user: 下面的汇编指令对应的C源代码是什么：

```asm
<decode_base64>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  sub    $0x40,%rsp
  mov    %rdi,-0x28(%rbp)
  mov    %rsi,-0x30(%rbp)
  mov    %rdx,-0x38(%rbp)
  movq   $0x0,-0x10(%rbp)
  movl   $0x0,-0x18(%rbp)
  mov    -0x28(%rbp),%rax
  mov    %rax,-0x8(%rbp)
  jmp    L178
L21:
  mov    -0x30(%rbp),%rax
  movzbl (%rax),%eax
  cmp    $0x40,%al
  jle    L139
  mov    -0x30(%rbp),%rax
  movzbl (%rax),%eax
  cmp    $0x5a,%al
  jg     L139
  mov    -0x30(%rbp),%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  sub    $0x41,%eax
  mov    %eax,-0x14(%rbp)
  jmp    L229
L139:
  mov    -0x30(%rbp),%rax
  movzbl (%rax),%eax
  cmp    $0x60,%al
  jle    L264
  mov    -0x30(%rbp),%rax
  movzbl (%rax),%eax
  cmp    $0x7a,%al
  jg     L264
  mov    -0x30(%rbp),%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  sub    $0x47,%eax
  mov    %eax,-0x14(%rbp)
  jmp    L229
L264:
  mov    -0x30(%rbp),%rax
  movzbl (%rax),%eax
  cmp    $0x2f,%al
  jle    L2
  mov    -0x30(%rbp),%rax
  movzbl (%rax),%eax
  cmp    $0x39,%al
  jg     L2
  mov    -0x30(%rbp),%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  add    $0x4,%eax
  mov    %eax,-0x14(%rbp)
  jmp    L229
L2:
  mov    -0x30(%rbp),%rax
  movzbl (%rax),%eax
  cmp    $0x2b,%al
  jne    L298
  movl   $0x3e,-0x14(%rbp)
  jmp    L229
L298:
  mov    -0x30(%rbp),%rax
  movzbl (%rax),%eax
  cmp    $0x2f,%al
  jne    L152
  movl   $0x3f,-0x14(%rbp)
  jmp    L229
L152:
  mov    -0x30(%rbp),%rax
  movzbl (%rax),%eax
  movsbl %al,%eax
  mov    %eax,%edi
  call   <isspace@plt>
  test   %rax,%rax
  jne    L180
  mov    -0x30(%rbp),%rax
  movzbl (%rax),%eax
  cmp    $0x3d,%al
  jne    L288
L180:
  addq   $0x1,-0x30(%rbp)
  jmp    L178
L288:
  mov    D33(%rip),%rax
  mov    (%rax),%edx
  mov    D76(%rip),%rax
  mov    %edx,(%rax)
  mov    $0xffffffff,%eax
  jmp    L28
L229:
  addq   $0x1,-0x30(%rbp)
  mov    -0x10(%rbp),%rax
  and    $0x3,%eax
  test   %rax,%rax
  jne    L193
  cmpq   $0x0,-0x38(%rbp)
  jne    L117
  mov    D33(%rip),%rax
  mov    (%rax),%edx
  mov    D76(%rip),%rax
  mov    %edx,(%rax)
  mov    $0xffffffff,%eax
  jmp    L28
L117:
  mov    -0x8(%rbp),%rax
  movzbl (%rax),%eax
  and    $0x3,%eax
  mov    %eax,%edx
  mov    -0x8(%rbp),%rax
  mov    %dl,(%rax)
  mov    -0x8(%rbp),%rax
  movzbl (%rax),%eax
  mov    %eax,%edx
  mov    -0x14(%rbp),%eax
  shl    $0x2,%eax
  or     %edx,%eax
  mov    %eax,%edx
  mov    -0x8(%rbp),%rax
  mov    %dl,(%rax)
  addq   $0x1,-0x10(%rbp)
  jmp    L178
L193:
  mov    -0x10(%rbp),%rax
  and    $0x3,%eax
  cmp    $0x1,%rax
  jne    L227
  cmpq   $0x0,-0x38(%rbp)
  jne    L235
  mov    D33(%rip),%rax
  mov    (%rax),%edx
  mov    D76(%rip),%rax
  mov    %edx,(%rax)
  mov    $0xffffffff,%eax
  jmp    L28
L235:
  mov    -0x8(%rbp),%rax
  movzbl (%rax),%eax
  and    $0xfffffffc,%eax
  mov    %eax,%edx
  mov    -0x8(%rbp),%rax
  mov    %dl,(%rax)
  mov    -0x8(%rbp),%rax
  movzbl (%rax),%eax
  mov    %eax,%edx
  mov    -0x14(%rbp),%eax
  sar    $0x4,%eax
  and    $0x3,%eax
  or     %edx,%eax
  mov    %eax,%edx
  mov    -0x8(%rbp),%rax
  mov    %dl,(%rax)
  cmpq   $0x1,-0x38(%rbp)
  jbe    L187
  mov    -0x8(%rbp),%rax
  add    $0x1,%rax
  movzbl (%rax),%edx
  mov    -0x8(%rbp),%rax
  add    $0x1,%rax
  and    $0xf,%edx
  mov    %dl,(%rax)
  mov    -0x8(%rbp),%rax
  add    $0x1,%rax
  movzbl (%rax),%eax
  mov    %eax,%edx
  mov    -0x14(%rbp),%eax
  shl    $0x4,%eax
  or     %eax,%edx
  mov    -0x8(%rbp),%rax
  add    $0x1,%rax
  mov    %dl,(%rax)
L187:
  addq   $0x1,-0x10(%rbp)
