> interesting that you have a separate row for "66+F2"

Yeah that's only for 0F38F0 to 0F38FF.

> Re 'custom size "xh"': AVX-512 also has fourth and eighth

Also AVX for VPMOVSX and VPMOVZX but those are handled differently. I probably should check if xh is actually redundant... EDIT: it's only needed for VCVTPS2PH, which is the only instruction with a half-sized destination.

> I think the prefix decoding is not quite right for x86-64: 26/2e/36/3e are ignored in 64-bit mode

Interesting, I need to check how they interact with the FS/GS prefixes (64/65).

> REX prefixes that don't immediately preceed the opcode (or VEX/EVEX prefix) are ignored

Oh, didn't know that!