user: 请显示与下面列出的汇编代码相匹配的C语言实现：

```asm
<decode_base64>:
  endbr64
  push   %r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  sub    $0x8,%rsp
  mov    %rdi,%r12
  movzbl (%rsi),%edi
  test   %dil,%dil
  je     L15
  mov    %rdx,%r13
  lea    0x1(%rsi),%rbp
  mov    $0x0,%r14d
  mov    $0x0,%ebx
  mov    $0x3e,%r15d
  jmp    L151
L272:
  lea    -0x61(%rdi),%eax
  cmp    $0x19,%al
  ja     L13
  movsbl %dil,%edi
  lea    -0x47(%rdi),%eax
  jmp    L7
L13:
  sub    $0x30,%edx
  cmp    $0x9,%dl
  ja     L287
  movsbl %dil,%edi
  lea    0x4(%rdi),%eax
  jmp    L7
L287:
  cmp    $0x2b,%dil
  je     L202
  mov    $0x3f,%eax
  cmp    $0x2f,%dil
  je     L7
  movsbl %dil,%edi
  call   <isspace@plt>
  test   %rax,%rax
  jne    L192
  cmpb   $0x3d,-0x1(%rbp)
  jne    L124
L192:
  mov    %rbp,%rcx
  jmp    L293
L124:
  mov    D3(%rip),%rax
  mov    (%rax),%edx
  mov    D10(%rip),%rax
  mov    %edx,(%rax)
  mov    $0xffffffff,%r14d
  jmp    L268
L202:
  mov    %r15d,%eax
  jmp    L7
L162:
  mov    D3(%rip),%rax
  mov    (%rax),%edx
  mov    D10(%rip),%rax
  mov    %edx,(%rax)
  mov    $0xffffffff,%r14d
L268:
  mov    %r14d,%eax
  add    $0x8,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
L169:
  cmp    $0x1,%rdx
  je     L236
  cmp    $0x2,%rdx
  je     L259
  cmp    $0x2,%r13
  jbe    L34
  movzbl 0x2(%r12),%edx
  and    $0xffffffc0,%edx
  or     %edx,%eax
  mov    %al,0x2(%r12)
  add    $0x1,%rbx
  add    $0x3,%r12
  sub    $0x3,%r13
  add    $0x1,%r14d
L293:
  movzbl (%rcx),%edi
  add    $0x1,%rbp
  test   %dil,%dil
  je     L268
L151:
  mov    %edi,%edx
  lea    -0x41(%rdi),%eax
  cmp    $0x19,%al
  ja     L272
  movsbl %dil,%edi
  lea    -0x41(%rdi),%eax
L7:
  mov    %rbp,%rcx
  mov    %rbx,%rdx
  and    $0x3,%edx
  jne    L169
  test   %r13,%r13
  je     L162
  movzbl (%r12),%edx
  and    $0x3,%edx
  shl    $0x2,%eax
  or     %edx,%eax
  mov    %al,(%r12)
  add    $0x1,%rbx
  jmp    L293
L236:
  test   %r13,%r13
  je     L12
  movzbl (%r12),%edx
  and    $0xfffffffc,%edx
  mov    %eax,%esi
  sar    $0x4,%esi
  or     %esi,%edx
  mov    %dl,(%r12)
  cmp    $0x1,%r13
  jbe    L200
  movzbl 0x1(%r12),%edx
  and    $0xf,%edx
  shl    $0x4,%eax
  or     %edx,%eax
  mov    %al,0x1(%r12)
L200:
  add    $0x1,%rbx
  add    $0x1,%r14d
  jmp    L293
L12:
  mov    D3(%rip),%rax
  mov    (%rax),%edx
  mov    D10(%rip),%rax
  mov    %edx,(%rax)
  mov    $0xffffffff,%r14d
  jmp    L268
L259:
  cmp    $0x1,%r13
  jbe    L131
  movzbl 0x1(%r12),%edx
  and    $0xfffffff0,%edx
  mov    %eax,%esi
  sar    $0x2,%esi
  or     %esi,%edx
  mov    %dl,0x1(%r12)
  cmp    $0x2,%r13
  jbe    L135
  movzbl 0x2(%r12),%edx
  and    $0x3f,%edx
  shl    $0x6,%eax
  or     %edx,%eax
  mov    %al,0x2(%r12)
L135:
  add    $0x1,%rbx
  add    $0x1,%r14d
  jmp    L293
L131:
  mov    D3(%rip),%rax
  mov    (%rax),%edx
  mov    D10(%rip),%rax
  mov    %edx,(%rax)
  mov    $0xffffffff,%r14d
  jmp    L268
L34:
  mov    D3(%rip),%rax
  mov    (%rax),%edx
  mov    D10(%rip),%rax
  mov    %edx,(%rax)
  mov    $0xffffffff,%r14d
  jmp    L268
L15:
  mov    $0x0,%r14d
  jmp    L268
```

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;
}
```