Compatibility test suites that use lib/compat.exp test interoperability
of two compilers that follow the same ABI, or of multiple sets of
compiler options that are not intended to affect binary compatibility.
They do not test conformance to a formal ABI; they check that things are
done consistently, not that they are done correctly.  They are meant to
complement ABI test suites, not to replace them.

Each test in these suites splits up functionality into two source files
that can be compiled with two different versions of GCC and with two
sets of compiler options.

  Override the default pairs of compiler options (from compat.exp) by
  defining the environment variable COMPAT_OPTIONS as:
  COMPAT_OPTIONS="[list [list {tst1} {alt1}]...[list {tstn} {altn}]]"
  where tsti and alti are lists of options, with "tsti" used by the
  compiler under test and "alti" used by the alternate compiler. The
  characters "[", "{", "}", and "]" are really in the string, as in
  "[list [list {-g} {-O3}] [list {-O3} {-g}]]".

  To use an alternate compiler, define the environment variable for the
  language you're testing; e.g., for C++ define ALT_CXX_UNDER_TEST to be
  the full pathname of an installed compiler.  That will be written to
  the site.exp file used by DejaGNU.  The default is to build each test
  with the compiler under test using the first of each pair of compiler
  options from COMPAT_OPTIONS.  When ALT_CXX_UNDER_TEST is "same", each
  test is built using the compiler under test but with combinations of
  the options from COMPAT_OPTIONS.  If the alternate compiler needs a
  different setting for LD_LIBRARY_PATH, specify it in the environment
  varible ALT_LD_LIBRARY_PATH.  If it needs different options to build
  a shared object, define those in ALT_SHOBJ_OPTIONS.

To run only the C++ compatibility tests, optionally specifying an
alternate compiler and/or sets of options, do from $objdir/gcc:

  rm site.exp
  make -k ALT_CXX_UNDER_TEST=<prefix>/bin/g++ \
    ALT_LD_LIBRARY_PATH=<prefix>/lib \
    COMPAT_OPTIONS="list as shown above" check-c++ \
    RUNTESTFLAGS="compat.exp"

A test that fails when the pieces are compiled with different compilers
but passes when the pieces are compiled with the same compiler
demonstrates incompatibility of the generated code or runtime support.
A test that fails for the alternate compiler but passes for the compiler
under test probably tests for a fix that is not present in the alternate
compiler.

There's a lot more that can be done with these tests.

  Increase the robustness of the existing tests:

   -  recognize target triples in dg-options strings in test files

   -  continue to split up tests that rely on language runtime support
      and add them to this set of tests

   -  make sure they work for testing with a simulator

   -  multilibs support?

  Build test pieces into more combinations that will be likely to find
  unusual errors:

   -  link using the alternate compiler; this requires support for
      LD_LIBRARY_PATH for the alternate compiler in the support for each
      language

   -  build pieces of the test into static libraries; this requires
      knowledge about how to build a static library for each target

  Continue to add tests to g++.dg/compat, based on existing tests for
  language runtime support and for test cases in PRs that didn't used to
  fit into the test suite.

  Set up similar tests in libstdc++-v3/testsuite.

  Set up similar tests for additional languages and libraries.

Janis Johnson, <janis187@us.ibm.com>
October 2002