Re: [RFA] Fix watchpoints when stepping over a breakpoint

Eli Zaretskii
2002-04-06T07:48:20Z

> Date: Fri, 5 Apr 2002 12:08:51 -0500
> From: Daniel Jacobowitz <drow@mvista.com>
>
> I think GDB ought to show that both the breakpoint and watchpoint have
> fired. At least, that's the behavior I would expect. I also thought
> that was what it would do, but I can't seem to make that happen.

Try using a hardware-assisted breakpoint, not a normal breakpoint.
Since the latter works by replacing the instruction with a breakpoint
opcode, you cannot have a breakpoint and a watchpoint at exactly the
same PC value, because doing so replaces the instruction that's
supposed to write into some data with the breakpoint opcode.

> Also bear in mind that if you have this sequence:
> - write to x
> - other instruction <--- breakpoint here
> You will stop based on the watchpoint, because the watchpoint happens
> first.

That's okay, since the instruction that writes to x is before the
breakpoint. In this case, I'd expect to have a watchpoint, then,
when I continue, I'd expect to hit the breakpoint.

> It's only if we expected a trap (single stepping for instance) that
> this does not work.

If this is limited to stepping, can we check whether we are stepping
instead of (or in addition to) the test for whether to ignore
breakpoints?

> Without my patch, we detect that we are at an address with a
> breakpoint, and don't even try to check our watchpoints.

If we change GDB to report both the breakpoint and watchpoint, the
problem would go away, no?

> [In fact, I'm having a great deal of trouble with hardware watchpoints
> surviving re-running. Remember that conversation from several months
> ago?

Yes. This is definitely wrong behavior, IMHO. IIRC, the problem is
that GDB doesn't initialize the ``old value'' correctly on the rerun,
and so when the watchpoint hits, it thinks it's a false positive,
because watchpoints are suppressed if the watched value doesn't
change.

> > More importantly, an introduction of a general-purpose mechanism to
> > ignore breakpoints is something that I consider to be dangerous,
> > because it is no longer limited to special situations such as
> > single-stepping.
>
> Well, we could just as easily call the flag "single_stepping"... That
> would probably limit abuse.

If all else fails, at least that, yes.