llvm dev - Oct 2016 - [RFC] Increase minimum supported GCC version for building LLVM to 4.8

According to the documentation at
http://llvm.org/docs/GettingStarted.html#software, compiling LLVM with GCC
requires at least version 4.7.0. However, there are apparently several
problems building current LLVM/Clang with gcc 4.7.X. This proposal is to
increase the minimum required GCC to 4.8.

This is split off of the thread started here:
   http://lists.llvm.org/pipermail/llvm-dev/2016-October/105742.html
which is a fairly recent regression in being able to compile LTO.cpp with
GCC 4.7.4, which I tracked down to a known GCC bug.

According to James Knight, it is not the only place where GCC 4.7.X cannot
compile the clang toolchain, which apparently hasn't been buildable by GCC
4.7.X in >3 months:
   http://lists.llvm.org/pipermail/llvm-dev/2016-October/105825.html

Renato also suggests bumping the minimum to 4.8, as that is the oldest
shipped version in most stable Linux distributions:
   http://lists.llvm.org/pipermail/llvm-dev/2016-October/105826.html

We currently have many buildbots building with GCC 4.8, but none with GCC
4.7.X, which is why this issue was not discovered previously.

Thoughts?

Thanks,
Teresa

-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |  408-460-2413
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+1. I've been bitten by the 4.7 STL's lack of std::map::emplace a few
times, so I support this.

On Wed, Oct 12, 2016 at 3:51 PM, Teresa Johnson via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> According to the documentation at http://llvm.org/docs/
> GettingStarted.html#software, compiling LLVM with GCC requires at least
> version 4.7.0. However, there are apparently several problems building
> current LLVM/Clang with gcc 4.7.X. This proposal is to increase the minimum
> required GCC to 4.8.
>
> This is split off of the thread started here:
>    http://lists.llvm.org/pipermail/llvm-dev/2016-October/105742.html
> which is a fairly recent regression in being able to compile LTO.cpp with
> GCC 4.7.4, which I tracked down to a known GCC bug.
>
> According to James Knight, it is not the only place where GCC 4.7.X cannot
> compile the clang toolchain, which apparently hasn't been buildable by
GCC
> 4.7.X in >3 months:
>    http://lists.llvm.org/pipermail/llvm-dev/2016-October/105825.html
>
> Renato also suggests bumping the minimum to 4.8, as that is the oldest
> shipped version in most stable Linux distributions:
>    http://lists.llvm.org/pipermail/llvm-dev/2016-October/105826.html
>
> We currently have many buildbots building with GCC 4.8, but none with GCC
> 4.7.X, which is why this issue was not discovered previously.
>
> Thoughts?
>
> Thanks,
> Teresa
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
> 408-460-2413
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>
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This makes sense to me. Especially since, in practice, we've already done it
;)

-Hal 

----- Original Message -----
> From: "Reid Kleckner via llvm-dev" <llvm-dev at
lists.llvm.org>
> To: "Teresa Johnson" <tejohnson at google.com>
> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>
> Sent: Wednesday, October 12, 2016 6:05:39 PM
> Subject: Re: [llvm-dev] [RFC] Increase minimum supported GCC version
> for building LLVM to 4.8
> +1. I've been bitten by the 4.7 STL's lack of std::map::emplace a
few
> times, so I support this.
> On Wed, Oct 12, 2016 at 3:51 PM, Teresa Johnson via llvm-dev <
> llvm-dev at lists.llvm.org > wrote:
> > According to the documentation at
> > http://llvm.org/docs/GettingStarted.html#software , compiling LLVM
> > with GCC requires at least version 4.7.0. However, there are
> > apparently several problems building current LLVM/Clang with gcc
> > 4.7.X. This proposal is to increase the minimum required GCC to
> > 4.8.
> 
> > This is split off of the thread started here:
> 
> > http://lists.llvm.org/pipermail/llvm-dev/2016-October/105742.html
> 
> > which is a fairly recent regression in being able to compile
> > LTO.cpp
> > with GCC 4.7.4, which I tracked down to a known GCC bug.
> 
> > According to James Knight, it is not the only place where GCC 4.7.X
> > cannot compile the clang toolchain, which apparently hasn't been
> > buildable by GCC 4.7.X in >3 months:
> 
> > http://lists.llvm.org/pipermail/llvm-dev/2016-October/105825.html
> 
> > Renato also suggests bumping the minimum to 4.8, as that is the
> > oldest shipped version in most stable Linux distributions:
> 
> > http://lists.llvm.org/pipermail/llvm-dev/2016-October/105826.html
> 
> > We currently have many buildbots building with GCC 4.8, but none
> > with
> > GCC 4.7.X, which is why this issue was not discovered previously.
> 
> > Thoughts?
> 
> > Thanks,
> 
> > Teresa
> 
> > --
> 
> > Teresa Johnson | 	Software Engineer | 	tejohnson at google.com |
> > 	408-460-2413
> 
> > _______________________________________________
> 
> > 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
-- 

Hal Finkel 
Lead, Compiler Technology and Programming Languages 
Leadership Computing Facility 
Argonne National Laboratory 
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On 10/12/2016 6:05 PM, Reid Kleckner via llvm-dev wrote:
> +1. I've been bitten by the 4.7 STL's lack of std::map::emplace a
few
> times, so I support this.
Same here. I still remember the disappointment about the emplace.

