[cfe-dev] Using C++11 in clang/tools/extra

[Chandler Carruth]

On Wed, Nov 28, 2012 at 9:58 PM, Eli Friedman <eli.friedman at gmail.com> wrote:
> On Wed, Nov 28, 2012 at 7:39 PM, Chandler Carruth <chandlerc at google.com> wrote:
>> On Wed, Nov 28, 2012 at 7:28 PM, Nick Kledzik <kledzik at apple.com> wrote:
>>> On Nov 28, 2012, at 10:45 AM, Marshall Clow wrote:
>>>> On Nov 28, 2012, at 7:15 AM, "Vane, Edwin" <edwin.vane at intel.com> wrote:
>>>>> It came up in a recent code review (http://llvm-reviews.chandlerc.com/D136) that C++11 features aren't allowed in the codebase. I can understand the reason for this in the LLVM and clang codebases as they're both widely used. However, would it be a problem to use C++11 officially in clang/tools/extra? It's not nearly as widely used and has less of a chance of impacting critical work in the community. Are there other problems I'm missing? If not, could the use of C++11 in clang/tools/extra be officially blessed?
>>>>
>>>> One of the problems with doing this (as we've found with lld), is that you need to link it against a set of clang libraries that are also built with C++11.
>>>
>>> I've had no problem building lld with C++11 and llvm with C++03.    The one change I did need to make to the llvm build was to build with -stdlib=libc++.  That is, use the new libc++ standard library for both projects.
>>
>> I suspect that what Marshall has hit is that libstdc++ has
>> ABI-breaking changes when compiled in C++11 mode vs. C++03 mode. I
>> don't know if anyone has audited libc++ for such changes, it took us
>> some time to find them in libstdc++, but they led to moderately
>> terrible bugs.
>
> IIRC, Howard tried to be pretty careful about making C++03 and C++11
> in libc++ binary-compatible (unlike libstdc++, which had intentional
> differences).  Of course, it's possible there are bugs.

That would be quite impressive, and very useful.

Anyways, there are folks that need to use llvm, lld, and
clang-tools-extra with libstdc++, and for them it is important to
build llvm in C++11 mode. Honestly, we should be building llvm in
C++11 mode whenever it is possible because of the performance
benefits, so I don't see this as a barrier anyways....