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

Zachary Turner via cfe-dev cfe-dev at lists.llvm.org
Thu Sep 1 12:35:44 PDT 2016


I guess what I'm getting at, is why do we even need to support 2 toolchains
with LLVM *at all*?  Why can't we just always upgrade to the newest one
unless there are serious issues with it?

There used to be two reasons that I'm aware of:

1) Licensing.  Community used to be Express, and Express used to be limited
in functionality.  But this is no longer the case.  Community is fine for
anyone to use, even corporations, for free, as long as it is for an open
source project.  IANAL, but it says it in Section 1.b.1 here
<https://www.visualstudio.com/support/legal/mt171547>.

2) Library toolchain mismatches.  It used to be that in order to link
against a library, it had to be compiled with the same toolchain.  So
someone whose application was built with VS 2012 couldn't link against LLVM
built with VS 2013, they would have to build LLVM themselves with VS2012,
so dropping support for 2012 would harm them.

But beginning with Visual Studio 2015, because of the UCRT, this is no
longer the case.  All versions of the CRT will be forward compatible.

So if we're going to upgrade to VS 2015, why not also just upgrade to VS 15
as soon as possible?  Once VS 2015 is the minimum, it doesn't seem like we
should worry too much about upgrading compiler versions anymore (aside from
the effort to actually do it and make sure it works and the compiler bugs
and issues are known about).

On Thu, Sep 1, 2016 at 12:24 PM Nico Weber <thakis at chromium.org> wrote:

