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

Tue Oct 31 13:57:45 PDT 2017

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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