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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 (J

by kmavm 15y ago

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."

nickik 15y ago
Let me give you a discription of the compiler and then you can tell me if I understand everything correct. You have a interpreter that for every basic block starts to record a trace. If you walk in to a basic block (or codeblock) the X-time with the same type you compile it for that type and you set a typeguard. Now everytime the interpreter goes trough there again it does a typecheck then either keeps interpreting or jumps into the compiled block. So if you have basic block in a loop you have to typecheck every time you go threw the loop.
- kmavm 15y ago
  Pretty close. Once we've translated to machine code, the interpreter doesn't do type checks; it blindly enters the translation cache for anything that has a translation, and the guards live in the tracelets themselves. The tracelets directly chain to one another in machine code, so they need embedded guards.
  - nickik 15y ago
    I see. Thx for answering all my questions. Good luck with outperfmorming the static compiler :)

by kmavm 15y ago

nickik 15y ago

Let me give you a discription of the compiler and then you can tell me if I understand everything correct. You have a interpreter that for every basic block starts to record a trace. If you walk in to a basic block (or codeblock) the X-time with the same type you compile it for that type and you set a typeguard. Now everytime the interpreter goes trough there again it does a typecheck then either keeps interpreting or jumps into the compiled block. So if you have basic block in a loop you have to typecheck every time you go threw the loop.

kmavm 15y ago
Pretty close. Once we've translated to machine code, the interpreter doesn't do type checks; it blindly enters the translation cache for anything that has a translation, and the guards live in the tracelets themselves. The tracelets directly chain to one another in machine code, so they need embedded guards.
- nickik 15y ago
  I see. Thx for answering all my questions. Good luck with outperfmorming the static compiler :)