Re: [gentoo-dev] impending c++11 clusterfuck?

[Michał Górny]

On Sun, 29 Nov 2015 19:56:04 -0800
"Gregory M. Turner" <gmt@×××××××.net> wrote:

> I'm quoting myself from bug #566328 here.  These were off-the-cuff
> remarks that got away from me and became a call-to-arms...
>
> (In reply to Michał Górny from comment #7)
> > This is never this simple. C++11 can change the ABI. So the point kinda is,
> > we need to ensure that all C++ libraries in a depgraph use the same C++
> > version.
>
> This is pretty awful when you really think about it.  I feel like I'm
> watching a train-wreck in super slow motion.

Well, it's not that bad actually. After some thinking, I figured out
they fixed most 98/11 incompatibilities around gcc 4.8/4.9, and left
only a few 'unlikely' to cause issues.

However, if one dep switches to C++11, it is quite likely to require
C++11 in its revdeps, and that's what happening with libsigc++
and other gtkmm libraries.

Plus, there's of course the classical issue of ABI incompatibility
between libstdc++ bundled with 4.9 and 5.1, and 5.2... so along with
switching g++ version, you soon start to have to rebuild random C++
libraries.

And the issue of supporting alternative C++ standard library
implementations -- like using libcxx with clang. They are of course
incompatible with GNU's ever-changing ABI.

> I'm not sure we're taking this seriously enough -- sooner or later it
> seems destined to become a major clusterfuck if we don't do something
> proactive about it now while the drawing-board is relatively
> uncluttered.
>
> The only thing I can think of that has this kind of two-way depgraph
> magic property are the major "abi" USE_EXPAND values (multilib-build
> and python-r1, in other words).
>
> But those rely on fancy framework-generated USE-flag deps, which seem
> like overkill and likely to incur unjustifiable user-experience-costs.

Yes, it is terrible. You end up introducing a lot of USE flags that
need to be manually switched along with gcc versions. If we start
splitting them between c++98 and c++11, we're quite likely to hit USE
flag conflicts between packages/developers which prefer one over
another.

> Perhaps a solution to this cxx11 clusterfuck can be found that works
> more like perl?  By that I mean, pick your poison (respectively, your
> cxx11 ABI of preference or your major perl version of choice), rely on
> inbuilt portage features do the trick most of the time, and, when it
> breaks, run "magically-fix-everything.sh," grab a caffeinated beverage
> or three and fire up your favorite VOD client while the mess gets
> magically cleaned up by robots somehow.

Sadly := can't help here since gcc switches occur independently of
package installs. And AFAIK revdep-rebuild doesn't help either.
-- 
Best regards,
Michał Górny
<http://dev.gentoo.org/~mgorny/>