[llvm-dev] [RFC] migrating past C++11

Tue Jan 22 13:44:54 PST 2019

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.

Timeline

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.

Upsides

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.

Toolchains

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

Release
Distro
Compiler
C++14 lang
2003-10
RHEL 3
GCC 3.2
❌
2005-02
RHEL 4
GCC 3.4
❌
2007-03
RHEL 5
GCC 4.1
❌
2010-11
RHEL 6
GCC 4.4
❌
2013-05
Debian 7 wheezy
GCC 4.7.2
❌
2013-12
RHEL 7
GCC 4.8
❌
2015-04
Debian 8 jessie
GCC 4.9.2
❌
2015-05
OpenBSD 5.7
LLVM 3.5
✅
2015-10
OpenBSD 5.8
LLVM 3.5
✅
2016-03
OpenBSD 5.9
LLVM 3.5
✅
2016-04
Ubuntu 14.04
GCC 4.8.2
❌
2016-04
Ubuntu 16.04
GCC 5.3.1
✅
2016-09
OpenBSD 6.0
LLVM 3.8
✅
2017-04
OpenBSD 6.1
LLVM 4.0.0
✅
2017-06
Debian 9 stretch
GCC 6.3.0
✅
2017-10
Ubuntu 17.10
GCC 7.2.0
✅
2017-10
OpenBSD 6.2
LLVM 5.0.0
✅
2018-04
Ubuntu 18.04
GCC 7.3.0
✅
2018-04
OpenBSD 6.3
LLVM 5.0.1
✅
2018-10
Ubuntu 18.10
GCC 8.1.0
✅
2018-??
Debian 10 buster
GCC 8.1.0
✅

The data comes from the following sources:
https://en.cppreference.com/w/cpp/compiler_support <https://en.cppreference.com/w/cpp/compiler_support>
https://packages.ubuntu.com/search?keywords=gcc <https://packages.ubuntu.com/search?keywords=gcc>
https://packages.debian.org/search?keywords=gcc <https://packages.debian.org/search?keywords=gcc>
https://access.redhat.com/solutions/19458 <https://access.redhat.com/solutions/19458>
https://www.openbsd.org/63.html <https://www.openbsd.org/63.html>
https://en.wikipedia.org/wiki/Clang <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.

Thanks,

JF
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