C++ compatibility testing

[Janis Johnson <janis187 at us dot ibm dot com>]

Phil and Benjamin,

I'm again looking into how to set up tests within the GCC testsuite to
build two pieces of a C++ test using different versions of GCC and then
linking them and running the test.  Phil, you mentioned that you might
try this (http://gcc.gnu.org/ml/gcc/2002-08/msg01259.html); please let
me know if you're doing something similar.  If you've got a scheme for
specifying a second compiler for the test tools to use, let me know what
it is.

I plan to use this to split up some existing tests of functionality of
the C++ runtime support from directories abi, eh, ext, init, and rtti
of gcc/testsuite/g++.dg.  Similar testing could be used in the
libstdc++-v3 testsuite for functionality of iostreams and other library
support.

After I've got things working with building simple relocatable objects
and linking them together, I'll try building the pieces of a test into
static and/or shared libraries in the hopes of finding different kinds
of problems.  That might be messy using dejagnu, though, and I only have
GNU/Linux systems on which to try out my changes.

This testing approach doesn't verify that the compiler is following an
ABI correctly, merely that it is consistent from one release to another.
On the other hand, it might be more likely to find weird compatibility
problems than will formal ABI test suites.  The two approaches
complement each other.

Janis