Another concern besides register file size is shuffle instructions, which can transfer any byte of a 512-bit register to any other (or any byte across two such

Another concern besides register file size is shuffle instructions, which can transfer any byte of a 512-bit register to any other (or any byte across two such registers for another instruction variant (vpermt2b), i.e. selecting from 128 bytes, and doing 64 such selections in one instruction). You can't emulate that via just two regular 256-bit uops, you need four (maybe more for blending the results together). And if you don't have the two-register table 256-bit variant (e.g. Tiger Lake doesn't, though for 512-bit of course; it splits it into three uops), that'd end up at a rather massive 12 uops.