The Hip-Hop Virtual Machine (HHVM) is a JIT compiler for executing PHP programs. It is used by some of the world’s largest websites such as facebook.com and wikipedia.org, among many others. At Facebook we have frequently been asked why we don't use LLVM as a back end for HHVM. Inspired by the success of Apple’s FTL we implemented an alternative back end using LLVM.
In this talk we will share what it took to hook LLVM in to HHVM from conception to running limited production traffic. We will cover changes to our internal IR and modifications we had to make to LLVM. We will discuss performance challenges we faced, peculiar bugs, and finally will discuss why we are not yet at the point of enabling the LLVM back end for production Facebook traffic.
