I want to know if it is possible to select different parts of my Fortran 95 routine to compile.
For example, if I pass certain flag to gfortran, then the compiler chooses which section to use for a certain function. I know I can do it using if inside the routine, but the drawback is that I don't want the program to run the if all the time due to speed concerns. I suppose solution should be similar to this one
I am working specifically with a program that calculates energies in a many-body system (say, a million). Then I don't want to put an if each time that I need to use a different energy definition at compilation time.
I hope this is possible and that my question is clear.
You can use the C like preprocessor with the -cpp command line option to your command line. That option is not turned on by default (As per Vladimir F comment below), although it looks like using the .F90 filename extension (i.e. capital F, instead of .f90) will do the trick without the -cpp option.
Details about the option:
https://gcc.gnu.org/onlinedocs/gfortran/Preprocessing-Options.html
Then you can do the same as you pointed out, so the:
#ifdef <some-var>
code when <some-var> is true
#elif defined(<other-var>)
code when <other-var> is true
#endif
As required.
There are more examples on this page with actual code.
Also, like with C/C++, you can define macros on your command line with the -D option:
gfortran -DCASE1=3 ...
This will define CASE1 with the value 3. If you do not specify the value, then 1 is automatically assigned to the macro. This is documented on the same page.