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

[Michael Snyder <msnyder@redhat.com>]

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.