[libcxx-commits] [libcxx] 61e8973 - [libc++] Simplify the configuration of the C++ ABI library

[Hans Wennborg via libcxx-commits]

On Thu, Apr 2, 2020 at 7:01 PM Louis Dionne <ldionne at apple.com> wrote:
>
>
>
> > On Apr 2, 2020, at 12:54, Hans Wennborg <hans at chromium.org> wrote:
> >
> > On Thu, Apr 2, 2020 at 8:21 AM Louis Dionne via libcxx-commits
> > <libcxx-commits at lists.llvm.org> wrote:
> >>
> >>
> >> Author: Louis Dionne
> >> Date: 2020-04-02T02:21:15-04:00
> >> New Revision: 61e89737c5daf7b65341936f88f68efb680cdcd4
> >>
> >> URL: https://github.com/llvm/llvm-project/commit/61e89737c5daf7b65341936f88f68efb680cdcd4
> >> DIFF: https://github.com/llvm/llvm-project/commit/61e89737c5daf7b65341936f88f68efb680cdcd4.diff
> >>
> >> LOG: [libc++] Simplify the configuration of the C++ ABI library
> >>
> >> This commit removes support for building against the system libc++abi,
> >> which was supported on Apple platforms. This is basically never what we
> >> want to do, since libc++ and libc++abi are coupled and building a trunk
> >> libc++ against an older libc++abi can lead to incompatibilities (and
> >> good luck debugging them!). It might have made some sense to support
> >> that when the monorepo did not exist, however I don't think this is
> >> anything but a footgun nowadays.
> >>
> >> Furthermore, based on the newly-made assumption that we're building
> >> against the monorepo libc++abi, we can simplify the search path logic
> >> for finding libc++abi.
> >>
> >> This area of our build system has a lot of technical debt accumulated,
> >> and it's surprisingly difficult to change. We've tried different things
> >> and failed several times in the past. I did test this change on our
> >> Docker image for the build bots and on Apple platforms, however it is
> >> possible that this breaks some unknown configuration, in which case it
> >> should be fine to revert this (so we can try again!).
> >
> > Well hello :-)
> >
> > This broke Chromium's packaging of Clang on Mac (see https://crbug.com/1067216).
> >
> > We build libc++ as part of that process, not because we ship it with
> > the compiler, but as part of the bootstrapping process: otherwise the
> > just-built Clang doesn't find the c++ headers. Until now we've been
> > building with -DLIBCXX_CXX_ABI=libcxxabi -DLIBCXX_CXX_ABI_SYSTEM=1
> > however with your patch that stopped working.
> >
> > I tried adding libcxxabi to -DLLVM_ENABLE_PROJECTS and dropping those
> > two flags, but now building libcxxabi failed for unclear reasons
> > ([1]).
> >
> > Would it be okay to revert your patch until we've figured this out?
>
> We've already applied a couple of patches on top and fixed other failing bots -- we're farther than we've ever been. Any chance of catching you on IM before we revert? I'm trying to repro your failure and I think I'll be able to submit a patch soon (max ~1h).

I just realized the failure I'm seeing is maybe
https://bugs.llvm.org/show_bug.cgi?id=42725

The clang used on that machine is identified by cmake as AppleClang
9.0.0.9000037. I'm not entirely sure how to upgrade that, so if you
have other ideas for how to fix, that would be great.