What is the difference between retq and ret?

Question

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?

`retq` is explicit about the operand size, but it's the same instruction. — Jester, Mar 07 '17 at 16:16

score 35 · Accepted Answer · answered Mar 07 '17 at 17:15

35

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).

answered Mar 07 '17 at 17:15

ephemient

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Does the stack pointer `rsp` gets updated when `retq`/`ret` is executed? Or that has to be done manually? – stillanoob Jun 20 '18 at 12:28
Obviously. It does pop, after all. – jv110 Jul 19 '18 at 18:09
1

@stillanoob the `ret` instruction does two things: (1) pops the stack (which modifies `rsp` by subtracting 4 or 8 from it) (2) alters the `rip` (because the popped value goes into the `rip`). – Fixee Sep 27 '19 at 20:20

What is the difference between retq and ret?

1 Answers1

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