Memmove in Assembly

Question

I'm trying to put this Memmove C code to assembly and don't get the supposed result.

I'm using x86-64 assembly on xubuntu and after debugging for 2 hours, I don't see where I'm wrong.

C memmove code:

#include <stdio.h> 

extern void * memmove(void *dest, void *src, size_t n);

int main () { 
  char str1[] = "Geeks";  // Array of size 6
  char str2[] = "Quiz";   // Array of size 5 

  puts("str1 before memmove "); 
  puts(str1); 

  /* Copies contents of str2 to sr1 */
  memmove(str1, str2, sizeof(str2));

  puts("\nstr1 after memmove "); 
  puts(str1); 

  return 0; 
} 
/*

void * memmove(void *dest, void *src, size_t n) { 

    char *d = (char *) dest; 
    char *s = (char *) src; 

    if(s == d)
        return dest;

    if(s < d) { 
        //copy from back 
        s=s+n-1; 
        d=d+n-1; 
        while(n--) { 
            *d-- = *s--; 
        } 
    } 
    else { 
        //copy from front 
        while(n--) 
            *d++ = *s++; 
    } 
    return dest; 
} */

Assembly code:

.globl memmove

# RDI = dest
# RSI = src
# RDX = n

# R8 = d
# R9 = s

memmove:
    mov     %rdi, %r8       # d = dest
    mov     %rsi, %r9       # s = src
    jmp     if_equal

if_equal:
    cmp     %r8, %r9        # s == d
    jz      retDest
    ja      else            # s > d
    jb      if_s_minor      # s < d

if_s_minor:
    add     %rdx, %r9       # s = s + n
    sub     $1, %r9         # s = s - 1

    add     %rdx, %r8       # d = d + n
    sub     $1, %r8         # d = d - 1

    jmp     while1

while1:
    cmp     $0, %rdx        # n > 0 ?
    jna     retDest         # if n <= 0 go to retDest
    sub     $1, %rdx        # n--

    movb    (%rsi), %cl     # *dst-- = *src--
    movb    %cl, (%rdi) 

    leaq    -1(%r8), %r8    # *d--
    leaq    -1(%r9), %r9    # *s--
    jmp     while1

else:
    jmp     while2

while2:
    cmp     $0, %rdx        # n > 0 ?
    jna     retDest
    sub     $1, %rdx        # n--

    movb    (%rsi), %cl     # *dst = *src
    movb    %cl, (%rdi) 

    leaq    1(%r8), %r8 # *d++
    leaq    1(%r9), %r9 # *s++
    jmp     while2

retDest:
    mov     %rdi, %rax
    ret

.end

It was supposed to show "Quiz" on the second print but it shows this:

str1 before memmove Geeks

str1 after memmove Qeeks

Answer 1

movb    (%rsi), %cl     # *dst = *src
movb    %cl, (%rdi) 
leaq    1(%r8), %r8 # *d++
leaq    1(%r9), %r9 # *s++
jmp     while2

The problem here is that your code changes the %r8 and %r9 registers, but the %rdi and %rsi registers that are used in the actual move stay the same, thus repeating copying the same byte again and again!

Same problem of course in the while1 code.