user: 下面的汇编指令对应的C源代码是什么：

```asm
<decode_base64>:
  endbr64
  push   %r14
  push   %r13
  push   %r12
  mov    %rdi,%r12
  movsbl (%rsi),%edi
  push   %rbp
  push   %rbx
  test   %dil,%dil
  je     L220
  mov    %rdx,%r13
  lea    0x1(%rsi),%rbp
  xor    %r14d,%r14d
  xor    %ebx,%ebx
  jmp    L225
  nopl   0x0(%rax)
L54:
  test   %r13,%r13
  je     L12
  movzbl (%r12),%eax
  shl    $0x2,%edx
  add    $0x1,%rbx
  and    $0x3,%eax
  or     %edx,%eax
  mov    %al,(%r12)
L91:
  movsbl 0x0(%rbp),%edi
  add    $0x1,%rbp
  test   %dil,%dil
  je     L236
L225:
  lea    -0x41(%rdi),%eax
  movsbl %al,%edx
  cmp    $0x19,%al
  jbe    L215
  lea    -0x61(%rdi),%eax
  cmp    $0x19,%al
  ja     L78
  sub    $0x47,%edi
  movsbl %dil,%edx
L215:
  mov    %rbx,%rax
  and    $0x3,%eax
  je     L54
  cmp    $0x1,%rax
  je     L201
  cmp    $0x2,%rax
  je     L68
  cmp    $0x2,%r13
  jbe    L12
  add    $0x1,%rbx
  add    $0x3,%r12
  sub    $0x3,%r13
  add    $0x1,%r14d
  movzbl -0x1(%r12),%eax
  add    $0x1,%rbp
  and    $0xffffffc0,%eax
  or     %edx,%eax
  mov    %al,-0x1(%r12)
  movsbl -0x1(%rbp),%edi
  test   %dil,%dil
  jne    L225
L236:
  pop    %rbx
  mov    %r14d,%eax
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  ret
  xchg   %ax,%ax
L78:
  lea    -0x30(%rdi),%eax
  cmp    $0x9,%al
  ja     L239
  add    $0x4,%edi
  movsbl %dil,%edx
  jmp    L215
L201:
  test   %r13,%r13
  je     L12
  movzbl (%r12),%eax
  mov    %edx,%ecx
  sar    $0x4,%ecx
  and    $0xfffffffc,%eax
  or     %ecx,%eax
  mov    %al,(%r12)
  cmp    $0x1,%r13
  je     L55
  movzbl 0x1(%r12),%ecx
  mov    %edx,%eax
  shl    $0x4,%eax
  and    $0xf,%ecx
  or     %ecx,%eax
  mov    %al,0x1(%r12)
L55:
  add    $0x1,%rbx
  add    $0x1,%r14d
  jmp    L91
L239:
  cmp    $0x2b,%dil
  je     L69
  mov    $0x3f,%edx
  cmp    $0x2f,%dil
  je     L215
  call   <isspace@plt>
  test   %rax,%rax
  jne    L91
  cmpb   $0x3d,-0x1(%rbp)
  je     L91
L12:
  mov    D18(%rip),%rax
  mov    $0xffffffff,%r14d
  mov    (%rax),%edx
  mov    D12(%rip),%rax
  mov    %edx,(%rax)
  mov    %r14d,%eax
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  ret
  nopl   (%rax)
L68:
  cmp    $0x1,%r13
  jbe    L12
  movzbl 0x1(%r12),%eax
  mov    %edx,%ecx
  sar    $0x2,%ecx
  and    $0xfffffff0,%eax
  or     %ecx,%eax
  mov    %al,0x1(%r12)
  cmp    $0x2,%r13
  je     L55
  movzbl 0x2(%r12),%ecx
  mov    %edx,%eax
  shl    $0x6,%eax
  and    $0x3f,%ecx
  or     %ecx,%eax
  mov    %al,0x2(%r12)
  jmp    L55
  nopw   0x0(%rax,%rax,1)
L69:
  mov    $0x3e,%edx
  jmp    L215
L220:
  xor    %r14d,%r14d
  jmp    L236
```

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;
}
```