user: What is the c source code of the assembly code below:

```asm
<ft_strjoin>:
  endbr64
  test   %rdi,%rdi
  je     L174
  push   %r13
  push   %r12
  push   %rbp
  mov    %rsi,%rbp
  push   %rbx
  sub    $0x8,%rsp
  test   %rsi,%rsi
  je     L234
  mov    %rdi,%rbx
  call   <ft_strlen@plt>
  mov    %rbp,%rdi
  mov    %eax,%r12d
  call   <ft_strlen@plt>
  lea    (%r12,%rax,1),%edi
  mov    %eax,%r13d
  call   <ft_strnew@plt>
  test   %rax,%rax
  je     L234
  test   %r12d,%r12d
  jle    L369
  lea    0x1(%rbx),%rdi
  mov    %rax,%rcx
  lea    -0x1(%r12),%edx
  mov    %r12d,%esi
  sub    %rdi,%rcx
  cmp    $0xe,%rcx
  jbe    L128
  cmp    $0x6,%edx
  jbe    L128
  cmp    $0xe,%edx
  jbe    L255
  mov    %r12d,%ecx
  xor    %edx,%edx
  shr    $0x4,%ecx
  shl    $0x4,%rcx
  xchg   %ax,%ax
L84:
  movdqu (%rbx,%rdx,1),%xmm0
  movups %xmm0,(%rax,%rdx,1)
  add    $0x10,%rdx
  cmp    %rcx,%rdx
  jne    L84
  mov    %r12d,%ecx
  and    $0xfffffff0,%ecx
  mov    %ecx,%edx
  cmp    %ecx,%r12d
  je     L369
  mov    %r12d,%esi
  sub    %ecx,%esi
  lea    -0x1(%rsi),%edi
  cmp    $0x6,%edi
  jbe    L118
L148:
  mov    (%rbx,%rcx,1),%rdi
  mov    %rdi,(%rax,%rcx,1)
  mov    %esi,%ecx
  and    $0xfffffff8,%ecx
  add    %ecx,%edx
  cmp    %ecx,%esi
  je     L369
L118:
  movslq %edx,%rcx
  movzbl (%rbx,%rcx,1),%esi
  mov    %sil,(%rax,%rcx,1)
  lea    0x1(%rdx),%ecx
  cmp    %ecx,%r12d
  jle    L369
  movslq %ecx,%rcx
  movzbl (%rbx,%rcx,1),%esi
  mov    %sil,(%rax,%rcx,1)
  lea    0x2(%rdx),%ecx
  cmp    %ecx,%r12d
  jle    L369
  movslq %ecx,%rcx
  movzbl (%rbx,%rcx,1),%esi
  mov    %sil,(%rax,%rcx,1)
  lea    0x3(%rdx),%ecx
  cmp    %ecx,%r12d
  jle    L369
  movslq %ecx,%rcx
  movzbl (%rbx,%rcx,1),%esi
  mov    %sil,(%rax,%rcx,1)
  lea    0x4(%rdx),%ecx
  cmp    %ecx,%r12d
  jle    L369
  movslq %ecx,%rcx
  movzbl (%rbx,%rcx,1),%esi
  mov    %sil,(%rax,%rcx,1)
  lea    0x5(%rdx),%ecx
  cmp    %ecx,%r12d
  jle    L369
  movslq %ecx,%rcx
  add    $0x6,%edx
  movzbl (%rbx,%rcx,1),%esi
  mov    %sil,(%rax,%rcx,1)
  cmp    %edx,%r12d
  jle    L369
  movslq %edx,%rdx
  movzbl (%rbx,%rdx,1),%ecx
  mov    %cl,(%rax,%rdx,1)
L369:
  test   %r13d,%r13d
  jle    L362
  movslq %r12d,%rcx
  lea    0x1(%rbp),%r8
  lea    -0x1(%r13),%edx
  mov    %r13d,%edi
  add    %rax,%rcx
  mov    %rcx,%rsi
  sub    %r8,%rsi
  cmp    $0xe,%rsi
  jbe    L287
  cmp    $0x6,%edx
  jbe    L287
  cmp    $0xe,%edx
  jbe    L91
  mov    %r13d,%esi
  xor    %edx,%edx
  shr    $0x4,%esi
  shl    $0x4,%rsi
  nopw   0x0(%rax,%rax,1)
L157:
  movdqu 0x0(%rbp,%rdx,1),%xmm1
  movups %xmm1,(%rcx,%rdx,1)
  add    $0x10,%rdx
  cmp    %rsi,%rdx
  jne    L157
  mov    %r13d,%esi
  and    $0xfffffff0,%esi
