[cfe-dev] [llvm-dev] Revisiting our informal policy to support two versions of MSVC

James Molloy via cfe-dev cfe-dev at lists.llvm.org
Wed Sep 7 07:59:36 PDT 2016

Hi all,

Firstly sorry I'm a bit late responding on this one. Internally to ARM we
build LLVM for Windows. Our current build cluster has only VS2013 installed
and as a result of this thread we've been working on getting VS2015
installed. This involves a certain amount of IT-wrangling as the cluster we
use is company-wide. There have been some hiccups regarding licensing of
MSVC professional (we can't use the community edition for the same reasons
mentioned by Paul previously) but we hoped to be ready in time for the 15th
September switchover date.

It's recently been realised that VS2013 and VS2015 are not ABI compatible
(something that really surprised me), and this means we have to synchronize
moving LLVM's build to VS2015 as well as upgrading a third party library
that we receive from the vendor in compiled library form. This is not
something we're capable of doing by September 15th.

We try really hard at ARM to hide our internal processes because we believe
that they're on the whole irrelevant to the community, however in this case
we'd be really stuck, unable to get production builds.

As this is an ABI-incompatible upgrade, and it's changing the informal
policy on upgrades, could we please have some more grace time? Ideally
another month, so the 15th October. If we haven't sorted it by then, it's
our problem.



On Thu, 1 Sep 2016 at 21:06 Mehdi Amini via cfe-dev <cfe-dev at lists.llvm.org>

> On Sep 1, 2016, at 1:05 PM, Reid Kleckner <rnk at google.com> wrote:
> On Thu, Sep 1, 2016 at 12:53 PM, Mehdi Amini via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>> Isn’t a big (the most) reason for supporting “old” toolchains to allow
>> downstream users to upgrade with some flexibility?
>> If I have a large codebase that is using LLVM (let say a few custom
>> backends), and is validated with “MSVC 2013”, I can upgrade to “2015” but I
>> will need some qualification/validation: this is not free and take some
>> time. If you drop aggressively supports for “old” toolchain it means that
>> I’m either stuck with an “old” LLVM or that I have to update earlier than
>> expected.
>> Isn’t this usually balanced in upstream LLVM to upgrade when there is a
>> real *benefit* to it?
>> I’m mentioning it because it seems to conflict with the "always upgrade
>> to the newest one unless there are serious issues with it” you mentioned
>> above.
> I agree, we should raise the minimum VS version requirement when the
> benefits to the LLVM community outweigh the costs of switching for major
> LLVM contributors and users. I think we'll always make that decision in the
> same way: by raising it on the mailing lists and discussing the pros and
> cons. That's basically what David said when he kicked this whole discussion
> off, anyway:
> """But if we find ourselves in a situation where asking folks to upgrade
> to a compiler which has been widely deployed soothes development for the
> greater LLVM community, we should consider dropping support for the older
> versions of that compiler."""
> I think everything is working as intended here.
> Right, to be clear there is no misunderstanding: I was absolutely not
> suggesting the opposite when answering Zach..
>> Mehdi
> We raised the VS 2013 upgrade issue, discussed it, determined that it was
> holding us back, and now we're doing the upgrade. If VS "15" brings major
> language compatibility improvements, I imagine we'll be having this same
> discussion again next year. If it doesn't, and supporting 2015 and "15" at
> the same time has the same cost, then we won't bother raising the floor for
> a while.
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