[libcxx-dev] Compiler support in libc++

[Louis Dionne via libcxx-dev]

Dear LLVM community,

I’ve wanted to address the topic of which compilers are supported by libc++
for a long time. LLVM documents that it supports GCC >= 5, Clang >= 3.5 and
other fairly old compilers. I think this makes a lot of sense for codebases
like LLVM and Clang, since it means you can bootstrap a compiler with your
system compiler in many cases. It’s also fairly easy to enforce that, since
you just have to code in a supported subset of C++.

However, for a library like libc++, things are a bit different. By its very
nature, libc++ needs to rely on a recent compiler in order to implement
most recent library features. Not being able to rely on a recent compiler
leads to problems:

   -

   Adding new features is significantly more complicated because we need to
   implement them conditionally on compiler support, not just on support for a
   C++ Standard. There can also be interactions between what compiler the
   library is built with and what compiler the headers are used with.
   -

   We accumulate technical debt around the code base. Some of these #ifdef
   code paths are not in use anymore, others don’t compile anymore or they
   contain bugs.
   -

   It creates a false sense of support: people think they can use a libc++
   built with e.g. Clang 3.5, but in reality doing so is a terrible idea. The
   library might not contain runtime support for features that will be
   advertised as available by the headers (the char8_t RTTI and the upcoming
   support for <format> come to mind). Those are serious ABI issues that
   you’ll only notice when trying to use the feature.


I think it’s important to stress that the current state of things is that
we don’t *actually* support much older compilers - the documentation claims
we do, but that is misleading. While things may happen to work on older
compilers, I wouldn’t recommend relying on that for anything serious, since
it’s mostly untested.

Furthermore, the actual value of supporting old compilers isn’t obvious.
Indeed, the best way of building libc++ is to bootstrap Clang and then
build libc++ with it, which is easily achieved with the LLVM Runtimes
build. Of course, we also support different shipping mechanisms (including
non-Clang compilers), but in all cases it should be reasonable to expect
that someone building libc++ at the tip is able to do so using a recent
compiler.

For all these reasons, I think we must adjust the official support policy
we currently document. Concretely, the following modified policy solves the
issues I mentioned above and makes it so that the stated support reflects
the reality of what we truly support:

   -

   At any given point in time, libc++ supports back to the latest released
   version of Clang. For example, if the latest major release of Clang is 14,
   libc++ (on main) supports Clang 14. When Clang 15 is released (and libc++
   15 with it), libc++ (on main) is free to assume Clang 15. As a result, any
   released libc++ will always support the previously (and the currently)
   released Clang, with the support window moving as newer Clangs are released.
   -

   We support the latest major release of GCC, as advertised on
   https://gcc.gnu.org/releases.html.
   -

   We support the latest major release of AppleClang.


The above policy is reasonable from libc++’s perspective, and it also
reflects what we test on a regular basis with the CI. Furthermore,
supporting up to the last release instead of requiring a trunk compiler
(like MSVC’s STL and libstdc++) gives vendors with alternate delivery
vehicles approximately 6 months to update their compiler if they want to
jump on the next release of libc++, which I think is an important property
to retain.

This message is both a heads up about the current state of things, an
explanation of where we (the libc++ contributors) want to end up, and an
invitation to have a discussion with the rest of the community.

I propose that we maintain our current level of support for older compilers
(i.e. keep things roughly building) until the next LLVM release, after
which the above policy would become official and libc++ development would
be allowed to assume a compiler as documented above. That would give
approximately 6 months (from now to the next release) for people managing
build bots to migrate to the Runtimes build, and approximately 6 months
(from the next release to the next-next release) for external users to
adjust to this policy if needed.

Thanks,

Louis

P.S.: There is no mention of other compilers besides Clang, AppleClang and
GCC above. That’s because no other compiler is tested on a regular basis,
so the status of support for other compilers is unknown. If you’d like to
add official support for a new compiler, I’ll be happy to help you set up
the required testing.
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