Hack and HHVM

Preface

For most of its history, Facebook has held internal hackathons every few months. For hackathons, engineers are encouraged to come up with ideas that aren’t related to their day jobs—they form teams and try to make something cool in the span of a day or two.

One hackathon in November 2007 resulted in an interesting experiment: a tool that could convert PHP programs into equivalent C++ programs and then compile them with a C++ compiler. The idea was that the C++ program would run a lot faster than the PHP original, as it could take advantage of all the optimization work that has gone into C++ compilers over the years.

This possibility was of great interest to Facebook. It was gaining a lot of new users, and supporting more users requires more CPU cycles. As you run out of available CPU cycles, unless you buy more CPUs, which gets very expensive, you have to find a way to consume fewer CPU cycles per user. Facebook’s entire web frontend was written in PHP, and any way to get that PHP code to consume fewer CPU cycles was welcome.

Over the next seven years, the project grew far beyond its hackathon origins. As a PHP-to-C++ transformer called HPHPc, in 2009 it became the sole execution engine powering Facebook’s web servers. In early 2010, it was open sourced under the name HipHop for PHP. And then, starting in 2010, an entirely new approach to execution—just-in-time compilation to machine code, with no C++ involved—grew out of HPHPc’s codebase, and eventually superseded it. ...