Trying to implement strlen in x86 GAS

Question

so I am very new (extremely new) to assembly programming and am trying to write a function that can calculate the length of a string.

I feel I have some issue with clearing out values in registers, or with the incrementation of the pointer, because the value that is getting returned is always "4571 + length" for me.

Basically, if I have string length 0, I get 4571 as the return value. If I have string length 6, I get 4577 as the return value, etc.

Here's my code, any help will be appreciated:

.globl my_strlen
    my_strlen:
        pushq %rbp
        movq %rsp, %rbp
        pushq %r12
        pushq %r13

        movq $0, %rax
        cmp $0, (%rdi)
        jne my_strlen_loop
        ret

    my_strlen_loop:
        inc %rax
        inc %rdi
        cmp $0, (%rdi)
        jne my_strlen_loop

        popq %r13
        popq %r12
        popq %rbp
        ret

Answer 1

There are two problems with this code.

First, the cmp instructions don’t specify a size, and neither operand is a register, so it's ambiguous. For most instructions (like mov $0, (%rdi)), GAS would refuse to assemble it, but cmp for some reason assembles to cmpl, comparing a dword. Change the mnemonic to cmpb explicitly.

Second, before the first ret, it doesn’t pop the registers that were pushed. It would be better to jump to the end (and have a single ret).