What is the difference between retq and ret?

Let’s consider the following program, which computes an unsigned square of the argument:

.global foo
.text
foo:
    mov %rdi, %rax
    mul %rdi
    ret

This is properly compiled by as, but disassembles to

0000000000000000 <foo>:
   0:   48 89 f8                mov    %rdi,%rax
   3:   48 f7 e7                mul    %rdi
   6:   c3                      retq   

Is there any difference between ret and retq?

Answer

In long (64-bit) mode, you return (ret) by popping a quadword address from the stack to %rip.

In 32-bit mode, you return (ret) by popping a dword address from the stack to %eip.

Some tools like objdump -d call the first one retq. It’s just a name, the instruction encoding is the same either way (C3).