C++ ABI library switching

[David Chisnall]

Hello the list,

I am currently working on making it possible to switch C++ ABI library.  We currently ship one of these in FreeBSD 9.0 (libsupc++), and will hopefully ship two (libsupc++ and libcxxrt) in 9.1 and one (libstdc++) in 10.

Currently, libc++ dynamically links to libcxxrt, libstdc++ statically links to libsupc++.  I would like to make us build libsupc++ as a .so and link libstdc++.so with -Wl,-f,libsupc++.so.1.  This makes it an auxiliary filter so, if libsupc++.so.1 exists, it will be used in place of the internal version.  This preserves ABI compatibility and allows us to switch between libcxxrt and libsupc++ with a libmap.conf entry, making it easy for users of 9.1 to test the libstdc++ and libcxxrt combination.

With this done, it's then possible to link both libstdc++ and libc++ into the same program, making the transition from libstdc++ to libc++ relatively painless (everything still needs recompiling, but it doesn't all need recompiling at the same time).  For 10.0, we can then ship a COMPAT version of libstdc++ that is linked against libcxxrt by default.

Any objections / suggestions of better approaches?

David