Re: STOPPED_BY_WATCHPOINT peculiarity

[Michael Snyder <msnyder@redhat.com>]

Daniel Jacobowitz wrote:
> 
> On Tue, Apr 02, 2002 at 05:01:45PM -0500, Daniel Jacobowitz wrote:
> > On Tue, Apr 02, 2002 at 01:54:47PM -0800, Doug Evans wrote:
> > > Something is not right, or at least confusing, in watchpoint-land.
> > >
> > > This code in infrun.c is odd:
> > >
> > >     /* It may be possible to simply continue after a watchpoint.  */
> > >     if (HAVE_CONTINUABLE_WATCHPOINT)
> > >       STOPPED_BY_WATCHPOINT (ecs->ws);
> > >
> > > STOPPED_BY_WATCHPOINT is a predicate.
> > > Therefore at first glance this code is pointless.
> > >
> > > Things are slightly less confusing by recognizing that in the
> > > process of computing STOPPED_BY_WATCHPOINT some debugging printf's
> > > may get printed.  e.g. grep for maint_show_dr in
> > > i386-nat.c:i386_stopped_data_address.
> > >
> > > nm-i386.h:
> > > #define STOPPED_BY_WATCHPOINT(W)       (i386_stopped_data_address () != 0)
> > >
> > > Is that all there is to it?
> > >
> > > If so, a comment should be added, maybe something like
> > >
> > > -    /* It may be possible to simply continue after a watchpoint.  */
> > > +    /* It may be possible to simply continue after a watchpoint.
> > > +       While at first glance this code is pointless, STOPPED_BY_WATCHPOINT
> > > +       is called in case there are any maintenance debugging printf's.  */
> > >      if (HAVE_CONTINUABLE_WATCHPOINT)
> > >        STOPPED_BY_WATCHPOINT (ecs->ws);
> >
> > What astonishing timing... I believe there is more going on here, and I
> > was in the middle of looking at this code just a moment ago.  See the
> > test failure on i386-linux in gdb.c++/annota2.exp (watch a.x).  We have
> > a problem actually correctly detecting that we are stopped by a
> > watchpoint.
> 
> Actually, I take that back.  STOPPED_BY_WATCHPOINT has nothing to do
> with the problem I'm working on.  This line came in far enough back
> that the public tree's CVS history appears to be useless; it's been
> through some reformats but that's it.  Might want to ask a Cygnus
> person to dig around.

Yo!   ;-)

1) History: the line was added in 1995 by Jim Kingdon.
I can't find any explanation in the changelogs or comments.

2) Yes, it is possible for a predicate to have side effects, 
and indeed several implementations of this one (eg. sparc)
cause a register to be read, and one (ia64) causes a register
to be modified.  Based on that, I don't think it is safe to
remove the line, without much more deliberation.