128-bit or 256-bit memsets via SIMD instructions are sufficient to saturate RAM bandwidth, so there wouldn't be much of a gain from having a dedicated instruction.
(By the way, x86 does have a dedicated instruction--rep stosb--but compilers differ as to how often they use it, for the reason cited above.)
(By the way, x86 does have a dedicated instruction--rep stosb--but compilers differ as to how often they use it, for the reason cited above.)