user: 下面的汇编指令对应的C源代码是什么：

```asm
<ip_cksum_add>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  sub    $0x30,%rsp
  mov    %rdi,-0x18(%rbp)
  mov    %rsi,-0x20(%rbp)
  mov    %edx,-0x24(%rbp)
  mov    -0x18(%rbp),%rax
  mov    %rax,-0x8(%rbp)
  mov    -0x20(%rbp),%rax
  shr    %rax
  mov    %eax,-0xc(%rbp)
  mov    -0xc(%rbp),%eax
  add    $0xf,%eax
  lea    0xf(%rax),%edx
  test   %eax,%eax
  cmovs  %edx,%eax
  sar    $0x4,%eax
  mov    %eax,-0x10(%rbp)
  mov    -0xc(%rbp),%eax
  cltd
  shr    $0x1c,%edx
  add    %edx,%eax
  and    $0xf,%eax
  sub    %edx,%eax
  cmp    $0xf,%eax
  ja     L10
  mov    %eax,%eax
  lea    0x0(,%rax,4),%rdx
  lea    D8(%rip),%rax
  mov    (%rdx,%rax,1),%eax
  cltq
  lea    D8(%rip),%rdx
  add    %rdx,%rax
  notrack jmp *%rax
L14:
  nop
  mov    -0x8(%rbp),%rax
  lea    0x4(%rax),%rdx
  mov    %rdx,-0x8(%rbp)
  mov    (%rax),%eax
  add    %eax,-0x24(%rbp)
  mov    -0x8(%rbp),%rax
  lea    0x4(%rax),%rdx
  mov    %rdx,-0x8(%rbp)
  mov    (%rax),%eax
  add    %eax,-0x24(%rbp)
  mov    -0x8(%rbp),%rax
  lea    0x4(%rax),%rdx
  mov    %rdx,-0x8(%rbp)
  mov    (%rax),%eax
  add    %eax,-0x24(%rbp)
  mov    -0x8(%rbp),%rax
  lea    0x4(%rax),%rdx
  mov    %rdx,-0x8(%rbp)
  mov    (%rax),%eax
  add    %eax,-0x24(%rbp)
  mov    -0x8(%rbp),%rax
  lea    0x4(%rax),%rdx
  mov    %rdx,-0x8(%rbp)
  mov    (%rax),%eax
  add    %eax,-0x24(%rbp)
  mov    -0x8(%rbp),%rax
  lea    0x4(%rax),%rdx
  mov    %rdx,-0x8(%rbp)
  mov    (%rax),%eax
  add    %eax,-0x24(%rbp)
  mov    -0x8(%rbp),%rax
  lea    0x4(%rax),%rdx
  mov    %rdx,-0x8(%rbp)
  mov    (%rax),%eax
  add    %eax,-0x24(%rbp)
  mov    -0x8(%rbp),%rax
  lea    0x4(%rax),%rdx
  mov    %rdx,-0x8(%rbp)
  mov    (%rax),%eax
  add    %eax,-0x24(%rbp)
  mov    -0x8(%rbp),%rax
  lea    0x4(%rax),%rdx
  mov    %rdx,-0x8(%rbp)
  mov    (%rax),%eax
  add    %eax,-0x24(%rbp)
  mov    -0x8(%rbp),%rax
  lea    0x4(%rax),%rdx
  mov    %rdx,-0x8(%rbp)
  mov    (%rax),%eax
  add    %eax,-0x24(%rbp)
  mov    -0x8(%rbp),%rax
  lea    0x4(%rax),%rdx
  mov    %rdx,-0x8(%rbp)
  mov    (%rax),%eax
  add    %eax,-0x24(%rbp)
  mov    -0x8(%rbp),%rax
  lea    0x4(%rax),%rdx
  mov    %rdx,-0x8(%rbp)
  mov    (%rax),%eax
  add    %eax,-0x24(%rbp)
  mov    -0x8(%rbp),%rax
  lea    0x4(%rax),%rdx
  mov    %rdx,-0x8(%rbp)
  mov    (%rax),%eax
  add    %eax,-0x24(%rbp)
  mov    -0x8(%rbp),%rax
  lea    0x4(%rax),%rdx
  mov    %rdx,-0x8(%rbp)
  mov    (%rax),%eax
  add    %eax,-0x24(%rbp)
  mov    -0x8(%rbp),%rax
  lea    0x4(%rax),%rdx
  mov    %rdx,-0x8(%rbp)
  mov    (%rax),%eax
  add    %eax,-0x24(%rbp)
  mov    -0x8(%rbp),%rax
  lea    0x4(%rax),%rdx
  mov    %rdx,-0x8(%rbp)
  mov    (%rax),%eax
  add    %eax,-0x24(%rbp)
  subl   $0x1,-0x10(%rbp)
  cmpl   $0x0,-0x10(%rbp)
  jg     L14
L10:
  mov    -0x20(%rbp),%rax
  and    $0x1,%eax
  test   %rax,%rax
  je     L1
  mov    -0x8(%rbp),%rax
  mov    (%rax),%eax
  shl    $0x8,%eax
  mov    %eax,%edi
  call   <htons@plt>
  mov    %eax,%edx
  mov    -0x24(%rbp),%eax
  add    %edx,%eax
  mov    %eax,-0x24(%rbp)
L1:
  mov    -0x24(%rbp),%eax
  leave
  ret
```

assistant: ```c
int ip_cksum_add(const void *buf, size_t len, int cksum) {
  uint16_t *sp = (uint16_t *)buf;
  int n, sn;
  sn = len / 2;
  n = (sn + 15) / 16;
  switch (sn % 16) {
  case 0:
    do {
      cksum += *sp++;
    case 15:
      cksum += *sp++;
    case 14:
      cksum += *sp++;
    case 13:
      cksum += *sp++;
    case 12:
      cksum += *sp++;
    case 11:
      cksum += *sp++;
    case 10:
      cksum += *sp++;
    case 9:
      cksum += *sp++;
    case 8:
      cksum += *sp++;
    case 7:
      cksum += *sp++;
    case 6:
      cksum += *sp++;
    case 5:
      cksum += *sp++;
    case 4:
      cksum += *sp++;
    case 3:
      cksum += *sp++;
    case 2:
      cksum += *sp++;
    case 1:
      cksum += *sp++;
    } while (--n > 0);
  }
  if (len & 1)
    cksum += htons(*(u_char *)sp << 8);
  return (cksum);
}
```