HHVM 3.19 is the new release this week and it ships with some interesting experimental features. First up, there is a "Retranslate All" feature to re-translate all profiled code into optimized translations after it hits a specified number of requests. This re-translate all can be used for getting better performance, quicker warmups, and more. This experimental feature will likely go on by default in HHVM 3.20.
Also experimental with HHVM 3.19 is a CLI server mode feature that runs a server for PHP/Hack CLI scripts so that a translation cache can be kept between script runs. hackfmt is also a new experimental feature as a formatting tool to reformat Hack code.
Other changes with HHVM 3.19 is PHP const arrays support, HHI headers for Fast Parse and Watchman, more performance improvements, and other bug fixes.
More details on HHVM 3.19 at hhvm.com.