[ridiculous_fish]

You are correct, it has nothing to do with alignment. rbx is a callee-saved register, so the callee saves it.

[thayne]

Do you mean thar rax is caller-saved?

[ridiculous_fish]

rax is indeed caller-saved, but I was referring to rbx. In this code:

    int bar(int a, int b) {
       int z = a * b;
       foo();
       return z;
    }

https://godbolt.org/z/a8Y35r

Where to put `z`? It has to be someplace where `foo` won't clobber it, like a callee-saved register - in this case ebx.

But now `bar` is responsible for saving the value of ebx for its caller. That explains the push and pop.

Alignment is unrelated; this is just about calling conventions.