i'm trying to learn some assembly (intel syntax, x86_64). I wrote this code to make a simple implementation of strcmp():
section .text
global ft_strcmp
ft_strcmp:
cmp byte [rsi], 0
je exit
mov cl, byte [rsi]
cmp byte [rdi], cl
jne exit
inc rsi
inc rdi
jmp ft_strcmp
exit:
xor rax, rax
mov al, [rdi]
sub al, byte [rsi]
ret
But trying it by calling ft_strcmp("Hello", "Hellooooo") return 145, whereas the real strcmp() return -1, and I cant seems to figure why. Am I wrong with my syntax, or is the way I try to do this ?
strcmp is supposed to return a 32-bit int in eax which is positive or negative according to whether the first string is greater or less. By doing an 8-bit subtract, the upper 24 bits of eax remain zero, so that the result is positive when viewed as a signed integer.
You want to do a 32-bit subtract, so you need both bytes in 32-bit registers with their upper 24 bits zeroed. This is efficient to do with movzx:
exit:
movzx eax, byte [rdi]
movzx ecx, byte [rsi]
sub eax, ecx
ret
If you didn't know about movzx, you could zero the whole register and then load the low byte:
exit:
xor eax, eax
mov al, [rdi] ; 'byte' is unnecessary, operand size inferred from register al
xor ecx, ecx
mov cl, [rsi]
sub eax, ecx
ret
(As a side comment, zeroing instructions like xor rax, rax can be replaced by xor eax, eax which is smaller and has the same effect: Why do x86-64 instructions on 32-bit registers zero the upper part of the full 64-bit register?)