How can I reconfigure deal.IIConfig.cmake?

[Kyle Schwiebert]

I decided to switch from compiling my user codes with clang instead of gcc, and I can successfully get the build to use clang with either

cmake . -DDEAL_II_CXX_COMPILER=clang

or

cmake . -DCMAKE_CXX_COMPILER=clang

but I get a bunch of warnings with like "warning: unknown warning option '-Wno-placement-new' [-Wunknown-warning-option]"

I would like to get the default clang flags that deal.II would have set up on installation. If it is relevant, I am running deal.II on ubuntu 20.04 through WSL1. I installed the back port of 9.4.0 with "sudo apt-get ..." I know that I could just edit the file, but I'm worried this would mess up something unrelated and I think I would benefit from using the flags the deal.ii devs think wise for clang.

Thank you for any insight you can offer.

Regards,

Kyle

[Alexander Kiselyov]

Hi Kyle,

Disclaimer: I'm not an expert in C++, but have some experience in maintaining packages, including ones of C++ software.

First, it's a bad idea to patch *Config.cmake files, they are meant to accurately describe the currently installed software. This is especially important because GCC and Clang ABIs aren't compatible, generally speaking. Bear in mind that C++ doesn't have any formal ABI standard, only best-effort compatibility within a single compiler vendor. I'd really advise against such experiments compared to a one-time deal.II rebuild with clang, even if it takes a couple of hours. You'll likely need to rebuild deal.II dependencies like MPI with clang, again because of potential ABI incompatibilities.

Concerning warnings and other flags, you can enable additional flags (e.g. `-fdiagnostics-color=always`) in CMakeLists.txt of your project using `target_compile_options` function after calling `DEAL_II_SETUP_TARGET`. However, deal.II should select supported flags during its build with the same compiler.

Finally, a practical advice about building deal.II yourself. Obviously it's very convenient to build in parallel (`-j <N>` option to `cmake --build` or `make`), but to be safe you need to have at least 2 GB of free RAM per build process.

Hope that helps.

Best regards,

Alexander

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[Kyle Schwiebert]

Alexander,

Thank you. That makes a lot of sense. I've been thinking it's about time to update to some version of 9.5 anyway and I'm thinking unlike when I first got deal.ii, I might be able to handle building from source like you suggest. I'll give it a go.  If it were possible to apt-get install the newest version of deal.II 9.5.1 but force it to use clang I would probably just do that, but despite looking all day, I have been unable to learn much about how that might be done.

Any other approach will not work as you pointed out. As you suggested, there are issues with the ABI and even if I compile my project with clang it's a big issue trying to link with deal.II which was compiled with gcc.

Regards,

Kyle