Re: frame->unwind->this_base()

Daniel Jacobowitz
2003-03-18T05:13:52Z

On Mon, Mar 17, 2003 at 08:05:58PM -0500, Andrew Cagney wrote:
> >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.

So in this case should we be hooking the get_frame_base() call to
return the computed DW_AT_frame_base? If so, we're going to need to go
through all the uses and computations of the frame base in all targets
for consistency. And what happens if we don't have DWARF-2
information?

I guess I just don't see how this evolves.