Re: RFA: frame id enhancement

[Andrew Cagney <ac131313@redhat.com>]

>> int
>> frame_id_inner (struct frame_id l, struct frame_id r)
>> {
>>   int inner;
>>   if (l.stack_addr == 0 || r.stack_addr == 0)
>>     /* Like NaN, any operation involving an invalid ID always fails.  */
>>     inner = 0;
>>   else
>>     /* Only return non-zero when strictly inner than.  Note that, per
>>        comment in "frame.h", there is some fuzz here.  Frameless
>>        functions are not strictly inner than (same .stack but
>>        different .code).  */
>>     inner = INNER_THAN (l.stack_addr, r.stack_addr);
>>   if (frame_debug)
>>     {
>>       fprintf_unfiltered (gdb_stdlog, "{ frame_id_inner (l=");
>>       fprint_frame_id (gdb_stdlog, l);
>>       fprintf_unfiltered (gdb_stdlog, ",r=");
>>       fprint_frame_id (gdb_stdlog, r);
>>       fprintf_unfiltered (gdb_stdlog, ") -> %d }\n", inner);
>>     }
>>   return inner;
>> }
>> 
>> 
>> does SPECIAL_ADDR add further ordering?  If it doesn't then the comment needs to be updated (and the description in "frame.h" clarified).

> 
> 
> Another good point.  Yes, it does in this case.  Two frames could both not use the stack but one will definitely move the special_addr.  I need to add a SPECIAL_INNER_THAN macro which can default to false and must be overridden by the platform.

Is there real value add in having SPECIAL_INNER_THAN though?  It would 
only be called by frame_id_inner.  Looking at how that method is used:

> frame.c:354:      if (frame_id_inner (id, this))
In frame_find_by_id:  Its sole purpose is to act as a short circuit for 
the unlikely case where the ID isn't present in the frame.  A stonger 
frame_id_inner has little value add.

> frame.c:1909:      && frame_id_inner (get_frame_id (this_frame),
In get_prev_frame:  Its a sainity check to detect what appears to be a 
badly corrupt stack.  Marginal value add?

> infrun.c:2094:      && (frame_id_inner (get_frame_id (get_current_frame ()),
Commented out.

> infrun.c:2383:  if (frame_id_inner (current_frame, step_frame_id))
Received a signal.  Given that a predicate to the call is:
       && INNER_THAN (read_sp (), step_sp))
the code's assumed that a signal modifies frame_id.stack_addr, so there 
is no value add.  It might be useful to clarify this assumption though.

> infrun.c:2477:        && frame_id_inner (step_frame_id,
It's the reverse of infrun.c:2383 where the inferior is falling out of a 
singnal trampoline, I think the assumptions again hold.

> infrun.c:2641:    if (!(frame_id_inner (current_frame, step_frame_id)))
"Trust me" there's no value add.  While the comment reads:
   /* In the case where we just stepped out of a function into the
      middle of a line of the caller, continue stepping, but
      step_frame_id must be modified to current frame */
The test also updates step_frame_id when switching between frameless 
stackless leaf function.  The extra test wouldn't fix that problem. 
I'll try to remember to add some comments to that code.

Andrew