()
{
    int a=4;
    int b=3;
    int c=a+b;
    b=c-a;
    a=c-b;
    return 0;
}

running in command line:

g++ -std=c++11 test.cpp
md5sum a.out

gives different hash:

64095263965d2d94ed2f305f99a2b25a  a.out

It shows that this orphan function which will never be called in my program has influenced the compiled code. Is there any way to tell gcc/g++ to remove orphan functions?

Merely recompiling the *exact* same source file will give a different hash too. GCC is not designed to always give the bit-for-bit same output file when you pass the same input file. Can you edit your question to include a more reliable test? — , Dec 11 '14 at 06:17
A function is never considered "orphaned" because its symbol is exported and available to other compilation units, which is something that gcc can't detect. — OmnipotentEntity, Dec 11 '14 at 06:17
So, is there any source code level tool providing such facility to export my codes (excluding standard header files) to minimal code? — barej, Dec 11 '14 at 06:23
I'm not sure I understand the point of this. Wouldn't it be better to just clean up your code by yourself? — nchen24, Dec 11 '14 at 06:25
@OmnipotentEntity what if our application is a final executive which is never used by any other application? — barej, Dec 11 '14 at 06:26
@nchen24 imagine there is a pile of codes and you cannot do it by yourself as you write code incrementally during a long time. so, in real situation finding orphaned functions is not such easy and simple. — barej, Dec 11 '14 at 06:28
@barej You can always use some terminal-fu like `nm ./a.out | c++filt | grep ' T ' | cut -f 3- -d' '` to get a list of the functions shown in the debug symbols, and grep for occurrences of those functions. — nchen24, Dec 11 '14 at 06:53

score 1 · Answer 1 · edited Dec 11 '14 at 06:28

When compiling use optimization option. The -O flag that is passed to the compiler

Change

g++ -std=c++11 test.cpp

TO

g++ -O1 -std=c++11 test.cpp

with this option the md5sum for both the program will be the same.

Program 1:

int main()
{
    int a=4;
    int b=3;
    int c=a+b;
    b=c-a;
    a=c-b;
    return 0;
}

and Program 2:

int test()
{

}

int main()
{
    int a=4;
    int b=3;
    int c=a+b;
    b=c-a;
    a=c-b;
    return 0;
}

Tell gcc/g++ compiler remove orphaned functions

1 Answers1