AIX 5.3 had buggy __thread support (see JDK-8176442).
That's why we introduced a macro USE_LIBRARY_BASED_TLS_ONLY and set it only to 1 on AIX (different to the other platforms supported by the OpenJDK).
However AIX 5.3 support was removed some time ago (currently we have a minimum requirement of AIX 7.X ) so the whole USE_LIBRARY_BASED_TLS_ONLY related coding could be removed.
That's why we introduced a macro USE_LIBRARY_BASED_TLS_ONLY and set it only to 1 on AIX (different to the other platforms supported by the OpenJDK).
However AIX 5.3 support was removed some time ago (currently we have a minimum requirement of AIX 7.X ) so the whole USE_LIBRARY_BASED_TLS_ONLY related coding could be removed.