user: 下面的汇编指令对应的C源代码是什么：

```asm
<decode_block4>:
  endbr64
  test   %esi,%esi
  je     L67
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  mov    %rdx,%rax
  mov    %rcx,%r9
  sub    $0x2,%esi
  mov    $0x0,%r8d
  jmp    L51
L63:
  mov    %edx,%r10d
  shr    $0x4,%r10w
  mov    %esi,%r11d
  and    $0x7fe,%r10d
  mov    %rax,%rcx
  sub    %r10,%rcx
  and    $0x1f,%dx
  je     L68
  lea    -0x1(%rdx),%ebp
  movzwl %dx,%edx
  lea    (%rax,%rdx,4),%rbx
L52:
  movzbl (%rcx),%edx
  shl    $0x18,%edx
  movzbl 0x1(%rcx),%r10d
  shl    $0x10,%r10d
  lea    (%rdx,%r10,1),%r10d
  movzbl 0x2(%rcx),%r13d
  movzbl 0x3(%rcx),%r12d
  add    $0x4,%rcx
  shr    $0x18,%edx
  mov    %dl,(%rax)
  mov    %r10d,%edx
  shr    $0x10,%edx
  mov    %dl,0x1(%rax)
  mov    %r13b,0x2(%rax)
  mov    %r12b,0x3(%rax)
  add    $0x4,%rax
  cmp    %rbx,%rax
  jne    L52
  movzwl %bp,%ebp
  lea    0x4(%r8,%rbp,4),%r8d
  mov    %rbx,%rax
L68:
  sub    $0x2,%esi
  test   %r11d,%r11d
  je     L25
L51:
  movzbl (%rdi),%edx
  shl    $0x8,%edx
  movzbl 0x1(%rdi),%ecx
  add    $0x2,%rdi
  add    %cx,%dx
  js     L63
  mov    %edx,%r10d
  shr    $0x8,%r10w
  mov    %r10,%r11
  and    $0xfe,%r11d
  add    %r9,%r11
  movzbl (%r11),%ecx
  shl    $0x8,%ecx
  movzbl 0x1(%r11),%r11d
  add    %r11d,%ecx
  and    $0x1,%r10d
  add    %ecx,%ecx
  or     %ecx,%r10d
  movzbl %ch,%ebx
  mov    %bl,(%rax)
  mov    %r10b,0x1(%rax)
  mov    %rdx,%r10
  and    $0xfe,%r10d
  add    %r9,%r10
  movzbl (%r10),%ecx
  shl    $0x8,%ecx
  movzbl 0x1(%r10),%r10d
  add    %r10d,%ecx
  mov    %esi,%r11d
  add    %ecx,%ecx
  and    $0x1,%edx
  or     %ecx,%edx
  movzbl %ch,%ebx
  mov    %bl,0x2(%rax)
  mov    %dl,0x3(%rax)
  add    $0x4,%rax
  add    $0x4,%r8d
  jmp    L68
L67:
  mov    %esi,%eax
  ret
L25:
  mov    %r8d,%eax
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  ret
```

assistant: ```c
int decode_block4(unsigned char *in, int length, unsigned char *out, unsigned char *lut) {
  unsigned char *look;
  unsigned int t3;
  unsigned short t0, t1, t2;
  int len = 0;
  while (length != 0) {
    t0 = (((in)[0] << 8) + ((in)[1]));
    in += 2;
    if ((t0 & 0x8000) == 0) {
      t1 = t0 >> 8;
      t2 = t1 & 0xFE;
      look = lut + t2;
      t2 = (((look)[0] << 8) + ((look)[1]));
      t1 &= 1;
      t2 <<= 1;
      t1 |= t2;
      do {
        (out)[0] = ((t1) >> 8) & 0xFF;
        (out)[1] = ((t1)) & 0xFF;
      } while (0);
      out += 2;
      t1 = t0 & 0xFE;
      look = lut + t1;
      t1 = (((look)[0] << 8) + ((look)[1]));
      t0 &= 1;
      length -= 2;
      t1 <<= 1;
      t1 |= t0;
      do {
        (out)[0] = ((t1) >> 8) & 0xFF;
        (out)[1] = ((t1)) & 0xFF;
      } while (0);
      out += 2;
      len += 4;
    } else {
      t1 = t0 & 0x1F;
      t0 &= 0x7FE0;
      t0 >>= 4;
      length -= 2;
      look = out - t0;
      while (t1 != 0) {
        t3 = (unsigned int)(((look)[0] << 24) + ((look)[1] << 16) + ((look)[2] << 8) + ((look)[3]));
        look += 4;
        t1 -= 1;
        do {
          (out)[0] = ((t3) >> 24) & 0xFF;
          (out)[1] = ((t3) >> 16) & 0xFF;
          (out)[2] = ((t3) >> 8) & 0xFF;
          (out)[3] = (t3)&0xFF;
        } while (0);
        out += 4;
        len += 4;
      }
    }
  }
  return len;
}
```