[llvm-dev] Compiler support in libc++

[Roman Lebedev via llvm-dev]

On Mon, Mar 8, 2021 at 9:59 PM Reid Kleckner via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> I think it's reasonable to raise the compiler version floor for libc++, but I think I would like to see a more relaxed policy with respect to clang. Maybe the last two releases of clang, so that a user of ToT libc++ with stable clang doesn't have to rush to upgrade clang as soon as it is released. If you support the last two releases, the user always has six months of lead time before updating, and libc++ never supports a compiler older than a year.
>
> I'll also point out that, I see a lot of support on this thread, but I see a lot of developer representation, and not much user representation. I have no idea how to effectively survey users of libc++, though.
+1.
>From user POV, supporting only the last two stable clang releases
is *the smallest reasonable guarantee*.


Roman

> Lastly, from Chromium's PoV, Chromium has an ancient NaCl toolchain, and we believe we may be using ToT libc++ with it. We have other reasons (C++17 for one) to want to either remove or update this compiler, so please don't consider this a blocker for libc++. I only mention it to show that users do sometimes inadvertently develop dependencies on old compilers.
>
> On Mon, Mar 1, 2021 at 9:41 AM Louis Dionne via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>>
>> 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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