Re: C++ in jemalloc

On Thu, Oct 5, 2017 at 3:01 PM, Warner Losh <imp_at_bsdimp.com> wrote:
> On Thu, Oct 5, 2017 at 11:59 AM, David Goldblatt <davidtgoldblatt_at_gmail.com>
> wrote:
>
>>  Hi all,
>>
>> The jemalloc developers have wanted to start using C++ for a while, to
>> enable some targeted refactorings of code we have trouble maintaining due
>> to brittleness or complexity (e.g. moving thousand line macro definitions
>> to templates, changing the build->extract symbols->rebuild mangling scheme
>> for internal symbols to one using C++ namespaces). We'd been holding off
>> because we thought that FreeBSD base all had to compile on GCC 4.2, in
>> order to support some esoteric architectures[1].
>>
>> The other day though, I noticed that there is some C++ shipping with
>> FreeBSD; /usr/bin/dtc and /sbin/devd (the former claiming in the HACKING
>> document that C++11 is a minimum for FreeBSD 11). This, combined with the
>> fact that ports now points to a modern gcc, makes me think we were
>> incorrect, and can turn on C++ without breaking FreeBSD builds.
>>
>> Am I right? Will anything break if jemalloc needs a C++ compiler to build?
>> We will of course not use exceptions, RTTI, global constructors, the C++
>> stdlib, or anything else that might affect C source or link compatibility.
>>
>> Thanks,
>> David (on behalf of the jemalloc developers
>>
>> [1] That being said, we don't compile or test on those architectures, and
>> so probably don't work there in the first place if I'm being honest. But
>> we'd also like to avoid making that a permanent state of affairs that can't
>> be changed.
>>
>
> For FreeBSD 10 and earlier, this would likely break all architectures that
> aren't x86. Starting in FreeBSD 11, arm and powerpc are supported by clang,
> but not super well. For FreeBSD 12, we're getting close for everything
> except sparc64 (whose fate has not yet been finally decided).
>
> So for the popular architectures, this arrangement might work. For building
> with external toolchains, it might also work. Some of the less popular
> architectures may be a problem.
>
> Does that help? It isn't completely cut and dried, but it should be helpful
> for you making a decision.
>
> Warner

To be clear, Warner is talking about C++11 code in jemalloc.  C++98
will work fine on all architectures, and I think most of C++03 will
too.  dtc(1) is allowed to use C++11 because it only builds on
architectures that support it.

-Alan