Why don't two binaries of programs with only comments changed exactly match in gcc?

Question

I created two C programs

1. Program 1

   int main()
   {
   }
   

2. Program 2

   int main()
   {
   //Some Harmless comments
            
   
   
           

Answer 1

The most common reason are file names and time stamps added by the compiler (usually in the debug info part of the ELF sections).

Try running

 $ strings -a program > x
 ...recompile program...
 $ strings -a program > y
 $ diff x y

and you might see the reason. I once used this to find why the same source would cause different code when compiled in different directories. The finding was that the __FILE__ macro expanded to an absolute file name, different in both trees.