Pre-compilation Flags

EPOCH uses precompiler directives to switch certain features of the code on or off. The precompiler directives all begin with a “#” character and look like:

#ifdef MY_PRECOMPILER_DIRECTIVE
 some_fortran_of_some_kind
#else
 some_other_fortran
#endif

They behave in a very simple manner. The precompiler runs BEFORE the Fortran compiler and, until it reaches a precompiler directive, it just creates a temporary file which is an exact copy of the source file. When it reaches a precompiler directive of this kind it treats the #ifdef commands as if/then/else statements. If MY_PRECOMPILER_DIRECTIVE was defined in the makefile then some_fortran_of_some_kind is pushed out to the temporary file. Otherwise some_other_fortran is written instead. The precompiler directives themselves are never output to the temporary file. Once then preprocessor has finished, it passes this temporary file to the Fortran compiler which can then compile it just like any other standard Fortran file.

When to use precompiler directives

• When adding properties to the particle structure.
• When adding time consuming calculations to the particle pusher.

Precompiler directives should be avoided when there is no significant performance gain or memory reduction to be made. Wherever possible, optional features should be controlled by parameters in the input deck.

The directive printing routine on code startup

When EPOCH starts it prints the precompiler directives that it was built with and what they mean. This isn’t required, but has proved very useful and is implemented in a very simple way. Just open the file src/housekeeping/welcome.F90 and find the subroutine compiler_directives. There are a large block of precompiler directives which read:

#ifdef TRACER_PARTICLES
   defines = IOR(defines, c_def_tracer_particles)
   WRITE(*, *) "Tracer particle support -DTRACER_PARTICLES"
#endif

Simply add a new element to the end of the list.

#ifdef MY_PRECOMPILER_DIRECTIVE
   defines = IOR(defines, c_def_my_precompiler_directive)
   WRITE(*,*) "My new physics -DMY_PRECOMPILER_DIRECTIVE"
#endif

You will also need to add c_def_my_precompiler_directive to the list of constants in src/shared_data.F90.