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

[Louis Dionne via libcxx-commits]

> 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).

Louis

> 
> Thanks,
> Hans
> 
> 1. https://logs.chromium.org/logs/chromium/buildbucket/cr-buildbucket.appspot.com/8884121514869999920/+/steps/package_clang/0/stdout