  mov    %esi,%edx
  cmp    %esi,%r13d
  je     L362
  mov    %r13d,%edi
  sub    %esi,%edi
  lea    -0x1(%rdi),%r8d
  cmp    $0x6,%r8d
  jbe    L66
L156:
  mov    0x0(%rbp,%rsi,1),%r8
  mov    %r8,(%rcx,%rsi,1)
  mov    %edi,%ecx
  and    $0xfffffff8,%ecx
  add    %ecx,%edx
  cmp    %ecx,%edi
  je     L362
L66:
  movslq %edx,%rcx
  movzbl 0x0(%rbp,%rcx,1),%esi
  lea    (%r12,%rdx,1),%ecx
  movslq %ecx,%rcx
  mov    %sil,(%rax,%rcx,1)
  lea    0x1(%rdx),%ecx
  cmp    %ecx,%r13d
  jle    L362
  movslq %ecx,%rsi
  add    %r12d,%ecx
  movzbl 0x0(%rbp,%rsi,1),%esi
  movslq %ecx,%rcx
  mov    %sil,(%rax,%rcx,1)
  lea    0x2(%rdx),%ecx
  cmp    %r13d,%ecx
  jge    L362
  movslq %ecx,%rsi
  add    %r12d,%ecx
  movzbl 0x0(%rbp,%rsi,1),%esi
  movslq %ecx,%rcx
  mov    %sil,(%rax,%rcx,1)
  lea    0x3(%rdx),%ecx
  cmp    %ecx,%r13d
  jle    L362
  movslq %ecx,%rsi
  add    %r12d,%ecx
  movzbl 0x0(%rbp,%rsi,1),%esi
  movslq %ecx,%rcx
  mov    %sil,(%rax,%rcx,1)
  lea    0x4(%rdx),%ecx
  cmp    %ecx,%r13d
  jle    L362
  movslq %ecx,%rsi
  add    %r12d,%ecx
  movzbl 0x0(%rbp,%rsi,1),%esi
  movslq %ecx,%rcx
  mov    %sil,(%rax,%rcx,1)
  lea    0x5(%rdx),%ecx
  cmp    %ecx,%r13d
  jle    L362
  movslq %ecx,%rsi
  add    %r12d,%ecx
  add    $0x6,%edx
  movzbl 0x0(%rbp,%rsi,1),%esi
  movslq %ecx,%rcx
  mov    %sil,(%rax,%rcx,1)
  cmp    %edx,%r13d
  jle    L362
  movslq %edx,%rcx
  add    %edx,%r12d
  movzbl 0x0(%rbp,%rcx,1),%ecx
  movslq %r12d,%r12
  mov    %cl,(%rax,%r12,1)
  add    $0x8,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  ret
  nopl   (%rax)
L234:
  xor    %eax,%eax
L362:
  add    $0x8,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  ret
  nopl   (%rax)
L174:
  xor    %eax,%eax
  ret
  nopl   0x0(%rax,%rax,1)
L128:
  mov    %r12d,%esi
  xor    %edx,%edx
  nopl   (%rax)
L270:
  movzbl (%rbx,%rdx,1),%ecx
  mov    %cl,(%rax,%rdx,1)
  add    $0x1,%rdx
  cmp    %rsi,%rdx
  jne    L270
  jmp    L369
  nopl   (%rax)
L287:
  mov    %r13d,%r13d
  xor    %edx,%edx
  nopl   (%rax)
L165:
  movzbl 0x0(%rbp,%rdx,1),%esi
  mov    %sil,(%rcx,%rdx,1)
  add    $0x1,%rdx
  cmp    %r13,%rdx
  jne    L165
  jmp    L362
L255:
  xor    %ecx,%ecx
  xor    %edx,%edx
  jmp    L148
L91:
  xor    %esi,%esi
  xor    %edx,%edx
  jmp    L156
```

assistant: ```c
char *ft_strjoin(char const *s1, const char *s2) {
  int s1_len;
  int s2_len;
  int i;
  char *ret;
  if (s1 && s2) {
    s1_len = ft_strlen(s1);
    s2_len = ft_strlen(s2);
    if ((ret = ft_strnew(s1_len + s2_len))) {
      i = -1;
      while (++i < s1_len)
        ret[i] = s1[i];
      i = 0;
      while (i < s2_len) {
        ret[s1_len + i] = s2[i];
        i++;
      }
      return (ret);
    }
  }
  return (0);
}
```