[llvm-dev] [RFC] Toolchain update policy (migrating LLVM past C++11)

[Chandler Carruth via llvm-dev]

Strong +1 on the process, I really do think it captures both the desire to
have *some* time input so we don't grow ever more stale, *and* the desire
to upgrade for a reason and with a clear understanding of the cost/benefit
to users.

On Wed, Jan 16, 2019 at 3:35 PM JF Bastien via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hi C++ enthusiasts!
>
> It’s a new year, so let’s try a new approach in getting LLVM’s codebase
> past C++11. Instead of discussing toolchain versions and whether C++14 or
> 17 is best, let’s just focus on one baby step: agreeing on a policy. This
> policy will be used to update our toolchain, hopefully warning people in
> LLVM 8 and actually moving past C++11 for LLVM 9.
>
> Good news! I believe we already have agreement on this policy. I went
> through all the discussions (again) and I think I captured everyone’s
> points of view and concerns. Here are the discussions:
>
>    - LLVM dev meeting 2018 BoF "Migrating to C++14, and beyond!"
>    <http://llvm.org/devmtg/2018-10/talk-abstracts.html#bof3>
>    - A Short Policy Proposal Regarding Host Compilers
>    <http://lists.llvm.org/pipermail/llvm-dev/2018-May/123238.html>
>    - Using C++14 code in LLVM (2018)
>    <http://lists.llvm.org/pipermail/llvm-dev/2018-May/123182.html>
>    - Using C++14 code in LLVM (2017)
>    <http://lists.llvm.org/pipermail/llvm-dev/2017-October/118673.html>
>    - Using C++14 code in LLVM (2016)
>    <http://lists.llvm.org/pipermail/llvm-dev/2016-October/105483.html>
>    - Document and Enforce new Host Compiler Policy
>    <http://llvm.org/D47073>
>    - Require GCC 5.1 and LLVM 3.5 at a minimum <http://llvm.org/D46723>
>
> When replying to this email, please avoid having the same discussions
> again. Please provide references to anything I might have missed. If you’re
> making a new point, say so. And don’t assume ill-will, I’m just trying to
> get us off C++11.
>
> I have a patch for you to review: https://reviews.llvm.org/D56819
>
> Here’s what it currently says our policy should be:
>
> +We intend to require newer toolchains as time goes by. This means LLVM's
> +codebase can use newer versions of C++ as they get standardized.
> Requiring newer
> +toolchains to build LLVM can be painful for those building LLVM, it will
> +therefore only be done through the following process:
> +
> +  * Generally, try to support LLVM and GCC versions from the last 3 years
> at a
> +    minimum. This time-based guideline is not strict: we may support much
> older
> +    compilers, or decide to support fewer ones.
> +
> +  * An RFC is sent to the `llvm-dev mailing list <
> http://lists.llvm.org/mailman/listinfo/llvm-dev>`_
> +
> +    - Detail upsides of the version increase (e.g. allow LLVM to use
> newer C++
> +      language or library features; avoid miscompiles in particular
> compiler
> +      versions, etc).
> +    - Detail downsides on important platforms (e.g. Ubuntu LTS status).
> +
> +  * Once the RFC reaches consensus, update the CMake toolchain version
> checks
> +    and this document. We want to soft-error when developers compile
> LLVM. We
> +    say "soft-error" because the error can be turned into a warning using
> a
> +    CMake flag. This is an important step: LLVM still doesn't have code
> which
> +    requires the new toolchains, but it soon will. If you compile LLVM
> but don't
> +    read the mailing list, we should tell you!
> +
> +  * Ensure that at least one LLVM release has had this soft-error. Not all
> +    developers compile LLVM tip-of-tree. These release-bound developers
> should
> +    also be told about upcoming changes.
> +
> +  * Turn the soft-error into a hard-error after said LLVM release has
> branched.
> +
> +  * Update the :doc:`coding standards<CodingStandards>` to explicitly
> allow the
> +    new features we've now unlocked.
> +
> +  * Start using the new features in LLVM's codebase.
>
> Thanks,
>
> JF
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