Which is these is "faster" depends greatly on whether you have the very rare memcpy-only workload, or if your program actually does something useful. Many people believe, often with good evidence, that the most important thing is for memcpy to occupy as few instruction cache lines as is practical, instead of being something that branches all over kilobytes of machine code. For comparison, see the x86 implementations in LLVM libc.

https://github.com/llvm/llvm-project/blob/main/libc/src/stri...