> On Thu, Sep 1, 2016 at 3:16 PM, Zachary Turner <zturner at google.com> wrote:
>
>> I don't see how the policy of supporting 2 versions is related to that
>> difficulty though. Whether we support 1 versions or 2 versions, 1 version
>> is still going to be deprecated every time a new version is released. So
>> this burden on upgrading buildbots doesn't seem to be much different.
>>
>> As long as chromium compiler version is not tied to llvm compiler version
>> (and if it did, that would be a chromium decision not an llvm one) this is
>> always going to be a problem whether llvm supports newest compiler or
>> newest 2 compilers
>
>
> I think in general the world is trying to update to new toolchains, it
> just takes a while. With LLVM's old policy, Chromium always updated faster
> than LLVM so far.
>
>
>>
>> On Thu, Sep 1, 2016 at 12:09 PM Nico Weber <thakis at chromium.org> wrote:
>>
>>> On Thu, Sep 1, 2016 at 3:03 PM, Zachary Turner <zturner at google.com>
>>> wrote:
>>>
>>>> I frequently see mention of how upgrading is problematic, is there
>>>> anyone here for whom upgrading msvc versions is problematic? It seems like
>>>> we keep talking in hypotheticals, but I'd like to hear from someone for
>>>> whom it is *actually* a problem, and why.
>>>>
>>>> Vs community is permissive enough now that licensing isn't an issue.
>>>> And every time this comes up it seems like we're saying "well it could be
>>>> hard for people..." but nobody ever says it actually is hard for them
>>>> specifically
>>>>
>>>
>>> I just said that in the mail you're replying to (?) Having to update our
>>> ~20 Windows bots so that they use a different MSVC than what's on there
>>> already for Chromium is going to be actually hard for us.
>>>
>>>
>>>> On Thu, Sep 1, 2016 at 11:16 AM Aaron Ballman via cfe-dev <
>>>> cfe-dev at lists.llvm.org> wrote:
>>>>
>>>>> On Thu, Sep 1, 2016 at 2:08 PM, Nico Weber <thakis at chromium.org>
>>>>> wrote:
>>>>> > On Thu, Sep 1, 2016 at 1:30 PM, Aaron Ballman <
>>>>> aaron at aaronballman.com>
>>>>> > wrote:
>>>>> >>
>>>>> >> On Thu, Sep 1, 2016 at 1:23 PM, Nico Weber via cfe-dev
>>>>> >> <cfe-dev at lists.llvm.org> wrote:
>>>>> >> > As mentioned upthread, we're still on update 2 for various
>>>>> reasons.
>>>>> >>
>>>>> >> Do you mind elaborating on those reasons?
>>>>> >
>>>>> >
>>>>> > Off the top of my head, clang-cl couldn't handle the code generated
>>>>> by the
>>>>> > midl compiler in that version until fairly recently, and we've been
>>>>> seeing
>>>>> > link.exe /INCREMENTAL failing intermittently (no reliable repro case
>>>>> > though).
>>>>>
>>>>> Thank you for the extra information.
>>>>>
>>>>> >
>>>>> >>
>>>>> >> I think we should require
>>>>> >> the latest updates to MSVC due to the number of issues the updates
>>>>> fix
>>>>> >> (esp regarding the newer language features that people keep using so
>>>>> >> frequently), but I've also not heard a concrete use case as to why
>>>>> we
>>>>> >> shouldn't.
>>>>> >
>>>>> >
>>>>> > Do you mean "require latest updates" in general, or just in this
>>>>> case?
>>>>> > Updating immediately every time a new MSVS release comes out would
>>>>> > definitely be tricky for us.
>>>>>
>>>>> I mean in general, but "immediate" wasn't what I had in mind. More
>>>>> like "within some reasonable time frame", for whatever definition of
>>>>> reasonable works for people. Given how often Microsoft fixes critical
>>>>> language bugs with newer features during an Update release, I'm hoping
>>>>> we can avoid sticking to older Updates if we are able to do so.
>>>>>
>>>>> ~Aaron
>>>>>
>>>>> >
>>>>> >>
>>>>> >>
>>>>> >> ~Aaron
>>>>> >>
>>>>> >> >
>>>>> >> > On Thu, Sep 1, 2016 at 1:07 PM, Robinson, Paul <
>>>>> paul.robinson at sony.com>
>>>>> >> > wrote:
>>>>> >> >>
>>>>> >> >> Hi Reid, first off thanks *very* much for all your help fixing
>>>>> >> >> 2013-related problems.  We really appreciate it.
>>>>> >> >>
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> Let me propose a target date of September 15 for advancing the
>>>>> minimum
>>>>> >> >> MS
>>>>> >> >> compiler to VS2015 Update 3.  Certainly my team should be ready
>>>>> by
>>>>> >> >> then. If
>>>>> >> >> anybody else needs a later date, in particular people who own
>>>>> Windows
>>>>> >> >> bots
>>>>> >> >> still using VS2013, please speak up.
>>>>> >> >>
>>>>> >> >> --paulr
>>>>> >> >>
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> From: Reid Kleckner [mailto:rnk at google.com]
>>>>> >> >> Sent: Wednesday, August 31, 2016 4:07 PM
>>>>> >> >> To: Robinson, Paul
>>>>> >> >> Cc: James Molloy; Nico Weber; llvm-dev; cfe-dev at lists.llvm.org
>>>>> >> >> Subject: Re: [llvm-dev] [cfe-dev] Revisiting our informal policy
>>>>> to
>>>>> >> >> support two versions of MSVC
>>>>> >> >>
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> I'd like to revisit this. As a person who spends a fair amount
>>>>> of time
>>>>> >> >> monitoring our VS 2013 buildbots, I would say that I am ready to
>>>>> throw
>>>>> >> >> in
>>>>> >> >> the towel on MSVC 2013. Since this discussion, I have committed
>>>>> five
>>>>> >> >> (!)
>>>>> >> >> workarounds for MSVC 2013:
>>>>> >> >>
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> # in llvm
>>>>> >> >>
