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Comment by fenomas | Hacker News Reader

TFA doesn't cover any cases where switch statements got optimized. It starts with the relevant code getting deopted, then the author tries a fix but the code is still deopted, then he removes the switch statements.

(Mind you I'm not saying switch statements are fast - they may well be slower than the alternatives even after being optimized. I'm just pointing out that TFA isn't about the performance of switch statements, it's about avoiding deopts.)

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The EcmaScript switch spec basically says that each case statement in turn is evaluated until one becomes equal or default is encountered, so to follow the spec the basic case is equal to a bunch of if-else's.

To optimize the runtime first has to figure out that all case values are constants (and insert guards to de-opt if it doesn't hold anymore), so in essence the machinery for running this code optimized has to be very conservative (and probably why they placed arbitrary limits that really aren't suited for emulators).

https://tc39.es/ecma262/multipage/ecmascript-language-statem...

TFA references a post that states that optimized switch statements in V8 are still just if-elseif-else and not jump tables, at least back when it was written. They might be faster than unoptimized JS, but certainly not as fast as jump tables. The author didn't even bother to split up the functions/switch blocks further when they learned about the requirements to get V8 to actually optimize a switch, as it seemed not worth it.

Sure, none of that is in doubt. But in my experience the performance cost of hot code getting deopted is so massive as to overwhelm virtually any other factor, so realistically, fixing the deopt is what solved TFA's problem. Of course getting rid of the switch statements was presumably an additional speedup, but since TFA doesn't compare those cases I guess that's neither here nor there.

The if-else behaviour comes from the EcmaScript spec, check the sibling comment.

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