libstdc++ and dual ABI

[Joerg Sonnenberger <joerg%bec.de@localhost>]

Hello everyone,
during the developer summit, I was made aware that we got the fancy dual
ABI for libstdc++ with the GCC 5 import. I thought that $#%#$% was
disabled from the start, but alas, it isn't.

What does dual ABI mean? Long story. std::string in libstdc++ had
a performance "feature" for ages where it avoided string copying by
using reference counting. This has a number of implications, one of them
is that in-place modifications will invalidate iterators. Since this
makes the behavior highly unpredictable especially in multithreaded
programs, C++11 forbids it. Since changing central parts of std::string
requires an ABI change, GCC decided to provide both old and new
interface in the same code and introduce a fancy hack in the name
mangler. Inline namespaces as used by libc++ wouldn't allow both ABIs to
coexist, since functions using std::string only in the return type
wouldn't be mangled different as that case is not allowed in C++.

Why is this bad? It means that if libfoo is compiled with -std=c++11 and
foobar is compiled with -std=c++03, they can't be linked together and
vice versa. While many libraries are starting to adopt C++11 features,
not everyone does. The dual ABI makes interaction between the standards
*worse*, just to avoid a major bump.

What should we do? I want to take the time before the 8.0 release and
switch GCC over to always use the new ABI. This means recompiling all
C++ code, but I consider it the lesser evil of having to deal with the
fallout in pkgsrc for years to come.

Joerg