emval_get_global() - can we make it faster?

[Lilit Darbinyan]

embind currently generates the following glue code for accessing the global object:

function emval_get_global() { return (function(){
    return Function;
  })()('return this')(); }

This function is called every time we access emscripten::val::global() from C++, e.g. 

val Object = val::global("Object");

It turns out it's pretty slow in SpiderMonkey - running this function in a loop 10,000 times we see that it takes on average 117 ms in SpiderMonkey compared to 17 ms in V8.

I shall raise this slowness issue with SpiderMonkey separately, but I am wondering if there is room for improvement here on the emscripten side as well - for example, would it make sense to cache this value, or make use of the new globalThis instead?

[Mathias Bynens]

+1 to using globalThis, and falling back to the current approach or a more robust polyfilling mechanism.

[Sergey Rubanov]

+1, this also unlocks usage of emval_get_global in scripts with unspecified `unsafe-eval` in CSP.

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[Alon Zakai]

Thanks everyone, definitely sounds like a good idea!

I opened https://github.com/emscripten-core/emscripten/pull/8904 for this.

- Alon

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