How JavaScript works: inside the V8 engine

Re:How JavaScript works: inside the V8 engine by zbjornson on Monday August 21, 2017 @05:12PM (#15067897)

You're right. In V8, values from a Float32Array are always converted to doubles upon access, even in cases like adding values from two Float32Arrays and storing them in another Float32Array where the result is provably the same. That costs a handful of cycles for each element.

Mozilla has a blog post about doing efficient float32 math: https://blog.mozilla.org/javascript/2013/11/07/efficient-flo...

V8 bug: https://bugs.chromium.org/p/v8/issues/detail?id=5541

Using a Float64Array is faster at the expense of memory usage. (I've never seen a case where the code generated by V8 bottlenecks on memory bandwidth fwiw.)

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Re:How JavaScript works: inside the V8 engine by fenomas on Monday August 21, 2017 @09:52PM (#15069159)

Thanks for the details, very useful!

I don't suppose you know if similar things are true for Int arrays? I think I once had a similar case there, where I was doing integer operations on a large set of ints, and regular arrays were faster than Int32. But it's hard to imagine there as well what the slowdown would be.

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Re:How JavaScript works: inside the V8 engine by zbjornson on Tuesday August 22, 2017 @12:31AM (#15069720)

Good question. I haven't looked at that case. V8 can detect and optimize small ints (smi, fit in uint32, i.e. all types of typedarrays), but it might still take the easy/slow path and convert to doubles. Relatively easy to look at with node.js and irhydra.

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