llvm dev - May 2018 - Using C++14 code in LLVM

Hi,

IMHO, it’s a good idea to move to C++14 first.

What do you think about doing this by two phases:

Phase1: require GCC >= 5 but build in C++11 mode (this will give time to
adapt build infrastructure to a new gcc)
Phase2: switch to C++14

Thanks,
Evgeny



From: llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of Reid
Kleckner via llvm-dev <llvm-dev at lists.llvm.org>
Reply-To: Reid Kleckner <rnk at google.com>
Date: Thursday, 10 May 2018 at 19:50
To: Zachary Turner <zturner at google.com>
Cc: llvm-dev <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] Using C++14 code in LLVM

The easy way not to have a three year discussion is to not worry about it for
another three years. :)

So, I think we should take the easy things on the table and just move to C++14
in the near future. It's just a matter of dropping support for building on
distros that only have GCC <5 (aka Trusty, which is from 2014 itself).
Let's do that and call it a day.

---
Aside: I'm always kind of amused by talk of moving to the next
"standard version" when the reality is that every C++ project is
always held back by the compilers and standard libraries that they actually use
in practice. We say LLVM requires C++11 which mandates a working set of
threading primitives, but in practice those don't exist on some platforms
that people would like us to support, so we end up maintaining the
LLVM_ENABLE_THREADING=0 build for them.

It seems more practical to simply list the minimum versions of supported
toolchains that are commonly used to build, i.e. GCC 5, MSVC 2015, Clang 3.N,
libc++ 3.N, libstdc++ 3.N, etc.

On Thu, May 10, 2018 at 11:36 AM Zachary Turner via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> 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.

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)?

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/

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:
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<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
>>
>>
_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
_______________________________________________
LLVM Developers mailing list
llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20180510/eb3a1a19/attachment.html>

> On May 10, 2018, at 12:25 PM, Evgeny Astigeevich via llvm-dev <llvm-dev
at lists.llvm.org> wrote:
> 
> Hi,
>  
> IMHO, it’s a good idea to move to C++14 first.
>  
> What do you think about doing this by two phases:
>  
> Phase1: require GCC >= 5 but build in C++11 mode (this will give time to
adapt build infrastructure to a new gcc)
> Phase2: switch to C++14
Sounds reasonable, here’s a patch:
https://reviews.llvm.org/D46723 <https://reviews.llvm.org/D46723>

> Thanks,
> Evgeny
>  
>  
>  
> From: llvm-dev <llvm-dev-bounces at lists.llvm.org
<mailto:llvm-dev-bounces at lists.llvm.org>> on behalf of Reid Kleckner
via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>>
> Reply-To: Reid Kleckner <rnk at google.com <mailto:rnk at
google.com>>
> Date: Thursday, 10 May 2018 at 19:50
> To: Zachary Turner <zturner at google.com <mailto:zturner at
google.com>>
> Cc: llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>>
> Subject: Re: [llvm-dev] Using C++14 code in LLVM
>  
> The easy way not to have a three year discussion is to not worry about it
for another three years. :)
>  
> So, I think we should take the easy things on the table and just move to
C++14 in the near future. It's just a matter of dropping support for
building on distros that only have GCC <5 (aka Trusty, which is from 2014
itself). Let's do that and call it a day.
>  
> ---
> Aside: I'm always kind of amused by talk of moving to the next
"standard version" when the reality is that every C++ project is
always held back by the compilers and standard libraries that they actually use
in practice. We say LLVM requires C++11 which mandates a working set of
threading primitives, but in practice those don't exist on some platforms
that people would like us to support, so we end up maintaining the
LLVM_ENABLE_THREADING=0 build for them.
>  
> It seems more practical to simply list the minimum versions of supported
toolchains that are commonly used to build, i.e. GCC 5, MSVC 2015, Clang 3.N,
libc++ 3.N, libstdc++ 3.N, etc.
>  
> On Thu, May 10, 2018 at 11:36 AM Zachary Turner via llvm-dev <llvm-dev
at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> 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.
>>  
>> 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)?
>>  
>> 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:
>>>>>> 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 <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
>>>>>> >> 
>>>>>> >> 
>>>>>> _______________________________________________
>>>>>> LLVM Developers mailing list
>>>>>> llvm-dev at lists.llvm.org <mailto:llvm-dev at
lists.llvm.org>
>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>_______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
<http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20180510/b1df7237/attachment-0001.html>

Last time this came up, there were a lot of people that were stuck on GCC
4.9 due to ABI reasons. I think forcing that upgrade is going to be the
most disruptive part of this, and I think that will really need a decent
amount of time. =[

On Thu, May 10, 2018 at 2:26 PM JF Bastien via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
>
> On May 10, 2018, at 12:25 PM, Evgeny Astigeevich via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hi,
>
> IMHO, it’s a good idea to move to C++14 first.
>
> What do you think about doing this by two phases:
>
> Phase1: require GCC >= 5 but build in C++11 mode (this will give time to
> adapt build infrastructure to a new gcc)
> Phase2: switch to C++14
>
>
> Sounds reasonable, here’s a patch:
> https://reviews.llvm.org/D46723
>
>
> Thanks,
> Evgeny
>
>
>
> *From: *llvm-dev <llvm-dev-bounces at lists.llvm.org> on behalf of
Reid
> Kleckner via llvm-dev <llvm-dev at lists.llvm.org>
> *Reply-To: *Reid Kleckner <rnk at google.com>
> *Date: *Thursday, 10 May 2018 at 19:50
> *To: *Zachary Turner <zturner at google.com>
> *Cc: *llvm-dev <llvm-dev at lists.llvm.org>
> *Subject: *Re: [llvm-dev] Using C++14 code in LLVM
>
> The easy way not to have a three year discussion is to not worry about it
> for another three years. :)
>
> So, I think we should take the easy things on the table and just move to
> C++14 in the near future. It's just a matter of dropping support for
> building on distros that only have GCC <5 (aka Trusty, which is from
2014
> itself). Let's do that and call it a day.
>
> ---
> Aside: I'm always kind of amused by talk of moving to the next
"standard
> version" when the reality is that every C++ project is always held
back by
> the compilers and standard libraries that they actually use in practice. We
> say LLVM requires C++11 which mandates a working set of threading
> primitives, but in practice those don't exist on some platforms that
people
> would like us to support, so we end up maintaining the
> LLVM_ENABLE_THREADING=0 build for them.
>
> It seems more practical to simply list the minimum versions of supported
> toolchains that are commonly used to build, i.e. GCC 5, MSVC 2015, Clang
> 3.N, libc++ 3.N, libstdc++ 3.N, etc.
>
> On Thu, May 10, 2018 at 11:36 AM Zachary Turner via llvm-dev <
> llvm-dev at lists.llvm.org> 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.
>
> 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)?
>
> 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
> >>
> >>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20180510/56c838c6/attachment.html>