frame->unwind->this_base()

[Andrew Cagney]

> On Mon, Mar 17, 2003 at 01:20:28PM -0500, Andrew Cagney wrote:
> 
>> 
> 
>> >>>GDB's frame code also makes available the get_frame_base() method. While 
>> >>>the default implementation returns get_frame_id().base, I think there is 
>> >>>going to need to be a per-frame frame->unwind->this_base method.
> 
>> >
> 
>> >>
>> >>get_frame_base() returns ->frame and NOT ->id.base.
> 
>> >
>> >
>> >OK, I'm definitely going around in confused little circles.  Don't the
>> >two statements above disagree?
> 
>> 
>> No.  See get_prev_frame() where it is defaulting ->frame to ->id.base.
>> 
> 
>> > The current get_frame_base does return
>> >->frame but you also say above that get_frame_base should return
>> >get_frame_id().base.
> 
>> 
>> No.  Default to get_frame_id().base.
> 
> 
> So is that supposed to be a statement about the future in the first
> paragraph?  It's sure not worded as one, no wonder I'm confused.

The present.  GDB historically has had FRAME_FP and frame->frame and 
their intended purposes were overloaded: per frame unique identifier, 
frame base pointer, ...

Frame ID provides a per-frame unique identifier.

>> >Conceptually, are frame->frame and frame->id.base supposed to be the
>> >same?
> 
>> 
>> No?
> 
> 
> Then could you enlighten me as to what the difference is supposed to
> be?

For dwarf2, check the spec where it discuss CFA (see CFI) and frame-base 
(see 3.3.5).

CFA + &function == frame_id
A per frame unique identifier that must be constant through out the 
lifetime of the frame.  CFI recommends the top-of-stack from the 
previous frame.

frame-base == get_frame_base()
What ever the debug info would like it to be.   The ISAs ABI will 
provide a strong set of guidelines though (if, for a framed function it 
doesn't match what the user expects, the'll likely complain :-).  It 
will often point into the middle of the stack frame.

Andrew