least portable C hello world program

hello.c

// only works on Compiler Explorer with x86-64 gcc
// gcc hello.c -o hello && ./hello

extern int(puts)(char const *);

char static *message = "Hello World!\n";

__attribute__((used)) void f(void)
{
  puts(message);
}

int main(void)
{
  asm("mov $0x402004, %edi");
  asm("call 0x401030");
  asm("xor %eax, %eax");
  asm("pop %rbp");
  asm("ret");

  while (1)
  {
    f();
  }
}