[sunfish]

I'm a Mozilla engineer involved in this. NEON support is very important and we're designing the spec to support it well.

CPUs that lack SIMD units can support the functionality (though not the performance of course), and there's even a polyfill library that can lower this API into scalar operations for SIMD-less browsers too.

[IvanK_net]

It would be great, if you could detect SIMDable operations in classic JS (e.g. in loops) and use SIMD for interpret them. I think that adding low-level features into a high-level language is not good practice.

[sunfish]

We will probably do that too at some point, but it won't replace explicit SIMD, just as widely-available auto vectorization support in C++ hasn't eliminated the need for explicit SIMD extensions there either.

One thing to keep in mind is that most programmers probably won't want to use this feature directly; it'll be used in libraries that expose higher-level APIs. It's still true that every feature we add increases overall clutter, but SIMD seems sufficently useful and sufficiently self-contained that it's worth the tradeoff.