cmake: link to MPI::MPI_Fortran using INTERFACE

[caitlinross]

combining fortran and cpp files in the same target was causing fortran flags to be applied to cpp files, which could cause build failures with clang. Fixes issue #3253

[vicentebolea]

@caitlinross thanks for the bugfix I think that this should go to our next patch release. I want to understand better this, I am trying to reproduce this locally but I cannot seem to hit the error, in which env do we get this build error?

[caitlinross]

I run into the issue on mac with AppleClang 14.0.3 and gfortran 13.1.0. I just learned that AppleClang has differences from normal clang, so maybe it's not generally an issue with clang?

[vicentebolea]

@caitlinross I see, do you think that a generator expression at the top cmake_fortran_flag that only applies the flag when is language:Fortran will do the the trick? That would be more concise.

[caitlinross]

I don't think so. So I tried

set(CMAKE_Fortran_FLAGS "${CMAKE_Fortran_FLAGS} $<$<COMPILE_LANGUAGE:Fortran>:-fallow-argument-mismatch>")

I get the following error:

CMake Error:
   Running

    '/opt/homebrew/bin/ninja' '-C' '/Users/caitlin.ross/dev/adios/fortran-fix/build' '-t' 'recompact'

   failed with:

    ninja: error: build.ninja:5867: bad $-escape (literal $ must be written as $$)
     FLAGS = $<$<COMPILE_LANGUAGE:Fortran>: -fallow-argument-mismatch> -O3 ...
             ^ near here

I don't think it's a mistake in the generator expression itself, but maybe I'm doing something else wrong there?

But I did a bit more digging and realized it's only happening when MPI is enabled. So if not building with MPI, it seems that -fallow-argument-mismatch is correctly only added to the f90 files and none of the cpp files. But when MPI is turned on, that flag is for some reason being added to the cpp files (but only adios2_f2c_adios_mpi.cpp and adios2_f2c_io_mpi.cpp, not the non-mpi f2c files). I have no idea why that flag is being added only for those 2 files.

[caitlinross]

It appears that flag is being added to those 2 cpp files from somewhere else. I commented out the set(CMAKE_Fortran_FLAGS "${CMAKE_Fortran_FLAGS} -fallow-argument-mismatch) line and I am still getting the error about the unknown argument when building those files.

[vicentebolea]

It appears that flag is being added to those 2 cpp files from somewhere else. I commented out the set(CMAKE_Fortran_FLAGS "${CMAKE_Fortran_FLAGS} -fallow-argument-mismatch) line and I am still getting the error about the unknown argument when building those files.

This is very odd since we only add this flag once at this CMakeLists.txt file. I think that the culcrip is when we link the library with the MPI::MPI_Fortran, I believe that this will inject the CMAKE_Fortran_FLAGS.

How about this approach. We keep the same targets but we do not set CMake_Fortran_FLAGS, we instead set that flag for all the targets in the file using the property: https://cmake.org/cmake/help/latest/prop_tgt/COMPILE_OPTIONS.html#prop_tgt:COMPILE_OPTIONS

We can use a generator expression to set the property only for Fortran files. This will be more concised. I wish I could try this but I cannot reproduce this in my setup. I tried different combinations and no luck.

[caitlinross]

So I tried a bunch of things that didn't work and finally found the really simple solution that was overlooked. In target_link_libraries MPI::MPI_Fortran should be INTERFACE not PUBLIC.

[vicentebolea]

Great! Thanks for getting to the bottom of this 👍