[libcxx-dev] [llvm-dev] Shipping custom libc++ on MacOS

[Louis Dionne via libcxx-dev]

> On Aug 6, 2021, at 09:34, Ken Cunningham <ken.cunningham.webuse at gmail.com> wrote:
> 
> 
> 
> On Thu, Jul 22, 2021 at 2:08 PM Louis Dionne via libcxx-dev <libcxx-dev at lists.llvm.org <mailto:libcxx-dev at lists.llvm.org>> wrote:
> 
> 
>> On Jul 10, 2021, at 09:33, Isuru Fernando via libcxx-dev <libcxx-dev at lists.llvm.org <mailto:libcxx-dev at lists.llvm.org>> wrote:
>> 
>> Hi Ken,
>> 
>> I'm in the same situation as you in conda package manager.
>> Note that with libcxx 12, https://reviews.llvm.org/D91517 <https://reviews.llvm.org/D91517> was merged where
>> codecvt with char8_t is added in the middle of a structure instead of at the end.
>> This means that you have to be careful that all applications link to the custom libc++.
>> 
>> For libcxx developers,
>> While we are discussing how to avoid ODR violations, is it possible to move the two lines at
>> https://github.com/llvm/llvm-project/blob/llvmorg-12.0.1/libcxx/src/locale.cpp#L210-L211  <https://github.com/llvm/llvm-project/blob/llvmorg-12.0.1/libcxx/src/locale.cpp#L210-L211>
>> to the bottom to avoid segfaults resulting from this?
> 
> Generally speaking, we make the assumption that there's a single copy of libc++ in use inside a final linked image. Or if you want to use libc++ as an implementation detail and link it statically, ensure it doesn't export any symbols and has absolutely no ABI surface outside of the executable where it's being used. Chrome does that.
> 
> Anything else is very brittle, and I'm reluctant to add workarounds that make it look like it works, because you may run into other issues that we won't be able to patch so easily.
> 
> Louis
> 
> 
> I had thought libc++ was more immune to this kind of problem given Apple's usual support to deploy on older systems.

It can't really be immune to those problems since they are byproducts of how linking works at a pretty fundamental level. Back-deploying to older platforms works nicely as long as you use the libc++ shipped with the system (we make sure of that), or that you "embed" libc++ into your application by linking it as a static archive and disregard the system libc++ altogether (Chrome does that). When you do something in-between, that's when things start failing.

> 
> If the libc++ structures have changed and are now incompatible with previous versions of libc++, would that now mean that an Apple build system using the new libc++ structures can no longer target a deployment target with an older libc++ (any system prior to the change)?

There was no ABI breaking change to the libc++ structures in https://reviews.llvm.org/D91517 <https://reviews.llvm.org/D91517>. We still support the same back-deployment targets as before.

Louis

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