Question: Is sub eax in code_asm unoptimized?

[Sewer56]

Hi; I have a quick question regarding code generation with code_asm.
I couldn't see anything related in issues list; so I figured I would ask here:

When I assemble with

a.sub(eax, 10)

I get the result:

0:  2d 0a 00 00 00          sub    eax,0xa 

I.e. It uses SUB (EAX, I32)

However, this can also theoretically be assembled with SUB (R32, I8)

0:  83 e8 0a                sub    eax,0xa

Which yields a smaller size in terms of code generation.

At first, I expected the EAX specific instruction to be faster, so I then had a look at uops.info; and it appears that the non-specialized instruction is measured to be faster on modern CPUs.

Namely, on modern architectures like Zen 3, SUB (R32, I8) clocks in at 0.25 throughput and SUB (EAX, I32) clocks in at 0.33. [Higher value is worse]. Intel CPUs follow the same trend.

I did cross reference these values with very well known Agner's CPU Optimization Guide; they matched; however Agner not have the measurements for SUB (EAX, I32) specifically, only SUB (R32, I8).

---

Note: In the Rust bindings, the code is auto-generated as:

#[rustfmt::skip]
impl CodeAsmSub<AsmRegister32, i32> for CodeAssembler {
	fn sub(&mut self, op0: AsmRegister32, op1: i32) -> Result<(), IcedError> {
		let code = if op0.register() == Register::EAX {
			Code::Sub_EAX_imm32
		} else if op1 >= i8::MIN as i32 && op1 <= i8::MAX as i32 {
			Code::Sub_rm32_imm8
		} else {
			Code::Sub_rm32_imm32
		};
		self.add_instr(Instruction::with2(code, op0.register(), op1)?)
	}
}

(Picking EAX when possible)

[wtfsck]

Yes, looks like that can be improved.