[llvm-dev] [RFC] migrating LLVM to C++14
Chandler Carruth via llvm-dev
llvm-dev at lists.llvm.org
Mon May 6 11:02:56 PDT 2019
I know you'll be shocked that we've slipped in our efforts. ;] I don't have
a super meaningful ETA update though -- a bunch of unknows have been found
and addressed, and again, I feel like we might finish this in roughly a
month.
On the flip side, I do want to clarify the immediate concern we have: we
need to be able to use LLVM libraries with libstdc++4.9. We can set
`-std=c++14` (or newer) w/o issue. So in some senses, if LLVM folks want to
move to C++14, that's fine. But if it involves moving the minimum supported
GCC version (and thus libstdc++ version tested on build bots) past
libstdc++4.9, any changes that break with that version would be a big
problem for us.
On Mon, May 6, 2019 at 11:51 AM JF Bastien <jfbastien at apple.com> wrote:
>
>
> On Apr 1, 2019, at 4:10 PM, JF Bastien via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>
>
> On Apr 1, 2019, at 4:07 PM, Chandler Carruth <chandlerc at gmail.com> wrote:
>
> On Mon, Apr 1, 2019 at 1:16 PM JF Bastien via llvm-dev <
> llvm-dev at lists.llvm.org> 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(GCC_SOFT_ERROR 5.1)
>> set(CLANG_MIN 3.5)
>> set(CLANG_SOFT_ERROR 3.5)
>> set(APPLECLANG_MIN 6.0)
>> set(APPLECLANG_SOFT_ERROR 6.0)
>> set(MSVC_MIN 19.1)
>> set(MSVC_SOFT_ERROR 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?
>>
>
> FWIW, despite making good progress, we're not quite finished.
>
>
>> If so, what does your timing look like?
>>
>
> We are significantly closer than when we last talked (two separate big
> milestones landed). We're basically on the last step which, for us, is
> switching to libc++. AFAIK, we don't have any major blockers, and folks are
> mostly trying to land the necessary changes. Optimistically, we'll finish
> in April. Conservatively, it might slip a week or two into May, but that
> seems unlikely at this point (there are few unknowns left).
>
>
> No problem, and thanks for the update. Let’s take another look in May.
>
> In the meantime, we can discuss the rest of the proposal. All that’ll be
> left for May is to rubber-stamp the patch.
>
>
> Hello all! It’s May and I was wondering: how are things going with the
> libc++ migration? Is everything ready and sufficiently settled so that we
> can migrate LLVM to C++14?
>
> Thanks!
>
>
> Thanks,
>>
>> JF
>>
>>
>>
>> On Jan 22, 2019, at 1:44 PM, JF Bastien via llvm-dev <
>> 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.
>>
>>
>> *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
>> - 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:
>>
>> *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://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
>>
>> 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
>>
>>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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