Open MP if OpenMP:...else - fortran

Question

Problem:
I have a some code that myself and a few others have been writing, I took the code and made it use mpi and openmp with great results (helps that I am running it on a Blue Gene/Q).
One thing I am not a fan of is that now I cannot compile the code without the -openmp directive because to get the speedup I needed I used reduction variables.
Example:
!$OMP parallel do schedule(DYNAMIC, 4) reduction(min:min_val)
....
min_val = some_expression(i)
....
!$OMP end parallel do
result = sqrt(min_val)
I am looking for something like:
!$OMP if OMP:
!$OMP min_val = some_expression(i)
!$OMP else:
if ( min_val .gt. some_expression(i) ) min_val = some_expression(i)
!$OMP end else
Anybody know of something like this? Notice that without -openmp the !$OMP lines are ignored and the code runs normally with the correct, er same, answer.
Thanks,
(Yes it is FORTRAN code, but its almost identical to C and C++)

Answer 1

To your exact question:
!$ whatever_statement
will use that statement only when compiled with OpenMP.
Otherwise, in your specific case, can't you just use:
!$OMP parallel do schedule(DYNAMIC, 4) reduction(min:min_val)
....
min_val = min(min_val, some_expression(i))
....
!$OMP end parallel do
result = sqrt(min_val)
?
I'm using this normally with and without -openmp quite often.

Answer 2

If you are willing to use pre-processed FORTRAN source file, you can always rely on the macro _OPENMP to be defined when using OpenMP. The simplest example is:
program pippo
#ifdef _OPENMP
print *, "OpenMP program"
#else
print *, "Non-OpenMP program"
#endif
end program pippo
Compiled with:
gfortran -fopenmp main.F90
the program will give the following output:
OpenMP program
If you are unwilling to use pre-processed source files, then you can set a variable using FORTRAN conditional compilation sentinel:
program pippo
implicit none
logical :: use_openmp = .false.
!$ use_openmp = .true.
!$ print *, "OpenMP program"
if( .not. use_openmp) then
print *, "Non-OpenMP program"
end if
end program pippo