[llvm-dev] Using C++14 code in LLVM
JF Bastien via llvm-dev
llvm-dev at lists.llvm.org
Thu May 10 13:26:17 PDT 2018
> On May 10, 2018, at 12:25 PM, Evgeny Astigeevich via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>
> Hi,
>
> IMHO, it’s a good idea to move to C++14 first.
>
> What do you think about doing this by two phases:
>
> Phase1: require GCC >= 5 but build in C++11 mode (this will give time to adapt build infrastructure to a new gcc)
> Phase2: switch to C++14
Sounds reasonable, here’s a patch:
https://reviews.llvm.org/D46723 <https://reviews.llvm.org/D46723>
> Thanks,
> Evgeny
>
>
>
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org <mailto:llvm-dev-bounces at lists.llvm.org>> on behalf of Reid Kleckner via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
> Reply-To: Reid Kleckner <rnk at google.com <mailto:rnk at google.com>>
> Date: Thursday, 10 May 2018 at 19:50
> To: Zachary Turner <zturner at google.com <mailto:zturner at google.com>>
> Cc: llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>>
> Subject: Re: [llvm-dev] Using C++14 code in LLVM
>
> The easy way not to have a three year discussion is to not worry about it for another three years. :)
>
> So, I think we should take the easy things on the table and just move to C++14 in the near future. It's just a matter of dropping support for building on distros that only have GCC <5 (aka Trusty, which is from 2014 itself). Let's do that and call it a day.
>
> ---
> Aside: I'm always kind of amused by talk of moving to the next "standard version" when the reality is that every C++ project is always held back by the compilers and standard libraries that they actually use in practice. We say LLVM requires C++11 which mandates a working set of threading primitives, but in practice those don't exist on some platforms that people would like us to support, so we end up maintaining the LLVM_ENABLE_THREADING=0 build for them.
>
> It seems more practical to simply list the minimum versions of supported toolchains that are commonly used to build, i.e. GCC 5, MSVC 2015, Clang 3.N, libc++ 3.N, libstdc++ 3.N, etc.
>
> On Thu, May 10, 2018 at 11:36 AM Zachary Turner via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>> If it's the only thing we can agree then I'll take it, but I just worry that 3 years from now we're going to start another 3 year discussion, so that any actual move to C++17 would end up taking double the time.
>>
>> Are the issues specific to C++17 additions to the standard library? What if you allow C++17 language features but not C++17 library features? I'm guessing this is too simple though and isn't sufficient to avoid the problems (which I don't know anything about, so you'll have to enlighten me)?
>>
>> On Thu, May 10, 2018 at 11:28 AM JF Bastien <jfbastien at apple.com <mailto:jfbastien at apple.com>> wrote:
>>>
>>>
>>>> On May 10, 2018, at 11:22 AM, Zachary Turner <zturner at google.com <mailto:zturner at google.com>> wrote:
>>>>
>>>> Windows has never been the issue. Honestly, MSVC on Windows is "fully C++17 conformant" [1].
>>>>
>>>> The issue has and always will be GCC. Given that a bump in any version of GCC has been (and will remain) difficult for some time, I propose that we skip C++14 and move to 17. We don't want to have a multi-year disccusion about this again any time soon, and from what I gather, nobody has any more reservations about moving to C++17 than they do about moving to C++14. They only have reservations about moving to anything at all. So if we're gonna move, we should go all the way.
>>>
>>> WebKit’s move to C++17 hasn’t been super smooth because of GCC / libstdc++ issues in both GCC 6 and GCC 7. It’s all fixable, but given LLVM's slow move out of C++11 I’d rather get C++14 now rather than a painful transition to C++17 that drags on as we discover issues.
>>>
>>>
>>>
>>>> Just my 2c.
>>>>
>>>> [1] https://blogs.msdn.microsoft.com/vcblog/2018/05/07/announcing-msvc-conforms-to-the-c-standard/ <https://blogs.msdn.microsoft.com/vcblog/2018/05/07/announcing-msvc-conforms-to-the-c-standard/>
>>>>
>>>> On Thu, May 10, 2018 at 11:18 AM Eric Christopher <echristo at gmail.com <mailto:echristo at gmail.com>> wrote:
>>>>> Once again, I'm totally down for this and think we should do it. I worry about windows, but ...
>>>>>
>>>>> Zach: How's windows c++14 support looking?
>>>>>
>>>>> -eric
>>>>>
>>>>> On Thu, May 10, 2018 at 10:01 AM JF Bastien via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> wrote:
>>>>>> Hi folks!
>>>>>>
>>>>>> Six more months have come and gone, and maybe we could move LLVM to C++14 now?
>>>>>>
>>>>>> The issues I picked out from the last discussion:
>>>>>> 1. Some folks want an official policy about compiler support before updating the standard version we use.
>>>>>> 2. Worries about which GCC version is available in which distro.
>>>>>> 3. Worries about MSVC.
