[flang-dev] Flang + CMake

[Michael Kruse via flang-dev]

If I understand correctly, [3] only renamed __FLANG_MAJOR__ to
FLANG_VERSION_MAJOR_STRING in its own internal configuration. This
name is not discoverable by invoking flang. Instead, flang's
preprocessor defines `__flang__`, `__flang_major__`, `__flang_minor__`
and `__flang_patchlevel__` which have not changed in [3].

Maybe CMake checks the upper case versions because classic flang sets
them? In any case, CMake should work with any unknown fortran
compiler, so you could file a bug with CMake.

Michael


Am Mi., 30. Juni 2021 um 11:36 Uhr schrieb Andrzej Warzynski via
flang-dev <flang-dev at lists.llvm.org>:
>
> [Flang = llvm-project/flang]
>
> Hello,
>
> I have been trying to use Flang with CMake and this integration seems to
> be dysfunctional right now. Has anyone been using/testing it?
>
> *Minimal reproducer*
> ```CMakeLists.txt
> project(hello_fortran Fortran)
> add_executable(hello hello.f90)
> ```
>
> *How to test*
> ```bash
>
> mkdir build && cd build
> cmake -G Ninja -DCMAKE_Fortran_COMPILER=<path-to-flang-or-gfortran> ../
> ```
>
> *The error*
> My minimal reproducer works fine with `gfortran`, but it fails with `flang`:
> ```bash
> CMake Error: Error required internal CMake variable not set, cmake may
> not be built correctly.
> Missing variable is:
> CMAKE_Fortran_PREPROCESS_SOURCE
> ```
>
> If I understand correctly, CMake fails to identify Flang because the
> driver does not define the following preprocessor variables currently
> expected by CMake [1]:
> * __FLANG
> * __FLANG_MAJOR__
> * __FLANG_MINOR__
> These macros were first discussed and added in [2], but then deleted in
> [3] in order to keep Flang consistent with the rest of LLVM.
>
> This seems to have been broken for a while, so I suspect that this has
> not been required by anyone recently?
>
> Thank you,
> -Andrzej
>
> [1]
> https://gitlab.kitware.com/cmake/cmake/-/blob/master/Modules/CMakeFortranCompilerId.F.in#L143-147
> [2] https://reviews.llvm.org/D84334
> [3] https://reviews.llvm.org/D94422
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