Re: [RFA/PATCH] breakpoint.c: fix until command

[Daniel Jacobowitz <drow@mvista.com>]

On Thu, Jan 02, 2003 at 05:44:42PM -0800, Michael Snyder wrote:
> Elena Zannoni wrote:
> > 
> > Elena Zannoni writes:
> > >  > Nevertheless, that is and has always been the intent.
> >  >  > If you're in factorial(5), and you say "until 100",
> >  >  > you don't stop until line 100 is hit by factorial(5).
> >  >
> >  >
> >  > I am tracking down this to something that changed between (ahem...)
> >  > 4.18 and 5.0. The code in breakpoint.c didn't change. Right now,
> >  > stepping the two gdb's side to side, I can see a difference in
> >  > get_prev_frame, because of a different value returned by
> >  > FRAME_CHAIN_VALID. :-( (i have not still stepped past that to see how
> >  > that could influence the until foo behavior, maybe it doesn't).
> >  >
> >  > The behavior you specify above is in 5.0 and not in 4.18, while the
> >  > 'until foo' works in 4.18 and is broken in 5.0.
> >  >
> >  > More digging.
> >  >
> >  > Elena
> > 
> > OK. The reason for which 'until foo' worked at all in 4.18 is totally
> > fortuitous.  It is because of this patch in breakpoint.c:
> > 
> > 1998-09-08  Jason Molenda  (jsm@bugshack.cygnus.com)
> > 
> >         * breakpoint.c (bpstat_stop_status):  Declare a bp match if the
> >         current fp matches the bp->fp OR if the current fp is less than
> >         the bp->fp if we're looking at a bp_step_resume breakpoint.
> > 
> > Index: breakpoint.c
> > ===================================================================
> > RCS file: /cvs/cvsfiles/src/gdb/breakpoint.c,v
> > retrieving revision 1.190
> > retrieving revision 1.191
> > diff -u -p -p -r1.190 -r1.191
> > --- breakpoint.c        1998/07/17 15:29:10     1.190
> > +++ breakpoint.c        1998/09/09 04:16:57     1.191
> > @@ -1506,7 +1506,9 @@ bpstat_stop_status (pc, not_a_breakpoint
> >        else if (DECR_PC_AFTER_BREAK != 0 || must_shift_inst_regs)
> >         real_breakpoint = 1;
> > 
> > -      if (b->frame && b->frame != (get_current_frame ())->frame)
> > +      if (b->frame && b->frame != (get_current_frame ())->frame &&
> > +          (b->type == bp_step_resume &&
> > +           (get_current_frame ())->frame INNER_THAN b->frame))
> >         bs->stop = 0;
> >        else
> >         {
> > 
> > Note that this added condition is always false for a bp_until type
> > breakpoint.  So, effectively we were invalidating the check of the
> > current frame vs. bp->frame. And we always stopped.
> > 
> > However, since we were not checking the frames, the case Michael wants
> > didn't work.
> > 
> > The patch above was reverted in 1999:
> > 
> > 1999-08-13  Jim Kingdon  <http://developer.redhat.com/>
> > 
> >         * breakpoint.c (bpstat_stop_status): Revert 1998-09-08 change
> >         to ->frame matching.  The change did not match the ChangeLog
> >         entry, looked fishy, and caused infinite stepping when running
> >         "next" from main on sparc w/ RH Linux.  Thanks to Jakub for the
> >         report.
> > 
> > the effect was that the frame matching check was re-enabled, and so
> > 'until foo' stopped working.
> > 
> > I don't think there is a way to have both behaviors work correctly.  I
> > thought of checking that the pc which you want to run until is in
> > the same function as the one of the selected frame, and in that case
> > enforce the check (by using a non-null frame for the bp_until),
> > otherwise use the null frame (which disables the check). But what would
> > be the correct behavior if you say:
> > 
> > "until bar" where bar is recursive, and you are in "bar" at the
> > moment?  This doesn't work currently. It seems intuitive that you
> > would stop the next time you enter "bar". Right now you end up at the
> > caller of "bar".
> > 
> > I think it is a matter of deciding which behavior is more useful.
> > 
> > (note that I tried to revert Jason's patch in stock 4.18 and 'until
> > foo' stopped working, i.e. it wasn't something else that broke between
> > 4.18 and 5.0)
> 
> You raise a good point.  The commands "until <line>" and "until <func>"
> are inconsistant.  Moreover the docs do not seem to describe this
> recursion behavior.  Maybe a conversation with a wider audience is
> in order (the gdb list)?  I'm sure I can't be the only one who 
> remembers that "until" behaved this way, and we shouldn't change
> the behavior precipitously.

Am I the only one getting the feeling that we have two useful behaviors
here; and that we should pick one for "until" but expose the other
under some other name or with some option?

-- 
Daniel Jacobowitz
MontaVista Software                         Debian GNU/Linux Developer