An Introduction to HHVM

HHVM (HipHop Virtual Machine) converts PHP code into a high-level bytecode (commonly known as an intermediate language). This bytecode is then translated into x64 machine code dynamically at runtime by a just-in-time (JIT) compiler. In these respects, HHVM has similarities to virtual machines for other languages including C#/CLR and Java/JVM.

A little big of history

In early 2008 Facebook began working on HipHop(now HPHP), a PHP execution engine; its original motivation was to convert Facebook massive PHP code base into C++ in order to save resources and increase the application performance. The original release was known as HPHPc a PHP to C++ compiler.

For next 2 years Facebook continued working on HipHop adding HPHPi ( a ‘developer mode’ version of HPHP) and HipHop debugger known as HPHPd, this allowed developers to watch and step through the code and interactively debug PHP applications running on the HipHop platform.

At it’s peak, HipHop PHP code showed up to 6x times better performance than its ZEND counterpart. However, there where several drawbacks to this first iteration of HipHop:

• HPHPc did not fully supported the PHP language, most noticeably the lack of support for create_function() and eval() –Honestly, I don’t see the lack of support for the last one as something bad– functions.
• Facebook developers had to maintain two difference engines (HPHPc and HPHPi) with resulted in duplication of efforts and waste of resources.
• Finally, HPHPc required a vastly different deployment process which will hurt adoption by PHP developers.

In light of these problems Facebook took two key actions on early 2010, the first was to open source the HipHop platform. – Open sourcing a project like this is a great way to build a community around the project and getting external help from that community –/

At the same time Facebook started the development of the modern version of HipHop, known as HHVM (HipHop Virtual Machine), HHVM improves the strengths of HPHPc and corrects many of the critical problems.

HHVM was build on top of HPHPc and it works by converting PHP code into high level bytecode(an intermediate language) this bytecode is the translated into machine code dynamically at runtime by a JIT (Just-In-Time) compiler.

If you are like me you probably have a vague recollection of the concepts of bytecode, machine code and Just-In-Time compilers, so let’s take a momentarily side step and quickly review these concepts and how the they play a key role on HHVM.

Bytecode, Machine code, JIT, Oh my …!

• Bytecode: Is a non human readable code designed for efficient execution by an interpreter or compiler. When HHVM first loads our project it converts all the PHP code into this intermediary form of bytecode; the bytecode generate is not particular to any type of architecture and could be portable among different systems.

• Machine code: Are a set of instructions designed to be executed directly by the CPU. If you ever played with Assembly (Who hasn’t !?) you know very well what machine code is, for those very few of you who haven’t had the pleasure of working on assembly or similar languages; machine code is generated by the compiler or interpreter and then handed to the CPU

• JIT (Just In Time) compiler: Just in time compilation is a technique used to improve performance of software, this is achieved by compiling bytecode during execution, the byte code is store in memory and then the JIT compiler loads and compiles as many sections of the byte code as needed.

The performance and speed gained by applying these techniques is what gives HipHop and subsequently HHVM its core strengths. Keeping a PHP code base while achieving performance comparable to compiled applications.

Currently HHVM supports PHP 5.4 almost on its entirety, however there are still numerous bugs that prevent some applications from running, for that reason Facebook has set as goal to have the top 20 open source PHP applications running on HHVM. The first popular application to achieve this was Wordpress.

What’s next

So now that we have a better understanding of what HHVM does and the advantages if running it we can start testing our applications on HHVM. In a subsequent post I will be covering the setup of a dedicated Vhost for HHVM, running benchmarks against your applications and eventually (fingers crossed) how to run Magento on HHVM.