>>>>>>
>>>>>> Instead of rehashing the compiler per distro surveys from previous discussion, and instead of talking bootstrap, let me offer three data points:
>>>>>> · WebKit is moving to C++17 <https://lists.webkit.org/pipermail/webkit-dev/2018-March/029922.html> (from C++14) right now †
>>>>>> · Chromium started moving to C++14 <https://groups.google.com/a/chromium.org/d/msg/cxx/ow7hmdDm4yw/eV6KWL2yAQAJ> in August of last year
>>>>>> · Firefox uses some C++14 <https://developer.mozilla.org/en-US/docs/Mozilla/Using_CXX_in_Mozilla_code>
>>>>>> What I get from this data: if your distro bundles a modern web browser, it already builds some C++14, somehow.
>>>>>>
>>>>>> The LLVM community has been talking about this for a while now, and I’m not aware of a policy coming to light. I don’t think we need a policy given the above data. So how about we… just kinda... move LLVM to C++14?
>>>>>>
>>>>>>
>>>>>> Thanks!
>>>>>>
>>>>>> JF
>>>>>>
>>>>>>
>>>>>> † the move to C++17 is very painful, but 14 has been working great in WebKit for quite a long time.
>>>>>>
>>>>>>
>>>>>> > Last time we discussed this, the consensus was "I think we can survive
>>>>>> > another year without generalized constexpr and variable templates".
>>>>>> >
>>>>>> > Well, we did indeed survive. And it's been exactly a year! So naturally,
>>>>>> > it only makes sense to revive this :)
>>>>>> >
>>>>>> >
>>>>>> >
>>>>>> > There's an active conversation going on in IRC right now, and it seems like
>>>>>> > there is more desire than there was last year.
>>>>>> >
>>>>>> > What are the main gains from allowing C++14?
>>>>>> > * Variable templates
>>>>>> > * Generalized constexpr
>>>>>> > * Return-type Deduction
>>>>>> > * Generic Lambdas
>>>>>> > * std::make_unique<> (the source of many build bot breakages)
>>>>>> >
>>>>>> >
>>>>>> > What are the main gains from allowing C++17? [1]
>>>>>> > * [[nodiscard]] attribute
>>>>>> > * structured bindings
>>>>>> > * constexpr-if
>>>>>> > * guaranteed copy elision
>>>>>> > * numerous new library types: optional, string_view, variant, byte,
>>>>>> > * numerous new algorithms: parallel algorithms, too many to list
>>>>>> > * Probably some more, but I just tried to hit the biggest ones.
>>>>>> >
>>>>>> >
>>>>>> > First, it seems like if we want to enable C++14 we need GCC >= 5.
>>>>>> > And if we want to enable C++17 we need GCC >= 7.
>>>>>> >
>>>>>> > With that out of the way, here were some of the issues that were raised
>>>>>> > last time:
>>>>>> >
>>>>>> > Issue: Ubuntu 14.04 LTS is on GCC 4.8.x, and we have to support it until
>>>>>> > end of life.
>>>>>> > Resolution: LTS is right around the corner, in 6 more months.
>>>>>> >
>>>>>> > Issue: Various other platforms have older GCCs as their system compiler,
>>>>>> > and it's annoying to upgrade.
>>>>>> > Question: Do any of these not have a port you can install? For example,
>>>>>> > NetBSD 7 appears to have GCC 5.3 as a port, if DistroWatch is any
>>>>>> > indication. It could be wrong though and I could also be misinterpreting
>>>>>> > it.
>>>>>> >
>>>>>> > Issue: If we're going to make people bootstrap a compiler, we might as well
>>>>>> > go all the way to C++17.
>>>>>> > Comment: I'm not opposed.
>>>>>> >
>>>>>> >
>>>>>> > Some questions / comments of my own:
>>>>>> >
>>>>>> > * Where is this policy about Ubuntu and LTS documented? Does this mean,
>>>>>> > for example, that we will not be able to use C++17 until 2023 (16.04 LTS
>>>>>> > has only GCC 5.3.1)? That seems a bit unreasonable. And there's no
>>>>>> > guarantee that 18.04 LTS will even have GCC 7 or higher either, so it could
>>>>>> > be 2025 or 2027.
>>>>>> >
>>>>>> > * We've asked people in the past to build a modern toolchain. For example,
>>>>>> > we did it with C++11 and Ubuntu 12.04 LTS. Is C++17 compelling enough to
>>>>>> > justify this again?
>>>>>> >
>>>>>> > * GCC 4.9 probably isn't sufficient to justify an increase for anyone, as
>>>>>> > it lacks two of the more sought-after features of C++14 (variable templates
>>>>>> > and generalized constexpr). So IMO we should require a bump to GCC 5 or
>>>>>> > higher, or not at all.
