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Comment by bonzini | Hacker News Reader

One byte instructions, with 8 registers as in the 8086, waste 8 opcodes which is 3% of the total. There are just five: "INC reg", "DEC reg", "PUSH reg", "POP reg", "XCHG AX, reg" (which is 7 wasted opcodes instead of 8, because "XCHG AX, AX" doubles as NOP).

One-byte INC/DEC was dropped with x86-64, and PUSH/POP are almost obsolete in APX due to its addition of PUSH2/POP2, leaving only the least useful of the five in the most recent incantation of the instruction set.

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I’m not sure I understand what you mean by “waste 8 opcodes.”

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.

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.

Traditionally in x86, only the first byte is the opcode used to select the instruction, and any further bytes contain only operands. Thus, since there exist 256 possible values for the initial byte, there are at most 256 possible opcodes to represent different instructions.

So if you add a 1-byte instruction for each register to zero its value, that consumes 8 of the possible 256 opcodes, since there are 8 registers. Traditional x86 did have several groups of 1-byte instructions for common operations, but most of them were later replaced with multibyte encodings to free up space for other instructions.

There are only 256 1-byte opcodes or prefixes available, if you take 8 of these to zero registers, they won't be available for other instruction, and unless you consider zeroing to be so important that they really need their 1-byte opcodes, it is redundant since you can use the 2-byte "xor reg,reg" instead, hence the "waste'.

In addition, you would need 16 opcodes, not 8, if you also wanted to cover 8 bit registers (AH/AL,...).

Special shout-out to the undocumented SALC instruction, which puts the carry flag into AL. If you know that the carry will be 0, it is a nice sizecoding trick to zero AL in 1 byte.

special mov 0 instruction times 8 registers. The opcode space, especially 1 byte opcode space, is precious so encoding redundant operations is wasteful.

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