Owen Yamauchi - Hack and HHVM: Programming Productivity Without Breaking Things

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For most of its history, Facebook has held internal hackathons every few months. For hackathons, engineers are encouraged to come up with ideas that arent related to their day jobs, form teams, and try to make something cool, in the span of a day or two.

In 2007, one hackathon in November 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, since 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. Facebooks entire web front-end 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 Facebooks 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to executionjust-in-time compilation to machine code, with no C++ involvedgrew out of HPHPcs codebase, and eventually superseded it. This just-in-time compiler, called the HipHop Virtual Machine, or HHVM for short, took over Facebooks entire web server fleet in early 2013. The original PHP-to-C++ transformer is gone; it is not deployed anywhere and its code has been deleted.

The origin of Hack is entirely separate. Its roots are in a project that attempted to use static analysis on PHP to automatically detect potential security bugs. Fairly soon, it turned out that the nature of PHP makes it fundamentally difficult to get static analysis thats deep enough to be useful. Thus the idea of strict mode was born: a modification of PHP, with some features removed (such as references), and a sophisticated type system added. Authors of PHP code could opt into strict mode, gaining stronger checking of their code while retaining full interoperability.

Hacks direction since then belies its origin as a type system on top of PHP. It has gained new features with significant effects on the way Hack code is structured, like async. It has added new features specifically meant to make the type system more powerful, like collections. Philosophically, its a different language from PHP, carving out a new position in the space of programming languages.

This is how we got where we are today: a modern, dynamic programming language with robust static typechecking, executing with just-in-time compilation on an engine with full PHP compatibility and interoperability.