HHVM is a follow-on effort to the static compiler. We got started long after HPHPc was in production; I started playing around in early 2010, and three of us (Jason, myself, and Drew Paroski) started earnestly putting in full-time work in summer of 2010.

I made up the term "tracelet." Do not use it when trying to sound intelligent. It roughly means "typed basic block," though the compilation units aren't quite basic blocks, and what we're doing isn't quite tracing, and ... so we thought it would be less confusing to just make up a name. Think "little tiny traces."

The bytecode is stack-based for a couple of different reasons, but it will probably remain stack-based because of compactness. Since instructions' operands are implicit, a lot of opcodes are 1, 3, or 5 bytes long. Facebook's codebase is large enough that its sheer compiled bulk can be problematic.

The translator doesn't turn the stack into SSA, instead turning it into a graph where the nodes are instructions and the edges are their inputs/outputs. You can sort of squint at this system and call it SSA where the edges are the "single assignments."