Difference between addq and movl, then addq

What is the functional difference between

addq (%rbx), %rax

and

movl (%rbx), %ecx addq %rcx, %rax

in assembly?

I think they do the same thing, but what is the functional difference?

1 Answer

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 :-)