user: Demonstrate the C code that generates the assembly output below:

```asm
<strsub>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  push   %rbx
  sub    $0x868,%rsp
  mov    %rdi,-0x858(%rbp)
  mov    %rsi,-0x860(%rbp)
  mov    %rdx,-0x868(%rbp)
  mov    %fs:0x28,%rax
  mov    %rax,-0x18(%rbp)
  xor    %eax,%eax
  movl   $0x0,-0x84c(%rbp)
  lea    -0x810(%rbp),%rdx
  mov    $0x0,%eax
  mov    $0xff,%ecx
  mov    %rdx,%rdi
  rep stos %rax,%es:(%rdi)
  mov    -0x858(%rbp),%rax
  mov    %rax,-0x830(%rbp)
  cmpq   $0x0,-0x858(%rbp)
  je     L34
  cmpq   $0x0,-0x860(%rbp)
  je     L34
  cmpq   $0x0,-0x868(%rbp)
  jne    L132
L34:
  mov    $0x0,%eax
  jmp    L80
L132:
  mov    -0x858(%rbp),%rax
  mov    %rax,%rdi
  call   <strlen@plt>
  mov    %eax,-0x840(%rbp)
  mov    -0x860(%rbp),%rax
  mov    %rax,%rdi
  call   <strlen@plt>
  mov    %eax,-0x83c(%rbp)
  mov    -0x868(%rbp),%rax
  mov    %rax,%rdi
  call   <strlen@plt>
  mov    %eax,-0x838(%rbp)
  mov    -0x840(%rbp),%eax
  cmp    -0x83c(%rbp),%eax
  jge    L145
  mov    $0x0,%eax
  jmp    L80
L9:
  mov    -0x84c(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x84c(%rbp)
  cltq
  mov    -0x810(%rbp,%rax,8),%rbx
  mov    -0x860(%rbp),%rax
  mov    %rax,%rdi
  call   <strlen@plt>
  cltq
  add    %rbx,%rax
  mov    %rax,-0x830(%rbp)
L145:
  mov    -0x860(%rbp),%rdx
  mov    -0x830(%rbp),%rax
  mov    %rdx,%rsi
  mov    %rax,%rdi
  call   <strstr@plt>
  mov    -0x84c(%rbp),%edx
  movslq %edx,%rdx
  mov    %rax,-0x810(%rbp,%rdx,8)
  mov    -0x84c(%rbp),%eax
  cltq
  mov    -0x810(%rbp,%rax,8),%rax
  test   %rax,%rax
  jne    L9
  cmpl   $0x0,-0x84c(%rbp)
  jne    L153
  mov    $0x0,%eax
  jmp    L80
L153:
  mov    -0x838(%rbp),%eax
  sub    -0x83c(%rbp),%eax
  imul   -0x84c(%rbp),%eax
  mov    %eax,%edx
  mov    -0x840(%rbp),%eax
  add    %edx,%eax
  mov    %eax,-0x834(%rbp)
  mov    -0x834(%rbp),%eax
  mov    %eax,%edi
  call   <malloc@plt>
  mov    %rax,-0x818(%rbp)
  cmpq   $0x0,-0x818(%rbp)
  jne    L72
  mov    $0x0,%eax
  jmp    L80
L72:
  mov    -0x858(%rbp),%rax
  mov    %rax,-0x830(%rbp)
  mov    -0x818(%rbp),%rax
  mov    %rax,-0x828(%rbp)
  movl   $0x0,-0x848(%rbp)
  jmp    L139
L116:
  mov    -0x838(%rbp),%eax
  mov    %eax,-0x844(%rbp)
  mov    -0x868(%rbp),%rax
  mov    %rax,-0x820(%rbp)
  jmp    L121
L46:
  mov    -0x830(%rbp),%rdx
  lea    0x1(%rdx),%rax
  mov    %rax,-0x830(%rbp)
  mov    -0x828(%rbp),%rax
  lea    0x1(%rax),%rcx
  mov    %rcx,-0x828(%rbp)
  movzbl (%rdx),%edx
  mov    %dl,(%rax)
L121:
  mov    -0x848(%rbp),%eax
  cltq
  mov    -0x810(%rbp,%rax,8),%rax
  cmp    %rax,-0x830(%rbp)
  jb     L46
  jmp    L79
L106:
  mov    -0x820(%rbp),%rdx
  lea    0x1(%rdx),%rax
  mov    %rax,-0x820(%rbp)
  mov    -0x828(%rbp),%rax
  lea    0x1(%rax),%rcx
  mov    %rcx,-0x828(%rbp)
  movzbl (%rdx),%edx
  mov    %dl,(%rax)
L79:
  mov    -0x844(%rbp),%eax
  lea    -0x1(%rax),%edx
  mov    %edx,-0x844(%rbp)
  test   %eax,%eax
  jne    L106
  mov    -0x83c(%rbp),%eax
  cltq
  add    %rax,-0x830(%rbp)
  addl   $0x1,-0x848(%rbp)
L139:
  mov    -0x848(%rbp),%eax
  cmp    -0x84c(%rbp),%eax
  jl     L116
  jmp    L63
L47:
  mov    -0x830(%rbp),%rdx
  lea    0x1(%rdx),%rax
  mov    %rax,-0x830(%rbp)
  mov    -0x828(%rbp),%rax
  lea    0x1(%rax),%rcx
  mov    %rcx,-0x828(%rbp)
  movzbl (%rdx),%edx
  mov    %dl,(%rax)
L63:
  mov    -0x830(%rbp),%rax
  movzbl (%rax),%eax
  test   %al,%al
  jne    L47
  mov    -0x828(%rbp),%rax
  movb   $0x0,(%rax)
  mov    -0x818(%rbp),%rax
L80:
  mov    -0x18(%rbp),%rdx
  sub    %fs:0x28,%rdx
  je     L168
  call   <__stack_chk_fail@plt>
L168:
  mov    -0x8(%rbp),%rbx
  leave
  ret
```

assistant: ```c
char *strsub(char *str, char *old, char *new) {
  int tl, ol, nl, sl, i = 0, j;
  char *gs[255] = {
      NULL,
  };
  char *sp = str;
  char *tn;
  char *tnp;
  if (str == NULL || old == NULL || new == NULL)
    return NULL;
  sl = strlen(str);
  ol = strlen(old);
  nl = strlen(new);
  if (sl < ol)
    return NULL;
  while (gs[i] = strstr(sp, old))
    sp = gs[i++] + strlen(old);
  if (i == 0)
    return NULL;
  tl = sl + i * (nl - ol);
  tn = (char *)malloc(tl);
  if (tn == NULL)
    return NULL;
  sp = str;
  tnp = tn;
  j = 0;
  while (j < i) {
    int tmp = nl;
    char *np = new;
    while (sp < gs[j])
      *tnp++ = *sp++;
    while (tmp--)
      *tnp++ = *np++;
    sp = sp + ol;
    j++;
  }
  while (*sp != '\0')
    *tnp++ = *sp++;
  *tnp = '\0';
  return tn;
}
```