[pcwalton]

> There is nothing stopping the Go compiler from using SIMD and automatic vectorization, it just doesn't yet.

Those optimizations would almost certainly reduce the speed of the Go compiler (requiring SSA form and aliasing info).

> There is nothing about the language that prohibits it from a very high level of optimization, indeed the language is generally sparse in a manner that allows for those optimizations.

Autovectorization is very sensitive to good output from alias analysis. This is where the const and restrict keywords in C, absent in Go, are useful. I think you will at least need runtime guards in Go, whereas they are not necessary in well-written C.