Request: llvm: add flang Fortran compiler

[scivision]

Output of brew config

N/A

Output of brew doctor

N/A

Description of issue

Flang the LLVM Fortran compiler has matured in LLVM 19 to where Flang can now compile most Fortran code, even for major projects. I would like to have "flang" be installed as part of "llvm" or at least make "flang" its own formula.

What I've been doing is to build Flang with a script, in case this is helpful to see what options are used.

reference, flang has progressed beyond thisN

[alebcay]

Some related work is currently happening in Homebrew/homebrew-core#191829.

[mathomp4]

So, I saw today that flang is now in Brew and decided to try it out. Unfortunately, I think I'm missing something:

❯ which flang-newv
/Users/mathomp4/.homebrew/brew/bin/flang-new
❯ cat test.F90
program main

end
❯ flang-new test.F90
Undefined symbols for architecture arm64:
  "std::exception::~exception()", referenced from:
      std::bad_variant_access::~bad_variant_access() in libFortranRuntime.a[19](edit-output.cpp.o)
  "std::terminate()", referenced from:
      ___clang_call_terminate in libFortranRuntime.a[5](assign.cpp.o)
  "typeinfo for std::bad_variant_access", referenced from:
      std::__1::__throw_bad_variant_access[abi:ue170006]() in libFortranRuntime.a[19](edit-output.cpp.o)
  "vtable for std::bad_variant_access", referenced from:
      std::__1::__throw_bad_variant_access[abi:ue170006]() in libFortranRuntime.a[19](edit-output.cpp.o)
   NOTE: a missing vtable usually means the first non-inline virtual member function has no definition.
  "___cxa_allocate_exception", referenced from:
      std::__1::__throw_bad_variant_access[abi:ue170006]() in libFortranRuntime.a[19](edit-output.cpp.o)
  "___cxa_begin_catch", referenced from:
      ___clang_call_terminate in libFortranRuntime.a[5](assign.cpp.o)
  "___cxa_throw", referenced from:
      std::__1::__throw_bad_variant_access[abi:ue170006]() in libFortranRuntime.a[19](edit-output.cpp.o)
  "___gxx_personality_v0", referenced from:
      /Users/mathomp4/.homebrew/brew/Cellar/flang/19.1.0/libexec/lib/libFortranRuntime.a[52](stop.cpp.o)
ld: symbol(s) not found for architecture arm64
flang-new: error: linker command failed with exit code 1 (use -v to see invocation)

I've tried this with both CC as Apple clang and with CC as brew llvm clang. Staring at the brew info llvm output I also tried:

flang-new -L/Users/mathomp4/.homebrew/brew/opt/llvm/lib -I/Users/mathomp4/.homebrew/brew/opt/llvm/include test.F90

flang-new -L/Users/mathomp4/.homebrew/brew/opt/llvm/lib/c++ -L/Users/mathomp4/.homebrew/brew/opt/llvm/lib/unwind -lunwind test.F90

flang-new -L/Users/mathomp4/.homebrew/brew/opt/llvm/lib/unwind -lunwind test.F90

But they all fail. So I'm obviously missing something.

NOTE: One issue could be that on this machine I'm not allowed to install to /opt. But that doesn't stop brew llvm clang from working:

❯ /Users/mathomp4/.homebrew/brew/opt/llvm/bin/clang test.c
❯ ./a.out
Hello, World!%

[scivision]

CMake isn't able to configure a trivial Fortran project with Homebrew Flang unless

LDFLAGS=-lc++ cmake -Bbuild

due to similar errors when detecting the Flang compiler

project(hello LANGUAGES Fortran)

Fails without the above LDFLAGS

[cho-m]

We could consider switching to default of LLVM_ENABLE_EH=OFF if that is what most distros/repositories do.

This will impact users who want to make use of C++ exceptions propagating through LLVM code, but I think majority of major repositories don't enable it anyways. Verified this for:

• Arch Linux
• Debian
• Fedora (aarch64 rpm)
• MacPorts
• NixOS
• Spack when built with +flang

EH is enabled for:

• Fedora (x86_64 rpm)
• Gentoo - I don't think they support Flang

[carlocab]

Is this -lc++ something that the Flang Homebrew package should automatically do as part of its self-defined toolchain?

This should be handled in Homebrew/homebrew-core#192611, which should hopefully merge some time this week.

[carlocab]

Homebrew/homebrew-core#192611 has been merged. You may need to brew update && brew reinstall flang to pick up the fix.

[mathomp4]

Ooh! First:

==> Caveats
Homebrew LLVM is built with LLVM_ENABLE_EH=ON, so binaries built by `flang-new`
require linkage to the C++ standard library. `flang-new` is configured to do this
automatically.

Nice.

And on my home mac:

❯ flang-new test.F90
❯ echo $?
0

So it works okay in the default /opt/homebrew area. I'll check my work laptop (where it's a non-standard install) on Monday.

[scivision]

Yes, #192611 fixes Flang for me. I no longer have to set LDFLAGS at all.

All I do now to use Flang is:

export FC=flang-new

Flang works to link Fortran-C-C++ binaries with AppleClang, Clang, and GCC.