The HipHop Virtual Machine (HHVM) is a JIT compiler and runtime for PHP. While PHP values are dynamically typed, real programs often have latent types that are useful for optimization once discovered. Some types can be proven through static analysis, but limitations in the aheadof-time approach leave some types to be discovered at run time. And even though many values have latent types, PHP programs can also contain polymorphic variables and expressions, which must be handled without catastrophic slowdown. HHVM discovers latent types by structuring its JIT around the concept of a tracelet. A tracelet is approximately a basic block specialized for a particular set of run-time types for its input values. Tracelets allow HHVM to exactly and efficiently learn the types observed by the program, while using a simple compiler. This paper shows that this approach enables HHVM to achieve high levels of performance, without sacrificing compatibility or interactivity.

Scripting languages are widely used to quickly accomplish a variety of tasks because of the high productivity they enable. Among other reasons, this increased productivity results from a combination of extensive libraries, fast development cycle, dynamic typing, and polymorphism. The dynamic features of scripting languages are traditionally associated with interpreters, which is the approach used to implement most scripting languages. Although easy to implement, interpreters are generally slow, which makes scripting languages prohibitive for implementing large, CPU-intensive applications. This efficiency problem is particularly important for PHP given that it is the most commonly used language for server-side web development.This paper presents the design, implementation, and an evaluation of the HipHop compiler for PHP. HipHop goes against the standard practice and implements a very dynamic language through static compilation. After describing the most challenging PHP features to support through static compilation, this paper presents HipHop's design and techniques that support almost all PHP features. We then present a thorough evaluation of HipHop running both standard benchmarks and the Facebook web site. Overall, our experiments demonstrate that HipHop is about 5.5× faster than standard, interpreted PHP engines. As a result, HipHop has reduced the number of servers needed to run Facebook and other web sites by a factor between 4 and 6, thus drastically cutting operating costs.