It's amazing to me that every CPU doesn't have specialized memory copying instructions given how common an operation this is and how it can be a bottleneck. We have hundreds of vector instructions to optimize all kinds of special cases but little for this most general thing?
Intel has `rep movs` which is recommended for general use, you can beat it for sizes < 128B using a specialized loop. A very good memcpy for general purposes would just branch over the size to either a specialized small copy or `rep movs` for larger sizes.
x86 does, it's called REP MOVS, and it copies entire cachelines at a time (if the size of the copy lets it.) AFAIK it has done this since the P6 family (Pentium II), and has only gotten better over time. As a bonus, it is only 2 bytes long. I believe the Linux kernel uses it as its default memcpy()/memmove().
It's funny enough that the REP family of instructions has existed at least since the original 8086/8088. It's very much a relic of the CISC heritage of the x86 ISA, and it was probably meant to make writing assembly code easier and to save space - rather to improve performance.
I remember it was actually quite common in early PC programs, but I don't remember well enough whether it was also generated by compilers, or just existed in hand-optimized assembly (which was of course extremely common back then).
As I tried to indicate, the microbenchmark-guided unrolled loops look better than the simple `rep` instruction because the benchmarks aren't realistic. They don't suffer from mispredicted branches, they don't experience icache pressure. Basically the microarchitectural reality is absent in a small benchmark.
There's also the fact that `rep` startup cost was higher in the past than it is now. I think it started to get really fast around Ivy Bridge.
This is all discussed in Intel's optimization manual, by the way.
Yes, some compilers emit AVX move instructions on memcpy or compiler-generated moves (e.g. pass by value struct). (In addition to the rep move mentioned in the sibling posts)
