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

Thu Apr 2 10:25:33 PDT 2020

I just pushed f5c1d4409e013f and updated the Chromium bug report. Let's continue the discussion at https://bugs.chromium.org/p/chromium/issues/detail?id=1067216 <https://bugs.chromium.org/p/chromium/issues/detail?id=1067216> to centralize things.

Louis

> On Apr 2, 2020, at 13:07, Louis Dionne via libcxx-commits <libcxx-commits at lists.llvm.org> wrote:
> 
> 
> 
>> On Apr 2, 2020, at 13:04, Hans Wennborg <hans at chromium.org> wrote:
>> 
>> 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.
> 
> No, it's not that. I know pretty much exactly what the issue is (I've repro'd), and I'm working on a patch. Once the dust has settled, we'll have to work together to get your libc++ build standardized, though, because LIBCXX_CXX_ABI_SYSTEM is not a publicly tweakable variable for libc++. I think I know what underlying issue you're working around and I think I know the proper fix, which wouldn't require building libc++ at all (since that's not really what you want anyway IIUC).
> 
> Louis
> 
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