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

Robinson, Paul via cfe-dev cfe-dev at lists.llvm.org
Tue Aug 2 12:49:08 PDT 2016

  -----Original Message-----
> From: cfe-dev [mailto:cfe-dev-bounces at lists.llvm.org] On Behalf Of Aaron
> Ballman via cfe-dev
> Sent: Tuesday, August 02, 2016 12:06 PM
> To: David Majnemer
> Cc: llvm-dev; cfe-dev
> Subject: Re: [cfe-dev] [llvm-dev] Revisiting our informal policy to
> support two versions of MSVC
> On Tue, Aug 2, 2016 at 1:24 PM, David Majnemer via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
> > Hello,
> >
> > Today we hit another VS 2013 breakage
> > <http://lab.llvm.org:8011/builders/sanitizer-
> windows/builds/26666/steps/run%20tests/logs/stdio>
> > which results us having to alter LLVM.
> >
> > While we have no documented policy of supporting two version of MSVC, we
> do
> > have an informal agreement that we should support the last two versions.
> >
> > I suggest that we alter our informal policy to the following:
> >
> > "If a compiler version keeps getting in the way and a newer compiler is
> > available, we should ask people to upgrade to that newer compiler."
> I think that's reasonable, but I also think this is no different than
> what we already do today, which is unfortunate. The lack of
> predictability of when we drop support for MSVC is something I would
> hope we can address. That's what the "last two versions" was hoping to
> achieve, but hasn't in practice since we've never actually adhered to
> that policy due to finding pain points to justify dropping early.
> Since this proposal is basically preserving the status quo, I don't
> think predictability needs to be solved in order to move forward with
> the proposal of dropping support for 2013.
> To me, the goal of having some degree of predictability with compiler
> versions is for people with out-of-tree projects to have a chance of
> knowing when support for a compiler may be dropped and plan
> accordingly. This isn't traditionally a problem with GCC (e.g.)
> because the stability of features and functionality is usually a bit
> higher than with MSVC, where major upgrades can be challenging and
> labor-intensive for some projects.

For my project, timing is everything.  We (and I could easily imagine,
for many downstream projects) lead time is important.  We've just had
a release branch, so making a *decision* right now that we *implement*
in 2-3 months is something we should find tolerable.  (We'd need to do
a bunch of internal validation, then deploy 2015 to all the developers
and all the bots... these things take time.)  I have already brought
this up with my management and we'll need to assign some resources to
it in order to start that evaluation.

> Maybe we can bring back the notion
> of  "last two versions" sometime in the future if MSVC functionality
> stabilizes a bit more.

You'd have to get the C++ committee to stabilize the language first. :-)
Seriously, MSVC 2013 would be distinctly less of a problem if people
weren't trying to use defaulted move constructors.  I think that's a
fine feature to want to be using, but it wasn't on the list back when
we decided to make 2013 the minimum version.  The developer community's
desired feature set has increased, so it's reasonable to debate moving
the minimum bar on that basis.

> > If we can support ten versions of MSVC with little burden, I don't see a
> > reason why we shouldn't.
> >
> > 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.
> Totally agreed.
> > In this case, dropping VS2013 allows us to use more C++11 features with
> > confidence.  Notably, move constructors will be synthesized instead of
> > having to be manually written (and kept in sync with data members
> getting
> > added).
> >
> > What do you all think?  Are folks still stuck on VS2013?
> Assuming that we don't have any major barriers to dropping MSVC 2013,
> I am okay with it.

Same here.  We will need to do our internal validation first, and
I'd bet we aren't the only ones.

> Additional request: can we also informally require
> the latest Update to be installed for whatever versions of MSVC we
> support (if we don't already)? I don't see a whole lot of value to
> keeping support for Update 1 & 2 when Update 3 is out.
> ~Aaron
> >
> > Thanks,
> > David
> >
> > _______________________________________________
> > LLVM Developers mailing list
> > llvm-dev at lists.llvm.org
> > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> >
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