Something I'm curious about is why JS developers don't have the option to send along some of this information themselves; type information, JS bytecode, etc. Given that we often have it on hand already (TypeScript) or could integrate it into our build process (webpack). Obviously plain JS needs to still work without all that, but it could be a compelling point of optimization for large-scale apps. Perhaps the JS bytecode just isn't standardized across browsers?
JS tried to add (optional) type hints in the ES4 standard that was never adopted (outside of tangential things like ActionScript 3).
It would be great if Typescript hints could pass right along to the JITs as useful optimization factors, but it currently sounds like TC39 would prefer not to recreate the disasters of ES4 and are staying out of type hints for the forseeable future.
(Well-typed code should prevent most JIT bailouts, at least. Typescript linters could possibly give even better "shape" warnings than they currently do, such as catching the issue the React team recently found of bailouts in the V8 engine due to "shapes" being built with "small integers" being reallocated to doubles at runtime. However, such lint warnings would probably be JIT engine specific, and maybe premature optimization in 90%+ of usages.)
Yeah it's probably not worth it to add types to JS the language, but what if you could ship standard metadata files similar to source-maps that only included type information, which browsers could leverage to speed up compilation?
In the long term, I hope that at least a compromise like the Python annotation approach could be reached (standardize the parsing, but ignore the semantics). Typescript type annotations share a surface level syntax with them in some ways, and also with ES4's type annotation attempt.
"Type maps" are an interesting idea. You could probably piggy back on/boostrap from WASM types.
Adding types would probably slow down the code as some optimizations done by the engine wouldnt be allowed. Kinda like when you use bitwise operations for optimization only to find out they actually slow down the code.
We could hower add Interface and optional types as syntax sugar for runtime checks.
One way to see it's more subtle than meets the eye: suppose the website defines a function F which is annotated as taking a string argument. The website itself only ever calls F with strings. But suppose I go to the website, open the console, and punch in F(1). Of course, this sort of thing could be guarded against, but the point is just that it would introduce more complexity than initially meets the eye.
Engines already have to do this - check the type of an argument before calling the compiled version of the function. What the OP is suggesting is simply prefilling the type and instruction caches with information that the build system was already able to gleen.
And that would probably amount to a "bailout". I'm only talking about providing hints that can be used to skip parts of the described process, not getting rid of it altogether.
i always wondered why it wasn't possible to use a js app in "record" mode and then export any witnessed type & profiling information from the JIT and deliver it alongside the source (like a sourcemap).
That's not the same thing; asm.js is a precursor to WASM which is a bytecode language that doesn't include garbage collection, and asm.js is actually an interpreter for it that's written in JS. The JS bytecode the article talks about is generated from JS and interpreted by C++.
In other words, it isn't practical to compile JS to WASM, and especially not to asm.js. The browser's JIT compilation of JS targets something completely different.
I was mostly replying to this: "JS developers don't have the option to send along some of this information themselves; type information, JS bytecode", that in JS you can send some type information with asm.js hints (or TypedArrays), but yes you don't send it directly to the C++ interpreter, but to the JS interpreter, which presumably will pass it further.
The point is, if you care about performance use asm.js. If you don’t then just use whatever standard path browsers provide and any optimization they do is gravy.
