[llvm-dev] [cfe-dev] Filesystem has Landed in Libc++

Thu Aug 9 10:53:45 PDT 2018

This isn't about C++20 and the <chrono> changes. I have no doubt any ABI
breaking changes W.R.T. that will be addressed.

During my work finishing up filesystem, I discovered/filed a number of
issues with ABI breaking concerns, in particular around directory_entry,
but perhaps other places as well (recursive_directory_iterator, various
calls in [fs.op.funcs]). These issues have not yet been addressed in the
standard, and these spec should be addressed before we commit to an ABI.

Note that we ofter put things into namespace std before they're considered
ABI stable, but up until now that has been limited to components specified
by a standard still in flight. Also note that libstdc++ also hasn't
committed to ABI stability for <filesystem> yet. This is the reason we put
the symbols in a separate static library.

/Eric

On Tue, Aug 7, 2018 at 2:15 PM Louis Dionne <ldionne at apple.com> wrote:

> Hi,
>
> My current understanding of the problem (based on
> https://reviews.llvm.org/D49774) is that we have a type, file_time_type,
> which is part of the ABI and is currently defined as
> std::chrono::time_point<_FileSystemClock>, where _FileSystemClock is an
> internal type represented using a __int128_t. However, C++20 will add a
> type called file_clock and redefine file_time_type to be
> std::chrono::time_point<std::chrono::file_clock> instead, which is an ABI
> break.
>
> Is this correct, and is this the only concern we have with respect to the
> ABI stability of Filesystem as currently included in the release branch for
> LLVM 7?
>
> If that is correct, then we can either
> (1) define file_time_type in a way that is forward-ABI-compatible with the
> definition that will be used in C++20 and patch LLVM 7 accordingly
> (2) do nothing and live with an ABI break in C++20
> (3) revert r338093 (and perhaps some patches that depend on it) in the
> LLVM 7 branch, and keep shipping Filesystem as being experimental
>
> (1) is ideal, but we have to confirm that my understanding is correct AND
> agree on a fix, soon.
> (2) is a no-go.
> (3) is the safest and simplest option.
>
> If we can't get a solution for (1) going very soon, I would like to
> request that we implement (3) and remove Filesystem from the
> non-experimental namespace in the LLVM 7 branch. This is unfortunate, but
> we're currently running towards (2), which is desirable for nobody.
>
> I suggest a deadline of Friday August 10th to have settled on a solution
> for (1) so as to give us time to implement it and review it in time for RC
> 2, on August 22nd.
>
> Louis
>
>
> > On Aug 2, 2018, at 03:46, Hans Wennborg <hans at chromium.org> wrote:
> >
> > I only saw this now after the llvm 7 branch was cut.
> >
> > It would be good to get this all figured out before the release :-)
> >
> > On Mon, Jul 30, 2018 at 10:19 PM, Louis Dionne via llvm-dev
> > <llvm-dev at lists.llvm.org> wrote:
> >> FWIW, I’d like for us to come to an agreement before the branch for
> LLVM 7.0
> >> is cut. How do others feel about this? Am I wrong when I claim that
> shipping
> >> an ABI-unstable feature in the std:: namespace is a deviation from
> normal
> >> practice? Am I overcautious when I say it’s asking for trouble?
> >>
> >> Eric, I know you’re busy and may not have time to do the work so I’m
> totally
> >> willing to chime in, but I’d like to have your thoughts on my objection
> >> first.
> >>
> >> Cheers,
> >> Louis
> >>
> >>
> >> On Jul 27, 2018, at 17:02, Louis Dionne via cfe-dev <
> cfe-dev at lists.llvm.org>
> >> wrote:
> >>
> >> Eric,
> >>
> >> I’m curious to know what the concerns are w.r.t. providing ABI
> stability for
> >> filesystem right now. What do you envision may require changing the ABI
> in
> >> the future?
> >>
> >> I feel like taking filesystem out of experimental/ without providing the
> >> usual guarantees provided by libc++ for non-experimental code may not
> be a
> >> good idea, as we’ll be pretending that we support filesystem when we
> really
> >> only half support it. In other words, I think the number of people that
> will
> >> start using filesystem while _consciously_ knowing that it is
> ABI-unstable
> >> (and what that means) is quite small, and that is making our users a
> >> disservice.
> >>
> >> Would it be possible to instead ship the parts we’re not quite sure we
> can
> >> keep ABI stable in the headers (with `_LIBCPP_HIDE_FROM_ABI`) for the
> time
> >> being, and lower them to the dylib eventually as things stabilize? This
> >> would allow us to ship filesystem with LLVM 7.0 without any compromise
> on
> >> the guarantees we make our users.
> >>
> >> I’m curious to know what you think of this suggestion.
> >> Louis
> >>
> >> On Jul 27, 2018, at 00:20, Eric Fiselier via cfe-dev
> >> <cfe-dev at lists.llvm.org> wrote:
> >>
> >> Hi All,
> >>
> >> I recently committed <filesystem> to trunk. I wanted to bring attention
> to
> >> some quirks it currently has.
> >>
> >> First, it's been put in a separate library, libc++fs, for now. Users are
> >> responsible for linking the library when they use filesystem.
> >>
> >> Second, it should still not be considered ABI stable. Vendors should be
> >> aware of this before shipping it. Hopefully all the standard and
> >> implementation bugs can be resolved by the next release, and we can
> move it
> >> into the main dylib.
> >>
> >> Third, libc++experimental no longer contains the symbols for
> >> <experimental/filesystem>, which is really just <filesystem> is
> disguise. If
> >> you've been using <experimental/filesystem> you now need to link
> libc++fs
> >> instead.
> >>
> >> Fourth, `<filesystem>` is technically available in C++11 and later. The
> >> implementation lives in the std::__fs::filesystem namespace, which is
> marked
> >> "inline" in C++17 but not before. We should consider documenting this
> as an
> >> extension to its use w/o C++17.
> >>
> >> Happy coding,
> >>
> >> /Eric
> >>
> >> [1]
> >>
> http://libcxx.llvm.org/docs/UsingLibcxx.html#using-filesystem-and-libc-fs
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> >>
> >>
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> >>
> >>
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