NULL selected/current frame; Was: [patch/rfc] Add frame_read_signed/unsigned_register(); convert h8300 to print_registers_info()

[Andrew Cagney <ac131313@redhat.com>]

>>  void
>> +frame_read_unsigned_register (struct frame_info *frame, int regnum,
>> +			      ULONGEST *val)
>> +{
>> +  frame_unwind_unsigned_register (get_next_frame (frame), regnum, val);
>> +}
> 
> 
> So, the register belonging to this frame.

Yes.  The old roughly equivalent function was get_saved_register.

> That means the register
> which would be in the hardware registers if this frame were current,
> right?

To be pedantic, no.  A frame's registers are ALWAYS found by unwinding 
get_next_frame(FRAME).  It just so happens that registers unwound from 
get_next_frame(current_frame) come from the register cache.

The difference is subtle but important.  current_frame isn't the special 
case, get_next_frame(current_frame) is.  Unfortunatly much of the GDB 
code treated ``current_frame'' as special creating unnecessary 
complexity and ongoing confusion.  Per generic_unwind_get_saved_register():

   /* Reached the the bottom (youngest, inner most) of the frame chain
      (youngest, inner most) frame, go direct to the hardware register
      cache (do not pass go, do not try to cache the value, ...).  The
      unwound value would have been cached in frame->next but that
      doesn't exist.  This doesn't matter as the hardware register
      cache is stopping any unnecessary accesses to the target.  */

   /* NOTE: cagney/2002-04-14: It would be nice if, instead of a
      special case, there was always an inner frame dedicated to the
      hardware registers.  Unfortunatly, there is too much unwind code
      around that looks up/down the frame chain while making the
      assumption that each frame level is using the same unwind code.  */

> Should we allow NULL to imply the current frame?

Definitly no :-)

There  is a bit of dogma here - there is always a frame.  The above 
should not be called with NULL.  Code that calls this checks that 
selected_frame != NULL.  I'll add a ``gdb_assert (frame != NULL)'' and a 
comment to that effect.

As for get_next_frame(FRAME), if that returns NULL, we've fallen off the 
inner most frame and should get the register value from the register 
cache.  (Oh, while get_next_frame(current_frame) will always return 
NULL, there may come a time when current_frame->next does not :-).

enjoy,
Andrew