user: 提供的汇编代码用C语言表示会是什么样子:

```asm
<decode_base64>:
  endbr64
  push   %r15
  push   %r14
  push   %r13
  push   %r12
  push   %rbp
  push   %rbx
  sub    $0x28,%rsp
  mov    %rdi,0x8(%rsp)
  mov    $0x0,%eax
  test   %rsi,%rsi
  je     L170
  mov    %rsi,%rbx
  cmpb   $0x0,(%rsi)
  jne    L139
L170:
  add    $0x28,%rsp
  pop    %rbx
  pop    %rbp
  pop    %r12
  pop    %r13
  pop    %r14
  pop    %r15
  ret
L139:
  mov    %rsi,%rdi
  call   <strlen@plt>
  lea    0x1(%rax),%edi
  mov    $0x1,%esi
  call   <calloc@plt>
  mov    %rax,0x18(%rsp)
  movzbl (%rbx),%edi
  test   %dil,%dil
  je     L178
  mov    $0x0,%r12d
  jmp    L140
L43:
  add    $0x1,%rbx
  movzbl (%rbx),%edi
  test   %dil,%dil
  je     L136
L140:
  movsbl %dil,%edi
  call   <is_base64@plt>
  test   %rax,%rax
  je     L43
  movslq %r12d,%rax
  movzbl (%rbx),%edx
  mov    0x18(%rsp),%rcx
  mov    %dl,(%rcx,%rax,1)
  lea    0x1(%r12),%r12d
  jmp    L43
L136:
  test   %r12d,%r12d
  jle    L61
  mov    0x18(%rsp),%r13
  lea    -0x1(%r12),%eax
  and    $0xfffffffc,%eax
  add    $0x5,%eax
  mov    %eax,0x14(%rsp)
  mov    0x8(%rsp),%rbx
  mov    $0x1,%ebp
  jmp    L153
L3:
  movsbl %dil,%edi
  call   <decode@plt>
  mov    %al,0x13(%rsp)
  movsbl 0x11(%rsp),%edi
  call   <decode@plt>
  mov    %al,0x11(%rsp)
  movsbl %r15b,%edi
  call   <decode@plt>
  mov    %al,0x12(%rsp)
  movsbl %r14b,%edi
  call   <decode@plt>
  mov    %eax,%ecx
  movzbl 0x11(%rsp),%eax
  shr    $0x4,%al
  movzbl 0x13(%rsp),%edx
  shl    $0x2,%edx
  or     %eax,%edx
  mov    %dl,(%rbx)
  cmp    $0x3d,%r15b
  je     L5
  movzbl 0x12(%rsp),%eax
  shr    $0x2,%al
  movzbl 0x11(%rsp),%edx
  shl    $0x4,%edx
  or     %eax,%edx
  mov    %dl,0x1(%rbx)
  lea    0x2(%rbx),%rbx
L29:
  cmp    $0x3d,%r14b
  je     L98
  movzbl 0x12(%rsp),%eax
  shl    $0x6,%eax
  or     %ecx,%eax
  mov    %al,(%rbx)
  lea    0x1(%rbx),%rbx
L98:
  add    $0x4,%r13
  add    $0x4,%ebp
  cmp    0x14(%rsp),%ebp
  je     L25
L153:
  mov    %r13,%rax
  movzbl 0x0(%r13),%edi
  movb   $0x41,0x11(%rsp)
  cmp    %ebp,%r12d
  jle    L167
  movzbl 0x1(%r13),%esi
  mov    %sil,0x11(%rsp)
L167:
  lea    0x1(%rbp),%edx
  mov    $0x41,%r15d
  cmp    %edx,%r12d
  jle    L38
  movzbl 0x2(%rax),%r15d
L38:
  lea    0x2(%rbp),%edx
  mov    $0x41,%r14d
  cmp    %edx,%r12d
  jle    L3
  movzbl 0x3(%rax),%r14d
  jmp    L3
L5:
  add    $0x1,%rbx
  jmp    L29
L178:
  mov    0x8(%rsp),%rbx
L25:
  mov    0x18(%rsp),%rdi
  call   <free@plt>
  mov    %ebx,%eax
  sub    0x8(%rsp),%eax
  jmp    L170
L61:
  mov    0x8(%rsp),%rbx
  jmp    L25
```

assistant: ```c
int decode_base64(unsigned char *dest, const char *src) {
  if (src && *src) {
    unsigned char *p = dest;
    int k, l = strlen(src) + 1;
    unsigned char *buf = calloc(l, sizeof(unsigned char));
    for (k = 0, l = 0; src[k]; k++) {
      if (is_base64(src[k])) {
        buf[l++] = src[k];
      }
    }
    for (k = 0; k < l; k += 4) {
      char c1 = 'A', c2 = 'A', c3 = 'A', c4 = 'A';
      unsigned char b1 = 0, b2 = 0, b3 = 0, b4 = 0;
      c1 = buf[k];
      if (k + 1 < l) {
        c2 = buf[k + 1];
      }
      if (k + 2 < l) {
        c3 = buf[k + 2];
      }
      if (k + 3 < l) {
        c4 = buf[k + 3];
      }
      b1 = decode(c1);
      b2 = decode(c2);
      b3 = decode(c3);
      b4 = decode(c4);
      *p++ = ((b1 << 2) | (b2 >> 4));
      if (c3 != '=') {
        *p++ = (((b2 & 0xf) << 4) | (b3 >> 2));
      }
      if (c4 != '=') {
        *p++ = (((b3 & 0x3) << 6) | b4);
      }
    }
    free(buf);
    return (p - dest);
  }
  return 0;
}
```