> I'm not sure that's true: rep movs is pretty fast these days.
That's only true if you target skylake and newer. If you target generic x86_64 then compilers will only emit rep mov for long copies due to some CPUs having a high baseline cost for it. There's some linker magic that might get you some optimized version when you callq memcpy, but that doesn't help with inlined copies.
That is irrelevant. The default target of compilers is some conservative minimum profile. Any binary you download is compiled for wide compatibility, not to run on your computer only.
That's only true if you target skylake and newer. If you target generic x86_64 then compilers will only emit rep mov for long copies due to some CPUs having a high baseline cost for it. There's some linker magic that might get you some optimized version when you callq memcpy, but that doesn't help with inlined copies.