I researched about string instructions that is supported in AVX or AVX2 ISA but I can not find any 256 bit string comparison instruction like SSE4.2 If there is any string comparison that I can not find where can I find them? Otherwise Why AVX/AVX2 ISA does not support 256 bit string instructions?
I also find out that AVX2 does not support mullo for unsigned short as a 16bit unsigned integer and I don't know the reason. Because it has been supported in SSE4.2.
256 bit string compare instructions - no, there are none in AVX/AVX2 (or AVX-512 for that matter) - just the 128 bit instructions in SSE 4.2.
mullo for unsigned short - this is not needed, since the result would be the same as with a signed short mullo. It's only the nulhi instruction that needs to exist in signed and unsigned variants.