[jsheard]

They did try to do SIMD in JS, and the proposal made it as far as being implemented in browsers, but it ended up being hideously convoluted in practice so it was killed in favor of WASM SIMD.

The MVP implementation of SIMD.js grew to about 10% of V8s entire codebase, just for the bare minimum "it runs" support without optimisations.

[davidmurdoch]

How large is WASM? and how convoluted are the implementations and the integrations now?

[jsheard]

Ballparking it based on the relative LOC of v8/src and v8/src/wasm, about 8%. That's for everything WASM though, not just the SIMD support, and it's a fully optimized and production-ready implementation.