  addl   $0x1,-0x18(%rbp)
  jmp    L178
L227:
  mov    -0x10(%rbp),%rax
  and    $0x3,%eax
  cmp    $0x2,%rax
  jne    L23
  cmpq   $0x1,-0x38(%rbp)
  ja     L166
  mov    D33(%rip),%rax
  mov    (%rax),%edx
  mov    D76(%rip),%rax
  mov    %edx,(%rax)
  mov    $0xffffffff,%eax
  jmp    L28
L166:
  mov    -0x8(%rbp),%rax
  add    $0x1,%rax
  movzbl (%rax),%edx
  mov    -0x8(%rbp),%rax
  add    $0x1,%rax
  and    $0xfffffff0,%edx
  mov    %dl,(%rax)
  mov    -0x8(%rbp),%rax
  add    $0x1,%rax
  movzbl (%rax),%eax
  mov    %eax,%edx
  mov    -0x14(%rbp),%eax
  sar    $0x2,%eax
  and    $0xf,%eax
  or     %eax,%edx
  mov    -0x8(%rbp),%rax
  add    $0x1,%rax
  mov    %dl,(%rax)
  cmpq   $0x2,-0x38(%rbp)
  jbe    L157
  mov    -0x8(%rbp),%rax
  add    $0x2,%rax
  movzbl (%rax),%edx
  mov    -0x8(%rbp),%rax
  add    $0x2,%rax
  and    $0x3f,%edx
  mov    %dl,(%rax)
  mov    -0x8(%rbp),%rax
  add    $0x2,%rax
  movzbl (%rax),%eax
  mov    %eax,%edx
  mov    -0x14(%rbp),%eax
  shl    $0x6,%eax
  or     %eax,%edx
  mov    -0x8(%rbp),%rax
  add    $0x2,%rax
  mov    %dl,(%rax)
L157:
  addq   $0x1,-0x10(%rbp)
  addl   $0x1,-0x18(%rbp)
  jmp    L178
L23:
  cmpq   $0x2,-0x38(%rbp)
  ja     L16
  mov    D33(%rip),%rax
  mov    (%rax),%edx
  mov    D76(%rip),%rax
  mov    %edx,(%rax)
  mov    $0xffffffff,%eax
  jmp    L28
L16:
  mov    -0x8(%rbp),%rax
  add    $0x2,%rax
  movzbl (%rax),%edx
  mov    -0x8(%rbp),%rax
  add    $0x2,%rax
  and    $0xffffffc0,%edx
  mov    %dl,(%rax)
  mov    -0x8(%rbp),%rax
  add    $0x2,%rax
  movzbl (%rax),%eax
  mov    %eax,%edx
  mov    -0x14(%rbp),%eax
  and    $0x3f,%eax
  or     %eax,%edx
  mov    -0x8(%rbp),%rax
  add    $0x2,%rax
  mov    %dl,(%rax)
  addq   $0x1,-0x10(%rbp)
  addq   $0x3,-0x8(%rbp)
  subq   $0x3,-0x38(%rbp)
  addl   $0x1,-0x18(%rbp)
L178:
  mov    -0x30(%rbp),%rax
  movzbl (%rax),%eax
  test   %al,%al
  jne    L21
  mov    -0x18(%rbp),%eax
L28:
  leave
  ret
```

assistant: ```c
int decode_base64(void *dest, char *src, size_t dstsize) {
  size_t processed = 0;
  int full_bytes = 0;
  unsigned char *dst = dest;
  while (*src) {
    int v;
    if (*src >= 'A' && *src <= 'Z')
      v = *src - 'A';
    else if (*src >= 'a' && *src <= 'z')
      v = 26 + *src - 'a';
    else if (*src >= '0' && *src <= '9')
      v = 52 + *src - '0';
    else if (*src == '+')
      v = 62;
    else if (*src == '/')
      v = 63;
    else if (isspace(*src) || *src == '=') {
      src++;
      continue;
    } else {
      errno = EINVAL;
      return -1;
    }
    src++;
    if (processed % 4 == 0) {
      if (dstsize <= 0) {
        errno = EINVAL;
        return -1;
      }
      dst[0] &= 0x03;
      dst[0] |= (v << 2) & 0xFC;
      processed++;
    } else if (processed % 4 == 1) {
      if (dstsize < 1) {
        errno = EINVAL;
        return -1;
      }
      dst[0] &= 0xFC;
      dst[0] |= (v >> 4) & 0x03;
      if (dstsize >= 2) {
        dst[1] &= 0x0F;
        dst[1] |= (v << 4) & 0xF0;
      }
      processed++;
      full_bytes++;
    } else if (processed % 4 == 2) {
      if (dstsize < 2) {
        errno = EINVAL;
        return -1;
      }
      dst[1] &= 0xF0;
      dst[1] |= (v >> 2) & 0x0F;
      if (dstsize >= 3) {
        dst[2] &= 0x3F;
        dst[2] |= (v << 6) & 0xC0;
      }
      processed++;
      full_bytes++;
    } else {
      if (dstsize <= 2) {
        errno = EINVAL;
        return -1;
      }
      dst[2] &= 0xC0;
      dst[2] |= v & 0x3F;
      processed++;
      dst += 3;
      dstsize -= 3;
      full_bytes++;
    }
  }
  return full_bytes;
}
```