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by adrianN 3045 days ago
I assume that JITs don't do very expensive optimizations because they have to do a trade off between execution speed and compilation time. JITs are also fairly blind on the first execution of a piece of code. Static optimizations are not made obsolete by the existence of JITs.
1 comments
fenomas 3045 days ago
Expensive optimizations like what? Can you give a before/after example of something such a tool might do?

(Note: I'm glossing over the case where one is using bleeding-edge syntax that a JS engine doesn't yet know how to optimize. In that case preprocessing out the new syntax is of course very useful, but I don't think this is the kind of optimization the GP comment was talking about.)

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seeekr 3045 days ago
JIT engines usually don't do static analysis. I'm not sure if that is because the cost for that is that much higher, but a hint towards why could be that the engine simply does not know which parts of the (potentially huge amounts of) code that was loaded is actually going to be needed during execution, so analysing all of it is likely to bring more harm than gain.

As an example for something that static analysis could have caught, take the example from the article about the "Argument Adaptation"[0]. Here the author uses profiling to learn that by matching the exact argument count for calling a function, instead of relying on the JS engine to "fix that", the performance can be improved by 14% for this particular piece of code. Static analysis could have easily caught and fixed that, essentially performing a small code refactoring automatically just like the author here did manually.

[0] http://mrale.ph/blog/2018/02/03/maybe-you-dont-need-rust-to-...

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fenomas 3045 days ago
I replied in main to your other comment, but regarding the "Argument Adaptation" issue it's maybe worth noting that I'm 90% sure V8 would have optimized this automatically if not for the subsequent issue (with monomorphism). I'm dubious that the former issue could be statically fixed as easily as you suggest, but either way I think it should be considered a symptom of the latter issue.
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