[llvm-dev] [RFC] migrating LLVM to C++14

via llvm-dev llvm-dev at lists.llvm.org
Thu Apr 4 01:40:18 PDT 2019

I don't think I made a fuss at the dev meeting, but with Google providing all the cover we could want, it didn't seem crucial. ☺
I'm told Sony's internal build farm will be up to scratch by the end of this week.  Our Windows bots are all on VS2017, and Linux is mostly on Ubuntu 16.04 with only one laggard still in progress.

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Philip Reames via llvm-dev
Sent: Tuesday, April 02, 2019 12:09 PM
To: JF Bastien; via llvm-dev
Subject: Re: [llvm-dev] [RFC] migrating LLVM to C++14

I was the other major objector at the dev meeting.  I can confirm that we've successfully migrated and no longer have any concerns with the proposed toolchain.

On 4/1/19 1:16 PM, JF Bastien via llvm-dev wrote:
Hello folks (except fans of April 1st: this is not a joke),

We discussed migrating past C++11<http://lists.llvm.org/pipermail/llvm-dev/2019-January/129452.html> recently and got consensus. This led us to bump our minimum toolchain requirements<https://reviews.llvm.org/D57264> to a point where C++14 was supported by all compilers. Following our policy<http://llvm.org/docs/DeveloperPolicy.html#toolchain>, I’m now suggesting that we make this official by creating a patch which will:

  *   Update the C++ standard version<http://llvm.org/docs/CodingStandards.html#c-standard-versions> to C++14, allowing the usage of the following language and library features:

     *   Binary literals<https://wg21.link/N3472>:

        *   Without restrictions.

     *   decltype(auto)<https://wg21.link/N3638>, Return type deduction for normal functions:

        *   As with auto<http://llvm.org/docs/CodingStandards.html#use-auto-type-deduction-to-make-code-more-readable>, only where it makes code more readable.

     *   Initialized/Generalized lambda captures (init-capture)<https://wg21.link/N3648>:

        *   Without restriction.

     *   Generic (polymorphic) lambda expressions<https://wg21.link/N3649>:

        *   As with auto<http://llvm.org/docs/CodingStandards.html#use-auto-type-deduction-to-make-code-more-readable>, only where it makes code more readable.

     *   Variable templates<https://wg21.link/N3651>:

        *   Without restriction.

     *   Member initializers and aggregates (NSDMI)<https://wg21.link/N3653>:

        *   Without restriction.

     *   More constexpr everywhere:

        *   Without restriction.

     *   Single quote as digit separator<https://wg21.link/N3781>:

        *   Not allowed.

     *   std::quoted<https://wg21.link/N3654>:

        *   Without restriction.

     *   std::exchange<https://wg21.link/N3668>:

        *   Without restriction.

  *   Change the minimum toolchain requirement from a “soft-error” to a “hard-error” such that CheckCompilerVersion.cmake now says:

     *   set(GCC_MIN 5.1)
set(CLANG_MIN 3.5)
set(MSVC_MIN 19.1)

  *   The above makes LLVM_TEMPORARILY_ALLOW_OLD_TOOLCHAIN useless until we try upgrading toolchain versions again.
  *   Use C++14 in the code somewhere.

When we last discussed this we said we’d revisit at the end of March 2019 and see if everyone was able to upgrade their toolchains. Remember that by now any build of LLVM with an old toolchain has received a “soft-error” which people had to read and disable using a CMake argument. This includes anyone building LLVM 8 because the change made it to the branch. There were plenty of discussions on Phabricator, on the mailing lists, and at the dev meeting. The was a PSA<http://lists.llvm.org/pipermail/llvm-dev/2019-February/130117.html> and emails were sent to all bot maintainers with older bots. At this point there’s no way that you didn’t know that you needed to upgrade you toolchain.

This would mean that LLVM 9 would require a newer toolchain.

Is anyone not ready for this move? If so, what does your timing look like?



On Jan 22, 2019, at 1:44 PM, JF Bastien via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Hello fans of the auto keyword!

We now have a policy on how LLVM toolchains get updated<https://reviews.llvm.org/rL351765>! Let’s put that policy to good use, and talk about how we’ll move all monorepo projects past C++11.

Previous 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>

Migrate to what?

I’m only proposing that we migrate to C++14 for now. If you want to propose C++17, please do the work required by the policy. In particular, document which toolchains this would require, and what features you’d unlock. As per policy, I want to start soft-errors when building LLVM 8, so that LLVM 9 can use more than C++11.


