Re: Understanding GDB frames

[Maxim Grigoriev <maxim@tensilica.com>]

>> I think distinguishing frames can be improved by using the third
>> argument of frame_id, which is a "special address". Most of the
>> implementations don't use it.

> What would you put there?

Let's look at the following frame ID template:

{
  stack_addr   = <frame pointer of the frame>
  code_addr    = <PC of the entry point of the frame function>
  special_addr = (Frame_Level << 24) | (<Return Address> & 0xffffff)	
}

Where   Frame_Level == frame_relative_level (frame) & 0xff .

I think it's going to cover 

1) Recursion ;

2)  void foo (void) { }
    void bar (void) { }

    main ()
    {
      int i;

      for (i = 0; i < 2; i++)
        {
          void (*f) (void) = i ? foo : bar;
          f ();
        }
    }

3) { foo(); foo(); }

4) All the cases when function is called from different levels;

But, this case is still a problem :

5) for (i = 0; i < 10; i++) foo ();

Anyway, doesn't it look like a reasonably inexpensive improvement ?

-- Maxim


Jim Blandy wrote:
> Maxim Grigoriev <maxim@tensilica.com> writes:
>   
>>> It's worth pointing out that the 'PC' in a frame ID isn't the current
>>> PC within the function.  It's always the PC of the function's entry
>>> point, so that stepping within a function doesn't cause the frame ID
>>> to change.  Usually it's a PC value returned by 'frame_func_unwind',
>>> which takes care of calling get_pc_function_start for you.
>>>       
>> We've been using a function return address instead of the PC of the
>> function's entry, and it works just fine.
>>     
>
> Hmm.  Consider:
>
>     void foo (void) { }
>     void bar (void) { }
>
>     main ()
>     {
>       int i;
>
>       for (i = 0; i < 2; i++)
>         {
>           void (*f) (void) = i ? foo : bar;
>           f ();
>         }
>     }
>
> So, main will call bar first, then foo.  Both calls will have
> identical incoming sp's, and identical return addresses.
>
> Using the callee's entry point is more direct; there's less
> opportunity for things to go wrong.  And the rest of GDB is already
> working to cover up the shortcomings of that approach.  :)
>
>   
>> I think distinguishing frames can be improved by using the third
>> argument of frame_id, which is a "special address". Most of the
>> implementations don't use it.
>>     
>
> What would you put there?
>
>