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

Reid Kleckner via llvm-dev llvm-dev at lists.llvm.org
Thu Sep 1 13:05:02 PDT 2016

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. 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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