Since some of the tests inherently produce platform-specific results, we have provided a way to supply platform-specific result comparison files. Frequently, the same variation applies to multiple platforms; rather than supplying a separate comparison file for every platform, there is a mapping file that defines which comparison file to use. So, to eliminate bogus test "failures" for a particular platform, you must choose or make a variant result file, and then add a line to the mapping file, which is src/test/regress/resultmap.
Each line in the mapping file is of the form
The test name is just the name of the particular regression test module. The platform pattern is a pattern in the style of the Unix tool expr (that is, a regular expression with an implicit ^ anchor at the start). It is matched against the platform name as printed by config.guess followed by :gcc or :cc, depending on whether you use the GNU compiler or the system's native compiler (on systems where there is a difference). The comparison file name is the name of the substitute result comparison file.
For example: some systems using older time zone libraries fail to apply daylight-saving corrections to dates before 1970, causing pre-1970 PDT times to be displayed in PST instead. This causes a few differences in the horology regression test. Therefore, we provide a variant comparison file, horology-no-DST-before-1970.out, which includes the results to be expected on these systems. To silence the bogus "failure" message on HPUX platforms, resultmap includes
which will trigger on any machine for which the output of config.guess includes -hpux. Other lines in resultmap select the variant comparison file for other platforms where it's appropriate.