On the x86, in the P4 times the best performing bulk operations essentially required using SIMD, and that SIMD hated you unless you aligned your memory accesses. The result was horrible bloated code to handle leading and trailing data and thus also a need to split off the implementations for small sizes. The unaligned access penalty is much lower now, and REP-prefixed operations have microcoded implementations that use the maximum memory access width (which you can’t do otherwise without SIMD instructions).

I’m curious about what the referenced code compiles down to, actually, because not only could GCC be auto-vectorizing it, it could be replacing it with a REP STOSQ or indeed a call to memset.