#### 6.4.8 Support for testing binary compatibility

The file `compat.exp` provides language-independent support for binary compatibility testing. It supports testing interoperability of two compilers that follow the same ABI, or of multiple sets of compiler options that should not affect binary compatibility. It is intended to be used for testsuites that complement ABI testsuites.

A test supported by this framework has three parts, each in a separate source file: a main program and two pieces that interact with each other to split up the functionality being tested.

`testname_main.suffix`
  Contains the main program, which calls a function in file `testname_x.suffix`.
`testname_x.suffix`
  Contains at least one call to a function in `testname_y.suffix`.
`testname_y.suffix`
  Shares data with, or gets arguments from, `testname_x.suffix`.

Within each test, the main program and one functional piece are compiled by the GCC under test. The other piece can be compiled by an alternate compiler. If no alternate compiler is specified, then all three source files are all compiled by the GCC under test. You can specify pairs of sets of compiler options. The first element of such a pair specifies options used with the GCC under test, and the second element of the pair specifies options used with the alternate compiler. Each test is compiled with each pair of options.

`compat.exp` defines default pairs of compiler options. These can be overridden 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. For example, with `[list [list {-g -O0} {-O3}] [list {-fpic} {-fPIC -O2}]]`, the test is first built with `-g -O0` by the compiler under test and with `-O3` by the alternate compiler. The test is built a second time using `-fpic` by the compiler under test and `-fPIC -O2` by the alternate compiler.

An alternate compiler is specified by defining an environment variable to be the full pathname of an installed compiler; for C define `ALT_CC_UNDER_TEST`, and for C++ define `ALT_CXX_UNDER_TEST`. These 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_CC_UNDER_TEST` or `ALT_CXX_UNDER_TEST` is `same`, each test is built using the compiler under test but with combinations of the options from `COMPAT_OPTIONS`.

To run only the C++ compatibility suite using the compiler under test and another version of GCC using specific compiler options, do the following from `objdir/gcc`:

         rm site.exp
         make -k \
           ALT_CXX_UNDER_TEST=${alt_prefix}/bin/g++ \
           COMPAT_OPTIONS="lists as shown above" \
           check-c++ \
           RUNTESTFLAGS="compat.exp"

A test that fails when the source files are compiled with different compilers, but passes when the files 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 bug that was fixed in the compiler under test but is present in the alternate compiler.

The binary compatibility tests support a small number of test framework commands that appear within comments in a test file.

`dg-require-*`
  These commands can be used in `testname_main.suffix` to skip the test if specific support is not available on the target.
`dg-options`
  The specified options are used for compiling this particular source file, appended to the options from `COMPAT_OPTIONS`. When this command appears in `testname_main.suffix` the options are also used to link the test program.
`dg-xfail-if`
  This command can be used in a secondary source file to specify that compilation is expected to fail for particular options on particular targets.