Re: AVR indirect_jump addresses limited to 16 bits

[Paul Schlie <schlie at comcast dot net>]

> From: Marek Michalkiewicz <marekm@amelek.gda.pl>
>> On Sun, Mar 20, 2005 at 04:29:01PM -0800, Richard Henderson wrote:
>> The easiest way is to do this in the linker instead of the compiler.
>> See the xstormy16 port and how it handles R_XSTORMY16_FPTR16.  This
>> has the distinct advantage that you do not commit to the creation of
>> an indirect jump until you discover that the target label is outside
>> the low 64k.
> 
> Looks perfect to me.  So we are not the first architecture needing
> such tricks...  AVR would need 3 new relocs, used like this:
> 
> .word pm16(label)
> 
> ldi r30,pm16_lo8(label)
> ldi r31,pm16_hi8(label)
> 
> and the linker can do the rest of the magic (add jumps in a section
> below 64K words if the label is above).
> 
> Cc: to Denis, as I may need help actually implementing these changes
> (you know binutils internals much better than I do).

- yup, and nicer than trying to play games with alignment, etc.,

  And just to double check, using the earlier example:

> int foo(int dest)
> {
>    __label__ l1, l2, l3;
>    void *lb[] = { &&l1, &&l2, &&l3 };
>    int x = 0;
> 
>    goto *lb[dest];
> 
> l1:
>    x += 1;
> l2:
>    x += 1;
> l3:
>    x += 1;
>    return x;
> }

  It would seem that the only time the pm16(label) address would ever
  be used, would as an initializing constant pointer value being assigned
  to a  _label_/function pointer variable; as a CALL/JUMP LABEL instruction
  would be used to call/jump-to the true entry point directly otherwise.
  (is that correct?)