user: What is the c source code of the assembly code below?

```asm
<decode_base64>:
  endbr64
  push   %rbp
  mov    %rsp,%rbp
  push   %rbx
  sub    $0x48,%rsp
  mov    %rdi,-0x38(%rbp)
  mov    %esi,-0x3c(%rbp)
  mov    %rdx,-0x48(%rbp)
  movl   $0x0,-0x20(%rbp)
  movq   $0x0,-0x18(%rbp)
  movl   $0x0,-0x1c(%rbp)
  mov    -0x3c(%rbp),%eax
  lea    0x3(%rax),%edx
  test   %eax,%eax
  cmovs  %edx,%eax
  sar    $0x2,%eax
  mov    %eax,%edx
  mov    %edx,%eax
  add    %eax,%eax
  add    %edx,%eax
  mov    %eax,-0x24(%rbp)
  mov    -0x3c(%rbp),%eax
  cltq
  lea    -0x1(%rax),%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cmp    $0x3d,%al
  jne    L20
  subl   $0x1,-0x24(%rbp)
L20:
  mov    -0x3c(%rbp),%eax
  cltq
  lea    -0x2(%rax),%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  cmp    $0x3d,%al
  jne    L51
  subl   $0x1,-0x24(%rbp)
L51:
  mov    -0x24(%rbp),%eax
  mov    %eax,%edi
  call   <malloc@plt>
  mov    %rax,-0x18(%rbp)
  cmpq   $0x0,-0x18(%rbp)
  jne    L47
  mov    $0x0,%eax
  jmp    L29
L47:
  movl   $0x0,-0x2c(%rbp)
  movl   $0x0,-0x28(%rbp)
  movl   $0x0,-0x20(%rbp)
  mov    -0x24(%rbp),%eax
  movslq %eax,%rdx
  imul   $0x55555556,%rdx,%rdx
  shr    $0x20,%rdx
  mov    %eax,%esi
  sar    $0x1f,%esi
  mov    %edx,%ecx
  sub    %esi,%ecx
  mov    %ecx,%edx
  add    %edx,%edx
  add    %ecx,%edx
  sub    %edx,%eax
  mov    %eax,%ecx
  mov    -0x24(%rbp),%eax
  sub    %ecx,%eax
  mov    %eax,-0x1c(%rbp)
  jmp    L81
L37:
  mov    -0x28(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x28(%rbp)
  movslq %eax,%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movzbl %al,%eax
  mov    %eax,%edi
  call   <b64_2_byte@plt>
  lea    0x0(,%rax,4),%ebx
  mov    -0x28(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movzbl %al,%eax
  mov    %eax,%edi
  call   <b64_2_byte@plt>
  sar    $0x4,%eax
  and    $0x3,%eax
  mov    %eax,%ecx
  mov    -0x2c(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x2c(%rbp)
  movslq %eax,%rdx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  lea    (%rbx,%rcx,1),%edx
  mov    %dl,(%rax)
  mov    -0x28(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x28(%rbp)
  movslq %eax,%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movzbl %al,%eax
  mov    %eax,%edi
  call   <b64_2_byte@plt>
  shl    $0x4,%eax
  and    $0xfffffff0,%eax
  mov    %eax,%ebx
  mov    -0x28(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movzbl %al,%eax
  mov    %eax,%edi
  call   <b64_2_byte@plt>
  sar    $0x2,%eax
  and    $0xf,%eax
  mov    %eax,%ecx
  mov    -0x2c(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x2c(%rbp)
  movslq %eax,%rdx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  lea    (%rbx,%rcx,1),%edx
  mov    %dl,(%rax)
  mov    -0x28(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x28(%rbp)
  movslq %eax,%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movzbl %al,%eax
  mov    %eax,%edi
  call   <b64_2_byte@plt>
  shl    $0x6,%eax
  and    $0xffffffc0,%eax
  mov    %eax,%ebx
  mov    -0x28(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x28(%rbp)
  movslq %eax,%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movzbl %al,%eax
  mov    %eax,%edi
  call   <b64_2_byte@plt>
  mov    %eax,%ecx
  mov    -0x2c(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x2c(%rbp)
  movslq %eax,%rdx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  lea    (%rbx,%rcx,1),%edx
  mov    %dl,(%rax)
L81:
  mov    -0x2c(%rbp),%eax
  cmp    -0x1c(%rbp),%eax
  jl     L37
  mov    -0x24(%rbp),%edx
  movslq %edx,%rax
  imul   $0x55555556,%rax,%rax
  shr    $0x20,%rax
  mov    %edx,%ecx
  sar    $0x1f,%ecx
