[akireu]

Why not? Fixed-size SIMD architectures use mostly the same operations, so if you target SSE2 initially, the code should run just fine on NEON. A runtime that ships a JIT compiler also has the unique opportunity to further optimize SIMD code by using more lanes or limiting the working set to the host platform's L1 cache size. Even the AOT compilers like GCC or clang emulate platform-specific intrinsics using generic vector ones. This should count for something, no?

[TinkersW]

They are similar but not the same, for instance SSE has movemask, but NEON does not, so it gets emulated(slowly) when targeting that platform. The cross lane ops are different enough that you might need to rewrite for other platforms. And then you run into situations where an instruction is very fast on one architecture but horribly slow on another because its basically emulated.

[akireu]

This isn't really relevant to wasm, though. You can't expect it to support platform-specific hacks just for SIMD, so you'll have to make do with the lowest common denominator anyway.