After almost a year development a new stable version has been made available.
SWI-Prolog 8.2 notably brings many of the advanced tabling support of XSB to SWI-Prolog: Well Founded Semantics, restraints, incremental tabling and shared tabling. It also makes rational numbers primary citizens. Using Google's tcmalloc reduces the memory footprint drastically on some multi-threaded workloads.
See announcement on Discourse for details.