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The SIMD proposal is in stage 4. It is released in V8, and behind a flag in Firefox.

It is also released in Firefox (x86 only, ARM is in the next release iirc)

You can see "Standardized features" and "In-progress proposals" in this table here: https://webassembly.org/roadmap/

It's a spec in addition to the initial we assembly spec. It's supported by clang/llvm, V8, firefox, and some standalone runtimes like wasmer.

SIMD for wasm is all 128bit operations. I've been working with it and so far it works pretty fast, but there's missing support for some operations. Namely lack of support for saturation/truncation operations.