user: Converte this assembly snippet into its equivalent C code:

```asm
<str_concat>:
  endbr64
  push   %r15
  push   %r14
  push   %r13
  mov    %rdi,%r13
  push   %r12
  push   %rbp
  push   %rbx
  mov    %rsi,%rbx
  sub    $0x8,%rsp
  call   <strlen@plt>
  mov    %rbx,%rdi
  mov    %eax,%r12d
  call   <strlen@plt>
  lea    0x1(%r12,%rax,1),%edi
  mov    %eax,%ebp
  mov    %eax,%r14d
  call   <malloc@plt>
  mov    %rax,%r8
  test   %r12d,%r12d
  jle    L239
  lea    0x1(%r13),%rdx
  lea    -0x1(%r12),%esi
  sub    %rdx,%rax
  cmp    $0xe,%rax
  jbe    L175
  cmp    $0x6,%esi
  jbe    L175
  cmp    $0xe,%esi
  jbe    L308
  mov    %r12d,%eax
  mov    %r13,%rdx
  shr    $0x4,%eax
  sub    %r8,%rdx
  sub    $0x1,%eax
  shl    $0x4,%rax
  lea    0x10(%r8,%rax,1),%rcx
  mov    %r8,%rax
  cs nopw 0x0(%rax,%rax,1)
L141:
  movdqu (%rax,%rdx,1),%xmm0
  add    $0x10,%rax
  movups %xmm0,-0x10(%rax)
  cmp    %rax,%rcx
  jne    L141
  mov    %r12d,%eax
  mov    %esi,%ecx
  and    $0xfffffff0,%eax
  mov    %eax,%r10d
  lea    (%r8,%r10,1),%rdi
  lea    0x0(%r13,%r10,1),%r9
  cmp    %eax,%r12d
  je     L321
  mov    %r12d,%r15d
  sub    %eax,%r15d
  lea    -0x1(%r15),%edx
  cmp    $0x6,%edx
  jbe    L37
L93:
  mov    0x0(%r13,%r10,1),%rdx
  mov    %rdx,(%r8,%r10,1)
  mov    %r15d,%edx
  and    $0xfffffff8,%edx
  mov    %edx,%ecx
  add    %edx,%eax
  add    %rcx,%rdi
  add    %rcx,%r9
  mov    %esi,%ecx
  cmp    %edx,%r15d
  je     L321
L37:
  movzbl (%r9),%edx
  mov    %esi,%ecx
  mov    %dl,(%rdi)
  lea    0x1(%rax),%edx
  cmp    %edx,%r12d
  jle    L321
  movzbl 0x1(%r9),%edx
  mov    %dl,0x1(%rdi)
  lea    0x2(%rax),%edx
  cmp    %edx,%r12d
  jle    L321
  movzbl 0x2(%r9),%edx
  mov    %dl,0x2(%rdi)
  lea    0x3(%rax),%edx
  cmp    %edx,%r12d
  jle    L321
  movzbl 0x3(%r9),%edx
  mov    %dl,0x3(%rdi)
  lea    0x4(%rax),%edx
  cmp    %edx,%r12d
  jle    L321
  movzbl 0x4(%r9),%edx
  mov    %dl,0x4(%rdi)
  lea    0x5(%rax),%edx
  cmp    %edx,%r12d
  jle    L321
  movzbl 0x5(%r9),%edx
  add    $0x6,%eax
  mov    %dl,0x5(%rdi)
  cmp    %eax,%r12d
  jle    L321
  movzbl 0x6(%r9),%eax
  mov    %al,0x6(%rdi)
L321:
  lea    0x1(%r8,%rcx,1),%rdx
L114:
  test   %ebp,%ebp
  jle    L318
  lea    0x1(%rbx),%rcx
  mov    %rdx,%rax
  lea    -0x1(%rbp),%r9d
  sub    %rcx,%rax
  cmp    $0xe,%rax
  jbe    L55
  cmp    $0x6,%r9d
  jbe    L55
  cmp    $0xe,%r9d
  jbe    L161
  mov    %ebp,%ecx
  xor    %eax,%eax
  shr    $0x4,%ecx
  shl    $0x4,%rcx
  nopw   0x0(%rax,%rax,1)
L214:
  movdqu (%rbx,%rax,1),%xmm1
  movups %xmm1,(%rdx,%rax,1)
  add    $0x10,%rax
