I have a question about using the extended registers as in r8, r9, r10, …. , I want to use them frequently in my program, but I’ve noticed a problem with code size.

For instance:

mov eax, DWORD [rdi+4]

☝️This generates the following machine code: `8b 47 04` that’s 3 bytes

But when I use an extended register like `r9`, it becomes:

mov eax, DWORD [r9+4]

Which now generates : `41 8b 41 04`, which has an extra byte prolly due to the prefix `41` , making it 4 bytes now

Why does using `rN` registers ie r8, r9, etc, result in larger code size compared to using lower registers like rdi?

Apart from code size, are there other performance related concerns , it’s it’s cache performance or execution cycles or others when using the `rN` registers instead of the lower registers?