[llvm-dev] Using C++14 code in LLVM

Thu May 10 12:40:57 PDT 2018

+1 to C++14.

On Thu, May 10, 2018 at 10:37 PM, Nathan Froyd via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
> Firefox's experience is that GCC 5 isn't going to cut it, especially
> if you move to MSVC 2017, because people are going to be quickly
> annoyed at the lack of relaxed constexpr function support, which is
> GCC 6+.
Are you sure? I think you meant GCC 4.9 isn't going to cut it, ...
...
annoyed at the lack of relaxed constexpr function support, which is
GCC 5.

https://gcc.gnu.org/projects/cxx-status.html.

Relaxing requirements on constexpr functionsN36525__cpp_constexpr >= 201304

Roman.

> You can get GCC 6 for Ubuntu 16.04; I have it on my machine, though
> it's strangely not listed on packages.ubuntu.com for 16.04.
>
> -Nathan
>
> On Thu, May 10, 2018 at 3:20 PM, via llvm-dev <llvm-dev at lists.llvm.org> wrote:
>> Well… Ubuntu 16.04 came with gcc 5, and deploying Visual Studio 2015 should
>> be a done deal in Windows shops, which suggests moving to C++14 should be no
>> problem.
>>
>>
>>
>> It's nice to see this week's version of MSVC supports C++17 but deploying
>> through corporate IT can take a while.  ("This week's version" because the
>> blog post is dated Monday.)
>>
>> --paulr
>>
>>
>>
>> From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of Zachary
>> Turner via llvm-dev
>> Sent: Thursday, May 10, 2018 3:00 PM
>> To: JF Bastien
>> Cc: via llvm-dev
>> Subject: Re: [llvm-dev] Using C++14 code in LLVM
>>
>>
>>
>> Consider me on board with the highest version we can come to an agreement on
>> :)
>>
>>
>>
>>
>>
>> On Thu, May 10, 2018 at 11:50 AM JF Bastien <jfbastien at apple.com> wrote:
>>
>>
>>
>> On May 10, 2018, at 11:35 AM, Zachary Turner <zturner at google.com> 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.
>>
>>
>>
>> Such a fatalistic view, let’s trust ourselves to be better next time ;-)
>>
>> But seriously: we can learn from moving to C++14, and use what we’ve learned
>> to move to C++17 faster next time. Also consider the code churn we’ll
>> encounter as we fix incompatibilities with C++11 / C++14, drop unnecessary
>> code, upgrade various uses, that will happen regardless of moving to C++17
>> and will take a little while to occur. There would be more of that type of
>> churn if we went straight to C++17.
>>
>>
>>
>>
>>
>> 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)?
>>
>>
>>
>> Mostly library so far, yes, but the GCC 6 support for C++17 language isn’t
>> great either:
>>
>>
>>
>>  - New auto rules for direct-list-initialization
>>
>>  - static_assert with no message
>>
>>  - typename in a template template parameter
>>
>>  - Nested namespace definition
>>
>>  - Attributes for namespaces and enumerators
>>
>>  - u8 character literals
>>
>>  - Allow constant evaluation for all non-type template arguments
>>
>>  - Fold Expressions
>>
>>  - Unary fold expressions and empty parameter packs
>>
>>  - __has_include in preprocessor conditional
>>
>>  - Differing begin and end types in range-based for\
>>
>>
>>
>> From: https://en.cppreference.com/w/cpp/compiler_support
>>
>>
>>
>> The only thing that’s really nice are fold expressions, and I hope they’re
>> not buggy in GCC 6.
>>
>>
>>
>> Otherwise the list is missing a good amount for C++17 language features, and
>> brings up the discussion of which GCC version is the minimum we mandate. I’d
>> rather avoid that discussion. Let’s assume GCC 6, if we get 7 then great,
>> but it doesn’t matter if we stick to C++14.
>>
>>
>>
>>
>>
>> On Thu, May 10, 2018 at 11:28 AM JF Bastien <jfbastien at apple.com> wrote:
>>
>>
>>
>> On May 10, 2018, at 11:22 AM, Zachary Turner <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/
>>
>>
>>
>> On Thu, May 10, 2018 at 11:18 AM Eric Christopher <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> 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:
>>
>> Some folks want an official policy about compiler support before updating
>> the standard version we use.
>> Worries about which GCC version is available in which distro.
>> 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 (from C++14) right now †
>> Chromium started moving to C++14 in August of last year
>> Firefox uses some C++14
>>
>> 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>
>>> wrote:
>>
>>>
>>
>>>>
>>
>>>> On Oct 4, 2016, at 2:10 PM, Reid Kleckner <rnk at google.com> wrote:
>>
>>>>
>>
>>>> On Tue, Oct 4, 2016 at 11:58 AM, Mehdi Amini <mehdi.amini at apple.com>
>>
>>>> wrote:
>>
>>>>
>>
>>>>>
>>
>>>>> On Oct 4, 2016, at 8:40 AM, Reid Kleckner via llvm-dev <
>>
>>>>> llvm-dev at lists.llvm.org> wrote:
>>
>>>>>
>>
>>>>> On Tue, Oct 4, 2016 at 8:29 AM, Zachary Turner <zturner at 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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