fortran - ISO_FORTRAN_ENV or -fdefault-real-8 to promote reals to double precision

Question

I've always been using the -fdefault-real-8 option of gfortran to automatically promote every single REAL declared anywhere in the program to double precision, along with any constant, e.g. 1.23. If I ever wanted to switch back to single precision, I only had to remove that option and recompile, without changing a single character in the source code.

At a point I started using ISO_FORTRAN_ENV module, since it allows me to use constants like INPUT|OUTPUT|ERROR_UNIT, as well as IOSTAT_END and IOSTAT_EOR and others (which seemed to be a good and easy move in the direction of portability, am I wrong?). From then on, I've been seeing and ignoring the following warning

Warning: Use of the NUMERIC_STORAGE_SIZE named constant from intrinsic module ISO_FORTRAN_ENV at (1) is incompatible with option -fdefault-real-8

since such incompatibility seems to have no effect so far.

Now I'd like to get rid of this warning if it is possible and worth it.

If I correctly understood, to avoid this warning I should give up on -fdefault-real-8 option and change every REAL to REAL(real64) and/or to REAL(dp) (provided that, in the latter case, the statement USE, INTRINSIC :: ISO_FORTRAN_ENV, dp => real64 is put in that unit), which is not a difficult task for sed or vim.

Nevertheless, it seems to me that this change wouldn't be the same as using -fdefault-real-8 option, since all constants would stay single precision as long as I don't add d0 to them. Assumed the -fdefault-real-8 option is removed and ISO_FORTRAN_ENV is used anywhere, is there any way to make any constant across the program behave as each had d0 suffix? Whether or not this is possible, have I correctly extrapolated that I can put the following lines in a single module which is used by all others program units, each of which can then use dp as kind type parameter?

USE, INTRINSIC :: ISO_FORTRAN_ENV
INTEGER, PARAMETER :: dp = real64

I would prefer this way since I could switch to real32 or real128 or whatever by changing only that line.

Answer 1

If you just want to silence the warning and you do not care about the implications -fdefault-real-8 has on storage association and some Fortran standard requirements, just do not import NUMERIC_STORAGE_SIZE from the module. For example,

USE, INTRINSIC :: ISO_FORTRAN_ENV, only: INPUT_UNIT,OUTPUT_UNIT,ERROR_UNIT