[llvm-dev] Using C++14 code in LLVM

[Mehdi Amini via llvm-dev]

> On Oct 3, 2016, at 10:38 AM, Justin Bogner via llvm-dev <llvm-dev at lists.llvm.org> wrote:
> 
> Zachary Turner via llvm-dev <llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>> writes:
>> A thread was started over the summer to discuss the timeline for bumping
>> LLVM up to Visual Studio 2015 to enable the use of various new language
>> features.  Currently the ETA for this is sometime in mid-October, so within
>> 2-3 weeks.
>> 
>> With this happening imminently, I thought it would be worth tossing this
>> out there and seeing what people think.
>> 
>> With VS on 2015, the major lagging compiler is going to be GCC.  Our
>> minimum GCC requirement is 4.7, which is quite old (almost 4 years for
>> anyone keeping count).  What are the challenges with pushing this forward?
>> 
>> With VS2015, the biggest remaining missing C++14 features are variable
>> templates, sized allocation, and extended constexpr.  But it gains generic
>> lambdas, init-captures, decltype(auto) return, and auto return without
>> requiring the trailing -> decltype syntax.
> 
> I'm not convinced that the cost is worth the gain for these features.
> Most of these are relatively straight forward (with more boilerplate) to
> work around in C++11, and I'd rather we not get into a situation where
> to bootstrap clang you need to build a C++11 compatible version so that
> you can build a C++14 compatible one so that you can build a C++17
> compatible one, etc. Consider that -std=c++17 isn't yet a valid flag on
> trunk clang, so if we bump to C++14 now then we're practically
> guaranteed that when we want to update to 17 we'll run into multi-stage
> bootstrapping problems.

I didn’t quite get this last part, can you elaborate please?

Thanks,

— 
Mehdi


> 
>> While GCC doesn't claim to "fully" support C++14 until 5.2 (which is only
>> about 1 year old), you can get all of the above features with GCC 4.9
>> 
>> So if we're willing to allow "partial" C++14 support in LLVM (i.e a
>> whitelisted set of features), we may be able to get many of the most useful
>> features by going to GCC 4.9, which is still a good 2.5 years old.
> 
> I'd be much happier getting to requiring "full" C++11 support than going
> to partial C++14 support - having partial support for C++11 has meant
> that many issues are only caught by bots, and I much prefer the compiler
> to complain when I use some feature we're not allowed to use yet.
> 
>> One potentially added benefit of this is that GCC supports <regex> in 4.9.
>> This might allow us to kill of llvm::Regex in favor of standardizing on
>> std::regex, as GCC is currently the only supported compiler without a regex
>> implementation.
>> 
>> Thoughts?
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org <mailto:llvm-dev at lists.llvm.org>
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev <http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20161003/483c4047/attachment-0001.html>