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

JF Bastien via llvm-dev llvm-dev at lists.llvm.org
Thu May 10 11:50:33 PDT 2018 


> 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 <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 <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:
>> 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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