[Bug c/38295] New: Support pointer difference as constant in static initializer

[gnu at behdad dot org]

The feature I'm proposing is not supported by the C standard, so I'm proposing
a gcc extension.

I am wondering if it's possible to make the difference of two function pointer
be a constant value if the two functions are defined as static and in the same
file.  That would allow for function pointer tables that live in .rodata
instead of .data.  Something like:

#include <stdio.h>

typedef int (*func_t) (void);

static int func_base (void)
{
  return 0;
}

static int func0 (void)
{
  return 0;
}

static int func1 (void)
{
  return 1;
}

size_t funcs[] = {
  func0 - func_base,
  func1 - func_base
};

int
main (void)
{
  int i;

  for (i = 0; i < 2; i++)
    printf ("%d: %d\n", i, ((func_t)(funcs[i] + func_base))());
}


-- 
           Summary: Support pointer difference as constant in static
                    initializer
           Product: gcc
           Version: 4.3.2
            Status: UNCONFIRMED
          Severity: enhancement
          Priority: P3
         Component: c
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: gnu at behdad dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=38295