  cmp    %rcx,%rax
  jne    L214
  mov    %ebp,%eax
  mov    %r9d,%edi
  and    $0xfffffff0,%eax
  mov    %eax,%r10d
  lea    (%rdx,%r10,1),%rcx
  lea    (%rbx,%r10,1),%rsi
  cmp    %eax,%ebp
  je     L213
  mov    %ebp,%r14d
  sub    %eax,%r14d
  lea    -0x1(%r14),%edi
  cmp    $0x6,%edi
  jbe    L327
L120:
  mov    (%rbx,%r10,1),%rdi
  mov    %rdi,(%rdx,%r10,1)
  mov    %r14d,%r10d
  and    $0xfffffff8,%r10d
  mov    %r10d,%edi
  add    %r10d,%eax
  add    %rdi,%rcx
  add    %rdi,%rsi
  mov    %r9d,%edi
  cmp    %r14d,%r10d
  je     L213
L327:
  movzbl (%rsi),%edi
  lea    0x1(%rax),%r10d
  mov    %dil,(%rcx)
  mov    %r9d,%edi
  cmp    %r10d,%ebp
  jle    L213
  movzbl 0x1(%rsi),%r9d
  mov    %r9b,0x1(%rcx)
  lea    0x2(%rax),%r9d
  cmp    %r9d,%ebp
  jle    L213
  movzbl 0x2(%rsi),%r9d
  mov    %r9b,0x2(%rcx)
  lea    0x3(%rax),%r9d
  cmp    %r9d,%ebp
  jle    L213
  movzbl 0x3(%rsi),%r9d
  mov    %r9b,0x3(%rcx)
  lea    0x4(%rax),%r9d
  cmp    %r9d,%ebp
  jle    L213
  movzbl 0x4(%rsi),%r9d
  mov    %r9b,0x4(%rcx)
  lea    0x5(%rax),%r9d
  cmp    %r9d,%ebp
  jle    L213
  movzbl 0x5(%rsi),%r9d
  add    $0x6,%eax
  mov    %r9b,0x5(%rcx)
  cmp    %eax,%ebp
  jle    L213
  movzbl 0x6(%rsi),%eax
  mov    %al,0x6(%rcx)
L213:
  lea    0x1(%rdx,%rdi,1),%rdx
L318:
  movb   $0x0,(%rdx)
  add    $0x8,%rsp
  mov    %r8,%rax
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
  nopw   0x0(%rax,%rax,1)
L55:
  mov    %r9d,%edi
  xor    %eax,%eax
  nopl   (%rax)
L80:
  movzbl (%rbx,%rax,1),%ecx
  mov    %cl,(%rdx,%rax,1)
  mov    %rax,%rcx
  add    $0x1,%rax
  cmp    %rdi,%rcx
  jne    L80
  jmp    L213
  nopl   (%rax)
L175:
  mov    %esi,%ecx
  mov    %r8,%rax
  sub    %r8,%r13
  lea    0x1(%r8,%rcx,1),%rsi
  nopl   (%rax)
L277:
  movzbl (%rax,%r13,1),%edx
  add    $0x1,%rax
  mov    %dl,-0x1(%rax)
  cmp    %rsi,%rax
  jne    L277
  jmp    L321
  cs nopw 0x0(%rax,%rax,1)
L239:
  mov    %rax,%rdx
  jmp    L114
L161:
  mov    %rbx,%rsi
  mov    %rdx,%rcx
  xor    %eax,%eax
  xor    %r10d,%r10d
  jmp    L120
L308:
  mov    %r12d,%r15d
  mov    %r13,%r9
  mov    %r8,%rdi
  xor    %eax,%eax
  xor    %r10d,%r10d
  jmp    L93
```

assistant: ```c
char *str_concat(char *s1, char *s2) {
  int size1 = strlen(s1);
  int size2 = strlen(s2);
  char *res = (char *)malloc(sizeof(char) * size1 + size2 + 1);
  char *pt_r = res;
  char *pt_s = s1;
  for (int i = 0; i < size1; i++) {
    *pt_r = *pt_s;
    pt_r++;
    pt_s++;
  }
  pt_s = s2;
  for (int i = 0; i < size2; i++) {
    *pt_r = *pt_s;
    pt_r++;
    pt_s++;
  }
  *pt_r = '\0';
  return res;
}
```