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

[Chandler Carruth via llvm-dev]

On Thu, Jan 17, 2019 at 1:49 AM Jonas Toth via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> +1 for keeping up to date :)
>
> Should the policy say something about old code as well?
>
> Do we consider it as good/reasonable to modernize our code once the new
> standards are allowed?
> I am thinking of clang-tidy modernization as an approach to modernize
> automatically and reduce manual burden.
> In general we aim for a consistent style in the code-base and a view words
> regarding this issue would be interesting in my opinion.
>
I think the coding standards used with new code should be a very separate
discussion. While the motivation here is about moving to C++14 and/or
C++17, what is actually being discussed is just the host toolchain, as that
is the part that requires tooling and other mechanical things we need to
get right along side any policy. (It also has much more impact on the
library *users* IMO, as opposed to primarily impacting LLVM *developers*.
While these overlap, they're not the same.)

I'd have a separate discussion about establishing coding standards and
actually updating the language standard when we have toolchains in place
that support it.


> Best, Jonas
> Am 17.01.19 um 00:35 schrieb JF Bastien via llvm-dev:
>
> 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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