[cfe-dev] RFC: A proposal to move toward using C++11 features in LLVM & Clang / bounding support for old host compilers

[Richard Smith]

On Thu, Oct 31, 2013 at 10:06 AM, Reid Kleckner <rnk at google.com> wrote:

> Chrome still ships with VS2010.  LLVM is only brought into Chrome via
> PNaCl, I think, but I don't think it's built with MSVC.  Maybe JF could say
> more.
>
> The initial release of 2012 had lots of miscompiles, most of which
> should've been fixed in update 1.  I think the story was the same for 2013.
>
> I'm not sure why Chrome hasn't moved up to 2012 now that bugs are fixed.
>  My best guess would be that LTO memory usage increased, breaking the
> 32-bit official release build.
>
> It sounds like WebKit will link in LLVM soon, and they may still be on the
> same toolchain trajectory as Chrome.
>
> As Doug mentioned, a lot of LLVM users live close to TOT, so moving to
> C++11 in two months is a much more immediate change for them than a release
> 8 months away.
>
> Personally, I'd be happy to move, but it's not much trouble for me.  :)
>  The only thing I don't like about C++11 so far from reading lld is to see
> std::move slapped on every return value under the sun.  =P
>

Hmm. There's 23 matches for 'return std::move' in lld. Of those, 12 uses of
std::move are redundant (and in fact harmful, since they're suppressing
NRVO). 8 others will become redundant in C++14. The other 3 are performing
some non-trivial ownership transfer.

I've been intending to add a warning to Clang for calls to std::move that
suppress NRVO. Seems like it'd have value here. And then you'd have to read
'return std::move(...);' less =)


> On Thu, Oct 31, 2013 at 9:41 AM, Douglas Gregor <dgregor at apple.com> wrote:
>
>>
>> On Oct 31, 2013, at 9:18 AM, Renato Golin <renato.golin at linaro.org>
>> wrote:
>>
>> On 31 October 2013 00:24, Eric Christopher <echristo at gmail.com> wrote:
>>
>>> The timeframe “2 whole years” might seem like a long time to us, but not
>>> everybody lives in the world where they adopt new toolsets so quickly.
>>>  That’s my concern about dropping VS 2010 support.  So this is both a
>>> question about how fast Visual Studio moves, but also the people who use
>>> Visual Studio.
>>>
>>> Agreed. I think the question here is whether or not it's reasonable for
>>> this change and less whether or not it's reasonable as a path for each
>>> release to then deprecate everything more than 2 years old. I'd like to get
>>> rid of VS2010 because I want the features of 2012 and few of the current
>>> people developing on windows have spoken up (and most of them positively),
>>> but you do quite a bit of work and maintenance with windows so your
>>> thoughts are definitely important here. Do you think it's reasonable?
>>>
>>
>> You guys are still taking it too literally... ;)
>>
>> Let's take one decision at a time. We seem not to have any reason to keep
>> VS2010 support. Check box.
>>
>>
>> Not quite :). At present, we (= Apple) still have some dependencies on
>> building top-of-tree Clang with VS2010. We’re currently investigating how
>> quickly we can move those to VS2012 or newer.
>>
>> - Doug
>>
>>
>>
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