[bonzini]

They occupy 8 of the possible 256 byte values. Together, those five cases used about 15% of the space.

Though I was forgetting one important case: MOV r,imm also used one-byte opcodes with the register index embedded. And it came in byte and word variants, so it used a further 16 opcodes bytes for a total of 56 one byte opcodes with register encoding.

[drob518]

Gotcha, thanks for clarifying. I was reacting to the word “waste” I guess. Surely, as you say, it consumes that opcode encoding space. Whether that’s a waste or not depends on a lot of other things, I suppose. I wasn’t necessarily thinking x86-specific in my original comment. But yea, if you try to zero every possible register and half-word register you would definitely consume lots of encoding space.