  sub    %ecx,%eax
  mov    %eax,%ecx
  add    %ecx,%ecx
  add    %eax,%ecx
  mov    %edx,%eax
  sub    %ecx,%eax
  cmp    $0x1,%eax
  jne    L88
  mov    -0x28(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x28(%rbp)
  movslq %eax,%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movzbl %al,%eax
  mov    %eax,%edi
  call   <b64_2_byte@plt>
  lea    0x0(,%rax,4),%ebx
  mov    -0x28(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movzbl %al,%eax
  mov    %eax,%edi
  call   <b64_2_byte@plt>
  sar    $0x4,%eax
  and    $0x3,%eax
  mov    %eax,%ecx
  mov    -0x2c(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x2c(%rbp)
  movslq %eax,%rdx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  lea    (%rbx,%rcx,1),%edx
  mov    %dl,(%rax)
  jmp    L7
L88:
  mov    -0x24(%rbp),%edx
  movslq %edx,%rax
  imul   $0x55555556,%rax,%rax
  shr    $0x20,%rax
  mov    %edx,%ecx
  sar    $0x1f,%ecx
  sub    %ecx,%eax
  mov    %eax,%ecx
  add    %ecx,%ecx
  add    %eax,%ecx
  mov    %edx,%eax
  sub    %ecx,%eax
  cmp    $0x2,%eax
  jne    L7
  mov    -0x28(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x28(%rbp)
  movslq %eax,%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movzbl %al,%eax
  mov    %eax,%edi
  call   <b64_2_byte@plt>
  lea    0x0(,%rax,4),%ebx
  mov    -0x28(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movzbl %al,%eax
  mov    %eax,%edi
  call   <b64_2_byte@plt>
  sar    $0x4,%eax
  and    $0x3,%eax
  mov    %eax,%ecx
  mov    -0x2c(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x2c(%rbp)
  movslq %eax,%rdx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  lea    (%rbx,%rcx,1),%edx
  mov    %dl,(%rax)
  mov    -0x28(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x28(%rbp)
  movslq %eax,%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movzbl %al,%eax
  mov    %eax,%edi
  call   <b64_2_byte@plt>
  shl    $0x4,%eax
  and    $0xfffffff0,%eax
  mov    %eax,%ebx
  mov    -0x28(%rbp),%eax
  movslq %eax,%rdx
  mov    -0x38(%rbp),%rax
  add    %rdx,%rax
  movzbl (%rax),%eax
  movzbl %al,%eax
  mov    %eax,%edi
  call   <b64_2_byte@plt>
  sar    $0x2,%eax
  and    $0xf,%eax
  mov    %eax,%ecx
  mov    -0x2c(%rbp),%eax
  lea    0x1(%rax),%edx
  mov    %edx,-0x2c(%rbp)
  movslq %eax,%rdx
  mov    -0x18(%rbp),%rax
  add    %rdx,%rax
  lea    (%rbx,%rcx,1),%edx
  mov    %dl,(%rax)
L7:
  mov    -0x48(%rbp),%rax
  mov    -0x24(%rbp),%edx
  mov    %edx,(%rax)
  mov    -0x18(%rbp),%rax
L29:
  mov    -0x8(%rbp),%rbx
  leave
  ret
```

assistant: ```c
unsigned char *decode_base64(unsigned char *src, int size, int *dest_sz) {
  int i, j, oct_cnt = 0;
  unsigned char *dest = 0;
  int dest_size;
  int iter = 0;
  dest_size = (size / 4) * 3;
  if (src[size - 1] == '=') {
    dest_size -= 1;
  }
  if (src[size - 2] == '=') {
    dest_size -= 1;
  }
  dest = (unsigned char *)malloc(dest_size * sizeof(unsigned char));
  if (!dest) {
    return 0;
  }
  i = 0;
  j = 0;
  oct_cnt = 0;
  iter = dest_size - (dest_size % 3);
  while (i < iter) {
    dest[i++] = (b64_2_byte(src[j++]) << 2) + ((b64_2_byte(src[j]) & 48) >> 4);
    dest[i++] = ((b64_2_byte(src[j++]) & 15) << 4) + ((b64_2_byte(src[j]) & 60) >> 2);
    dest[i++] = ((b64_2_byte(src[j++]) & 3) << 6) + b64_2_byte(src[j++]);
  }
  if (dest_size % 3 == 1) {
    dest[i++] = (b64_2_byte(src[j++]) << 2) + ((b64_2_byte(src[j]) & 48) >> 4);
  } else if (dest_size % 3 == 2) {
    dest[i++] = (b64_2_byte(src[j++]) << 2) + ((b64_2_byte(src[j]) & 48) >> 4);
    dest[i++] = ((b64_2_byte(src[j++]) & 15) << 4) + ((b64_2_byte(src[j]) & 60) >> 2);
  }
  *dest_sz = dest_size;
  return dest;
}
```