[LLVMdev] Preserving accurate stack traces with optimization?

[Reid Kleckner]

On Wed, Oct 30, 2013 at 8:58 PM, Eric Christopher <echristo at gmail.com>wrote:

>
> On Oct 30, 2013 7:25 PM, "Philip Reames" <listmail at philipreames.com>
> wrote:
> >
> > On 10/30/13 7:09 PM, Philip Reames wrote:
> >>
> >> David, Quentin - Thanks for the feedback.  Responses inline.
> >>
> >> On 10/30/13 11:21 AM, David Blaikie wrote:
> >>>
> >>> Actually CCing Eric.
> >>>
> >>>
> >>> On Wed, Oct 30, 2013 at 11:00 AM, Quentin Colombet <
> qcolombet at apple.com> wrote:
> >>>>
> >>>> Philip,
> >>>>
> >>>> Thanks for the clarification.
> >>>>
> >>>> As far as I can tell, there is currently no way to preserve a full
> and accurate stack trace while utilizing most of LLVM’s optimization
> abilities.
> >>>>
> >>>> The work on debug information may help you get the information you
> need, but I do not think we will provide information on stack frames that
> have been removed via inlining or tail call.
> >>>
> >>>
> >>> In theory, at -gmlt we should emit enough debug info to give you
> accurate stack traces including inlined frames. Tail calls I assume we
> can't do anything about.
> >>
> >> Tail calls I'm not too worried about.  I'm reasonably sure that our
> existing optimizer doesn't do any tail call optimizations.  Given that,
> turning them off doesn't worry me too much performance wise.
> >>
> >> First, thank for you for mentioning the -gmit option.  I had been
> completely unaware of that.  I'll have to dig into the implementation and
> usage a bit.  Can you point me to any documentation?  A quick google search
> didn't turn up anything.
> >>
> >> Can you clarify two things for me?  First, is the intent that -gmit
> *always* provide accurate stack traces?  (modulo bugs of course)  If so,
> what is your subjective opinion on how close it comes to meeting that goal
> today?  (i.e. how much help would we need to contribute to get it to a
> solid state?)
> >
> > David - Redoing my google searching with "gmlt" (i.e. the correct
> spelling) I see more useful links.  From what I can gather, -gmlt emits a
> strict subset of the debug information you'd see at -g.  Are you saying
> that "-g" should always be sufficient for accurate stack traces - even in
> the face of inlining?  This would be ideal for us.
> >
>
> It should yes. Any time where it can't is a bug in either our
> implementation or in DWARF itself. :)
>
What about tail calls?  I didn't think we knew how to handle that.
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