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

Thu May 10 11:27:58 PDT 2018

> 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/ <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:
> 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 <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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