+1

-Krzysztof

On Wed, Oct 12, 2016 at 03:51:07PM -0700, Teresa Johnson via llvm-dev
wrote:
> According to the documentation at
> http://llvm.org/docs/GettingStarted.html#software, compiling LLVM with GCC
> requires at least version 4.7.0. However, there are apparently several
> problems building current LLVM/Clang with gcc 4.7.X. This proposal is to
> increase the minimum required GCC to 4.8.
Fine with me.

Joerg

+1 from me.

But which version of 4.8.x?  4.8.0 was released in March 2013 while 4.8.5 is
June 2015 (see https://gcc.gnu.org/releases.html
<https://gcc.gnu.org/releases.html>)

Thats an awfully long time between those dates, so i can’t imagine everyone
being on 4.8.5, but shouldn’t we aim for the highest possible one if we’re
bumping versions anyway?

Looking at Ubuntu 14.04 LTS (http://packages.ubuntu.com/search?keywords=gcc-4.8
<http://packages.ubuntu.com/search?keywords=gcc-4.8>), appears that 4.8.4
is available for many platforms, so if possible that might be a good version to
do.

Cheers,
Pete
> On Oct 12, 2016, at 4:28 PM, Joerg Sonnenberger via llvm-dev <llvm-dev
at lists.llvm.org> wrote:
> 
> On Wed, Oct 12, 2016 at 03:51:07PM -0700, Teresa Johnson via llvm-dev
wrote:
>> According to the documentation at
>> http://llvm.org/docs/GettingStarted.html#software, compiling LLVM with
GCC
>> requires at least version 4.7.0. However, there are apparently several
>> problems building current LLVM/Clang with gcc 4.7.X. This proposal is
to
>> increase the minimum required GCC to 4.8.
> 
> Fine with me.
> 
> Joerg
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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Teresa Johnson via llvm-dev <llvm-dev at lists.llvm.org>
writes:
> According to the documentation at
> http://llvm.org/docs/GettingStarted.html#software, compiling LLVM with GCC
> requires at least version 4.7.0. However, there are apparently several
> problems building current LLVM/Clang with gcc 4.7.X. This proposal is to
> increase the minimum required GCC to 4.8.
-1 unless it's particularly hard to fix the incompatibilities that have
been introduced or there are strong pain points in supporting 4.7.4. We
should only increase the minimum version of our host compilers if we
have strong motivation to do so.

On Wed, Oct 12, 2016 at 9:14 PM, Justin Bogner via llvm-dev <
llvm-dev at lists.llvm.org> wrote:
> Teresa Johnson via llvm-dev <llvm-dev at lists.llvm.org> writes:
> > According to the documentation at
> > http://llvm.org/docs/GettingStarted.html#software, compiling LLVM with
> GCC
> > requires at least version 4.7.0. However, there are apparently several
> > problems building current LLVM/Clang with gcc 4.7.X. This proposal is
to
> > increase the minimum required GCC to 4.8.
>
> -1 unless it's particularly hard to fix the incompatibilities that have
> been introduced or there are strong pain points in supporting 4.7.4. We
> should only increase the minimum version of our host compilers if we
> have strong motivation to do so.
>
We do: Everyone we care about has already done so, including not just all
developers, but all users anyone can find.

Nobody is testing or ensuring it builds with 4.7, nor is there any
compelling reason anyone can give to do so

Testing with 4.7 would actually require people go and install non-default
compilers for their long term supported distros in the buildbots, etc
(that's a pain point right there).

Last i looked, 3.9 did  not even build with gcc 4.7, and not a single
complaint, bug filed, etc.

(note: I haven't double checked this, it was definitely true at one point).

Even the case where someone noticed that Teresa points out is an odd case,
and not a normal usage.

I think the above is a pretty strong motivation, even if it's not the
typical motivation of "it adds x feature we want".

We can certainly offer support for 4.7  if we want to, but if we are
offering 4.7 just because 4.8 doesn't have magical new features that we
would upgrade to, that seems wrong.
We should offer support for 4.7 because someone actually wants it, and we
care about that someone.
Otherwise, it fails the very trivial test of "who cares if we support 4.7
or not"
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