user: What would the C source look like for the assembly code provided here?

```asm
<tcp4_checksum>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  push   %r13
  push   %r12
  push   %rbx
  sub    $0x68,%rsp
  mov    %rdi,-0x58(%rbp)
  mov    %rsi,%rax
  mov    %rdx,%rsi
  mov    %rsi,%rdx
  mov    %rax,-0x70(%rbp)
  mov    %rdx,-0x68(%rbp)
  mov    %rcx,-0x60(%rbp)
  mov    %r8d,-0x74(%rbp)
  mov    %fs:0x28,%rax
  mov    %rax,-0x28(%rbp)
  xor    %eax,%eax
  mov    %rsp,%rax
  mov    %rax,%rbx
  mov    D3(%rip),%rax
  mov    (%rax),%eax
  movslq %eax,%rdx
  sub    $0x1,%rdx
  mov    %rdx,-0x40(%rbp)
  movslq %eax,%rdx
  mov    %rdx,%r12
  mov    $0x0,%r13d
  movslq %eax,%rdx
  mov    %rdx,%r10
  mov    $0x0,%r11d
  cltq
  mov    $0x10,%edx
  sub    $0x1,%rdx
  add    %rdx,%rax
  mov    $0x10,%edi
  mov    $0x0,%edx
  div    %rdi
  imul   $0x10,%rax,%rax
  mov    %rax,%rcx
  and    $0xfffffffffffff000,%rcx
  mov    %rsp,%rdx
  sub    %rcx,%rdx
L35:
  cmp    %rdx,%rsp
  je     L36
  sub    $0x1000,%rsp
  orq    $0x0,0xff8(%rsp)
  jmp    L35
L36:
  mov    %rax,%rdx
  and    $0xfff,%edx
  sub    %rdx,%rsp
  mov    %rax,%rdx
  and    $0xfff,%edx
  test   %rdx,%rdx
  je     L27
  and    $0xfff,%eax
  sub    $0x8,%rax
  add    %rsp,%rax
  orq    $0x0,(%rax)
L27:
  mov    %rsp,%rax
  add    $0x0,%rax
  mov    %rax,-0x38(%rbp)
  movl   $0x0,-0x44(%rbp)
  mov    -0x38(%rbp),%rax
  mov    %rax,-0x30(%rbp)
  lea    -0x58(%rbp),%rax
  lea    0x2(%rax),%rcx
  mov    -0x30(%rbp),%rax
  mov    $0x1,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x1,-0x30(%rbp)
  mov    -0x44(%rbp),%eax
  add    $0x1,%eax
  mov    %eax,-0x44(%rbp)
  lea    -0x58(%rbp),%rax
  lea    0x1(%rax),%rcx
  mov    -0x30(%rbp),%rax
  mov    $0x1,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x1,-0x30(%rbp)
  mov    -0x44(%rbp),%eax
  add    $0x1,%eax
  mov    %eax,-0x44(%rbp)
  mov    -0x30(%rbp),%rax
  movb   $0x0,(%rax)
  addq   $0x1,-0x30(%rbp)
  addl   $0x1,-0x44(%rbp)
  lea    -0x58(%rbp),%rcx
  mov    -0x30(%rbp),%rax
  mov    $0x1,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x1,-0x30(%rbp)
  mov    -0x44(%rbp),%eax
  add    $0x1,%eax
  mov    %eax,-0x44(%rbp)
  mov    -0x74(%rbp),%eax
  add    $0x10,%eax
  mov    %eax,%edi
  call   <htons@plt>
  mov    %al,-0x46(%rbp)
  lea    -0x46(%rbp),%rcx
  mov    -0x30(%rbp),%rax
  mov    $0x1,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x1,-0x30(%rbp)
  mov    -0x44(%rbp),%eax
  add    $0x1,%eax
  mov    %eax,-0x44(%rbp)
  lea    -0x70(%rbp),%rcx
  mov    -0x30(%rbp),%rax
  mov    $0x1,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x1,-0x30(%rbp)
  mov    -0x44(%rbp),%eax
  add    $0x1,%eax
  mov    %eax,-0x44(%rbp)
  lea    -0x70(%rbp),%rax
  lea    0x1(%rax),%rcx
  mov    -0x30(%rbp),%rax
  mov    $0x1,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x1,-0x30(%rbp)
  mov    -0x44(%rbp),%eax
  add    $0x1,%eax
  mov    %eax,-0x44(%rbp)
  lea    -0x70(%rbp),%rax
  lea    0x2(%rax),%rcx
  mov    -0x30(%rbp),%rax
  mov    $0x1,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x1,-0x30(%rbp)
  mov    -0x44(%rbp),%eax
  add    $0x1,%eax
  mov    %eax,-0x44(%rbp)
  lea    -0x70(%rbp),%rax
  lea    0x3(%rax),%rcx
  mov    -0x30(%rbp),%rax
  mov    $0x1,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x1,-0x30(%rbp)
  mov    -0x44(%rbp),%eax
  add    $0x1,%eax
  mov    %eax,-0x44(%rbp)
  mov    -0x6c(%rbp),%eax
  shl    $0x4,%eax
  mov    %eax,%edx
  mov    -0x68(%rbp),%eax
  add    %edx,%eax
  mov    %al,-0x45(%rbp)
  lea    -0x45(%rbp),%rcx
  mov    -0x30(%rbp),%rax
