Is there a technical restriction why JS JITs don't transparently apply SIMD to existing loops like normal compilers (=autovectorization)? Or is it in the cards to build it on top of SIMD.js?
Autovectorization is hard and fragile in the best of times -- tight stable loops with no (or few) branches and little/no memory access that may be aliased, etc.
For a dynamic language JIT this is pretty much infeasible (as I understand it). Every loop might have branches for guards/bailing out due to deopts, and at least in JS, TypedArrays are allowed to alias eachother.