When object support was first added to HHVM, a class named "Instance" was introduced (deriving from ObjectData) to represent instances of user defined classes. Since then, things have evolved and HPHPc and HPHPi have been retired, and now there really is no needed to have ObjectData and Instance be separate classes anymore. As a first step towards merging ObjectData and Instance together, this diff puts their definitions in the same .h file and puts their implementations in the same .cpp file. A few small changes were necessary to fix issues with cyclical includes: (1) Repo/emitter related parts of class.cpp and class.h were moved to class-emit.cpp and class-emit.h; (2) the contents of "vm/core_types.h" was moved to "base/types.h"; and (3) a few functions that didn't appear to be hot were moved from .h files and the corresponding .cpp files.
HipHop VM for PHP 
HipHop VM (HHVM) is a new open-source virtual machine designed for executing programs written in PHP. HHVM uses a just-in-time compilation approach to achieve superior performance while maintaining the flexibility that PHP developers are accustomed to. HipHop VM (and before it HPHPc) has realized > 5x increase in throughput for Facebook compared with Zend PHP 5.2.
HipHop is most commonly run as a standalone server, replacing both Apache and modphp.
Installing
You can install a prebuilt package or compile from source.
Running
You can run standalone programs just by passing them to hhvm: hhvm my_script.php.
HipHop bundles in a webserver. So if you want to run on port 80 in the current directory:
sudo hhvm -m server
For anything more complicated, you'll want to make a config.hdf and run sudo hhvm -m server -c config.hdf.
Contributing
We'd love to have your help in making HipHop better. If you run into problems, please open an issue, or better yet, fork us and send a pull request. Join us on #hhvm on freenode.
If you want to help but don't know where to start, try fixing some of the Zend tests that don't pass. You can run them with hphp/test/run. When they work, move them to zend/good and send a pull request.
Before changes can be accepted a Contributors Licensing Agreement (pdf - print, sign, scan, link) must be signed.
Licence
HipHop VM is licensed under the PHP and Zend licenses except as otherwise noted.