Hacker News Reader

Thank you! That makes quite a few things clear.

As a naïve question, how does SBCL (Common Lisp compiler) generate such a wicked fast code? Other than actor model, it is also dynamically typed and hot code loading is one of the headline features there.

View on HN

When I say hot code loading, I’m really specifically referring to how that works in the BEAM, which is a more sophisticated mechanism than simply compiling/generating code on the fly. The biggest problem though is that it prevents most forms of dead code elimination. It also means that you can never assume anything about how a function will be called, because at any point new code could be loaded that calls it differently. You can still optimize such code with a tracing JIT, but in Wasm (at least currently) that’s not even an option.

Without knowing too much detail about SBCL specifically, I suspect that they use a combination of clever static analysis and specialization to unlock a lot of that speed. That way functions can be specialized/inlined where beneficial, but new code can safely call the unoptimized versions when loaded at runtime, but even hot loaded code could be specialized with a JIT on hand. The big reason we made the trade off with hot code loading though is due to the restrictions that Wasm imposes - there’s no particular reason we couldn’t support it otherwise. In general it is rarely used in production BEAM apps in my experience, so from my perspective it seemed like an opportunity to stop paying for a feature unused and gain something in return.

Thank you for detailed reply here and elsewhere in the thread. That probably took as much time as writing a new blog post, so it is really appreciated.

As mentioned before, as a newbie student of Elixir this is all very exciting. Please keep up the good work!

Reply on news.ycombinator.com