Also, why is the ASM version so much faster if the glue code doesn't make much difference? I suppose it matters at the margins...
The remark on the glue code only applies to my entry (the second). The assembly entry uses a completely different (and much more complicated) algorithm than the C entry.
(To check, I wrote a Go program that doen nothing else than fmt.Print($largebody) in a for loop and it tops out at around 10GB/s)
In Rust this memcpy intrinsic is required (for the core language, not just the standard library) because Rust clearly has arbitrarily large data structures you can literally copy, so, somebody has to take responsibility for doing that work, Rust says it's the compiler back-end's job. On a toy CPU the memcpy might really just be a trivial read->register->write loop because that's adequate.