  mov    $0x1,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x1,-0x30(%rbp)
  mov    -0x44(%rbp),%eax
  add    $0x1,%eax
  mov    %eax,-0x44(%rbp)
  lea    -0x70(%rbp),%rax
  lea    0xc(%rax),%rcx
  mov    -0x30(%rbp),%rax
  mov    $0x1,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x1,-0x30(%rbp)
  mov    -0x44(%rbp),%eax
  add    $0x1,%eax
  mov    %eax,-0x44(%rbp)
  lea    -0x70(%rbp),%rax
  lea    0xd(%rax),%rcx
  mov    -0x30(%rbp),%rax
  mov    $0x1,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x1,-0x30(%rbp)
  mov    -0x44(%rbp),%eax
  add    $0x1,%eax
  mov    %eax,-0x44(%rbp)
  mov    -0x30(%rbp),%rax
  movb   $0x0,(%rax)
  addq   $0x1,-0x30(%rbp)
  mov    -0x30(%rbp),%rax
  movb   $0x0,(%rax)
  addq   $0x1,-0x30(%rbp)
  addl   $0x2,-0x44(%rbp)
  lea    -0x70(%rbp),%rax
  lea    0xe(%rax),%rcx
  mov    -0x30(%rbp),%rax
  mov    $0x1,%edx
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  addq   $0x1,-0x30(%rbp)
  mov    -0x44(%rbp),%eax
  add    $0x1,%eax
  mov    %eax,-0x44(%rbp)
  mov    -0x74(%rbp),%edx
  mov    -0x60(%rbp),%rcx
  mov    -0x30(%rbp),%rax
  mov    %rcx,%rsi
  mov    %rax,%rdi
  call   <memcpy@plt>
  mov    -0x74(%rbp),%eax
  cltq
  add    %rax,-0x30(%rbp)
  mov    -0x74(%rbp),%eax
  add    %eax,-0x44(%rbp)
  mov    -0x44(%rbp),%edx
  mov    -0x38(%rbp),%rax
  mov    %edx,%esi
  mov    %rax,%rdi
  call   <checksum@plt>
  mov    %rbx,%rsp
  mov    -0x28(%rbp),%rdx
  sub    %fs:0x28,%rdx
  je     L4
  call   <__stack_chk_fail@plt>
L4:
  lea    -0x18(%rbp),%rsp
  pop    %rbx
  pop    %r12
  pop    %r13
  pop    %rbp
  ret
```

assistant: ```c
uint16_t tcp4_checksum(struct ip iphdr, struct tcphdr tcphdr, uint8_t *tcp_options, int tcp_opt_len) {
  uint16_t svalue;
  char buf[IP_MAXPACKET], cvalue;
  char *ptr;
  int chksumlen = 0;
  ptr = &buf[0];
  memcpy(ptr, &iphdr.ip_src.s_addr, sizeof(iphdr.ip_src.s_addr));
  ptr += sizeof(iphdr.ip_src.s_addr);
  chksumlen += sizeof(iphdr.ip_src.s_addr);
  memcpy(ptr, &iphdr.ip_dst.s_addr, sizeof(iphdr.ip_dst.s_addr));
  ptr += sizeof(iphdr.ip_dst.s_addr);
  chksumlen += sizeof(iphdr.ip_dst.s_addr);
  *ptr = 0;
  ptr++;
  chksumlen += 1;
  memcpy(ptr, &iphdr.ip_p, sizeof(iphdr.ip_p));
  ptr += sizeof(iphdr.ip_p);
  chksumlen += sizeof(iphdr.ip_p);
  svalue = htons(sizeof(tcphdr) + tcp_opt_len);
  memcpy(ptr, &svalue, sizeof(svalue));
  ptr += sizeof(svalue);
  chksumlen += sizeof(svalue);
  memcpy(ptr, &tcphdr.th_sport, sizeof(tcphdr.th_sport));
  ptr += sizeof(tcphdr.th_sport);
  chksumlen += sizeof(tcphdr.th_sport);
  memcpy(ptr, &tcphdr.th_dport, sizeof(tcphdr.th_dport));
  ptr += sizeof(tcphdr.th_dport);
  chksumlen += sizeof(tcphdr.th_dport);
  memcpy(ptr, &tcphdr.th_seq, sizeof(tcphdr.th_seq));
  ptr += sizeof(tcphdr.th_seq);
  chksumlen += sizeof(tcphdr.th_seq);
  memcpy(ptr, &tcphdr.th_ack, sizeof(tcphdr.th_ack));
  ptr += sizeof(tcphdr.th_ack);
  chksumlen += sizeof(tcphdr.th_ack);
  cvalue = (tcphdr.th_off << 4) + tcphdr.th_x2;
  memcpy(ptr, &cvalue, sizeof(cvalue));
  ptr += sizeof(cvalue);
  chksumlen += sizeof(cvalue);
  memcpy(ptr, &tcphdr.th_flags, sizeof(tcphdr.th_flags));
  ptr += sizeof(tcphdr.th_flags);
  chksumlen += sizeof(tcphdr.th_flags);
  memcpy(ptr, &tcphdr.th_win, sizeof(tcphdr.th_win));
  ptr += sizeof(tcphdr.th_win);
  chksumlen += sizeof(tcphdr.th_win);
  *ptr = 0;
  ptr++;
  *ptr = 0;
  ptr++;
  chksumlen += 2;
  memcpy(ptr, &tcphdr.th_urp, sizeof(tcphdr.th_urp));
  ptr += sizeof(tcphdr.th_urp);
  chksumlen += sizeof(tcphdr.th_urp);
  memcpy(ptr, tcp_options, tcp_opt_len);
  ptr += tcp_opt_len;
  chksumlen += tcp_opt_len;
  return checksum((uint16_t *)buf, chksumlen);
}
```