Difference Between Addq and Movl, Then Addq

The first one pulls a full quadword (64 bits) out of memory at location rbx and adds that to the rax register.

The second pulls a longword (only 32 bits) from that same location and stores it into ecx (zeroing the top half of rcx). Then it adds rcx to rax.

So I would say the main difference is that the second snippet is not adding the full quadword in memory to rax, only the longword.

The first (one-liner) code sample would be more akin to:

movq    (%rbx), %rcx
addq    %rcx,   %rax

although even that is not strictly identical since it changes rcx. To make it an even closer match, you could save and restore rcx as part of the process:

push    %rcx
movq    (%rbx), %rcx
addq    %rcx,   %rax
pop     %rcx

Although then, of course, you've changed rsp (albeit temporarily) and it requires you to have a stack actually set up (likely, but not absolutely the case) so you may be better off just sticking with the one-liner :-)