> Unaligned accesses are not a problem on x86(-64), but they will segfault
(bus error, IIRC) on other architectures such as SPARC, unfortunately.

On x86(-64) this kills performance and makes the optimization be senseless.

> FWIW, on x86/x64 gcc often generates identical code for x = y and
memcpy(x, y, 8).

The code can be identical, but the time will differ significantly for aligned and non-aligned data.