What is HHVM?
HHVM is an open-source virtual machine designed for executing programs written in Hack. HHVM uses a just-in-time (JIT) compilation approach to achieve great runtime performance while maintaining amazing development flexibility.
HHVM supports the Hack programming language. We are moving fast, making changes daily and releasing often. If you notice a regression in the typechecker or the runtime, please open issues when you find them.
The JIT Compiler
Rather than directly interpret or compile source code directly to C++, HHVM compiles Hack into an intermediate bytecode. This bytecode is then translated into x64 machine code dynamically at runtime by a just-in-time (JIT) compiler. This compilation process allows for all sorts of optimizations that cannot be made in a statically compiled binary, thus enabling higher performance of your Hack programs.