#include <stdint.h>
#include <stdio.h>

int
main (int argc, const char *argv[])
{
  const char *p;
  uint64_t n0, n1;
  int n;

  p = (void *)-1;
  /* warning: cast from pointer to integer of different size [-Wpointer-to-int-cast] */
  n0 = (unsigned int)p;         /* Not good.  It's masked.  */
  printf ("0: %p : %016llx\n", p, n0);
  n1 = (uintptr_t)p;            /* Good.  It's kept as is.  */
  printf ("1: %p : %016llx\n", p, n1);

  n = -1;

  /* works by sign extend, but warning for different size. */
  /* warning: cast to pointer from integer of different size [-Wint-to-pointer-cast] */
  p = (void *)n;
  printf ("ok:   %p\n", p);

  /* works by sign extend, explicitly request sign extend. */
  p = (void *)(intptr_t)n;
  printf ("good: %p\n", p);

  /* works by no sign extend.  */
  p = (void *)(uintptr_t)n;
  printf ("ok:   %p\n", p);

  /* doesn't works by no sign extend.  */
  p = (void *)(uintptr_t)(unsigned int)n;
  printf ("bad:  %p\n", p);

  return 0;
}

$ ./a.exe 
0: ffffffffffffffff : 00000000ffffffff
1: ffffffffffffffff : ffffffffffffffff
ok:   ffffffffffffffff
good: ffffffffffffffff
ok:   ffffffffffffffff
bad:  00000000ffffffff