Fix building ponyc on DragonFly BSD

[mneumann]

• LLVM on DragonFly does not ship with libatomic; it has to be installed
  manually via pkg ins cxx_atomics.

• Note that ponyc has to be build with gcc as stdatomic.h from
  cxx_atomics fails to work with LLVM (in a prior attempt I tried to
  compile ponyc with LLVM).

• Pass -latomic to the linker when targeting DragonFly. Also add
  -L/usr/local/cxx_atomics as search path.

• Until a new version of google/benchmark is released, you'll need
  the patch from this PR and apply it to gbenchmark to build
  ponyc.

Superseeds: #3654 (which on it's own, never worked)

[SeanTAllen]

Does you gcc comment mean it wouldn't build with clang on dragonfly? Is there clang for dragonfly?

[mneumann]

@SeanTAllen Yes, it does not build with clang. If you try to (by setting env CC=clang10 CXX=clang++10), you'll get tons of these error messages:

/home/mneumann/Dev/ponyc/src/libponyrt/actor/actor.c:32:19: error: address argument to atomic operation must be a pointer to a
      trivially-copyable type ('_Atomic(unsigned char) *' invalid)
  uint8_t flags = atomic_load_explicit(&actor->flags, memory_order_relaxed);
                  ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/local/cxx_atomics/stdatomic.h:138:5: note: expanded from macro 'atomic_load_explicit'
    __atomic_load (__atomic_load_ptr, &__atomic_load_tmp, (MO));        \
    ^              ~~~~~~~~~~~~~~~~~
/home/mneumann/Dev/ponyc/src/libponyrt/actor/actor.c:32:11: error: initializing 'uint8_t' (aka 'unsigned char') with an expression of
      incompatible type 'typeof (*__atomic_load_ptr)' (aka '_Atomic(unsigned char)')
  uint8_t flags = atomic_load_explicit(&actor->flags, memory_order_relaxed);
          ^       ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

This seems to be an issue with clang itself. Does this work on FreeBSD?

[mneumann]

@SeanTAllen It actually compiles with LLVM/clang, but not with the cxx_atomics package. The problem is that the clang/llvm port on DragonFly does not ship with the libatomic.a and stdatomic.h files (FreeBSD does). Someone on dragonflybsd #irc then suggested to use the cxx_atomics library instead. The stdatomic.h file of cxx_atomics is from the GCC project ("This file is part of GCC"), and it does not work with LLVM. LLVM itself ships with it's own version of that header file, just not on DragonFly.

I once got it to compile with LLVM (https://github.com/mneumann/ponyc/tree/dragonfly-fix-compile), but that was rather a hack.
IIRC clang automatically added "-latomic" when linking, even though, it did not use any functions from libatomic (on x86). GCC on the other hand requires -latomic to be specified explicitly.

I will ask the maintainer of the cxx_atomics port, if he can also include the LLVM version of stdatomic.h, which would solve the problem.

[SeanTAllen]

Thanks @mneumann.

[ponylang-main]

Hi @mneumann,

The changelog - fixed label was added to this pull request; all PRs with a changelog label need to have release notes included as part of the PR. If you haven't added release notes already, please do.

Release notes are added by creating a uniquely named file in the .release-notes directory. We suggest you call the file 3676.md to match the number of this pull request.

The basic format of the release notes (using markdown) should be:

## Title

End user description of changes, why it's important,
problems it solves etc.

If a breaking change, make sure to include 1 or more
examples what code would look like prior to this change
and how to update it to work after this change.

Thanks.

[SeanTAllen]

@mneumann if you have time to add release notes to this, that would be great. otherwise i will merge later this week and add release notes myself.

[SeanTAllen]

Release notes added.

[SeanTAllen]

Thanks @mneumann