>>>>> >> >> $ git log --author=rnk --grep=2013  --after='Aug 4 2016'
>>>>> --oneline
>>>>> >> >>
>>>>> >> >> 21a8ade Fix the MSVC 2013 build by using Elf_Word instead of
>>>>> making a
>>>>> >> >> local typedef
>>>>> >> >>
>>>>> >> >> 27e101d Revert "Add an optional parameter with a list of undefs
>>>>> to
>>>>> >> >> extendToIndices"
>>>>> >> >>
>>>>> >> >> e8beddd Make vec_fabs.ll pass with MSVC 2013
>>>>> >> >>
>>>>> >> >> ca77873 [AMDGPU] Give enum an explicit 64-bit type to fix MSVC
>>>>> 2013
>>>>> >> >> failures
>>>>> >> >>
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> # in clang
>>>>> >> >>
>>>>> >> >> $ git log --author=rnk --grep=2013  --after='Aug 4 2016'
>>>>> --oneline
>>>>> >> >>
>>>>> >> >> 18235a5 Try to work around an MSVC 2013 bug around defaulted
>>>>> default
>>>>> >> >> ctors
>>>>> >> >>
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> I'm pretty sure I'm missing instances where I helped others
>>>>> commit
>>>>> >> >> workarounds as well. So, I'd really like to drop 2013, probably
>>>>> >> >> sometime
>>>>> >> >> next month.
>>>>> >> >>
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> That said, I'd also like to echo Paul's sentiment that it'd help
>>>>> if
>>>>> >> >> people
>>>>> >> >> were less adventurous in their uses of C++11. New language
>>>>> features may
>>>>> >> >> look
>>>>> >> >> nice, but ultimately you may end up wasting my time and yours
>>>>> when I
>>>>> >> >> come
>>>>> >> >> and revert your change.
>>>>> >> >>
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> On Thu, Aug 4, 2016 at 7:52 AM, Robinson, Paul via llvm-dev
>>>>> >> >> <llvm-dev at lists.llvm.org> wrote:
>>>>> >> >>
>>>>> >> >> I've heard from another group within Sony that they had "a
>>>>> number of
>>>>> >> >> problems" with VS2015 update 2, and strongly recommend going
>>>>> straight
>>>>> >> >> to
>>>>> >> >> update 3.  My immediate team has initiated a request but it
>>>>> hasn't gone
>>>>> >> >> through yet.
>>>>> >> >>
>>>>> >> >> --paulr
>>>>> >> >>
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> From: James Molloy [mailto:james at jamesmolloy.co.uk]
>>>>> >> >> Sent: Wednesday, August 03, 2016 1:54 AM
>>>>> >> >> To: Nico Weber; Robinson, Paul
>>>>> >> >> Cc: llvm-dev; cfe-dev at lists.llvm.org
>>>>> >> >> Subject: Re: [cfe-dev] [llvm-dev] Revisiting our informal policy
>>>>> to
>>>>> >> >> support two versions of MSVC
>>>>> >> >>
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> Hi,
>>>>> >> >>
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> This sounds like a decent idea to me. However we use 2013 for
>>>>> all our
>>>>> >> >> windows builds at the moment and it will take around 2 weeks to
>>>>> upgrade
>>>>> >> >> the
>>>>> >> >> installations on our cluster. We're pushing this hard to get it
>>>>> done
>>>>> >> >> soon so
>>>>> >> >> we don't get caught short, but a grace period would be much
>>>>> >> >> appreciated.
>>>>> >> >>
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> Cheers,
>>>>> >> >>
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> James
>>>>> >> >>
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> On Tue, 2 Aug 2016 at 21:24 Nico Weber via cfe-dev
>>>>> >> >> <cfe-dev at lists.llvm.org> wrote:
>>>>> >> >>
>>>>> >> >> On Tue, Aug 2, 2016 at 3:49 PM, Robinson, Paul via cfe-dev
>>>>> >> >> <cfe-dev at lists.llvm.org> wrote:
>>>>> >> >>
>>>>> >> >> For my project, timing is everything.  We (and I could easily
>>>>> imagine,
>>>>> >> >>
>>>>> >> >> for many downstream projects) lead time is important.
>>>>> >> >>
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> In Chromium land, we've so far been able to use the same
>>>>> compiler we
>>>>> >> >> use
>>>>> >> >> to build Chrome to build clang. Currently that's MSVS2015 update
>>>>> 2, and
>>>>> >> >> it
>>>>> >> >> took quite a while to update from 2013 to 2015 due bugs in 2015
>>>>> and
>>>>> >> >> whatnot.
>>>>> >> >> So I agree that it's useful to support older MSVS versions for
>>>>> some
>>>>> >> >> time.
>>>>> >> >> For this reason, requiring update 3 would be inconvenient for
>>>>> us, but
>>>>> >> >> 2015u2
>>>>> >> >> would be no problem by now. It would've been a problem if 2015
>>>>> had been
>>>>> >> >> required shortly after it was released.
>>>>> >> >>
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> Nico
>>>>> >> >>
>>>>> >> >> _______________________________________________
>>>>> >> >> cfe-dev mailing list
>>>>> >> >> cfe-dev at lists.llvm.org
>>>>> >> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>>> >> >>
>>>>> >> >>
>>>>> >> >> _______________________________________________
>>>>> >> >> LLVM Developers mailing list
>>>>> >> >> llvm-dev at lists.llvm.org
>>>>> >> >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>>> >> >>
>>>>> >> >>
>>>>> >> >
>>>>> >> >
>>>>> >> >
>>>>> >> > _______________________________________________
>>>>> >> > cfe-dev mailing list
>>>>> >> > cfe-dev at lists.llvm.org
>>>>> >> > http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>>> >> >
>>>>> >
>>>>> >
>>>>> _______________________________________________
>>>>> cfe-dev mailing list
>>>>> cfe-dev at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>>>
>>>>
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