user: 请从给定的汇编指令推导出C源代码:

```asm
<conv_8str_to_6str>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  sub    $0x30,%rsp
  mov    %rdi,-0x18(%rbp)
  mov    %esi,-0x1c(%rbp)
  mov    %rdx,-0x28(%rbp)
  mov    %rcx,-0x30(%rbp)
  movl   $0x0,-0xc(%rbp)
  movl   $0x0,-0x8(%rbp)
  movl   $0x0,-0x4(%rbp)
  jmp    L81
L25:
  mov    -0x4(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  mov    %eax,%edi
  call   <find_6val@plt>
  mov    %al,-0xd(%rbp)
  cmpl   $0x6,-0xc(%rbp)
  je     L108
  cmpl   $0x6,-0xc(%rbp)
  jg     L91
  cmpl   $0x4,-0xc(%rbp)
  je     L7
  cmpl   $0x4,-0xc(%rbp)
  jg     L91
  cmpl   $0x0,-0xc(%rbp)
  je     L103
  cmpl   $0x2,-0xc(%rbp)
  je     L52
  jmp    L91
L103:
  movzbl -0xd(%rbp),%eax
  lea    0x0(,%rax,4),%ecx
  mov    -0x8(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x28(%rbp),%rax
  add    %rdx,%rax
  mov    %ecx,%edx
  mov    %dl,(%rax)
  movl   $0x2,-0xc(%rbp)
  jmp    L91
L52:
  mov    -0x8(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x28(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%edx
  movzbl -0xd(%rbp),%eax
  shr    $0x4,%al
  mov    %eax,%esi
  mov    -0x8(%rbp),%eax
  movslq %eax,%rcx
  mov    -0x28(%rbp),%rax
  add    %rcx,%rax
  or     %esi,%edx
  mov    %dl,(%rax)
  mov    -0x8(%rbp),%eax
  cltq
  lea    0x1(%rax),%rdx
  mov    -0x28(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%edx
  movzbl -0xd(%rbp),%eax
  shl    $0x4,%eax
  mov    %eax,%esi
  mov    -0x8(%rbp),%eax
  cltq
  lea    0x1(%rax),%rcx
  mov    -0x28(%rbp),%rax
  add    %rcx,%rax
  or     %esi,%edx
  mov    %dl,(%rax)
  movl   $0x4,-0xc(%rbp)
  addl   $0x1,-0x8(%rbp)
  jmp    L91
L7:
  mov    -0x8(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x28(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%edx
  movzbl -0xd(%rbp),%eax
  shr    $0x2,%al
  mov    %eax,%esi
  mov    -0x8(%rbp),%eax
  movslq %eax,%rcx
  mov    -0x28(%rbp),%rax
  add    %rcx,%rax
  or     %esi,%edx
  mov    %dl,(%rax)
  mov    -0x8(%rbp),%eax
  cltq
  lea    0x1(%rax),%rdx
  mov    -0x28(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%edx
  movzbl -0xd(%rbp),%eax
  shl    $0x6,%eax
  mov    %eax,%esi
  mov    -0x8(%rbp),%eax
  cltq
  lea    0x1(%rax),%rcx
  mov    -0x28(%rbp),%rax
  add    %rcx,%rax
  or     %esi,%edx
  mov    %dl,(%rax)
  movl   $0x6,-0xc(%rbp)
  addl   $0x1,-0x8(%rbp)
  jmp    L91
L108:
  mov    -0x8(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x28(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%ecx
  movzbl -0xd(%rbp),%edx
  mov    -0x8(%rbp),%eax
  movslq %eax,%rsi
  mov    -0x28(%rbp),%rax
  add    %rsi,%rax
  or     %ecx,%edx
  mov    %dl,(%rax)
  movl   $0x0,-0xc(%rbp)
  addl   $0x1,-0x8(%rbp)
  nop
L91:
  addl   $0x1,-0x4(%rbp)
L81:
  mov    -0x4(%rbp),%eax
  cmp    -0x1c(%rbp),%eax
  jl     L25
  mov    -0x8(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    -0x30(%rbp),%rax
  mov    %edx,(%rax)
  nop
  leave
  ret
```

assistant: ```c
void conv_8str_to_6str(char *str8, int str8_len, char *str6, int *str6_len) {
  int bits_left = 0;
  int byte_at = 0;
  for (int i = 0; i < str8_len; i++) {
    unsigned char b1 = find_6val(str8[i]);
    switch (bits_left) {
    case 0:
      str6[byte_at] = b1 << 2;
      bits_left = 2;
      break;
    case 2:
      str6[byte_at] = str6[byte_at] | b1 >> 4;
      str6[byte_at + 1] = str6[byte_at + 1] | b1 << 4;
      bits_left = 4;
      byte_at = byte_at + 1;
      break;
    case 4:
      str6[byte_at] = str6[byte_at] | b1 >> 2;
      str6[byte_at + 1] = str6[byte_at + 1] | b1 << 6;
      bits_left = 6;
      byte_at = byte_at + 1;
      break;
    case 6:
      str6[byte_at] = str6[byte_at] | (b1);
      bits_left = 0;
      byte_at = byte_at + 1;
      break;
    }
  }
  *str6_len = byte_at + 1;
}
```