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Re: [rfa/mips] Stop backtraces when we've lost the PC

On Thu, Mar 11, 2004 at 06:47:04PM -0500, Andrew Cagney wrote:
> >On Thu, Mar 11, 2004 at 03:51:11PM -0500, Andrew Cagney wrote:
> >
> >>>>>I hypothesize that if two consecutive frames, regardless of their type,
> >>>>>claim to save the PC register at the same location, then unwinding is
> >>>>>hosed.
> >>
> >>>
> >>>It would need to do a deep analysis of the location (think about a 
> >>>register window architecture), hence I don't know that there's that much 
> >>>cost benefit.
> >>> Something simpler such as a list of functions known to
> >>>terminate the stack might be more useful.
> >
> >
> >Er, no.  frame_unwind_register tells us where, relative to the current
> >machine state, the register is saved.  If it returns lval_register and
> >real_regnum == O7_REGNUM, then that means it leaves in
> >read_register(O7_REGNUM) at this moment, not that it did at some point
> >in the past.  Isn't that the point of the recursive unwinder?
> "Er, no". to which part?  I'll assume the first half of the first half.
> I suspect you're violently agreeing with me here - you're describing 
> what I ment by a deep analysis of the location - tracking things all the 
> way back to where in the inferior the value is.   The architecture 
> vector will need to be changed, the existing function deprecated, and 
> new methods implemented.  The introduction of "struct location" (or 
> whatever) would then see it changed again. Given it is all for a 
> marginal edge case (and to cover up breakage in glibc), I don't see any 
> cost benefit in doing this.

OK.  It was just a thought :)  It seems reasonable that whatever kind
of location frame_unwind_register returns (which you're right, is
likely to change) could naturally be returned by frame_unwind_pc also.
But it would require playing with the interfaces pretty severely, so
I'll just table the idea unless I run into this again somewhere else.

> I think a more useful mechanism is for there to be a table of "start" 
> functions that the user could manipulate (but would default to values 
> specified by the OSABI).

I'm not sure how useful that would really be; we seem to handle the
entry points OK at the moment.  And it couldn't be used for this case
since we do want to backtrace past clone in some circumstances.

Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer

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