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Re: GCC version bikeshedding

On 6 August 2014 11:20, Richard Biener wrote:
> On Wed, Aug 6, 2014 at 12:08 PM, Jonathan Wakely <> wrote:
>> On 6 August 2014 10:06, Jakub Jelinek wrote:
>>> On Wed, Aug 06, 2014 at 11:04:14AM +0200, Richard Biener wrote:
>>>> > - libstdc++ ABI changes (it is a significant user visible change,
>>>> >   if you rebuild everything, no extra effort is needed, but otherwise
>>>> >   if you want some C++ code built with older compilers work together
>>>> >   with code built with newer compilers, it might require source code
>>>> >   changes (the abi_tag attribute additions where needed and warning
>>>> >   suggest to put those at), at least that is my current understanding
>>>> >   of the plans
>>>> But that's only with -std=c++11?  Which had no compatibility
>>>> guarantees before?
>>> No, AFAIK it is also -std=c++98.  At least my understanding was that
>>> std::list and std::string are going to change ABI (and get new abi_tag)
>>> in all C++ modes.  Jonathan/Jason/Paolo, is that right?
>> Correct. We want C++03 code to continue to be able to interoperate
>> with C++11 code.
>> It's an ABI change for all modes (but not a SONAME change because the
>> old and new definitions will both be present in the .so).
> Ugh.  That's going to be a nightmare to support.  Is there a configure
> switch to change the default ABI used?

You'll need to define a macro to get the old versions of the affected
classes, we can add a configure switch to make that the default.

>  That is, on a legacy system
> can I upgrate to 5.0 and get code that interoperates fine with code
> built with 4.8?  (including ABI boundaries using the affected classes?
> I suspect APIs with std::string passing are _very_ common, not
> sure about std::list)
> What's the failure mode the user will see when linking against a
> 4.8 compiled library with a std::string interface using 5.0?

There will be a linker error as the types and the interface functions
will have different mangled names.

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