It's true, I was considering only the original mov rax, 1 case, which I'm pretty sure compilers don't generate: it's just a useless encoding of mov eax, 1. Giv

It's true, I was considering only the original mov rax, 1 case, which I'm pretty sure compilers don't generate: it's just a useless encoding of mov eax, 1. Given that 64-bit immediate math also causes this though, compilers might generate it (I think this is a minor missed optimization by gcc though: it could have used add eax, 1 instead.