I’m not sure why someone of facebook’s scale would want JIT instead of AOT. That’s a lot of servers all JITing the same thing that could be done once at build time.
JIT compilation has the opportunity to do profile-guided optimization at runtime. JIT compilation is also simpler when distributing an application to non-identical servers, as it can optimize for the exact hardware it is running on.
HHVM's predecessor at FB was an AOT PHP to C++ transpiler called HPHPc. There are a lot more optimization opportunities with JIT compilation when dealing with dynamically typed languages like PHP.
