DIIS

DevHeads IoT Integration Server•17mo ago

Marvee Amasi

Impact of Using Extended Registers (r8, r9, etc.) on Code Size and Performance

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]

mov eax, DWORD [rdi+4]

This generates the following machine code:

8b 47 04

8b 47 04

that's 3 bytes

But when I use an extended register like

r9

r9

, it becomes:

mov eax, DWORD [r9+4]

mov eax, DWORD [r9+4]

Which now generates :

41 8b 41 04

41 8b 41 04

, which has an extra byte prolly due to the prefix

41

41

, making it 4 bytes now

Why does using

rN

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

rN

registers instead of the lower registers?