Re: short pointers (32 bit) in 64 bit apps

[Nicholas Sherlock <n dot sherlock at gmail dot com>]

On 20/05/2010 3:06 p.m., Ian Lance Taylor wrote:
> I don't see how you can possibly avoid ALU operations given your
> design goals (i.e., permitting the pointers to exist in memory areas
> other than the low 4GB).  You are just asking for a way to make the
> compiler generate them automatically.
>
> Heck, even if you don't like C++, you can do it yourself with macros.

Here's the relative-pointer idea expressed as macros, too:

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

typedef uint32_t relptr;

struct s {
  relptr p;
  int elem;
};

#define GET_RELPTR(p) \
  ((void*) ((p) + (uintptr_t) &(p)))

#define SET_RELPTR(p, q) \
  (p) = (relptr) ((char*) (q) - (char*) &(p))


int main(void) {

  struct s arr[100];

  arr[0].elem=143;
  arr[99].elem=42;

  SET_RELPTR(arr[0].p, &arr[99]);
  SET_RELPTR(arr[99].p, &arr[0]);

  printf("%d\n", ((struct s *) GET_RELPTR(arr[0].p))->elem);
  printf("%d\n", ((struct s *) GET_RELPTR(arr[99].p))->elem);

  return 0;
}

A relptr can point to anywhere else within the same object the pointer 
is stored in, so long as the object is smaller than 4GB.

Cheers,
Nicholas Sherlock