At the LLVM dev meeting BoF, the room already agreed to move past C++11. Late March 2019 was proposed as a time when we’d start migrating, pending large contributors’ readiness. I’m sticking to that timeline, we’ll see if everyone is ready at the end of March. I nonetheless want to get the soft-errors into the LLVM 8 branch so that we give a sufficient heads-up to developers who only compile releases.


One clear upside of dropping older toolchains: they don’t even support C++11 very well. We have a handful of workarounds left in ADT (particularly around type traits) and I’d like to get rid of them.

The compiler versions I propose allow us to use all of C++14, which includes:

  *   Binary literals<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3472.pdf>
  *   decltype(auto), Return type deduction for normal functions<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3638.html>
  *   Initialized/Generalized lambda captures (init-capture)<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3648.html>
  *   Generic (polymorphic) lambda expressions<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3649.html>
  *   Variable templates<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3651.pdf>
  *   Member initializers and aggregates (NSDMI)<http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2013/n3653.html>
  *   A bunch of new constexpr language and library features
  *   Various other language and library features
See CppReference<https://en.cppreference.com/w/cpp/compiler_support> for details.

Of these, I think polymorphic lambdas are the big feature. Of course, just like Almost Always Auto, we should use such things only where it makes sense.


We’re currently mandating:

  *   Clang 3.1 (released 2012/05)
  *   Apple Clang 3.1 (released 2012/05)
  *   GCC 4.8 (released 2013/03)
  *   Visual Studio 2015 (Update 3) (released 2016/06)
I propose instead:

  *   Clang 3.5 (released 2014/07) to get -std=c++14 instead of -std=c++1y
  *   Apple Clang 6.0 (released 2014/07) to match clang 3.5
  *   GCC 5.1 (released 2015/04) because C++14 mostly came to be in GCC 5
  *   Visual Studio 2017 (released 2017/03) so that we get extended constexpr and NSDMI
Version information from:

  *   Clang http://releases.llvm.org<http://releases.llvm.org/>
  *   Apple clang https://trac.macports.org/wiki/XcodeVersionInfo and https://en.wikipedia.org/wiki/Xcode#Latest_versions
  *   GCC https://gcc.gnu.org/releases.html
  *   MSVC https://en.wikipedia.org/wiki/Microsoft_Visual_Studio and https://docs.microsoft.com/en-us/cpp/visual-cpp-language-conformance

My previous attempts pointed out that WebKit / Chromium / Firefox are all past C++11 (WebKit is moving to C++17<https://lists.webkit.org/pipermail/webkit-dev/2018-March/029922.html> (from C++14), Chromium started moving to C++14<https://groups.google.com/a/chromium.org/d/msg/cxx/ow7hmdDm4yw/eV6KWL2yAQAJ>, Firefox uses some C++14<https://developer.mozilla.org/en-US/docs/Mozilla/Using_CXX_in_Mozilla_code>). This means that platforms which distribute a modern browser can already bootstrap a browser. That’s a nice datapoint, but isn’t sufficient for platforms which compile / use LLVM (especially as a library).

Here’s a table from the LLVM dev meeting BoF detailing version info for distros that seemed relevant:




C++14 lang



GCC 3.2




GCC 3.4




GCC 4.1




GCC 4.4



Debian 7 wheezy

GCC 4.7.2




GCC 4.8



Debian 8 jessie

GCC 4.9.2



OpenBSD 5.7

LLVM 3.5



OpenBSD 5.8

LLVM 3.5



OpenBSD 5.9

LLVM 3.5



Ubuntu 14.04

GCC 4.8.2



Ubuntu 16.04

GCC 5.3.1



OpenBSD 6.0

LLVM 3.8



OpenBSD 6.1

LLVM 4.0.0



Debian 9 stretch

GCC 6.3.0



Ubuntu 17.10

GCC 7.2.0



OpenBSD 6.2

LLVM 5.0.0



Ubuntu 18.04

GCC 7.3.0



OpenBSD 6.3

LLVM 5.0.1



Ubuntu 18.10

GCC 8.1.0



Debian 10 buster

GCC 8.1.0


The data comes from the following sources:

  *   https://en.cppreference.com/w/cpp/compiler_support
  *   https://packages.ubuntu.com/search?keywords=gcc
  *   https://packages.debian.org/search?keywords=gcc
  *   https://access.redhat.com/solutions/19458
  *   https://www.openbsd.org/63.html
  *   https://en.wikipedia.org/wiki/Clang
  *   https://releases.llvm.org<https://releases.llvm.org/>
I haven’t documented FreeBSD / NetBSD / Fedora / MacOS / MSVC, and nobody complained at the BoF. I’d like to understand if we should care about documenting these: ideally the toolchain update policy would list which platforms need to be considered and how far back in time is relevant.




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