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Re: [RFA] Artifical dwarf2 debug info
On Mon, Dec 16, 2002 at 02:56:55PM -0500, Andrew Cagney wrote:
>
> >>>If I'm scanning this code correctly, all we would need to do would be
> >>>to connect set_unwind_by_pc to the CFI machinery. No, it's more
> >>>complicated than that, we still call both FRAME_CHAIN and
> >>frame_pc_unwind;
> >>>I'm not entirely clear on how frame_saved_regs_id_unwind works.
> >>>Similarly in get_prev_frame.
> >
> >>
> >>FRAME_CHAIN is going away.
> >>
> >>The steps are broadly:
> >> pc = pc-unwind (next_frame)
> >> if (not an edge case like dummy frame where the id doesn't need to
> >> be unwound because the frame can be identified using the callee's ID)
> >> id = id-unwind (next_frame);
> >> create frame from pc/id setting new unwind methods using pc.
> >>(frame_saved_regs_id_unwind is there to keep code that just implements
> >>frame chain working.).
> >
> >
> >Great!
> >
> >
> >>>But what I'd like to see is something like you've sketched above.
> >>>Probably check first for dummy frame, then for sigtramp frame, then for
> >>>CFI frame, and then fall back to the defaults.
> >
> >>
> >>Yes. Should the choices/order be hardwired or specified by the
> >>architecture though? I.e., iterate over a list of possible frames that
> >>are specified by the architecture.
> >
> >
> >Hmm, I'm not sure. Do we have any architectures that would want to
> >specify their own frame types? In such a way that using this CFI
> >approach wouldn't suffice?
>
> Well, I'd not want to be the one enabling CFI on all architectures.
> That code needs some serious cleanups.
Yes. I'd like to start turning this on for other architectures, and I
suspect it'll come on for i386 when Mark K. really gets his teeth into
bringing that together with the x86-64 port. Hopefully it will clean
up over time.
> As for own frame types, a SIGTRAMP frames are one case.
Hmm, good point, that could be handled by an architecture-dependent
list of frame types.
> >>The catch is that it needs to unwind the PC before anything else. That
> >>way it can correctly set the type. Like I said, patch for that pending.
> >
> >
> >Right. I really appreciate all your cleanups in this area. I have
> >some work to do on FRAME_CHAIN_VALID but I'll sit on it for a while,
> >until I see what this looks like when you're done revamping the
> >unwinders. (That's the backtrace-to-or-through-main conversation from
> >some months ago.)
> >
> >Back to the patch at the beginning of this thread - do you think this
> >view of fake CFI information is feasible? Any comments on Michal's
> >patch?
>
> It's feasible. It may long term solve another problem. Apparently GDB
> needs to generate, at run time, debug info for things like Java. It may
> also be easier to handle this case by implementing direct functions and
> not going via CFI.
>
> That actual code, though, is a mess. It is adding another edge case to
> code that shouldn't have to handle anything at all.
Where's the case you're concerned about - are you refering to code in
the tdep file or in dwarf2cfi.c?
> BTW, exactly is the difference between a prologueless and frameless
> function? The prologue case appears to be checking for a push -> the
> reverse of frameless.
The patch doesn't talk about frameless functions particularly - it
checks for _has_prologue, and generates an FDE based on whether it does
or not...
--
Daniel Jacobowitz
MontaVista Software Debian GNU/Linux Developer