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

Fri May 11 01:37:20 PDT 2018

>  It's nice to see this week's version of MSVC supports C++17 but
deploying through corporate IT can take a while.

But this does apply to LLVM versions, to, doesn't it ? What's the problem
with LLVM 9 using c++17 if you can only develop based on LLVM 5 at your
shop anyways ?

-------
Jean-Michaël Celerier
http://www.jcelerier.name

On Thu, May 10, 2018 at 9:20 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:
>
>    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>
> 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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>
>
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