>>>>>> >
>>>>>> > * Clang 6 supports all of C++20, and it builds with only C++11, so we
>>>>>> > shouldn't have to worry too much about the problem of needing to "daisy
>>>>>> > chain" compilers to finally get the latest version of LLVM building. "GCC
>>>>>> > 4.8 -> Clang 6 - > Clang ToT" should hold up through C++1z.
>>>>>> >
>>>>>> > * While we obviously can't be tied to the versioning of every single distro
>>>>>> > out there, some are "bigger" than others. Which are big enough that
>>>>>> > warrant serious consideration? The ones I found are (and I did my best to
>>>>>> > aggregate all this, but please correct me if anything is incorrect or
>>>>>> > misrepresented):
>>>>>> >
>>>>>> > OpenBSD - Ships with GCC 4.2.1 anyway. They are already having to
>>>>>> > bootstrap something, so the proposal here does not change anything, because
>>>>>> > even current LLVM doesn't compile with GCC 4.2.1
>>>>>> >
>>>>>> > CentOS & RHEL - No version of Distro, including trunk, has GCC >= 4.8.5
>>>>>> > (are there ports?)
>>>>>> >
>>>>>> > Debian - Minimum version 9 for GCC >= 5 (are there ports for earlier
>>>>>> > releases?)
>>>>>> >
>>>>>> > Fedora - Minimum version 24 for GCC >= 5, minimum version 26 for GCC >= 7
>>>>>> >
>>>>>> > Ubuntu - Minimum LTS 16.04 for GCC >= 5
>>>>>> >
>>>>>> > NetBSD - Version 7 has GCC 4.8.4 by default, but contains port for 5.3.0
>>>>>> >
>>>>>> > FreeBSD - Minimum Version 11 for GCC >= 5
>>>>>> >
>>>>>> > So, thoughts?
>>>>>> >
>>>>>> >
>>>>>> > [1] - Note that we'd need to wait a few more revs for MSVC before allowing
>>>>>> > C++17, but given that it's becoming easier and easier to bump the minimum
>>>>>> > MSVC version, I'm discounting this as a factor, as MSVC will not really be
>>>>>> > the bottleneck in any real sense.
>>>>>> >
>>>>>> > On Tue, Oct 4, 2016 at 2:15 PM Mehdi Amini <mehdi.amini at apple.com <http://apple.com/>> wrote:
>>>>>> >
>>>>>> >>
>>>>>> >> On Oct 4, 2016, at 2:10 PM, Reid Kleckner <rnk at google.com <http://google.com/>> wrote:
>>>>>> >>
>>>>>> >> On Tue, Oct 4, 2016 at 11:58 AM, Mehdi Amini <mehdi.amini at apple.com <http://apple.com/>>
>>>>>> >> wrote:
>>>>>> >>
>>>>>> >>>
>>>>>> >>> On Oct 4, 2016, at 8:40 AM, Reid Kleckner via llvm-dev <
>>>>>> >>> llvm-dev at lists.llvm.org <http://lists.llvm.org/>> wrote:
>>>>>> >>>
>>>>>> >>> On Tue, Oct 4, 2016 at 8:29 AM, Zachary Turner <zturner at google.com <http://google.com/>>
>>>>>> >>> wrote:
>>>>>> >>>>
>>>>>> >>>> I ask because many of these LTS distros are notoriously slow at updating
>>>>>> >>>> their packages. While some people may think C++14 doesn't provide enough
>>>>>> >>>> bang for the buck to justify bumping to GCC 4.9, C++17 definitely does. But
>>>>>> >>>> at that point we're going to be talking about GCC 6.1 or 6.2, which is
>>>>>> >>>> going to be significantly harder unless we want to wait 5-7 years, and I
>>>>>> >>>> suspect people won't.
>>>>>> >>>>
>>>>>> >>>
>>>>>> >>> If by "notoriously slow" you mean they don't bump their toolchain
>>>>>> >>> versions at all, then yeah. We just wait until the LTS release is at
>>>>>> >>> end-of-life before dropping it.
>>>>>> >>>
>>>>>> >>>
>>>>>> >>> That’s the first time I read about this policy: we support every linux
>>>>>> >>> LTS distribution till their end-of-life? Only Ubuntu? Do you have a pointer
>>>>>> >>> where it is documented / discussed?
>>>>>> >>> (Note that Ubuntu LTS is 5 years AFAIK.)
>>>>>> >>>
>>>>>> >>
>>>>>> >> Sorry, I didn't mean to refer to the LTS support lifetime. I just meant we
>>>>>> >> support the last LTS until we can reasonably expect users to have upgraded
>>>>>> >> to the new one. If there's an LTS release every two years, then we want to
>>>>>> >> keep supporting them for at least three years to give people a year to
>>>>>> >> upgrade.
>>>>>> >>
>>>>>> >>
>>>>>> >> OK, got it.
>>>>>> >>
>>>>>> >> Thanks for clarifying!
>>>>>> >>
>>>>>> >> Mehdi
>>>>>> >>
>>>>>> >>
>>>>>> _______________________________________________
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