What are the prospects for a Flang / F18 port to Windows

[matthew-brett]

I see that the last commit here was a few years ago - can I ask what the prospects are of a working Windows port of any version of Flang, in the nearish future? What would it take to make it happen? And where is the best place to look for news?

[isuruf]

There are 2 compilers.

1. classic flang (based off of PGI)
2. llvm flang (f18)

This repository is for classic flang and there's some movement recently to add windows support. See for eg: flang-compiler#464

LLVM flang is not ready for code generation even for Linux although windows support is there for what is already there. So once LLVM flang is ready for Linux it will be for Windows.

[matthew-brett]

Can you comment on the future of Classic Flang? I see from https://github.com/flang-compiler/flang that it "continues to be maintained, but the plan is to replace Classic Flang with the new Flang in the future." That seems to imply it may not be a solid basis for future planning. Is that right? How far away is the Windows port?

Do you have any feeling for how long it will be before LLVM Flang is ready for tentative production use?

[isuruf]

That seems to imply it may not be a solid basis for future planning. Is that right? How far away is the Windows port?

I think it'll not go away anytime soon. It's fine to use it as it is IMO. It'll take several months for the Windows port.

Do you have any feeling for how long it will be before LLVM Flang is ready for tentative production use?

No idea about that.

[h-vetinari]

@matthew-brett: Can you comment on the future of Classic Flang?

I haven't seen something official, but one of the contributors was very clear:

xoviat: Classic flang will be killed immediately after a windows version of f18 (new flang) is available.

@matthew-brett: Do you have any feeling for how long it will be before LLVM Flang is ready for tentative production use?

Isuru knows more than me on this, but from watching the development from afar, I'd say at least another year or two - though they did figure out some long-standing driver integration problems recently (docs - note that they haven't updated their certificate correctly for the new domain, alternatively on github). Their development tracker is here

[matthew-brett]

@isuruf @h-vetinari - any update on progress towards Windows Flang? An ETA for anything useable? We find ourselves really needing a Windows Fortran compiler, for the new Scipy Meson builds ...

[isuruf]

LLVM flang is still not ready. It can't produce object code yet. Good news though that they have a windows buildbot.
Classic flang is coming along slowly. Progress at https://github.com/flang-compiler/flang/pulls?q=is%3Apr+windows+is%3Aclosed
I'm hoping at the end of the year we can have something useful upstream.
You can try to use the flang=11 from conda-forge in the meantime, which came from @xoviat's fork.

[h-vetinari]

LLVM is starting to get there (e.g. cf. e.g. here, which will then still need upstreaming); with any luck, LLVM 14 (in ~6 months) might be workable already.

[matthew-brett]

Thanks both - pinging @rgommers because we've been talking about Scipy Windows builds.

[rgommers]

Thanks for the ping, and great to hear about the progress. Testing the conda-forge compiler with SciPy sounds useful.

because we've been talking about Scipy Windows builds.

We should summarize the conversation and the host of present and future options somewhere soon (on a SciPy issue seems best).

[isuruf]

You can follow the progress at flang-compiler#1154 (comment)

[matthew-brett]

Nice! Thanks.