Genode and clang

[Piotr Tworek]

Hello Genodians,

A couple of months ago I started wondering how hard would it be to add
Genode support to clang. Since I nowadays have bit of extra free time
on my hands I decided to give this a try. As one may guess adding
support for a new platform to a complex codebase like llvm/clang is a
relatively big undertaking so I decided that for the first stage I want
to limit the scope of the work a bit.

1. Clang running on already supported "host" platform (in my case
Linux) to compile code for Genode on x86_64, x86_32, arm_v7a and
arm_v8a architectures. No support for running clang and all the llvm
utils on Genode itself.
2. Try to use as many llvm tools as possible (llvm-ar instead of GNU
ar, llvm-objcopy instead of GNU objcopy, etc), but don't try to replace
GCC toolchain completely if it requires major effort (ex clang
integrated as instead of GNU as, ld.lld instead of ld.bfd, etc).
3. Use libstdc++, libsupc++, libgcc instead of respective llvm
replacements.

With those goals I started hacking and fast forwards those few months
I've mostly achieved my goal. I currently have Clang 11.0.0 port able
to compile working binaries for all the target architectures when using
base-hw and base-linux. Obviously this did require a bunch of changes
in Genode codebase itself, but most issues found were relatively simple
and generally not specific to clang itself. For example clang is much
better at spotting unused variables in the code, see the following PR:
https://github.com/genodelabs/genode/pull/3938

When using clang I also managed to stumble upon some code relying on
undefined behaviour, which happens to produce different results in
clang than it does in GCC, ex:
https://github.com/genodelabs/genode/pull/3941

Overall the number of problems encountered was lower than I expected,
which I think puts Genode codebase in very positive light.

As one may easily guess there is still a lot of code Genode project
consists of which I haven't even tried building, yet.

The reaason I'm writing this email is to ask if Genode project would be
potentially interested in having clang support in the main repo? I know
that officialy supporting another compiler might not be an interesting
prospect for the project and I won't mind if the answer is no. Its
enough for me if generic fixes I make get merged :). If there is an
interest in clang support however, I'll be happy to spend a few
evenings cleaning up my work and making a few more PRs out of it. If
needed I can also push my remaing clang support patches in the
existing, slightly chaotic form, to some branch so Genode developers
can see how much work having clang support in Genode would entail.

/ptw