Re: Display of read/access watchpoints when HAVE_NONSTEPPABLE_WATCHPOINT

[Orjan Friberg <orjan dot friberg at axis dot com>]

Eli Zaretskii wrote:
> Anyway, from your description, it is quite clear that if a target
> defines HAVE_NONSTEPPABLE_WATCHPOINT, GDB must call
> target_stopped_data_address before it disables the watchpoint and
> steps over it, or else the target end should store the necessary info
> somewhere and deliver it when target_stopped_data_address is called.
>
> In other words, it sounds like a bug.

I had missed a previous thread on the subject, initiated by Paul Koning 
(starting at 
http://sources.redhat.com/ml/gdb-patches/2003-05/msg00506.html).  Paul 
sent me a patch for 5.3 which no longer applies cleanly, and with his 
blessings I'm trying to update his patch to the current CVS.

Doing that, I encountered a problem with read watchpoints (rwatch 
command): in insert_breakpoints, before calling insert_bp_location, the 
current value of the watched expression is read.  Then in 
bpstat_stop_status, when the watchpoint has hit, there's the following 
code snippet:

case WP_VALUE_CHANGED:
  if (b->type == bp_read_watchpoint)
    {
      /* Don't stop: read watchpoints shouldn't fire if
         the value has changed.  This is for targets
         which cannot set read-only watchpoints.  */
      bs->print_it = print_it_noop;
      bs->stop = 0;
      continue;
    }
  ++(b->hit_count);
  break;

So, if we're "read watching" a variable that indeed has changed since we 
read it before inserting it we decide not to stop.  Here's another 
example (foo is rwatched):

  volatile int foo = 0;
  int a;

  a = foo;   /* foo read; stops.  */
  foo = 1;
  a = foo;   /* foo read; doesn't stop because value changed.  */


The "Don't stop" comment is consistent with the behaviour, so it seems 
like this is intended.  What am I missing here?

--
Orjan Friberg
Axis Communications