[flang-dev] Flang + CMake

[Romero, Nichols A. via flang-dev]

Andrzej,

Thanks for raising awareness of the program.

I think someone will need to add support for Flang in CMake. I don’t have a good sense of who big of a project that will be. This is what Intel had to do for the new LLVM-based compiler:
https://gitlab.kitware.com/cmake/cmake/-/merge_requests/5594

I am not sure if there is a quick hack for it, the merged request above for the Intel LLVM-based compiler was a good chunk of CMake code.


From: flang-dev <flang-dev-bounces at lists.llvm.org> on behalf of Andrzej Warzynski via flang-dev <flang-dev at lists.llvm.org>
Reply-To: Andrzej Warzynski <andrzej.warzynski at arm.com>
Date: Wednesday, June 30, 2021 at 11:36 AM
To: "flang-dev at lists.llvm.org" <flang-dev at lists.llvm.org>
Subject: [flang-dev] Flang + CMake

[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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