Why does -Wabi-tag complain when -std=c++03?

[Jonathan Wakely]

On 24 March 2016 at 17:29, Jeffrey Walton <noloader@gmail.com> wrote:
> On Thu, Mar 24, 2016 at 1:02 PM, Jonathan Wakely <jwakely.gcc@gmail.com> wrote:
>> On 24 March 2016 at 16:43, Jeffrey Walton wrote:
>>> There are lots of folks who have experienced the issue; check out
>>> http://bugs.debian.org/cgi-bin/pkgreport.cgi?tag=libstdc%2B%2B-cxx11;users=debian-gcc@lists.debian.org.
>>> Debian happened to say "f**k it, we'll just recompile everything under
>>> the new ABI and be done with it". But they experienced the corner-case
>>> issues too. I don't know why they did not consult with the GCC folks.
>>
>> They did. Rebuilding everything is the right choice.
>
> It was an incomplete remediation. Debian still has open bugs because
> of it, and we are fielding reports because of it.
>
>>> This step was not obvious to me at all. I don't ever recall having to
>>> define a library macro for a library:
>>>
>>>     $ g++ -D_GLIBCXX_USE_CXX11_ABI=0 -c test.cxx -o test-v1.o
>>>     $ g++ -D_GLIBCXX_USE_CXX11_ABI=1 -c test.cxx -o test-v2.o
>>>
>>> Naively, because of the blog post on the coexistent implementations, I
>>> was looking for an option like -dual-abi to do it for me.
>>
>> Which is a leap that doesn't in any way follow from a literal reading
>> of the blog post!
>
> Right. We've had similar on other platforms for years and even on
> Linux. For example, with Apple, we can build fat binaries. Even the
> kernel has FatELF (http://lwn.net/Articles/359070/).
>
> This situation seems like a perfect application of the technologies,
> but it seems to be missing from the solution. Hence the reason I
> refrained from the leap, and asked what to do/how do I do it.

I've written up the basic steps of how to do what I really, really
suggest you do not try to do at http://kayari.org/cxx/dualabi.html

The last section explains why a simple compiler switch probably won't
work: most code will need non-trivial refactoring to make it work,
which a compiler could not do on the fly if you simply provide a
command-line option.