user: 这段汇编代码用C语言表示是什么样的：

```asm
<merge>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  sub    $0x30,%rsp
  mov    %rdi,-0x18(%rbp)
  mov    %rsi,-0x20(%rbp)
  mov    %rdx,-0x28(%rbp)
  mov    %ecx,-0x2c(%rbp)
  cmpq   $0x0,-0x18(%rbp)
  jne    L77
  mov    -0x20(%rbp),%rax
  mov    %rax,-0x8(%rbp)
  jmp    L27
L77:
  cmpq   $0x0,-0x20(%rbp)
  jne    L81
  mov    -0x18(%rbp),%rax
  mov    %rax,-0x8(%rbp)
  jmp    L27
L81:
  mov    -0x20(%rbp),%rdx
  mov    -0x18(%rbp),%rax
  mov    -0x28(%rbp),%rcx
  mov    %rdx,%rsi
  mov    %rax,%rdi
  mov    $0x0,%eax
  call   *%rcx
  test   %eax,%eax
  jns    L102
  mov    -0x18(%rbp),%rax
  mov    %rax,-0x10(%rbp)
  mov    -0x2c(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  mov    (%rax),%rax
  mov    %rax,-0x18(%rbp)
  jmp    L116
L102:
  mov    -0x20(%rbp),%rax
  mov    %rax,-0x10(%rbp)
  mov    -0x2c(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x20(%rbp),%rax
  add    %rdx,%rax
  mov    (%rax),%rax
  mov    %rax,-0x20(%rbp)
L116:
  mov    -0x10(%rbp),%rax
  mov    %rax,-0x8(%rbp)
  jmp    L53
L58:
  mov    -0x20(%rbp),%rdx
  mov    -0x18(%rbp),%rax
  mov    -0x28(%rbp),%rcx
  mov    %rdx,%rsi
  mov    %rax,%rdi
  mov    $0x0,%eax
  call   *%rcx
  test   %eax,%eax
  jns    L45
  mov    -0x2c(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x10(%rbp),%rax
  add    %rdx,%rax
  mov    %rax,%rdx
  mov    -0x18(%rbp),%rax
  mov    %rax,(%rdx)
  mov    -0x18(%rbp),%rax
  mov    %rax,-0x10(%rbp)
  mov    -0x2c(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  mov    (%rax),%rax
  mov    %rax,-0x18(%rbp)
  jmp    L53
L45:
  mov    -0x2c(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x10(%rbp),%rax
  add    %rdx,%rax
  mov    %rax,%rdx
  mov    -0x20(%rbp),%rax
  mov    %rax,(%rdx)
  mov    -0x20(%rbp),%rax
  mov    %rax,-0x10(%rbp)
  mov    -0x2c(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x20(%rbp),%rax
  add    %rdx,%rax
  mov    (%rax),%rax
  mov    %rax,-0x20(%rbp)
L53:
  cmpq   $0x0,-0x18(%rbp)
  je     L115
  cmpq   $0x0,-0x20(%rbp)
  jne    L58
L115:
  cmpq   $0x0,-0x18(%rbp)
  je     L113
  mov    -0x2c(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x10(%rbp),%rax
  add    %rdx,%rax
  mov    %rax,%rdx
  mov    -0x18(%rbp),%rax
  mov    %rax,(%rdx)
  jmp    L27
L113:
  mov    -0x2c(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x10(%rbp),%rax
  add    %rdx,%rax
  mov    %rax,%rdx
  mov    -0x20(%rbp),%rax
  mov    %rax,(%rdx)
L27:
  mov    -0x8(%rbp),%rax
  leave
  ret
```

assistant: ```c
char *merge(a, b, cmp, offset)
char *a;
char *b;
int (*cmp)();
int offset;
{
  char *ptr, *head;
  if (a == 0) {
    head = b;
  } else if (b == 0) {
    head = a;
  } else {
    if ((*cmp)(a, b) < 0) {
      ptr = a;
      a = (*(char **)(((unsigned long)a) + offset));
    } else {
      ptr = b;
      b = (*(char **)(((unsigned long)b) + offset));
    }
    head = ptr;
    while (a && b) {
      if ((*cmp)(a, b) < 0) {
        (*(char **)(((unsigned long)ptr) + offset)) = a;
        ptr = a;
        a = (*(char **)(((unsigned long)a) + offset));
      } else {
        (*(char **)(((unsigned long)ptr) + offset)) = b;
        ptr = b;
        b = (*(char **)(((unsigned long)b) + offset));
      }
    }
    if (a)
      (*(char **)(((unsigned long)ptr) + offset)) = a;
    else
      (*(char **)(((unsigned long)ptr) + offset)) = b;
  }
  return head;
}
```