1.asm

%define EQUAL_EACH 1000b
%define NEGATIVE_POLARITY 010000b

EXPORT __strcmp_sse42
    mov rax, rdi
    sub rax, rsi ; rax = rdi - rsi
    sub rsi, 16
 
@B:
    add rsi, 16
    
    ; compare strings
    MovDqU    xmm0, dqword[rsi]
    PcmpIstrI xmm0, dqword[rsi + rax], EQUAL_EACH | NEGATIVE_POLARITY
    jc @F
    
    ; testing for EOL
    pxor xmm0, xmm0
    PcmpIstrI xmm0, dqword[rsi], EQUAL_EACH
    jnz @B
 
    ; the strings are equal
    mov rax, 0
    ret
@F:
    ; subtract the first different bytes
    add rax, rsi
    movzx rax, byte[rax + rcx]
    movzx rsi, byte[rsi + rcx]
    sub rax, rsi
    ret