Re: Understanding GDB frames

Jim Blandy
2007-05-22T23:56:20Z

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?