Re: frame->unwind->this_base()

Daniel Jacobowitz
2003-03-19T15:32:27Z

On Wed, Mar 19, 2003 at 10:24:37AM -0500, Andrew Cagney wrote:
>
> >>True dwarf2 debug info or that .eh_frame stuff (i'm curious)?
> >
> >
> >Hmm, I thought it would write out .debug_frame without DWARF-2 but
> >peering at the GCC source I seem to be wrong again. So just .eh_frame.
>
> So using .eh_frame is along the same lines as using ia64's libunwind.
> .eh_frame just happens to be implemented using something very like
> dwarf2's unwind code.
>
> >In any case, we'll parse both, so I stand by my statement. We'll have
> >.eh_frame even without normal debug info.
>
> I think there needs to be separate eh-frame and cfi-frame but with a
> common implementation. That way the subtle, but important, differences
> are clear.

What's your perception of the difference? Just based on the different
on-disk format? The information contained is essentially the same. I
certainly don't object, though. This will be painless.

> >>For stabs to work, it needs FRAME_LOCALS_ADDRESS(); and
> >>FRAME_LOCALS_ADDRESS() relies on the prologue analyzer (since frame ID
> >>won't correspond to `frame-base') for the computation of the correct
> >>value; and that means unwinding the same frame two ways. Outch.
> >
> >
> >Yeah...
> > - if we have CFI use it to find the frame address. Does this
> >become the frame ID?
> > - if we have dwarf2 debug and CFI, then we don't need to do prologue
> >analysis; CFI should give us everything we need
> > - if we have stabs debug and CFI, then we do need to do prologue
> >analysis to get FRAME_LOCALS_ADDRESS
> > - if we have either kind of debug info and no CFI then we need to do
> >prologue analysis; for dwarf2 we'll also need to calculate the frame
> >base from DW_AT_frame_base in order to use it to find locals
> >
> >Is that about right?
>
> Yes. Try the following higher-level view of the problem:
>
> On the left is the unwinder. It exports methods to obtain the frame's
> ID and the registers. It can be implemented using CFA, EH, libunwind,
> prologue analysis, ...; and the implementation is selected based on the
> low-level unwind information, or lack there of.
>
> On the right is the local variable code and that needs a frame-base /
> frame-locals-address / .... It uses high-level debug info and unwound
> register values to compute that base. It can be implemented using
> dwarf2's frame_base, or prologue analysis (stabs), or ...; and the
> implementation is selected based on the frame's high-level debug info.
>
> The mess occures when the high-level RHS frame-locals-address starts
> assuming the flavour of the low-level LHS unwinder and, consequently,
> tries to directly exploit that knowledge. For instance, a RHS prologue
> based frame-locals-address assuming that the LHS is also prologue based,
> and hence, can directly access the LHS's prologue analysis cache.
>
> It can be `fixed' two ways:
>
> - refusing to allow that sharing of data, forcing the RHS
> frame-locals-address to re-analyze the prologue.
>
> - make it possible to tease out the prologue analysis object so that
> both the LHS and RHS can share it.
>
> I guess the second is it.

I agree. Personally, what I would like would be to generate something
structurally/conceptually equivalent to the unwind data from the
prologue reader. This requires a bit of rewriting and a great deal of
testing/patience.